Monday, June 29, 2015 | 2 a.m.
Las Vegas is one of the driest cities in the country, receiving an average of just 4 inches of rain per year.
Current system includes:
■ 90 detention basins
■ 596 miles of channels and storm drains
■ 129 miles of natural washes
Still to be built
■ 31 detention basins
■ 210 miles of channels and storm drains
■ 25 years estimated completion
But if enough rain falls in the right place in a short amount of time, the valley’s desert landscape quickly can be inundated with raging flood waters.
With no tornadoes or hurricanes to contend with, floods are Las Vegas’ most damaging and most deadly natural disaster.
Las Vegas experienced four floods in 2014, two of which were declared emergencies by the National Weather Service, the most severe category of alert.
Most flooding happens during the monsoon season, from July to September, but floods can strike any time of year.
The problem was worse 30 years ago, when the valley’s drainage system consisted of several natural washes and a piecemeal network of city flood control channels. In 1985, with the valley on the verge of a growth spurt, state legislators decided a more organized approach was needed and formed the Clark County Regional Flood Control District to oversee urban and rural lands in the valley, on Mount Charleston and in smaller communities such as Mesquite and Moapa.
Since its formation, the district has spent $1.7 billion building a flood control system that today includes dozens of detention basins and hundreds of miles of channels. The system is about 75 percent complete. During intense storms, there still are parts of town where floods submerge streets.
Why does Las Vegas flood?
The geography of the Las Vegas Valley basin is similar to a bowl tipped slightly toward its edge. Storm water flows downhill through the valley from west to east, converging in the Las Vegas Wash before pouring through the Clark County Wetlands and into Lake Mead.
1. The desert landscape and sparse vegetation do little to absorb water flowing over it.
2. The desert soil also is averse to water, with some stretches of natural land absorbing so little water, it may as well be asphalt.
3. The valley’s urban sprawl of roads, shopping centers and neighborhoods means there’s lots of paved land that’s little help absorbing or slowing storm flows. “Water just hits and runs off,” said Steve Parrish, general manager of the Clark County Regional Flood Control District. “When you combine all of those things with an intense storm, the result is flooding.”
Who monitors flooding?
The Flood Control District’s job is to make sure water flows from one side of the valley to the other in as orderly a fashion as possible, away from areas with lots of buildings or people. The system is designed to handle a 100-year storm, which has a 1 percent chance of happening in any given year.
How is the system funded?
In 1986, voters approved a quarter-cent sales tax increase to fund the regional flood control district. The dedicated funding is unique compared with most flood districts, which are paid for with annual appropriations from local governments. Predictable revenue allows the Clark County Regional Flood Control District to plan its construction over decades and tackle projects in a more efficient manner.
Several tools are used to carry and control storm water
■ Detention basins
Detention basins act as temporary storage when water pours into the valley faster than it can drain out to the lake. The basins fill during storms, then slowly divert the water to channels at a manageable rate. The detention basins typically span 10 to 50 acres and are up to 50 feet deep.
Most homes in Las Vegas aren’t in designated flood zones, meaning homeowners aren’t required to buy federal flood insurance. But buying a flood policy, which can cost as little as $300 to $400 a year for homes in low to moderate risk areas, is worth considering. The flood control district offers resources to help homeowners buy flood insurance and research the history of flooding near their homes.
The development of the regional flood control network has resulted in more than 51 square miles of land in Clark County being removed from federally designated flood zones, saving people millions of dollars annually on insurance premiums.
■ Storm drains
Storm drains are the feeder system of the flood control network, using curbs and gutters to carry water off streets and into underground drains that lead to flood control channels.
Seven large concrete channels form the backbone of the regional flood control system, collecting water from storm drains and detention basins. The channels converge in the Las Vegas Wash and flow to Lake Mead. During storms, the otherwise dry channels can fill up quickly and create rapid currents that move thousands of cubic feet of water per second. In most areas, the channels are open and uncovered, allowing water to flow easily into them over their sides. Where flood channels meet roads or developments, the flow is routed underground through four-sided concrete box culverts.
Gaps in the system
Despite the progress made over the past three decades, there still are many parts of the valley that remain unconnected to the regional drainage system, causing streets to flood when heavy rains arrive. The gaps in the system are caused by the piecemeal measures put in place by cities before the flood control district was formed, as well as continued development on the edge of the valley that’s being built faster than the flood district can keep pace. For instance, a stretch of Grand Teton Drive in the northwest suffered frequent flooding because of a one-mile gap in the drainage system. The gap recently was fixed, thanks to $15 million of construction to install new drains under the road.
The most recent deaths due to flooding came in 2012 when a pair of people were killed in separate incidents during a summer of heavy storms. A Henderson teenager died after being swept into a flood channel on Pittman Wash. At the Desert Rose Golf Course, a landscaper died after being swept away by flood waters. Flood officials say there are a few basic steps people can take to stay safe when the waters start rising.
Don’t drive into flood waters
Most injuries and deaths from flash floods come from motorists who don’t heed warnings or roadblocks and drive onto submerged streets. While from a distance flood water may not look like it’s moving fast, it can travel as fast as 30 miles per hour — fast enough to lift a car and send it tumbling off the road. Drivers “think they’re in this big, heavy car and that they’re almost invincible,” Parrish said. “They don’t understand that cars can float. If the doors are shut and water gets above the bottom of the car, there’s a buoyancy effect.”
Stay away from channels and basins
Flood control facilities are scattered throughout the valley, with detention basins near soccer fields, dog parks and walking trails. The basins are permanently marked with no trespassing signs, but the warnings are especially important to heed when the basins start to fill with water. Water flows rapidly through the flood control system, making it nearly impossible for even experienced swimmers to escape if they fall into a channel. Banks around some of the channels are unlined and can become unstable during storms, increasing the risk that someone trying to get a close look at the storm waters may fall in.