Consultant: FEMA CTP4 mapping nearing submittal; HEC‑RAS and USGS data used

Eric Griffin, Development Review Coordinator, told the Board that the District and consultant Wood Rodgers, Inc. are finalizing the FEMA CTP No. 4 floodplain mapping submittal and expect to submit materials to FEMA within weeks. The project covers hydrologic and floodplain mapping for roughly 36 miles across eight Placer County streams under an existing Professional Services Agreement.

Griffin introduced Harvey Oslick, the Wood Rodgers project manager for CTP No. 4, who outlined technical aspects of the work. When asked to define hydrology, Oslick said, "Hydrology is science of determining the flow rate or volume of water at particular locations within the study area." He explained that current modeling used HEC‑RAS (an updated successor to older HEC‑2 modeling) and relied on USGS topographic data from 2017–2020; the team also conducted terrestrial LiDAR surveys at select structures rather than collecting full new topography.

Board members asked whether removal of debris from the American River following a past Hell Hole dam failure would affect downstream floodplain elevations. Staff said the debris removal improved local channel conveyance and that the change in conveyance was expected to be highly localized with no anticipated downstream impacts. Griffin and consultant staff said the CTP4 update includes both areas with prior studies for comparison and "new" areas where no previous mapping exists.