Churchill County Commissioners agree to new floodplain mapping
December 26, 2017
With the 2017 water season still on everyone's minds, the general manager of the Carson Water Subconservancy District asked Churchill County commissioners to submit a letter to his board requesting a flood flow study below Lahontan Reservoir.
Ed James said the mapping has been done for the Carson River until it reaches the reservoir. He said FEMA (Federal Emergency Management Administration) funded the upper river floodplain mapping, but the new request will update the mapping for the Carson River corridor in Churchill County, which was last completed 40 years ago. With preplanning and mitigation, though, an emergency task force was able to prevent the Carson River from experiencing widespread flooding.
"A lot of work has been done to handle the flows this year," James said.
According to James, he recommended the commissioners request a study that will calculate peak flows below the reservoir that meet FEMA requirements. He also said the study would look at scenarios that look at flood events based on incremental years such as 20-, 50- or 100-years and how water is measured flowing from the reservoir into the river.
"This is one of the criteria needed to know in future planning," James said.
Commission chairman Pete Olsen, who had a major role in this year's flood mitigation, said the task force could've used more information on its planning earlier this year.
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"We could've used better data," Olsen said. "We were using older maps like on Bravo 16."
Olsen agreed with James, pointing out data has changed, but the chairman said the county has a "good handle" on preparation.
"Using new numbers gives us an idea of what we can do," Olsen added.
The commission approved the letter and Olsen submitted it last week to the CWSD. In the letter, Olsen said, "the Bureau of Reclamation has changed its operation methods for Lahontan Reservoir, furthering the need to update mapping of the area.
"The recalculation of peak flows below Lahontan Reservoir will enable Churchill County to evaluate flood preparation for our community. This data will also be helpful in aiding the county with decisions to pursue new flood maps or develop an area drainage master plan."
The commission learned the Bureau of Reclamation will begin LIDAR (Light Detection and Ranging mapping) in February of the Carson River corridor. According to the National Oceanic and Atmospheric Administration, "LIDAR is a remote sensing method that uses light in the form of a pulsed laser to measure ranges (variable distances) to the Earth. These light pulses — combined with other data recorded by the airborne system — generate precise, three-dimensional information about the shape of the Earth and its surface characteristics."
The data gleaned from Reclamation's mapping will provide additional information where flood water flows when released from the reservoir.
The emergency task force, along with the Truckee Carson Irrigation District, used previous information and historical date to mitigate potential flooding in March when the water district and Churchill County constructed an emergency weir and spillway off the V-line canal east of Diversion Dam. Subsequently, the water flowed into the desert and onto Bravo 16, a Navy training range, and then east under U.S. Highway 50.
The water filled Carson Lake, and the construction of the "Big Dig" or a deep, wide channel then carried the water under U.S. Highway 50 north of Grimes Point and then toward the Stillwater National Refuge and Carson Sink.
Although the Sierra had record snowfall, James told commissioners earlier this month "the river functioned like it's supposed to. We had some flooding in Lyon County and had a lot of runoff."