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Roaring Fork Bed Material Entrainment

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Roaring Fork Bed Material Entrainment

Study Area: Statewide
Period of Project: October 2000-December 2001
Project Number: CO423
Project Chief: John G. Elliott
Cooperator: Town of Basalt, Colorado

BACKGROUND:

The Town of Basalt, Colorado, is located at the confluence of the Fryingpan River with the Roaring Fork River, in Eagle and Pitkin Counties. The Roaring Fork River is a single-thread, steep-gradient, cobble/boulder-bed river that presents a natural hazard to the town, nearby residential structures, several bridges and roadways, and the municipal waste-treatment facility. The hazard originates from high velocity snowmelt runoff and a high bedload transport rate that cause intermittent shifts in the channel form as well as scour and deposition of coarse-grain bars and islands. Recent urban and recreational development on the flood plain, earlier attempts to confine the Roaring Fork channel with retaining walls and levees, and flow obstructions such as bridge piers have altered the hydrology, hydraulics, sediment transport and sediment deposition in the river at Basalt.Recent engineering studies developed hydraulic models and estimated flood-hazard areas for the Roaring Fork River channel and flood plain at Basalt (McLaughlin Water Engineers, 2000; Matrix Design Group, 2000). However, these studies did not address sediment entrainment, transport, and deposition processes.

OBJECTIVES:

1. To evaluate a previously surveyed reach of the Roaring Fork River for the purpose of estimating bed-material entrainment potential. The reach begins just upstream from the Highway 82 Upper Bypass Bridge and extends approximately one mile downstream.  The study reach includes the confluence of the Fryingpan River and three bridges.
2. To determine bed-material particle-size statistics at several locations in the study reach where entrainment or deposition potentially could occur. 
3. To estimate bed-material entrainment potential from the channel and adjacent alluvial bars, if present, at 14 previously surveyed cross sections for several flood-discharge scenarios.