Lake level regulation, shoreline erosion and shore protection : Flathead Lake, Montana Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tt44pq19z

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  • Concern regarding increased coastal erosion has heightened amid growing acceptance of global warming and associated sea-level rise. This study examines shoreline erosion in Flathead Lake, Montana due to five decades of artificially elevated lake levels. It provides a model to investigate coastal erosion associated with sea-level rise. The natural water level fluctuation in Flathead Lake is regulated witha dam, which has elevated the entire annual fluctuation approximately 0.7 m on average. The annual rise in lake level is held for an extended duration at a regulated "full pool" level which is approximately 3 m above its natural base elevation, and the total transgression time for a complete cycle of lake level change has been increased from 100 days to over 300 days. Nearshore environments in Flathead Lake, are described as dissipative or reflective on the basis of a surf similarity parameter, grain size, morphology, number of breaking waves and angle of wave incidence. The relative resistance to foreshore and backshore erosion caused by anthropogenic lake level regulation is compared between the two nearshore configurations. Reflective systems are characterized by dynamic gravel beach faces and steep inshore shelves armored by wave-washed cobble. In contrast dissipative systems are characterized by sand-sized substratum, broad flat inshore shelves, and the presence of multiple linear bars approximately 350 m offshore. Five decades of regulated lake levels have resulted in extensive shoreline erosion (970 ha on the north shore of the lake) and a general reshaping of both types of nearshore environments, although dissipative shorelines have eroded faster. The presence of docks and other man-made structures on reflective beaches have accelerated erosion by intercepting longshore gravel transport. Shoreline erosion has ceased on the west side of the north shore as the varial zone, (that area between the maximum and minimum regulated lake levels), has reached an equilibrium slope. In contrast erosion and shoreline retreat continues on the east side, although the rate has slowed over the years in response to the development of a near equilibrium varial zone profile similar to that of the west side. However shoreline retreat is quite variable even along short stretches of shoreline. Variability in shoreline retreat results from three erosive processes, undercutting, endstripping and overwash. Undercutting and endstripping occur when waves scour an exposed bank, while overwash results when water is forced over the top of the shoreline bank or berm. Therefore shoreline elevation and morphology determine the type of erosion process, and the rate of shoreline retreat. Localized accretion also occurred due to sediment entrapment by drift logs bordering the shoreline. Drift logs naturally protect the shoreline from direct wave attack and once buried, provide new recruitment area for riparian vegetation. Alteration of lake level fluctuation has resulted in a transfer of annual wave energy from base elevations corresponding to the pre-dam nearshore shelf, to those corresponding with the limnetic foreshore environment, thereby inducing lake-wide erosion. The most extensive retreat has occurred along the low-lying dissipative north shore of the lake. Measured shoreline retreat along the north shore is compared to the retreat predicted by the Bruun Rule. The Bruun Rule under-predicted the actual measured shoreline retreat by as much as an order of magnitude. The redistribution of annual wave energy due to regulated lake level fluctuation is the main factor contributing to erosion, a factor that is not accounted for with the Bruun Rule. Therefore the redistribution of annual incident wave energy is examined in terms of an increased transgression time, a reduced range in lake level fluctuation, and an elevated lake level. Both transgression time and range in water level fluctuation affect the distribution of incident wave energy and were found to be more important to shoreline erosion than an elevated water level. An alternative regulation scheme incorporating both concerns for hydropower production and lake regulation is proposed and examined in terms of reducing erosion. Altering the transgression time of lake level fluctuation by changing the timing and increasing the rate of lake level drawdown, results in a significant reduction in the amount of annual wave energy that reaches the eroding full pool shoreline, thereby reducing the potential for shoreline erosion lake-wide. A lowered regulated full pooi lake level would decrease the intensity of overwash further reducing shoreline retreat related to that process of erosion. Natural gravel beaches can provide the backshore of coastal environments with a protective buffer from wave erosion. A case study is presented of the conceptual design and utility of an artificial perched gravel beach used to stabilize an eroding backshore on the reflective east shore of the lake. Boulders and cobbles were used as a stable platform to perch beach gravels above the previously eroding profile. A longshore exchange of gravel within the beach compartment was incorporated into the conceptual design. The structure has performed successfully during the three year monitoring period as exemplified by the lack of backshore erosion and the maintenance of the perched profile.
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