Evaluation of the Ecological and Hydraulic Conditions of the Resilient St. Vrain Project (Longmont, Colorado)
Sharon Bywater-Reyes1*, Keaton MacMillan1, Antonio Reveles-Hernandez2, Sarah Holland1, Mimi Dunda1, Nathan Duggins3, Jeremy Burton1
1University of Northern Colorado, Greeley, Colorado
2University of Wyoming, Laramie, Wyoming
3Front Range Community College, Longmont, Colorado
We assessed the effectiveness of the city section of the Resilient St. Vrain Project (Longmont, Colorado) by characterizing: 1) the health of the riparian system in terms of vegetation-channel-flow relationships, 2) geomorphic condition in terms of complexity and bed mobility, and 3) the ability of the St. Vrain to withstand future 100-year flow events. We found the functional traits of the revegetation plan to have variable outcomes regarding diversity and roughness, depending on the implementation location and time. We also found channel slope and grain size to be adjusting to the reconstruction. Repeat grain size measurements showed coarsening throughout the river, which may impede bed mobility at moderate flows. Slope was as designed (0.003) for the entire study area, but with some local variations (concave-up sections near Price Road and pools near Dickens Farm) likely responsible for grain-size patterns as the profile adjusts.
Vegetation surveys indicated certain species demonstrated characteristics expected to enhance sediment aggradation. Plum required a force two times higher than that of a Sandbar Willow to bend, indicating its potential to influence hydraulics through drag. Invasive Russian olive and Tamarisk were found in restored reaches, and are very rigid. In contrast, most native species (Cottonwood, Sandbar Willow, and Peachleaf Willow) are flexible. Certain reaches have much higher vegetation roughness than others, particularly lower reaches, which is likely to cause deposition. Many naturally-recruited cottonwood seedlings also contribute to vegetation roughness, with the City’s plan to remove seedlings indefinitely. Nevertheless, the channel design has the capacity to carry a 100-year flood. Shear-stress divergence from hydraulic modeling predicts locations of grain-size changes and depositional areas. This work contributes to our understanding of channel design techniques within urban areas.