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Merging New Coastal Floodplain Mapping with Effective Riverine Flood Zones
Krista Conner, Coastal Scientist, BakerAECOM
The process of merging new flood hazard study information into an existing map is commonly referred to as “tying in” the new information.  For the California Coastal Analysis and Mapping Project (CCAMP), the new coastal floodplain mapping will be tied into effective riverine flood zones. In general terms, those tie-ins occur in areas that transition from being controlled by flooding from the coastal flood source to being controlled by flooding from riverine or rainfall events.  Within this transition area, there is typically a risk of both types of flooding.  However, for the purpose of the FEMA FIRM, the “controlling” flooding source is the flooding source with the greater flood elevation associated with a 1-percent-annual-chance flood event. 
 
The goal of the “tying in” process is the creation of seamless floodplain mapping across the transition area.  Each riverine flooding source is reviewed individually to determine the exact location of the limit of the coastal floodplain.  This control limit of the coastal floodplain is located where the coastal base flood elevation (BFE) is equal to the riverine BFE.  For a riverine floodplain mapped on the effective FIRM as Zone A without BFEs, the Zone A area is typically retained on the FIRM, and is not replaced with coastal mapping. This is because the Zone A riverine flooding presents a greater flood hazard than the hazard associated with coastal flooding. Whereas, in highly developed floodplains, as in the case throughout the San Francisco Bay area, structures such as a culvert mitigate coastal flooding.
 

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