FEMA Flood Studies
FEMA needed a detailed hydrologic, hydraulic, and engineering assessment to guide potential revisions to its mapping of special flood hazard areas in Santa Clara County, California. Specifically, Watsonville Slough and its tributary, Struve Slough, as well as a portion of Salsipuedes Creek, were mapped Zone A on the County's effective Digital Flood Insurance Rate Map (DFIRM).
ESA led a detailed engineering assessment of the study reaches and a hydrologic assessment to determine peak discharge. A thorough review of topographic data and acquired base mapping was supplemented by ESA to meet FEMA standards. Hydrologic and hydraulic analyses using HEC-RAS software, the rational method, and the regional regression method were conducted for the study reaches in Watsonville for the 10-, 2-, 1-, and 0.2-percent-annual-chance flood events to establish flood elevations and regulatory floodways for Struve and Watsonville Sloughs. For Salsipuedes Creek, ESA surveyed the left bank levee to evaluate the available freeboard and the effective floodplain mapping accuracy. ESA developed a Technical Support Data Notebook (TSDN) from interim work products per FEMA Guidelines, inputting information from the annotated work map onto a digital topographic work map using FEMA DFIRM specifications.
ESA assisted FEMA Region IX to assess the flood hazards for Watsonville Slough and its tributary, Struve Slough, as well as a portion of Salsipuedes Creek.