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Clearwater Hydrology (CH) prepared a flooding and water supply assessment for Green Gulch Farm (GGF) in Marin County, CA. In-stream ponds, culverts, and cement-lined channels were installed at the farm as part of historical ranching and farming activities. These structures created an extremely complex scenario for flooding risk assessment. Flooding characteristics for the complex, existing conditions at the farm were analyzed for the 10-yr., 50-yr. and 100-yr. rainstorms. The assessment included a field inspection of channel hydrologic and hydraulic conditions; companion geotechnical evaluations of detention pond embankment stability; structural evaluations of channel modifications; and an extensive topographic survey of Green Gulch Creek, its southern tributary and the valley floor. The topographic survey was organized to generate a site map, including the location, relative elevations, and interconnections of the stream/conveyance system and the adjoining floodplain.
The refined, topographic base map provided channel geometric data for a coupled hydraulic (HEC-RAS) and hydrologic (HEC-HMS) modeling of floodwater pathways, inundation extent and depths of flooding for each of the three selected flood events. A patchwork of culverts, sump areas, detention ponds, concrete-lined channel segments, and disparate overland flow zones provided a challenging context for flood modeling. The primary products of the study were detailed, scaled maps of the extent of inundation for each flood event. Building finished floor elevations and depths of inundation potentially affecting each structure were documented, and recommendations were made regarding citing of future GGF development and an eventual restoration of a sustainable fluvial geomorphic setting. Historical water use and supply source data- for both surface impoundments and groundwater wells- were compiled and assessed. The flood mapping and inundation information are providing important information to guide the development of a new Master Plan for GGF.
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