Physical System Dynamics and

White Phosphorus Fate and

Transport, 1994, Eagle River Flats,

Fort Richardson, Alaska

Daniel E. Lawson, Lewis E. Hunter, Susan R. Bigl,

Beth M. Nadeau, Patricia B. Weyrick and John H. Bodette August 1996

CRREL REPORT 96-9
Abstract: Eagle River Flats (ERF) is a subarctic estuarine salt marsh where human and natural forces are

causing significant changes in the environment. Multiple internal and external forces govern the physical

and chemical processes by actively altering surface

conditions, sometimes in unpredictable ways. ERF is

also used as an artillery range by the U.S. Army, where

past use has resulted in white phosphorous (WP) contamination of the sediments within ponds and mudHow to get copies of CRREL technical publications:

Department of Defense personnel and contractors may order reports through the Defense Technical Information Center:

DTIC-BR SUITE 0944

8725 JOHN J KINGMAN RD

FT BELVOIR VA 22060-6218

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WWW http://www.dtic.dla.mil/

All others may order reports through the National Technical Information Service:

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Telephone 1 703 487 4650

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A complete list of all CRREL technical publications is available from:

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For information on all aspects of the Cold Regions Research and Engineering Laboratory, visit our World Wide Web site:

http://www.crrel.usace.army.mil

Cover: Aerial view of In-Between Gully (center) with stranded ice floes adjacent to it. WP-contaminated

sediments can be transported by ice pulled from ponds during tidal flooding.

flats. Bottom-feeding waterfowl ingest this WP, which

causes rapid death. This report documents analyses

of the physical environment, describing the nature of

the physical systems and factors controlling them. It

includes data on sedimentation, erosion and hydrology. These investigations provide knowledge necessary to designing and evaluating remedial technologies. They also help determine the system’s capacity

to naturally attenuate the WP contamination.
CRREL Report 96-9

Physical System Dynamics and

White Phosphorus Fate and

Transport, 1994, Eagle River Flats,

Fort Richardson, Alaska

Daniel E. Lawson, Lewis E. Hunter, Susan R. Bigl,

Beth M. Nadeau, Patricia B. Weyrick and John H. Bodette August 1996

Prepared for

U.S. ARMY ALASKA, FORT RICHARDSON, ALASKA

and

U.S. ARMY ENGINEER DISTRICT, ALASKA

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
ii

PREFACE

This report was prepared by Dr. Daniel E. Lawson, Research Physical Scientist,

Geological Sciences Division, Susan R. Bigl, Research Physical Scientist, Civil and

Geotechnical Engineering Research Division, Lewis E. Hunter, Research Physical

Scientist, Geological Sciences Division, Beth M. Nadeau, Physical Scientist, Geological Sciences Division, Patricia B. Weyrick, Physical Science Technician, Snow and Ice

Division, and John H. Bodette, Civil Engineering Technician, Civil and Geotechnical

Engineering Research Division, Research and Engineering Directorate, U.S. Army

Cold Regions Research and Engineering Laboratory.

Funding and support for this research was provided through the U.S. Army

Engineer District, Alaska, by the U.S. Army Alaska, Fort Richardson, Alaska. The

authors thank William Gossweiler, Douglas Johnson, William Smith and Laurie

Angell for helpful discussions, logistical support and assistance in the field, as well

as for their continuing support, and Dr. Charles Racine, Marianne Walsh and Charles

Collins for numerous discussions on the Eagle River Flats project and their assistance

in the field and laboratory. Also thanked are Dennis Lambert, Troy Arnold and Ron

Bailey for their assistance in the field, and Cora Farnsworth for her contribution in

preparing the report. Earlier versions of this report have been reviewed by Larry

Gatto and Jeffrey Strasser.

The contents of this report are not to be used for advertising or promotional

purposes. Citation of brand names does not constitute an official endorsement or

approval of the use of such commercial products.
iii

CONTENTS

Page

Preface ................................................................................................................... ii

Executive summary ............................................................................................. vii

Introduction .......................................................................................................... 1

Environmental conditions .................................................................................. 1

Background and previous studies .................................................................... 3

Study objectives ................................................................................................... 11

Study sites and methods ..................................................................................... 11

Transects........................................................................................................... 11

Sedimentation and erosion ............................................................................ 11

Gully erosion and recession .......................................................................... 15

Water quality parameters .............................................................................. 17

WP transport sampling .................................................................................. 21

Bathymetric profiling ..................................................................................... 21

Length of data record ..................................................................................... 22

Tidal hydrology and inundation ....................................................................... 22

Water quality parameters and tidal flats hydrology ...................................... 27

Source waters .................................................................................................. 28

Coastal sites ..................................................................................................... 28

Central sites ..................................................................................................... 29

Otter Creek....................................................................................................... 30

Sediment transport and sediment sources ....................................................... 31

Sedimentation ...................................................................................................... 33

Erosion dynamics ................................................................................................ 37

Driving mechanism and system response .................................................. 37

Erosion and recession rates ........................................................................... 37

Implications for remediation methods ........................................................ 45

Gully hydraulics and discharge ........................................................................ 46

Gully profiles ................................................................................................... 46

Gully discharge ............................................................................................... 47

White phosphorus erosion and transport ........................................................ 49

Bathymetric monitoring and white phosphorus deposition ......................... 53

Bathymetric profiles ....................................................................................... 53

Potential areas of WP deposition .................................................................. 54

Ground water information ................................................................................. 54

Conclusions .......................................................................................................... 57

Literature cited ..................................................................................................... 57

Appendix A: TSS analysis .................................................................................. 60

Appendix B: Seasonal summary of erosion rates ........................................... 61

Appendix C: Tidal inundation data for Bread Truck, Parachute and

Spring gullies .............................................................................................. 62
iv

Page

Appendix D: Preliminary estimates of gully water discharge and

sediment flux .............................................................................................. 63

Abstract ................................................................................................................. 64

ILLUSTRATIONS

Figure

1. Upper Cook Inlet region, showing ERF and several other esturine

salt marshes ............................................................................................ 1

2. General distribution of ponds and the primary drainage system ......... 2

3. Distribution of primary landform–vegetation units ............................... 3

4. 1994 aerial photographs of ERF .................................................................. 4

5. Aerial photographs of ERF .......................................................................... 7

6. Drainage pattern analysis of 1993 aerial photograph of C and D

areas ......................................................................................................... 11

7. Transect locations ......................................................................................... 12

8. Sedimentation measurement techniques .................................................. 12

9. Paint layer depth within block of sediment is measured to define

sedimentation rates ............................................................................... 13

10. Pond sedimentation stations ....................................................................... 14

11. Sedimentation station in a pond site.......................................................... 14

12. Example of sedimentation station at intermittent pond location

showing layout of cup and plate sampler ......................................... 15

13. Gully headwall and lateral wall erosion sites .......................................... 15

14. Examples of gully erosion sites .................................................................. 16

15. Location of surveyed longitudinal cross sections of gullies ................... 17

16. Locations of instrumentation recording water quality parameters

and water depths ................................................................................... 18

17. Active hydrostations during the 1994 season ........................................... 19

18. Schematic of hydrostation showing layout .............................................. 20

19. Platform and instrumentation measuring discharge characteristics

at the Bread Truck Gully location ....................................................... 20

20. Plankton net used for collection of WP in transport through

gullies ...................................................................................................... 21

21. Location of selected bathymetric profiling transect lines ....................... 22

22. Typical high-tide curve showing period of sedimentation .................... 24

23. Water elevations as measured by pressure transducer for two

flooding tide events ............................................................................... 24

24. Wind characteristics and effects on tide heights at Parachute and

Tanker gullies, 6–8 October 1994......................................................... 24

25. Relationship of water rise time to peak water elevation at Bread

Truck Gully ............................................................................................ 25

26. Water elevation record at Parachute Gully relative to elevations

required to flood selected landforms ................................................. 25

27. Peak water elevation vs. flooding time at Bread Truck Pond,

September 1994 ...................................................................................... 25
Figure Page

28. Peak water elevation vs. runoff time at Bread Truck Gully, June–

November 1994 ...................................................................................... 25

29. Relationships among the duration of tidal flooding and tidal

delay with gully length and distance from gully headwall

to Knik Arm............................................................................................ 26

30. Delay in peak high tide from Anchorage to ERF sites, June through

November ............................................................................................... 26

31. Hydrostation record at Mortar Gully ........................................................ 27

32. Variation in water quality parameters at two gully locations

during 7–8 September flooding cycle ................................................. 28

33. Hydrostation record at Spring Gully ......................................................... 30

34. OBS levels at Parachute Gully during flooding periods in June,

September, October and November 1994 .......................................... 30

35. Hydrostation record at Otter Creek ........................................................... 31

36. TSS measurements at Parachute Gully through four tidal cycles

in September 1994.................................................................................. 32

37. TSS measurements at coastal sites during single tidal cycle, 3–4

November ............................................................................................... 32

38. Typical TSS variation through a flooding cycle ....................................... 32

39. Maximum monthly TSS measurements .................................................... 32

40. Coastal vs. river TSS measurements .......................................................... 32

41. Velocity profile for Eagle River on 5 September 1994 with

estimates of discharge ........................................................................... 33

42. Comparison of Eagle River profiles from 17 May and 5

September 1994 ...................................................................................... 33

43. Net seasonal sedimentation rates at sites sprayed with paint in

August 1993 ............................................................................................ 34

44. Sediment accumulation and elevation along transect 19 on

Racine Island .......................................................................................... 34

45. Relation between total sediment accumulation recorded in

September 1994 and vegetative cover at sites sprayed with

paint in 1992 ........................................................................................... 35

46. Accumulated sedimentation with distance inland from coast

along line 16 in northwest Area A ...................................................... 35

 47. Sediment accumulation and elevation along transect line 18 in

Area A ..................................................................................................... 35

48. Summary of sediment accumulation in pond locations during

summer 1994 .......................................................................................... 36

49. Longitudinal profiles along two gully thalwegs ...................................... 38

50. Gully scarp locations for measuring erosion rates ................................... 39

51. River bank erosion at the River Section-North ........................................ 42

52. Eastward view of eroded cuspate embayment at monitored

section of B-Gully where white phosphorus was recorded

in transport ............................................................................................. 42

53. Examples of slope failure ............................................................................ 43

54. Rotational slump block failure at River Section-North ........................... 44

55. Sites of observed or suspected riverbank erosion.................................... 44

v
Figure Page

56. Drainages within C/D Area near permanent ponds with arrows

showing locations of anticipated progressive erosion ..................... 45

57. Water flowing over gully headwall at Parachute Gully plunge

pool .......................................................................................................... 46

58. Velocity during flood and ebb in a single tidal cycle .............................. 47

59. Current velocity measurements for tidal inundation and ebb

during the period 5–9 October ............................................................ 48

60. Comparison of peak velocities at the Spring, Bread Truck and

Parachute gullies ................................................................................... 48

61. Gully cross sections at the location of flow probes in Bread Truck

and Parachute ........................................................................................ 49

62. Location maps for WP.................................................................................. 50

 63. Examples of freeze-on to pond ice at ERF................................................. 51

64. Stranded ice blocks around the In-between Gully .................................. 52

65. Knik Arm in the vicinity of ERF ................................................................. 53

66. Bathymetric map of the Knik Arm near the mouth of the River

and adjacent to the coast of ERF.......................................................... 54

67. Bathymetric profiles ..................................................................................... 55

68. Locations of piezometers and monitoring well on ERF .......................... 56

69. Plots of piezometer cluster data ................................................................. 56

70. Ground water monitoring well levels and Hydrolab data at

B-Gully .................................................................................................... 57

TABLES

Table

1. Summer erosional processes ....................................................................... 5

2. Winter erosional processes .......................................................................... 5

3. Summer transport processes ....................................................................... 6

4. Winter transport processes.......................................................................... 6

5. Summer depositional processes ................................................................. 6

6. Winter depositional processes .................................................................... 6

7. Controls on physical processes ................................................................... 6

8. Specifications of sensors used in water quality parameter

measurements ........................................................................................ 18

9. Extent of data record for various measurement parameters.................. 23

10. Pressure sensor elevations and peak high tide delay relative to

predicted time at Anchorage gauge.................................................... 27

11. Characteristic temperature and salinity of Knik Arm and Eagle

River source waters during flood and ebb portions of selected

flooding events ...................................................................................... 29

12. Gross sedimentation rates ........................................................................... 29

13. Comparison of predicted and measured number of flooding events

reaching critical heights during summer and fall 1994 ................... 35

14. Summary of gully and river erosion monitoring ..................................... 38

15. Preliminary estimates of delay before natural pond drainage .............. 43

16. Discharge calculations for October and November 1994 ....................... 46

vi
EXECUTIVE SUMMARY

Eagle River Flats (ERF) is an estuarine salt marsh where natural and human forces

are causing changes in the physical environment. The natural system is governed by

a high tidal range, glacial river influences with a high sediment influx, a subarctic

coastal climate and an active tectonic setting that make ERF susceptible to rapid and

unpredictable changes. Ground explosions from the U.S. Army’s use of ERF as an

artillery range also cause physical changes to the terrain, hydrology and surface

drainage. Smoke-producing rounds detonated above or on the ground have introduced White Phosphorus (WP), which is now distributed in the near-surface sediments of the ponds and marshes (Racine et al. 1992a).

Previous studies have shown that the unusually high duck mortality at ERF is

linked to ingestion of WP particles by bottom-feeding waterfowl (Racine et al.

1992a,b). This study aims at understanding the physical environment of ERF to

evaluate the fate and transport of WP, which is critical to developing remedial

technologies for clean-up.

In this complex and dynamic environment, it is extremely difficult to predict the

effects remedial measures will have on the physical system, and vice versa. Thus,

knowledge of the overall system response, site-specific conditions, and process

interactions is required before appropriate decisions on remedies can be made. In

addition, natural attenuation of WP contamination can result from the site’s physical

and chemical processes, but the potential for this attenuation, and the length of time

for attenuation to take place, requires an understanding of the physical processes. So,

salinity, temperature, dissolved oxygen (DO), pH and turbidity were monitored to

assess tidal and river dynamics during the flood and ebb cycle. Hydrological controls,

including drainage pattern, channel gradients, channel cross sections and flow

velocity, were also analyzed to calculate ebb discharge, sediment and WP particle

transport rates, and to model sediment and water flux.

Tidal height fluctuations were measured in 1994 in concert with water level

changes in eight gullies and the Eagle River where it discharges into ERF. Tidal

currents measured in three gullies indicate that velocities are higher during ebb than

during flood; thus, sediment transport and channel erosion are potentially greatest

during ebb. Eagle River provides an avenue for tidal waters to inundate the innermost

reaches of the Flats. Preliminary analyses show that sedimentation in the northern

two-thirds of ERF is tidally dominated, whereas the southern one-third is river

dominated. Sedimentation ranged from several millimeters per year on levees, 10–

15 mm per year on mudflats, and up to 20–40 mm per year in ponds. Sediment is

derived predominantly from the Knik Arm.

Suspended sediment concentrations were measured in the Eagle River, tidal

gullies draining the ponds and mudflats, and the Knik Arm to define the sediment

sources and to determine the timing of sediment transport. During a tidal cycle, TSS

increases steadily with the flooding tide and decreases at a slower rate during the ebb.

Seasonally, TSS values in gully and Knik Arm waters increase from spring to fall.

Continuously recorded optical backscatter values of turbidity show a similar seasonal relationship, but with a high degree of variability.

Surveyed longitudinal profiles of tidal gully thalwegs revealed that the gullies are

receding into the mudflats and ponds. There are several sharp increases in gradient

(knickpoints) in each gully, marking channel adjustment to the progressive extension

of the tidal gully into the mudflats and ponds. Two gully headwalls, one on the

western side of Bread Truck and the other near the pond complex between Bread

Truck and C ponds, are advancing at a rate sufficient to increase the drainage of those

ponds in the next 15–30 years, if the current average rates persist.

Chemical analyses of sediment trap and plankton net samples indicate that WP

does move and redeposit in ERF proper, and is transported as well through tidal

vii
gullies to the Eagle River and Knik Arm. Bathymetric profiling of Knik Arm located

several potential sites where WP-contaminated sediments may be deposited.

The physical system of ERF is dynamic and changing over seasons, years and

decades. The relative importance of erosional and depositional processes vary from

area to area in response to tidal and river hydrology. Natural sedimentation rates are

high and may aid in the infilling of dredged areas and provide a natural method of

WP burial, thereby reducing the risk of WP ingestion by waterfowl. The drainage

system is actively changing through expansion of tidal gullies into adjacent mudflats

and ponds, with certain pond areas likely to be drained in the next 10–20 years.

Erosion will continue to release WP from mudflat and pond sediments over time,

while existing ice and water transport mechanisms will move WP particles into tidal

gullies, the Eagle River and Knik Arm. Whether these particles survive mechanical

transport and persist in Knik Arm is unknown, but chemical analyses of deposits in

the river and the Arm, coupled with laboratory flume tests of WP particle resistance

to abrasion, could determine if transport may physically destroy them.

Our investigations of the physical system have significantly improved knowledge

of the processes actively changing ERF and determining the flux of sediment and

water in the system. Additional work is required to address the basic issue of whether

these initial data and process relationships are representative of the variability in the

ERF physical system. Further evaluation of the potential for natural attenuation of WP

is also required. Site-specific investigations of contaminated areas requiring

remediation are still necessary to develop criteria and a methodology to evaluate the

potential effectiveness of remedial technologies at each location.

viii
INTRODUCTION

Eagle River Flats (ERF) is an 865-ha tidal flat

and salt marsh at the mouth of Eagle River on

Knik Arm, northeast of Anchorage, Alaska (Fig.

1). The tidal flat has been used as an artillery

range since the early 1940s. Previous work by

CRREL has shown that an unusually high mortality of migratory waterfowl, particularly ducks,

is attributable to the ingestion of elemental white

phosphorus (WP) particles (Racine et al. 1992a,b).

White phosphorus particles were

introduced by smoke rounds detonated during military training

(Racine et al. 1992a, 1993). Particles

of WP are now present in nearsurface sediments at numerous locations in ERF, most notably in

pond and marsh bottom sediments

where dabbling ducks ingest them

during feeding.

This report describes the results

of 1994 investigations of the physical component of the Eagle River

Flats ecosystem. These studies included analyses of the tidal and

river hydrology that are critical for

evaluating remedial technologies

for WP contamination. Proposed

remedies, which include removal

of WP by dredging, in-situ degradation by temporary or permanent

drainage and drying of ponds, and

reduced risk by use of covers (possibly geotextiles), sedimentation

and burial, will affect the ERF

physical ecosystem in largely unknown ways. Studies of specific

sites to be remediated are critical and their influence on the whole physical system must be considered.

ENVIRONMENTAL CONDITIONS

Eagle River Flats is one of several estuarine

tidal flats and salt marshes in the upper Cook

Inlet region of south-central Alaska (Fig. 1). Located at the mouth of the Eagle River, ERF is

Physical System Dynamics and

White Phosphorus Fate and Transport, 1994,

Eagle River Flats, Fort Richardson, Alaska

DANIEL E. LAWSON, LEWIS E. HUNTER, SUSAN R. BIGL,

BETH M. NADEAU, PATRICIA B. WEYRICK AND JOHN H. BODETTE

Figure 1. Upper Cook Inlet region, showing ERF and several other estuarine salt marshes (shaded areas).

N

0 20 miles

0 30 km Study

Area

Salt

Marsh

Palmer

Flats

Goose

Bay

Knik R.

Eagle

R.

Eklutna L.

Fire I.

Susitna Flats

Chickaloon

Flats

Beluga R.

Trading

Bay

Kalgin I.

 INLET

Kenai

Skilak L. Kenai L.

KENAI PENINSULA

TURNAGAIN ARM

Eagle River

Anchorage Flats

FT. RICHARDSON

Kenai R.

Potter

Marsh

KNIK

ARM

Matanuska R.

Redoubt

Bay

COOK

UPPER

C

H

U

G

A

C

H

M

N

T

S.

R. Susitna

Prince William

Sound
about 2.8 km wide at the coast and

narrows inland. This subarctic region

has a transitional maritime to continental climate, with moderate annual

temperatures (daily mean 1.9°C;

minimum mean –2.2°C) and precipitation (330–508 mm; Evans et al.

1972). ERF is inundated by the semidiurnal macrotidal fluctuations (9 to

11 m) in Knik Arm and the resulting

overflow from the Eagle River as it

meets the rising tide.

The Eagle River drains a 497-km2

basin in the Chugach Mountains,

13% of which is covered by glaciers,

yet it takes only a few percent glacier cover to significantly modify the

runoff and sediment yield of the

drainage basin (e.g., Lawson 1993).

Glaciers modify peak discharges, the

timing and volume of hourly, daily

and seasonal discharges, the lag between precipitation and the increase

in runoff it causes, and long-term

trends in annual discharge of the basin (e.g., Gurnell and Clark 1987,

Lawson 1993).

Because of this glacial influence,

the primary melt season of July and August sees

the maximum and peak discharges. River discharge and sediment fluxes vary daily, seasonally and annually, primarily because of glacial

meltwater fluxes (Lawson 1993). Massive quantities of silt- and clay-size particles are transported

in suspension by the large rivers draining

glacierized basins in the Alaska Range, Chugach

Mountains and Kenai Peninsula (e.g., Susitna,

Knik and Matanuska river basins, Fig. 1) and are

discharged into Knik Arm. These materials can

remain suspended in the Arm for extended periods and flow into intertidal wetlands during tidal

inundation.

Average discharge of Eagle River from 1966 to

1981 was about 14.7 m3/s, with a peak discharge

of 177 m3/s having occurred on 18 September

1967 (USGS 1981). During a 10-year observation

period (1966–1975), average monthly river discharge was observed to be lowest in February (1.4

m3/s), with a minimum of 0.68 m3/s having occurred between 29 January and 5 February 1974.

The highest monthly discharges came during July

and August, with mean discharges of 46.9 and

43.5 m3/s respectively. However, owing to the

variability in glacier melting and rainfall, average

discharge for these months ranged widely from

24.7 to 62.9 m3/s, with peak discharges exceeding

100 m3/s when rainfall-induced floods took place

during high melt periods (USGS 1981).

The Eagle River cuts approximately through

the middle of ERF. The ponds and marshes primarily drain into it through vegetated channels

and drainageways, and through unvegetated tidal

gullies that form a dendritic network (Fig. 2).

Where gullies and drainageways do not follow a

dendritic course, their location appears to be controlled by relict drainage networks abandoned

during the evolution of ERF (see Fig. 4a). The

northern and coastal 20% of ERF is drained

through gullies that discharge directly into Knik

Arm. Surface water enters ERF from uplands on

the east, west and southern boundaries; uplands

are composed of glacial deposits and are covered

by spruce and birch forests.

As with other estuarine salt marshes in Cook

Inlet (e.g., Hanson 1951, Vince and Snow 1984,

Rosenberg 1986), vegetation grows in zones that

are commonly determined by elevation and related to the landforms of ERF (Fig. 3). We believe

this relationship to be a function of flooding frequency, salt tolerance, drainage capacity and sedi-

   



0 500 m

Eagle River


Permanent Pond


Intermittent Pond


Drainage Gully

N

Knik Arm Upland

Upland

Area D

Pond Beyond

Bread Truck Pond

C-Pond

Area A

Racine


Island

Area B

Rt.


Bravo


Bridge

Figure 2. General distribution of ponds and the primary drainage

system.

2
mentation rates. Levees, mudflats, marshes and

shallow ponds (to 50 cm depth) approximately

parallel the Eagle River and coastline (Racine et

al. 1992a). Freshwater ponds or shrub bogs lie

along the uplands on the northeast and southwest.

BACKGROUND AND

PREVIOUS STUDIES

The fundamental sedimentary and hydrological processes of subarctic and arctic tidal flats and

salt marshes are not well understood (e.g., Boon

1975, Frostick and McCave 1979, Carling 1981,

Collins et al. 1987, Dionne 1988, Stoddart et al.

1989, Reed 1990, Andrew and Cooper 1993). The

roles of glacial rivers and glacial-marine sediment

discharges in tidal flat and salt marsh sedimentology have received only limited attention

(Ovenshine et al. 1976a,b; McCann et al. 1981;

Bartsch-Winkler and Ovenshine 1984). Neither the

sediment flux nor sediment budget of tidal flats

and salt marshes are sufficiently defined to link

their hydrology and sedimentology (e.g., Frostick

and McCave 1979, Kohsiek et al. 1981).

Short-term sediment accumulation is related to elevation, and thus to the frequency

and depth of inundation and annual sediment supply, but eustatic rise in sea level

may be more important to long-term rates

(e.g., Krone 1987; Allen 1990a,b; French

1991, 1993; McLaren et al. 1993; French and

Spencer 1993). The texture and composition of sediment deposited in tidal flats and

salt marshes vary in space because of tidal

channels and gullies as well as tidal currents (e.g., Carling 1982, Allen 1992).

The sediment balance, the accretion or

erosion of sediment, in tidal flats and salt

marshes has been estimated using shortterm vertical sedimentation rates (e.g.,

Harrison and Bloom 1977, Richard 1978,

Letzch and Frey 1980, Stoddart et al. 1989).

Longer-term estimates have been made by

dating subsurface horizons (e.g., Hubbard

and Stebbings 1968, Allen and Rae 1988)

and creating radionuclide profiles (e.g.,

Keene 1971, Bloom 1984, Kearney and Ward

1986). Both estimates, however, may be misleading owing to compaction after deposition in the former case and time-dependent

accretion rates in the latter (French 1993).

The initial analyses of ERF sedimentation and erosion (Lawson and Brockett 1993,

Lawson et al. 1995) indicate that the forces of

erosion and deposition are actively changing the

morphology of the drainage system, ponds and

mudflats. Past migration of the Eagle River has

apparently reworked a significant amount of sediment, as indicated by meander scars and channel

cut-offs (Fig. 4b). Limited sampling during early

studies has shown that WP-bearing sediments

from gullies probably move into the Eagle River

during tidal ebb, potentially increasing the accessibility of WP particles to migratory waterfowl,

invertebrates, fish or other organisms. We also

evaluated factors controlling these physical processes in the tidal flats, but the data are still insufficient to assess the short- and long-term response.

Processes of erosion, transport and deposition

vary seasonally (Tables 1 to 6). Their importance

may likewise vary across ERF in response to a

number of internal factors (Table 7). The relationship of these factors to physical processes are

under investigation; our understanding of the process relationships are presented in subsequent sections of this report.

External controls (Table 7) are more difficult to

define because they are affected by a number of











































































































































































 











Mudflats

Sedge Marsh

Pond/Marsh

Sedge Meadow

Border

352000

6803000

6802000

6801000

6800000

6799000

353000 354000 356000 355000



























Figure 3. Distribution of primary landform–vegetation units

(from Racine et al. 1993).

3
other factors, most of which lie beyond the scope

of our analysis. In particular, tectonic, and especially earthquake, activity is virtually impossible

to predict; however, its impact on the physical

system of ERF may be enormous.

Eagle River Flats was last severely struck by

the 1964 Alaskan Earthquake (magnitude 9.2 on

the Richter Scale [Cohen et al. 1995]), with both

sedimentary and tectonic subsidence affecting the

site’s elevation (Small and Wharton 1969) and

thus its physical processes. A tectonic drop of

about 0.6–0.7 m was recorded in the Anchorage

region (Brown et al. 1977). Subsidence caused by

sediment liquefication and solidification probably

reduced the pond elevations by an amount similar to that found near the Portage area of

Turnagain Arm (Ovenshine et al. 1976a). Since

the earthquake, the region has been rising at about

1.5 cm/year, although the rate has been decreasing over time (Brown et al. 1977).

How the physical system responded to these

changes in elevation is unknown; however, an

increase in pond water depths may have resulted

in an increase in sedimentation rates, in keeping

with what happened in Turnagain Arm (Ovenshine et al. 1976a). In addition, after the elevation

drop, the tidal flat hydrology would be out of

equilibrium, bringing a response from the physical system. Disequilibrium would have been maintained by the uplift following the earthquake.

a. Typical active gully erosion following courses of relief

drainageways.

Figure 4. 1994 aerial photographs of ERF (originally color

infrared).

4
b. Meander scars along the side of

the Eagle River.

Figure 4 (cont’d).

Table 1. Summer erosional processes.

Morphological

unit Processes

Marshes Currents (rare) Wind waves (rare)

Ponds Wind waves Tidal currents

Wind currents Debris impacts (e.g., logs)

Ducks and other Bioturbation

bottom-feeding

organisms

Gullies Currents Ground water

–tidal –piping

–runoff –sapping

Overland flow Gravitational slope processes

–sheet –slump

–rill –block collapse

Wind waves –sediment gravity flow

Mudflats Currents Debris impacts (e.g., logs)

–wind Rain drop impact

–tidal Bioturbation

Overland flow

–sheet

–rill

Levees Currents Debris impacts

–tidal Wind waves

–river

Coast Current scour Debris impacts

Wind waves Overland flow

Table 2. Winter erosional processes.

Morphological

unit Processes

Marshes Ice plucking

Ponds Freeze-on and ice plucking

Ice shove

Ice scour

Gullies Plunge pool undercutting

Freeze–thaw cycling

Ice segregation and thaw

Ice directed current scour

Mudflats Ice plucking

Ice shove (floating–expansion)

Ice scour

Ice cover confined scour

Freeze–thaw cycling

Levees Ice scour

Freeze–thaw cycling

Ice shove

Ice-directed current scour

Coast Ice plucking

Ice shove (floating–expansion)

Ice scour

Ice block confined scour

Freeze–thaw cycling

Current scour

Wind waves

5
Table 3. Summer transport

processes.

Currents (river)

Suspended load

Bedload

Saltation

Wind currents (pond–mudflats)

Suspended load

Bedload

Gravitational slope processes

Currents (tidal–gully)

Flood

Ebb

Ground water

Piping

Table 4. Winter transport processes.

Ice floes

Freeze-on

Freeze-in

Ice shove

Frazil ice

Anchor ice

Freeze-on

Freeze-in

Table 5. Summer depositional processes.

Morphological unit Processes

Ponds Suspension sedimentation

–settling-out

–vegetation trapping

Gullies Suspension sedimentation

–settling-out

Bedload deposition

Sediment gravity flows

Slumping

Mudflats Suspension sedimentation

–settling-out

–vegetation trapping

Marshes Suspension sedimentation

–vegetation trapping

Levees Suspension sedimentation

–settling-out

–vegetation trapping

Table 6. Winter depositional processes.

Morphological

unit Processes

Ponds Ice entrapment and in-situ melting

Mudflats Ice freeze-on and in-situ melting

Snow filtering and in-situ melting

Ice growth entrapment and in-situ melting

Ice cover confined suspension settling-out

Levees Ice freeze-on and in-situ melting

(sediment–organics)

Snow filtering and in-situ melting

Ice growth entrapment and in-situ melting

Ice cover confined suspension settling-out

Gullies Ice growth entrapment and in-situ melting

Ice cover confined settling-out

Table 7. Controls on physical processes.

Internal

River

Tides

Glacial sediment–water sources

Substrate material properties

Vegetation

Sediment influx–efflux

Weather–climate

Human activity

Ground water conditions

External

Earthquakes

Tectonic activity

Eustatic sea level rise

Isostatic rebound

Subarctic climate

Glaciers

Surface and ground water systems

Other important external forces include the regional surface and ground water systems, subarctic climate, eustatic rise in sea level, and glaciers as sediment and water sources (Table 1).

Each of these factors must be considered when

interpreting the physical dynamics of the Flats.

Tidal and river water dynamics control the

amount of material available for deposition in

ERF and affect the locations and rates of erosion.

The amount of sediment transported in flood waters is a primary factor determining the erosion

and transport of sediments during ebb. This capacity changes seasonally, as sediment discharge

is reduced in winter, while water temperature

decreases and salinity increases, causing a net

increase in water density. Higher density water

can erode and transport more sediment during

the winter, although the total volume transported

is reduced.

Aerial photographs show that tidal gullies have

significantly extended into the mudflats by

headward erosion (Fig. 5). Short- or long-term

treatments for WP contamination may be compromised if such large-scale physical changes occur shortly after their implementation. The aerial

photos also show large-scale changes resulting

from riverine processes, which include abandonment of a primary river channel in the southern

part of the Flats in the last 40 years, channel migration of various amounts from near the Route

Bravo bridge north to the river mouth at Knik

Arm, and the apparent burial of marsh and gully

terrain in the Racine Island area.

It is not known whether the seasonal and annual variability of sediment discharge in the glacial rivers is evident in the tidal waters of Knik

Arm. The Eagle River, as do other glacial rivers,

transports an insignificant amount of suspended

6
a. 1950.

Figure 5. Aerial photographs of ERF.

7
b. 1967.

Figure 5 (cont’d). Aerial photographs of ERF.

8
c. 1993.

Figure 5 (cont’d).

9
sediment during the fall through early spring,

when glacial discharge is limited primarily to

ground water sources (e.g., Lawson 1993). Samples

from along the coast in Knik Arm in January,

March and November of 1994 indicated that sediment remains in suspension through the winter,

but in lesser quantities than during the summer.

The continued presence of suspended sediment

during winter is important, because it is the only

source available for deposition in ERF during that

time.

Expanding the above discussion, we note that

aerial photographs from 1950, 1967 and 1993 reveal changes in the channel pattern entering ERF.

Two main channels entered the Flats in 1950, but

by 1967 the southern channel was abandoned and

the northern channel diverged northwest of the

Route Bravo bridge. The two primary channels

entering the Flats today are partially braided and

have a divergent pattern characteristic of an alluvial fan (Fig. 5c). Both characteristics indicate a

significant decrease in channel gradient near the

bridge, leading to a reduction in carrying capacity and the deposition of sediments in transport,

as well as a fundamental change in the channel

configuration (e.g., Leopold et al. 1964). In addition, the 1993 image shows a well-defined change

in surface texture where these channels merge

into a single meandering channel (Fig. 5c). This

textural change probably bespeaks a change in

physical characteristics such as elevation, gradient or the grain size and mineralogy of the substrate materials.

In addition, landforms in this part of ERF are

mainly levees, vegetated marshes and abandoned

channels and point bars, except in the center of

the Racine Island area (Fig. 5c). Here, an abandoned gully within the center of an intertidal pond

exists, while surrounding it, the deposits are relatively featureless and probably consist of overbank

deposits from Eagle River flooding. In contrast,

the northern two thirds of ERF have landforms

more typical of tidal flats near river mouths elsewhere in this region (e.g., Ovenshine et al. 1976b).

Both the tributary channel and the meandering main channel deepen downstream, becoming

incised by over 5 m. Near-vertical scarps and evidence of recent slumping characterize river banks

that are actively eroding. Aerial photographic

analyses reveal that, historically, banks have receded several tens of meters at some locations.

The relatively tight meander loops in the lower

section of the river have significantly changed

over the last 40 years. Meander scars and abandoned point bars are common along the length of

the active channel and are especially evident in

photographs of the southern reaches of the Flats

(Fig. 5c). Channel changes are a natural progression resulting from the erosion and recession of

the outer banks of meander bends, deposition of

sediments as point bars in the inner parts of each

bend, and a general downstream migration of the

channel (e.g., Allen 1982). At some locations, a

breaching of tight meander loops causes their

abandonment.

Channel changes remobilize bank sediments,

which may be transported either downstream or

upstream, depending on the timing of collapse.

In the idealized river model, they would move to

the next point bar and be redeposited; however,

redeposition will be controlled by the interaction

among tidal and river processes, particle grain

size, and water properties affecting density (e.g.,

temperature and salinity) and flocculation. Sediments may therefore be redeposited in the ponds

and mudflats during flooding tides, or they may

be transported towards the Knik Arm during the

ebb cycle.

Gullies in the mudflats have also lengthened

and deepened during the last 40 years, in some

cases extending over 200 to 300 m. A significant

extension of gullies toward the ponds and marshes

is particularly evident in Figures 5b (1967) and 5c

(1993). Headwalls recede during flood or ebb flow,

as water is funneled into and out of the ponds

and mudflats through the drainage system. The

cause of gully expansion is not proven. But the

tectonic activity during and following the 1964

Alaskan earthquake or large-scale river channel

changes caused by flooding can cause the gully

expansion. We must further quantitatively analyze historical aerial photographs and deposits in

the alluvial landforms to find its cause.

Tributary channels with dendritic patterns,

called vegetated drainageways, drain water from

the mudflats and ponds into the gullies (Fig. 6). A

second set of channels intercepted by the active

tributary system unconformably crosses other

landforms, including ponds (such as C and Bread

Truck) where they lie below the pond water surface. Their pattern is irregular and unrelated to

the active gully drainage system. We interpreted

these secondary channels as relict drainages that

reveal significant changes to the ERF drainage

system in the past. The cause of such change is

unknown, but may also be related to larger scale

forces such as river avulsion, flood-induced channel migration, or earthquake-induced subsidence.

10
This report summarizes the current

results of this study, synthesizing previous work of 1992 and 1993 with the 1994

investigations, and addresses elements

of Objective 1.

STUDY SITES

AND METHODS

Transects

We established our transects to include

representative morphological units, with

11 transects set out in May 1992 (Lawson

and Brockett 1993), 1 in June 1993, and

11 in 1994 (Fig. 7). We surveyed the

transects using an electronic theodolite

to determine ground surface elevations

relative to local benchmarks and the

UTM grid. Grab samples of surface sediment (to 5-cm depth) were taken at each

survey point along the transects for analyzing the grain size distribution using

standard sieving and hydrometer techniques. These data are necessary for us

to delineate surfacial textural trends and

to evaluate the relative importance of

the tidal and riverine sediment sources

to sedimentation and landform development.

Sedimentation and erosion

Sedimentation rates in ponds and marshes are

difficult to measure accurately because only a

slight disturbance of the water column will resuspend the fine-grained bottom sediments. If these

sediments are caught in sediment traps, sedimentation will be over-sampled and the resulting rates

high. We, therefore, used several methods to measure primary sedimentation rates and related surface erosion and secondary rates of resedimentation resulting from resuspension of pond bottom

materials. Rates of sedimentation and surface erosion at mudflat, levee and tidal gully sites were

measured using other techniques.

We used “sedimentation stakes” to measure

erosion and deposition at transect points where

the surface was wet or standing water was present

(Fig. 8). These stakes consist of a rod and a square,

rigid plate (about 7 cm2) that slides freely on the

rod as used by Ovenshine et al. (1976a,b) in

Turnagain Arm, Alaska. Erosion depth is defined

by the increase in distance between the top of the

rod and the top of the plate, as measured periodiFigure 6. Drainage pattern analysis of 1993 aerial photograph of C

and D areas.

STUDY OBJECTIVES

The objectives of this multi-year investigation

are to:

1. Conduct quantitative analyses of ERF’s

physical system dynamics, particularly hydrological, sedimentological and hydraulic processes controlling erosion, transport, deposition and burial

of WP-bearing sediments. We will develop a conceptual framework for the ERF physical ecosystem and the fate and transport of WP particles,

including the potential for off-site migration.

2. Evaluate the short- and long-term response

and sensitivity of the physical processes and terrain to modifications caused by proposed remedial technologies, particularly temporary or permanent pond drainage, dredging, capping,

geotextiles or others as proposed.

3. Assess the effects of the physical system on

the effectiveness and stability of proposed remedial measures for WP contamination and develop

strategies for restoring areas of dredging, artificial drainage, or other proposed measures.

11
cally to the nearest 0.5 mm. The amount of accretion is the thickness of sediment deposited on the

plate surface. The difference between these two

readings defines the net sedimentation (or erosion) rate. Monthly rates were measured in July,

August and September 1992 following each monthly period of tidal

flooding, while seasonal rates were

measured in May and September of

both 1993 and 1994.

The amount of accretion at transect survey points was also monitored by spraying an area (~30 × 30

cm) on the ground surface with

pavement marking paint and locating the corners of this area with wire

survey flags (see also Vince and

Snow 1984). Net accumulated sediment is measured by cutting and removing a ~2- × 2-in. (~0.03- × 0.03-

m) block of sediment with a putty

knife (Fig. 9). The thickness of sediment above the paint layer is then

measured to the nearest 0.5 mm.

Paint was applied at certain sites in

June 1992, August 1992, August 1993

and May–June 1994. In contrast to

sedimentation stakes that are broken or removed during winter, the

wire flags and the persistence of buried paint horizons allow us to acquire a continuous record of net sedimentation

rates. Use of the method allows us to account for

sediment compaction through time.

Sedimentation rates in ponds were also measured at 10 sites following tidal inundations in

Figure 7. Transect locations.

a. Sedimentation stake with plate that slides freely on the rod.

Figure 8. Sedimentation measurement techniques.



0 500 m

Eagle River


Permanent Pond


Intermittent Pond


Drainage Gully


Transect Line

Line 14

Line 6

Line 21

Line 22

Line 23

Line 10

Line 18

Line 19

Line 20

Line 24

Line 2

Line 12

Line 15

Line 1

Line 16

Line 17

Line 3 Line 11

Line 5

Line 4

Line 8

Line 9

Line 7

N

12
b. Wire flags mark an area of painted ground.

Figure 8 (cont’d).

Figure 9. Paint layer depth within block of sediment is measured to define sedimentation rates.

June, August and September 1992, May and September 1993 and May and September 1994 (Fig.

10). Samplers were installed in eight additional

sites in 1994. Gross pond deposition was measured in a sediment trap consisting of a 4-in.-

diam. (10.2-cm-diam.) schedule-40 PVC pipe end

cap glued to a short length of 2-in.-diam. (5-cmdiam.) schedule-40 PVC pipe. The pipe was inserted into the pond until the bottom of the cup

was in contact with the bed (Fig. 11b and 12).

Sediment trapped in the cup includes new sediment brought into the ponds by tidal inundation

and river currents, and materials resuspended by

wind waves, dabbling ducks or other mechanisms.

13
We measured the quantity of accumulated sediment to the nearest 0.5 mm by inserting a graduated scale into the sediments at three places. After measurement, the sediment in the cup is

cleared or saved in plastic bags for

WP analysis.

Net sedimentation rate in ponds

is measured using a thin, rigid plastic plate of about 30 cm square that

is pushed gently into the pond bottom until the plate surface is flush

with the pond bottom surface. The

corners of the plate are secured by

aluminum tent pegs (Fig. 11 and 12).

Sediment in suspension settles onto

this plate, but this sediment can also

be reworked and resuspended by

wind or other mechanisms, thereby

delineating a net rate. Plates become

covered by sediment and aquatic

vegetation in 1 to 2 weeks.

Initially, in 1992, a stake with a

millimeter-graduated scale on its

face was inserted into the pond bottom, and the height of the pond bottom and the water column were measured to provide a second tally of

the net sedimentation rate (Fig. 11a).

However, surface waves and glare

commonly made this a difficult measurement to obtain. In addition, aquatic vegetation on the bed commonly covered the base of the

scale. For these reasons, only water levels are

measured on these scales.



0 500 m

Eagle River


Permanent Pond


Intermittent Pond


Drainage Gully


Sedimentation Station 

N

1

2

3 4 7

6

5 8 9

15

10

12

11

13

14

Area B

Illume Pond

Amoeba Pond

Figure 10. Pond sedimentation stations.

Figure 11. Sedimentation station in a pond site. A meter stake (a), cup sampler (b) and

plate sampler (c) are shown in position. The plate sampler is located between the two

survey flags.

14
Gully erosion and recession

Gullies draining the ponds and mudflats are

actively extending inland by headwall and lateral

wall erosion. We established 57 sites to evaluate

retreat rates (Fig. 13)—11 sites were established

in June 1992, 3 in June 1993, 3 in September 1993,

1 in May 1994 and 39 in July 1994. At each site,

stakes were driven into the ground along a straight

line at known distances from one another and

from the crest of a gully scarp (Fig. 14). A “hub”

stake was set at a known distance from this stake

line. The distance between the hub and the crest

Figure 12. Example of sedimentation station at intermittent pond location showing

layout of cup and plate sampler.

Figure 13. Gully headwall and lateral

wall erosion sites.

1992 Hub Site


1993 Hub Site


1994 Hub Site

0 500 m

N

Bread Truck

Parachute

In-between

B

Racine Island

River-South

River-Meander

River-North

Tanker

Illume Pond

Coastal 5

Coastal 6

Coastal 7

Coastal 4

Coastal 1

15
of the gully scarp was measured across the top of

each line stake with a tape measure. We measured the position of the gully scarp by lowering

a plumb bob by string from the tape measure, so

that a horizontal distance could be read where

the string and measuring tape met. We then placed

flagged wire stakes at each point of measurement

along the headwall. By repeating these measurements, we could monitor changes in scarp geometry with time while providing seasonal and annual data on gully recession and erosion rates.

Recession was measured in September 1992,

June and September 1993, and May, September

and November 1994. The September measurements delineate summer rates, while those of May

or June delineate winter rates. The November 1994

16

Figure 14. Examples of gully erosion sites. Line stakes are established along a line

approximately parallel to the gully scarp. Hub stakes are located away from the line

stakes and were used as the origin of measurements to scarp.

b. Aerial view of Coastal 6 Gully.

a. B-Gully
data will define the amount of erosion occurring

between September and the initial period of

freezeup. Repeat measurements at points without any retreat (as indicated by the continuing

presence of wire flags) show that they are reproducible to ± 2–5 cm. The accuracy, however, is

limited by how well the crest of the gully scarp

can be defined; the gully scarp is highly irregular

and, therefore, accuracy is probably limited to

±10 cm in the worst case.

Longitudinal cross-sections of nine gullies were

surveyed in 1993, beginning at or near the Eagle

River and continuing into the ponds (Fig. 15).

Gradients were calculated from these data for

characteristic reaches of each gully. Gradients

were incorporated into empirical estimates of

gully discharge at surveyed cross sections for

maximum or bankfull ebb (Lawson et al. 1995).

These data signify the relative stability of the

drainage system, and therefore may provide information to allow us to assess both short- and

long-term trends in gully erosion and pond stability.

Water quality parameters

Various types of data have been collected at

several sites to help us assess the surface hydrology and water quality of ERF. A majority of sampling sites were in small

plunge pools at the heads of tidal

gullies throughout the Flats (Fig. 16).

The Mortar and Spring gully sites

are located along drainages that receive tidal waters directly from Knik

Arm. The sites at Bread Truck, Parachute, B, Tanker and Otter gullies

are located in drainages that flow to

the Eagle River. The Otter Creek site

represents drainage from the southwestern uplands as well as Area B

mudflats. The two final sites represent the primary source waters: the

Eagle River upstream of where it enters ERF, and the Knik Arm just

north of the coast (Fig. 16).

At the nine gully locations (hydrostation sites) (Fig. 16), a basic suite

of water characterization data was

collected at 4-minute intervals. These

data include temperature (measured

by thermistor or a Hydrolab H2O

multiparameter sensor, or both); salinity, pH, redox and dissolved oxygen (Hydrolab); turbidity (OBS3 optical backscatter sensor); and water surface

elevation (Druck pressure transducer) (Table 8).

From August to September 1993, data were collected at three sites: Parachute Gully, Bread Truck

Gully and Eagle River. These three sites were

reestablished in May 1994, and other sites were

added later as equipment became available

throughout the 1994 season (Fig. 17). In addition,

500-mL water samples were collected for analyzing Total Suspended Sediments (TSS) during flood

and ebb cycles at each hydrostation and in the

nearshore zone of Knik Arm.

During the fall of 1994, two sensors were added

to measure current velocity and water level. A

Marsh–McBirney electromagnetic current probe

was mounted about 1 m above the bed at the

Bread Truck, Parachute and Spring gully sites

(Fig. 16). Ultrasonic sensors by Campbell were

installed at Otter and Spring gullies and the

weather station adjacent to Knik Arm to monitor

flooding duration and height above the mudflats.

Water characterization data were also collected

using Seabird CTD (mdl 19) Profilers (Table 8)

over shorter time periods at a variety of sites in

gullies and the Knik Arm (Fig. 16). Also, a Seabird model SBE 26 wave and tide gauge was positioned north of Area D to record water depth and



0 500 m

Eagle River


Permanent Pond


Intermittent Pond


Drainage Gully


Surveyed Gully

N

D

B A

E

C

F Eo

F1

Fo

Co C2

Figure 15. Location of surveyed longitudinal cross sections of gullies.

17
Table 8. Specifications of sensors used in water quality parameter measurements.

Instrument type Sensor Accuracy Resolution

Hydrolab Temperature ± 0.15°C 0.01°C

(H2O Multiprobe) pH ± 0.2 units 0.01 units

Specific conductance

Fresh water ± 0.0015 to 0.1 mS/cm* 0.001 mS/cm

Salt water ± 0.15 to 1.0 mS/cm* 0.01 mS/cm

Salinity ± 0.2 ppt 0.1 ppt

Dissolved oxygen ± 0.2 ppm 0.01 ppm

Redox ± 20 mV 1 mV

Depth ± 0.45 m water 0.1 m water

CRREL thermistor Temperature ± 0.02°C 0.01°C

Druck (PDCR 950) Pressure (water depth) ± 0.008 m water 0.001 m water

OBS-3 Turbidity ± 100 mV 1 mV

5 V = 2000 FTU

Marsh-McBirney Water current (velocity) ± 6.10 cm/s 2.13 cm/s

(model 512)

Cambell Ultrasonic Distance (water depth) ± 1 cm 0.05 cm

(model UDG01)

Seagauge wave/tide Pressure (water depth) ± 0.003 m water 0.0015 m water

(model SBE 26-03) Temperature ± 0.02°C 0.01°C

Seacat CTD Pressure (water depth) ± 0.75 m water 0.045 m water

(model SBE 19-03) Temperature ±0.01°C 0.001°C

Conductivity ± 0.001 S/m 0.0001 S/m

OBS [5 V = 2000 FTU] ± 100 mV 3 mV

* Depends on which of three auto-adjusting ranges is employed.

River

Rt.


Bravo


Bridge

0 500 m

N

Hydrolab & ISCO Water Sampler


Hydrolab


ISCO


CTD


Wave & Tide Gauge


Weather Station

Otter Gully

In- between

Otter


Creek

Coast

Spring Gully

Coastal 6

Tanker

B-Gully

Parachute

Bread Truck

Mortar

Figure 16. Locations of instrumentation recording water quality parameters and water depths.

18
temperature of Knik Arm waters and to continuously record data from August through September 1993 and from May through November 1994.

At hydrostation sites, the sensors were mounted

on a metal crossbar attached to a stake driven

into the gully bed and positioned in plunge pools,

where they would remain wet at all times, at an

elevation of 15 to 45 cm above the bed. Flow

probes were mounted about 1 m above the bed

on separate stakes downstream of the plunge pool

at Spring and Parachute gullies, and on a separate crossbar in a higher position at the Bread

Truck Gully hydrostation (Fig. 16). Data were collected at 5-second intervals and averaged for a 4-

minute output frequency. Wires from the sensors

were bundled and strung to a Campbell CR10

datalogger, which was positioned on a floating

platform next to the gully (Fig. 18).

Each platform was constructed of a 4-ft (1.2-m)

square plywood deck with a wood frame made

from 2- × 8-in. (~5- × 20-cm) studs. Large metal

eyehooks were screwed into the frame on each

corner. One and one-half meter sections of 0.75-

in. (2-cm) -diameter steel pipe were mounted verx x

xxx 	 xxx

xxx 		 xxx

x x xxx

xxx xx

xxx

xxx

xx


xx

xxxx xxx xxxx

xxx xx

xxxx xxx xxxx

xxxx

xxx x xx x xx x x x x x x x xx xx x x xxxxx xxxx x xxxxx

oooo ooo ooo

 xxx xxxx

xxxx xxx xxxx

+++ +++

+++ +++

xxx xx xxx	 xxx	 

xxxx	 xxx 	 xxxx

xxx	 xx 				 xxxx		xxx	 xxxx

xxx xxxxx x xx 				 xxxx		xx	 xxxx

x

+++ ++++

ooo oooo

oooo oooo

xxxx

10 10 10 10 10 10 10 20 20 20 20 20 20 20

East
 –––––––––

Coast

Mortar Gully


Bread Truck

Parachute

B-Gully

River

Spring Gully

Tanker Gully

Otter Gully

Otter Creek

West
 –––––––––

1994

xxx TSS Hydrolab ooo Ultrasonic +++ Flow

May Jun Jul Aug Sep Oct Nov

Bread Truck

Parachute

River

1993

Figure 17. Active hydrostations during the 1994 season.

Bread Truck

Parachute

River

East

Coast

Mortar Gully

Bread Truck

Parachute

B-Gully

River

West

Spring Gully

Tanker Gully

Otter Gully

Otter Creek

10 20

May

10 20

Jun

10 20

Jul

10 20

Aug

10 20

Sep

10 20

Oct

10 20

Nov

1994

1993

TSS Hydrolab Ultrasonic Flow

19
tically in 20-gal. (75-L) cans filled with concrete,

and the eyehooks mounted on the platform’s

wood frame lowered over them. Foam filled the

inner space of the deck framework and provided

flotation so that the platform could move vertically up and down the steel pipes during tidal

flood and ebb (Fig. 19). The platforms allowed us

to establish monitoring stations without setting stakes or foundations in

the ordnance-bearing mudflat sediments; however, the platform’s main

task is to keep the datalogger and related devices dry during the highest

flood levels. Dataloggers were installed in NEMA plastic enclosure

boxes mounted on 2-in. (~5-cm) vertical steel pipes. Solar panels of 18-W

output were fastened to the pipes

above the dataloggers to recharge the

12-V external battery. Ultrasonic sensors were also mounted on an arm

attached to the support pipe and aimed

downward towards the mudflat surface, which was covered by a metallic

plate pinned to the ground.

An ISCO Model 3700 water sampler was also mounted on the platform at each site, except B-Gully (Fig.

18). A tygon tube extended from each

sampler to an intake screen positioned on the

same crossbar bearing the electronic sensors. The

sampler was programmed to obtain 500-mL water samples at specific intervals through the flood

and ebb cycles. These samples were processed for

TSS concentration using vacuum techniques and

45-μm glass microfiber filters following procedure 2540D in Standard Methods for the ExaminaHydrolab H20
 - temperature


- depth


- salinity


- pH


- dissolved oxygen


- redox

Thermistor - temperature

Pressure Sensor

OBS

ISCO

- water depth

- optical backscatter


 (turbidity)

- water sampler


 intake

Solar Panel

Data Logger

ISCO Sampler

Figure 18. Schematic of hydrostation showing layout.

Figure 19. Platform and instrumentation measuring discharge characteristics at the

Bread Truck Gully location.

20
tion of Water and Wastewater (APHA, AWWA, WEF

1992). Our project modifications to this procedure are described in Appendix A.

WP transport sampling

A plankton net, 3 m long, with an opening of 1

m and a mesh size of 80 μm, was used to sample

suspended and bed load sediments transported

in the gullies during ebb. We then analyzed these

samples by standard laboratory methods for the

presence of WP particles.

The plankton net was tied to stakes driven into

the gully bottom. It extended down-channel in

the direction of ebb flow (Fig. 20). A cup at the

apex of the net collected sediments that were

trapped from water flowing down the gully. Nets

were deployed in the Parachute Gully in May

1994, in the Bread Truck, Parachute and B gullies

in September 1994, and again in the B-Gully in

October 1994. These nets were set during multiple flooding tides to document whether or not

WP was being transported out of the ponds and

mudflats (see White Phosphorus Erosion and Transport section). In addition, grab samples were collected from recent deposits in various gullies

across ERF on 4 June, 6–8 October and 6 November 1994. Samples of pond and mudflat sediments

frozen-on to ice cover fragments were also collected in Areas C and D in November 1994.

Bathymetric profiling

Bathymetry of the Knik Arm near the Eagle

River mouth was recorded for the first time this

year to allow us to develop a baseline for assessing erosion and deposition and to help us identify potential areas for WP deposition. Bathymetry was measured using a Lowrance fathometer

with a 50-kHz transducer–receiver. On 27 and 28

July, 20.1 km of fathometer transects were run in

a 2-km-wide portion of Knik Arm just offshore of

the Eagle River Flats (Fig. 21).

We made our profiles using a 14-ft (~4.3-m)

inflatable skiff. The transducer was mounted on

the transom with angle irons, screwed onto a

wooden block made from a 2- × 4-in. (5- × 10-cm)

stud that was tightened to the hull with C-clamps.

The mounting bracket was wedged so that the

bracket hung vertically off the stern, and the transducer–receiver was mounted on the bracket such

that its surface was approximately parallel to the

water surface.

We located and recorded our transects using a

land-based Total Station. Seven benchmarks were

established along the coast for setting up the surveying equipment to track the skiff. Personnel in

the skiff pointed a prism and rod towards shore.

Communication between shore and skiff was

maintained to relay navigation commands using

hand-held VHF radios.

Figure 20. Plankton net used for collection of WP in transport through gullies.

21
Survey data have given us highly accurate

bathymetric data. Water depth can be read to

within 10 cm, while survey data provide us with

the ability to compensate for variations in tidal

height (±3 m) during bathymetric profiling. The

timing of bathymetric profiling was centered

around the predicted high tide in Anchorage, providing us with an appropriate 4-hour window of

operation each day. This time was chosen to take

advantage of maximum flooding so that transect

lines would not be hindered by bars in Knik Arm

and, more importantly, when tidal and fluvial

currents are minimum around slack high tide.

Elevation data collected during profiling allow us

to subtract tidal effects from the raw bathymetric

data and adjust the bathymetric records to mean

Anchorage sea level. Bathymetry is therefore estimated to have accuracy that is better than a meter.

All data are standardized to mean Anchorage sea level, as are all other surveyed data in ERF.

Length of data record

Table 9 summarizes the periods when

various types of field data were acquired.

The starting dates for acquisition reflect

both the development of new methods

for these investigations and expansion

to analyze a larger proportion of the

physical system across ERF. Gaps in the

records of water quality sensors, other

than during the winter period, resulted

from either instrumentation failure or

disruption by natural processes.

TIDAL HYDROLOGY

AND INUNDATION

Erosional, transport and depositional

processes are complexly related to Knik

Arm tidal dynamics and Eagle River hydrology. Although sufficiently high tides

can flood all of ERF, lower high tides

may be supplemented by the river’s discharge or wind-induced surges (or both)

and flood more area to a greater depth

than the predicted tide height indicates.

An understanding of the tidal and river

dynamics is critical to the evaluation of

the physical system and WP attenuation.

Several factors influence the timing, magnitude and duration of flooding, each of

which must be evaluated since flooding

drives many of the surface processes and modulates the moisture-dependent decomposition of

WP particles (Walsh et al. 1995).

A typical flooding tidal cycle begins with a rise

in the water level in the Eagle River and gullies.

As the tide reaches its peak, the water floods over

the banks of the gullies, letting it flow over a large

area, causing a decrease in the rate of water level

rise (Fig. 22). Once the peak is reached, there is a

period of slack high tide when the water is

dammed upon the Flats. During ebb, the water

begins to drain nearest the coast and progresses

inland. The channeling of ebb flow into tidal gullies, combined with the dammed fresh water, results in a longer runout time than flooding prior

to complete inundation.

The damming of the river water during the

flood tide causes the river to slow, pool, and evenN

Coastal 1

Coastal 2

Coastal 3

Coastal 4

Coastal 5

Coastal 6

Coastal 7

New 


Coastal

Line A

Line D

Line L

Line H

Line F

Eagle R. Mouth

0 500 m

Selected Transect Line


Location of Echo-Sounding


Transect Line

A1

A2

A3

A4

A5

A6

B1

B2

B3

B4

B5

B6

C1

C2

C3

C4

C5

C6

D1

D2

D3

D4

D5

D6

E1

E2

E3

E4

E5

E6

F1

F2

F3

F4

F5

F6

Figure 21. Location of selected bathymetric profiling transect lines.

22
Table 9. Extent of data record for various measurement parameters.

a. Discrete samples.

1992 1993 1994

M JN JY A S M JN JY A S M JN JY A S O N

White phosphorus

Sample type

Sediment grab x x x xxx xx xx xx

Sediment trap x xx x xxx

Plankton net xx xxxx xxx

Water xxx xx

Bedload trap xxx

Ice x

TSS

Knik Arm xx xx

Eagle River xx x x

ISCO xx xxx xx xxx x xx xxx xxx xx xxxx x xx xxx xx

Ice xx

Grain size x xxxx

= measurement taken 1992 1993 1994

M JN JY A S M JN JY A S M JN JY A S O N

Sedimentation

Transect 1–11

12

14–24

Station 1–9

13, 14, 15

11, 12, Illume, Amoeba, Area B

Erosion

Site 1–11

12–14 

15–17 

18

19–57

Hydrostation†

TSS†

CTD

Mortar Gully

Otter Gully

Knik Arm

Coastal 6 Gully

Otter Creek

In-Between Gully

Snow Depth†

Weather Station, Parachute

Gully, Bread Truck Gully

Velocity†

Spring Gully, Parachute

Gully, Break Truck Gully

Wave and Tide Guage

Weather Station

† see Figure 18

S S S S

b. Continuous data.

23
tually reverse its flow direction in the

upper Flats. This damming effect causes

the water of the river to rise higher than

the elevation of water in the gullies and

reverses flow in the drainage system. We

saw this happen in September and October 1994 during flooding events with a

predicted high tide of at least 4.0 m. The

damming effect may be accentuated during spring snowmelt or mid- to late-summer runoff peaks caused by a large volume of glacial melt in the Eagle River.

Water depths measured at the hydrostations and the tidal gauge in Knik Arm

show that during the September and October flood tides, the peak water elevation on the Flats exceeded the predicted

tidal height for Anchorage by more than 0.9 m in

some events (Fig. 23).

During periods of lower high tides, water

depths matched well with the predicted Anchorage high tide values. Strong winds in Knik Arm

and Cook Inlet can, however, cause either a higher or lower high tide elevation, depending on

wind direction. As an example, let’s observe two

tidal events during the nights of 6–7 October that

were predicted to be of the same height (Fig. 24).

On 6 October, strong southerly (180°) winds blowing up Cook Inlet into Knik Arm amplified tide

height on the Flats; however, in the latter case,

northerly winds off the Flats produced a peak

Figure 22. Typical high-tide curve showing period of sedimentation.

Figure 23. Water elevations as measured by pressure

transducer for two flooding tide events.

Figure 24. Wind characteristics and effects on tide heights at

Parachute and Tanker gullies, 6–8 October 1994.

2

4

6

8

00:00

6 Oct 1994

12:00

6 Oct 1994

00:00 (hr)

7 Oct 1994

Bread Truck Gully

Otter Creek

Parachute Gully

River

Anchorage Predicted

Elevation (m) Elevation (m)

Wind Speed 

(m/s)

0

5

10

15

Wind Direction Elevation (m)

2

4

6

12:00

6 Oct 1994

0:00

7 Oct 1994

12:00

7 Oct 1994

0:00

8 Oct 1994

12:00 (hr)

8 Oct 1994

0

180°

360°

Tide 

Prediction Parachute

Otter Gully

Runout

Slack


High


Tide

Peak

Elevation

Time

Optimum


Sedimentation


Conditions

Sedimentation Decreasing with Time

24
height slightly below that predicted, reducing the

inundation. Higher tides attributable to wind

surge increase the height and volume of flood water and prolong the period of runoff during ebb.

The time between the start of water rise and

peak high tide was calculated for each hydrostation site. We found a linear relationship between water rise time and peak elevation (Fig.

25). Then, average elevations for tidal flooding

across the Flats were estimated from the surveyed

transects (Fig. 7). These data reveal that ponds

will begin to flood at a tidal height of 4.6 m, that

mudflats are inundated at 4.9 m and that levees

are covered at 5.2 m (Fig. 26). These elevations

also yielded a linear relationship between the period of flooding (i.e., time when water elevation

was greater than 4.6 m) and peak water elevation

(Fig. 27). How often each landform is flooded in a

given period can be estimated using these elevations. The length of time available for sedimentation during tidal inundation is directly related to

Peak Water Elevation (m)

Water Rise Time (min.)

0

40

80

120

160

3456

Figure 25. Relationship of water rise time to peak

water elevation at Bread Truck Gully.

2

4

6

8 Oct 15 Oct 22 Oct 29 Oct

Elevation (m)

1994

Cover Levees (5.21 m)

Cover Mudflats (4.87 m)

Inundate Ponds (4.6 m)

Peak Water Elevation (m)

Flooding Time (min.)

0

50

100

150

4.8 5.0 5.2

Figure 27. Peak water elevation vs. flooding time at

Bread Truck Pond, September 1994.

Figure 26. Water elevation record at Parachute Gully

relative to elevations required to flood selected landforms. Open circles represent the time and elevation

of predicted high tide at Anchorage.

600

400

200

0

34 56

Peak Water Elevation (m)

Runoff Time (min)

Jun

Jul

Aug

Sep

Oct

Nov

Figure 28. Peak water elevation vs. runoff time at

Bread Truck Gully, June–November 1994.

the peak elevation and each landform’s threshold

for flooding.

During flooding, ponds have relatively high

water levels, increasing the area available for sedimentation. Successive flooding tides move new

material into the ponds and higher deposition

rates can be maintained for extended periods.

Pond drainage is restricted by the gullies’ crosssectional area, channel roughness and drainage

density, which limit the volume of water that can

escape from the pond. This produces a bottleneck

where water in the pond remains dammed despite turbulent, fast flowing conditions in the gully

heads. The response is a nonlinear relationship

between pond elevation and runoff time (Fig. 28).

Pond sedimentation results mainly from settling-out of sediment suspended in the water column (Table 5); sediments are presumably deposited during and immediately following slack high

tide (Fig. 22). Ponds commonly did not completely

drain between consecutive tides during a high

Jun

Jul

Aug

Sep

Oct

Nov

25
26

flooding cycle, allowing even longer sedimentation times. This may be significant because ponded

water has low turbulence and reduced particulate transport capacity. Increased runoff time may

therefore allow for increased sedimentation. In

addition, the mixing of flood waters with the pond

and marsh waters increases the amount of sediment suspended in the ponds and marshes, and

is the primary source of newly deposited materials.

The timing of flooding is a function of the elevation of the levees and mudflats surrounding

the gullies, the distance of the gully headwall

from the coast, and gully length. Water moving

down the river can also alter the timing of flooding inland from the coast, depending upon its

volume relative to the volume of tidal waters.

Analyses indicate that tidal flooding is best described by a polynomial regression (R2 = 0.97) for

the relationship between gully length and tidal

delay (Fig. 29a). A good correlation (R2 = 0.89)

also describes the relationship between tidal delay and gully headwall distance from the coast

(Fig. 29b). A similar comparison using tidal duration indicates little relationship with either gully

length or distance to Knik Arm.

The characteristic tidal response observed at

our hydrostation sites is governed by gully characteristics (i.e., sinuosity, gradient, cross-sectional

area, perimeter and channel roughness) that influence the tidal inundation rate in the gullies.

Thinking intuitively, we feel that the distance to

Knik Arm also must be an important factor, since

the Arm is the primary source of water flowing

into the gullies; however, secondary controls on

water dynamics in the gullies modulate the individual site responses.

Water level measurements at the river and gully

sites (Fig. 16) verified our observations that the

flood cycle normally starts with a progressive rise

in the river channel and adjacent gullies before

the mudflats gradually flooded. Flooding begins

first in the coastal mudflats on Knik Arm while

water is moving progressively inland up the Eagle

River channel. The delay in flooding at the

hydrostation sites from that predicted for Anchorage is illustrated by data from June through

November (Fig. 30).

Gully Length (m)

Time (min.)

40

80

120

160

0 2000 3000

Tidal Delay

 

Tidal Duration

1000

Distance to Knik Arm (m)

Time (min.)

40

80

120

160

0 2000 4000 6000

Tidal Duration

Tidal Delay

a. Gully length vs. tidal duration and delay. Regression line for tidal duration is expressed by y =

–8E-06x2 + 0.0437x + 79.818 (R2 = 0.6964); and

for tidal delay, y = 7E-06x2 – 0.0187x + 35.887

(R2 = 0.9655).

b. Distance to Knik Arm vs. tidal duration and delay. Regression line for tidal duration is expressed by

y = 1E-06x2 – 0.004x + 112.72 (R2 = 0.1283); and

for tidal delay y = 5E-07x2 + 0.0005x + 22.94 (R2 =

0.8915).

Figure 29. Relationships among the duration of tidal flooding and tidal delay with gully length and distance from

gully headwall to Knik Arm.

Time (min.)

0

20

40

60 June

July

August

September

October

November

Mortar

Bread 

Truck

Tanker

Spring 

Gully

Parachute

BGully

Otter Gully

Otter 

Creek

Figure 30. Delay in peak high tide from Anchorage to

ERF sites, June through November.
27

Spring Gully and Mortar Gully are generally

first to reach peak high tide because they are

close to the coast and independent of river influence (Table 10). Otter Creek has the greatest lag

time owing to incomplete drainage of the creek

between flooding tides and distance from the

coast. During periods of flooding tides from August to November, the tide was still rising at Otter Creek while the rest of the stations recorded ebb. This order of peak high tides

between sites varies slightly from month to

month, indicating that additional factors

affect the rate of tidal inundation.

WATER QUALITY PARAMETERS

AND TIDAL FLATS HYDROLOGY

Knowing water quality parameters during tidal flood and ebb is critical to understanding the interplay of tidal flood waters

and river discharge and its influence on

sedimentation in the Flats. The hydrostation

data plotted in Figure 31 bespeak the parameters controlling sedimentation across

the Flats. These data can be used to determine critical hydrological elements, such

as duration of inundation (depth), river vs.

tidal water domination (salinity and temperature), and sediment transport (OBS).

For example, Figure 31a presents water

quality data for about 1 month (18 August

to 14 September) where an earlier sequence

of high tides was only high enough to fill

the gullies, and a later sequence flooded

onto the mudflats. The record shows a daily

sinusoidal water temperature pattern when

tides are low (29 August–1 September). Perturbations to this regular temperature pattern and fluctuations in the salinity and OBS signify tidal flooding and mixing of different water

masses, as will be discussed later in more detail.

Data from a single flood and ebb cycle show

the basic relationships of water quality parameters and inundation (Fig. 32). Initially, as the

water level in the gully rises during flood, parameters remain at values characteristic of the incom

Table 10. Pressure sensor elevations and peak high tide delay relative to predicted time at Anchorage gauge.

Sensor Average delay time

elevation (min)

Site* (m) Jun Jul Aug Sep Oct Nov

Mortar Gully 2.79 19.1 20.9 25.4 25.8 23.0

Bread Truck Gully 2.76 22.3 21.0 18.9 27.1 29.9 28.2

Tanker Gully 3.08 24.9 26.2 27.8

Spring Gully 1.60 21.1 22.5 27.5 23.4 23.5

Parachute Gully 3.29 25.3 23.6 26.1 30.4 29.7 33.5

B-Gully 3.11 31.8 30.5 30.0

Otter Gully 2.87 4.0 25.9 31.3 30.1 31.0

Otter Creek 2.20 38.4 38.5 53.5 47.6 43.4

* Arranged by increasing distance from Knik Arm Coast.

21 Sep 25 Sep 3 Oct 11 Oct 19 Oct

0

2

0

10

20

0

20

0

1000

2000

29 Sep 7 Oct 15 Oct

Salinity (‰) Temp. (°C) Depth (m) OBS (mg/L) OBS (mg/L)

18 Aug 22 Aug 26 Aug 30 Aug 3 Sep 7 Sep 11 Sep 15 Sep

0

2

0

10

20

0

20

0

1000

2000

Depth (m) Salinity (‰) Temp. (°C)

a. 18 August–14 September 1994.

b. 21 September–18 October 1994.

Figure 31. Hydrostation record at Mortar Gully.
ing water. When the water crests over the gully

headwall (flooding depth mark), temperature and

salinity change rapidly to values characteristic of

the arriving flood water—either tidal, riverine or

a mixture. During ebb, the parameters gradually

change to levels that are, first, a mixture of flood

waters, to one with more local water characteristics. Knik Arm waters are generally the dominant

incoming water mass in the Mortar (Fig. 32a),

Spring, Bread Truck and Tanker gullies. A more

complex mixture of both riverine and Knik Arm

waters occurs in the Parachute, B (Fig. 32b) and

Otter gullies. Otter Creek often receives backed-up water from the Eagle

River, but during the highest tidal

events, Knik Arm waters penetrate

even to this most southern part of the

Flats, as will be discussed later.

Source waters

The characteristic signature of Knik

Arm water varies through the season

as air temperature and discharge from

glacial meltwater in the Eagle River

and in the rivers draining Knik Arm

change (Table 11). Knik Arm water

temperature reflects the seasonal rise

and fall in air temperature. During

May and October flooding events,

temperatures were in a similar range

(8–9°C), higher in September (11–

12°C), and lower in November (about

1°C). Salinity was highest in May (15–

16 ppt) prior to the start of the glacial

meltwater season, lowest in September when glacial discharge had a large

influence (5–6 ppt), and increased

gradually during October (8–10 ppt)

and November (11–13 ppt) as the discharges from rivers decreased.

The Eagle River is the primary incoming freshwater surface source for

the Flats, with both glacial and seasonal changes (Table 12, see Fig. 36).

While salinity, i.e., specific conductance, remains at near-zero (0.1 ppt)

throughout the season, changing specific conductance levels document the

relative influences of glacial sources.

Higher conductance levels occur during early and late season when the

ground water and local runoff components are greater than the glacial

input, and solute concentrations in the

glacial outflow are not diluted by meltwater

(Sharp et al. 1995). Daily undulations in the specific conductance and temperature curves in midsummer mark the diurnal meltwater discharge

variations from the Eagle Glacier.

Coastal sites

The two coastal sites where flood and ebb are

controlled by Knik Arm must be treated separately, since the Mortar Gully drains ponds in Area

D, and Spring Gully is fed by an upland spring

and the northwestern-most section of Area A.

18:00

7 Sept 1994

00:00 (hr)

8 Sept 1994

0

2

4

0

500

1000

3

6

9

10

12

14

Flooding depth

 Knik

Temp.

 Knik

Salinity

Salinity (‰)

Temperature (°C)

Depth (m)

OBS (mg/L)

18:00

7 Sept 1994

00:00 (hr)

8 Sept 1994

0

2

4

0

1000

2000

3

6

9

10

12

14

Flooding Depth

Temp.

Salinity

 Knik

 Knik

Salinity (‰) Depth (m)

Temperature (°C) OBS (mg/L)

1994

a. Mortar Gully

b. B-Gully

Figure 32. Variation in water quality parameters at two gully locations during 7–8 September flooding cycle.

28
The Mortar Gully record (Fig. 31) displays the

expected pattern of decreasing temperatures from

mid-August to mid-October as air temperatures

drop. This decrease in temperature, coupled with

increases in salinity, causes the water to become

denser, thereby increasing its capacity to transport sediment. Salinity changes at Mortar Gully

are cyclic, opposite to the tidal flooding sequence,

becoming more saline between flooding periods.

This saline condition is likely caused by the discontinued exchange of water between Knik Arm

and the ponds, allowing the salinity to increase in

the ponds and the plunge pools at the gully heads

through evaporation and stagnation. During

flooding, the salinity drops to the relatively low

levels of Knik Arm water.

Spring Gully, with significant freshwater input, maintains salinity levels near zero ppt when

tides are low, but has increasing salinity semidiurnally, marking periods of tidal inundations

(Fig. 33). During fall flooding (6–8 September, 5–8

October), the salinity increase is longer as the

higher flooding levels cause Knik Arm water to

remain impounded in the northernmost section

of Area A when freshwater spring discharge is

apparently lower.

Central sites

Hydrostation sites in the central area of the

Flats monitor gullies that hydrologically link

ponds to the Eagle River. Observed patterns are

cyclic and generally consistent from mid-summer

to fall. Temperature decreases steadily through

the period, while salinity levels during floods increase in response to the decreasing glacial meltwater input. Increased salinity between flooding

events in Parachute and Tanker gullies means

that their waters become stagnant and evaporate.

Table 11. Characteristic temperature and salinity of Knik Arm and Eagle River source

waters during flood and ebb portions of selected flooding events.

 Knik Arm Eagle River

Temperature (°C) Salinity (ppt) Temperature (°C) Salinity (ppt)

Date Flood Ebb Flood Ebb Flood Ebb Flood Ebb

May

25 8.2–7.5 7.5–7.8 — — 6.2–6.5 6.5–8.0 0.1 0.1

26 8.7–8.0 8.0–8.3 15.5–16.3 7.1–7.2 7.2–8.5 0.1 0.1

September

7–8 11.4–12.4 12.4–12.2 5.8–6.2 5.5–7 5.7–6.2 — —

8–9 11.5–12.5 12.5–12.2 5.9–6.5 7.3–7.0 7.0–6.8 0.1 0.1

October

5–6 8.2–8.9 8.9–8.8 6.3–6.1 6.1–6.2 0.1 0.1

6–7 8.3–8.8 8.8–8.7 8.2–10.0 10.0–9.9 6.2–5.8 5.8–5.5 0.1 0.1

7–8 8.0–8.6 8.6–8.4 5.0–4.5 4.5–4.0 0.1 0.1

November

3–4 0.8–1.6 1.6–1.5 11.2–12.7 12.7–12.6 — — — —

4–5 0.7–1.7 1.7–1.3 11.4–13.0 13.0–12.8 — — — —

5–6 0.4–1.3 1.3–1.0 11.6–13.0 13.0–12.9 — — — —

Table 12. Gross sedimentation rates. Measurements for seasons

through winter 1993–1994 are from transect lines 1–12 only, Summer 1994 measurements from lines 1–24.

Range (mm)

Morphological unit Sum 92 Win 92–93 Sum 93 Win 93–94 Sum 94

Levee 1–6 1–12 0–6 0–11 0–15

Vegetated mudflat 1–6 1–16 1–14 1–21 1–30

Unvegetated mudflat 1–13 7–12 1–8 6–10 1–17

Pond – plate 1–4 8–28 6–26 2–17 1–21

Pond – cup 2–9 8–26 6–19 1–13 2–20

Marsh ND ND ND ND 1–20

Gully 1–77 20–33* 0–19 0–16 1–19

* Based on small sample size.

29
30

Salinity remains more uniform at the remaining

sites—Bread Truck, B and Otter gullies—and this

uniformity reflects the continuous flow of water

out of the respective pond sources, each of which

has ground water influx from springs. In general,

the peak turbidity levels, which are consistent

with those of Knik Arm, increased during flooding events through the season (e.g., Fig. 34).

Otter Creek

Hydrostation data from the Otter Creek site

appear to confirm our 1993 hypothesis that the

dominant source water for this area is both Eagle

River and the upland springs that feed Otter

Creek, which enters ERF from the west. Salinity

remained very low throughout the summer, indicative of the freshwater discharge from the uplands southwest of Area B (Fig. 35).

During tides without floods, water temperatures drop semi-diurnally to levels

similar to those recorded in Eagle River

(Table 11). This behavior shows that

when high tide levels are low, Knik Arm

waters lack momentum and time to penetrate deep into the ERF river drainage

system (i.e., Area B); however, backpressure from inflowing tidal water appears to dam the Eagle River and divert

it into Otter Creek and the southwestern reaches of the Flats (Fig 35a). During the extremely high tides of late fall,

Knik Arm waters did have enough energy and elevation to penetrate deep

into this area, as indicated by the salinity increases in Otter Creek at that time

(Fig. 24). The salinity increase at Otter

Creek during the November floods is

longer and more pronounced (Fig. 35c).

The snow cover on the Flats at that time

retarded the runoff during ebb because

of its capacity to store flood water. Its

slow release probably was the primary

cause of the longer period of high salinity. Drainage through the porous

snow is much slower than normal ebb

runoff without snow.

18 Aug 26 Aug 30 Aug 3 Sep 7 Sep

0

2

4

0

10

20

20

0

2000

4000

22 Aug 4 Oct 8 Oct

Temp. (

0

°C) Salinity (‰) Depth (m) OBS (mg/L)

Figure 33. Hydrostation record at Spring Gully.

2 Nov 6 Nov 10 Nov

0

1000

2000

2 Oct 6 Oct 10 Oct

0

1000

2000

3 Sep 7 Sep 11 Sep 0

1000

2000

20 Jun 24 Jun 28 Jun

0

1000

2000

OBS (mg/L) OBS (mg/L) OBS (mg/L) OBS (mg/L)

Figure 34. OBS levels at Parachute Gully during flooding periods

in June, September, October and November 1994.
31

SEDIMENT TRANSPORT

AND SEDIMENT SOURCES

Sediments are deposited in ponds and mudflats

mainly through suspension settling from pond

and flood waters (Table 5). Sediment is derived

from two external sources: the Eagle River and

Knik Arm. Internally, the erosion of riverbanks,

gullies, pond bottoms and mudflats by wind and

water currents, ice, bottom feeding waterfowl, and

other mechanisms provides remobilized materials.

Concentrations of suspended sediment vary

seasonally, through monthly tidal inundation periods as well as through single tidal cycles (from

flood to ebb) (Fig. 36). These concentrations appear to vary with distance from the river and

coastal sediment sources (Fig. 36 and 37). Total

suspended sediment (TSS) measurements also

vary among sites through a single tidal cycle,

depending upon local runoff variations, pond storage and how effectively the gully moves water

out of the local area (Fig. 37).

A typical pattern in TSS response through a

flooding cycle is a sharp increase owing to highly

turbid flood water and resuspended gully bed

sediment (Fig. 38). TSS values remain high through

slack high tide and the start of ebb, but abruptly

decrease during ebb runoff. The length of ebb

runoff depends on how well the gully or network

of gullies transfers water stored in the ponds and

mudflats. The greater the volume of water stored

in an area, the longer it will take for the gullies to

effectively drain the ponds. The decrease in suspended concentration may reflect the loss of material by deposition on mudflats and in ponds.

TSS concentrations vary seasonally in the Eagle

River, being high through July and August, with

18 Aug 22 Aug 26 Aug 3 Sep 7 Sep 11 Sep 15 Sep 0

2

0

10

20

0

20

0

2000

4000

30 Aug

Depth (m) Salinity (‰) Temp. (°C) OBS (mg/L)

a. 18 August–14 September 1994.

1 Oct 5 Oct 9 Oct 13 Oct

0

2

4

0

10

20

0

20

0

2000

4000

29 Oct 2 Nov 6 Nov 10 Nov

Salinity (‰) Depth (m) Temp. (°C) OBS (mg/L)

b. 1–13 October 1994. c. 29 October–9 November 1994.

Figure 35. Hydrostation record at Otter Creek.
32

TSS (mg/L)

0

1000

2000

6 Sept 7 Sept 8 Sept 9 Sept 10 Sept

1994

Parachute Gully

Bread Truck

Mortar

Tanker

TSS (mg/L)

0

1000

2000

3000

12:00 18:00 00:00 (hr)

3-4 November 1994

1600

800

0

6

5

4

3

TSS (mg/L)

Elevation (m)

TSS

Elevation

18:00

6 Oct 1994

00:00 (hr)

7 Oct 1994

Figure 36. TSS measurements at Parachute Gully

through four tidal cycles in September 1994.

Figure 37. TSS measurements at coastal sites during

single tidal cycle, 3–4 November.

Figure 38. Typical TSS variation through a flooding

cycle.

glacially induced sediment peaks in mid-June and

early August (Fig. 39). However, the relative levels of TSS concentrations in the river are quite

low compared with simultaneous values measured at mudflat gully sites and in Knik Arm.

These data tell us that Knik Arm, and not the

Eagle River, is the main external sediment source

for ERF. Coastal TSS measurements exceed those

at the river throughout the monitoring period

and show a general increase in sediment concentration through the season (Fig. 40). We observed

gradual early season increases, as well as sharper,

late-season increases in the peak TSS values of

both the Knik Arm and gully sites. Late-season

increases in TSS also correlate with increases in

OBS turbidity measurements at the hydrostations,

as will be described subsequently.

Trends in TSS related to location in the Flats

were not observed until the October and November cycles, when TSS values of sites near the coast

were generally lower than at other sites, but were

comparable to those observed in Knik Arm waters (Fig. 39). The extremely high TSS values at

inland sites during the fall flooding cycles probably reflect a stronger influence of sediment transport from internal sources such as riverbank and

gully erosion, perhaps ascribable to freeze–thaw

effects on bank and mudflat materials.

Unfortunately, the USGS stopped monitoring

of the Eagle River by 1981, so that there is no

continuous record of water or sediment discharge.

However, the channel profile at Bravo Bridge was

measured twice during the summer of 1994 (17

4000

3000

2000

1000

0

4 May 3 Jun 3 Jul 4 Aug 1 Sep 1 Oct 31 Oct

TSS (mg/L)

Coast ISCO

Bread Truck

Parachute

Otter Gully

Otter Creek

Spring Gully

Mortar Gully

Tanker

River

Figure 39. Maximum monthly TSS measurements.

Coast-Grab

Coast-ISCO

River

Site

4000

2000

0

TSS (mg/L)

4 May 3 Jun 3 Jul 2 Aug 1 Sep 1 Oct 31 Oct 30 Nov

Figure 40. Coastal vs. river TSS measurements.
33

May and 5 September), the latter as part of an

exercise to measure the velocity profile of the

Eagle River (Fig. 41). Velocity was measured about

every meter across the bridge at approximately

30-cm (1-ft) depth increments. The water stage in

early September is relatively high because of seasonal runoff from Eagle Glacier, and velocity

ranged from below 0.2 m/s to about 1.9 m/s with

a discharge of 21.9 m3/s. TSS values measured at

the river ISCO averaged 44.6 mg/L with a peak

of 52.0 mg/L. Using these values, we estimated

average and peak sediment flux to be 0.9 and 1.1

kg/s respectively.

In May the channel profile was measured at

Bravo Bridge, yielding a cross-sectional area of

9.7 m2 (Fig. 42). TSS measurements during May

averaged 11.1 mg/L with a peak of 30 mg/L.

River discharge and sediment flux during the

channel survey can be estimated if the discharge

is assumed proportional to the cross-sectional area

of the river. In May it was 40.6% smaller than in

September. Using this assumption, we calculated

a river discharge of 8.9 m3/s, while the estimated

average and peak sediment fluxes were 0.34 and

0.45 kg/s. This estimated discharge is similar to

the 1966–1975 average of 6.8 m3/s for the month

of May from the USGS (USGS 1981).

SEDIMENTATION

Suspension settling occurs during slack high

tide and the early stages of the ebb cycle. In

ponded areas, it also happens between tides when

water turbulence is low and fine particles settle

out of the water column. Therefore, the longer





































Peak =


Ave.=


Peak =


 Ave. =


21.9 m3/s


52.0 mg/L


44.6 mg/L 


1.1 kg/s


0.9 kg/s


0.2 m/s

Water Discharge:


Sediment Concentrations:


 


Sediment Discharge:


 


Contour Interval:

1.0

0.8

0.6

0.4

0.2

0.8

1.0

0.6

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

South Pier

North Pier

Distance (m)

–5 0 5 10 15 20 25 30 35

6.0 

5.5 

5.0 

4.5 

4.0 

3.5 

Elevation (m)

Figure 41. Velocity profile for Eagle River on 5 September 1994 with

estimates of discharge (sediment and water).













 Water Level and Bed Profiling Date:


 May 17, 1994 (cross-sectional area = 9.7 m2)


 September 5, 1994 (cross-sectional area = 24.0 m2)

–5 0 5 10 15 20 25 30 35

Distance (m)

Left Pier

Right Pier

8 

6 

4 

2 

0 

Elevation above Sea Level (m)

Figure 42. Comparison of Eagle River profiles from 17 May and 5

September 1994.
quiet water conditions can be maintained, the

greater is the time for deposition. The mixing and

exchange of flood water with pond and marsh

water increases the amount of suspended sediment in the ponds and marshes, and provides the

primary source for deposition with each flooding

tide. Additional processes in gullies include

bedload deposition, sediment flows and slumping; these materials, however, may be remobilized and transported either into the pond areas

or into the Eagle River, depending upon tidal

stage.

Controls on sedimentation rates at any particular location include elevation, vegetation that

traps sediment, and the frequency, height and

duration of inundations by sediment-laden waters. The distance from the source water also plays

a role in some areas, as sediment can be deposited or eroded during flood and ebb, altering its

concentration. Local variability in deposition may

also result from ice rafting of sediment plucked

from other locations within the Flats.

Sedimentation rates vary with morphology and,

in a general sense, elevation (Table 12). The overall trend in rates is an increase from levees to

mudflats, ponds, marshes and gully bottoms, with

the more heavily vegetated areas having higher

rates of accumulation. Typical annual sedimentation rates range from about 10–30 mm among the

various landform types.

Net accumulation rates are generally higher

during the “winter” 8-month period from September to May than during the “summer” 4-month

period from May to September. This difference

may reflect the respective lengths of the seasons,

an increase in available sediment as indicated by

TSS values during the early winter months (Fig.

39), the trapping of sediment by the snow and ice

cover, or the number of flooding events. Net accumulation of sediment above paint layers

through August 1994 in Area C and D mudflats

and levees are shown in Figure 43. Most winter

values fall in the range 2–15 mm, while the summer values range between –5 and 6 mm. The

negative summer values may result from compaction or erosion, or from the variability in sedimentation in the area from which measurements

are taken.

The relationship of sedimentation rate to elevation is a response to the number of times the

sites are inundated. Table 13 depicts the number

of events that would reach critical elevations to

flood different landforms, based on predicted tide

heights at the Anchorage tide gauge. The number

of inundations is highest for ponds and decreases

with the increasing elevations of mudflats to

levees, thereby correlating with the general decrease in sedimentation rates (Table 12). An example of decreasing sedimentation rate with increasing elevation is revealed in the summer 1994

data for Transect 19 on Racine Island (Fig. 44).

Sediment accumulation along the northern part

of the transect that crosses a low-lying, abandoned

oxbow is much higher than in the elevated, vegetated mudflat to the south.

The number of flooding events that we show

in Table 13 is a conservative estimate, since predicted tidal elevations do not account for higher

tidal elevations from tidal amplitude increases

caused by the geometry of Knik Arm (e.g.,

McCann 1980a,b; Syvitski et al. 1987, p. 163), river

discharge, ice cover or wind. If the augmentation

Sediment Accumulation (mm)

–10 0 10 20

Winter

1993-94

Summer

1994

Individual 

Average

Figure 43. Net seasonal sedimentation rates at sites

sprayed with paint in August 1993 (winter 93–94

represents September 1993 through May 1994, summer 94 represents May 1994 to September 1994).

Circles represent individual sedimentation measurements; the triangle and dashed line show the averages.

Distance (m)

Accumulation (mm) 8

16

24

0 200 400

0

2

4

6

Elevation (m)

Accumulation

Elevation

North

Figure 44. Sediment accumulation and elevation along

transect 19 on Racine Island. Northern lower-lying

section is located within an abandoned oxbow.

34
of the tidal effects by such factors is considered,

the number of inundations increases (Table 13).

There were significantly more tidal floods at the

Bread Truck hydrostation during the 1994 season

than were predicted by the tide chart elevations.

The increase was on the order of twice that predicted for the summer period. The increase during the fall was much less evident and correlates

with a seasonal reduction in river discharge.

The amount of vegetative cover in the mudflats

influences the sediment accumulation rates. Sediment accumulation vs. vegetation cover at sites

first established in 1992 are correlated in Figure

45. Gross sedimentation rates at these longestestablished sites had 2-year total accumulations

of 6–18 mm at sites with less than 60–65% vegetative cover, and 26–32 mm at sites with a vegetative cover greater than 70%.

Sedimentation rates also vary with distance

from source waters. A decrease in sedimentation

rate with distance inland is best documented along

transect 16 (Fig. 46), which extends southward

from Knik Arm in coastal Area A (Fig. 7). Decreased velocities of Knik Arm waters during

flooding reduces their ability to transport sediment, causing rapid sedimentation near the coast,

with less material available further inland. The

same effect is often seen along the scarp of many

gullies where flow conditions change rapidly as

flood waters overtop gully banks. Sedimentation

rates in general decrease with distance from the

Eagle River inland toward the marshes abutting

the uplands (Fig. 47).

Table 13. Comparison of predicted and measured number of flooding events

reaching critical heights during summer and fall 1994. Measured data based on

depth transducer at Bread Truck Gully.

Number of flooding events

Anchorage predicted Measured

Critical height Summer 1994* Fall 1994* Summer 1994 Fall 1994

Inundate ponds (4.6 m) 16 21 52 23

Cover mudflats (4.87 m) 8 11 18 18

Cover levees (5.21 m) 0 5 4 7

*Summer 1994 (22 May to 15 Sept).

*Fall 1994 (16 Sept. to 9 Nov).

Vegetative Cover (%)

Sediment Accumulation (mm) 10

20

30

0 20 40 60 80 100

Figure 45. Relation between total sediment accumulation recorded in September 1994 and vegetative cover at sites sprayed with paint in 1992.

Cumulative Distance on Line (m)

Sediment Accumulation (mm)

0

2

4

6

8

400 800 1200

Coast Upland

Figure 46. Accumulated sedimentation with distance inland from coast along line 16 in northwest

Area A.

Distance (m)

Accumulation (mm)

8

16

24

0 200 400 600

0

2

4

6

Elevation (m)

Accumulation

Elevation

East

Figure 47. Sediment accumulation and elevation along

transect line 18 in Area A. East end of line is located at

river levee.

35
36

Sedimentation rates in the ponds are

also quite high (Table 12). The amount

of sediment that originated by resuspension of bottom sediments in ponds

ranged from 8–16 mm for the summer

of 1992, or approximately equivalent

to the thickness of new sediments deposited in the ponds during the same

period. During the winter 1992–93 period, sedimentation from resuspension

ranged from 2–15 mm, or amounts

similar to those of the summer 1993.

Winter 1993–94 resuspension rates

could not be calculated because the

plate collection devices were not

cleared in the fall of 1993. Using the

sedimentation stations established in

1994, we found that summer resuspension sedimentation ranged from 0–

7 mm (slightly less than in previous

years).

Pond sedimentation rates vary in

space and time across the Flats because

of the interactions of processes and

physical constraints described earlier.

Summer sedimentation rates for monitored ponds across ERF are summarized in Figure 48. During the summer

of 1994, the highest sedimentation rates

(15–20 mm) were recorded in the

ponds of Bread Truck, Area C and Area

A (Fig. 48a, b). Sedimentation rates at

intermittent ponds ranged from 1 to

15 mm per summer; however, the rate

at station 1 (Fig. 10) was anomalously

higher, having 24 mm per summer.

EROSION DYNAMICS

Gully erosion is one of the most visible mechanisms that is currently modifying and reshaping the Flats. Similarly, Eagle River migration also causes

considerable change (Tables 1–4). Because of the rapid dynamics associated with river and gully erosion, the

rates of erosion and recession need to

be quantified and integrated into our

modeling of the physical system. This

quantification is critical because numerous gullies are experiencing headward erosion towards several of the

most contaminated ponds on ERF.

 

 

   N

1994 Pond Sediment


Accumulation (mm/summer):


—————————————

0-05


5-10


10-15


15-20


0 500 m













0-5 mm/summer

5-10 mm/summer

10-15 mm/summer

15-20 mm/summer

0 500 m

N

a. Northwest tile.

b. Northeast tile.

Figure 48. Summary of sediment accumulation in pond locations

during summer 1994.
37

Driving mechanism

and system response

Gully and river erosion in ERF may best be

described as a complex response to external and

internal factors (Table 7), particularly the 1964

Alaskan earthquake. Brown et al. (1977) concluded

that the Anchorage area subsided 60–70 cm during the earthquake, followed by a post-seismic

recovery of about 1.5 cm/yr. However, gravity

and tide gauge data indicate that the rate of uplift

has been decreasing. Accordingly, up to 45 cm (or

about 65%) of the recovery may have occurred

through September 1994. The physical system

therefore has been and now is complexly responding to an initially rapid reduction in base level,

which is now being followed by a gradual rise as

isostatic recovery takes place.

The ponds’ and mudflats’ initial response to

subsidence probably was a significant increase in

deposition in them, as the potential energy decreased with the lowering of the surface elevation

closer to that of mean Anchorage sea level. The

lower elevation increased flooding frequency, and

the volume of water entering the Flats increased

the amount of sediment entering ponds and the

time available for suspension settling in the ponds,

marshes and mudflats. Deposition will augment

the rate of surface rise and thus may reduce the

length of time for recovery in base level than

that expected from tectonic adjustment

alone.

The subsequent uplift and increase in

flood water volume also induced gully erosion across ERF; this is evident by an increase in gully entrenchment, the formation

of knickpoints* and progressive headwall

migration into the mudflats and ponds. The

most prominent knickpoints are the steep

headwalls at plunge pools in gully heads.

As uplift continues, the gradient of the gully

bed steepens relative to mean tide. Gully

incision results from scour of the bed as the

volume and rate of discharge increase. Gully

knickpoints migrate up-gradient, reducing

the slope of the longitudinal profile by

scour and progressively lowering the downchannel gradient (Fig. 49). Gully incision and

headwall erosion will continue as long as

the overall base level is lower than the one

that existed prior to the 1964 earthquake;

once tectonic stability is attained, an equilibrium gully slope profile will develop and

gully extension should cease.

Erosion and recession rates

Headwall erosion is progressively elongating gullies into the mudflats, towards the ponds.

Eagle River is also laterally migrating, transporting the eroded sediment down-river towards Knik

Arm.

The amount of recession of scarp crests is

highly variable within a gully, as well as among

the individual sites (Table 14, App. B). At each

site, some parts of the scarp crests did not retreat at all, and in fact, extension was measured

where tension cracks developed at an early stage

of collapse. This variability in recession rates is

evident in plots of the scarp crest location with

time (Fig. 50 and 51). Recession rates for the

September 1993 flooding cycle were a maximum

of 1.4 m, while 3.8 m was observed at Bread

Truck Gully during the fall of 1994 (Table 14,

Fig. 50d). Recent monitoring of erosion along

the Eagle River recorded recession rates of 2.3

m during the summer and 2.0 m during the fall

of 1994 (Fig. 51), with maximum recession rates

of 3.7 m. Headward erosion in 1994 resulted in

  



        



 



0 500 m

N

1994 Pond Sediment


Accumulation (mm/summer):


—————————————

0-05


5-10


10-15


15-20


c. Southern tile.

* Knickpoints are locations of abrupt slope change in the

longitudinal channel or gully profile and represent system

disequilibrium.

Figure 48 (cont’d).
38

Table 14. Summary of gully and river erosion monitoring.

Recession range (m)

Area Position* Sum 92† Win 92 Sum 93 Win 93 Sum 94 Fall 94 Net erosion

Area A

 (coast) H ———— 0–0.47 0–1.17 0–1.09

L— — — — 0–1.01 0–1.85 0–2.76

 (interior) H ———— 0–1.59 — 0–1.59

L— — — — 0–1.01 0–1.17 0–1.17

Area B H — — — —0 0 0

L— — — — 0 0 0

Area C H 0.09–3.06 0.17–0.37 0.04–3.79 0.13–1.35 0–1.16 0 –1.17 0.04–9.30

L 0–4.90 0–6.30 0.17–9.80 0–2.08 0–2.57 0–1.03 0.27–8.17

Area D H 0.01–6.95 0.05–0.35 0–4.31 0–0.61 0–2.10 0–3.77 0.04–4.61

L 0–2.37 0–2.00 0–3.12 0–2.29 0–2.23 0–3.77 0–8.15 **

Racine Isl. H — — — —0 0 0

L— — — — 0 0 0

River L ———— 0–2.32 0–2.03 0–3.69

* H —Head wall; L—Lateral wall.

† Summer (Sum) represents period May to September; Winter (Win) represents period September to May.

** Measured change since Fall 1993.

Distance (m)

–2

–1

0

1

2

3

4

5

0 200 400 600 800

Elevation (m)

Headwall

Plunge Pool

To Eagle R.

Gradients

(m/m)

0.0099 0.0061 0.0044 0.0027 0.0045

0.0063

0.00380.010

– 0.0014

E0 1 F F 0

Distance (m)

1

2

3

4

5

0 50 100 150 200 250 300 350

Elevation (m)

Headwall

Knik Pt.

Plunge Pool

Gradients

(m/m)

0.0165 0.0070 0.0081 0.0042 0.0143 0.0059 – 0.0044 0.0014

To Eagle R.

C0 C2

a. C0 to C2.

b. E0 to F0 to F1.

Figure 49. Longitudinal profiles along two gully thalwegs (see Fig. 13). Gradients

along sections of the gullies are indicated.
39

variable recession rates. No erosion was measured on Racine Island or Area B in 1994. Field

observations in Area B showed that slopes are

heavily vegetated and more gently sloping than

in other sections of ERF. Lateral erosion in 1994

also had values ranging from 0 to 3.7 m (Table

14). The greatest change measured thus far was

the extension of a shallow (0.5 m) gully by nearly

45 m into the mudflats near Lawson’s Pond during the 1992–93 winter (Fig. 50e); little change has

been observed since that time.

Recession by lateral erosion characterizes the

B-Gully site, located about 75 m below the headwall (Fig. 50a). Erosion forms cuspate embayments

that gradually enlarge, with a maximum measured

recession of 8.2 m (Fig. 52). This site is significant

because WP has been found in gully transport,

but additional study is needed to see if WP is

being flushed out of C-Pond or introduced by

erosion and collapse of these bank sediments.

Parachute Gully (Fig. 50b) is a good example

of the processes of headward erosion and recession. Up to 9.3 m of headward recession (~3.79

m/year; Table 14) but only 1.6 m of lateral recession have been monitored at this site since 1992.

Recession has centered around a cratered drainageway that has become incised and enlarged.

Two additional drainages, Coastal 6 and Bread

Truck, exhibit variable head and lateral recession

rates characteristic of ERF (Fig. 50c and 50d). Bread

Truck Gully receded headward 0–3.5 m (up to 1.9

m/year) and laterally 0–8.2 m since monitoring

began in 1993 (Table 14). Coastal 6 Gully receded

0–2.8 m headward and 0–1.2 m laterally in a single

summer season of 1994. Upwards of 1.9 m of

headward and 1.2 m of lateral recession occurred

here in early fall.

Rapid lateral erosion of about 3.7 m was recorded at River Section-North between July and

November of 1994 (Fig. 53). Erosion takes place

along large rotational slumps that are undercut

by the river (Fig. 54). To start, large extension

cracks develop parallel to the scarp, gradually

leading to catastrophic failure. The location of the

crack becomes the new scarp of the riverbank.

Several locations where the river is meandering

and slope processes are consuming mudflat sediments are given in Figure 55. Material at these

sites is entering directly into the Eagle River and

subsequently into Knik Arm.

a. B-Gully.

Figure 50. Gully scarp locations for measuring erosion rates.

27 Jun 1992

22 Sep 1992

02 Jun 1993

26 Aug 1993

22 Sep 1993

14 May 1994

17 Sep 1994

12 Nov 1994

Hub Stake

Line Stake

B

132

131

120

123

102

43

92

A
0 5 m

275

276

282

284

283

288

291

296

303 298

297

290

27 Sep 1993

25 Jul 1994

17 Sep 1994

12 Nov 1994

Hub Stake

Line Stake

b. Parachute Gully.

c. Coastal 6 Gully.

Figure 50 (cont’d). Gully scarp locations for measuring erosion rates.

27 Jun 1992

22 Sep 1992

02 Jun 1993

26 Aug 1993

22 Sep 1993

15 May 1994

17 Sep 1994

12 Nov 1994

Hub Stake

Line Stake

0 5 m

85

178

181 34

33

32

84

37

94

31

42

40
Scarp Profile

Gully

Thalweg

Stakes

Hub

0 10 m

Original (27 May '92)

Summer 1 (22 Sep '92)

Winter ( 2 Jun '93)

Summer 2 (22 Sep '93)

Measured Points

Figure 50 (cont’d).

e. In-Between Gully.

d. Bread Truck Gully.

0 5 m

27 Sep 1993

25 Jul 1994

17 Sep 1994

12 Nov 1994

Hub Stake

Line Stake

383

387

384

177

163

164

162 158

159

157

151

150

41
0 5 m

22 Jul 1994

14 Sep 1994

12 Nov 1994

Hub Stake

Line Stake

406

401

400

Figure 51. River bank erosion at the River Section-North.

Figure 52. Eastward view of eroded cuspate embayment

at monitored section of B-Gully where white phosphorus was recorded in transport.

Recession rates vary because erosion takes

place as rapid, short-duration events (Tables 1

and 2). Recession is mostly caused by current

scour of the lower, unvegetated portion of gully

walls during ebb tide when water velocities are

high. Because the uppermost 20–30 cm of material is consolidated and rootbound, this soil and

root mat fail only after an erosional niche of

approximately 0.5 m or deeper is cut below it.

Along steep scarps, blocks of consolidated, root

bound sediments fall and roll into the gully bottoms, remaining intact until currents eventually

break them down either by rolling them along

the bed or by scour (Fig. 53). The lateral walls of

gullies, which tend to develop low-angle slopes

as headwalls, progressively recede inland, failing mainly by rotational slumping, but also by

debris flow; both processes introduce sediment

into the gully channel. On gentler slopes, blocks

of rootbound material can remain intact as they

are transported by these mudflows. This sequence of events happens at different times and

rates, and therefore recession varies within as

well as among the study sites (Table 14).

If our measurements of the average recession

rates are representative of longer-term trends,

then erosion may be expected to extend gullies

into ponds over the next 15–50 years (Table 15).

42
a. Plunge pool of Mortar Gully.

b. Gully scarp of Coastal 6 Gully.

Figure 53. Examples of slope failure.

Table 15. Preliminary estimates of delay before natural pond drainage.

Distance Max. Max. recession Min. time before

Gully to pond (m) recession (m) rate (m/yr) natural drainage (yr)

Bread Truck 65–100 3.4 1.9 35–50

In-Between 40 9.8 2.3 17–20

Parachute ~80 9.3 3.1 25–30

43
Figure 54. Rotational slump block failure at River

Section-North.

Knik Arm

0 500 m

Permanent Pond


Intermittent Pond


River Bank Erosion Site

N

Figure 55. Sites of observed or suspected

riverbank erosion.

44
The two ponds most likely to be naturally

drained by headward gully extension are Bread

Truck in an estimated 35–50 years and C-Pond

in an estimated 17–25 years (Fig. 56). Further

analyses of both modern and historical recession rates are required to determine if the 1992–

1994 data are representative of longer-term

rates.

Implications for

remediation methods

An understanding of gully recession enables

us to better assess the effectiveness of the proposed remedial measures for WP contamination. In a positive way, headward recession will

eventually drain C-Pond, Bread Truck Pond and

Pond Beyond, thereby producing drying conditions that will favor a natural degradation of

WP (see Walsh et al. 1995). Gully erosion, however, could threaten the integrity of capping

measures (i.e., geotextiles and AquaBlock) if

these are used in the wrong places. Erosion will

undercut the capping material and damage or

destroy its effectiveness.

Gullies are also routes for pond outflow and

therefore are potential pathways for migration of

WP off-site. As the gullies continue headward

expansion into contaminated ponds and mudflats,

WP-bearing sediment will be introduced into the

water draining the ponds. As gullies encroach on

pond margins, erosion will probably increase runoff, stimulating increased scour, and once reaching unvegetated pond bottoms, increase the rate

of gully expansion within them.

Dredging has also been proposed as a potential method to remove WP from pond sediments.

This method will, however, affect the physical

properties of the dredged areas and will affect

hydrological conditions, including drainage. Gullies intercepting a dredged area may deepen and

extend rapidly through those areas because of the

loss of vegetation and compaction of sediments.

Extension could be rapid, and move large

amounts of material while destroying the ponds.

N

BT

BG

PC

MG

C-Pond

Bread Truck


 Pond

Pond Beyond

35-50 years

17-20 years

25-30 years

E a g l e

R i v e r

0 500 m

Figure 56. Drainages within C/D Area

near permanent ponds with arrows showing locations of anticipated progressive erosion. Where rates have been measured, estimated time to natural draining of ponds

is indicated (MG–Mortar Gully, BT–

Bread Truck Gully, PC–Parachute Gully

and BG–B Gully).

45
46

GULLY HYDRAULICS

AND DISCHARGE

Gully profiles

Longitudinal profiles of nine gullies, surveyed

along their central channel or thalweg, have several characteristics in common. Each gully in the

area east of Eagle River has moderate to relatively steep slopes downstream of an eroding

headwall (e.g., Fig. 49). The headwall region has

a nearly vertical face ranging from 1.2 m to over

2.0 m in height. At the base of the headwall is a

depression or plunge pool scoured by water flowing over it (Fig. 57). These plunge pools range in

depth from about 0.5 to over 2.0 m.

Channel gradients are steepest near the

Eagle River (1.0 to 2.0 m/km). The steep bed

slope here probably results from the large

range in tidally controlled variations in water

level and discharge in the river proper. Upgradient of this steep section, the gradient

shallows (0.3 to 0.7 m/km), producing a concave upward profile that then progressively

steepens with distance to the base of the

headwall (Fig. 49). At one or more locations

in the gully, short segments of the channel

exhibit the sharp increases in gradient

(knickpoints) that signify that the bed profile

is not in equilibrium with the existing hydrological conditions. Further, they indicate active scour of the bed as the gully thalweg

deepens and the channel gradient lessens.

Channels above each headwall are lined by

vegetation and the bed slope ranges from 0.1 to

0.6 m/km, generally being less than gradients in

the gully proper. With distance, slopes gradually

reverse in direction and dip towards the ponds

across the unvegetated mudflats. The divide represents a critical threshold beyond which gully

extension will capture the pond and accelerate its

drainage. As erosion continues, the lower threshold for pond drainage may extend the duration of

and possibly the volume of water flow through

the gully, further accelerating erosion and

headwall recession.

Figure 57. Water flowing over gully headwall at Parachute Gully plunge pool.

Table 16. Discharge calculations for October and November 1994.

Spring Gully Bread Truck Parachute

Cross-sectional area (m2) 26.5 5.5 9.8

Peak flow (cm/s)

Flood 51.82 84.17 64.97

Ebb 177.06 146.41 75.71

Mean flow (cm/s)

Flood 15.07 21.92 10.48

Ebb 66.06 69.42 50.31

Peak discharge (m3/s)

Flood 13.73 4.63 6.65

Ebb 46.92 8.05 7.42

Mean discharge (m3/s)

Flood 3.99 1.21 1.03

Ebb 17.51 3.82 4.93
47

18:00

6 Oct 1994

00:00 (hr)

7 Oct 1994

–200

–100

0

100

2

4

6

0

1000

2000

3000

Elevation

TSS

Velocity

Velocity (cm/s)

Elevation (m)

TSS (mg/L)

18:00

6 Oct 1994

00:00 (hr)

7 Oct 1994

–200

–100

0

100

4

6

0

500

1000

1500

2000

TSS

Elevation

Velocity

Velocity (cm/s)

Elevation (m)

TSS (mg/L)

18:00

6 Oct 1994

00:00 (hr)

7 Oct 1994

–200

–100

0

100

4

6

0

1000

2000

3000

TSS

Elevation

Velocity

Velocity (cm/s)

Elevation (m)

TSS (mg/L)

c. Parachute Gully.

b. Bread Truck Gully.

a. Spring Gully.

Figure 58. Velocity during flood and ebb in a single tidal cycle.

Gully discharge

The volume of water that flows through

a gully is mainly a function of the height

of tidal inundation, including its supplement by the river and any additive effects

of wind, ice and any other factor that may

reduce gully cross-sectional area. The velocity range and magnitude of the flood

and ebb currents determine the flux of

sediment and water from the ponds and

mudflats, as well as gully erosion.

The velocity of tidal flood and ebb currents at the Bread Truck, Spring and Parachute gullies differs greatly in magnitude

among these sites, but the peak ebb velocity in each instance is greater than the

peak flood velocity (Table 16). Velocity

asymmetry is greatest between the ebb

and flood in Spring Gully and the smallest in Parachute Gully (Fig. 58). The magnitude also changes with the height of

flooding (Fig. 59), the lower peak and

range in velocities occurring during the

lower elevation flooding tides. Velocities

are greatest in Spring Gully, with moderate values recorded in Bread Truck Gully

and the lowest values in Parachute Gully

(Table 16).

The flood and ebb asymmetry determines the net transport of sediment. During flood, gully water levels generally rise

passively before spreading onto the

mudflats (Fig. 60). Then, the amount of

sediment in suspension is probably limited to the amount in the water source,

mainly Knik Arm. In contrast, higher

velocities during ebb mean increased turbulence and potential for entraining sediment and thereby resulting in a net flux

out of ERF. More energy is also available

to cause bank and bed scour across the

mudflats and within the gullies.

Discharge has been calculated for the

Bread Truck, Parachute and Spring gullies, where current meters were located 1

m above the bed. The cross-sectional areas in each gully (Fig. 61) were used with

the peak and mean velocities to estimate

the discharge (Q) (Table 16, App. D) by

Q = VA

where V is velocity and A is the crosssectional area.
48

100

0

–100

–200

6

4

2

100

0

–100

–200

X Direction

Y Direction

Composite Vector

Ultrasonic 

Data

5 Oct 7 Oct 9 Oct

Elevation (m) Flow (cm/s) Flow (cm/s)

1994

100

0

–100

–200

6

4

2

100

0

–100

–200

X Direction

Y Direction

Composite Vector

5 Oct 7 Oct 9 Oct

Elevation (m) Flow (cm/s) Flow (cm/s)

1994

5

100

0

–100

–200

6

4

2

100

0

–100

–200

X Direction Y Direction

Composite Vector

5 Oct 7 Oct 9 Oct

Elevation (m) Flow (cm/s) Flow (cm/s)

1994

5

a. Spring Gully. b. Bread Truck Gully.

c. Parachute Gully.

Figure 59. Current velocity measurements for tidal inundation and ebb during the period 5–9 October. Flow

velocity is in the x and y directions; the composite velocity vector and flood–ebb elevations are shown.

200

100

0

4 56

Maximum Water Elevation (m)

Velocity (cm/s)

Spring Gully

Bread Truck

Parachute

Maximum Water Elevation (m)

200

100

0

4 56

Velocity (cm/s)

Spring Gully

Bread Truck

Parachute

a. During flood. b. During ebb.

Figure 60. Comparison of peak velocities at the Spring, Bread Truck and Parachute gullies. Note the variability

from site to site, which may be related to the area drained by each gully.
49

Data for October and November 1994 were

divided into their flood and ebb components using the tidal inundation data in Appendix C. Calculations were based on average flood velocities

ranging from 13.4 to 32 cm/s and ebb velocities

ranging from 45 to 124.6 cm/s. Peak velocity

ranged from 37.4 to 71.1 cm/s during flood and

72.0 to 176.2 cm/s during ebb.

Using the measured average and peak velocities, we estimated flood and ebb discharge for

other months without measurements (App. D).

Average velocities of 25.0 and 76.1 cm/s were

used for Bread Truck Gully calculations, 27.1 and

47.0 cm/s for Parachute Gully, and 15.5 and 101.9

cm/s for Spring Gully.

Average discharge values in Bread Truck Gully

were about three times greater during ebb than

flood (~4.1 vs. 4.3 m3/s), with a peak ebb discharge of 8.0 m3/s. The difference in flood and

ebb discharge at Parachute was slightly less overall (~1.2 vs. 2.6 m3/s), peaking at 4.0 m3/s. At

Spring Gully, the difference is over 4 to 1 (~1.2 vs.

5.8 m3/s); peak ebb is 9.7 m3/s.

Peak and average TSS concentrations collected

over the same time intervals were then combined

with discharge calculations to make rough estimates of flood and ebb sediment fluxes (App. D).

Ebb sediment flux exceeded flood sediment flux

in each case, ranging from 2 to 3 times more during ebb. Peak sediment flux during ebb ranged

from 33.7 kg/s at Spring Gully, to 11.2 kg/s at

Parachute Gully, to 10.9 kg/s at Bread Truck

Gully.

The calculated ebb and flood sediment flux

data suggest that there should be a net transfer of

sediment from ERF into Knik Arm, if these calculations are representative of the summer and this

relationship holds during the winter when we

don’t have any data. Our sedimentation data indicate a higher overall rate during times of ice

and snow cover. Without data on runoff, including sediment discharge, it is difficult to examine

this problem further.

WHITE PHOSPHORUS

EROSION AND TRANSPORT

White phosphorus may be eroded and transported with sediment by wind, river and tidal

currents in summer. During winter, ice forming

on the ponds and mudflats as frazil and anchor

ice within both freshwater and saltwater bodies

has the potential to entrain and transport sediments with WP (Fig. 62). It is critical to determine

if transport of WP by these processes is quantitatively important in moving it to other locations in

ERF proper or elsewhere in the Eagle River and

Knik Arm. Ice and water may deposit WP-bearing sediments where waterfowl, fish, invertebrates

or other potential receptors may ingest them.

WP-contaminated sediments transported as

suspended or bedload in gullies were sampled

using a plankton net (Fig. 19). Recent gully bottom deposits were also sampled as surrogates for

0 2 4 6 8 10 12 14 16 18

Distance (m)

0 

1 

2 

3 

4 

5 

0 

1 

2 

3 

4 

5 

0 

1 

2 

3 

4 

5 

1 

2 

3 

4 

5 

Elevation (m)

Bread Truck Gully

Parachute Gully

Spring Gully

Spring Gully

a

b

c

d

Figure 61. Gully cross sections at the

location of flow probes in Bread Truck

and Parachute. Two cross sections from

Spring Gully represent the gully profile

at the hydrolab (c) and the flow probe (d).
sampled to determine if in fact ice was transporting WP-contaminated pond sediments (Fig. 63).

Sediment and WP may be moved when the ice

and frozen sediments are lifted off of the mudflat

during flooding tides and drift with the wind and

tidal currents (Fig. 64a). During tidal ebb, the ice

can strand on the mudflat surface, and it often

piles up at the heads of gullies (Fig. 64b).

The plankton net, gully bottom, pond and ice

floe samples each carried WP-contaminated sediments (Fig. 62b and 63). WP concentration ranged

from 0.0013 to 0.0493 mg/kg in three out of four

plankton net samples taken during ebb discharge

through B-Gully, following the four highest flood

cycles in September. Four out of eight samples

obtained from sediments frozen to ice floes yielded

WP concentrations that ranged from 0.001 to 0.013

mg/kg. Two resuspended sediment samples from

ponds in Area A and C ranged from <0.0014 to

0.003 mg/kg. These latter samples show that WP

can be resuspended, but this may be limited since

no samples from other sites have yet tested positive for WP.

Both the suspended load and bedload transport samplings and those from ice floes are significant. The ice floe samples that we obtained

while walking along a transect from the EOD pad

to the Bread Truck Gully site are the first from

ERF. Samples were collected if the ice floes en-

            



































































































































































































M u d f l a t s

E a g l e

R i v e r

C


Pond

M

a r s h

Bread


Truck


Pond

Unvegetated


Mudflat

Levee Not Detected


Detected

a. Positive results of previous sampling of gully

bottom sediments.

N

Eagle River


Permanent Pond


Intermittent Pond


Drainage Gully


Ice Sample (no detection)


Ice Sample (detection)


Sediment Station (detection)


Plankton Net

0 500 m

0.001

 0.0045

0.013

0.003

<0.0014

6803000

355000356000

6801000

0.0043

0.001

b. Positive results of 1994 sampling

of sediment and water samples by

gully currents and ice floes.

Figure 62. Location maps for WP.

material in transport. Sediment in each sedimentation trap at the pond sites was sampled to see if

WP particles were being resuspended and redeposited in ponds.

In addition, during November 1994, recently

transported and deposited ice floes having sediments frozen to their bottom surfaces were

50
a. Vegetated pond bottom sediments frozen onto ice near the In-between Gully.

b. Sediment layer frozen onto an ice block at Otter Gully.

Figure 63. Examples of freeze-on to pond ice at ERF.

countered had sediment frozen to them. The number of positive detections (~50%) out of the total

number of samples indicates that this may be

extremely important. This mechanism could entrain areas of pond bottom material of several

square meters or more, with thicknesses of tens of

centimeters. Ice floes move inland as well as out

into Knik Arm and thereby can redistribute WPcontaminated materials. Further analyses are required to determine if these results are representative.

The plankton net samples are important because they point to water actively transporting

WP-contaminated sediment from C-Pond or the

51
52

surrounding mudflat, or both. Although other

samples in 1994 from Bread Truck and Parachute

gullies proved negative, they were taken during

high tides of less than 5.03 m. Each flood when

the B-Gully samples proved positive exceeded

5.15 m, so the former samples may not be representative.

The positive analyses for WP-contaminated

sediments on ice floes and in gully runoff give

some indication that migration of WP within the

site and off-site is taking place. They clearly show

the need for further investigation of WP transport

and redeposition, including active deposition areas of Knik Arm.

a. Aerial view showing distribution of ice blocks stranded during ebb tide.

b. Ice block pile along high point near gully margin.

Figure 64. Stranded ice blocks on the southern edge of In-between Gully.
53

BATHYMETRIC MONITORING

AND WHITE PHOSPHORUS

DEPOSITION

Bathymetric monitoring of Knik Arm near the

mouth of Eagle River provides us with important

information on the submarine and estuarine environment adjacent to ERF. Locations within this

area may be sites where sediment and WP is deposited. Once identified, these areas should be

sampled for analysis of WP.

The Knik Arm near the mouth of the Eagle

River is 9–10 km wide (Fig. 65), having large tidal

cycles (±9 m) that cause the Arm to alternate between being tidally and fluvially dominated. At

low tide, this portion of Knik Arm essentially

becomes a large anastomosing stream with a main

channel located directly off the coast of ERF.

Highly turbulent flow with large standing waves

are common during low tide and as tidal flooding

starts.

Bathymetric profiles

Examples of the bathymetric transect lines (Fig.

20) from which Figure 66 is developed are shown

in Figure 67. The main channel just off ERF can be

identified on each of the five bathymetric profiles. This channel changes from a broad (~500

m), smooth profile about 14 m deep along the

northeastern corner of ERF to a more deeply incised (>16 m) and narrow channel (<200 m) near

the mouth of the Eagle River. The asymmetrical

profile has steeper slopes nearshore than those

further out into the Arm (Line D, Fig. 67). Line F

(Fig. 67), located near the northwestern corner of

ERF, shows that the channel is wider (near 1 km)

and appears to merge with a secondary channel

at the intersection of Lines D and H. This secondary channel is located about 1–2 km offshore of

ERF. It is characterized by sharp scarps, an uneven or scabbed bed profile with 3–5 m of relief,

and shallower depth than the main channel. The

main channel appears to handle most of the discharge during ebb, and low tide in particular,

when two large intertidal bars in the center of

Knik Arm are exposed (Fig. 65 and 67). The

scabbed profile of the secondary channel suggests

that localized and intense scour occurs in it.

The intertidal bar nearest ERF is located 0.2–

1.2 km offshore and is greater than 1.5 km wide.

Goose

Bay

KNIK

ARM

Eagle

Bay

Eagle

R.

Sixmile

Lake

Otter

Lake

Eagle River

Flats

0 1 mile

18

Mud

60

60 30

18

30

18

30

18

18

18

18

18

18

18

Figure 65. Knik Arm in the vicinity of ERF.
The second bar extends north beyond the area

that we have monitored. Both intertidal bars represent large zones of temporary sediment storage

within Knik Arm.

A third, smaller intertidal bar forms a sharp

peninsula that protrudes northwest into the Knik

Arm from the northeast bank of the Eagle River

mouth. Transect Line L (Fig. 67) turns at this bar

across the Eagle River mouth, showing it to have

a relatively flat bed and a depth of only 2–3 m

below Anchorage mean sea level.

Potential areas of

WP deposition

The bathymetric data show that several areas

in Knik Arm are potential sites of sediment and

WP deposition. These areas include the

two intertidal bars mentioned previously

(Fig. 66). In addition, there may be deposition along the coastline of ERF in zones

where eddies develop during tidal inundation and in the early stages of ebb. Sediment appears to be accumulating along

the upper beach slope as well. These sediments probably come from suspension

settling near slack high tide and during

the initial ebb.

Because our data indicate that WPcontaminated sediment is being transported by ice and water into gullies that

drain into the Eagle River, we recommend

that further analyses of offshore areas in

Knik Arm be undertaken. These studies

should include sampling of suspected areas of deposition for analysis of WP.

GROUND WATER

INFORMATION

Data on ground water conditions in

ERF are severely limited, but suggest that

conditions may vary widely. We currently

have four sources of data available (Fig.

68): piezometer measurements from three

areas in 1994 and Druck pressure transducer measurements from a single shallow well in 1993. We established 4 piezometers in Bread Truck, 9 on a transect

from C-Pond to the Eagle River and 15

piezometers in four groups in the C-Pond

area for this study (Fig. 68). The other

piezometer groups consisted of three to

five piezometers per set, located at different depths below the ground surface, depending

on site conditions. Each cluster provides data on

vertical gradients. Between the initial installation

and first measurement, three clusters went dry as

the surface elevation of C-Pond lowered. These

groups were then reestablished within the intermittently flooded area of the pond; they were

measured twice after flooding in early September

(Fig. 68). A single cluster in the permanently

ponded area of C-Pond (Fig. 68) was measured

three times during the summer.

Piezometers in C-Pond recorded elevated water levels in September whenever the sites were

flooded (Fig. 69). Piezometers that were set deepest within the muddy substrate of the ponds generally recorded the highest level of elevation above

N

Coastal 1

Coastal 2

Coastal 3

Coastal 4

Coastal 5

Coastal 6

Coastal 7

New 


Coastal

Eagle R.


Mouth

0 500 m

Bathymetric Contour Line


(CI = 2m)


Location of Echo-Sounding


Transect Lines

A1

A2

A3

A4

A5

A6

B1

B2

B3

B4

B5

B6

C1

C2

C3

C4

C5

C6

D1

D2

D3

D4

D5

D6

E1

E2

E3

E4

E5

E6

F1

F2

F3

F4

F5

F6

14

4

4

12 10 8

8

8

6 4

10

6

6

4

0

0

Bar

S

Bar econdary Channel

Figure 66. Bathymetric map of the Knik Arm near the mouth of

the River and adjacent to the coast of ERF. Datum is mean

Anchorage sea level.

54
the surrounding pond surface and indicate that

C-pond was an area of ground water discharge.

The variability in water table elevation through

time was only recorded from the monitoring well

in B-Gully (Fig. 68 and 70). These data show that

the water table fluctuates as a function of tidal

flooding frequency in B-Gully, even when tides

do not fully inundate the mudflats. There is, therefore, a subsurface hydraulic connection between

tidal water and ground water at this site. However, there are no data available to determine the

extent of this effect, and since the site was monitored only in 1993, these data cannot be correlated with our 1994 piezometer data.

The scanty data on ground water conditions

suggest that tidal inundation interacts with the

ground water system, but the precise nature of

that interaction is unknown. There is a clear need

for analysis of ground water conditions before its

effects on WP remediation can be interpreted.

Ground water fluctuations may be critical to measures requiring drainage and drying of sediment.

0

4

8

12

16

20

Secondary

Channel

Main

Channel

Bed

North South

Depth (m)

LINE D

0

4

8

12

16

20

Secondary Channel

North South

Bed

Depth (m)

LINE F

Main Channel

0

4

8

12

16

20

Secondary

Channel

Main

Channel

North South

Bed

Depth (m)

LINE H

0

4

8

12

16

20

West

Turn Turn

Bed

Main

Channel

River

Mouth

West

Depth (m)

LINE L

0

4

8

12

16

20

Main Channel Bed

South North

Depth (m)

LINE A

Figure 67. Bathymetric profiles. Refer to Figure 20 for profile locations.

55
56

Group 3

Group 1

Group 4

Group 2 

S2 S1 S4 S3

S5

S7 S6 S8

S9

C1

C4 C3

C2

Eagle River


Permanent Pond


Intermittent Pond


Drainage Gully


1993 Monitoring Well

1994 Piezometer Location


(C. Racine)


1994 Piezometer Location


(M. Walsh)


1994 Piezometer Location


(C. Collins)

0 500 m

6803000

6801000

356000355000

N

Figure 68. Locations of piezometers and monitoring well on ERF.

1.0

0.5

0

3 Sep 7 Sep 11 Sep 15 Sep

Group 1

30 cm

60

90

Height in Piez. (mm)

Depth in Ground (cm)

1.0

0.5

0

3 Sep 7 Sep 11 Sep 15 Sep

Group 2

30 cm

60

80

Height in Piez. (mm)

Depth in Ground (cm)

2.0

1.0

0

3 Sep 7 Sep 11 Sep 15 Sep

Group 3

20 cm

25

30

35

60

Height in Piez. (mm)

Depth in Ground (cm)

2.0

1.0

0

3 Sep 7 Sep 11 Sep 15 Sep

Group 4

15 cm

30

40

60

Height in Piez. (mm)

Depth in Ground (cm)

a. Cluster 1.

b. Cluster 2.

c. Cluster 3.

d. Cluster 4.

Figure 69. Plots of piezometer cluster data.

60
57

CONCLUSIONS:

SYSTEM DYNAMICS

AND REMEDIATION

Eagle River Flats is an estuarine salt marsh

whose physical environment is undergoing progressive and significant changes. These changes

result from the interaction and response of multiple physical processes to external forces such as

tectonics, isostatic adjustment, macrotides, glacial river discharges and a cold climate. The location of ERF in an active earthquake zone adds to

its complexity and to the potential for future rapid,

but probably unpredictable, physical changes. If

the area subsides again because of an earthquake,

major changes in the hydrology and terrain can

be expected. While pond areas may increase because of subsidence, the drainage system and river

channel may also respond with increases in rates

and locations of erosion if the base level is altered. In addition, ground explosions and

cratering during the use of ERF as a military firing range since the early 1940s have caused physical changes to the terrain, hydrology and surface

drainage. Craters act to divert and capture surface flow, causing changes in drainage and sediment storage patterns that can locally increase

erosion and enhance gully expansion.

The inherent complexity of this dynamic environment makes it extremely difficult to predict

what effects potential remedies for WP contamination will have on the physical system and, conversely, what short- and long-term effects the

physical system will have on the effectiveness of

proposed remediation. Understanding both the

system’s response to remediation and whether

5.5

4.5

3.5

14 Jul 24 Jul 3 Aug 13 Aug 23 Aug

Elevation (m)

Top

of Well

Well

Water

Gully

Water

Figure 70. Ground water monitoring well levels

and Hydrolab data at B-Gully. Elevations of top

and bottom of monitoring well estimated at 4.78

and 3.78 m respectively (from C. Racine).

the remediation process will be enhanced or hindered by the physical environment is critical to

their success in this dynamic environment. Further, site-specific process analyses of tidal flat hydrology, as well as continued measurements of

erosion and sedimentation, will allow us to determine if this system will naturally attenuate the

WP contamination.

LITERATURE CITED

Andrew, J. and G. Cooper (1993) Sedimentation

in a river dominated estuary. Sedimentology, 40:

979–1017.

Allen, J.R.L. (1982) Sedimentary structures: Their

character and physical basis. Volume 1, 2. Developments in Sedimentology, vol. 30A, B. New York:

Elsevier.

Allen, J.R.L. (1990a) Constraints on measurements

of sea-level movements from salt marsh accretion

rates. Journal of the Geological Society London, 147:

5–7.

Allen, J.R.L. (1990b) Salt marsh growth and stratification: A numerical model with special reference to Severn estuary, S.W. Britain. Marine Geology, 95: 77–96.

Allen, J.R.L. (1992) Large-scale textural patterns

and sedimentary processes on tidal salt marshes

in Severn Estuary, Southwest Britain. Sedimentary

Geology, 81: 299–318.

Allen, J.R.L. and J.E. Rae (1988) Vertical salt marsh

accretion since the Roman period in the Severn

estuary, southwest Britain. Marine Geology, 83: 225–

235.

APHA, AWWA, WEF (American Public Health

Association, American Water Works Association,

Water Environment Federation) (1992) Method

2540D total suspended solids dried at 103–105°C.

Standard Methods for the Examination of Water and

Wastewater, p. 2–56.

Bartsch-Winkler, S. and A.T. Ovenshine (1984)

Macrotidal subarctic environment of Turnagain

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APPENDIX A: TSS ANALYSES

TSS samples were analyzed in a field laboratory following the standard procedures, with minor modifications to the methods that were relevant to this project.

Our procedure differed in the following ways.

1. Glass fiber filters were not pre-washed, dried and weighed. Control tests

performed in 1993 and 1994 determined that the weight changes were insignificant.

2. Oven temperature remained between 103 and 105°C since volatile solids

were not measured. Temperature was checked and recorded daily as was the

balance calibration.

3. Owing to the small particle sizes of sediment, filters readily clogged. We

minimized filtration time by carefully pouring the majority of settled sample into a

graduated cylinder, recording the volume, then vigorously shaking the remaining

portion and quickly transferring it to a graduated cylinder. This volume was recorded and added to the initial value. The sample container, graduated cylinder

and filter funnel were thoroughly rinsed with distilled water, with all washes

added to the filtered sample.

4. The final cycle of drying, cooling and reweighing of filters was deemed

unnecessary for the concentration ranges encountered.

60
APPENDIX B: SEASONAL SUMMARY OF EROSION RATES

Recession range (m)

SITE POS S92 W92 S93 W93 S94 F94 NET

River-Meander 32 L ————— 0–0.59 0–59

33 L — — — — — 0–1.62 0–1.62

34 L — — — — — 0 0

River-North 36 L — — — — — 00

37 L — — — — 1.02–2.32 1.05–2.03 2.59–3.69

38 L — — — — — 0 0

39 L — — — — — 0 0

Racine Island 31 L — — — — — 00

29 H — — — — — 0 0

30 L — — — — — 0 0

River South 28 L — — — — — 00

Area A Near Coast

Coastal 6 52 L ———— 0.01–0.91 0.23–1.85 0.93–2.76

55 L — — — — 0–1.0 0–0.63 0–1.65

53 H — — — — 0–0.02 0–0.74 0–0.74

54 H — — — — 0–0.27 0–0.64 0.25–0.58

Coastal 7 56 L ———— 0–0.47 0–0.71 0–1.65

57 H — — — — 0.12–0.43 0.1–1.17 0.07–1.09

Area A Inland

Coastal 5 45 L ———— 0–0.62 — 0–0.62

47 L — — — — 0–1.0 — 0–1.10

48 L — — — — 0–0.35 — 0–0.35

43 H — — — — 0–0 — 0

44 H — — — — 0–0.53 — 0–0.53

46 H — — — — 1.33–1.59 — 1.33–1.59

49 H — — — — 0 — 0

Tanker 40 L ———— 0 0–1.17 0–1.17

42 L — — — — 0 0–0.27 0–0.27

41 H — — — — 0 0 0

Area D—In Between 7 H 0.01–1.09 0.05–0.35 0–2.79 0–0.61 0–0.25 — 0.13–2.14

9 H 0.01–6.95 0.05–0.1 1.7–4.31 — 0.04–0.19 — 0.04–4.61

6 L 0.05–2.37 0–2 0.35–2.34 0–2.29 0–0.4 — 0.02–4.37

8 L 0.002–0.142 0–0.95 0–0.41 0–0.37 0–0.03 — 0.08–0.56

10 L 0–0.16 0–0.5 0–0.03 0–0 0–0 — 0.3–0.041

11 L 0.02–0.52 0–0.4 0–1.02 0–0 0–0.05 — 0.02–0.96

Bread Truck 16 H ———— 0.99–2.1 0.065–1.39 20.03–2.97

18 H — — — — 0–1.2 0–0.77 0–1.44

19 H — — — — 0.18–1 0–3.77 0–3.54*

20 H — — — — 0–0.03 0.01–0.04 0.03–0.05

12 L — — 0.84–3.12 0–1.29 0.03–0.69 — 8–5.1

13 L — — 0–1.68 0–0.93 0–0.6 — 0–3.33

14 L — — 0.01–1.16 00.03–1.42 0–1.02 — 0.66–2.37

15 L — — — 0–2.34 0.03–1.44 0–0.62 0.44–8.15

17 L — — — 0.43–1.99 0–2.23 0–0.52 0.02–2.6

Area D—Coastal 1 26 L ————— 0–0.82 0–0.82

26 H — — — — 0–0.78 0.07–1.65 0.19–1.65

27 H — — — — 0–0 0–0.4 0–0.04

25 H — — — — 0 0.41–0.9 0.41–0.9

Area C

B–Gully 1 L — 0.17–4.5 0.70–1.82 0.13–2.08 0–2.57 0–0.97 0.12–7.32

2 L 0–4.9 0–1.95 0.24–0.92 0–1.37 0–1.16 0–1.03 0.27–8.17

Parachute 3 L 0.13–0.61 0–0.37 — 0.16–0.75 0.02–6.02 0–0.71 0.3–1.63

5 L 0.1–1.314 0.12–0.75 0.17–0.21 0–0.7 0–0.4 0 0.49–1.5

4 H 0.09–3.06 0.09–3.06 0.04–3.79 0.24–1.35 0–0.07 0–1.17 0.04–9.30

* Records initial phase of extension prior to collapse.

L = lateral erosion, H = headward erosion.

61
APPENDIX C: TIDAL INUNDATION DATA FOR

BREAD TRUCK, PARACHUTE AND SPRING GULLIES

Start rise Peak End runout

Site Julian time Elev. (m) Julian time Elev. (m) Julian time Elev. (m)

Bread Truck 274.704 2.959 274.749 3.479 274.793 2.959

275.201 2.962 275.251 3.671 275.31 2.962

277.74 2.995 277.835 5.287 278.265 *3.28

278.268 3.28 278.349 5.091 278.757 *3.205

278.76 3.205 278.86 5.606 279.29 *3.326

279.293 3.326 279.388 4.671 279.782 *3.397

279.785 3.397 279.885 5.933 280.326 *3.395

280.329 3.395 280.413 5.206 280.815 *3.046

280.818 3.046 280.913 5.307 281.36 *3.112

281.363 3.112 281.446 4.956 281.843 *2.999

281.846 2.999 281.943 5.1 282.407 *3.05

282.415 3.05 282.482 4.268 282.885 *2.955

307.213 3.174 307.304 5.263 307.696 *3.158

307.699 3.158 307.796 5.82 308.246 *3.092

308.251 3.092 308.332 5.115 308.724 *3.099

308.726 3.099 308.826 5.688 309.288 *3.143

209.29 3.143 309.365 5 309.438 3.166

309.757 3.191 309.857 5.769 310.16 3.107

310.315 3.085 310.396 4.987 310.468 3.063

310.796 3.037 310.89 5.263 311.34 3.096

311.343 3.096 311.429 4.748 311.729 3.017

311.84 3.017 311.915 4.786 311.988 3.012

Parachute 275.746 3.603 275.776 4.094 275.976 3.603

276.246 3.603 276.288 4.261 276.513 3.603

276.743 3.603 276.801 4.838 276.979 3.606

277.26 3.61 277.318 4.897 277.524 3.606

277.76 3.602 277.835 5.206 278.285 *3.642

278.288 3.642 278.349 5.044 278.729 3.614

278.779 3.612 278.868 5.49 279.31 *3.685

279.313 3.685 279.385 5.23 279.799 *3.669

279.801 3.669 279.89 5.719 280.346 *3.678

280.349 3.678 280.413 5.049 280.835 *3.625

280.838 3.625 280.915 5.288 281.379 *3.668

281.382 3.668 281.446 5.023 281.865 *3.619

281.868 3.619 281.943 5.158 282.435 *3.64

282.438 3.64 282.479 4.325 282.774 3.614

306.693 3.697 306.776 5.407 307.235 *3.619

307.238 3.619 307.301 5.119 307.715 *3.643

307.718 3.643 307.799 5.653 308.271 *3.693

308.274 3.693 308.329 5.049 308.749 *3.625

308.751 3.638 308.829 5.541 309.313 *3.742

309.315 3.742 309.368 4.926 309.668 3.619

309.785 3.628 309.868 5.514 310.34 *3.685

310.343 3.685 310.401 4.958 310.818 *3.659

310.821 3.659 310.89 5.196 311.382 *3.734

311.385 3.734 311.435 4.705 311.865 *3.641

311.868 3.641 311.918 4.738 312.135 3.623

Spring Gully 277.71 1.888 — — 278.229 2.202

278.243 2.206 278.349 5.019 278.735 *2.047

278.738 2.047 — — 279.265 *2.351

279.268 2.354 — — 279.763 *2.303

279.765 2.303 — — 280.307 *2.332

280.31 2.332 280.41 5.027 280.79 *2.304

280.793 2.034 — — 281.335 *2.265

281.338 2.265 281.438 4.986 — —

308.224 1.88 308.329 4.921 308.701 1.839

308.704 1.839 — — 309.054 2.205

309.26 1.908 309.365 4.823 309.46 1.907

309.735 1.879 — — 310.265 1.908

310.288 1.9 310.396 4.902 310.765 *1.798

310.768 1.798 — — 311.324 *1.872

311.326 1.872 311.426 4.623 311.807 *1.785

311.81 1.785 311.915 4.657 312.365 *1.806

* Water elevation never reached base level before next tidal inundation.

62
APPENDIX D: PRELIMINARY ESTIMATES OF

GULLY WATER DISCHARGE AND SEDIMENT FLUX

Tidal Velocity (cm/s) TSS (mg/L) Discharge (m3/s) Sediment flux (kg/s)

Month stage Peak Average Peak Average Peak Average Peak Average

Bread Truck Gully; Cross-sectional area = 5.5 m2

May Flood NA 25.0 739 170 NA 1.38 NA 0.23

Ebb NA 76.1 1148 669 NA 4.19 NA 2.80

June Flood NA 25.0 715 227 NA 1.38 NA 0.31

Ebb NA 76.1 1175 512 NA 4.19 NA 2.14

September Flood NA 25.0 1714 429 NA 1.38 NA 0.59

Ebb NA 76.1 1423 467 NA 4.19 NA 1.95

October Flood 71.1 22.6 1930 847 3.9 1.24 7.55 1.05

Ebb 133.0 81.0 1329 758 7.3 4.46 9.72 3.38

November Flood 68.6 29.9 2152 1186 3.8 1.64 8.12 1.95

Ebb 146.0 74.0 1363 885 8.0 4.07 10.94 3.60

Parachute Gully; Cross-sectional area = 9.8 m2

May Flood NA 27.1 983 222 NA 1.49 NA 0.33

Ebb NA 47.0 1507 485 NA 2.59 NA 1.25

June Flood NA 27.1 638 250 NA 1.49 NA 0.37

Ebb NA 47.0 854 221 NA 2.59 NA 0.57

September Flood NA 27.1 1740 646 NA 1.49 NA 0.96

Ebb NA 47.0 1840 373 NA 2.59 NA 0.96

October Flood 37.4 13.4 2733 1752 2.1 0.74 5.62 1.29

Ebb 76.0 45.0 2073 723 4.2 2.48 8.67 1.79

November Flood 56.4 32.2 3102 2420 3.1 1.77 9.62 4.29

Ebb 72.0 49.0 2838 1285 4.0 2.70 11.24 3.46

Spring Gully; Cross-sectional area = 26.5 m2

May Flood NA 15.5 1850 420 NA 0.85 NA 0.36

Ebb NA 101.9 1439 689 NA 5.60 NA 3.86

June Flood NA 15.5 5626 1765 NA 0.85 NA 1.50

Ebb NA 101.9 1888 834 NA 5.60 NA 4.67

September Flood NA 15.5 1458 763 NA 0.85 NA 0.65

Ebb NA 101.9 1586 608 NA 5.60 NA 3.41

October Flood 59.7 24.5 2733 1752 3.3 1.35 8.97 2.36

Ebb 176.2 124.6 2538 619 9.7 6.85 24.60 4.24

November Flood 51.8 17.2 3049 1757 2.8 0.95 8.69 1.66

Ebb 162.4 84.5 3776 1883 8.9 4.65 33.73 8.75

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August 1996

Physical System Dynamics and White Phosphorus Fate and Transport,

1994, Eagle River Flats, Fort Richardson, Alaska

Daniel E. Lawson, Lewis E. Hunter, Susan R. Bigl, Beth M. Nadeau,

Patricia B. Weyrick and John H. Bodette

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road CRREL Report 96-9

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U.S. Army Alaska, Fort Richardson, Alaska

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Anchorage, Alaska

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76

Erosion Processes Sedimentation Tidal flat

Factors Remediation Salt marsh WP

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

Eagle River Flats (ERF) is a subarctic estuarine salt marsh where human and natural forces are causing

significant changes in the environment. Multiple internal and external forces govern the physical and chemical

processes by actively altering surface conditions, sometimes in unpredictable ways. ERF is also used as an

artillery range by the U.S. Army, where past use has resulted in white phosphorous (WP) contamination of the

sediments within ponds and mudflats. Bottom-feeding waterfowl ingest this WP, which causes rapid death.

This report documents analyses of the physical environment, describing the nature of the physical systems and

factors controlling them. It includes data on sedimentation, erosion and hydrology. These investigations

provide knowledge necessary to designing and evaluating remedial technologies. They also help determine

the system’s capacity to naturally attenuate the WP contamination.