LAS VEGAS WASH—DYNAMICS OF WATER AND SEDIMENT IN SEVERE FLOODING

The Las Vegas Wash illuminates the dramatic effects of urban development on a desert drainage system. Once seasonally active with broad floodplains, the Las Vegas Wash has become a perennial stream. The wash is the largest of three tributaries in Nevada flowing into Lake Mead—the other two being the Virgin and Muddy rivers. Stretching over a distance of approximately 12 miles, the wash begins in the southeastern portion of Las Vegas and ends at the Las Vegas Bay on Lake Mead. The wash carries water returns to the lake in the form of urban runoff, shallow groundwater seepage, storm water, and reclaimed water released from the valley's three water-reclamation facilities .

The Las Vegas Wash has experienced severe channel-bed degradation and bank erosion over the past 50 years, causing the channel planform, or pattern, to evolve; wetlands to diminish; and water quality to decline. Due to the short-term surges of floodwaters associated with storm events, the channel has undergone sudden and sharp erosion.

DRI’s Jennifer Duan, specialist in computational hydroscience and engineering, is using a groundbreaking, advanced computational model, which she has previously developed, to simulate the hydrodynamic processes affecting the severely damaged Las Vegas Wash. This investigation was conducted under a Cooperative Agreement between the U.S. Army Corps of Engineers and DRI. The goal of this work was to understand flood paths running through the Las Vegas Valley, and with this knowledge, design bank-protection structures that could prevent future bank erosion and improve overall stability and function in the wash.

Dr. Duan with other DRI researchers, including Drs. Dong Chen and Li Chen, Jen Weller, Ryan Sharrer, and Shane Rotter, used this two-dimensional, hydrodynamic model to predict the natural processes of bank erosion, channel migration, and movement and deposition of sediment through the wash. Not only was the model used to analyze and identify natural processes at work in the Las Vegas Wash, but it also will be used to predict channel responses to proposed restoration projects.

This initial analysis of the Las Vegas Wash was needed to measure the amount of sediment that is transported during base and storm flows and to evaluate the impact of high-volume turbulent storm flow on geomorphic processes in the wash. DRI scientists have conducted field measurements of flow and sediment transport monthly in the Las Vegas Wash, a gravel-bed stream, at three cross sections since 2003. These cross sections were selected because they were relatively stable, and it was possible to wade into the water at these areas. Because of the severe bed degradation and bank erosion, other locations were either inaccessible or too unstable.

With Jennifer Duan’s model, the magnitude of a 100-year flood was simulated to visualize flow-field and bank-erosion processes, as well as sediment-transport processes expected to occur around dike structures. Because, significant sediment transport usually occurs during high flow, determining the extent of bed-load transport during storm events was necessary to assess properly the effect of storm flows on geomorphic processes. Investigations have shown that the banks of the Las Vegas Wash have suffered incredible degradation of approximately 4.9–5.9 feet during the 10 years between 1989 and 1999.

Floods can dramatically alter water channels, large and small, over the course of a single flood event, producing devastating consequences. Following such events, communities and land managers must determine the best ways to correct the damage and restore the waterway to an appropriate level of service and function. Computational approaches utilizing mathematical models, such as Dr. Duan’s ENSED2D model, continue to afford DRI opportunities to play instrumental roles in the restoration of numerous waterways in Nevada and worldwide.

For additional information,
contact Jennifer Duan, DRI associate research professor.  
Office—702.862.5452
755 E. Flamingo Road
Las Vegas , NV 89119
Jennifer.Duan@dri.edu