Date of Award

1985

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

J.R. Reid

Abstract

Shoreline erosion is a problem at Lake Sakakawea and Lake Audubon, North Dakota. Land is lost, water quality is adversely affected, and reservoir storage capacity is decreased.

Instrumentation of the eastern shores of both lakes began in 1983 to quantify bank erosion by process (e.g., wave erosion, rainsplash and runoff, and frost-thaw failure). Other data gathered included: pool level fluctuations; wind velocity, direction, and duration; precipita tion; soil moisture; frost penetration; freeze-thaw cycles; and geology (e.g., texture, clay mineralogy and structure).

The magnitude of shoreline erosion is highly variable, especially within Lake Sakakawea. For the interval of May 1983 through August 1984 the banks receded between 0.6 and 5.9m (0.5 to 4.6m/yr). Measurement of aerial photographs for 1966 to 1976 yielded similar average recession rates (0 to 4.3m/yr).

The predominant activating cause of bank recession at Lake Sakakawea is wave erosion; it is responsible for about 87 percent of total bank recession. The most important variables include: pool level; wind velocity, direction and duration; bank orientation, geology, geometry and vegetation cover; natural rip-rap; offshore bathymetry; and near-shore islands. Results indicate that banks that are shorter than Sm, which face the north or northeast, and are composed of well-jointed till or mudstone, have the highest recession rates, especially during high pool levels. At Lake Audubon, the most important activating factors are lake ice-shove and subsequent wave erosion. These factors caused most of the 0.8 to 1.4m (0.7 to l.2m/yr) of bank recession.

Because of the nearly vertical banks, the effects of rain on the primary sediment and bedrock are minor. Most of the 2 to 52mm of bank slope erosion at each lake occurred in colluvium at the toe of the banks. This colluvium, primarily derived from sublimation and thaw failure, ranged from 0.13 to 3.30m 3 per metre of shoreline at Lake Sakakawea in spring 1984. Thaw failure accounted for about 13 percent of total bank recession and was greatest for those banks facing west and northwest, and which are composed of well-jointed till or mudstone. Measured colluvium volumes for Lake Audubon varied from 0.7 ta l.Sm3 per metre of shoreline.

Erosion at Lake Sakakawea begins in late winter as frost sublimates. The loosened aggregates accumulate as a thin apron at the foot of steep banks. Spring thaw results in slab failures, followed by earthflows and mudflows. As summer approaches, the lake rises until maximum pool level is reached sometime in mid-summer. Storm waves easily erode the loose calluvium along the base of the banks. If all the colluvium is eroded, the waves can remove the primary sediment or bedrock, effectively under cutting the banks. At the top of such banks, extensional joints are initiated. The joints expand until bank failure releases the stresses. Bank failure continues until a relatively stable profile has formed. Late summer to early winter is an extended period of relative quiescence, after which time release of aggregates by sublimation again occurs.

Ultimate bank recession at Lake Sakakawea primarily depends upon the wave energy that reaches unprotected banks. Thus, as long as the pool level is not kept at or below about 562.0m msl., the beaches and banks will not stabilize and bank recession will continue.

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