Physical Processes and Natural

Attenuation Alternatives for

Remediation of White Phosphorus

Contamination, Eagle River Flats,

Fort Richardson, Alaska

Daniel E. Lawson, Lewis E. Hunter and Susan R. Bigl December 1996

CRREL REPORT 96-13
Abstract: This report describes the results of investigations into the role of tidal flat physical systems in the

natural attenuation of white phosphorus (WP) contamination in Eagle River Flats (ERF) on Fort Richardson,

Alaska. Waterfowl feeding in ponds and marshes here

ingest the WP and die. These investigations found that

natural attenuation and in-situ degradation of the WP

could result from certain physical phenomena operating

within the ERF ecosystem. Specifically, the on-going

erosion and headward recession in the gullies will drain

large areas of contaminated ponds in an estimated 1 to

10 years. Lowering of water levels should lead to in-situ

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Cover: Oblique aerial photograph of the head of Bread Truck Gully. This gully is eroding headward at an average

rate of 21.6 m annually, and will probably drain Bread Truck Pond (top of photo) within 1 to 2 years. A hydrostation

platform (1.3 × 1.3 m) is located to the lower left.

WP degradation and natural attenuation as pond sediments dry. Annual sedimentation rates in some ponds

and marshes are sufficient to bury WP in several years

or more and thereby reduce the exposure to feeding

waterfowl. Ice and water are also effective transporters

of WP, moving it about ERF and into Eagle River and

eventually into Knik Arm where its fate is unknown.

Certain areas of ERF will require artificial drainage, but

natural conditions can be restored following treatment.

Recommendations are made for the use of natural

attenuation and additional studies that are required to

ensure the successful clean-up of ERF.
CRREL Report 96-13

Physical Processes and Natural

Attenuation Alternatives for

Remediation of White Phosphorus

Contamination, Eagle River Flats,

Fort Richardson, Alaska

Daniel E. Lawson, Lewis E. Hunter and Susan R. Bigl December 1996

Prepared for

U.S. ARMY ALASKA

ENVIRONMENTAL RESOURCES DEPARTMENT

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory

®

US Army Corps

of Engineers®

Cold Regions Research &

Engineering Laboratory
PREFACE

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

Lewis E. Hunter, Research Physical Scientist, and Susan R. Bigl, Research Physical

Scientist, Geological Sciences Division, Research and Engineering Directorate, U.S.

Army Cold Regions Research and Engineering Laboratory.

Funding and support for this research was provided by the U.S. Army Alaska,

Environmental Resources Department, Directorate of Public Works, Fort Richardson,

Alaska, through the U.S. Army Engineer District, Anchorage. Technical review was

provided by Dr. Steven Arcone and Dr. Jeffery Strasser, both of CRREL.

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

Laurie Angell for helpful discussions, logistical support and assistance in the

field, and their continuing support of our work, as well as Dr. Charles Racine, Charles

Collins and Marianne Walsh for numerous scientific discussions and assistance

in the field and laboratory on the Eagle River Flats project. They also thank Troy

Arnold, Ron Bailey, Dennis Lambert, Beth Nadeau and Pat Weyrick for their assistance, Jodie Strasser and Audrey Krat for data analysis and plotting and Cora

Farnsworth for her contribution to report preparation.

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.

ii
CONTENTS

Page

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

Executive summary.................................................................................................. vi

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

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

Results of previous physical system investigations ........................................... 5

Study sites and methods ......................................................................................... 13

Sedimentation transects...................................................................................... 13

Pond sedimentation measurements ................................................................. 16

Gully erosion and headwall recession ............................................................. 20

Historical aerial photographic analyses .......................................................... 21

Water quality parameters ................................................................................... 21

Hydrostation configuration ............................................................................... 24

WP transport and resuspension ........................................................................ 24

Data acquisition record....................................................................................... 26

Intrinsic remediation ............................................................................................... 28

External forcing .................................................................................................... 28

Gully erosion and discharge .............................................................................. 28

Gully erosion and scarp recession rates........................................................... 31

Historical analysis of the drainage system ...................................................... 44

Natural attenuation by pond drainage ............................................................ 46

Sedimentation and natural attenuation of WP by burial ................................... 47

Tidal inundation .................................................................................................. 47

Sediment transport and sediment sources ...................................................... 52

Landform sedimentation rates .......................................................................... 53

Pond transects and stations ............................................................................... 56

White phosphorus fate, transport and migration ............................................... 59

Knik Arm and WP deposition ................................................................................ 61

Conclusions and recommendations ...................................................................... 62

Conclusions .......................................................................................................... 62

Recommendations ............................................................................................... 63

Literature cited.......................................................................................................... 64

Abstract ...................................................................................................................... 67

ILLUSTRATIONS

Figure

1. Location of ERF and other salt marshes in upper Cook Inlet ............... 2

2. 1995 aerial photograph of ERF showing the dendritic drainage

 network ................................................................................................. 3

3. Drainage network in the C and D areas of ERF ....................................... 4

4. Distribution of primary landform–vegetation units ............................... 4

5. Locations of gullies, ponds and areas ........................................................ 7

6. Aerial photograph of Racine Island taken 16 August 1995 showing

remnant channel surrounded by overbank deposits ...................... 8

7. Aerial photographs of ERF .......................................................................... 10

8. 1992 aerial photograph of northeastern ERF ............................................ 13

iii
Figure Page

 9. Mudflat sedimentation transect locations ................................................. 14

10. Sedimentation measurement techniques .................................................. 15

11. Wedge-shaped sediment block used to measure vertical accretion ..... 16

12. Pond sedimentation measurement locations............................................ 17

13. Locations of 1992–1994 pond sedimentation stations ............................. 19

14. Example of sedimentation station at an intermittent pond

location showing layout of cup and plate sampler ......................... 19

15. Example of layout of hub and line stakes at River-North erosion

site ........................................................................................................... 20

16. Location of gully headwall and lateral wall erosion sites ...................... 21

17. 1995 locations of instrumentation recording water quality

parameters and water depths ............................................................. 22

18. Extent of data collection at hydrostation sites from 1993–1995 ............. 23

19. Hydrostation layout at a gully location .................................................... 25

20. Plankton net used for collecting WP in water transport ......................... 26

21. Sampling coverages for various types of field data ................................ 27

22. Relationship between peak velocity and maximum water elevation

at selected sites ..................................................................................... 29

23. Summary of gully scarp recession ............................................................. 31

24. Measurements of bank erosion along Eagle River at the RiverNorth site ............................................................................................... 34

25. Bank of Bread Truck Gully showing ice growth formation ................... 35

26. Aerial view of Parachute Gully in October 1995 showing

accelerated headward recession ......................................................... 36

27. Aerial view of gullies where recession and ebb flow have been

modified by surface craters................................................................. 36

28. Aerial view of Bread Truck Gully in October 1995.................................. 38

29. Eastward view of eroding cuspate embayments at B-Gully .................. 39

30. Thick laminated peat layer at Coastal 6 .................................................... 39

31. Sediment flow at Bread Truck Gully ......................................................... 40

32. Historical gully recession ............................................................................ 41

33. Spatial comparison of headward and lateral recession monitored at

selected hub-line stake erosion sites .................................................. 43

34. Aerial photographs depicting the morphologic changes in the

Eagle River where it enters the ERF .................................................. 45

35. Predictions for pond drainage by selected gullies in Areas C,

Bread Truck and C/D .......................................................................... 46

36. Ice rafting at ERF .......................................................................................... 47

37. Example of a flooding tide cycle ................................................................ 49

38. Water quality data from the Eagle River site showing the effect

of water damming caused by a flooding tide .................................. 49

39. Water elevation record showing elevations required to flood

selected landforms and tidal amplification at Parachute

Gully above predicted high tide at Anchorage................................ 50

40. Relationship between water rise time and peak water elevation

at several gully sites ............................................................................. 50

41. Tidal flooding time as a function of peak flood elevation at

selected gullies ..................................................................................... 51

iv
Figure Page

42. Runoff time vs. peak water elevation at Bread Truck Gully .................. 52

43. Coastal vs. river TSS measurements .......................................................... 52

44. Maximum monthly TSS comparison at all sites ....................................... 53

45. Typical TSS variation through a flooding tide ......................................... 53

46. Net seasonal accumulation rates at sites spray-painted in the

C and D areas ........................................................................................ 54

47. Variations in sedimentation rate as a function of landform type .......... 55

48. Relationship between total sediment accumulation and vegetation

cover at sites sprayed with paint in 1992 and measured in

the fall of 1994 ....................................................................................... 55

 49. Sediment accumulation along mudflat lines sprayed with paint in

early summer 1994 ............................................................................... 56

50. Pond sedimentation measured along transects from May to

September 1995 ..................................................................................... 57

51. Average sediment accumulation from May to September 1995 at

pond sedimentation station transects, without data points

suspected of disturbance ..................................................................... 58

52. Fall 1995 measurements of accumulation at sedimentation stations

established in 1992 ............................................................................... 59

53. Relationship between WP detected in plankton net samples and

peak elevation of flooding event sampled........................................ 60

54. Bathymetry of Knik Arm near the mouth of the Eagle River and

adjacent to the coast of ERF ................................................................ 61

55. Recommendations for site remediation .................................................... 62

TABLES

Table

1. Controls on physical processes ................................................................... 5

2. Erosional processes ...................................................................................... 5

3. Transport processes ...................................................................................... 6

4. Depositional processes ................................................................................. 6

5. Historical aerial photography of Eagle River Flats.................................. 21

6. Specifications of sensors used in water quality parameter

measurements ....................................................................................... 22

7. Summary of gully velocity and discharge data ....................................... 29

8. Summary of scarp recession ....................................................................... 30

9. Summary of long-term gully erosion rates at selected sites ................... 44

10. Gross sedimentation rates ........................................................................... 54

11. Comparison of predicted and measured number of flooding

events reaching critical heights during summer 1994 and

summer 1995 ......................................................................................... 55

12. May to September 1995 sediment accumulation along pond

sedimentation transects ....................................................................... 58

13. 1995 positive WP results from pond sedimentation station cup

samples .................................................................................................. 60

v
EXECUTIVE SUMMARY

This report describes the results of the 1995 investigations of the physical ecosystem of Eagle River Flats (ERF) on Fort Richardson, Alaska. These investigations

focused on the role of the physical system in the natural attenuation of white phosphorus (WP) contamination in this salt marsh area. Specifically, we evaluated 1) whether

the processes of gully erosion, headward recession and drainage of contaminated

ponds could result in a natural attenuation and in-situ degradation of WP contamination over a significant proportion of ERF, and 2) whether natural sedimentation

would bury contaminated pond sediments sufficiently to form a barrier to feeding

waterfowl. In addition, the erosion and potential for off-site transport of WP was

examined.

Several important conclusions result from this and previous years’ work:

1. Physical system processes can naturally attenuate and remediate a significant

proportion of the WP in the ERF ecosystem.

2. Gully erosion and headwall recession towards contaminated ponds will begin

to drain large areas in about 1 to 10 years. The resultant lowering of pond surfaces can

lead to in-situ WP degradation and natural attenuation from drying of surface

sediments. These processes are a cost-effective alternative to remediating the site by

expensive removal methods such as dredging. Historical analyses and field measurements indicate that Bread Truck Pond will probably begin draining in 1 year, while

C/D-Pond, Lawson’s Pond and a large area of C-Pond will begin to drain in 10 to 15

years or less.

3. Annual sedimentation in certain ponds and marshes can bury WP in several

years to a depth sufficient to reduce the exposure risk for feeding waterfowl. Artificial

enhancement of sedimentation by introducing additives to increase flocculation may

increase the effectiveness of the sediment barrier in some locations, while reducing

the time required to build up an adequate thickness. Sedimentation (natural and

enhanced) is a cost-effective alternative to the physical removal of WP from

waterfowl feeding areas, particularly in areas of C/D-Pond, Lawson’s Pond and APond. Gully erosion and extension may subsequently drain and dry these areas,

thereby permanently remediating them.

4. Ice is an effective erosion and transport medium for WP; large, sometimes thick

slabs of WP-contaminated pond bottom sediments can be removed by ice plucking

and transported to other locations in ERF during tidal flooding.

5. WP-bearing sediments are eroded from ponds and drainages by ice and water,

and are subsequently transported by currents into the Eagle River and off-site into

Knik Arm where their fate is unknown. Contaminated sediments are also transported

to other locations within ERF.

6. Gully erosion, recession and pond sedimentation rates at Racine Island Pond are

insufficient to naturally remediate the site in the short term. The pond floods frequently at low tidal heights and its sediments are rich in organics. It therefore appears

that Racine Island Pond can be remediated most cost-effectively through artificial

drainage and temporary berm containment to permit long-term drying and in-situ

WP degradation.

On the basis of the investigations to date, the following recommendations are

made:

1. Cost-effective remediation can be accomplished to a significant degree by

allowing the physical system to remove or isolate WP contamination.

vi
2. WP contamination of Bread Truck Pond and 50% or more of C-Pond, possibly

including Lawson’s Pond, should be treated by natural or enhanced drainage and

subsequent in-situ WP degradation by drying.

3. Sedimentation and burial of WP will be effective in removing it from feeding

waterfowl in several years; in a decade or less, burial will reduce waterfowl mortality

in the C, Lawson’s and C/D ponds.

4. Racine Island Pond may be effectively remediated by gully extension, artificial

pond drainage and pumping, and long-term containment with a temporary berm,

which will permit in-situ WP degradation by extended drying of pond bottom

sediments. By removing the berm after WP has attenuated, the pond environment

will be restored.

5. Erosion and recession rates, pond sedimentation, ground water, pond drainage

and drying, and WP degradation and attenuation should be monitored to ensure that

remediation is taking place. Continued analyses of the physical system will allow us

to assess the rate and effectiveness of natural remediation and to evaluate if and where

additional remedial actions are needed. Where artificial remediation techniques are

applied, we suggest detailed site investigations to assess ecosystem impacts and how

natural processes will influence the remediation measures.

6. WP migration and contamination in Knik Arm should be evaluated, focusing on

areas of nearshore zones and mid-Arm bars where there is a potential for WP

exposure to receptors.

7. The potential for natural WP attenuation by mechanical abrasion during transport by gully and tidal currents should also be evaluated.

vii
INTRODUCTION

Eagle River Flats (ERF) is a 865-ha salt marsh

at the mouth of the Eagle River on the Knik Arm,

northeast of Anchorage, Alaska (Fig. 1). The salt

marsh 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). WP particles were introduced by smoke-producing compounds in devices detonated during military training (Racine et al. 1992a, 1993). WP is now present

as particles and is also adsorbed to near-surface

sediments at numerous locations in ERF, most

notably in pond and marsh bottom sediments

where dabbling ducks ingest it during feeding. In

addition, WP-bearing sediment can be remobilized and transported to other locations within or

external to ERF where it is redeposited (Lawson

et al. 1996) and may pose a threat to unidentified

receptors.

Our studies in 1993 and 1994 (Lawson et al.

1995, 1996) show that the physical system, and

specifically the processes of erosion and recession

of gullies draining the contaminated ponds and

mudflats, can naturally attenuate WP contamination by increasing exposure and drying time of

pond-bottom sediments. Until this takes place,

natural sedimentation may reduce waterfowl exposure risks to WP ingestion by producing a thick

sediment barrier between the surface and contaminated sediments.

Therefore, in 1995, we focused our efforts on

understanding the rates at which these processes

would alter the pond environments, removing or

blocking the WP pathway to migrating waterfowl. We also examined WP erosion and transport to evaluate the potential for its off-site migration. These studies included analyses of the

tidal and river hydrology and associated factors

critical to evaluating proposed remedial technologies for WP contamination. This report describes

the results of 1995 investigations of the physical

ecosystem of Eagle River Flats and its role in the

natural attenuation or intrinsic remediation of WP

contamination.

ENVIRONMENTAL CONDITIONS

Eagle River Flats is one of several estuarine salt

marshes in upper Cook Inlet, south-central Alaska

(Fig. 1). Located at the mouth of the Eagle River,

ERF is 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). Inundation results

from both the semi-diurnal macrotidal fluctuations of 9 to 11 m in Knik Arm and the resultant

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 that significantly affect 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

Physical Processes and Natural Attenuation Alternatives

 for Remediation of White Phosphorus Contamination,

Eagle River Flats, Fort Richardson, Alaska

DANIEL E. LAWSON, LEWIS E. HUNTER AND SUSAN R. BIGL
F P O

resultant increase in runoff, and long-term trends

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

Clark 1987, Lawson 1993). Because of this glacial

influence, maximum and peak discharges usually

occur during the primary melt season of July and

August. Sediment transport and sediment flux

have diurnal, seasonal and annual variations

caused primarily by glacial meltwater influx into

the river basin (Lawson 1993). Similarly, massive

quantities of silt- and clay-size particles are transported in suspension to Knik Arm by the large

rivers draining into Cook Inlet from glacierized

basins in the Alaska Range, Chugach Mountains

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

Matanuska river basins, Fig. 1). These materials

remain suspended for an extended time and are

transported into the ERF salt marsh by tidal inundation (Lawson et al. 1996).

The Eagle River cuts approximately through

the middle of ERF, with the ponds and marshes

primarily draining through vegetated channels

or drainageways and 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 evolution of ERF (Fig. 3). The northern fifth of ERF

along the coast is drained through gullies that

discharge directly into Knik Arm. The east, west

and southern boundaries of ERF are defined by

uplands composed of glacial deposits 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 bands or

zones that approximately parallel the river and

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

N

Figure 1. Location of ERF and other salt marshes in upper Cook Inlet.

2
Figure 2. 1995 aerial photograph of ERF showing the dendritic drainage network.

uplands adjacent to ERF (Fig. 4, Racine et al.

1993). They commonly vary in location by elevation and thereby are related to the landforms of ERF as a function of flooding frequency,

salt tolerance, drainage capacity and sedimentation rates. Levees, mudflats, marshes and shallow ponds (to 50 cm depth) are aligned approximately parallel to the Eagle River and

coastline (Racine et al. 1992a). Freshwater ponds

or shrub bogs bordered the upland along the

northeast and southwest portions of ERF.

3
Mudflats

Sedge Marsh

Pond/Marsh

Sedge Meadow

Border

352000

6803000

6802000

6801000

6800000

6799000

353000 354000 356000 355000























Figure 4. Distribution of primary landform–

vegetation units (after Racine et al. 1992a).

Figure 3. Drainage network in the C and

D areas of ERF.

4
Table 2. Erosional processes.

a. Summer.

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

b. Winter.

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

RESULTS OF PREVIOUS PHYSICAL

SYSTEM INVESTIGATIONS

Our previous investigations have focused on

characterizing the processes and factors affecting the physical environment of ERF (Tables 1

to 4) and evaluating the fate and transport of

WP, both of which are critical to developing a

conceptual model for identifying appropriate

methods of WP remediation and site restoration (Lawson et al. 1996).

The processes of erosion, transport and deposition vary across ERF, with responses to multiple internal and external factors over seasons,

years and decades (Table 1). External controls

on the physical system are difficult to define

because their effects may last several decades or

more and exert considerable control on internal

factors that influence process relationships. The

effects of earthquakes and other tectonic activity are difficult to predict; however, their impact on the ERF physical system may be enormous.

The 1964 Alaskan Earthquake (magnitude 9.2

on the Richter scale) resulted in both sedimentary and tectonic subsidence, which modified

the site’s elevation and the processes operating

therein (e.g., Ovenshine et al. 1976a; Brown et

al. 1977; Combellick 1990, 1991, 1994; Savage

and Plafker 1991). A tectonic drop of about 0.6–

0.7 m relative to mean sea level was recorded in

the Anchorage region (Brown et al. 1977). Subsidence caused by sediment liquefaction and

solidification during the earthquake probably

Table 1. Controls on physical

processes.

Factor

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

reduced the pond bottom elevations an additional,

but unknown, amount, as it did in the Portage

area of Turnagain Arm (Ovenshine et al. 1976a).

Post-seismic adjustments have resulted in an

episodic uplift (Savage and Plafker 1991), which,

according to tide-gauge data, was initially rapid

5
Table 3. Transport processes.

Summer Winter

Currents (river) Ice floes

–suspended load –freeze-on

–bedload –freeze-in

–saltation –ice shove

Wind currents (pond–mudflats) Frazil ice

–suspended load –anchor ice

–sedload –freeze-on

Gravitational slope processes –freeze-in

Currents (tidal–gully)

–flood

–ebb

Ground water

–piping

(~2 cm/year; Brown et al. 1977) and now may be

in the range of about 1.0 mm/year (Savage and

Plafker 1991).

Long-term responses of the physical system to

these changes in elevation, and hence base level

relative to sea level, are unknown. An increase in

flooding volume and duration may lead to an

increase in pond sedimentation (e.g., Ovenshine

et al. 1976a); however, the increased tidal prism

(depth and volume) over the Flats might provide

more energy for erosion as water is funneled into

gullies during ebb tide. These responses remain

under investigation.

Tidal dynamics and river hydrology control

the amount of sediment that enters and drains

from ERF, as well as the locations and rates of

erosion and sedimentation. The amount of sediment transported in tidal flood waters is also a

primary factor determining scour and sediment

transport during ebb (Tables 2 and 3). Flooding

tides transport large volumes of sediment onto

the Flats during the summer, but the volume decreases substantially during the fall (Lawson et

al. 1996). Transport changes seasonally as glacial

input to Knik Arm decreases at freezeup and ambient air temperatures drop. During winter

months, sediment and water discharge in the Knik,

Matanuska, Susitna and Eagle river catchments

decrease. This reduces the sediment available, despite lower water temperatures that increase its

density, transport capacity and potential for movement of sediment into and within ERF.

Peak tidal flood heights across ERF, in the Eagle

River and at the coast are delayed 20 to 40 minutes relative to the peak predicted at Anchorage

(Lawson et al. 1996). The height is also generally

0.5 m or more greater than the Anchorage datum,

reflecting, at least in part, the funneling of water

out of Cook Inlet into the narrower Knik Arm

(i.e., Syvitski et al. 1987). Tidal flooding of the

Bread Truck, C, A and Racine Island areas may be

Table 4. Depositional processes.

Morphological

unit Summer Winter

Ponds Suspension sedimentation Ice entrapment and in-situ melting

–settling-out

–vegetation trapping

Gullies Suspension sedimentation Ice growth entrapment and in-situ melting

–settling-out Ice cover confined settling out

Bedload deposition

Sediment gravity flows

Slumping

Mudflats Suspension sedimentation Ice freeze-on and in-situ melting

–settling-out Snow filtering and in-situ melting

–vegetation trapping Ice growth entrapment and in-situ melting

Ice cover confined suspension settling

Marshes Suspension sedimentation

–vegetation trapping

Levees Suspension sedimentation Ice freeze-on and in-situ melting

–settling-out (sediment–organics)

–vegetation trapping Snow filtering and in-situ melting

Ice growth entrapment and in-situ melting

Ice cover confined suspension settling out

6
Gullies Ponds Areas

1—Racine A—Racine B—B-area

2—B-Gully B—C-Pond RI—Racine Island

3—Parachute C—Lawson’s C—C-area

4—In-Between D—Bread Truck EOD Pad

5—Bread Truck E—C/D-Pond C/D—C/D-area

6—Coastal 1 F—Pond Beyond BT—Bread Truck

7—Mortar G—D-Pond D—D-area

8—Coastal 4 H—A-Pond Coastal East

9—Coastal 5 I—B-Pond Coastal West

10—Coastal 6 A—A-area

11—Coastal 7

12—Tanker

13—Otter Creek

N

0 500 1000 m

Coastal West

A

Coastal East

BT

C/D

D

C

RI

B

EOD


 Pad

Coastal West

A

Coastal East

BT

C/D

D

C

RI

B

EOD


 Pad

Clunie Pt.

Clunie


Creek

Rt. Bravo


Bridge

I

13

9 10

11

6

G

7

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8

5 E

3

4

2

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C

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1

12

H

Clunie Pt.

Clunie


Creek

Rt. Bravo


Bridge

I

13

9 10

11

6

G

7

F

8

5 E

3

4

D

2

B

C

A

1

12

H

Figure 5. Locations of gullies, ponds and areas.

enhanced seasonally by the discharge of the glacially fed Eagle River, particularly by snowmelt

and precipitation in the river’s watershed (Fig 5).

In contrast, inundation along the coast is a function of tidal height, which may be influenced by

the direction and velocity of the wind, ice cover

and storm-driven surges in Knik Arm.

Tidal current measurements indicate that velocities are higher during ebb than during flood,

and thus sediment transport and channel erosion

are potentially greatest during ebb (Lawson et al.

1996). Peak velocities within gullies ranged from

about 0.8 to 1.9 m/s in 1994 and varied from site

to site and with the elevation of tidal flooding.

The velocity variations reflect differences in gully

width, depth, roughness and network configuration.

The Eagle River provides access for tidal waters to inundate the innermost reaches of the Flats.

High tides increase river stage by tidal damming

and reversing flow in the river. Sedimentation in

the northern two-thirds of ERF is tidally dominated, whereas the southern one-third appears to

be river dominated. Tidally dominated sedimentation ranges from several millimeters per year

on levees, to 10–15 mm/year on mudflats, and up

to 20–40 mm/year in ponds. Sedimentation has

formed an alluvial fan at the head of ERF (Fig. 6),

indicating that deposition here is mainly derived

from overbank flooding of the river.

Suspended sediment concentrations measured

in 1994 in the Eagle River, tidal gullies draining

the ponds and mudflats, and Knik Arm show that

most of the sediment is derived from Knik Arm

tidal waters (Lawson et al. 1996). The Total Suspended Solids (TSS) in the waters of ERF vary

with tidal stage, location, source and season. TSS

values in the glacially fed Eagle River vary seasonally from peaks of 100–700 mg/L between

breakup in May and freezeup in October. Apparently, there are two seasonal highs each year, the

first during snowmelt runoff and the second during the peak glacial melt season. In contrast, TSS

values of Knik Arm water range from about 1000

to 2800 mg/L from May to October. The high TSS

values of Knik Arm reflect its source from large

glacial rivers (e.g., Susitna, Knik and Matanuska),

which have higher sediment discharges than the

Eagle River. During a tidal cycle, TSS measurements at gully sites increase steadily through the

flooding tide and decrease at a slower rate during

the ebb. Seasonally, TSS values in gully and Knik

Arm waters increase from spring to fall; the cause

of this increase is unknown.

Gullies are actively extending into the mudflats

and ponds in the Bread Truck, C, A, D and C/D

7
Figure 6. Aerial photograph of Racine Island taken 16 August 1995 showing remnant channel

surrounded by overbank deposits.

areas (Fig 5). Headwalls and adjacent lateral

walls receded at variable rates: 0.1 to 4.9 m during summer 1992, 0.4 to 6.3 m during winter 1992–

93, 0.0 to 9.8 m during summer 1993, 0.0 to 2.3 m

in winter 1993–94, and 0.0 to 2.6 m in summer

1994. Large tidal floods in 1993 and 1994 caused

the highest headwall recession rates. Two gully

headwalls, one on the western side of Bread Truck

Pond and the other near the pond complex between Bread Truck Pond and C-Pond, were advancing at a mean rate during 1993–1994 that was

sufficient to cause increased drainage of these

ponds in 15 to 20 years (Lawson et al. 1996).

Surveying of longitudinal profiles of tidal gully

thalwegs revealed nonuniform gradients that reflect their progressive and episodic elongation into

the mudflats and ponds, and a lack of equilibrium with present conditions. Gully gradients are

8
relatively steep near their mouths, but are reduced

in the lower and mid-reaches. Channel gradients

then steepen in the uppermost reaches up to a

nearly vertical headwall that ranges in height

from 1 to 2.5 m. Incipient drainageways from

ponds and mudflats into tidal gullies have very

low gradients. Several sharp increases in gradient

(knickpoints) occur in each gully, marking channel adjustment as the tidal gully progressively

extends into the mudflats and ponds.

Aerial photographs from 1950, 1967 and 1993

reveal changes in the distributary channel pattern

entering ERF (Fig. 7). Three main channels entered the Flats in 1950, but by 1967 the westernmost channel was abandoned and the eastern and

middle channels diverged just northeast of the

Route Bravo bridge. These two primary channels

were characterized in 1967 by partial braiding

and a divergent pattern characteristic of an alluvial fan (Fig. 7b). Sediments in the fan area are

relatively featureless overbank materials deposited by Eagle River flooding. Only the center of

the Racine Island area (Fig. 6) is characterized by

intertidal ponds and an abandoned gully. The

westerly channel was abandoned in September

1995, during flooding of the Eagle River as channel scour shifted the easterly channel to the northeast.

The northern two-thirds of ERF has landforms

typical of tidal flats near river mouths (e.g.,

Ovenshine et al. 1976b) that consist mainly of

levees, vegetated marshes and abandoned channels and point bars. Significant changes in the

relatively tight meander loops in this lower section of the river have also taken place over the last

40 years. 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). Meander scars, abandoned meander loops, and point bar deposits are

common along the length of the active channel

and are evidence of such changes in the past.

Tributary channels with dendritic patterns,

herein called vegetated drainageways, drain water from the mudflats and ponds into the tidal

gullies (Fig. 3 and 8). A second set of channels,

intercepted by the active tributary system,

unconformably crosses other landforms, including ponds (such as C and Bread Truck). Their

pattern is irregular and unrelated to the active

gully drainage system. These secondary channels

are parts of former drainageway and gully systems that are now inactive and therefore relict.

Their fragmentary presence is a sign of significant changes to the drainage system in the past.

The cause of such change is unknown, but events

such as river avulsion, channel migration or earthquake-induced subsidence may cause abandonment of drainageways and gullies.

Chemical analyses of a limited number of sediment trap and plankton net samples indicate that

WP undergoes suspension, transport and redeposition in ERF, as well as transport through tidal

gullies into the Eagle River and Knik Arm. Tidal

and wind-driven currents can scour pond bottoms, resuspending and transporting WP, whether

as individual particles or sorbed to sediment. Erosion and scour of vegetated drainageways and

gullies can also entrain sediment containing WP

and transport it in suspension or as bedload. In

addition, pond and mudflat sediments can freeze

to the base of the ice cover during its growth, and

subsequently be rafted during tidal inundation.

Bathymetric profiling offshore of ERF shows no

evidence of deposition of ERF sediments as a delta

at the mouth of the Eagle River. WP entering

Knik Arm is potentially diffused by tidal and

river currents, but there are several sites where

WP-contaminated sediment might be deposited

(Lawson et al. 1996). These sites include intertidal

bars and nearshore deposits where waterfowl may

feed.

Previous investigations suggest that natural attenuation of WP contamination is possible, but

that further work is needed to assess the length of

time over which this may happen and to identify

specific locations where natural attenuation is the

remedial method of choice. The relative importance of erosion and deposition processes varies

from area to area in response to tidal and river

hydrology; however, changes in the system over

seasons, years and decades are possible.

Pond sedimentation rates appear to be sufficiently high in some areas to bury WP, thereby

reducing the exposure risk for waterfowl naturally. The drainage system is also changing as

tidal gullies expand across mudflats into ponds.

Certain ponds are likely to drain in 20 years or

less. Drier conditions will permit in-situ degradation and natural remediation of WP contamination. Erosion will, however, continue to release

WP from mudflat and pond sediments over time,

while ice and water transport mechanisms will

move WP into tidal gullies, the Eagle River and

9
a. 1950.

Figure 7. Aerial photographs of ERF.

10
Figure 7 (cont’d).

b. 1967.

11
Figure 7 (cont’d). Aerial photographs of ERF.

c. 1993.

12
Figure 8. 1992 aerial photograph of northeastern ERF.

subsequently Knik Arm. Sediment entering Knik

Arm may be widely dispersed by estuarine processes, or concentrated in bars and other deposits. Which will happen is unknown. In addition,

whether WP survives transport to actually persist

in Knik Arm is unknown. Laboratory flume tests

of WP particle abrasion could determine if transport destroys such particles.

STUDY SITES AND METHODS

Sedimentation transects

Sedimentation rates were measured along

transects across representative landforms of ERF,

with 11 transects being set out in May 1992 (Lawson and Brockett 1993), 1 in June 1993 and 11 in

1994 (Fig. 9). Their locations and elevations were

13
surveyed using an electronic theodolite. Grab

samples of surface sediment (to 5 cm depth) were

taken at each survey point along the transects,

and their grain size distributions were analyzed

using standard sieving and hydrometer techniques. These data delineated textural trends in

surface materials and the relative importance of

tidal and river sediment sources to sedimentation

and landform development (Lawson and Brockett

1993).

Sedimentation stakes were used to measure

surface aggradation or erosion on transects where

the surface was wet or standing water was temporarily present (Fig. 10). These stakes consisted

of a rod and a square, rigid plate (about 7 cm2)

that slides freely on it, following 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 periodically 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. 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

1993, 1994 and 1995.

Sedimentation on levee and mudflat surfaces

was 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).

Paint was applied at certain sites in June and

August 1992, August 1993, and May–June 1994.

Net accumulation was measured by cutting and

removing a block of sediment with a putty knife

or spatula (Fig. 11). The thickness of sediment

above the paint layer was then measured to the

nearest 0.5 mm. In contrast to the sedimentation

stakes that were commonly broken or removed

by ice during winter, the wire flags and buried

paint horizons enabled us to acquire a continuous

record of net sedimentation rates.



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

Figure 9. Mudflat sedimentation transect locations.

14
a. Sedimentation stake with plate.

Figure 10. Sedimentation measurement techniques.

b. Wire flags mark an area of painted ground.

15
Pond sedimentation

measurements

Sedimentation rates in permanent ponds and

marshes are difficult to measure accurately because a slight disturbance of the water column

can resuspend the fine-grained materials covering their bottoms. Transects were established in

May 1995 for detailed measurements of sedimentation rates in six ponds (Fig. 12), while isolated

sites were previously established in 1992, 1993

and 1994. Two orthogonal transects were usually

established in each pond, with traps located about

every 20 m along them. Two methods were used

in permanent ponds, one to measure gross sedimentation rates and another to evaluate resedimentation caused by resuspension of pond and

marsh sediments by wind, waterfowl and other

processes.

Gross (total) sedimentation rates in ponds were

measured at 18 sites following tidal inundations

in June, August and September 1992, and in May

and September 1993, 1994 and 1995 (Fig. 13). Gross

deposition from tidal and river inundation, as

well as resuspended bottom sediments, was measured in a sediment trap consisting of a 4-in.-

diameter (10.2-cm-diameter) schedule-40 PVC

pipe end cap glued to a short length of 2-in.-

diameter (5-cm-diameter) schedule-40 PVC pipe

at the isolated pond stations, and a 16-cm-diameter end cap at pond transect stations. The pipe

was inserted into pond sediments until the bottom of the cup was in contact with the bed (Fig.

14). Sediment trapped in the cup included new

sediment brought into the ponds by tidal inundation and river currents, and materials resuspended

by wind waves, dabbling ducks or other mechanisms. The thickness of accumulated sediment

was measured to the nearest 0.5 mm by inserting

a graduated scale into it at three places. After

measurement, the sediment in the cup was cleared

or saved for analysis of WP concentration.

Net sedimentation rates at isolated sites and

transect sites were measured using a thin, rigid

plastic plate of about 30 cm square that was

pushed gently into the pond bottom until its surface was flush with the bottom surface. The corners of the plate were secured by aluminum tent

pegs (Fig. 14). Sediment in suspension settled onto

Figure 11. Wedge-shaped sediment block used to measure vertical accretion. Note subhorizontal laminations.

16
N



 



















































 





























 

 

























































 

































 







 







































































































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0 200 400 m y

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Pond Sedimentation Station


Wet Shale 



yy

yy





yy

yy



 Intermittent Pond


Permanent Pond

Figure 12. Pond sedimentation measurement locations.

a. Locations of pond transects.

b. Transects in Bread Truck, C/D, C and Lawson’s ponds.

17
y

y

yy

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Pond Sedimentation Station


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 y

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Permanent Pond yy





yy

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0 300 m

N

c. Racine Island Pond transects.

d. A-Pond transects.

Figure 12 (cont’d). Pond sedimentation measurement locations.

18
yy

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0 500 1000 m

Permanent Pond


Intermittent Pond


Sedimentation Station











yyy

yyy

6

5 10

9 8

7

3

4

2

1

15

14

13

16

17

18

12

11

Figure 13. Locations of 1992–1994

pond sedimentation stations.

Figure 14. Example of sedimentation station at an intermittent pond location showing layout of cup and

plate sampler.

19
this plate, but this sediment can also be reworked

and resuspended by wind or other mechanisms,

thereby delineating a net rate. The thickness of

sediment at three locations was measured to the

nearest 0.5 mm using a graduated scale.

Gully erosion and

headwall recession

Tidal gullies draining ponds and mudflats are

actively extending inland by erosion at their heads.

We established 86 sites between June 1992 and

May 1995 to evaluate retreat rates. At each site,

stakes were driven into the ground along a line at

known distances from one another, typically parallel to the crest of the gully scarp, and a

“hub” stake was set at a known distance

from this stake line (Fig. 15). The distance between the hub and the crest of

the gully scarp was measured across the

top of each line stake with a tape measure. The position of the gully scarp crest

was identified by lowering a plumb bob

on a string from the tape measure, so

that the horizontal distance could be read

where the string and measuring tape

met. Flagged wire stakes were then

placed at each point of measurement

along the headwall. We periodically relocate the stakes when scarp recession

takes place (Fig. 16) and either removes

or threatens them. Repeated measurements using this technique allow us to

monitor changes in scarp geometry with

time, as well as the rate of gully recession.

Recession rates were measured in

September 1992, 1993, 1994 and 1995,

in May 1993, 1994 and 1995, in November 1994 and late October 1995. The

September measurements delineate

summer rates, while those of May or

June delineate winter rates. The October

1995 and November 1994 data show the

amount of erosion since the end of summer measurements and the initial period of freezeup. Repeated measurements at points without any retreat (as

indicated by the continuing presence of

wire flags) indicate that they are reproducible to ±2–5 cm. Their accuracy, however, is limited by how well the crest of

the gully scarp can be defined, the shape

of which is highly irregular. In the worst

case, accuracy is probably limited to ±10 cm.

Recession data are presented as a range of maximum recession rates measured orthogonally to

the gully or river at each site. This method differs

from previous years’ method, where erosion measurements were reported as collected in the field

and did not account for apparent distances created by the method (Lawson and Brockett 1993;

Lawson et al. 1995, 1996). These rates tended to be

higher than actual wherever hub to line measurements were made at an acute angle to the scarp,

thus recording an oblique distance. Measurements

reported herein were taken from scaled summary

plots and summarize the range in recession rates,

Figure 15. Example of layout of hub and line stakes at River-North

erosion site.

20
not absolute values based upon each hub to line

stake measurement.

Historical aerial

photographic analyses

During 1995, multiple sets of aerial photographs

were acquired to increase our historical coverage

of Eagle River Flats (Table 5). Our current aerial

photography archive provides a 45-year record of

surficial changes at ERF from 8 August 1950 to 9

October 1995. The images are of variable quality

and range from the highly blurred 1967 black and

white images to the high resolution natural color

photographs of 9 October 1995.

We measured historical changes by studying

aerial photographs taken at approximately decade intervals (1950, 1960, 1972, 1984–86 and 1995).

The actual photography set used was determined

by our ability to detect gully headwall scarps. The

headwalls of B-Gully, Parachute Gully, In-Between Gully, Bread Truck Gully, Mortar Gully

and Coastal 5 Gully were mapped to document

their recessional histories.

Average rates of headward recession were calculated by transposing the position of the gully

headwall scarps onto gully base maps produced

from the 9 October 1995 aerial photographs. The

distance of headward recession was measured as the centerline distance within each

gully between known points. The average

rates were then determined by dividing the

distance of centerline recession by the number of years between each measure.

Water quality parameters

Various types of water quality data have

been collected at multiple sites to assess the

surface hydrology and water quality of ERF.

The majority of sampling sites were in small

plunge pools at the heads of tidal gullies;

three were newly located in ponds in 1995

in some of the most heavily contaminated

locations of Eagle River Flats (Fig. 17). The

site at Mortar Gully receives tidal waters

directly from Knik Arm, whereas Bread

Truck, Parachute, B and In-Between gullies

are located in drainages that flow into the

Eagle River and therefore have mixed fresh

and tidal water sources. Racine Island Gully

receives most of its water influx from the

Eagle River during the summer. The three

pond locations where measurements were

recorded—C, A and Racine ponds—are distributed in the eastern, western and southern sections of ERF. The two remaining sites represent

the primary source waters: the Eagle River upFigure 16. Location of gully headwall and lateral wall erosion

sites.

Table 5. Historical aerial photography of

Eagle River Flats.

Year Date Type

1950 8 August B/W

1953 27 June B/W

1957 12 July B/W

1960 30 August B/W

1962 17 May B/W

1967 Unknown B/W

1972 July False color IR

1972 9 August B/W

1974 7 May B/W

1977 Unknown False color IR

1978 August False color IR

1984 12 August False color IR

1986 5 October True color

1986 12 September LANDSAT TM (Digital)

1988 4 August B/W

1991 21 June Color IR

1992 22 May Color IR

1992 2 August Color IR

1993 8 July B/W Orthophotograph

1994 30 August Color IR

1995 16 August Color IR

1995 9 October True color

N

1992 Gully Erosion Hub Sites


1993 Gully Erosion Hub Sites


1994 Gully and River Erosion Hub Sites


1995 Gully Erosion Hub Sites 

0 500 1000 m

Rt. Bravo


Bridge

Coastal 7

Coastal 6 Coastal 5

Tanker

River North

Coastal 4

Coastal 1

Bread Truck

Parachute

B Gully

Racine Island

River South River Meander

Illume Pond

In-Between Gully

21
stream of where it enters ERF, and in Knik

Arm in the nearshore zone, about 200 m

north of the Eagle River mouth.

A suite of water characterization data was

collected at 4-minute intervals at each site.

These data include salinity, pH, redox, dissolved oxygen, turbidity and water surface

elevation (Table 6). Figure 18 shows the times

of data collection since 1993. Water samples

were collected with an ISCO suction sampler automatically at various intervals before, during and following flood tides. 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, generally following procedure

2540D in Standard Methods for the Examination of Water and Wastewater (APHA, AWWA,

WEF 1992). Modifications to this procedure

were described in Lawson et al. (1996).

A Marsh–McBirney electromagnetic flow

probe was used to measure current velocity

during ebb and flood in selected tidal gullies. A unit was also installed in C-Pond to

N

C Pond

Coast Mortar

Bread Truck

Parachute

B Gully

A Pond

Racine Pond

Racine Gully River Hydrolab & ISCO Water Sampler


Plankton Net


CTD


Wave & Tide Gauge


Weather Station


ISCO Water Sampler

0 500 1000 m

In-Between

C Pond

Figure 17. 1995 locations of instrumentation recording water

quality parameters and water depths. Locations of weather

station and plankton net sampling sites are also shown.

Table 6. 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

D&A 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)

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

 (model UDG01)

Seabird Wave/Tide Pressure (water depth) ± 0.003 m water 0.0015 m water

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

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

 (model SBE 16) Temperature ±0.01°C 0.001°C

Conductivity ± 0.001 S/m 0.0001 S/m

Dissolved oxygen ± 0.1 mL/L 0.01 mL/L

pH ± 0.1 units

OBS [5 V=2000 FTU] ± 100 mV 3 mV

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

22
Figure 18. Extent of data collection at hydrostation sites from 1993–1995.

measure tidal and wind currents. The sensor was

mounted about 1 m above the bed in gullies at the

Bread Truck, Parachute, In-Between, Mortar and

B sites (Fig. 17). Ultrasonic sensors were used to

measure flooding on the mudflats near the Mortar, Bread Truck, Parachute, In-Between and

Racine gullies, and at the weather station coastal

sites.

The same suite of water quality parameters

was collected at the Knik Arm coastal site using a

Seabird SBE 16 Profiler. In addition, a Seabird

SBE 26 wave and tide gauge was located here to

record water depth and temperature in Knik Arm

(Table 6). The same location was used in 1993 and

1994 (Fig. 17).

TSS Hydrolab Ultrasonic Flow 



yyy

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

May Jun Jul Aug Sep Oct Nov





yyyy













yyyy



yy





y

y

yy







1994

1993

Bread Truck

Parachute

River

Coast

Mortar Gully

Bread Truck

Parachute

B-Gully

River

Spring Gully

Tanker Gully

Otter Gully

Otter Creek

East

West

23
Figure 18 (cont’d). Extent of data collection at hydrostation sites from 1993–1995.

Hydrostation configuration

The instrumentation and samplers for water

quality analyses were identically set up at each

gully and pond site (hydrostations). Sensors and

the suction sampler intake screen were mounted

on a stake driven into the bottom of the pond or

gully under investigation. Mounting hardware

was identical at each hydrostation to permit us to

precisely position them in the water column. Sensors in gully sites were placed in plunge pools to

keep them wet at all times, usually about 15 to 45

cm above the bed. Current probes were mounted

about 1 m above the bed on separate stakes located downstream of the plunge pool within a

reasonably symmetrical gully cross section. Ultrasonic sensors were mounted on an arm attached

to a metal stake located about 10 m from the

platform. Each was aimed downward towards

the mudflat surface, upon which a plate was

pinned to provide a stable, reflective surface.

Instrumentation, including the ISCO sampler,

was located on a floating platform either on the

mudflat next to gully sites or within the ponds

(Fig. 19). Each platform was constructed of a 4-ft

(1.2-m) square plywood deck mounted on a 2- ×

8-in. (about 5- × 20 -cm) wood frame. Large metal

eyehooks were screwed into the frame on each

corner. Five-foot (1.5-m) sections of 0.75-in. (2-

cm) -diameter steel pipe were mounted vertically

in 20-gal. (75.7-L) cans filled with concrete, and

the eyehooks mounted on the platform’s wood

frame lowered over them. Foam filling the inner

space of the deck framework provided flotation

so that the platform could move vertically up and

down the steel pipes during tidal flood and ebb,

keeping the datalogger and related devices dry

during the highest flood levels. Dataloggers were

installed in NEMA plastic enclosure boxes

mounted on 2-in. (about 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.

WP transport and resuspension

Plankton nets of 3-m length with an opening of

1 m and a mesh size of 80 μm were placed at four

gully sites, three of which were previously monitored in 1994 (Fig. 17). The plankton net was tied



























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y

y





























yy

yyyyyyyyyyyyyyy

yyy

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yy

yyyyyyyyyyyyyyyyyyyyy

yyy

yyyyyyyyyyy





yy

yy

















yyyyyyyyy

y

yyyyyyyyyyyyy

y

y

















yy

yyyyyyyyyyyyyyyyyyyyyyyyyy













yy

yyyyyyyyyyyyyyyyyyyyyyyyyy





yyyy

Coast

Mortar Gully

Bread Truck

In-Between

Parachute

B-Gully

Racine Gully

River

A-Pond

C-Pond

Racine Pond

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

May 1995 Jun Jul Aug Sep Oct Nov

TSS Hydrolab Ultrasonic Flow 



yy

24
Hydrolab 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

a. Platform and instrumentation at Bread Truck Gully.

Figure 19. Hydrostation layout at a gully location.

b. Schematic showing layout of intrumentation in water and collection devices on a

floatation platform.

25
F P O

Figure 20. Plankton net used for collecting WP in water transport.

to stakes driven into the gully bottom and extended down-channel in the direction of ebb (Fig.

20). A cup at the apex of the net collected the

sediments that were transported into it. Each net

was in place during multiple flooding tides each

month (Fig. 21). Sediment was collected following each ebb tide during a monitored tidal cycle

and analyzed by a commercial laboratory for WP

following standard laboratory methods (USEPA

1995).

Sediments collected in sediment traps from

transects within C, A, Racine Island, C/D and

Lawson’s ponds were also analyzed for WP (Fig.

12). These analyses provide data on resuspension

of WP by wind, tidal currents and waterfowl activity.

Data acquisition record

Figure 21 summarizes the periods when various types of field data were acquired. The multiple starting dates for acquisition reflect both the

implementation of new methods and expansion

to additional sites in ERF. Gaps in the records of

water quality sensors, other than during the winter, resulted from either instrumentation failure

or disruption by natural processes.

26
1992 1993 1994 1995

M J J A S M J J A S M J J A S O N M J J A S O D

Grain Size

White Phosphorus


Sample Type:


Sediment Grab


Sediment Trap


Plankton Net


Water


Bedload Trap


Ice

TSS:


Ice

= Measurement Taken

a. Continuous data.

b. Discontinuous data.

Figure 21. Sampling coverages for various types of field data.

1992 1993 1994 1995

M J J A S M J J A S M J J A S O N M J J A S O N

Sedimentation:


Mudflat Transect 1-11


12


14.24


Pond Station 1-9


11-18


Pond Transects


Erosion:


B-Gully


Parachute


In-Between


Bread Truck


Coastal 1


Mortar


Coastal 5


Coastal 6


Coastal 7


Tanker


Illume Pond


River-North


River- Meander


River South


Racine Island


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, Bread Truck Gully


Wave and Tide Gauge


Weather Station

= Measurement Taken

*See Figure 18

27
INTRINSIC REMEDIATION:

POND DRAINAGE BY GULLY

EROSION AND EXTENSION

Erosion and the inland extension of tidal gullies are among the most visible mechanisms currently modifying the ERF physical system. This

activity is critical because our initial assessments

(Lawson et al. 1996) suggested that gully extension could drain contaminated ponds, create drying conditions conducive to in-situ WP degradation (Walsh et al. 1995), and result in a natural

attenuation in a relatively short time. The length

of time for drainage to occur will be a function of

several factors and may vary across the Flats. The

1964 Alaskan Earthquake appears to have had

the single greatest effect on ERF, compared to all

other external and internal factors. It significantly

altered the hydrological system, which in turn

has initiated major changes in drainage, gully erosion and extension, and pond and mudflat sedimentation.

External forcing

The effects of major tectonic events in the

greater Anchorage area are reasonably well known

and best documented following the 1964 earthquake (Hansen 1965, Plafker et al. 1971). Stratigraphic analyses of coastal marshes indicate that

large earthquakes (>Mw = 9.0) have a recurrence

interval of 600–800 years (Combellick 1993, 1994),

while magnitude 8.0 or greater earthquakes are

more frequent (230–460 years; Nishenko and Jacob

1990). Tectonic disturbance associated with such

events causes co-seismic subsidence or uplift (depending on site position relative to local displacement), which is followed by a post-seismic recovery in the opposite direction (e.g., Savage and

Plafker 1991). These crustal movements cause adjustments in surficial processes, which are manifested at ERF by the interactions of the internal

factors listed in Table 1.

Leveling data collected over the decade following the earthquake of 1964 show that Anchorage subsided 60–70 cm during it (Brown et al.

1977). Brown et al. (1977) estimated that there had

been about 20 cm of post-seismic recovery by

1975, although this is not apparent in the tide

gauge records (Savage and Plafker 1991). Cohen

et al. (1995) and Cohen (1996) suggest that uplift

in the first decade following the earthquake was

rapid and has subsequently decreased; Savage

and Plafker (1991) estimate the current uplift rate

to be around 1.0 ±2.2 mm/yr. Considerable research is still required to resolve the absolute magnitude and nature of the tectonic response to the

1964 earthquake.

The complex way in which the physical system

of ERF responded to this disturbance is difficult

to reconstruct. In a previous report, we speculated that the recent erosion and stepped profile

in the gully gradients reflected a dynamic equilibrium related to changes in base level vs. mean

sea level (Lawson et al. 1996). However, it is questionable as to whether or not the physical system

would respond in such a dynamic manner to a

0.6- to 0.7-m change in base level given the

macrotidal range of 9–11 m for Knik Arm. We

now feel that erosion and sedimentation rates are

a response to increased tidal flooding, coupled to

sedimentary subsidence. Both the increase in frequency of flooding and larger volumes of water

inundating the Flats are important factors that

would increase the amount of sediment entering

ponds while also increasing the volume and intensity of drainage during ebb.

Gully erosion and discharge

The volume of water that flows through each

gully is controlled by the height and duration of

tidal flooding, as supplemented by river discharge and the additive effects of wind, ice and

other factors. The range and magnitude of ebb

velocity is critical to determining both the flux of

sediment and water into and out of the ponds

and mudflats, and the ability of those currents to

scour and resuspend pond, mudflat and gully

sediments.

The velocity of tidal flood and ebb currents

at Mortar, Bread Truck, B, Parachute and InBetween gullies ranges between 0.75 and 2.07 m/s

(Table 7). The peak velocity at each site changes

with the height of flooding (Fig. 22), the lower

peak and range in velocities occurring during the

lower elevation flooding tides. Flow velocities are

greatest at B-Gully, with moderate values recorded

at Bread Truck, In-Between, Mortar and Parachute

gullies.

Peak ebb velocity is greater than peak flood

velocity (Table 7, Fig. 22). This asymmetry in flow

velocities, and therefore discharge, determines

when erosion and sediment transport will take

place and to what magnitude. During tidal flooding, gully water levels rise passively before spreading out onto the mudflats and suspended sediment loads increase as Knik Arm waters flood the

gullies, mudflats and ponds. In contrast, higher

velocities during ebb dictate an increase in turbu28
Table 7. Summary of gully velocity and discharge data.

Monthly range Monthly range Monthly range

in average Peak Average Peak in average in average

Tide velocity velocity TSS TSS water discharge sediment discharge

Gully stage (cm/s) (cm/s) (mg/L) (mg/L) (m3/s) (kg/s)

B Flood 8.50–15.83 42.39 280–780 2251 0.50–0.94 0.14–0.53

Ebb 9.04–75.41 207.5 0.54–4.47 0.20–1.81

Parachute Flood 7.63–26.67 66.96 422–972 2518 0.96–3.36 0.30–1.99

Ebb 4.56–52.47 96.02 0.57–6.60 0.18–2.79

In-Between Flood 2.23–15.45 75.16 292–999 2787 0.16–1.11 0.08–1.04

Ebb 3.46–25.07 97.29 0.25–1.81 0.12–1.62

Bread Truck* Flood 22.6–29.9 71.1 170–1186 2152 1.24–1.64 0.23–1.95

Ebb 74.0–81.0 146 4.07–4.46 1.95–3.60

Mortar Flood 2.75–3.26 15.34 251–856 1973 0.12–0.15 0.03–0.12

Ebb 7.75–9.93 31.62 0.35–0.45 0.08–0.30

*Based on 1994 measurements.

200

100

0

456

Maximum Water Elevation (m)

Velocity (cm/s)

In-Between Gully

Mortar

Parachute

B Gully

Bread Truck

Figure 22. Relationship between peak velocity and maximum water elevation at selected sites.

a. Flood currents

In-Between Gully

Mortar

Parachute

B Gully

Bread Truck

200

100

0

45 6

Maximum Water Elevation (m)

Velocity (cm/s)

(207)

b. Ebb currents

29
Table 8. Summary of scarp recession (m).

a. Seasonal.

Summer 92 Winter 92–93 Summer 93 Winter 93–94 Summer 94 Winter 94–95 Summer 95 1992–1995

Site H L H L H L H L H L H L H L H LB — 0.1–0.3 — 0.2–0.3 — 0.3–0.8 — 0.1–0.6 — 0.1–1.1 — 0.1–0.8 — 0.2–0.8 — 1.0–5.3Parachute 0.3–3.2 0.2–0.5 0.5–1.3 0.6–0.7 0.1–3.3 <0.1–0.2 0.6–2.7 0.6–0.7 1.7–2.0 0.7–1.2 0.3–1.7 0.1–2.3 0.1–3.4 0.3–0.6 1.1–12.1 1.3–1.8*In-Between <0.1–1.0 <0.1–0.5 <0.1–0.8† <0.1 <0.1–0.8 0.8 0.1–0.3 <0.1 0.1–0.6 ~0.8 <0.1–0.2 <0.1–0.5 <0.1–1.2 <0.1 0.2–2.8 0.2–1.2Tanker <0.1 0.1–0.8 <0.1 <0.1–0.3 <0.1–0.5 <0.1–0.5 <0.1–0.5 0.3–0.9

Bread Truck 0.3–5.0 0.1–0.3 0.1–>1.1 0.1–2.2 1.0–11.7 0.1–0.3 0.3–20.0 0.1–2.0 2.0–33.3 0.6–3.0Coastal 5 0.3–1.5 <0.1–0.6 0.4–6.4 0.6–1.0 0.3–16.9 0.5–0.7 0.6–25.9 1.0–1.7Mortar 0.3–1.7 0.5–1.1 0.3–1.3 <0.1–1.2 ~1.1–1.6 0.5–2.2Coastal 6 ~0.9 1.0–1.8 ~0.3 0.3–0.4 0.7–1.2 0.9–1.1 ~1.9 1.1–2.7

Coastal 1 1.5–1.7 <0.1–0.3 0.5–0.8 <0.1–0.7 1.1–1.4 <0.1–0.3 0.8–3.1 <0.1–0.7Coastal 7 1.0–1.2 0.3–1.0 0.4–0.9 ~0.6 0.2–0.8 ~0.5 1.3–2.4 0.9–1.1River-North — ~3.2 — 3.6–4.6 — 1.1–2.5 — 3.3–7.4H = Headward; L = Lateral.* Maximum total erosion measured from scarp outline of May 1992; local lateral erosion was greater where associated with lateral expansion accompanying headward migration of

headwall.

† Shallow trough (<0.5 m deep) extended ~ 40 m.

b. Annual.

1992–1993 1993–1994 1994–1995 1992–1995

Site Headward Lateral Headward Lateral Headward Lateral Headward LateralB — 0.1–1.5 — 0.1–1.1 — 0.2–0.8 — 1.0–5.3Parachute 0.8–4.5 0.3–0.7 0.4–2.8 0.6–1.0 0.3–2.0 0.6–3.2 1.1–12.1 1.3–1.8*In-Between <0.1–1.1 <0.1–0.5 0.1–0.8 0.1–1.1 <0.1–1.3 ~0.1 0.2–2.8 0.2–1.2Tanker <0.1–0.5 0.3–0.7 <0.1–0.5 0.3–0.9Bread Truck 0.3–5.0 0.2–2.2 0.8–31.7 0.3–1.3 2.0–33.3 0.6–3.0Coastal 5 0.4–7.6 0.9–1.0 0.6–25.9 1.0–1.7Mortar 1.1–1.6 0.5–2.2 ~1.1–1.6 0.5–2.2Coastal 6 0.6–1.0 1.0–1.8 ~1.9 1.1–2.7Coastal 1 0.8–2.0 <0.1–0.7 0.8–3.1 <0.1–0.7Coastal 7 0.8–2.0 ~1.0 1.3–2.4 0.9–1.1River-North 1.1–7.4 — 3.3–7.4* Maximum total erosion measured from scarp outline of May 1992; local lateral erosion was greater where associated with lateral expansion accompanying headward migration of

headwall.

30
lence and the potential for transporting greater

amounts of sediment. Perhaps more importantly,

additional energy is released from the stored potential energy of the elevated flood waters, causing scour and sediment entrainment. Thus, tidal

discharge during ebb is more erosive than during

flood.

Discharge (Q) was estimated for each monitored gully using the peak and mean velocities by

Q = VA

where V is velocity and A is the cross-sectional

area.

Average discharge values calculated for each

gully ranged from 0.12 to 3.36 m3/s during flood

and 0.25 to 6.60 m3/s in ebb (Table 7). Peak

ebb discharge at each site ranged from 1.16

to 12.30 m3/s. Peak and average TSS concentrations collected over the same time

intervals provide rough estimates of flood

and ebb sediment fluxes through gullies

(Table 7).

These data indicate ebb sediment flux is

greater than the flood sediment flux, with

average sediment flux ranging from 0.03

to 1.99 kg/s during flood and 0.08 to 2.79

kg/s during ebb (Table 7). This difference

in flux reflects the ebb and flood asymmetry. The significance of the data in terms of

sediment transfer is not as clear. These data

suggest that, over time, there should be a

net transfer of sediment into Knik Arm.

However, they do not account for sediment flux via flood waters that overflow

the Eagle River and gully banks and levees

and, therefore, may be misleading.

Gully erosion and

scarp recession rates

Gully erosion and recession were monitored to assess current rates of extension

into ponds and the potential for pond

drainage and in-situ WP degradation.

These short-term rates were compared with

long-term trends analyzed on aerial photographs of selected gullies. These historical rates also define variations in gully extension across ERF and are an independent

check on field measurements.

Modern rates

The amount of recession of scarp crests

is highly variable within a particular gully,

as well as among all gully sites (Table 8).





yy

yy 



 



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yy 



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yyyyy

yyy

yyyy

yy

N

0 5 10 m

May to Nov 1995


Sep 1994 to May 1995
 ySep 1993 to Sep 1994 



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yyyyy

yyyyyyy

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yy

y

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yyy

yy



yyyy

yyyy

0 5 10 m





yy

yyyyMay 1995 to Oct 1995


May 1994 to May 1995


June 1993 to May 1994


May 1992 to Jun 1993 



N

a. Bread Truck Gully.

b. Parachute Gully.

Figure 23. Summary of gully scarp recession.

Maximum recession rates ranged from 0.3 to 3.2

m during May–September 1992, 0.1 to 1.3 m during September 1992 to June 1993, and 0.1 to 3.3 m

during June–September 1993. During the winter

of 1993, erosion rates ranged from 0.1 to 5.0 m, 0.1

to 2.0 m in the summer of 1994, 0.1 to 11.7 m

during the winter of 1994, and 0.1 to 20.0 m during the summer of 1995. Net headward recession

ranged from 0.1 m to about 33 m between 1992

and 1995, with a maximum of 33 m occurring

since the spring of 1994 at Bread Truck Gully (Fig.

23a). At each site, however, some parts of the

scarp crests did not retreat at all.

Extension of the hub to scarp distance was

measured where tension cracks developed at an

31
yy

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yyy

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y

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yy

y

yyyy

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0 5 10 m

yyy





May 1995 to Oct 1995


July 1994 to May 1995

N

c. Coastal 5 Gully.

d. Mortar Gully.

e. Coastal 6 Gully.

Figure 23 (cont’d). Summary of gully scarp recession.































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yy

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yyy

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yy

yyyy

yyyyyy

yyyyyyyy

yyyyyyyyyy

yyyyyyyyyyy

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N

0 5 10 m

May 1995 to Oct 1995


July 1994 to May 1995





yyy

yyy







Mortar


TBM





 



yy

y

























yy

yyy

yy













yyy

yyyyy

yyyyyy

yyyyyy

yyyyy



yyyyyy





yyy

y

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N

0 5 10 m

Nov 1994 to Oct 1995


July 1994 to Nov 1994 



yy

yy





32
yyy

yy

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yyyyyyyyyyy

yy

yyy

y

  yyy yyy 









yyyyyyyyyyyyyyyyy

yyyyy

yyyy

yy

N

0 5 10 m





yy

yy

yyyy

Current Surface


1994 Winter Ice-Push Ridge


May to Oct 1995


Nov 1994 to May 1995


July to Nov 1994

f. Coastal 1 Gully

g. Coastal 7 Gully.

h. River-North site.

Figure 23 (cont’d).

















yy

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yyyy

yyyyyy

yyyyyyyy

yyyyyyyy

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yy

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N

0 5 10 m

Nov 1994 to Oct 1995


July 1994 to Nov 1994 



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N

0 5 10 m

Nov 1995 to Oct 1995


July 1994 to Nov 1995





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33
a. North view showing extension crack in foreground.

b. Rotational slump blocks visible to right of scarp.

Figure 24. Measurements of bank erosion along Eagle River at the River-North site.

early stage of collapse (Fig. 24). This extension

was particularly evident in May of 1995 following the first tidal flooding cycle (15–17 May 1995)

after thawing of the ground began. A shallow

snow cover in the winter allowed frost to penetrate deeply. Intermittent flooding and subsequent pond drainage are favorable to ice growth

(Fig. 25). The subsequent freezing of interstitial

water and growth of needle ice, as well as ice

shove, probably reduced the cohesiveness of the

bank sediments and increased their susceptibility

to erosion during the May flood and ebb cycles.

The spatial variability in recessional rates is

evident in the plots showing the sequential recession of scarp crest location through time (Fig. 23).

One of the first gullies monitored at ERF was

34
Figure 25. Bank of Bread Truck Gully showing ice growth formation.

a. Lateral wall view. Ice armor is caused by repeated flooding and subsequent freezing in gully.

b. Headwall view showing ice coating formed by freezeup of waters flowing into gully.

35
a. Area A.

Figure 27. Aerial view of gullies where recession and ebb flow have been modified by surface craters.

Figure 26. Aerial view of Parachute Gully in October 1995 showing accelerated headward recession that

has accompanied capture of flow through craters.

36
Parachute Gully on the west side of Area C (Fig.

16). Since monitoring began in 1992, Parachute

Gully (Fig. 23b and 26) has undergone up to 12.1

m of headwall recession but only 1.8 m of lateral

recession (Table 8). Recession has centered around

a drainageway that has become incised and enlarged where multiple craters were once situated.

In this situation, the craters became hydraulically

connected by localized scour during ebb; the direction of gully extension is dictated by their presence. There was similar enhanced erosion along

drainageways in Area A (Fig. 27a) and B-Gully

(Fig. 27b).

Bread Truck and Coastal 5 gullies bracket the

mouth of the Eagle River and are the sites with

the maximum rates of headward recession (Fig.

16, 23a and c). Bread Truck Gully (Fig. 28 and

23a) was first monitored in 1993 when headward

recession ranged from 0.3 to 5.0 m and lateral

recession was on the order of 0.2 to 2.2 m (Table

8a). In 1995, headward recession accelerated to

11.7 m during the May 1995 flooding tide (representing the winter 94–95 erosion column of

Table 8a) and an additional 20 m during the

1995 summer season. These rates do not appear

to be exceptional or unique; Coastal 5 Gully

receded by 17 m during the summer of 1995

and a total of about 26 m since the hub and line

stakes were established in July of 1994.

Recession at the B-gully site, monitored since

1992, is the result of lateral erosion. The monitored section lies along the margin of a peninsula

b. B-Gully.

Figure 27 (cont’d).

37
between two tributaries to the main gully, about

75 m downchannel of the gully headwall. Gully

erosion is characterized by the formation of cuspate embayments that have gradually enlarged

since 1992, with a maximum of 5.3 m change (Table

8, Fig. 29). This site is significant because we have

repeatedly detected WP in transport within ebb

waters (Lawson et al. 1995), as well as in fauna

sampled within the gully (Bouwkamp 1995). The

source of this WP is unknown at this time; however, it is likely to have either eroded out of the

gully headwall or been flushed from C-Pond.

Data (Tables 8) indicate that, in general, lateral

recession is lower (often by an order of magnitude) than headward recession. The exceptions

are Mortar and Coastal 6 gullies,

where wide plunge pools below their

respective headwalls are expanding

laterally through bank collapse and

rotational slumping (Fig. 23d and 30).

Figure 23h shows one section of

the monitoring network at the RiverNorth site, where slope processes are

consuming mudflat sediments and

introducing material directly into the

Eagle River (Fig. 24). Rapid lateral

erosion on the order of 3.2 m was

recorded at this site between July and

November of 1994, and there was up

to 7.4 m of recession by late October

1995. Erosion occurs along large rotational slumps that fail because of

river undercutting and removal of

sediment supporting the toe of the

slope (Fig. 24b). Prior to failure, large

extension cracks develop parallel to

the scarp, gradually leading to

catastrophic failure (Fig. 24a). The

location of the crack becomes the new

scarp of the riverbank.

Recession rates are variable because the erosional processes are

rapid, short-duration events (Table

2). Recession appears to be caused

mostly by currents scouring the

lower, unvegetated portion of the

gully walls during ebb tide when water velocities are highest. Because the

uppermost 20–30 cm of material is

consolidated and root-bound (Fig.

30), this soil and root mat are undermined and only fail after an erosional niche of approximately 0.5 m or

deeper is cut below it. In the gullies

themselves, current scour during ebb tide removes

material from the toe of the slope, thereby removing the base of the slope and causing slumping

and sediment flow (Fig. 31).

Eroded sediments form deposits in the gullies

that are eventually transported into the Eagle

River. Along steep scarps, blocks of consolidated,

root-bound sediments fall and roll into the gully

bottoms and remain intact until currents eventually break them apart. The lateral walls of gullies,

which tend to develop low-angle slopes as headwalls recede inland, fail mainly by rotational slump

flow, or by creeping slowly as a mudflow into the

gully channel. On gentler slopes, blocks of rootbound material remain intact as they are transFigure 28. Aerial view of Bread Truck Gully in October 1995.

38
Figure 29. Eastward view of eroding

cuspate embayments at B-Gully.

Figure 30. Thick laminated peat layer at Coastal 6 forms the threshold to channel downcutting and

headward recession.

39
40

ported downslope by slow-moving mudflows that

are active in the latter stages of the ebb cycle.

Historical rates

Historical rates of gully extension by headward

erosion have been reconstructed for B, Parachute,

In-Between, Mortar, Bread Truck and Coastal 5

gullies (Fig. 32; Table 9). Long-term average recession rates over the past 45 years range from 3.6

m/year at Mortar Gully to 13.7 m/year at the

Coastal 5 Gully. Recession ranged from 1.1 to 3.5

m/year between 1950 and 1960 and increased to

between 2.1 and 14.7 m/year between 1960 and

1972. Recession continued to increase between

1972 and 1986, when rates ranged from 5.1 to 23.6

m/year. In the decade of 1986 to 1995, recession

rates have remained high but have in general

slowed slightly, ranging from 3.9 to 21.6 m/year.

The aerial photographic record suggests that

there was little physical change before the 1964

earthquake. At four of the six locations studied,

recession rates reached their peak between 1972

and 1986. The recent high rate of recession at InBetween Gully reflects a rapid, shallow distributary that advanced into the mudflat by about 40 m

during the 1992–93 winter (Lawson et al. 1996),

which increases the 9-year average by 4.4 m/year.

Therefore, the last decade of recession at In-Between Gully may be best represented by a rate of

2.9 m/year (Table 9). The other gullies that

have shown a recent acceleration of recession are those of Bread Truck, which has been

the site of considerable monitoring (see below), and Coastal 6. Recession at Bread Truck

Gully has been accelerating in an unpredictable manner as the gully headwalls have encroached on Bread Truck Pond and entered

into unvegetated mudflats that are intermittently ponded.

Recent vs. long-term

rates of recession

The modern rates of net headward and lateral recession measured using the hub and

line stake technique are depicted in Figure 33

to show the spatial patterns in gully erosion

along a transect from the mouth of the Eagle

River inland to the head of the Flats. Nearest

the coast, sites Coastal 1 and Coastal 7 have

experienced more headward than lateral recession. In the mid-coastal regions, Mortar

and Coastal 6 have experienced similar rates

of lateral and headward recession, with lateral erosion dominating slightly as these gullies have been widening by bank collapse and

slumping. Slightly inland from the mouth of

the river, Bread Truck and Coastal 5 gullies

have been experiencing drastic, yet rather

Figure 31. Sediment flow at Bread Truck Gully. Saturated sediment fails when erosion during ebb tide

removes material from the slope toe.
b. Mortar Gully.

Figure 32. Historical gully recession (see Table 9 for corresponding recession rates).

a. B-, Parachute

and In-Between

gullies.

41
c. Bread Truck Gully.

d. Coastal 5 Gully.

Figure 32 (cont’d). Historical gully recession (see Table 9 for corresponding recession rates).

42
symmetrical, retreat patterns governed by rapid

headwall recession. Moving further inland,

Tanker and In-Between gullies have also experienced unequal headward and lateral erosion and

recession. The rates of erosion at sites north of

Parachute Gully appear to be mirrored on either

side of the Eagle River. The reasons for this symmetry are unknown, but are probably related to

the nature of flow patterns during ebb drainage

and how effectively water is channeled into incipient feeder channels inland of the gully heads.

Comparison of the recent erosion (Table 8) and

historical data (Table 9) shows considerable annual variability over the short term, but over the

long term (approximately decades), rates appear

relatively consistent. The high rates of headwall

recession at Bread Truck (33 m in approximately

2 years) and Coastal 5 (26 m in approximately 1.5

years) are close to the 11-year average rates of

21.6 and 18.4 m/year, respectively, as determined

from historical photograph analyses (Fig. 32). Similarly, the headward recession of 12.1 m in 3 years

Figure 33. Spatial comparison of headward and lateral recession monitored at selected hub-line stake

erosion sites.

43
at Parachute Gully (Tables 8) is almost identical

to the 3.9-m/year average since 1986; the 1.6-m

rate observed at Mortar is lower than the 11-year

average of 3.9 m/year (Fig. 23d, 32b; Table 8b).

Since our hub and line stakes at B-Gully record

only lateral erosion, the current rates are not comparable to the 11-year average of 3.9 m/year of

headward recession.

Historical analysis

of the drainage system

The surface morphology and drainage patterns

of ERF have undergone significant change over

the last 45 years, with large scale changes particularly evident since the 1964 earthquake. The photographic coverage from 8 August 1950, 27 June

1953 and 30 August 1960 shows an apparently

stable environment prior to 1960. The only indicators of on-going change are the incised meanders of the Eagle River where vegetation was absent and some slumping was evident. Small,

low-relief channels drained into tidal gullies,

which were eroding slowly into the mudflats

(Table 9).

During the 1950s and early 1960s, the mudflat

areas of Racine Island were largely undissected

and had a mostly complete cover of vegetation

(interior sedge meadow; Racine and Brouillette

1995a). The C, C/D and D areas were mostly

vegetated (halophytic herb meadow; Racine and

Brouillette 1995a), except where dissected by narrow, relict drainageways that extended into freshwater marsh (bulrush) in the areas where the C-,

Bread Truck and Pond Beyond ponds now occur

(Fig. 3, 4 and 5). Open or standing water covered

the drainageways that feed Mortar, Bread Truck,

Parachute and B gullies in the early 1960s. In

Parachute and B gullies, open water covered what

now are gullies to within about 100 to 150 m of

the Eagle River. At Mortar and Bread Truck, the

narrow relict drainageways were arranged in a

dendritic pattern penetrating into emergent

sedge marsh (Racine and Brouillette 1995a). They

formed a tight drainage network along the

boundary of what is now Bread Truck Pond and

Pond Beyond, and where there is at present standing water. Relict drainageways directed water

flow through an abandoned meander present

in Bread Truck Pond. Small abandoned pools are

also apparent in several of these relict gullies in

locations where headwall recession is active

Open water first became evident on photos in

1967 in the areas near Clunie Creek and Clunie

Point (Fig. 5). There were also several small bodies of water in the C/D and D areas, where small

sedge bogs are now located. The major gullies of

Bread Truck and Mortar were also actively eroding into the mudflats by 1967 (Fig. 7, 32b and c;

Table 9). By 1974, areas in C-Pond, the channel in

Bread Truck Pond, and large relict drainages in

Pond Beyond and C/D-Pond had additional open

water and, by 1984, ponds had dimensions similar to the present. Pond expansion and gully

recession rates (Table 9) appear to have been greatest during the late 1970s and early 1980s at a time

when isostatic uplift rates were high (~1.5 cm/

year; Brown et al. 1977).

The Eagle River exhibited major changes only

where it enters ERF; these changes are probably

related to floods of the river, rather than tidal

inundation. In 1950, two well-developed channels, which bifurcated about 850 m southeast of

the Route Bravo Bridge, entered ERF (Fig. 34a).

The northern channel was straight and confined

by the uplands, while the south channel appears

slightly subordinate in discharge volume and

had a meandering pattern. Between 1957 and

1960, the south channel had eroded and captured

Table 9. Summary of long-term gully erosion rates

at selected sites.

Duration Distance Rate

Gully (years) (m) (m/yr)

B 1950–60 10 35.4 3.5

1960–72 12 70.8 5.9

1972–86 14 330.4 23.6

1986–95 9 35.4 3.9

Total 45 472.0 10.5

Parachute 1950–60 10 11.8 1.2

1960–72 12 82.6 6.9

1972–86 14 70.8 5.1

1986–95 9 35.4 3.9

Total 45 200.6 4.5

In-Between 1950–60 10 23.6 2.4

1960–72 12 99.1 8.3

1972–86 14 94.4 6.7

1986–95 9 66.1 7.3

Total 45 283.2 6.3

Mortar 1950–60 10 21.4 2.1

1960–72 12 25.2 2.1

1972–86 14 82.0 5.9

1986–95 9 35.3 3.9

Total 45 164.0 3.6

Bread Truck 1950–60 10 34.3 3.4

1960–72 12 34.3 2.9

1972–86 14 251.5 18.0

1986–95 9 194.3 21.6

Total 45 514.4 11.4

Coastal 5 1950–60 10 11.4 1.1

1960–72 12 175.8 14.7

1972–84 12 225.1 18.8

1984–95 11 202.6 18.4

Total 45 614.9 13.7

44
Figure 34. Aerial photographs depicting the morphologic changes in the Eagle River where it enters the

ERF.

1950 1960

1972 1993

45
all of the flow in the Eagle River; the north

channel was inundated only during tidal flooding (Fig. 34b). The southern channel remained the

principal discharge until sometime between 1967

and 1972, when flow was reestablished in the

north channel (Fig. 34d). Multiple channels then

remained active throughout the 1980s and 1990’s

(Fig. 34c); however, in September 1995, flooding

of the Eagle River resulted in abandonment of the

southern channel as the northern channel was

deeply scoured and eroded eastward into the upland bluffs.

Natural attenuation

by pond drainage

Given the combined short-term and long-term

recessional data, we revise our previous prediction of how long it will take for the ponds to be

drained naturally (see Lawson et al. 1996). These

refined recessional data (Tables 8 and 9) indicate

that Bread Truck pond will start to drain in 2 to 3

years or less. In about 6 to 7 years, In-Between

Gully will drain parts of C and Lawson’s ponds,

and in about 14 to 16 years, Parachute Gully will

drain parts of C-Pond (Fig. 35a). Previously, more

limited short-term data suggested that that Bread

Truck Pond would begin to drain in 35 to 50

years, while the In-Between and Parachute gullies would begin to drain C-Pond in 17 to 20 years

and 25 to 30 years respectively (Fig. 35b). The

drastic change in our predictions reflects the increase in data obtained on the highly variable

short-term rates of erosion, and the longer-term

historical rates. The historical data average the

variability seen in the short-term observational

data and are considered more accurate for predicting future gully behavior.

Gullies are routes for pond outflow and therefore potential pathways for WP transport. As the

gullies continue headward erosion into WP-con0 200 400 m

Erosion Site


Predicted Erosion


Direction


Relict Drainage

E a g l e

R i v e r

N

Pond Beyond

Bread Truck


Pond

C Pond

MG

BT

PC

BG

2-3 Years

6-7 Years

14-16 Years

0 200 400 m

Erosion Site


Predicted Erosion


Direction


Relict Drainage

E a g l e

R i v e r

N

Pond Beyond

Bread Truck


Pond

C Pond

MG

BT

PC

BG

35-50 Years

17-20 Years

25-30 Years

a. 1995. b. 1994.

Figure 35. Predictions for pond drainage by selected gullies in Areas C, Bread Truck and C/D.

46
taminated areas, fresh sediment will be exposed

and introduced into them, potentially increasing

the amount of WP in ebb discharge. Gully erosion

may also threaten the integrity of other remedial

methods. For example, capping measures (i.e., geotextiles and AquaBlock) may be undercut and

damaged. Similarly, hydrological and physical

changes to the ponds and mudflats may result

from dredging and cause rapid extension of the

gullies into the dredged areas.

SEDIMENTATION AND

NATURAL ATTENUATION

OF WP BY BURIAL

Suspension settling through flocculation takes

place during slack high tide and through the early

stages of the ebb cycle. In ponded areas, it also

happens between tidal floods when turbulence is

low, allowing fine-grained particles to settle out

of the water. Therefore, the longer calm conditions can be maintained, the greater is the time

over which deposition can occur. The mixing and

exchange of tidal flood water with existing pond

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

provides the primary source for deposition.

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

traps sediment (e.g., Reed 1995), and the frequency, height and duration of inundations by

sediment-laden waters (e.g., Vince and Snow 1984;

Reed 1989, 1995; Reed and Cahoon 1992; French

and Spencer 1993). The distance from the source

also plays a role in some areas, as sediment can be

deposited or eroded during flood and ebb, altering the amount in suspension. Large amounts of

sediment may be deposited locally from ice rafting of materials plucked from other locations

within the Flats (Fig. 36).

Tidal inundation

Several factors (see Lawson et al. 1996) influence the timing, magnitude and duration of tidal

flooding and runoff, which in turn control sediment influx into ponds. The typical tidal flooding

cycle begins with a rise in the water level in the

Eagle River and the gullies. This water gradually

floods over the banks of the gullies and river,

a. Sediment layer frozen onto an ice block transported during the November 1994 flooding tides.

Figure 36. Ice rafting at ERF.

47
b. Ice-rafted sediment deposited at the head of Mortar Gully.

Figure 36 (cont’d). Ice rafting at ERF.

displacing the water over a large area, and causing a decrease in the rate of water level rise (Fig.

37). River waters are dammed, causing the river

to reverse its direction in the upper Flats (Fig.

38). This damming effect causes the elevation of

the river to rise higher than the elevation of water

in the gullies and reverses flow direction in the

drainage systems. The effect may be accentuated

during peak discharge of the river. Water depths

measured at the hydrostations and the tide gauge

in Knik Arm show that the peak elevation on the

Flats exceeded the predicted tidal height for Anchorage by up to 0.7 m throughout the 1995 summer season (Fig. 39). The effect is to increase the

volume of water and hence sediment available

for sedimentation.

At peak flood or slack high tide, water is

ponded upon the Flats. The mixing of flood waters with the existing pond and marsh waters

increases the amount of suspended sediment. In

those ponds such as Racine Island Pond, which

have the highest proportion of fresh river water

inundation, suspended sediment concentrations

are less than where tidal sources dominate.

Flood waters first begin to drain near the coast

at the start of ebb tide, and water levels inland

progressively decrease. The duration of outflow

during ebb is greater than inflow during flood.

Pond sedimentation results mainly from settling-out of sediment suspended in the water column; sediments are deposited in ponds and mudflats during and immediately following slack high

tide (Fig. 37b). Its duration and the height and

volume of the flood water therefore determine

sedimentation rates.

The length of time available for sedimentation

during tidal inundation is directly related to peak

elevation and each landform’s threshold for flooding. The time between the start of water rise and

peak high tide was measured for each gully

hydrostation and compared to the peak water

elevation at each site. We found a linear relationship between water rise time and peak elevation

(Fig. 40). Average elevations for tidal flooding

across the Flats were estimated from surveyed

transects (Fig. 9). These data indicate that most

drainageways feeding ponds begin to flood at a

tidal height of about 4.6 m above sea level, mud48
Runout

Slack


High


Tide

Peak

Elevation

Time

Optimum


Sedimentation


Conditions

Sedimentation Decreasing with Time

River

(16 May ‘95)

20

10

0

Temperature (°C)

100

0

OSB (mV)

0.3

0.2

0.1

0

Spec. Cond.

(mS/cm)

5.5

5.0

00:00 12:00 24:00

Elevation (m)

b. Schematic showing a typical tidal

cycle and period of sedimentation

during the ebb tide event.

Figure 38. Water quality data

from the Eagle River site showing the effect of water damming

caused by a flooding tide.

B Gully

(14 Jun ‘95)

30

20

10

Temperature (°C)

1000

500

OBS (mg/L)

0

20

10

0

Salinity (‰)

5

4

00:00 12:00 24:00

Elevation (m)

Start of

Mudflat Flooding

a. B-Gully on 14 June 1995.

Figure 37. Example of a flooding tide cycle. Note that rate of tidal rise decreases following bankfull stage when

water begins to flood adjacent mudflats.

49
6

4

2

24 Aug ‘95 31 7 Sep 14

Parachute Gully

Pressure

Sensor

Elevation (m)

Anchorage Predicted

High Tide

Levees (5.21 m)

Mudflats (4.87)

Ponds (4.6)

Figure 39. Water elevation record showing elevations required to flood selected landforms and

tidal amplification at Parachute Gully above predicted high tide at Anchorage.

160

120

80

40

0

Water Rise Time (min)

3 0 3 5 4 0 4 5 5 0 5 5

Bread Truck Gully

R = 0.96487 2

R = 0.953314 2

160

120

80

40

0

Water Rise Time (min)

3 0 3 5 4 0 4 5 5 0 5 5

In-Between Gully

160

120

80

40

0

Water Rise Time (min)

3.0 3.5 4.0 4.5 5.0 5.5

Peak Water Elevation (m)

B Gully

(8 points off graph)

R = 0.166957 2 Figure 40. Relationship between water

rise time and peak water elevation at

several gully sites. Note that x-axis for

Racine Gully differs from other graphs.

50
160

120

80

40

0

Water Rise Time (min)

3.0 3.5 4.0 4.5 5.0 5.5

Parachute Gully

R = 0.912567 2

160

120

80

40

0

Water Rise Time (min)

12 3 45

Peak Water Elevation (m)

Racine Gully

(7 points off graph)

R = 0.255974 2

Figure 40 (cont’d).

flats are inundated at 4.9 m and levees are covered at 5.2 m (Fig. 39). Only at Racine Island was

there a difference; here, survey data indicate a

4.35-m threshold for pond flooding at the south

end. The former 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. 41). Estimates

of how often each landform is flooded in a given

time period can be based upon these elevations.

The timing of flooding is controlled by the elevation of the levees and mudflats surrounding

the gullies, the distance of the gully headwall

from the coast, and the 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 show that tidal flooding is best described

by a polynomial regression (Lawson et al. 1996).

Pond drainage is restricted by gully parameters (cross-sectional area, channel roughness and

drainage density) that limit the volume of water

that can escape from the pond. This produces

a bottleneck effect where water in the pond remains dammed despite turbulent, fast flowing

conditions in the gully heads. The response is a

nonlinear relationship between water elevation

Duration of Flood (min)

Racine Gully

480

240

0

Parachute Gully

480

240

0

B Gully

480

240

0

In-Between Gully

480

240

0

Peak Water Elevation (m)

Bread Truck Gully

480

240

0

4.4 4.8 5.2 5.6 6.0

Figure 41. Tidal flooding time as a function of peak

flood elevation at selected gullies.

51
4000

2000

0

4 May ‘95 3 Jun 3 Jul 2 Aug 1 Sep 1 Oct 31 Oct

TSS

(mg/L)

Coast

River

Figure 43. Coastal vs. river TSS measurements. The large spike in mid-September indicates a flooding event in

the Eagle River watershed.

and runoff time (Fig. 42), which, in effect, extends

the time for sedimentation. Drainage is incomplete between consecutive tides exceeding 5.0 m,

furthering the time available for sedimentation.

Sediment transport

and sediment sources

Sediment transported in suspension from Knik

Arm is the primary source for ponds, and it varies seasonally as well as during single tidal cycles

(from flood to ebb) (Lawson et al. 1996). TSS measurements in Knik Arm exceed those at the river

(which are quite low) throughout the year and

show a general increase in sediment concentration through the summer season (Fig. 43). In contrast, TSS concentrations vary seasonally in the

Eagle River, being relatively high through June

and September, with peaks in mid-June and early

August during snowmelt in the mountains and

720

600

480

360

240

120

0

3.0 3.5 4.0 4.5 5.0 5.5

Peak Water Elevation (m)

Runoff Time (min)

Bread Truck Gully

maximum glacial runoff (Fig. 43; Lawson et al.

1996). The higher sediment concentrations in Knik

Arm reflect the high sediment influx from the

large glacierized basins of the Susitna, Matanuska

and Knik rivers (Fig. 1). The increase in TSS values in Knik Arm during fall and winter probably

reflect the effects of colder water and increased

salinity (both of which affect its viscosity). In the

Flats, reduced discharge in fall and winter from

the Eagle River minimizes dilution by fresh water

with a much lower sediment concentration, allowing more saline water with a greater sediment

concentration to enter the ponds and mudflats.

Sediment flux into and out of the ponds and

mudflats varies monthly as tidal height, flood

duration and runoff time vary (Fig. 44). TSS concentrations vary among the gullies through a

single tidal cycle and reflect distance inland from

the coast, as well as spatial variations in runoff,

Figure 42. Runoff time vs. peak water

elevation at Bread Truck Gully. The

change in slope reflects the effect of

pond drainage where runoff is extended

once ponds are flooded.

52
pond storage capacity and the capacity of the gully

to drain water from the ponds and mudflats. These

differences in sediment flux into and out of the

ponds and mudflats result in a spatial variability

in deposition, which is compounded by the effects

of localized differences in elevation and vegetation.

A typical pattern in TSS response, as measured

at the heads of gullies through a flooding cycle, is

a sharp increase because of the influx of highly

turbid flood water from Knik Arm and resuspended gully bed sediment (Fig. 45). TSS values

remain high through slack high tide and the start

of ebb. TSS then abruptly decreases during ebb

runoff. The duration of ebb runoff depends on

the ability of the gully or network of gullies to

transfer water stored in the ponds and mudflats.

The greater the volume of water stored in an area,

the longer it will take for gullies to effectively

drain the ponds. The decrease in suspended sediment concentration during ebb reflects the loss of

material by deposition on mudflats and in ponds,

as well as the mixing of the tidal waters with

sediment-poor pond and river waters.

Landform sedimentation rates

Sedimentation rates vary with morphology and,

in a general sense, elevation (Table 10). The overall trend is an increase in rate from levees to

mudflats, to ponds, to marshes and to gully bottoms, with the more heavily vegetated areas of

mudflats having higher rates of accumulation than

those that are not vegetated. Typical annual sedimentation ranges from about 10 to 30 mm among

the various landform types.

Net accumulation is 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 probably

Figure 44. Maximum monthly TSS

comparison at all sites. Dashed lines

connect data from pond sites.

1600

1200

800

400

0

6

5

4

3

Elevation (m)

TSS

(mg/L)

6 Oct ‘94 7 Oct

Figure 45. Typical TSS variation

through a flooding tide.

A Pond

In-Between Gully

B Gully

Bread Truck Pond

C Pond

Mortar

Parachute

Racine Pond

Racine Gully

River

+ Coast

+

+

+ +

+

+

4000

3000

2000

1000

0

TSS

(mg/L)

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

53
Winter ‘94-’95

Summer ‘95

– 20 0 20 40 60

Sediment Accumulation (mm)

Avg. Sedimentation

for Time Period

Individual Sedimentation Measurements

Figure 46. Net seasonal accumulation rates at sites spray-painted in the C and D areas.

Table 10. Gross sedimentation rates.

Range (mm)

Sum. Win. Sum. Win. Sum. Win. Sum.

Morphological unit 1992 1992–93 1993 1993–94 1994 1994–95 1995

Levee 1–6 1–12 0–6 0–11 0–15 0–30 0–17

Vegetated mudflat 1–6 1–16 1–14 1–21 1–30 0–31 0–13

Unvegetated mudflat 1–13 7–12 1–8 6–10 1–17 3–21 0–17

Pond—plate 1–4 8–28 6–26 2–17 1–21 1–16 1–37

Pond—cup 2–9 8–26 6–19 1–13 2–20 9–40 1–39

Marsh ND ND ND ND 1–20 2–10 1–13

Gully 1–77 20–33* 0–19 0–16 1–19 3–50 6–60

* Based on small sample size.

Measurements for seasons through winter 1993–94 are from transect lines 1–12 only, Summer

1994 and 1995 measurements from lines 1–24 (Fig. 9).

reflects their respective length of measurement,

the seasonal increases in TSS within the source

waters during the early winter months (Fig. 44),

the trapping of sediment by the snow cover, and

the number of flooding events. Net accumulation

measurements from the paint-layer technique vary

seasonally for the periods of September 1994

through May 1995 (winter) and May 1995 through

September 1995 (summer) in Area C and Area D

mudflats and levees (Fig. 46). Most winter values

fall in the range of 2 to 18 mm (average 10 mm),

while the summer values range between –2 and

+14 mm (average 6 mm).

The relationship of sedimentation rate to elevation reflects the number of times the sites are

inundated. The number of measured flooding

events exceeds that predicted from tidal elevations at Anchorage because of several factors,

including tidal amplitude caused by the geometry of Knik Arm (e.g., Syvitski et al. 1987, p. 163),

river discharge, ice cover and wind. There was

significantly more tidal flooding at each hydrostation site during both 1994 and 1995 than predicted by using the Anchorage datum (Table 11).

Given this scenario, floods should far exceed that

number again in 1996.

During summer, about twice as many floodings

occur than predicted, but fewer take place in winter, presumably because of the seasonal reduction in river discharge (Table 11). Flooding of the

various landforms is therefore also affected. The

number of inundations is highest for ponds, being less on mudflats and levees, thereby correlating with the general decrease in sedimentation

rates that characterizes this respective sequence

of landforms (Fig. 47).

The amount of vegetation covering the mudflats

also influences the sediment accumulation rates

(Fig. 48). Gross sedimentation rates from 1992 to

1994 had total accumulations of 6–18 mm at sites

54
Table 11. Comparison of predicted and measured number of flooding events

reaching critical heights during summer 1994 and summer 1995.

Number of flooding events

Summer 1994 Summer 1995 Summer 1996

Critical height Predicted Measured Predicted Measured Predicted

Inundate ponds (4.6 m) 16 52 33 38 44

Cover mudflats (4.87 m) 8 18 10 22 25

Cover levees (5.21 m) 0 4 0 1 2

Summer 1994 is 22 May–15 September; summer 1995 is 5 May–11 September; predicted means

based on Thompson tide table at Anchorage; measured means based on depth transducer at

Bread Truck gully; time periods are intervals between sedimentation measurements.

with less than 60–65% vegetative cover, and 26–

32 mm at sites with a vegetation cover of greater

than 70%.

Sedimentation rates also vary with distance

from source waters. A decrease in sedimentation

rate with distance inland is illustrated by transect

16 data (Fig. 49a), which extends southward from

Knik Arm into coastal Area A (Fig. 9). The decreased velocities of Knik Arm waters during

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

The same effect is often seen along the scarp of

many gullies where flow conditions change rapidly as flood waters overtop their banks. Sedi40

30

20

10

0

Sediment Accumulation (mm)

Pond Vegetated

Mudflat

Unvegetated

Mudflat

Levee

Summer ‘92

Winter ‘92-’93

Summer ‘93

Winter ‘93-’94

Summer ‘94

Winter ‘94-’95

Summer ‘95

Figure 47. Variations in sedimentation rate

as a function of landform type.

35

30

25

20

15

10

5

0 20 40 60 80 100

Vegetative Cover (%)

Sediment Accumulation (mm)

Figure 48. Relationship between total sediment accumulation and vegetation cover at

sites sprayed with paint in 1992 and measured in the fall of 1994.

55
mentation rates decrease with distance from the

Eagle River inland toward the marshes adjacent

to the uplands (Fig. 49b). We also observed high

accumulation rates at a low elevation oxbow area

on the north end of Racine Island that floods more

frequently than adjacent mudflats (Fig. 49c).

Pond transects and stations

Pond sedimentation transects were newly established in 1995 in Bread Truck, C/D, C,

Lawson’s, A and Racine Island ponds (Fig. 12).

The plates and cups were measured in September

at each station on these transects, except in C/D,

where only cups could be measured. Sediment

that accumulated at the new pond sites during

this first summer was liquefied and contained a

high organic fraction that we did not differentiate

from the mineralogical fraction. At sites where an

algal bloom was present in the water column

above the measuring devices, we either gently

removed or moved the material enough to see the

sediment surface and obtain the measurement.

50

40

30

20

10

0 100 200 300 400

Distance (m)

Accumulation (mm)

Fall ‘94

Spring ‘95

Fall ‘95

S N

500

50

40

30

20

10

0 200 400 600 800

Distance (m)

Accumulation (mm)

Fall ‘94

Spring ‘95

Fall ‘95

E W

50

40

30

20

10

0 100 200 300 400

Distance (m)

Accumulation (mm)

Fall ‘94

Spring ‘95

Fall ‘95

N S

500

Figure 49. Sediment accumulation along mudflat lines sprayed with paint in early summer

1994 (see Fig. 9).

a. Transect line 16.

b. Transect line 14.

c. Transect line 19.

56
Accumulation (mm) 40

20

40 80 120 160 200

Distance (m)

North

0

40

20

0

Cup

West Plate

Bread Truck Pond

Accumulation (mm)

40

20

40 80 120 200 240

Distance (m)

0 160

West

CD Pond

Cup

Accumulation (mm) 40

20

40 80 120 200 240

Distance (m)

North

0 160

40

20

0

Cup

West Plate

C Pond

Accumulation (mm) 40

20

20 40 60 80 100

Distance (m)

North

0

40

20

0

Cup

West Plate

Lawson’s Pond

Accumulation (mm) 40

20

40 80 120 160 200

Distance (m)

North

0

40

20

0

Cup

West Plate

A Pond

Accumulation (mm)

40

20

0

Cup

West (cross line) Plate

Racine Island Pond

40

20

50 100 150 200 250

Distance (m)

West (deep channel)

0

Figure 50. Pond sedimentation measured along transects from May to September 1995.

a. Bread Truck Pond.

b. C/D Pond.

 c. C-Pond.

d. Lawson’s Pond.

 e. A-Pond.

 f. Racine Island Pond.

Overall, accumulation amounts were consistent along the transects, ranging from 5 to 20 mm

(Fig. 50). There were only a few anomalously

high readings among the otherwise consistent

data, which we believe were probably the result

of human or wildlife-induced disturbance.

Gross sedimentation amounts as measured by

the cups, without the anomalously high data

points (Table 12), indicate that Lawson’s Pond,

C/D Pond and C Pond have higher averages than

the other three ponds (Fig. 51). Lawson’s Pond

and C/D Pond have less open water and more

57
vegetation to help trap sediment. Net sedimentation rates measured by the plates show that Lawson’s Pond has the highest average, but with a

large variability. The three ponds—Bread Truck,

Racine Island and C—had similar net rates of accumulation, while A-Pond accumulations were less.

Longer-term net accumulation rates over 3

years were measured at the isolated sedimentation stations (Fig. 13 and 52). At these sites, the

material in about the upper 5 mm is liquefied,

while the lower portion of the accumulated sediment has undergone dewatering and compaction,

resulting in a consistency that resists penetration

with a millimeter rule. Accumulation amounts

were somewhat higher in C Pond than in Bread

Truck Pond, but in both cases, amounts appear to

Table 12. May to September 1995 sediment accumulation along pond sedimentation transects.

Sediment accumulation (mm)

All measurements Without disturbance

Cup Plate Cup Plate

Pond Avg Std Avg Std Avg Std Avg Std

Bread Truck 5.0 2.2 4.2 1.9 — — — —

C/D 17.7 8.0 — — 15.6 4.2 — —

C 11.9 7.7 8.2 7.8 9.2 2.3 5.4 2.4

Lawson’s 12.3 9.3 14.3 10.9 9.5 4.1 11.7 8.3

A 7.7 7.8 3.7 5.0 4.6 1.5 2.3 0.9

Racine Island 4.3 2.3 4.7 1.5 — — — —

Left columns include all measurements; right omits sites with disturbance.

Accumulation (mm)

20

15

10

5

0

Bread

Truck

C/D Pond C Pond Lawson’s A Pond Racine Is.

Fall ‘95

Cup Measurements

20

15

10

5

0

Bread

Truck

C/D Pond C Pond Lawson’s A Pond Racine Is.

Fall ‘95

Plate Measurements

Accumulation (mm)

a. Fall 1995 cup measurements.

b. Fall 1995 plate measurements (no measurements made at C/D Pond).

Figure 51. Average sediment accumulation from May to September 1995 at pond sedimentation station transects,

without data points suspected of disturbance. One standard deviation is shown with error bars.

58
be such that, in another 6 to 9 years, the upper

uncontaminated sediment should provide a barrier deep enough to keep dabbling ducks from

reaching contaminated material beneath it.

WHITE PHOSPHORUS FATE,

TRANSPORT AND MIGRATION

WP-contaminated sediment is eroded and

transported from the ponds, mudflats and gully

walls by a variety of processes operating throughout the year (Tables 2 and 3). The processes are

primarily wind, river and tidal currents, and various types of slope erosion. Animals, waterfowl

and humans may likewise disturb and mobilize

WP in ponds.

Both ice and water entrain and transport WPcontaminated sediments. Wind and water currents in ponds can resuspend WP-contaminated

sediments, as evidenced by analyses of resuspended materials in sediment traps. Gully transport during ebb moves these sediments into the

Eagle River. Samples of material trapped by plankton nets record WP-contaminated sediment transport during ebb tide. Analyses of samples from

sediments adhering to ice floes are evidence that

ice can erode and transport WP-contaminated

sediments.

Field sampling of sediment in transport in a

natural setting, rather than in a controlled laboratory, is difficult in the intertidal environment and

was not possible in Eagle River or Knik Arm during tidal ebb. Given the fine particle size of both

the suspended and bed loads, it is likely that these

sediments are transported into Knik Arm where

their fate is unknown. Whether WP erosion, transport and redeposition constitute a quantitatively

important biologic hazard cannot be assessed because of the limited number of samples acquired

during this study and the generally random nature of WP occurrence.

We gauged WP migration by analyzing material in sediment traps within contaminated permanent ponds, and by analyzing the materials

collected in plankton nets from the bedload and

suspended load of gully runoff from the ponds

during ebb tide. Plankton nets in the centers of

gullies provide the only data available on transport and migration of WP into the Eagle River

and Knik Arm. The sediment traps in ponds provide data on localized WP movement by resuspension. Sediment trap and surface samples,

which were taken in previous years (Lawson et

al. 1995, 1996), provide data on mudflat scour of

WP during runoff. Ice samples were obtained in

1994, but weather conditions in late 1995 and early

1996 did not produce ice conditions suitable to

floe analyses. Previous work indicated that ice

floes cause WP erosion and transport (Lawson et

al. 1996).

Plankton nets in four gullies draining contaminated ponds and mudflats were used to trap sediments during the monthly tidal flooding events

of May through October (Fig. 17 and 21b). Of 138

samples, WP was detected in 44 (32%), indicating

WP transport during ebb tide in all tidal flooding

events exceeding 5.1 m elevation (Fig. 53). This

relationship to elevation is consistent with the

more limited 1994 results (Lawson et al. 1996).

The quantities detected in these samples were

typically less than 0.1 μg/g. The B-Gully site is

the only exception to this trend, with values ranging from less than 0.1 to about 0.9 μg/g. The

reasons for this difference are not clear; however,

the upper reaches of B-Gully are undergoing lateral and headward erosion into a heavily cratered

and highly contaminated part of the mudflats (Fig.

27) (Racine et al. 1993, Racine and Brouillette

1995b). It is therefore possible that the plankton

40

30

20

10

0

Accumulation (mm)

S3 S7

S8

S9

S5

S6

C Pond BT Pond

Figure 52. Fall 1995 measurements of

accumulation at sedimentation stations

established in 1992.

59
Table 13. 1995 positive WP results from pond sedimentation station cup samples.

Time Area Station no. Conc. (μg/g)

May B-Pond S-18 0.0671

C-Pond S-3 Pan N 0.0175

S-3 Pan S 0.0320

S-8 0.0225

S-9 0.0260

Bread Truck Pond S-6 0.0268

Sept. Lawson’s Pond LP-4 0.0008

Bread Truck Pond BT-1 0.0003

BT-6 0.0008

BT-9 0.0023

Racine Is. Pond RI-2 0.0203

net samples are recording the erosion of these

WP-bearing sediments and that the higher concentrations reflect the lack of mixing and dilution

that occur during longer distance transport.

Sediments trapped in cups on pond transects

revealed some internal movement of WP as the

result of resuspension of pond and mudflat materials during both the winter and summer seasons

(Fig. 12 and 13; Table 13). Analytical results were

less than 0.1 μg/g, but quantities detected were

generally greater in May samples of “winter” activity than in September samples of the “summer” season. The six WP detections in the May

analyses were from a total of 12 trap samples,

whereas the September analyses were from 90

samples. These detected values are consistent with

those of pond sediment traps that were analyzed

previously (Lawson et al. 1996).

In summary, the plankton net samples show

that water is actively transporting WP in the gullies. The origin of the WP is probably contaminated sediments released by erosion of gully walls,

or by resuspension and transport of pond and

mudflat sediments of the C, Bread Truck, Lawson’s

and C/D ponds. About one-third of the samples

acquired during tidal events exceeding 5.1 m in

1995 contained WP, similar to 1994. Previous

samples in 1993 from the Bread Truck and Parachute gullies proved negative for WP, but were

collected during tides of less than 5.03 m when

the potential for erosion and transport were less.

Resuspension of WP during winter and summer tells us that we need to consider remobilization and potential movement of WP to uncontaminated areas. Data are, however, limited and

the amount of WP migrating within and out of

ERF cannot be quantified.

Further measurements of summer and winter

transport should be made. The high level of WP

found in a B-Pond sample this year is a good

Figure 53. Relationship between WP detected in plankton net samples

and peak elevation of flooding event sampled. Open circles are samples

in which no WP was detected. White Phosphorous Concentration (μg/g)

0.2

0.1

0

0.2

0.1

Elevation (m)

4.5 5.0 5.5 6.0

0.9

0

B Gully

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.2

0.1

0

Bread Truck Gully

4.5 5.0 5.5 6.0

0

In-Between Gully

Parachute Gully

60
example of why such monitoring is important;

the B-Area has been considered clean of WP

and this sampling result shows that WP migration can spread contamination within the Flats.

KNIK ARM AND WP DEPOSITION

The Knik Arm near the mouth of the Eagle

River is 9–10 km wide and has large tidal cycles

(±9 m). At high tide, Knik Arm fills and spills

over into ERF. Currents are relatively weak by

comparison to those at low tide. At low tide, this

portion of Knik Arm essentially becomes a braided

stream system with a main channel located directly off the coast of ERF. Rapid flow in this

channel produces highly turbulent conditions with

large standing waves. As tidal flooding begins,

conditions change rapidly, with transitory areas

of severe turbulence and scour and back-eddies

with minimal turbulence—conditions enhancing

sedimentation.

Bathymetric analyses of Knik Arm offshore of

ERF and near the mouth of Eagle River in 1994

(Lawson et al. 1996) provided data that are integral to identifying potential locations where WP

may be deposited after migration out of ERF

proper. These data showed various locations in

Knik Arm, such as intertidal bars and other

areas, where sedimentation is active. Such areas

are migratory and can change extent and location

over time.

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 54. Bathymetry of Knik Arm near

the mouth of the Eagle River and adjacent

to the coast of ERF.

61
These areas in Knik Arm are therefore sites

where samples of sediment should be examined

for WP. They include two intertidal bars, a bar at

the mouth of the Eagle River, and nearshore

zones north and south of the Eagle River (Fig.

54). Because they are periodically exposed, these

sites may be locations where receptors can find

WP. Further analyses are required to evaluate

whether WP is being preserved in these deposits,

and, further, whether the dynamics of Knik Arm

can cause potential exposure of WP to waterfowl

or other receptors. Repeated bathymetric profiling could also define net rates of sedimentation

or erosion in Knik Arm.

CONCLUSIONS AND

RECOMMENDATIONS

Conclusions

The results of the 1995 and previous years’

investigations lead to several important conclusions:

N

0 500 1000 m

yy

y

Q¢Q¢

Q¢Q¢QQ¢¢Q¢

Q¢

y

y



yy 

yy 

yy

y

yyyy

y

yy

y

yy





y

y

y

yyy

y

yy



yyy

y

yy

y

y

y

Q¢

Q¢ QQ¢¢

Q¢Q¢

Q¢

Q¢Q¢Q¢QQ¢¢QQ¢¢QQ¢¢

Q¢Q¢Q¢QQ¢¢QQ¢¢

Q¢Q¢Q¢

Q¢Q¢

Q¢Q¢QQ¢¢QQ¢¢QQ¢¢

Q¢Q¢

Q¢

Q¢Q¢QQ¢¢QQ¢¢

Q¢

Q¢Q¢Q¢QQ¢¢Q¢QQ¢¢QQ¢¢

Q¢QQ¢¢Q¢Q¢Q¢

Q¢Q¢Q¢Q¢Q¢Q¢

QQ¢¢Q¢

Q¢Q¢

Q¢

Q¢

Drainage and Containment


Natural/Enhanced Sedimentation and Burial


Natural Gully Drainage and In Situ Attenuation


Combination of Natural Sedimentation and Drainage


WP Removal


Ponds

Figure 55. Recommendations for site remediation.

0 500 1000 m

Drainage and Containment

Natural/Enhanced Sedimentation and Burial

Natural Gully Drainage and In Situ Attenuation

Combination of Natural Sedimentation and Drainage

WP Removal

Ponds

62
1. Physical system processes can produce a

natural attenuation or in-situ degradation and removal of WP from a significant portion of the

ERF ecosystem (Fig. 55).

2. Gully erosion and headwall recession will

begin to drain large areas of contaminated ponds

in about 1 to 10 years, potentially resulting in insitu WP degradation and attenuation from drying; it is a cost-effective alternative to artificial

remediation (Fig. 35a). Historical photographic

analyses, field data and process analyses indicate

that Bread Truck Pond will probably begin draining in 1 year; C/D-Pond, Lawson’s Pond and a

large area of C-Pond will begin draining in 10 to

15 years or less.

3. Sedimentation rates appear sufficient to bury

WP in certain ponds and can reduce the risk of

feeding waterfowl being exposed to WP contamination.

4. Natural sedimentation, perhaps artificially

enhanced in some ponds by flocculation or other

means, is a cost-effective alternative that removes

WP from feeding waterfowl through burial. Areas of C/D-Pond, Lawson’s Pond and A-Pond

are sites where sedimentation and burial are important in reducing exposure (Fig. 55). Sedimentation and burial may be the best solution in isolated and remote ponds where other techniques

would be too costly. Erosion and recession of gullies draining C/D-Pond and Lawson’s Pond may

subsequently drain these areas and produce permanent remediation.

5. Ice growth in ponds can be sufficiently thick

that freezing extends into the sediments of the

pond bottom. In so doing , it is possible that large

areas of sediment, including those contaminated

with WP, will be pulled from the bottom when the

ice cover is lifted by incoming tides or wind. Ice

floes have been observed with dimensions of over

7 to 8 m square, onto which sediments of up to 30

cm thick were frozen. Wind and water currents

move these floes throughout ERF during the flooding tide and move them out into the Eagle River

and Knik Arm during ebb tide. Both ice floes with

pond sediments adhering to them and deposits of

sediments from such ice were sampled; WP was

detected in several of these samples. It is, therefore, possible that large areas of contaminated sediments (hot spots) could be entrained and transported by ice floes to other areas of ERF, as well as

into Knik Arm.

6. Ice and water entrain and transport WP-contaminated sediments within the ponds, mudflats

and gullies of ERF. Wind and water currents in

ponds can resuspend WP contaminated sediments,

as evidenced by analyses of resuspended materials in sediment traps. Gully transport during ebb

moves these sediments into the Eagle River.

Samples of material trapped by plankton nets

record WP-contaminated sediment transport during ebb tide. Analyses of samples from sediments

adhering to ice floes are evidence that ice can erode

and transport WP-contaminated sediments. Field

sampling of sediment in transport in a natural

setting, rather than in a controlled laboratory, is

difficult in the intertidal environment and was not

possible in Eagle River or Knik Arm during tidal

ebb. Given the fine particle size of both the suspended and bed loads, it is likely that these sediments are transported into Knik Arm where their

fate is unknown. Whether WP erosion, transport

and redeposition make up a quantitatively important biologic hazard cannot be assessed because of

the limited number of samples acquired during

this study and the generally random nature of WP

occurrence in ERF.

7. Racine Island Pond has neither high gully

erosion and recession rates, nor high sedimentation rates. It also floods at a relatively low tidal

elevation (4.35 m). Because the pond bottom sediments are rich in organics, a longer period is required for drying. Therefore, it appears that Racine

Island can be effectively remediated and readily

restored through artificial drainage or pumping

of the pond, following construction of a temporary berm to contain the former pond area and

permit long-term drying to degrade the WP. By

removing the berm after in-situ degradation is

complete, the pond environment can be restored.

Recommendations

Based upon our investigations in 1995 and previous years, we recommend the following (Fig.

55):

1. Cost-effective remediation can be accomplished across a large area of ERF by allowing the

physical system to remove or isolate WP contamination.

2. WP contamination of Bread Truck Pond and

50% or more of C-Pond, including Lawson’s Pond,

should be treated by natural (or enhanced) drainage and in-situ WP degradation by drying of the

former pond bottoms.

3. Sedimentation and burial of WP will, in certain ponds, effectively remove it from feeding

waterfowl over the short term. Over the long term,

burial will reduce waterfowl mortality during

natural pond drainage, especially in parts of C63
Pond, Lawson’s Pond and C/D-Pond. It should

be considered a method of natural attenuation,

particularly in isolated, small, remote ponds where

other methods are too costly.

4. Racine Island Pond should be remediated by

artificial gully extension, pond drainage and

pumping, and long-term containment with a temporary berm to permit in-situ WP degradation by

drying. By removing the berm, the pond environment can be naturally restored.

5. Gully erosion and recession, pond sedimentation, pond drainage, and ground water levels

and WP degradation and attenuation should continue to be monitored to ensure that remediation

is taking place as predicted by the physical system analyses.

6. WP migration and contamination in Knik

Arm should be evaluated, focusing on sampling

areas of the nearshore zones and mid-Arm bars

where there is a potential for WP exposure to

receptors.

7. The potential natural attenuation of WP

as the result of mechanical abrasion during transport by gully and tidal currents should be examined.

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Physical Processes and Natural Attenuation Alternatives for Remediation of

White Phosphorus Contamination, Eagle River Flats, Fort Richardson, Alaska

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

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road CRREL Report 96-13

Hanover, New Hampshire 03755-1290

U.S. Army Alaska

Environmental Resources Department

Directorate of Public Works

Fort Richardson, Alaska

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Erosion Pond drainage Tidal flat processes

Natural attenuation Sedimentation White phosphorous

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

This report describes the results of investigations into the role of tidal flat physical systems in the natural

attenuation of white phosphorus (WP) contamination in Eagle River Flats (ERF) on Fort Richardson, Alaska.

Waterfowl feeding in ponds and marshes here ingest the WP and die. These investigations found that natural

attenuation and in-situ degradation of the WP could result from certain physical phenomena operating within

the ERF ecosystem. Specifically, the on-going erosion and headward recession in the gullies will drain large

areas of contaminated ponds in an estimated 1 to 10 years. Lowering of water levels should lead to in-situ WP

degradation and natural attenuation as pond sediments dry. Annual sedimentation rates in some ponds and

marshes are sufficient to bury WP in several years or more and thereby reduce the exposure to feeding

waterfowl. Ice and water are also effective transporters of WP, moving it about ERF and into Eagle River and

eventually into Knik Arm where its fate is unknown. Certain areas of ERF will require artificial drainage, but

natural conditions can be restored following treatment. Recommendations are made for the use of natural

attenuation and additional studies that are required to ensure the successful clean-up of ERF.