Year of Award

2011

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Andrew Wilcox

Commitee Members

Johnnie Moore, Lisa Eby

Keywords

bedload, Blackfoot, channel geometry, Clark Fork, confined, dispersing, DREAM, HEC, hydrology, modeling, Montana, processes, rates, removal, reservoir erosion, sediment budget, sediment characteristics, sediment transport, sediment waves, suspended, thres

Abstract

Measurements of bedload transport in combination with suspended sediment and discharge data collected by the USGS illustrate the rates, magnitudes and processes by which reservoir sediment evacuated the Milltown Reservoir after the 2008 removal of Milltown Dam from just downstream of the confluence of the Blackfoot and Clark Fork Rivers, Montana. Mobilized sediments transported as a series of distinct waves and the speed at which the waves moved downstream was dependent upon the grain size of the sediment. Sand and smaller sized particles were transported out of the reservoir rapidly both as bedload and in suspension with different thresholds for incipient motion between the confined Blackfoot and unconfined alluvial Clark Fork Arms of the reservoir. Bedload sediments, gravel and larger sized, transported downstream as a dispersing and translating wave. Sediment budget calculations, both volumetric and transport derived, illustrate the one-dimensional HEC-6 and DREAM-1 model’s inaccuracy in predicting sediment transport in unconfined alluvial deposits while both models predicted accurately confined channel geometry transport settings. Results from sampling and modeling demonstrate that the most important factors in reservoir sediment transport are the channel geometry, hydrology, and grain size and location of the reservoir sediment; which in turn determine distance, processes and timing of transport.

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© Copyright 2011 James Walden Johnsen