Bank recession processes,rates, and prediction,Lake Sakakawea,North Dakota,U.S.A. |
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| Institution: | 1. Department of Biology and Geology, Universidad de Almería, Carretera de Sacramento s.n, La Cañada de San Urbano, Almería 04120, Spain;2. Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, United Kingdom;3. Departamento de Física de la Materia Condensada, Cristalografía y Mineralogía, Universidad de Valladolid, Paseo de Belén, 7, 47011 Valladolid, Spain;4. Centro de Estudios Ambientales y del Mar, CENM-naturaleza, Alcántara, 5, Cartagena, Murcia 30394, Spain |
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| Abstract: | Bank recession at twenty stations along the eastern end of 286 km long Lake Sakakawea, North Dakota, measured regularly between 1983 and 1986, showed a recession range from essentially zero to almost 9 m yr−1 with an average of 1.6 m yr−1. Between 1969 and 1979, rates were greater, the declining rate a normal consequence of the factors affecting bank recession. Rates of recession vary with seasons, with 78% of the annual recession occurring during the warmer season (May–October) as a result of wave erosion. The remaining 22% occurs during thaw failure in spring.Prediction of ultimate (500 yr) bank recessions attempted using historical recession data, the three years of detailed field measurements, and regression analysis of the erosion factors. An equation, presented here, requires field measurement of bank orientation and height, batch width and composition, and slope of the offshore area. Effective fetch must be measured, as well. The resulting equation for the monthly warm season recession rate is: Rs=154.9 −, (18.8 √ A+ 25.1 √ B +10.1 √ c +6.9 √ 0+5.0 √ E + 1.1 √ F) where: A = sine of angle between bank orientation and dominant wind; B = offshore slope angle; C = beach width; D = bank height; E = effective fetch; and F = percentage of coarse beach clasts. The colder season recession percentage is: Rw = Rs (2.05D = 0.043G) − 2/100] where D is bank height and G the bank orientation with respect to sun. The total yearly predicted recession (cm yr−1) is: Rt = 6 (Rw) + 6 (Rs) 6 being the number of months of “cold” and “warm” weather at Lake Sakakawea.For this rate to be projected into the future, it is necessary to assume a declining rate of recession, employing a parabolic trend. From this, the ultimate bank recession for Lake Sakakawea is predicted to range up to 542 m. |
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