|Abstract or Summary
- 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
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