Non-perennial streamflow and sediment mobilization in a semi-arid rangeland slated for development

Urbanization has profound impacts on stream morphology and streamflow patterns. Predicting these impacts in semi-arid areas is limited by the lack of information on pre-development streamflow, as non-perennial streams are not well monitored in semi-arid areas. This research focuses on the ephemeral stream West Stroh Gulch, located south of Denver, Colorado, U.S. This grassland watershed was used for grazing during the pre-development study period of June 2020 – August 2023. During this time, time-lapse photography at five locations along the stream network along with radar-rainfall data were used to determine which storm events did, or did not, lead to a streamflow response. Out of 115 recorded storm events, 32 led to streamflow. A classification tree indicated that streamflow was due to storm events with higher rainfall intensities (maximum 30-minute intensity above 4.3 mm/hr and 60-minute intensity above 6.2 mm/hr) and 7-day antecedent rainfall above 20.3 mm. A two-dimensional hydrodynamic model (SRH-2D) enabled comparison of the impacts of predicted flows through a reach of interest. The model used a digital elevation model developed using structure-from-motion techniques and drone aerial imagery. Storm Water Management Model (SWMM) generated peak flows were used to simulate impacts of different sized storms (Water Quality Event (WQE), 2-, 5-, 10-, 50-, and 100-year storms) and development and stormwater management scenarios (pre-development, developed undetained, and developed with distributed detention). Results predicted that the smallest but most frequent storms (WQE and 2-year storms) would cause increased flow and boundary shear stress post-development. For larger storms, distributed detention facilities reduced post-development flows below pre-development and were well below the undetained development, which consistently had the largest flow and potential for sediment mobilization. The findings contribute to understanding of ephemeral streamflow in rangelands and streamflow regime alterations as drivers of sediment mobilization with urban development.