Capacity crowd views flood plan
Johnson Lane is built on what is essentially a giant sand dune deposited over millennia by winds blowing Carson River deposits up against the mountains.
If the wind was able to deposit that sand, it doesn’t take much water to send it back down the hill and into residents’ yards and homes.
On Tuesday night, about 200 residents turned out to hear a presentation on a draft Johnson Lane area drainage master plan prepared by JE Fuller and Lumos & Associates.
County Engineer Erik Nilssen was inundated by calls after the flashfloods in 2014 and 2015.
“I got a lot of calls from people saying ‘Do something, build something, fix something,’” he said. “Well what? Where? How much is it going to cost? What’s the best way to do it? None of those questions were answered.”
He said the drainage plan will help provide those answers.
“There’s ABC, all the way to R and S on steps to mitigate the flood situation,” he said. “Plus, once we get this approved, and we have a mitigation plan we’re more likely to get FEMA money for construction.”
The plan won’t be a panacea, but it will help show federal flood officials what’s needed.
“We can show we’ve analyzed this is the best route,” he said. “It is still difficult to get money for those types of projects. We’ve put a couple in over the last couple of years and have been unsuccessful. But this gets us closer.”
The price tag for the improvements is one of the reasons the county is going to require federal help.
A dam across Johnson Lane Wash could cost $4.9 million.
Building a series of basins to contain the Hot Springs Mountain and Buckbrush flooding would cost $7.3 million, according to information presented to residents on Tuesday.
Water is coming down into Johnson Lane from a dozen different directions including Hot Springs Mountain and the Pine Nuts, according to the study
Engineers studied a 27-square-mile area. Buckbrush and Johnson Lane washes alone account for 14 square miles of drainage.
Unlike the Carson River, water flows across the slope in different channels, which is harder to define.
“Active alluvial fans are subject to some degree of flowpath uncertainty during flood events and are generally characterized by a distributory drainage network that becomes shallower in the downstream direction.”
According to the study, the most likely source of the sand is flood deposits from the Carson River.
That material was deposited during large floods and then carried by prevailing winds northeast to the slopes of the surrounding mountains.
“The thick accumulation of sand on the mountain slopes provides a near endless supply that will impact the Johnson Lane area for the foreseeable future unless infrastructure is put in place to mitigate the issue.”
Johnson Lane’s slope ranges from 4 percent near the mountains to less than 1 percent.
“There is a fairly distinct slope break from 3-4 percent to less than 2 percent near Kayne Avenue, which can result in a change in flooding patterns and deposition of sediment.”