Floodplain Management Plan

On December 2, 2011, the City of Boise adopted the Ada County Emergency Management Agency’s All Hazards Mitigation Plan. In the plan, the City of Boise identified 16 different Initiatives it would pursue related to mitigation of seismic, flood and wildfire hazards in the city.

As required by the City’s participation in the National Flood Insurance Program’s Community Rating System annual recertification program, Boise is reporting on our progress with each of the flood-related initiatives identified in the plan under Activity 510. This report will also address any flood events which occurred in the past year, any changes in our capabilities that could impact implementation, and consider recommendations for changes or enhancement.

Action Plan Initiative Progress

Initiative #B-1 | Esther Simplot Flood Channel

A flood study of the Boise River between Main St. and Veteran’s Memorial Park bridges is underway and expected to result in a project to modify overbank areas in the park to reduce flood potential in both Garden City and Boise City.


Boise has continued to work with adjacent irrigation companies, the park developers and their consultants in a plan to modify the river bank and adjacent city-owned park properties on the north side of the river to increase the conveyance capacity of the river and overbank area to reduce potential flooding of developed areas on both sides of the river. In 2014, a Conditional Letter of Map Revision (CLOMR) was prepared and submitted to FEMA that documents a “no impact” condition for Esther Simplot Park construction and reconstruction of the river inlet structure for Farmers Union Canal.  It was approved by FEMA later that year. With the park’s final completion coming to an end, the final Letter of Map Revision (LOMR) will be submitted to FEMA for approval late this year.

With the LOMR approval the Flood Insurance Rate Maps (FIRM) will be modified in this area to include all improvements and increased conveyance channels.

Initiative #B-5 | Flood Containment Facility Maintenance

Continue to maintain foothills flood containment structures located within our gulches (Hulls, Stuart, Crane Creek, Cottonwood Creek etc.) and the flumes that run through parts of the city.


Boise has continued its normal high quality maintenance of all foothills containment facilities. This involves annual inspection, clean-out, weed control, bank and fencing maintenance and inlet/outlet repairs as needed.

Initiative #B-6 | Maintain Boise's Compliance

Maintain Boise’s compliance and good standing under the National Flood Insurance Program (NFIP).


Boise has continued to administer the floodplain development regulations with diligence and accuracy.  Public Works Engineering staff coordinate with PDS Planning and Building staff to review all development applications in the floodplain for compliance with the standards.  We require Elevation Certificates for all new construction and substantial improvements and maintain copies on file in the office.  This year, we issued 15 final Elevation Certificates.  We also work with applicants in the processing of Letters of Map Revision (LOMR’s) as necessary.  This year, five LOMR’s were approved for multiple locations within the city.

  1. Harris Ranch Subdivision #16 (amended)
  2. ACHD Bridge replacement on S. Maple Grove (south of Overland)
  3. Pepper Hills Subdivision
  4. Five Mile Creek reroute (south of Gowen Road)
  5. Harris Ranch Estates Subdivision #16

Initiative #B-7 | Maintain City's Classification

Continue to maintain/enhance the City’s classification under the Community Rating System (CRS).


Boise continues to participate in the CRS classification program with the goal of maintaining our Class 6 rating.  Our annual public outreach letters include detailed information including a flood map, the history and the nature of flood events in Boise, how the floodplain is regulated, how to prepare for and maintain safety in flood events and a detailed list of contacts. Boise provides proof of all the monitoring, record keeping, reporting, system maintenance and training necessary to maintain our Class 6 rating. In 2016, Boise received its five-year CRS visit at which time we were moved into the 2013 Coordinator’s manual.

Initiative #B-15 | Coordinate with Local Experts

Whenever possible, coordinate with local experts and employ natural environmental processes in mitigation activities that increase ecosystem resilience and reduce the impacts of flooding on the built environment.


Public Works Engineering staff is resolved in ensuring our riverbanks are not completely rocked and is using techniques to soften the bank repairs, when applicable, with vegetation and natural techniques. There have been several projects that river barbs, live stumps, Fabric Encapsulated Soil Lifts (FESL), and natural stone, have been used to mitigate riverbank projects.

Flood Events from 2017-2019

Boise City has no repetitive loss areas and has experienced only minor, isolated flooding this spring (2019). A stretch of the greenbelt south of Lake Harbor was damaged during peak flows when a tree fell in the river, temporarily altering the watercourse. There were no flood events or damage in the Spring of 2018, which was a below average water year. In the Spring of 2017, however, a heavy snow pack and unusually high dam release rates did cause significant damage to banks and greenbelt pathway facilities along the river. Boise obtained a FEMA grant and the appropriate floodplain permits for repair of those areas.  That work has since been substantially completed.

Changes in Capabilities that Could Affect Implementation

Boise has not experienced any negative changes in the capabilities of staff or staffing levels that would affect our ability to continue to implement the Action Plan for flood mitigation. In fact, we have increased our capabilities. Our Public Works Assistant City Engineer obtained his Certified Floodplain Manager certificate in 2017 and continues to take FEMA floodplain management classes in anticipation of an increase to structures added within the floodplain once the new FEMA FIRM panels get formally adopted. This is an enhancement of our staff capabilities for floodplain management plan administration.

Recommendations for Changes or Enhancements

This was the eighth year after adoption of the new Ada County All Hazards Mitigation Plan/Floodplain Management Plan in 2011. It was comprehensively updated in 2015 and was subject to minor update in 2017. Our efforts remain focused on Initiative #B-1, the Esther Simplot Park Flood Management project. This is a multi-year planning and construction project and we will continue to refine the plan through 2019 and will submit final elevation survey data to FEMA after completion of the park. No changes or additions to the Action Plan are recommended at this time.

In conjunction with adoption of the new Floodplain Maps, the Planning Team recommends updating the Floodplain Chapter of the Development Code as well as application forms, templates, and checklists. The intent is to provide more clarity on the regulations, requirements, and risks associated with development in the floodplain.

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