Gardnerville board members decided Tuesday night to host another public workshop (date not yet determined) on the Pine Nut Creek and Cottonwood/Martin Slough re-study before submitting to Douglas County any proposed changes to the existing flood hazard maps.
The county is currently appealing the Federal Emergency Management Agency's new flood insurance rate maps for East Valley, which put thousands of parcels not previously in the flood zone into areas requiring flood insurance.
To avoid a similar problem within the town boundaries, Gardnerville hired Manhard Consulting to study flooding within the town and develop new maps.
On Tuesday, Manhard estimated that the Pine Nut Creek drainage peaks at about 4,300 cubic feet per second in a 100-year event, versus FEMA's existing estimate of about 6,600 cfs.
"With the methods used at the time, in the 1970s, predictions were very conservative," said Manhard hydrologist/engineer Denny Peters.
But Manhard's new study also accounts for downstream flows of Pine Nut not included in FEMA's models. Manhard's new maps show portions of Chichester Estates in the "AE" and "AO" 100-year flood zones, which require insurance, and a vast swath of the subdivision in the "Shaded-X" 500-year zone, which doesn't require insurance.
"Chichester is currently in no flood zone and could become littered with a variety of flood zones," said Peters.
To mitigate the situation, Peters proposed a large channel be constructed around the subdivision to divert flows into the Martin Slough. Additionally, he estimated that detention basins upstream could reduce Pine Nut's peak-flow in downtown Gardnerville from 3,100 cfs to about 2,500 cfs.
"It will probably be a combination of storage upstream and conveyance of water around the town," explained Gardnerville Manager Jim Park.
Manhard also estimated Cottonwood Slough's peak-flow, where it splits off the East Fork of the Carson River, at about 7,090 cfs in a 100-year event, slightly higher than FEMA's 7050 cfs.
But Manhard estimated 100-year peak-flows much lower where Cottonwood Slough splits into Martin Slough at Lampe Park. Peters said FEMA assumed a percentage split of 3,900 cfs continuing down Cottonwood Slough and about 3,250 cfs in Martin Slough.
Accounting for irrigation ditches and other flow "break-out areas" upstream, Manhard estimated about 2,200 cfs in Cottonwood Slough after Lampe, and only about 400 cfs in Martin Slough. If accepted by FEMA, the lower peak-flows would reduce the flood zones in the area, Peters said.
He said the county is considering a Valley-wide re-mapping effort that may include Gardnerville's study. He said FEMA may not accept the town's new maps because they're limited to the town boundaries.
"One thing we don't want is to start and then drop the ball on this," said Board Vice Chair Mike Philips.
"It's important we get these maps corrected as quickly as possible," added board member Paul Lindsay.
The board directed staff to collaborate with both the county and Town of Minden before the public workshop.
"Everybody will be above water then," said Board Chair Tom Cook.
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