Assessing Flow Alteration and Channel Enlargement Due to Augmentation and Dam-regulation at Hog Park Creek in the Southern Rocky Mountains
B.S. (ESS-Watershed Science) 2013 Colorado State University, Fort Collins, CO, USA 80523-1476
To assess flow alterations without pre-dam flow records, the US Geological Survey's Precipitation-Runoff Modeling System (PRMS) simulated natural flows for historic (1980-1999), contemporary (1995-2015), and future (2040-2059) periods. A regionalization technique transfers calibrated parameters to Hog Park Creek model parameterization from Encampment River model parameterization.
The three greatest dam-induced flow alterations are 1) faster flood fall rates, 2) greater winter and spring monthly flow magnitudes, and 3) greater 7-day low flow. Both warmer-wetter and -drier climate-induced flow alteration scenarios predict 1) increased annual runoff and 2) more frequent and flashier peak flows. Additionally, the warmer-wetter scenario predicts a shift to earlier peak and central timing of flows; whereas the warmer-drier scenario predicts no changes in timing, but decreased winter flow magnitudes. Extreme climate-induced flow alterations are minor compared to augmentation and dam-regulation.
Geomorphic response is predicted by the qualitative channel response model for increased flow with sediment loads less than sediment transport capacity: channel widening, discontinuous bed degradation with pool erosion and riffle erosion/deposition, bed material coarsening, and a decrease in slope (Lane, 1955; Brandt 2000). From 2006-2015, riffle cross-sections (XS) exhibit little change (increase area of 0.3 m2). Pools located near the maximum point of scour exhibit substantial widening (increase area of 3 m2). Despite nonlinear, episodic channel widening and discontinuous bed erosion, there is a continuous decrease in water surface elevation (3 cm decade-1) throughout the reach. Flood inundation area decreased on the order of 100 m2 m-1 of stream decade-1. Changes in hydraulics can be related to ecologic effects such as riparian vegetation establishment and growth.
Findings from the coupling of hydrologic and hydraulic models also support the theory that alluvial channel form is most influenced by bankfull discharge, which in this case is equivalent to the 1.5-year flood (Dunne and Leopold, 1978). Based on a connection between bankfull discharge and corresponding 1.5-year floods, channel enlargement began near a pre-dam bankfull dimension of 3.8 m3 s-1 and increased to 4.7 m3 s-1 in 2006 and 5.5 m3 s-1 in 2015. Though there is a high degree of uncertainty, one possible trajectory of channel enlargement is to a bankfull discharge of 5.8 m3 s-1, which is the observed 1.5-year flood and effective discharge for the last 30-year period since dam enlargement.
Advisor: Steven Fassnacht
John Stednick (Watershed Science)
Greg Butters (Soil and Criop Sciences)