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Public Works

 

Flood Control - Flash flooding /Local Flood Hazards                                                 Updated November 26, 2012

 


Photo by John Locher - Car and driver in flood water

It can be difficult, especially for the 1,000 plus people who move into Henderson every month, to believe that southern Nevada residents face the very real threat of flooding, especially flash flooding, year round.  Flash flooding is most likely during our monsoon season of July through September when we experience our summer rainfall

 

Photo by John Locher

Turn Around, Don't Drown
Hard as it might be to believe at first, yes, there is a significant risk of flooding in Henderson, Nevada during one of these usually brief but very intense summer thunderstorms.

Some of the areas prone to flooding are obvious such as washes, arroyos, ravines and gullies, but others such as such as the plains lying below mountain valleys are not.

Photo by John Gurzinski

Photo by John Gurzinski of  flood water

To help you identify the risks associated with specific regions, FEMA (Federal Emergency Management Agency) has published maps of areas that are prone to flooding.  These FIRM (Flood Insurance Rate Maps) indicate two basic flood zones in the Henderson area: Zone A are those areas prone to flooding during the 100-year flood while Zone X indicates the areas not prone to flooding.

Zone A is further subdivided into additional flood designations based on what is known about the flood zone such as Zone AE where a detailed study has determined the water surface elevation for the flood zone.  Zone AO are areas where the depth of flow has been calculated. 

Zone X areas are the areas that are not prone to flooding. The two types of Zone X areas are shown on the FIRM by shading. Un-shaded Zone X areas indicate areas outside the 500-year flood plain. Shaded Zone X areas are the areas outside the 100-year flood plain but within the 500-year flood plain. A 100- or 500- year flood plain is simply the geographic area the 100- or 500- year flood occurs in.

To determine the flood zone where you live, work, or play, visit the Clark County Regional Flood Control District at: http://gustfront.ccrfcd.org/ParcelInFloodZone/default.aspx/. You will need to provide a street number and street name. When entering data, do not use road, street, avenue, et cetera. You can also search by Assessor Parcel Number (APN). 

Flash Flooding can happen without apparent warning. The sky is blue above, but it is raining off in the distance. It doesn't seem dangerous.  Suddenly there is water everywhere. What just happened? Where did all the water come from?  Several factors can be involved, but the most basic is the rain water did not soak into the earth, but instead runs off,  The runoff collects in low-lying areas and rapidly flows downhill. Flash floods most often occur in normally dry areas that have recently received precipitation, but may be seen anywhere downstream from the source of the rainfall, even dozens of miles from the source.

Most people tend to underestimate the dangers of flash floods. Flash floods are extremely dangerous because of their sudden nature. Being in a vehicle provides little to no protection against being swept away; it may make people overconfident and less likely to avoid the flash flood. More than half of the fatalities attributed to flash floods are people swept away in vehicles when trying to cross flooded intersections. It takes very little water to carry away even the larger SUV type vehicles. The occupant in the vehicle above was safely rescued.

Our southwestern desert is especially dangerous for both hikers and vehicles from the sudden onslaught of water from isolated thunderstorms. These rains fill poorly-absorbent and often clay-like dry riverbeds. A moving flood will usually be headed by a debris pile that may have wood branches and/or logs. Deep slot canyons can be especially dangerous to hikers as they may be flooded by a storm that occurs on a mesa miles away, sweeping through the canyon, making it difficult to climb up and out of the way to avoid the flood.