Local and state government officials, along with residents, attended a seven-hour workshop about alluvial fan flooding at the Carson City Sheriff’s Office on Wednesday – good timing before the forecasted spring storm hits the Sierra ridges this week.
Collaborated by the U.S. Army Corps of Engineers and the Nevada Division of Water Resources, the seminar introduced weather and geological experts, whom presented an overview of issues associated with alluvial fan flooding in Nevada, geographical hazards, triggering storm types, and master drainage plans.
The seminar was developed under the Nevada Silver Jackets program.
Regional speakers included climatologist Nina Oakley of the Western Regional Climate Center at the Desert Research Institute in Reno; and Douglas County Engineer Erik Nilssen.
Oakley discussed how the state’s meteorological events contributes to alluvial fan flooding.
“We don’t have many snowpack events like we’ve had recently,” she said. “There are hazards we don’t know about, now that we’re coming into spring and warmer weather.”
As for these last few months experiencing vicious rain and snow storms, the area went through an atmospheric river system – narrow-shaped storms carrying moisture from outside of the tropics – a typical occurrence for October through March, Oakley said.
Now that spring has arrived, northern and western regions of the state will experience cut-off low storms – a bowl-shaped, low-pressure system that brings long stretches of cool, cloudy, showery weather – for the remainder of the season, she said.
Once June hits, the area can expect more thunderstorms this year before another cut-off low revisits in the fall.
“We haven’t had many this year but they’re prevalent in the west,” Oakley said. “It’s an interesting impact for Nevada. Our last three spring seasons have been more wet than average.”
Although these weather patterns are normal for the area, Oakley said as population increases, there’s more alluvial pressure to build and prepare for storms.
Some residents who live in flood-prone areas are in hope developers will install a lot with proper drainage systems to help relieve results from storms.
But according to Nevada Statue 278.461, a proposal to divide land for transfer or development into four lots or less "shall prepare a parcel map and file the number of copies, as required by local ordinance, of the parcel map with the planning commission or its designated representative or, if there is no planning commission, with the clerk of the governing body; and pay a filing fee in an amount determined by the governing body".
Douglas County engineer Nilssen said if four lots or less are created, the parcels will be used for residential, commercial or industrial purposes. An agency may require off-site access, street alignment, surfacing and width, water quality, water supply and sewerage provisions only as necessary and consistent with the existing use of any land zoned for similar use which is within 660 feet of the proposed parcel.
“For infrastructure, the developer pays for it up front, or the county would have to come in and have the infrastructure installed," he said. "It is more expensive for the County to retrofit the drainage and other utility improvements after the area is built out than for the developer to install it up front."
Douglas County experienced four flash flood events at Johnson Lane in summer 2014, with 144 lots affected, costing $927,000 of public infrastructure and more than $1 million in private property damages, he said.
And the flash flood during summer 2015 cost more than $2 million in damage to public infrastructure.
As a result of damages from flash floods over the years, especially in Douglas County, Nilssen proposed a master drainage plan.
The ideas consider identifying capital projects in areas going through development, identifying probable costs and quantities, and brainstorming alternative designs.
“Area drainage master plans generally have an alternatives assessment, but we are not doing one with the Johnson Lane master plan," Nilssen said. "Instead of doing an alternatives option, master plan will consider the effectiveness of soft improvements including vegetation restoration, contour ditches, individual lot management plans or private parcel best management practices."
That consists of vegetation restoration, contour ditches, individual lot management plans, or private parcels.
There’s also consideration of building a 400-acre flood control reservoir on Smelter Creek to protect residents, but that would cost $7 million. He said the main priority is to provide storm water utility to perform routine culvert maintenance.
“This master plan is more about identifying risks and mitigating flood risks,” he said. “Not mapping a floodplain.”
The cost of storm water utility fee for residents is narrowed by the user’s demand placed on the drainage system.
Nilssen said this fee isn’t to be confused with a tax or flat fee. He said the utility is flexible as it can be adjusted with major transitions in programs, and also stable as it’s not as dependent on the whims of the annual budgetary process as taxes.
This action is already in place in Carson City’s utility bills.
“This fee is not going to fix flood problems, either,” he said. “It’s more going toward grants and projects for improvement.”
One of Oakley’s take-home messages included high intensity rainfall can happen at anytime, regardless of the season.
The seminar included a discussion on non structural flood proofing techniques and planning measures, presented by USACE and Pacific Advanced Civil Engineering, Inc.
The seminar will also be presented in Las Vegas on Friday.
Updated Thursday, April 15, 10:58 a.m.