## Theses and Dissertations

1986

Thesis

#### Degree Name

Master of Science (MS)

Geology

J.R. Reid

#### Abstract

Shoreline erosion is a major problem at Lake Sakakawea, North Dakota. Instrumentation along the eastern shore was initiated in 1983 to measure shoreline recession and determine the processes responsible, with the ultimate goal being the development of a relatively simple equation to predict the rate of recession which is better than the model currently used by the U.S. Army Corps of Engineers.

For the twenty stations, the present rate of recession ranges from 0.2 to 4.3 m/y. Approximately 78 percent of the yearly recession occurs during the warm months (May-October) due to wave erosion. Erosion over the cold months occurs as a result of thaw failure.

The most important variables associated with shoreline recession include: bank height, effective fetch, offshore slope angle, beach width, mean grain size, percentage of coarse beach clasts, angle between the shoreline and dominant wind, and bank orientation with respect to the sun. These variables, along with the average monthly rate of recession, were submitted to regression analysis.

Because the rates of recession are seasonally dependent a separate equation was developed for warm and cold season recession. The warm season recession equation, in cm/mo, is:

1a) Rs = 141.53 – [17.2√A + 8.44√B + 25.08√C + 10.4√D] where A= angle between the dominant wind and the shoreline, B= bank height, C= offshore slope angle, and D= beach width. This equation exceeds the 95 percent confidence level with an r² of 59.3 percent. This equation is preferred to equation 1b:

1b) Rs=154.9-[18.8√A + 25.12√B + 10.06√C + 6.91√D + 5.03√E + 1.1√F] which includes effective fetch (E) and percentage of coarse clasts (F). This equation exceeds the 75 percent confidence level only. Both equations, however, produce similar results.

The cold season rate is:

2) Rw = Rs [(2.05 (bank height) + 0.043 (bank orientation) - 2)/100, where bank orientation is with respect to the sun. This analysis exceeds the 99 percent confidence level and produces an r² of 46 percent. The yearly rate of recession (cm/yr) is the sum of the warm and cold season recession multiplied by their active months.

3) Rt= 6(Rs) + 6(Rw)

For future bank recession it was assumed that the rate of recession will decrease with time. Thus, an equation was developed that incorporated the present yearly recession rate and the formula for a parabola. Recession calculated from the equations predicts cumulative recession up to 495m over the 500-year life of the reservoir.

Although these equations are a significant improvement over the template method in use by the U.S. Army Corps of Engineers, further testing is necessary to determine their applicability to other inland bodies of water.

COinS