willow creek no-rise analysis
PROJECT TYPE
Waterway & Floodplain Management
CLIENT
Private Landowner
LOCATION
Caldwell, ID
project description
A private landowner built a structure encroaching into the floodway in Middleton, Idaho, without a floodplain development permit, in violation of the floodplain ordinance. WCG was contracted by the landowner to assist with achieving compliance with the floodplain ordinance and completing a no-rise analysis. WCG coordinated with FEMA and the Canyon County Floodplain Administrator and Engineer during our floodplain modeling effort to assist the landowner with obtaining compliance while maintaining the existing location of the structure. WCG was able to certify a no-rise condition by providing mitigation excavation in the floodway to offset the hydraulic impact from the structure.
WCG overcame several challenges during this project. It was evident that extensive changes to the FEMA mapped floodplain had occurred as a result of the subdivision development. WCG compared surveyed cross-sections and LiDAR data to the effective model to determine that the flood fringe was likely filled from material excavated in the floodway. Therefore, the LiDAR data was used to replace affected areas for 8 model cross-sections in the existing conditions model to more accurately reflect the current topography. WCG provided the landowner’s surveyor with pertinent data necessary to complete the elevation certificate and receive a building permit.
key services list:
Hydraulic Modeling in HEC-RAS
Agency Coordination with FEMA and Canyon County
Hydraulic memorandum documenting methodology
LiDAR and ground survey comparisons
Flood conveyance mitigation design