Project: Martin Slough Letter of Map Revision
Client: Douglas County, Nevada
Location: Towns of Minden and Gardnerville, Nevada
Description:
Martin Slough is a distributary of East Fork Carson River and also receives flows from Pine Nut Creek. It passes through a significant portion of the developed and undeveloped areas along the east side of the Towns of Minden and Gardnerville, Nevada.
Challenge: After substantial development of the area in and around the Martin Slough floodplain, the Flood Insurance Rate Maps were seriously out of date. That, coupled with the relatively coarse modeling used to estimate base flood water-surface elevations and the five-foot contour mapping used to define the floodplain boundary, indicated that the floodplain boundaries were not current. Furthermore, changes to the floodplain mapping could potentially affect a number of property owners along Martin Slough. Finally, the hydraulics are relatively complex at both the upstream and downstream ends of the study reach, requiring substantial analysis to determine the direction of flow so that appropriate cross sections could be developed.
Solution: A combination of field-surveyed cross sections and sections derived from the 2004 Douglas County LiDAR survey were used to construct an existing conditions model of the Martin Slough floodplain from Stockyard Road to Toler Avenue. Numerous changes to the floodplain were evident, both from fill placed along the floodplain fringe and from street crossings of Martin Slough. All of these changes were included in the one-dimensional HEC-RAS model. Because of changes to the principal direction of flow resulting from modifications to the profile grade of U.S. 395, the resulting modeling was substantially different than the original modeling done in 1982.
After review by Federal Emergency Management Agency (FEMA) engineers, FEMA directed Douglas County to develop a floodway for Martin Slough. A regulatory floodway indicates the portion of the floodplain where placement of fill or other changes is not allowed without mitigation that results in a no-rise state. The floodway was required because changes to the Martin Slough floodplain resulted in a rise exceeding the allowable one foot in portions of the study reach.
The floodway analysis was completed and the project report, modeling, and supporting documentation were revised and submitted to FEMA for re-review.