The Federal Clean Water Act requires large and medium-sized towns across the United States to take steps to reduce polluted stormwater runoff. The law was applied in two phases. The first phase addressed large cities. The second phase, often referred to as ”Phase II,” requires medium and small cities, fast growing cities, and those located near sensitive waters to take steps to reduce stormwater. In Douglas County, the Clear Creek area and parts of Johnson Lane are subject to these regulations. The Clear Creek Stormwater Management Plan and the Johnson Lane Stormwater Management Plan provide additional information on the steps being taken to manage stormwater.
What causes polluted stormwater runoff?
Polluted stormwater runoff generally happens anywhere people use or alter the land. People going about their daily lives are the number one source of stormwater pollutants. Most people are unaware of how they impact water quality. Some common examples include over fertilizing lawns, excessive pesticide use, not picking up pet waste, using salt or fertilizer to de-ice driveways, letting oil drip out of their vehicles and littering. Developed areas in general, with their increased runoff, concentrated numbers of people and animals, construction and other activities, are a major contributor to non-point source pollution, as are agricultural activities. Other contributors include forest harvesting activities, roadways, and malfunctioning septic systems.
Flash Floods Versus Riverine Floods
Douglas County experiences two hydrologically distinct types of floods: riverine flooding, typical of the Carson River during times of spring snowpack melt or rain-on-snow events, and flash floods, typical of the recent Johnson Lane events. The County participates in FEMAs Community Rating System (CRS), that incentivizes communities to pursue floodplain management strategies to reduce flood damage and support flood insurance. Floodplain maps have been developed, and many people who live within the “100-year floodplain” are encouraged, and sometimes required, to purchase flood insurance policies. In the recent flash flood events around Johnson Lane, many people affected were not in an identified 100-year floodplain. This area typifies an alluvial fan: a gently sloping, fan-shaped landform created over time by deposition of eroded sediment, common at the base of mountain ranges in arid and semiarid regions such as much of the western United States (National Research Council, 1996). Building on alluvial fans in the western United States has occurred as development reaches the outskirts of urbanized areas. While riverine flooding usually follows well-defined boundaries and has more predictable behavior, alluvial fan floods move laterally and can migrate during the course of a flood, and may not even follow the same path in subsequent floods.
According to FEMA (FEMA-165, February 1989):
“An often overlooked ‘hazard’ is the tendency to underestimate both the potential and severity of alluvial fan flood events. The infrequent rainfall, gently-sloping terrain, and often long time spans between successive floods contribute to a sense of complacency regarding the existence of possible flood hazards. Though the intense rainstorms which produce fan floods occur randomly, they nevertheless can develop very rapidly at any time, and can recur with any frequency.”
Alluvial fan or flash flooding does not play by the same rules as riverine flooding, as recognized by FEMA in 1989. Residents must be aware that while some floodplain boundaries are well-defined (Carson River floodplain), other areas of the Valley present unpredictable flood hazards such as flash floods. Residents are encouraged to discuss different flood insurance policies with their insurance agents, attend Nevada’s Flood Awareness Week events in November, and visit County staff at the outreach session to be held at the CVIC Hall from 3.30 to 7.00 on Thursday, August 6.