Ice Jam Flooding on the Missouri River

Near Williston, North Dakota

James L. Wuebben and John J. Gagnon September 1995

CRREL REPORT 95-19
For conversion of SI units to non-SI units of measurement consult Standard

Practice for Use of the International System of Units (SI), ASTM Standard E380-

93, published by the American Society for Testing and Materials, 1916 Race St.,

Philadelphia, Pa. 19103.

Cover: Spring breakup on the Yellowstone River near the confluence with the

Missouri River.

Abstract

This investigation focused on ice-related flooding along the Missouri River, just

below the confluence with the Yellowstone River near Williston, North Dakota.

This area is at the upper end of Lake Sakakawea. With the closure of Garrison

Dam in 1953, Lake Sakakawea began filling, reaching operational levels in

1965. Changes in the hydraulics, sedimentation and ice regime of the Missouri

River caused by the impoundment have led to an increase in the potential for

overbank flooding. This report describes the ice regime assessment that was

conducted to characterize ice jam flooding, the development of a method to

predict the potential for ice jam occurrence and severity, and potential flood

mitigation measures.
CRREL Report 95-19

Ice Jam Flooding on the Missouri River

Near Williston, North Dakota

James L. Wuebben and John J. Gagnon September 1995

Prepared for

U.S. ARMY ENGINEER DISTRICT, OMAHA

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
ii

PREFACE

This report was prepared by James L. Wuebben, Research Hydraulic Engineer, and

John J. Gagnon, Civil Engineering Technician, of the Ice Engineering Research Division,

Research and Engineering Directorate, U.S. Army Cold Regions Research and Engineering Laboratory. Funding was provided U.S. Army Engineer District, Omaha, under reimbursable work order ENH 2654, Ice Jam Flooding Problem in the Vicinity of the BufordTrenton Irrigation District near Williston, North Dakota.

The authors thank Stephen L. DenHartog and Kathleen White of CRREL, and Roger

L. Kay and Dwight Olson of the Omaha District for technically reviewing the report.

They also thank David Deck, formerly of CRREL, and Kevin O’Brien of USAED, Omaha

Williston Resource Office, for their assistance in field data collection.

The contents of this report are not to be used for advertising or promotional purposes. Citation of brand names does not constitute an official endorsement or approval of

the use of such commercial products.
iii

CONTENTS

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

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

Site description ...................................................................................................................... 1

Background ............................................................................................................................ 2

Hydraulics ......................................................................................................................... 2

Hydrology .......................................................................................................................... 2

Sedimentation ................................................................................................................... 4

Field data collection .............................................................................................................. 5

Historical information ..................................................................................................... 5

Winter 1992 field observations ....................................................................................... 7

Analysis................................................................................................................................... 9

March 1986 ice jam simulation ....................................................................................... 10

Ice-affected water levels .................................................................................................. 12

Ice jam potential ................................................................................................................ 14

Mitigation techniques ........................................................................................................... 16

Nonstructural techniques ................................................................................................ 17

Structural measures .......................................................................................................... 20

Emergency measures ....................................................................................................... 23

Mitigation techniques for the Buford–Trenton Irrigation District ............................ 23

Summary and conclusions ................................................................................................... 24

References ............................................................................................................................... 25

ILLUSTRATIONS

Figure

1. Location of the Buford–Trenton Irrigation District ................................................... 1

2. Details of the Buford–Trenton Irrigation District ...................................................... 3

3. Flow and stage record during the 1992 breakup ...................................................... 9

4. Longitudinal profile of a typical breakup ice jam..................................................... 10

5. Simulation of the longitudinal profile of the March 1986 ice jam in the Middle

Bottom area .............................................................................................................. 11

6. Simulation of the longitudinal profile of a fully developed ice jam throughout

the Buford–Trenton area ........................................................................................ 11

7. Rating curves for open water, ice-covered and ice-jammed conditions ............... 13

8. Map of Beaurivage River ice cutting ........................................................................... 18

9. Isometric drawing of the proposed Cazenovia Creek ice control structure ......... 20

10. Plan view of the Allegheny River ice boom ............................................................... 21

TABLES

Table

1. Missouri River discharge frequencies below the Yellowstone River

confluence ................................................................................................................ 3

2. Missouri River breakup discharge frequencies below the Yellowstone River

confluence ................................................................................................................ 4

3. Area flooded by different water stages for West Bottom and Middle Bottom .... 4

4. Analysis of ice jam potential for the Missour River near the Buford–Trenton

Irrigation District .................................................................................................... 15

5. Ice jam potential rating factors..................................................................................... 16

6. Summary of costs for existing ice boom installations .............................................. 21

Figure 1. Location of the Buford–Trenton Irrigation District.

INTRODUCTION

The Buford–Trenton Irrigation District is

located along the Missouri River about 15 miles

upstream of the city of Williston, North Dakota,

on the extreme western edge of the state. This

area is at the upper end of the Garrison Dam–

Lake Sakakawea Project within the Omaha District of the U. S. Army Corps of Engineers. With

the closure of Garrison Dam in 1953, Lake Sakakawea began filling, reaching its maximum normal pool elevation of 1850 feet above mean sea

level (msl) in 1965. Changes in the hydraulics,

sedimentation and ice regime of the Missouri

River caused by the impoundment of Lake Sakakawea have led to a rise in groundwater levels

and an increased potential for overbank flooding

in the Buford–Trenton Irrigation District. This report describes an ice regime assessment that was

conducted to characterize ice jam flooding in the

vicinity of the Buford–Trenton District and to

identify potential flood mitigation measures.

SITE DESCRIPTION

The Buford–Trenton Irrigation District (hereafter called the “District”) was constructed in the

early 1940s with joint assistance from the Bureau

of Reclamation and the Department of Agriculture. The District is located approximately 170

miles upstream of Garrison Dam, at the upper

end of Lake Sakakawea and just downstream of

the mouth of the Yellowstone River (Fig. 1).

The District is divided by bends in the MisIce Jam Flooding on the Missouri River

Near Williston, North Dakota

JAMES L. WUEBBEN AND JOHN J. GAGNON

Trenton

Buford

West

Bottom

Middle 

Bottom

Zero

Bottom

Williston

Missouri

River

Yellowstone

River

N

Sakakawea

Lake

East

Bottom

Lake

Trenton

Subject to

Controlled Inundation
souri River into four areas: East, Middle, West

and Zero Bottoms (Fig. 1). The following project

description was taken from Design Memorandum No. MGR-146 “Buford–Trenton Irrigation

District: Backwater and Drainage Problems”

(USACE 1978):

The project consists of a pumping plant, the main

canal and lateral system and all other features

needed to deliver water to each farm unit. This

also includes the drainage system carrying the

return flows from the irrigated land back to the

Missouri River and Lake Trenton. When fully developed the project consisted of about 16,800

acres of which 10,000 were irrigable. In 1958 the

Corps acquired the East Bottom for the Garrison

Dam-Lake Sakakawea project... This reduced the

total acreage of the District to about 10,100 acres

and the irrigated acreage to about 7100 acres.

BACKGROUND

The Missouri River above Williston has a

drainage area of approximately 164,500 square

miles, with roughly 70,000 square miles contributed by the Yellowstone River and 90,000 square

miles by the Missouri above the confluence with

the Yellowstone. The Missouri River discharge

below the confluence, based on daily mean values, ranges from about 3,000 to 22,000 cfs during

the fall freezeup and mid-winter periods. Mean

flows for the months of December through February range from about 10,000 to 11,000 cfs. At a

flow of 10,000 cfs, the Missouri River has a water

surface width on the order of 500–1000 ft, a thalweg depth of 10–20 ft and water velocities of 1–3

ft/s. Water surface slopes are relatively flat

(0.00002 or less) below the Route 85 bridge (cross

section 1552.7). Above the Route 85 bridge the

water surface slope is on the order of 0.00011,

while on the lower Yellowstone River it is about

0.00018.

Hydraulics

The hydraulic analysis portion of this investigation relied heavily on the HEC-2 Water Surface

Profiles computer program (USACE 1990) with

the ice cover analysis option. This option provides the user with the capability to determine

water surface profiles for streams with stationary

floating ice covers. In addition, a utility program

called ICETHK (Wuebben and Gagnon, in prep.)

was employed to facilitate the use and interpretation of the HEC-2 ice option.

Verified, open-water HEC-2 data sets, obtained from the Omaha District, were employed

as the base for the ice analysis. Since the field data

collection program indicated that significant ice

events were driven by ice breakup on the Yellowstone River and that the ice on the Missouri River

above the confluence typically ran several weeks

later, the Yellowstone River HEC-2 data file was

merged with the Missouri River data file at the

confluence. The Missouri River above the confluence was not included in the model, except as a

tributary source of water inflow.

Figure 2 shows the locations of cross sections

used by the Omaha District to monitor sediment

aggradation. These cross sections also correspond

to some of the cross sections used to evaluate

water surface profiles along the rivers using the

HEC-2 computer program (USACE 1990). However, the HEC-2 data file contained several cross

sections intermediate to those ranges, as well as

cross sections farther downstream on the Missouri

and upstream on the Yellowstone. The HEC-2 cross

section numbers correspond to 1960 Missouri

River mileage.

The modeled area ranged from cross section

number 1497.11, which is roughly 50 miles below

Williston and well into Lake Sakakawea, to

1594.38, about 10 river miles upstream of Fairview,

Montana, a distance of about 100 miles. Points of

interest include the Route 85 bridge at 1552.70,

Hurley Bend at approximately 1570, Ryder Bend

near 1578 and the Yellowstone River confluence at

roughly 1582.

Hydrology

An analysis of open-water conditions along the

Missouri River from Fort Peck Dam to Garrison

Dam was conducted by the Omaha District

(USACE 1978). USGS discharge records are available for the Missouri River at Williston for the period from 1929 to 1965, when it was discontinued.

The flow record was extended through 1975 by

transposing the combined flow records for the

Missouri River at Culbertson, Montana, and the

Yellowstone River at Sidney, Montana. The results

of an annual peak discharge–frequency analysis

based on those records are summarized in Table 1.

The results of the 1978 discharge–frequency analysis have since been extended through 1984, again

using the Culbertson and Sidney gage data. These

values, also presented in Table 1, show that extending the record has resulted in a lowering of predicted discharge values, ranging from about 5% at a

two-year event to 9% at the 500-year event. Both

sets of values are presented for comparison, since

an analysis of peak discharges occurring during

2
3

Figure 2. Details of the Buford–Trenton Irrigation District. The cross sections are the locations used by the

Omaha District to monitor sediment aggradation. The locations where ice bridged across the river on 7 March

1992 are also shown.

the month of March contained in the 1978 report

(USACE 1978) will be used later in the report.

While these discharge values are appropriate

for determining open-water flood flows, the analysis of spring-breakup-related flooding requires

information on flow magnitudes during past

breakup events. Unfortunately the actual flows

or even the exact dates when ice cover breakup

and ice jamming have occurred in the past are

unknown. In the absence of more detailed historical information, spring breakup flows were taken to be the first major peaks in flow occurring

during March or early April. These events may

not accurately represent the date of actual ice cover breakup events and their discharge magnitudes, but they will provide a conservative estimate for use in further analysis.

The estimated breakup discharge frequencies

presented in Table 2 were developed by ranking

these combined flows, plotting them on log-probability paper and fitting a curve by eye. The period of record ranged from 1966, after Lake Sakakawea reached its maximum normal pool elevation,

through 1990. For comparison, discharge–frequency values for peak flows in March developed by the Omaha District (USACE 1978) are

also included. The March values are somewhat

higher than the estimated breakup discharges

since they consider the maximum discharge in

the calendar month rather than the first signifiTable 1. Missouri River discharge

frequencies below the Yellowstone River confluence.

Return interval Discharge* Discharge†

(years) (cfs) (cfs)

2 95,000 90,000

5 130,000 120,000

10 160,000 150,000

25 210,000 190,000

50 260,000 251,000

100 340,000 300,000

500 440,000 400,000

* After USACE (1978).

† Based on data provided by Roger L. Kay,

Omaha District, June 1992.

N

Ice Bridges

1E 1F

1582.07

1583.07

1584.17

1581.31

1580.77

1579.45

1578.03

1576.38

1575.68

1574.74

1574.16

1570.30

1569.24

1568.19

1566.39

1565.44

1564.48

1563.22

East

Bottom

Lake

Trenton

Sakakawea

Lake

Trenton

Buford

Zero

Bottom

West

Bottom

Middle 

Bottom

Williston
4

Table 2. Missouri River breakup

discharge frequencies below the

Yellowstone River confluence

(USACE 1978).

March

Return Interval Discharge discharge

(years) (cfs) (cfs)

2 32,000 36,000

5 67,000 80,000

10 92,000 125,000

25 130,000 170,000

50 160,000 220,000

100 195,000 280,000

discharge relationships at critical points along the

river. These relationships are used to establish design elevations for irrigation control structures. In

a report dealing with the design of channel blocks

to prevent Missouri River water from backing up

into the main drainage ditches of the Irrigation

District (USACE 1978), the Omaha District estimated that the long-term sediment inflow for the Missouri River at Culbertson, Montana, averaged

13,500,000 tons per year, and for the Yellowstone

River at Sidney, Montana, the average was

41,500,000 tons per year. The suspended sediment

at the Culbertson gaging site averaged 45% sand,

50% silt and 5% clay. For the Yellowstone River at

Sidney, the percentages were 35, 60 and 5%, respectively. Bed material at Culbertson had a mean

grain size (D50) of 0.28 mm, while at Sidney the

D50 was 0.25 mm. Based on an average deposition

density of 70 lb/ft3, the measured sediment inflow

rate between 1964 and 1975 was 260,000 acre-ft, or

about 23,600 acre-ft/yr.

The aggradation analysis contained in the

USACE (1978) report is being updated based on

surveys of sediment deposits made through 1989

and a review of sediment transport data. Preliminary results of a study by the U.S. Geological Survey on the Missouri River at Bismarck, North Dakota, indicates that the procedures previously

used by the Corps of Engineers for collecting and

analyzing sediment load data at Bismarck, and

possibly elsewhere, may have overpredicted sediment loads by an average of 30%. The updated

aggradation analysis, described in a draft report

(USACE 1992), found that approximately 486,000

acre-ft of sediment were deposited in the reach between the confluence and Tobacco Garden Creek

(river mile 1512) between 1956 and 1988, or about

15,200 acre-ft/yr.

The deposition of sediment in the calmer headwaters of Lake Sakakawea has resulted in a progressive loss of channel capacity and an upward

shift in the stage–discharge relationship for the

Missouri River in the Buford–Trenton area. The

Omaha District (USACE 1992) found that from

1965 through the mid-1970s, Missouri River

stages for a discharge of 40,000 cfs had shifted upwards by 2.5, 3.3 and 5.0 ft at water level gages 5A,

6 and 7, respectively. Water level gage 5A is located

just below the Yellowstone River confluence, near

HEC-2 cross section 1581.31. Water level gage 6 is

adjacent to the West Bottom at cross section

1576.38, while gage 7 is adjacent to the Middle Bottom at cross section 1566.39.

Since the mid-1970s, aggradation in the vicinity

Table 3. Area flooded by different water stages for West Bottom

and Middle Bottom. Missing values were not reported in the original reference.

Elevation West Bottom Middle Bottom

(ft msl) (acres) (acres)

1858 — 0

1859 — 50

1860 0 1450

1861 — 1950

1862 200 2400

1863 — 2800

1864 450 —

1865 700 —

1866 1000 —

1868 1800 —

1870 3500 —

cant discharge peak. Further, the March discharge frequencies were developed using a logPearson Type III distribution rather than a

log-normal distribution, and the period of record

analyzed covered 1929–1975. As discussed previously, when the all-season discharge–frequency

relationship was extended to cover the period

from 1929 to 1984, the predicted ten-year discharge value was reduced by about 5%.

In their 1978 analysis of flooding problems

within the Buford–Trenton Irrigation District

(USACE 1978), the Omaha District estimated the

area flooded by different water stages for the

West Bottom and Middle Bottom areas. These

were based on stage–discharge rating curves at

HEC-2 cross sections 1563.5 and 1570.0 and topographic maps of the area. The results of this

analysis are presented in Table 3.

Sedimentation

The Omaha District investigated channel

aggradation in the Missouri River at the upper

end of Lake Sakakawea to predict future stage–
of the Irrigation District has continued at a rate of

about 1 ft per 6–7 years. Total stage increases from

1965 to 1988, for a discharge of 40,000 cfs, have

been 4.6, 5.2 and 6.6 ft at Gages 5A, 6 and 7, respectively. For the period from 1988 through 2055, an

additional 5–10 ft of deposition is expected to occur in the reach from the confluence of the Missouri and Yellowstone Rivers downstream

through the Irrigation District to river mile 1530.

Deposition depths in excess of 30 ft can be expected farther downstream. Stages, for a discharge of

40,000 cfs, are expected to rise at an average rate of

1 ft per 20 years at Gage 5A, 20 years at Gage 6,

and 17 years at Gage 7. Total stage increases between 1990 and 2055 would be about 2.7, 3.2 and

3.8 ft, respectively, for Gages 5A, 6 and 7.

FIELD DATA COLLECTION

The objectives of the field program were to

monitor ice, weather and runoff conditions in order to anticipate potential ice problems during the

spring of 1992, and to collect additional data necessary to identify potential short- and long-term

flood mitigation measures. The area studied extends along the Missouri River from Lake Sakakawea to the railroad bridge approximately 3

miles upstream of the Fort Union National Historic Site, and along the Yellowstone River upstream

to Glendive, Montana.

In addition to direct measurements and observations, a significant component of the data collection involved interviews with local residents to

obtain their recollections and opinions on ice-related flooding in the Buford–Trenton area. The

first several site visits concentrated on gathering

historical information through such discussions,

while the remaining site visits focused on documenting winter ice conditions.

Historical information

Ice-related flooding tends to be local and highly

site specific. While ice jams may be relatively common at a given site, they cannot be predicted with

certainty in any given year, and they may be totally absent at other sites nearby. Further, ice jams

often occur when flow rates are relatively low,

perhaps no more than a 0.5 exceedance probability discharge,* and water levels are normally high

only in the vicinity of the ice and in a backwater

zone upstream. Their relatively small geographic

extent (perhaps a few river miles) and short duration (from a few hours to a few days for breakup

events) make it unlikely that detailed field information will have been gathered at most sites.

Even in cases where hydrographic gaging records

exist for a site, ice effects on the gage rating curve,

the location of the gage relative to the ice accumulation, the potential for gage freezeup because of

cold weather, or direct ice action on the gage can

reduce its reliability for ice events.

Without prior field observation it is difficult to

predict where, or even if, ice jams will form along

a river. Because ice jams are site specific, it is generally not possible to extrapolate from stage data

for other sites along the river. Hence, in an analysis of ice-related flooding it is often necessary to

resort to other sources of historical data, sources

that are often overlooked or regarded as unreliable for the analysis of open-water flooding. During this study, no significant ice run or jam

occurred, and a series of interviews with local

residents comprised virtually the only source of

information on ice jam processes in the area.

Freezeup

A primary question asked in regard to freezeup was whether the river froze in a manner similar to a lake or a puddle, in which the ice thickens

gradually and smoothly, or whether floes came

floating down the river and accumulated to form

a cover. A common response was that the river

froze in place, leaving a smooth cover, but others

recalled seeing numerous floes accumulating to

form a rough cover.

Winter ice conditions

Ice conditions were reported to be generally

smooth with some rough areas. Ice thickness estimates were generally on the order of 2–2.5 ft, although some residents reported thicknesses as

high as 3 and 4 ft on occasion. Ice growth calculations based on thermal growth of single-layer ice

indicate that the ice thickness in this area might

reach 2.5 ft about once in five years, and 3 ft less

than once in 50 years. The 50% exceedance ice

thickness would be just over 2 ft. It is possible that

these thicker estimates were made in areas where

ice floes have, in some years, accumulated to

form a multi-layer cover.

Breakup

It appears that, in most years, breakup on the

Missouri River in the vicinity of Williston, North

Dakota, is driven by events on the tributary Yellowstone River. In response to warmer weather

5

* This is new terminology corresponding to a recurrence interval of two years.
6

and increasing runoff, the ice on the Yellowstone

begins to break up and run several weeks prior to

breakup on the Missouri. The breakup of the Yellowstone River then proceeds downstream in a

series of ice jamming and release events. Eventually the breakup front on the Yellowstone reaches

the Missouri and proceeds farther downstream

through the area of the Buford–Trenton Irrigation

District towards Williston. During an ice run, it

was reported that very large ice floes pass down

river. One resident commented that the ice floes

typically appeared to be 2.5–3 ft thick and

“...gym-size in area.”

The ice on the Missouri River upstream of the

confluence area typically remains in place for approximately two weeks after the Yellowstone River runs, in large part due to the small, steady discharges maintained by the release schedule of

Fort Peck Dam in Montana. While spring runoff

on the Yellowstone River rises to values on the

order of 20,000–40,000 cfs, the Missouri River at

Fort Peck is typically held below 10,000 cfs until

after the Yellowstone River flood peak has

passed.

Ice jam locations

A number of residents commented that ice

jams in the vicinity of the Buford–Trenton Irrigation District form in the same locations year after

year but with varying severity. Since the ice normally starts running (and jamming) on the Yellowstone River two or three weeks before the ice

run begins on the Missouri River, ice runs from

the upstream portion of the Missouri River were

felt to be of little consequence to the Buford–Trenton area.

Once the ice run on the Yellowstone River

reaches the Missouri River, it often jams in the

confluence area. This jam causes few problems in

the District. As on the Yellowstone River, however, the ice marches downstream in a series of jam

and release events. Once a jam in the confluence

area releases, subsequent jams are likely to occur

in the vicinity of Ryder Point on the West Bottom

(between cross sections 1576.38 and 1578.03) and

the Hurley Bend in the Middle Bottom (cross section 1569.24). Ice jams were reported to be normally between 0.5 and 2.5 miles in length, but as

long as 4 miles on occasion.

One resident, Clarence Johnsrud, also mentioned a former jam site between these two locations, in the bend immediately upstream of the

Hurley Bend, but he stated that this ceased to be a

problem when this bend was isolated by a manmade channel cutoff in 1958. This cutoff is located

between cross sections 1569.24 and 1574.16. One

of the prime locations for ice jams to form is at a

transition from a steeper to a milder-sloped reach.

Such a cutoff channel would have significantly

increased the slope within its limits but led to a

significantly greater reduction in slope when the

ice reached the natural channel in the Hurley

Bend area. The cutoff channel could exacerbate

the potential for ice jams in that lower reach.

Ice jam events

At times the dates of ice jam events estimated

by local residents differed by a year or so. For example, several persons mentioned a relatively

large event in the spring of 1951. However, Bob

Bearce (a resident of the West Bottom) said that he

knew this event had occurred in 1952 because the

river was flooded on 28 March 1952—the day his

son was born. In a subsequent discussion, Clarence Johnsrud (a resident of the Middle Bottom)

also recalled that the event was in 1952, not 1951.

This example shows that some uncertainty is to

be expected, especially for events 30–40 years

ago. Therefore, the dates of the ice-related events

cited below may not be exact, and not all events

may have been recorded.

There were six reported jams in the last 40

years. The earliest was in 1952, the most recent in

1986. Based on six jams in 40 years, the jamming

frequency would be 6/40 = 0.15, or about once in

seven years. Since the backwater condition

caused by the formation of Lake Sakakawea can

have a significant impact on ice jam formation

and since some long-past events may not have

been recorded or recalled, we might instead use

only the last 20 years of record. In that case, jam

frequency would be 5/20, or once in four years.

Reported years with flooding included the following.

1952. According to Roger Bearce, the ice jam

flood in 1952 covered much of the West Bottom.

Prior to breakup the measured ice thickness was

approximately 32 in., and there had still been

roughly 10 in. of snow on the ground one week

prior to breakup (USDOC 1953). In a review of the

floods of 1952, the Weather Bureau (USDOC 1953)

indicated that the peak stage of 17.76 ft at the

Williston Gage occurred at 0820 on 1 April but

that the ice was still moving. Flood stage for this

gage was set at 20 ft. The stage had receded to

15.8 ft by 1400 but then once again rose to 16.9 ft

by 1630 on the same day in response to an ice jam

below the gage.

USGS records for 1 April 1952 show a mean

daily flow of 124,000 cfs, but it is uncertain what
7

discharge was present while the jam was in place.

The fact that the highest stage occurred while the

ice was observed to be in motion may either indicate that the peak flow was due to the release of

an upstream ice jam, in which case the flow situation was analogous to a dam-break wave, or possibly that there was an unrecorded ice jam downstream of the gage.

1972. An ice jam in March caused water to back

into the main drains, flooding most of the Middle

Bottom and half of the West Bottom. Thirty-one

families had to move out for five days.

1975. An ice jam event occurred at some unrecorded time during the spring. Also, in June the

highest open-water flood levels experienced

since closure of Garrison Dam occurred.

1976. Ice scars found on trees on the Floyd Ryder property in the West Bottom (near HEC-2

cross section 1577.15) were thought by local residents to have been made during an older event,

which perhaps took place in 1976. The scars were

approximately 9 ft above the water surface on 26

June 1990 (determined using a hand level). Based

on a HEC-2 simulation of the water surface profile for that day, the tree scars would be at an approximate elevation of 1870 ft msl. However, this

elevation would also correspond to computed

water levels during the 1986 event, making the

true date of the scars uncertain without a tree ring

analysis.

1978. High ice jam flood levels occurred in

1978. According to Clarence Johnsrud, “Larsen

lost out.” The Larsen property was located on the

Middle Bottom. Clarence Johnsrud also recalled

that although there was substantial overbank

flooding, most of the ice remained in the channel

in the Middle Bottom reach of river during this

event.

1986. The most recent ice jam event occurred in

1986. During this event, water almost overtopped

a lateral in the West Bottom near HEC-2 cross section 1578.03. A local resident recalled that it took

about 11 hours from the time the initial jam

formed until water reached the top of the road.

Before overtopping the road on top of the lateral,

however, the jam released. The ice then passed

downstream and re-jammed in the Hurley Bend

area in the Middle Bottom. Tree scars found in

this area, which local residents suggested might

have been from the 1976 event, were at an elevation of approximately 1870 ft msl. The road on top

of the lateral in this area has comparable elevations, suggesting that the tree scars may have

been from the 1986 event.

On 1 March 1986, water was said to have

reached the top of the Borlaug Bridge on the Middle Bottom, which corresponded to an elevation of

1870 ft msl. This location coincides approximately

with HEC-2 cross section 1567.44. (A Corps of Engineers survey in the spring of 1992 indicated that

the top of road at this site, as defined by a photograph of the event, was about 1868.5 ft msl).

According to Bob Bearce, a resident of the Middle Bottom, the jam extended from approximately

halfway between HEC-2 cross sections 1564.48

and 1565.44 to about cross section 1569.24. This

would give a jam length of about 4.2 miles. He

estimated that ice floes at this site were about 18

in. thick.

During June 1990, photos were taken of severely scarred trees at the Bauste property, which is

downstream of the Hurley Bend on the Middle

Bottom. Since that time, most of these trees have

been cut down. The scars were very regular in

both their orientation to the river and in the elevations of the top of scarring between numerous

trees as determined by hand level. The regularity

in scar orientation and elevation suggest damage

by ice (or perhaps debris) rather than by animals.

The scars extended from approximately 4 to 6 ft

above the ground surface on a relatively level terrace near HEC-2 cross section 1564.48. The top elevation of the tree scars was approximately 1860 ft

msl, which is slightly below the ground elevation

at the corral buildings on the property. Local residents thought that the scars were from the 1986 ice

breakup event, and water during that event was

said to have come near to a corral building at the

site.

Winter 1992 field observations

The plan for the 1991-92 winter field program

included a series of site visits to document ice

thickness and type, ice bridging and jamming

locations, and other site characteristics necessary

for the formulation of ice-related flooding mitigation techniques. To anticipate changing ice conditions or possible ice breakup events, we also monitored weather conditions and forecasts at Williston and several sites in Montana for any indication of an increase in runoff that could initiate

breakup on either the Yellowstone or Missouri

Rivers.

Winter ice conditions

Observations during 1992 showed that, with

rare exceptions, the ice in the study area was a

smooth, single-layer ice sheet not unlike that
kawea to the confluence with the Yellowstone River was essentially clear of ice. Exceptions included

some small areas of shore-attached ice in shallow,

slow-moving reaches and some ice floes held in

place by bridge piers. Locations where ice remained bridged across the river on the morning of

7 March were recorded from the air by Kevin

O’Brien of the Williston Resource Office (Fig. 2).

While these bridging locations were remnant ice

locations rather than ice jams, they do indicate locations of reduced ice transport capacity. Further,

there were remnant ice bridges at two known jam

sites, the confluence area and near the Bauste

Ranch on the Middle Bottom.

Only provisional, uncorrected discharge information for the 1991-92 winter is currently available from the U.S. Geological Survey (USGS). The

Omaha District’s Missouri River Bulletin lists the

gage on the Missouri River at Culbertson, Montana, as ice affected through 6 March. According

to the USGS records, discharge peaked at 17,500

cfs on the Missouri River at Culbertson at 2300 on

5 March, while the gage on the Yellowstone River

at Sidney, Montana, peaked at 18,000 cfs at 1400 on

the same day. Mean daily discharges for Culbertson and Sidney on 5 March are listed as 17,200 and

8790 cfs, respectively.

Since the travel time for water passing each of

these gages to reach the Buford–Trenton area is on

the order of one day, these values can be used to

approximate discharge on the Missouri River below its confluence with the Yellowstone River.

While not exact, the flow reaching the District can

be estimated by adding the mean daily values for

each of the two upstream gages and adding one

day to the date. Recognizing this assumption, and

the fact that the flow data are still provisional, we

could estimate that the Missouri River flow

peaked sometime during the sixth of March and

that the mean daily value was on the order of

26,000 cfs. Based on the monthly discharge–frequency analysis conducted by the Omaha District

(USACE 1978), a flow peak of 26,000 cfs in the

month of March would be exceeded during

roughly seven out of ten years, making this a relatively common flow event.

Figure 3 shows the estimated flow record based

on combining the records from the gages at Culbertson and Sidney, Montana, as described above.

Since the flow records are provisional and uncorrected for ice effects, the sharp reductions in discharge on the sixth and tenth of March may be

due, at least in part, to the ice cover going out near

the gages at Sidney and Culbertson, respectively.

8

formed on a lake. In some areas the ice was clear,

black ice that was apparently formed in place. In

other areas the ice cover was made up of large

pans of ice that had formed elsewhere, floated

downstream and gently accumulated through juxtaposition to form a single layer of floes, which

subsequently froze together to form a smooth ice

cover. Near the Fort Union trading post on the

Missouri River there was some rough or lumpy

ice, but it was unclear whether this resulted from

upturned frazil pans or the remnants from beaver

trapping activity.

By the end of January, both the Missouri and

Yellowstone Rivers were almost completely ice

covered, with an average thickness of 18 in. and a

range of measured thickness from 13 to 21 in. On

the Yellowstone River at the Route 23 bridge near

Sidney, Montana, there had been a small breakup

ice run, and the ice had gouged the left bank of the

river. Due to subsequent warm, sunny weather

and a lack of snow, these ice conditions degraded

such that by the first of March there were extensive

open-water areas on the Yellowstone, and the ice

that remained was significantly thinner and very

weak due to internal decay by solar radiation. The

maximum measured ice thickness at this time was

13 in. The reduced thickness and areal extent of the

ice cover meant that lesser ice volumes would be

available to accumulate in an ice jam on the Missouri, and what ice remained would be quite

weak. It was clear that if conditions continued as

they were, the volume and strength of the ice cover

would make severe ice jamming and flooding unlikely. Even if the ice cover did not melt out prior to

the arrival of the spring runoff, it would have little

capacity to form significant jams.

Breakup

In early March, in cooperation with Kevin

O’Brien of the Corps of Engineers Williston Resource Office (WRO), several local residents were

enlisted to notify us if the ice began to move. On 3

March 1992, a small, peaceful ice run was observed

on the Yellowstone River upstream of Sidney,

Montana, at the Seven Sisters Boat Landing. Ice

downstream from this point on the Yellowstone

and Missouri Rivers was extremely rotted, and

numerous open leads had formed with little apparent increase in runoff.

During the morning of Saturday, 7 March 1992,

the Williston Resource Office relayed a message

from one of the river observers, Delbert Dishon,

that ice had begun to move in the confluence area.

By Sunday the Missouri River from Lake Saka-
9

26 × 103

16

18

20

22

24

140 144 148 152 156 160

Julian Day (water year)

Discharge (cfs)

Elevation (ft)

1848

1847

1846

Flow

Stage

Figure 3. Flow and stage record during the 1992 breakup.

Figure 3 also shows stage data for the water level

gage located at the Route 85 bridge over the Missouri River. This gage (located at HEC–2 cross section 1552.70) shows an increase in stage during the

decrease in discharge beginning on 6 March,

which suggests a short-term ice stoppage or jam

downstream. The record also reveals a drop in

stage of over 3 ft after 9 March, reflecting the release of a jam or loss of ice cover in reaches farther

downstream. No gage records were available between this gage and the confluence area. Lake

Sakakawea water levels during the breakup period varied gradually from 1820.8 ft msl on 3 March

1992 to 1821.2 ft msl on 9 March 1992.

The ice cover in the Buford–Trenton area went

out prior to any large increase in water discharge,

reflecting a loss of ice strength. No significant ice

accumulations were observed, fluctuations in the

water surface elevation were generally less than 3

ft, and the bulk of the fluctuations were negative,

decreasing from pre-breakup levels. All flows remained well within the riverbanks, no flooding

occurred and at no time was there any indication

that there was a significant potential for flooding.

While it was fortunate that no damaging ice-related flood event occurred in 1992, it was unfortunate for the purposes of this study that the 1991-92

ice season was uneventful, yielding little information on the character of spring breakup on the Yellowstone and Missouri Rivers above Williston.

ANALYSIS

In contrast to open-water flooding, where high

water levels directly result from excessive water

discharge, ice-affected flooding results from added resistance to flow and blockage of flow caused

by accumulations of ice. The formation of an ice

cover or ice jam on a river roughly doubles the

wetted perimeter of a wide channel. The added

resistance to flow caused by the ice cover, along

with the reduction in flow area caused by the ice,

results in higher stages than a comparable openwater discharge would produce. This is particularly true for ice jams, which can cause flood stages

comparable to rare open-water events despite discharge recurrence intervals on the order of two

years or less (exceedance probabilities on the order of 0.5 or greater).

An ice jam is defined as a stationary accumulation of fragmented ice or frazil that restricts flow

(IAHR 1986). These accumulations include freezeup jams as well as breakup jams. Freezeup jams

are formed by the collection of pieces of floating

ice during the periods of relatively steady flow experienced when the ice cover initially forms early

in the winter season. Breakup jams, on the other

hand, form during the often highly unsteady flow

conditions when the ice cover breaks up because

of a significant rainfall, snowmelt or other increase

in runoff. The longitudinal profile of a typical

breakup ice jam is shown in Figure 4. In the case of

the Missouri River along the Buford–Trenton Irrigation District, it is the breakup jam scenario that

is of concern.

Most breakup ice jams are the result of ice moving downstream until it encounters a significant

reduction in water surface slope or a strong, intact

downstream ice cover or other surface obstruction. Downstream of the jam the flow may be uniform (at least in a reach-averaged sense). At the

downstream end, or toe, of the jam, the ice accumulation results in a gradually varied flow profile

in the transition reach as the water depth increases

towards the deeper normal flow depth associated

with the thicker, rougher ice conditions. If the ice
10

Figure 4. Longitudinal profile of a typical breakup ice jam.

jam is long enough, a fully developed or equilibrium jam reach may form, in which ice and flow

conditions are relatively uniform. From the upstream end, or head, of the jam, flow depths again

gradually decrease towards the lower flow depths

associated with the open-water conditions upstream. Further information on ice and ice jam

processes can be found in EM 1110–2–1612

(USACE 1982a).

Ice-related flooding tends to be local and highly site specific. Without prior field observations, it

is difficult to predict where, or even if, ice jams

will form along a river. Unfortunately no significant ice jams occurred during the current study

period. Thus, analyses of ice-related flooding

must rely heavily on the historical data obtained

through the visits with local residents described

earlier. One of the best-documented events (and

also the most recent) occurred in March 1986. The

jams associated with this event were severe, nearly overtopping laterals in both the Middle and

West Bottoms, and they provide a good test of our

ability to simulate ice jam processes in this reach

of the Missouri River. The 1986 event will also

provide a basis for simulating potential flood levels for other ice jam events.

Lacking field data, it is very difficult to predict

where, and with what severity, jams will form

along a river. Major obstacles to be overcome include estimating the appropriate ice conditions at

a particular site. Analysis is often limited to estimating the upper and lower limits of probable

stages. If a jam is known (or assumed) to form at a

given location, it is possible to estimate the maximum resulting flood levels. It can be shown that

for a given scenario of water discharge and ice

conditions, the maximum water levels will occur

within the equilibrium portion of the jam described earlier. Since ice and flow conditions are

relatively uniform within the equilibrium reach, it

is fairly simple to estimate the water levels in this

portion of the jam. Depending on where a jam

forms and whether there is a sufficient upstream

ice discharge to form a jam long enough to develop an equilibrium reach, actual water levels may

be less and the estimate will be conservative.

March 1986 ice jam simulation

For the 1986 event we have information on the

ice jam that occurred in the vicinity of Hurley

Bend in the Middle Bottom. We know the maximum stage and the approximate locations of the

head and toe of the jam, and we know that most of

the ice remained in the channel and that the jam

released while at the maximum stage. Further,

knowing that the maximum stage was reached on

March 1st allows us to estimate the water discharge and Lake Sakakawea stage on that day. The

discharge was estimated to be 59,500 cfs on the

Missouri River and 45,000 cfs on the Yellowstone

River, while the Lake Sakakawea stage was taken

to be 1833.2. Below the toe of the jam (cross section

1563.22), it was assumed that there was an intact

ice sheet 1.5 ft thick, and above the head of the jam

(cross section 1570.13) there was open water. The

roughness of the ice sheet and jam were unknown,

so the default values supplied by the ICETHK program (Wuebben and Gagnon, in prep.) were assumed. These values include n = 0.025 for sheet ice

and the default n-value calculation scheme for ice

jams contained in the ICETHK program. This calculation scheme is based on an empirical predictive relation developed to describe the ice accumulation roughness data of Nezhikovskiy (1964).
11

Figure 6. Simulation of the longitudinal profile of a

fully developed ice jam throughout the Buford–

Trenton area.

1880

1840

1800

Elevation (ft)

1500 1520 1540 1560 1580 1600

River Miles

Water Surface

Bed

Bottom of Ice

Route 85

Bridge

Figure 5. Simulation of the longitudinal profile of

the March 1986 ice jam in the Middle Bottom

area.

Using this information the HEC–2 computer

program (USACE 1990) was used to calculate the

resulting water surface profile as shown in Figure

5. The area of zero ice thickness, appearing as

a nearly vertical line through the ice cover near

HEC–2 cross section 1552.7, corresponds to the

location of the Route 85 bridge. Because the HEC–

2 ice option is actually a modification of the standard bridge option, ice cannot normally be simulated at cross sections where the bridge code

appears in a data file. Since bridge widths are

quite small relative to the river lengths typically

modeled with HEC–2, the absence of ice in the

bridge throat has a very localized effect on the

computed water surface profile. Except in the immediate vicinity of a bridge, the effect of deleting

the ice cover over such a short distance is normally negligible. Another option is to delete the

bridge from the simulation if the ice effects are determined to be of greater significance.

In this jam, water reached the elevation of the

road at the Borlaug Bridge site in the Middle Bottom. A Corps of Engineers survey found this location, as identified in a photograph, to have an elevation of 1868.5. Although it is difficult to determine accurately from the figure, the computed

water surface elevation at the nearest cross section

to this site, 1567.44, was 1868.75 ft msl. Considering that no ice-related calibration of the openwater-verified HEC–2 data file was conducted, the

agreement of the ice jam simulation with the

known water surface elevation is surprisingly

good.

Had we not been able to obtain information on

the location of the toe of the jam, we would have

had little alternative but to assume that a fragmented ice cover existed throughout the river and

that it was free to thicken into an equilibrium jam

in response to forces imposed by the flowing water. As described earlier, this assumption would

result in the maximum possible water levels for a

given discharge. A simulation of this condition

was also run (Fig. 6), and the computed water surface elevation in the vicinity of the Borlaug Bridge

was 1870.13. Thus, assuming a fully developed ice

jam below the known (or estimated) toe of the jam

resulted in a computed water surface elevation

1.38 ft higher than that computed using the known

toe location. Areas closer to the toe of the jam

would have been more severely affected by the

difference in assumptions, while points farther

upstream would have seen lesser or no effect.

1880

1840

1800

Elevation (ft)

1500 1520 1540 1560 1580 1600

River Miles

Water Surface

Bed

Bottom of Ice

Route 85

Bridge
It must be reiterated that calculations assuming an equilibrium ice jam constitute the maximum possible water surface elevation for a given

discharge. Actual water levels are most often less.

Lower actual water levels can be found at locations close to the toe of the jam where the jam may

not be fully developed. If there is enough ice to

form an equilibrium jam, the actual water levels

will also be lower than computed. If the discharge

continues to rise or the strength of the ice accumulation deteriorates, a point can be reached where

no stable jam is possible, and a lower, open-water

rating curve again applies. The equilibrium model also assumes that a supply of broken ice is

available. With lower discharges and stronger ice,

the ice cover may remain intact and not subject to

thickening into a jam.

Ice-affected water levels

The first step in the analysis of ice-affected water levels was a year-by-year review of flow

records to determine the expected breakup discharge. Actual water surface profiles lie somewhere between the limiting conditions of open

water, a solid cover of sheet ice and a fully developed equilibrium ice jam. The solid ice cover case

would represent the minimum ice-affected stage,

while the equilibrium ice jam case would represent the maximum stage possible for a given

discharge. If we consider the range of possible

Missouri River discharges during the breakup

period, we can categorize ranges of flow from

discharges too low to cause breakup of the ice

cover to discharges where all ice would move

downstream without jamming. These categories

might be based on personal observations, observations by local residents, notes on nearby gaging

records, sharp breaks in the trend of continuous

stage measurements or other sources of information.

Rating curves have been developed at several

locations for discharges up to 92,000 cfs. This

range would include events as large as a ten-year

spring ice cover breakup period flows, as well as

two-year open-water flows. Figure 7 shows conditions near cross sections 1564.48 and 1568.19 in

the Middle Bottom, cross section 1578.03 in the

West Bottom and 1581.31 just downstream of the

Yellowstone confluence area. The curves represent open-water, ice-covered and ice-jammed

conditions in order of increasing stage. Based on

a review of air temperature records for the winters of 1970-71 through 1991-92, it would appear

that the midwinter ice thickness would normally

be greater than the 1.5-ft-thick ice measured during 1991-92 but less than the 32-in.-thick ice reported in 1952 (USDOC 1953). The ice jam curves

in Figure 7 represent the maximum water surface

elevations possible for the range of discharges

covered, assuming that an unlimited supply of

ice has formed an equilibrium jam throughout

the study area.

Based on a typical freezeup flow of about

10,000 cfs, a spring discharge on the order of

25,000 cfs or greater would be required to initially

dislodge a strong ice cover. This value is based on

a rule of thumb that the stage must rise three to

four ice thicknesses above the freezeup stage to

initiate the breakup and run of a strong ice cover.

An increase from 10,000 to 25,000 cfs, with a continuous ice sheet, would result in an increase in

stage on the order of 5 ft in most areas within the

District. A deteriorated ice cover can release with

lesser increases in flow, but such events do not

normally result in significant ice jams. Below that

discharge, then, it might be assumed that the

stage–discharge relation would follow the sheet

ice curve. Above that discharge, stages would

tend towards the ice jam curve, assuming that

conditions approaching an equilibrium jam were

possible.

At a somewhat higher discharge, the trend of

increasing stage with discharge would begin to

flatten out. Because of the wide floodplains

throughout most of the area, once the raised lateral ditches were overtopped, the channel stage

would be substantially stabilized. For example, at

the Borlaug Bridge we know that the 1986 jam

with an estimated discharge of 59,500 cfs nearly

overtopped the lateral. The stage would certainly

have leveled off near the elevation of the lateral,

or even dropped if the lateral had been breached,

had the jam not collapsed and released. Figure 7b

appears to show that the stage would continue to

increase to an elevation of at least 1872 in this

area, but that is due to a fairly coarse HEC–2 data

file in which the apparent low elevation on the

lateral is somewhat higher than that at the Borlaug Bridge. By the time the ten-year breakup period flow (or two-year open water flow) is

reached, the ice jam rating curves indicate that

the laterals in both the Middle and West Bottoms

would be overtopped.

In view of the limited length of historic jams,

however, actual stages for these flow ranges

would likely be less than shown, since a fully developed equilibrium jam would not exist throughout the area. Most reported jams were 0.5–2.5

12
13

Figure 7. Rating curves for open-water, ice-covered and ice-jammed conditions.

1864

1860

1856

1852

Elevation (ft)

100 1

Open

Ice Sheet

Ice Jam

1872

1868

1864

Elevation (ft)

0 20 40 60 80 100 103

Open

Ice Sheet

Ice Jam

1860

1856

1876

1872

1868

Elevation (ft)

0 20 40 60 80 100

Open

Ice Sheet

Ice Jam

1864

1860

1876

1872

1868

Elevation (ft)

0 20 40 60 80 100 × 103

Flow (cfs)

Open

Ice Sheet

Ice Jam

1864

1860

a. Cross section 1564.48.

b. Cross section 1568.19.

c. Cross section 1578.03.

d. Cross section 1581.31.
miles long. Further, if discharge continued to increase after a jam was formed, a discharge magnitude would eventually be reached where a stable

jam would no longer be possible, the jam would

release and the ice would pass downstream. Stages would then return to the open-water curve. As

a result, extrapolation of ice-induced stages to

more extreme events is generally not reliable,

particularly if such limiting factors as floodplain

flow and jam release cannot be defined.

The jam release discharge remains unclear, as

it depends not only on water discharge but also

on the quantity of ice and the strength of the ice

accumulation. In 1986 we know that two jams released with discharges no higher than about

60,000 cfs. Five of the six known jams occurred

with discharges between 30,000 and 78,000 cfs.

The 1952 event had a mean daily flow of 120,000

cfs and a peak discharge of 170,000 cfs in the same

time frame as the jam event, but it is unclear

whether the jam was still in place or had (more

likely) released prior to that peak flow. Experience and data from other rivers indicate that the

maximum discharge during an ice jam event is

usually no more than a two-year open-water

flow, which in this area would be 90,000 cfs. A

ten-year breakup period discharge is approximately 92,000 cfs. It is suggested here that the

maximum ice jam discharge should lie at or below this ten-year breakup period flow. Beyond

that level, ice jams should become unstable, and

the water levels would return to nearly open-water levels.

Ice jam potential

Although the prediction of ice jam occurrence

and severity is still beyond the state of the art, it is

sometimes possible to rate the likelihood of ice

jams based on historical data. Such a prediction

mechanism could prove useful in estimating the

potential for ice jam formation in a given year,

both for early warning of potential flooding and

for determining whether advance measures to

limit ice-related flood damage are advisable.

Based on interviews with local residents and a review of literature, six historic ice jam events were

identified in the Buford–Trenton area (1952, 1972,

1975, 1976, 1978 and 1986). To review the winter

characteristics leading up to significant ice jam

events, weather and hydrologic data from 1970

through the present were reviewed. The year preceding the closure of Garrison Reservoir, the ice

jam year of 1952 and the randomly selected year

of 1960 were also included.

As shown in Table 4, factors examined included

freezing degree-days, snowfall, breakup period

water discharge Qb and Garrison stage. Freezing

degree-days and snowfall are used to reflect the

thickness and strength of ice on the river. Freezing

degree-days can be used in a relatively simple

equation to predict ice thickness:

h = c (FDD)0.5

where h = calculated solid ice thickness

FDD = accumulated degree-days of freezing

(°F)

c = empirical constant to account for wind

exposure and snow cover.

This constant was calibrated for the Williston area

during the essentially snowless winter of 1991-92

to a value of 0.60. Using this value for the winter of

1952 resulted in a predicted ice thickness of 31 in.

The measured ice thickness in 1952 was 32 in., and

there was about 10 in. of snow on the ground one

week prior to breakup (USDOC 1953). For winters

with greater snowfall this value might overpredict

ice thickness and thus may be considered conservative.

The next term in the table (FDDmax) is the water

year Julian day (days since October 1st) when the

FDD term began to decrease, taken as an indicator

of net melting of the snow–ice cover. Low values

of this term often indicate that the weather

warmed gradually in the spring and that significant ice deterioration and melting may have occurred prior to any large increase in runoff. The

term in the next column (Qmax) indicates the estimated date of the spring runoff causing breakup

in a given year. Lacking direct field observations

of ice breakup for the period of record, this term

had to be estimated from discharge records, an

approach that leaves some uncertainty. The date

of the maximum river discharge, Qmax, when

compared to FDDmax, can be used to reflect the arrival of significant spring runoff relative to warm

weather in the District.

In the Williston area the local weather that governs the thickness and strength of the river ice is

relatively uncoupled from the weather in the

mountains that governs the snowmelt runoff that

produces most breakup discharges. If the weather

warms up in the Buford–Trenton area well in advance of snowmelt runoff in the mountains, the

ice is likely to be too thin or rotten to pose a significant flood threat. On the other hand, an increase

in runoff from upstream early in the winter may

encounter a strong, resistant ice cover requiring a

14
15

greater, more rapid discharge increase to initiate

breakup.

As mentioned previously, it takes a certain

magnitude of discharge and stage increase to release an ice cover and allow it to move downstream. If the increase in discharge is rapid or the

ice is deteriorated, the required increase in stage

may be slight, but for gradually rising discharges

the required increase in stage may be equivalent

to three or four ice thicknesses. For the typical

freezeup and midwinter discharge of about 10,000

cfs in the study area, a spring runoff event in excess of 25,000 cfs should be required to break up a

strong ice cover. The required breakup discharge

(Qb) varies, however, with the actual freezeup discharge for a given year, as well as variations in the

other terms listed in Table 4. At very high discharges, greater than about 90,000 cfs, no stable ice

jams would be able to form. The situation would

then revert to an open-water flood scenario.

Two terms are included to account for snowfall

characteristics. Ideally the snow-related portion of

the prediction scheme would use the depth of

snow remaining on the ground prior to breakup

as an index, but historic records of this parameter

are not available. Instead we have used two terms,

the total snowfall for the season and the timing of

the snowfall, as an indicator. The timing of the

snowfall is listed as being E for early, L for late or

VL for very late. The climate of the Williston area is

such that, in many years, the snow has completely

melted from the river ice surface prior to breakup.

In addition to potential thinning of the ice cover

through thermal melting, this allows solar radiation to penetrate and decay the internal structure

of the ice. In this manner, even a relatively thick ice

cover can be weakened to reduce or eliminate the

ice jam flooding potential.

A factor is also included in the table to reflect

the elevation of Lake Sakakawea (Garrison Reservoir stage), since the most common location for ice

jam formation is the transition from a steeper to a

milder energy slope such as that presented by a

river flowing into a lake or reservoir. Garrison Reservoir reached its normal operating levels in 1965

and since that time has been a potential factor in

the occurrence and location of ice jam formation.

In addition to listing the magnitudes of these

various terms, an effort was made to discriminate

whether the values indicated a high, medium or

low potential for ice-related flooding. The criteria

for ranking the terms are provided in Table 5.

Table 4. Analysis of ice jam potential for the Missouri River near the Buford–Trenton Irrigation

District.

Calculated

ice Snow- Garrison

Known FDD thickness FDDmax Qmax Qmax–FDDmax Qb fall Snow stage

Year jam (°F-days) (in.) (J.D.) (J.D.) (days) (Kcfs) (in.) timing* (ft) Σ+ Σ– Σ±

1952 X 2750++† 31 178+ 180+ 2++ 124.0– 25 E– Low– 6 3 2.0

1960 2000 27 165 174 9 100.0 15– – 1798.0– 1 4 0.25

1970 2030 27 182+ 160 –22– – 26.5 38 L+ 1837.0 2 2 1.00

1971 2444 30 177+ 166 –11– – 58.0+ 18– – L+ 1842.0+ 4 4 1.00

1972 X 2600++ 31 160 167 7++ 75.0 33 1844.0+ 5 0

1973 1650– – 24 147– 157 10 30.0+ 7– – E– 1843.2+ 2 6 0.33

1974 2260 29 180+ 186+ 6++ 31.0+ 20 1838.8 5 0

1975 X 1800 25 180+ 175+ –5++ 30.0+ 48++ L+ 1838.6 8 0

1976 X 1800 25 165 175+ 10 43.0+ 28 E– 1840.5+ 3 1 3.00

1977 1900 26 140– 170 30– – 25.0 15– – E– 1836.0 0 6 —

1978 X 3000++ 33 170+ 176+ 6++ 87/0 15 E 1828.0– 6 4 1.50

1979 3300++ 34 180+ 170 –10– – 60.0+ 37 L+ 1835.5 5 1 2.50

1980 1900 26 165 151– –14– – 26.0 25 1837.6 0 3 —

1981 1100– – 20 135– 145– 10 28.0 5– – E– 1829.9 0 8 —

1982 2300 29 160 147– –13– – 40.0+ 60++ L+ 1829.2– 4 4 1.00

1983 1422– – 23 174+ 166 –8 21.0– 29 VL+ 1841.1+ 3 3 1.00

1984 1960 27 168+ 174+ 6++ 19.5 30 L+ 1839.1 5 1 5.00

1985 2307 29 158 156 –2++ 25.0 18– – L+ 1838.4 3 3 1.00

1986 X 2000 27 145– 152– 7++ 59.5+ 35 L+ 1836.3 4 2 2.00

1987 1018– – 19 179+ 168 –11– – 17.0 16– – E– 1840.3+ 2 6 0.33

1988 1517– – 23 167+ 176+ 9 17.0– 16– – E– 1832.9– 2 7 0.29

1989 2300 29 175+ 167 –8 40.5+ 32 E– 1820.7– 2 2 1.00

1992 918– – 18 147– 158 11– – 26.0 <18– – E– 1821.5 0 9 —

* E = early, L = late, VL = very late.

† + = high correlation with ice jamming, – = lower correlation with ice jamming.
16

Table 5. Ice jam potential rating factors.

Indicated potential

Term Range Low High

Fahrenheit freezing 918 / 3300 <1700 >2600

 degree-days

FDDmax 135 / 180+ <150 >165

Qmax 145 / 175 <155 >170

Qmax – FDDmax –13 / +30 <–8 or >+10 >–5 or <+7

Qb (K cfs) 17 / 124 <25 or >90 >30 or <70

Garrison stage (ft) 1798 / 1844 <1835 >1840

Total snowfall (in.) 5 / 60 <20 >40

Snowfall timing N.A. <5 in. after >10 in. after JD=90

JD=90 > 5 in. after JD=120

runoff sufficient to cause breakup came when the

ice was still thick and strong. Because of the low

topographic relief within the Irrigation District,

relatively small jams will cause overbank flooding,

as the overbanks are generally within 5 ft or so of

normal winter stages.

Techniques for mitigating ice-jam-related floods

can be grouped into those measures that are designed and implemented to alleviate future problems and those that take place in response to a jam

already in existence. Advance measures range

from alteration of river ice formation processes to

provisions for structural containment of ice and

flood waters. Although the mild ice conditions encountered during 1991-92 required no response for

mitigation of ice-related flooding, short-term advance measures would appear to be technically

feasible. Since there are a relative few problem jamming locations in the study area, and in view of the

smooth ice cover and favorable weather conditions, structural weakening of the ice (either mechanically or by dusting) could ease the passage of

the ice run through these resistant reaches. Had the

advance preparations failed and ice jams formed,

emergency flood-fighting measures would also

have been possible.

The smooth, single-layer ice sheet observed on

the river during the winter of 1991-92 indicates

that ice growth (and hence the total ice volume

available to form a spring breakup jam) is dominated by thermal growth once the river is ice covered, rather than by accumulation or jamming processes during the freezeup period. Based on the

observations of 1991-92 and the information collected in the historical review, it is clear that longterm mitigation techniques will have to focus on

the spring breakup period rather than the control

of ice formation. Basic flood control alternatives

Based on these criteria, values for the various

terms that would indicate a high correlation with

ice jamming are indicated by a “+” in the table,

while values associated with a lesser risk are denoted with a “–.” Note that three terms, FDD,

Qmax–FDDmax and snowfall magnitude, are double weighted.

In the final three columns the number of “+”

and “–” symbols have been added and a ratio calculated. Values less than one would indicate lesser

ice jam flooding potential, while values greater

than one would indicate a greater potential. The

lowest ratio for a known ice jam is 1.5 in 1978, but

three years in which no jams were reported (1974,

1979 and 1984) have values greater than 1.5. The

highest ratio, 5.00, occurred in 1984, which had no

reported jam. However, the breakup period discharge was only 19,500 cfs, less than the flow required to initiate ice cover breakup. While these

criteria are empirical and approximate (there is no

precedent for this prediction scheme), their correlation with past ice jam events is quite clear. More

importantly, tabulating these or comparable terms

during future winter seasons should give a useful

indication of the potential for ice-related flooding

in the spring.

MITIGATION TECHNIQUES

While the winter of 1991-92 was warmer than

normal and had less snowfall and no significant

ice jamming, the ice breakup observed may still be

indicative of breakup processes in a majority of

years. We were able to identify only six significant

ice jam events in the last 40 years, although more

events may have occurred. For those years in

which jams did occur, the weather was colder

and/or snowier than average, and an increase in
could include ice retention to prevent or control

breakup, flow containment or diversion to reduce

flood damage, and ice weakening to reduce jamming potential.

Nonstructural techniques

The most common location for ice jams to form

is in an area where the slope of the river changes

from a steeper to a milder slope. Since gravity is

the driving force for an ice run, when the ice

reaches the milder slope it loses its impetus and

stalls against a more stable downstream ice cover.

Another common location for ice jam formation

is the point where a tributary stream enters a larger river. Smaller rivers normally respond to increasing runoff more quickly than large rivers.

The ice cover on a smaller river will typically

break up and run until it reaches the strong, intact

ice cover on the larger river, where the slope is

normally milder as well. The ice run stalls at the

confluence, forming a jam and backing up water

on the tributary stream. Riverbends are also frequently cited as ice jam instigators. While riverbends may contribute to jamming by forcing the

moving ice to change its direction and by causing

the ice to impact the outer shoreline, slope is

again a factor. Riverbends are nature’s way of

controlling river slope, with a straight reach between two points being the steepest possible

course, and the slope decreasing as the bend severity increases.

On the Missouri River in the Buford–Trenton

area, all three of these ice jam instigators are

present. At the lower end of the District as the

Missouri enters Lake Sakakawea, the energy

slope decreases as the river enters the still waters

of the lake. Near the upper end of the District, the

Yellowstone River typically breaks up several

weeks prior to the Missouri River, and the first ice

jam in the vicinity of the District is often in the

confluence area of the two rivers. Between these

two locations the District is separated by a series

of riverbends, two of which are common jam locations (Ryder and Hurley bends).

Since jamming in each of these areas arises

when the ice run loses a portion of its driving

force due to a reduction in slope, the situation

might be improved either by increasing the driving forces or by reducing the resisting forces. First

we will look at means of reducing the resisting

forces by weakening the ice cover. Weakening the

ice cover has been employed in numerous areas

to reduce flood potential or to limit its severity.

Methods of ice weakening can be grouped into

methods that mechanically weaken ice and those

that speed ice deterioration.

Mechanical weakening

Ice weakening has been accomplished by using mechanical cutting techniques on rivers such

as the Rideau in Ottawa, Ontario, and the Beaurivage near Quebec City, Quebec, among others.

Recently the Finnish National Board of Waters

and Environment, through its District Office in

the town of Kokkola, has been developing icecutting equipment for the purpose of mechanical

ice weakening.* The prototype device has a cutting wheel 2.5 m (8.2 ft) in diameter and 0.2 m

(0.66 ft) wide. It is towed behind a tracked, amphibious vehicle. In 30-cm- (1-ft-) thick ice it can

cut ice at a speed of 4 km/hr (2.5 mph). With 60-

cm ice the speed is reduced to about 2 km/hr

(1.25 mph), and with 120-cm ice the speed is approximately 1 km/hr (0.6 mph). Once testing is

complete, they plan to have these ice cutters manufactured and marketed commercially.

As reported in Deugo (1973), ice cutting is an

annual activity on the Rideau River in Ottawa,

Ontario. A 7.5-mile reach above the confluence

with the Ottawa River is cleared of ice during a

two-week period beginning in late February in

anticipation of the spring freshet. Ten thousand

lineal feet of ice are cut in critical locations during

this time using a mechanical saw. They are able to

clear 7.5 miles of river with only 2 miles of actual

cutting. The unit is a circular saw powered by a

60-hp engine and is capable of cutting to a depth

of 2 ft. The ice cutter is used to cut slots parallel to

the water flow approximately 50 ft from each

shoreline. Once cutting is complete and spring

runoff approaches, explosives are used to dislodge the ice cover throughout the reach, beginning at the downstream end. The combined costs

of the cutting and blasting program in the City of

Ottawa is approximately $180,000 in 1985 Canadian dollars.

In relatively straight reaches of a river, simply

cutting slots down both sides of the river to release the ice cover from the riverbanks may be

sufficient, while in curved reaches more elaborate

cutting patterns may be required. Jolicoeur et al.

(1984) reported on tests of ice trenching performed in an effort to find a cheaper, safer and

more environmentally acceptable alternative to

the explosive demolition of ice. In those tests a

series of five cutting patterns were used to allow

17

* Personal communication, M. Ferrick, CRREL.
18

N

St. Etienne

Village

Quebec City

Beaurivage

River

Location of

Ice Jam Priming

Flow

Figure 8. Map of Beaurivage River ice cutting. (After

Jolicoeur et al. 1984.)

an evaluation of their performance in a riverbend. A general map of the area along with a

depiction of the cutting patterns is shown in Figure 8.

The patterns were cut in the ice using an unmodified Case DH4 trencher. This trencher has a

chain width of 15 cm and an in-line trenching

speed of about 8 m/min (26 ft/min). Trenching a

600-m (2000-ft) bend using the five patterns took

eight hours, or a normal working day. Although

all cutting patterns worked well, the authors felt

that the most efficient pattern was the one consisting of an unconnected “V” pattern shown farthest downstream in Figure 8.

Jolicoeur et al. (1984) concluded that trenching

breaks more ice than blasting, costs about a third

as much, has no adverse environmental effects

and is far safer. Ice trenching has been used as an

advance measure on the Beaurivage River annually since 1984.* Because significant ice strength is

required to support the large trenching machine

initially employed, ice cutting on the Beaurivage

River since 1986 has been conducted with a specially designed “Aquaglace” ice trencher that

was lighter and safer to operate on thin or weak

ice (Belore et al. 1990).

Each of these reports has discussed ice cutting

that involved completely cutting through the ice,

allowing the cut slot to fill with water. With the

smooth, uniform ice sheet present in the Buford–

Trenton area, however, it would be possible to cut

a trench that doesn’t quite penetrate the cover, allowing the slot to remain dry. This would be an

advantage if cutting were conducted early

enough in the season such that water in the

trenches could freeze and defeat the operation.

On the other hand, if warm sunny weather was

anticipated, the slots could be cut completely

through the ice since the low albedo of water in

the trenches could absorb additional solar radiation, leading to accelerated ice decay.

Ice deterioration

Ice deterioration occurs both through thermal

melting and through decay of its internal structure. With higher air temperatures, higher sun

angles and longer days in the spring, the winter

snow cover and the top surface of the ice begin to

melt, forming a water layer. With the reflective

snow cover gone, this water layer absorbs more

solar radiation, and deterioration of the ice structure along crystal boundaries can take place. The

ice becomes progressively thinner and weaker,

and given time the ice will melt in place.

One technique that has been used to accelerate

the decay of an ice cover is dusting. The term dust

is used here to denote any dark material that can

be spread on the ice in a thin layer so as to absorb

solar radiation and thereby accelerate the deterioration process. Dusting has been used in the

Omaha District to reduce ice-related flooding potential on the Platte River in Nebraska (USACE

1979). Moor and Watson (1971) described the use

of dusting on the Yukon River in Alaska. They

found that dusting a river reach subject to perennial jamming two to three weeks prior to breakup

weakened the ice sufficiently so that no jams

formed there during years when dusting was

performed. Ideally the dust should be applied as

early as possible to provide maximum deterioration time but after the last snowfall (which would

cover the dust and render it useless).

Dusting materials can include such substances

as fly ash from the burning of coal or pit run sand.

Moor and Watson (1971) suggested an application rate of 0.5 lb/yd2 for sand and 0.35 lb/yd2 for

fly ash. Sinotin (1973) cited values of 0.18 lb/yd2

for 0.04-in. material and 0.92 lb/yd2 for 0.2-in.

material. Problems with aerial dusting identified

by Moor and Watson (1971) include moisture (the

material must be dry enough to flow freely and

not freeze), wind (even a slight breeze can cause

drifting of fine materials and require low release

altitudes) and ice or glazed snow surface (the

dust can drift after landing). The potential for adverse environmental impacts of the dusting material must also be considered.

Instead of aerial dusting, it is also possible to

* Personal communication, B. Michel, Universite de Laval,

Quebec City, Canada.
pump a slurry of water and riverbed sediments

onto the ice, at least for rivers with silt or sand

bed material. Moor and Watson (1971) described

tests in which dusting coverage similar to aerial

dusting was obtained by pumping. While the cost

of treating a 100-ft-wide strip was less than for

aerial application ($0.09 per lineal foot in 1970

dollars), a disadvantage is the time required to

cover large areas, as required in most ice jam prevention applications.

Flow control

The control of river discharge has been used

both to increase ice cover stability and prevent

jamming and to intentionally cause river ice

breakup. Flow control is often used during the

fall freezeup period to allow a thinner, more stable ice cover to form in a reach where natural flow

velocities would cause ice floes to underturn or

shove and thicken into a jam. This form of flow

control may also be used to improve the performance of ice control structures by providing

reduced, steady flows during ice cover formation

(Deck 1984). Similarly, flow control during the

spring could be used to prevent ice cover breakup, delay breakup until significant deterioration

of the ice cover has occurred, or reduce peaks in

water discharge and resultant flooding. This

approach, however, requires a dam with sufficient storage capacity located near enough to the

flooding problem area.

In the case of the Buford–Trenton area, flow

control is employed by Fort Peck Dam on the

Missouri to reduce flood peaks during the breakup of the Yellowstone River. This also increases

the tendency of the Missouri River to break up

several weeks after the Yellowstone River and

limits the supply of ice feeding any ice jams that

might occur within the Irrigation District during

the breakup period on the Yellowstone River.

During the 1986 event, Fort Peck releases were

reduced from 12,000 cfs on 1 March to 1,000 cfs on

9 March to lessen flooding (USACE 1986).

Unfortunately Fort Peck is far upstream from the

Buford–Trenton area and can control only a small

portion of the flow causing flooding (the drainage area above Fort Peck Dam is 57,500 mi2 vs.

164,500 mi2 above Williston). Further, close monitoring of conditions on the Yellowstone is a must,

since the travel time of water from Fort Peck Dam

to the District (about 200 river miles) is on the order of four days.

Flow control, in the form of a rapid increase of

discharge from a dam, has also been tested as a

technique to fracture an ice cover and cause premature breakup (Ferrick and Mulherin 1989). The

rapid release of a pulse of water is intended to

cause the ice cover to fracture and run with a

smaller increase in stage than required for most

natural runoff events. This method could be helpful in the case of a tributary stream that jams as it

encounters an intact ice cover at its confluence

with a larger river. With adequate flow control on

the larger river, it might be possible to cause the

ice on the larger river to break up prior to the

smaller river, providing room for the smaller river’s ice run to enter. The dam release must be

quite rapid and near to the problem site so that a

sharply defined wave passes through the ice. Although clearing the ice cover on the Missouri River prior to breakup on the Yellowstone River

should reduce the threat of ice jam flooding, Fort

Peck dam is much too far away to provide such a

pulse of water.

Icebreaking

Icebreaking is another nonstructural alternative for clearing a channel or weakening its ice

cover in advance of a natural breakup. As described in the flow control section above, icebreaking on the Missouri River in advance of the

breakup of the Yellowstone River ice cover would

help mitigate ice-related flooding. The use of icebreaking ships is highly effective but requires

suitable vessels on-site, which is not the case in

the Buford–Trenton area. In rivers that are isolated from navigable waterways or too shallow for

navigation by icebreaking ships or tugs, air-cushion vehicles (Kankakee and St. Lawrence Rivers)

or amphibious landing craft (Buffalo River) have

also been used.

Early warning

While not a flood mitigation measure, an early

warning system can be quite useful in minimizing flood losses. The ice jam potential rating

scheme presented earlier can be used to provide

some indication of whether ice-related flooding is

likely. In addition, there are numerous water level

gages along the Missouri and Yellowstone Rivers

in the vicinity of the Irrigation District that are either no longer in use or not active during the winter months. Reactivating gages in the District and

for a good distance upstream, and adding telephone, radio or satellite relays, could provide an

early warning system for increased flows, ice

movement or increasing stages. Direct ice motion

sensors could also be added to the gaging stations

19
to indicate the onset of ice motion leading to

breakup of the ice cover.

Structural measures

Structural control measures include the construction of permanent or temporary structures

either to prevent ice-related flooding or to lessen

damages should a flood event occur. Such measures include means to control ice formation, restrain ice movement, and contain or divert flood

flows, either alone or in combination.

Flood containment

Flood containment normally refers to levees

that physically separate the river from property

to be protected. While levees are expensive and

normally limited to the protection of densely

populated areas, the Buford–Trenton area is currently protected by the levee-like irrigation ditches. The level of protection against ice-related

flooding provided by this system of ditches is unclear. The 1986 ice jam event nearly overtopped

the ditches in both the Middle and West Bottoms.

The maximum flow during that event was approximately 60,000 cfs, well below the two-year

open-water discharge of 90,000 cfs and somewhat

less than the five-year breakup period discharge

of 67,000 cfs. Their function as flood protection

levees might be economically upgraded by identifying sections vulnerable to overtopping and

raising them to provide a more uniform level of

protection along the length of the system.

Ice control structures

Fixed structures can be used to stabilize an ice

cover or prevent the downstream movement of

broken ice. Stabilization of an ice cover prevents

premature breakup, allowing additional time for

the ice cover to weaken and melt in place and reducing the supply of ice for jamming downstream. Perham (1983) reviewed a wide variety of

ice sheet retention structures, including piermounted booms, stone groins, artificial islands,

timber cribs, dams and weirs.

An ice control dam obstructs the passage of ice

and can provide a reservoir to control ice until it

eventually melts. Weirs and low-head overflow

dams raise upstream water levels and promote

stable ice covers upstream. At some sites they can

also aid in controlling ice breakup in a manner

somewhat similar to dams. A 9-ft-high overflow

weir was built for ice control on the Israel River in

New Hampshire by the New England Division of

the Corps of Engineers (USACE 1973, 1982b). The

170-ft-long weir was constructed with concretecapped gabions, with four, 4-ft-wide by 8-ft-deep

gaps to allow fish passage during the open-water

months. During the winter the gaps are blocked

with stop logs or metal gates to develop an ice retention pool (Axelson 1991). Completed in 1982,

the project cost $300,000, for a unit cost of about

$1800/ft.

An ice control structure consisting of a 5-ft-high

fixed-crest weir with a Bascule gate for sediment

passage was constructed on Oil Creek by the Pittsburgh District of the Corps of Engineers in 1989.

The structure was 351 ft in length, including the

45-ft-wide gate, and had a supplementary ice

boom located 75 ft upstream to increase its ice retention capability. Levees were constructed on

both upstream banks to contain the Standard

Project Flood. The total project cost was $2.2 million, for a unit cost of about $6300/ft.

A low-head (6-ft) weir for ice breakup control

has been physically modeled and designed for

Cazenovia Creek near Buffalo, New York (USACE

1985, Gooch and Deck 1990). As shown in Figure

9, the proposed weir has nine ice retention piers

rising above its crest and is bordered by a highlevel, bypass floodway to help limit hydraulic

forces that could cause the upstream ice to be extruded through the piers. This project, designed

by the Buffalo District of the Corps of Engineers,

has an estimated cost of $2 million. With a struc20

Figure 9. Isometric drawing of the proposed Cazenovia Creek

ice control structure. (From Gooch and Deck 1990.)
21

tural width of 650 ft, the cost would come to

about $3100/ft, but the project cost also includes

excavation to enlarge the pool upstream of the

structure as well as the construction of a 200-ftwide floodway to route flows around the structure at higher discharges.

Belore et al. (1990) described an ice control

structure on the Lower Credit River in Canada

consisting of concrete piers placed at approximately 7-ft spacings across the main river channel. The structure was designed to stop the downstream movement of ice at breakup and make use

of the available channel and floodplain storage.

Ice booms, the most widely used type of ice

retention structure, are essentially a series of logs,

timbers or pontoons tethered together and strung

across a river to control the movement of ice (Fig.

10). In some locations, however, the boom elements are supported by fixed piers. Ice booms are

most commonly used to stabilize or retain an ice

cover in areas where flow velocities are 2.5 ft/s or

less and relatively steady.

While conventional ice booms are normally

used to promote ice cover formation during

freezeup and during midwinter in areas of marginal stability, if properly designed they can have

application in some breakup situations. The Lake

Erie ice boom located at the head of the Niagara

River has been employed for many years to keep

lake ice floes from passing into the river, causing

flooding and disrupting hydroelectric plant operations. Perham (1983) recounted a description of

an experimental ice boom built on the Chaudiere

River in Quebec:

The boom was like a horizontal rope ladder with

steel structural channel sections for rungs. The

spaces between the rungs were filled with wooden blocks. The two parallel 25-mm-diameter

wire ropes were anchored to heavy concrete

structures at each shore. The arrangement was

expected to retain ice until a flow of 207 m3/s

(four-year flood) was reached.

Costs for ice booms can vary with river size

and ice conditions. Table 6 summarizes the costs

for a number of flexible ice boom installations as

compiled by Perham (1976). The costs listed have

not been converted to a common year but correspond to costs at the time of design and construction as indicated in the table; costs for structures

in Canada are in Canadian dollars. The only

Corps of Engineers structure is the St. Marys River ice boom, which had a unit cost of about $212/

ft of river width spanned in 1975. More recently

the Pittsburgh District constructed the Allegheny

River ice boom shown in Figure 10. This boom

Table 6. Summary of costs for existing ice boom installations. (After Perham 1976.)

Length Cost Unit cost

Body of water (ft) Year ($M) ($/ft)

St. Lawrence River, 16,830 1960 0.8 48

Ogdensburg, New York–

Prescott, Ontario

Lake Erie, Niagara River, 8,800 1964 0.9 103

Buffalo, New York

Beauharnois Canal, 17,000 1964 1.5 92

St. Lawrence River,

Beauharnois, Quebec

Copeland Cut, 750 1974 0.25 333

St. Lawrence River,

Massena, New York

Riviere des Prairies, 2,300 1975 0.25 109

Montreal, Quebec

Lake St. Francis, 7,200 1975 2.0 278

Valleyfield, Quebec

St. Marys River, Sault 2,400 1975 0.5 212

Ste. Marie, Michigan

cost approximately $900,000 to construct in 1982.

It spans a river that is 540 ft wide, for a unit cost of

about $1700/ft. The Montreal ice control structure,

on the St. Lawrence River, is a rigid ice boom that

uses floating steel booms or stop logs set between

concrete piers to collect ice floes and stabilize an

ice cover. The 1.27-mile-long structure cost approximately $18 million in 1964-65 (Perham 1983),

or about $2700/ft.

Groins, dikes or jetties can be used to constrict

channel width, raise water levels and reduce upstream velocities to promote the formation of a stable ice cover (Cumming–Cockburn and Associates

1986, Janzen and Kulik 1979). Sometimes the ice

retention capacity of such structures is enhanced

Figure 10. Plan view of the Allegheny River ice boom.

(From Perham 1983.)
with upstream ice booms (Perham 1983). At

breakup the reduced river width can improve the

stability of unfrozen ice accumulations, making

them thinner and less of an impediment to flow,

and it might even prevent breakup of the solid ice

cover (Cumming–Cockburn and Associates

1986). Such structures can also create a storage

area for ice floes at breakup, reducing the volume

of ice passing downstream. Opposing dikes have

been used for ice control on the Burntwood River

in Manitoba and on the Pasvik River in Norway

(Perham 1983, Janzen and Kulik 1979).

Artificial islands of soil or rock or both have

also been employed in many areas, including the

St. Lawrence River near Montreal, to help hold

ice in place, just as natural islands often do (Perham 1983). A smaller variation on the use of artificial islands is the construction of stone-filled timber cribs to anchor an ice sheet. An example of

using timber cribs for controlling ice can be found

on the Narraguagus River in Cherryfield, Maine

(Perham 1983). Three cribs, oriented in a triangular pattern, are located about 125 ft upstream of a

7-ft-high dam and spillway.

A longitudinal dike or series of structures

aligned with the flow along the center of a channel can be employed to decrease the effective

width of a river (Cumming–Cockburn and Associates 1986). Since the river width that an ice cover or accumulation must arch across is reduced,

stability is increased and a thinner cover with

lower water levels may result. Channel divider

structures can be used to promote early ice cover

formation, delay ice cover breakup, and initiate

and maintain stable breakup accumulations at

desirable locations. A divided channel design

was incorporated into the Burntwood River

diversion project in Manitoba (Cumming-Cockburn and Associates 1986).

Channel modifications

Ice jams tend to form in areas of change in

slope, riverbends, slow-moving pools and constrictions. Channel modifications can be used to

improve the passage of ice through such reaches.

An example of such a channel modification can

already be found in the Buford–Trenton area in

the form of a channel cutoff constructed by the

Corps of Engineers in the late 1950s between cross

sections 1569.24 and 1574.16. This cutoff isolated

a sharp riverbend, eliminating a long length of

channel and significantly increasing the local

slope. Such cutoffs must be used with care, however, due to both the potential adverse effects on

river morphology and the possibility that a new

ice jam location may be created downstream

where the artificially steepened cutoff reenters the

natural channel.

Another form of channel modification is a diversion channel to divert excess flood waters

around a jam or beyond an area to be protected. A

diversion channel can also be used in conjunction

with an ice control structure to improve its performance. For example, if an ice control dam or weir

were used to control a breakup ice run, a high-level diversion could be used to limit the discharge

reaching the structure, reducing river stages to

prevent local flooding and ensuring the stability of

the ice being retained. Such a diversion can be designed to remain dry except during flood events

so that it will be available to function as an openwater channel and not add to the ice supply reaching downstream ice jam sites.

Instead of diverting excess flood waters, it is

also possible to divert or store ice during breakup

in areas where it will cause less damage. Even

without human intervention, significant volumes

of ice are often left behind during an ice run in side

channels, on floodplains or simply grounded on

the riverbanks. By developing low overbank areas

where ice can easily leave the channel during

breakup, perhaps supplemented by dikes or

booms to redirect ice movement, the volume of ice

passing downstream can be substantially reduced.

Ice anchors are an additional form of channel

modification that can be used to control ice. While

ice control dams and weirs are constructed to slow

the flow of water by raising water levels, ice anchors are essentially pools excavated in rivers to

help stabilize ice during formation or breakup.

During freezeup the pool can help to form an ice

cover early in the winter season and obstruct the

passage of ice floes to downstream areas. During

the breakup period the pool can serve to moderate

stage fluctuations caused by rising discharge, thus

maintaining a stable ice cover in a manner similar

to an ice control weir or dam. An ice anchor constructed on the Bow River near Calgary, Alberta,

involved the excavation of about 140,000 yd3 of

bed material, resulting in a pool approximately

650 ft long, with a top width of about 330 ft and an

average depth of about 11 ft (Andres and Fonstad

1982). On rivers with significant transport of sediment, however, such dredged pools could be refilled and rendered useless in a short period of

time.

22
Thermal control

Thermal ice control methods use either heat

obtained from a nearby source (such as power

plant cooling water) or heat contained in the

water body itself to melt ice. Because water reaches its maximum density at a temperature of about

39°F, colder water in lakes tends to stratify above

warmer water. An ice cover can form on the water

surface even though the water at depth is still well

above freezing. When present, this warm water

can be brought to the surface quite economically

using air bubblers, pumps or flow enhancers.

While this natural source of warm water is common on lakes and reservoirs, warm water can exist at depth on some slow-moving rivers as well.

Water even a fraction of a degree above freezing

can be quite effective in melting ice over a period

of days or weeks. With water velocities on the order of 1–3 ft/s in the Missouri, however, such a

warm water source is unlikely except downstream in Lake Sakakawea.

External heat sources can include the cooling

water effluent from fossil or nuclear power plants,

or even groundwater. Recently a thermally based

ice control solution was developed on the Kankakee River near Wilmington, Illinois (Deck 1986). A

siphon system was constructed to extract warm

water from a nuclear power plant’s cooling pond

and place it in the river through a system of diffusers. Unfortunately no ready source of warm water

is available in the Buford–Trenton area.

Emergency measures

Predicting when, or even if, an ice jam will occur at a given location in a given year is rarely possible. The section dealing with ice jam potential

provides a scheme for predicting whether ice jams

are likely in the Buford–Trenton area based on

weather and flow conditions, but should ice jams

occur unexpectedly or in an area where no advance measures were taken, it may be necessary to

resort to so-called emergency measures. Once a

jam is in place, there are generally few mitigation

alternatives available. The Ice Engineering Manual (USACE 1982a) cites four methods: mechanical

removal, dusting, blasting and icebreaking with

ships. Dusting has already been discussed as an

advance measure, but because of the time required for dusting to be effective, it is not normally effective once a spring breakup jam is in place.

The use of icebreaking ships is highly effective but

requires suitable vessels present on-site, which is

not the case in the Buford–Trenton area.

Blasting, in an appropriate application, can be

quite effective. However, the primary purpose of

blasting is to break an ice cover or to loosen an ice

jam so that it is free to move, and there must be

enough flow passing down the river to transport

the ice away from the site and an open-water area

downstream to receive the ice. Since the primary

driving force available to break ice by blasting is

the large gas bubble resulting from the blast and

not the shock waves, the charges must be weighted and placed under the ice cover. Blasting is not

a quick, easy solution. It requires planning, acquisition of explosives and hole-drilling equipment,

and a crew of perhaps 11 people (USACE 1982a).

A properly outfitted crew might be able to blast

two rows of charges along about one-half mile of

river per day. Safety and environmental concerns

must also be addressed.

Removing an ice jam mechanically simply

means taking the ice out of the river and placing it

elsewhere. This method directly relieves the

cause of flooding but is neither cheap nor fast. In

February 1978 it cost approximately $11,500 to

make a 2600-ft channel with a backhoe (USACE

1982a). Other mechanical removal operations

have employed such equipment as bulldozers,

excavators and draglines. Because of the time required to excavate ice with conventional equipment and safety concerns, this approach is normally limited to midwinter jams on small

streams.

Mitigation techniques for the Buford–Trenton

Irrigation District

A number of ice-related flood mitigation techniques have been reviewed in terms of technical,

but not economical, feasibility. Given the climate

of the Williston area and the characteristics of the

river ice regime, a program of ice weakening

should be feasible as either a short-term advance

measure or a long-term ice control program. Ice

weakening efforts should be focused on the river

bendways near Ryder Point and the Hurley Bend,

which have been frequent jam locations in the

past. In terms of structural control measures,

flood flow containment is already a reality for a

flow approaching a five-year recurrence interval

spring breakup flood due to the existence of the

raised irrigation ditches along much of the river

in the Buford–Trenton Irrigation District. It might

be possible to improve the level of protection offered by these ditches by raising low spots and

ensuring their structural adequacy.

The confluence area of the Missouri and Yellowstone Rivers is considered to be a good poten23
tial site for controlling ice during breakup. According to local residents, problem breakups typically consist of a 0.5- to 2-mile-long jam (though

sometimes as long as 4 miles), which propagates

slowly in a series of jam and release cycles down

the Yellowstone and continuing into the study

reach on the Missouri River. Flood flows and ice

breakup on the Missouri River generally occur

several weeks after the Yellowstone River ice run.

One of the natural jamming points for the Yellowstone River ice run is in the confluence area, and

based on interviews with local residents concerning jams in that area, the jams have not caused

significant damage in the past. Causing the jam to

remain in this area would surely alleviate downstream ice jam flooding. Ideally the ice should be

retained and allowed to melt at this site. The observation that the spring breakup ice run propagates tens of miles while the jams remain relatively short and constant in length indicates that significant melting is a common feature at breakup

and can be used to advantage in conjunction with

structural control.

Numerous types of structures or stream alterations could be used for ice control in the confluence area. Rock-filled timber cribs, spur dikes, ice

control weirs, ice booms and a dredged ice retention pool are potential alternatives. If necessary a

high-level flood bypass or diversion channel

could be incorporated into the design to ensure

proper performance of the structure and to reduce flood levels in the vicinity. Numerous existing water level gages that are either inoperable in

winter or totally abandoned could be used as

sites for detecting ice movement and providing

advance warning of breakup or flooding.

Most ice control projects pursued by the Corps

of Engineers in the past have been conducted under the general authority of Section 205 of the

1948 Flood Control Act, as amended. That act authorized the expenditure of funds for the construction of small projects for flood control and

related purposes not specifically authorized by

Congress. More recently the Water Resources Development Act of 1986, Title XI, Section 1101 (U.S.

Congress 1986) directed the Secretary of the

Army to undertake a program of research and

community assistance for the control of ice and

ice-induced streambank erosion. Further, the act

authorized the Secretary of the Army to:

...provide technical assistance to units of local

government to implement local plans to control or break up such ice. As part of such authority, the Secretary shall acquire necessary

ice-control or ice-breaking equipment, which

shall be loaned to units of local government together with operating assistance, where appropriate.

The act also authorized and directed the Secretary of the Army to undertake a series of demonstration projects for the structural control of ice,

projects that were to be exempt from the cost-sharing provisions of Section 103 of the Water Resources Development Act of 1986. Under this act the

Corps of Engineers would have been authorized

and funded to assist local communities in analyzing their ice-related flooding and streambank erosion problems, develop measures to control ice

and mitigate damages, and lend specialized “icebreaking” equipment that could have been used to

implement some of the advance or emergency

measures described above. At some sites, structural ice control measures would also have been implemented. Unfortunately the necessary funding

was never appropriated.

SUMMARY AND

CONCLUSIONS

This report has reviewed historical and recent

information on ice processes and ice-related flooding in the vicinity of the Buford–Trenton Irrigation

District near Williston, North Dakota. Based on

that information, an evaluation of the ice regime of

the Missouri and Yellowstone Rivers was conducted to assess the potential severity of ice-related

flooding. A scheme for estimating the potential for

ice-related flooding in any given year was outlined, along with a suggestion that existing water

level gages be reactivated for winter activity to

provide an early warning mechanism for ice

breakup and flooding. Finally, a number of structural and nonstructural flood control measures

were reviewed from a technical applicability

standpoint. Mitigation techniques recommended

for further, detailed study include a program of ice

weakening in advance of spring breakup, improvements to existing irrigation ditches to allow

them to afford an increased level of protection as

pseudo-levees, and structural ice control or diversion on the Yellowstone River near its confluence

with the Missouri River.

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September 1995

Ice Jam Flooding on the Missouri River ENH 2654

Near Williston, North Dakota

James L. Wuebben and John J. Gagnon

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road CRREL Report 95-19

Hanover, New Hampshire 03755-1290

U.S. Army Engineer District, Omaha

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161.

This investigation focused on ice-related flooding along the Missouri River, just below the confluence with the

Yellowstone River near Williston, North Dakota. This area is at the upper end of Lake Sakakawea. With the

closure of Garrison Dam in 1953, Lake Sakakawea began filling, reaching operational levels in 1965. Changes

in the hydraulics, sedimentation and ice regime of the Missouri River caused by the impoundment have led to

an increase in the potential for overbank flooding. This report describes the ice regime assessment that was

conducted to characterize ice jam flooding, the development of a method to predict the potential for ice jam

occurrence and severity, and potential flood mitigation measures.

Ice jams

Ice prediction

Ice modeling

Ice control

Ice jam flooding

Ice jam flood mitigation