The vital role of Pittman Wash
Although typically hot and dry most of the year, Southern Nevada can experience intense rainfall which cannot be absorbed quickly by the hard desert soil. This rainwater runs off rapidly over the surface of the land and collects as if flows through the region from west to east, and from a higher to lower elevation, putting Henderson directly in the path of most of the valley’s rainwater flow.
Long-time residents will remember how twenty years ago this rapid rainwater runoff would cause flash floods that created impassable roads and damaged property, and even resulted in fatalities. Since 1960, the area has experienced at least 11 major floods that resulted in more than a million dollars in property damage. In that same period, 31 lives were lost in 21 separate flash flood events.
In order to find solutions to better control flooding, the Regional Flood Control District was formed in 1985 to create a comprehensive Master Plan for Clark County. That plan includes the construction and/or management of 85 detention basins and approximately 560 miles of channels and underground storm drains (of which 130 miles are natural washes) throughout Clark County. To date, an area of 51square miles has been removed from federally-identified FEMA flood zones.
Pittman Wash is essential to this flood control network, channeling the rainwater runoff that collects in Henderson and from the higher elevations across the valley on to Las Vegas Wash and eventually Lake Mead. Because the major washes are dry most of the year, many residents are unaware of the severe flood potential or never see flooding occur until it is too late.
Why it is necessary to protect Pittman Wash
Because it is at the lowest end of the flood control network, the lower reaches of the Pittman Wash are subject to dangerous and destructive storm flows. Documented storm events in the past six to seven years have resulted in substantial erosion in the area of Pittman Wash between the Union Pacific Railroad (UPRR) trestle and Arroyo Grande Boulevard, particularly to the trestle piers, gabion-lined slopes and natural slopes.
The undercutting of the UPRR structure and gabion-slope protection occurred from 2-year and 5-year storm events with stormwater flows ranging from 367 to 1,194 cubic feet per second. The wash must be able to accommodate a 100-year flood event that is projected to create flows of approximately 6,659 cubic feet per second, or nearly 5 times the flow that caused significant erosion in the documented storms.
The following video shows stormwater flows resulting from a 2-year event with flow depth of 1.15 feet and flow of approx 375 cubic feet per second.