Logan Floodplain Modeling & Mapping
Project Summary
This project for Logan City, Utah was completed over two phases. First, flooding events experienced by the City of Logan brought into question the accuracy of the regulatory floodplains shown on the effective Flood Insurance Rate Maps (FIRMs). Therefore, the City contracted ACE to complete the following tasks:
- Determine the accuracy of the regulatory floodplains and existing levee systems
- Update the regulatory flood hazard information through a Physical Map Revision (PMR)
- Confirm existing known and identify any new flooding problems
- Develop a coordinated plan of flood mitigation and channel restoration/stabilization improvements
Before the Phase I PMR was submitted to FEMA, widespread flooding occurred in Cache County following record snowmelt in the spring/early summer of 2011. Flood hazard modeling/mapping developed under Phase I of this study was successfully utilized to implement flood fighting strategies during flood events along both Blacksmith Fork River and Logan River in May and June of 2011. Following the 2011 flood, ACE was contracted to assess flood damages, prepare construction plans for channel restoration/stabilization improvements under the NRCS Emergency Watershed Protection (EWP) program; and update the PMR submittal to reflect all stream restoration projects.
Details
- Client/Owner: Logan City Public Works Department
- Project Location: Logan, Utah
- Project Completion Date: March, 2022
- Phase I Project Components:
- Hydrologic investigation using PEAKFQ and current USGS gage data that revised regulatory discharges for Logan River, and confirmed applicability of effective FEMA discharges for Blacksmith Fork River
- Hydraulic modeling, using HEC-RAS, of approximately 20 miles of the Logan River, the Blacksmith Fork River, and Spring Creek, that include numerous flow splits and divided flow paths, lateral weirs, and hydraulic structures such as bridges, culverts, and levees
- Preparation of digital floodplain work maps that include delineation of the 100-year and 500-year floodplains and one-foot rise floodway
- Evaluation of existing levees, including the coordination of a geotechnical/subsurface evaluation to determine whether or not the levee embankments can be certified as providing protection from the 100-year flood
- Documentation supporting preparation of topographic mapping, field reconnaissance, hydrologic and hydraulic analyses, levee evaluations, and flood hazard mapping for a Physical Map Revision (PMR) submittal
- Preparation of digital flood hazard data in DFIRM format
- Phase II Project Components:
- Detailed field reconnaissance effort to support the geomorphologic evaluation and collect bed and bank material soils samples
- Geomorphologic assessment and sediment transport analysis of current conditions
- Formulation and preparation of channel restoration/stabilization and flood mitigation plan for Logan River, Blacksmith Fork River and Spring Creek
- Hydraulic design of all channel restoration/stabilization and flood mitigation plan components, including riprap toe protection, rock grade controls, river edge restoration to improve riverine ecology and aquatic/fishery habitat
- Hydraulic design of the 1700 S Bridge replacement over the Blacksmith Fork River
- Hydraulic design of the Blacksmith Fork berm to provide flood protection for the Country Manor Subdivision
- Preparation of preliminary designs, construction cost estimates, and an implementation/prioritization plan
- Preparation of construction plans and specifications for selected plan components
- Hydraulic modeling and flood hazard mapping of as-built conditions, including preparation of a PMR application to FEMA
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