Floodplains

Preliminary Floodplain Maps
New Preliminary Floodplain Maps available for review.

Boise East Floodplain Delta | Boise East Floodway Delta

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Floodplain Information

Boise City is crossed by a large freestone river and several foothills gulches that are susceptible to annual flooding events that pose threats to life and safety and can cause significant property damage. 

The Boise River system drains a very large area on the west and south sides of the Sawtooth Mountains. Although three dams (Anderson Ranch, Arrowrock and Lucky Peak) control the release of water, unusual climatic events such as heavy winter snowfall and warm wet springtime conditions can and have caused release rates from the dams to be high enough to create widespread local flooding in the Boise area. 

In addition, sudden and severe thunderstorms over the foothills can cause flash flooding on the various gulches that drain into the Boise urbanized area. Ongoing development within the City and County continues to displace natural areas that have historically functioned as flood storage.

Flood Insurance


Boise City participates in the National Flood Insurance Program (NFIP) that makes federally backed flood insurance available for all structures, whether or not they are located within the floodplain. 

  • More than 25% of NFIP claims are filed by properties located outside the 100-year floodplain, also known as the Special Flood Hazard Area (SFHA). 
     
  • Following the purchase of flood insurance, NFIP imposes a 30-day waiting period, so residents should purchase insurance before the onset of the rainy season to ensure coverage during the flooding season.
     
  • Membership within NFIP (and the availability of flood insurance to City residents) requires the City to manage its floodplain in ways that meet or exceed the standards set by FEMA. 
     
  • NFIP insures building with two types of coverage: structural and contents. Structural coverage includes walls, floors, insulation, furnace and other items permanently attached to the structure. Contents coverage may be purchased separately to cover the contents of an insurable building. Flood insurance also pays a portion of the costs of actions taken to prevent flood damage.

Floodplain Understanding and Regulation


Maintaining the flow capacity in streams that cross City properties requires cooperation and assistance to prevent flooding and bank erosion. Following are some suggestions and information for understanding the ways that floodplains function and how the City regulates the floodplain in order to protect property and lives, while affording citizens the ability to obtain floodplain insurance.

  • Do not dump or throw anything into ditches or streams
    A plugged channel cannot carry water, and when it rains, the excess water must go somewhere. Trash and vegetation dumped into a stream degrades water quality of both the stream itself and its receiving waters, and every piece of trash contributes to flooding.

    The City and Ada County Highway District have adopted and enforce regulations that prohibit dumping material into any natural or manmade component of the drainage system. 
     
    • Observations of dumping debris or other objects into street drains should be reported to the Ada County Highway District at 387-6325. 
    • Dumping into Foothills Gulch Facilities (Cottonwood, Hulls, Crane and Sand Creeks), streams, drainage ways, or rivers should be reported to the Boise City Public Works Department at 608-7150.
       
  • Remove debris, trash, loose branches and vegetation
    Keep banks clear of brush and debris to help maintain an unobstructed flow of water in stream channels. Do not, however, remove vegetation that is actively growing on a stream bank. Streamside vegetation is tightly regulated by local, state and federal regulations. Before undertaking any removal of streamside vegetation, contact the Boise City Planning and Development Services Department at (208) 608-7100.
     
  • Obtain a floodplain development permit and/or building permit, if required
    To minimize damage to structures during flood events, the Boise City requires all new construction in the floodplain to be anchored against movement by floodwaters, resistant to flood forces, constructed with flood-resistant materials and flood-proofed or elevated so that the first floor of living space, as well as all mechanical and services, is at least 1 foot above the elevation of the 100-year flood.

    These standards apply to new structures and to substantial improvements of existing structures. The City defines a Substantial Improvement as any reconstruction, rehabilitation, or addition to an existing structure, the cost of which exceeds 50 percent of the structure's appraised or market value (whichever the builder chooses to use).

    Additionally, most other types of development within the floodplain also require a floodplain development permit, such as grading, cut and fill, installation of riprap and other bank stabilization techniques. City staff are available to undertake site visits, if requested, to review flood, drainage and sewer issues. Contact the Boise City Planning & Development Services Department at (208) 608-7100 for further information and prior to undertaking any activity within the floodplain.
     
  • Recognize the natural and beneficial functions of floodplains to help reduce flooding
    Floodplains are a natural component of the Boise City and Treasure Valley environment. Understanding and protecting the natural functions of floodplains helps reduce flood damage and protect resources. When flooding spreads out across the floodplain, its energy is dissipated, which results in lower flood flows downstream, reduced erosion of the streambank and channel, deposition of sediments higher in the watershed and improved groundwater recharge. Floodplains are scenic, valued wildlife habitat, and suitable for farming. Poorly planned development in floodplains can lead to streambank erosion, loss of valuable property, increased risk of flooding to downstream properties and degradation of water quality.
     
  • Reduce risk of damage to homes
    Practical and cost-effective methods for reducing or eliminating the risk of flooding are available to property owners whose homes have experienced damage from flooding in the past, or may experience damage in the future.

    Such techniques include elevation of the home, relocating the home to higher ground, constructing floodwalls or berms, flood-proofing and protecting utilities.
    Contact Boise's Planning & Development Services department at (208) 608-7100 and the Federal Emergency Management Agency (FEMA) Region X at (425) 487-4600 for more information.

    During times of flooding, homes that have not been retrofitted can be protected during emergencies by the installation of sandbags. For further information about sandbags and the locations of sites where sandbags are available during flooding, contact Ada City-County Emergency Management (208) 377-6645 or visit their website at www.accem.org. In case of an emergency, call 911.
     
  • City Floodplain Information Services
    The City can determine the relationship of a particular property to the floodplain, including:
    1. Whether the property is located within the Special Flood Hazard Area; 
    2. Flood Insurance Rate Map (FIRM) Zone for property; 
    3. Base Flood Elevation for property, if available; and 
    4. Whether the property is located within the Floodway. 

Contact the Boise City Planning & Development Services Department at (208) 608-7100 for further information. 

Flood Hazard Information


During Boise's typical hot, dry summer it is difficult to envision flooding as a problem. However, for those that live in or directly below the foothills gulches, flash flooding related to thunderstorms is possible this time of year. In the spring, flooding of the Boise River becomes a possibility. Extensive property damage and even loss of life can occur from flooding. Property owners in a designated Special Flood Hazard Area (SFHA) should make every effort to protect themselves and their property from flood loss.

  • Mapping & Geographic Information System (Online Mapping)
    The City of Boise's  Mapping & Geographic Information System has the floodplain boundary maps available for viewing and printing. Property owners can research floodplain mapping data online.

    Using the Online Map: After opening Public Work's mapping page, scroll down to the link to "Boise Interactive Mapping System." Once the map opens, click on the "Planning & Zoning" folder to access the floodplain information. For problems with the online map, contact Jim Hetherington.
     
  • Planning Staff
    Boise City Planning staff can also provide floodplain information. Call 608-7100.
     
  • Ada County Emergency Management
    The Ada County Emergency Management organization has a wide range of flood plain information, including a "Flood Preparedness" page

    The document "Are You Ready for the Flood?" describes Boise's past flooding events, discusses a range of property protection and personal safety measures that can be taken to minimize risk of damage or injury, provides more information about the National Flood Insurance Program and gives local contact information for additional resources. A two-page handout "Emergency Preparedness Pointers" is also available from the ACCEM webpage. 
     
  • Additional Links
     Federal Emergency Management Agency (FEMA) 

Flood Hazards, Flood Insurance and Flood Protection in Boise


The Flood Insurance Rate Map (FIRM) for the City of Boise identifies properties that are within or in close proximity to special flood hazard areas (SFHA) that are subject to flooding. Such areas are associated with the floodplain of the Boise River and Boise River tributaries including foothills gulches such as Cottonwood Creek, Crane Creek, Hulls Gulch, Sand Creek (Stuart Gulch) and Squaw Creek. The Boise River and its tributaries, as well as the various foothills gulches have all caused damaging flooding in past years and will inevitably do so again.

National Flood Insurance Program and Community Rating System
The City of Boise is an active participant in the National Flood Insurance Program (NFIP), which enables owners of property within floodplain areas to purchase flood insurance. The City also participates in the Community Rating System (CRS) Program, which rates the community's performance in administering the NFIP. 

  • Currently, Boise has a CRS rating of 6, which qualifies City flood insurance policy holders with U.S. companies for a 20% reduction in their flood insurance premiums. Foreign insurance companies, such as Lloyds of London are not obligated to provide the discount. 
     
  • The City must re-certify its CRS Program annually in order to maintain its classification. One of the elements of re-certification is a community outreach program and that is the primary purpose of this correspondence.

The City's Planning & Development Services Department (PDS), located on the second floor of City Hall, maintains copies of the current FIRM, which were effective February 19, 2003. Copies of the current FIRM and other floodplain documents are also available at the public library. PDS Staff is available to assist the public in determining whether or not specific properties are within a SFHA. We can also help you determine the estimated depth of flooding in your area.

Flood Insurance
Mortgage lenders generally require flood insurance for properties within a SFHA. Citizens that own property outright are not required to purchase flood insurance and some elect not to, primarily because of the cost of the insurance premiums. All properties within SFHA should be covered by flood insurance as the primary protection against loss of property from flooding.

Mitigating Flood Damage 
In addition to purchasing flood insurance, there are physical improvements that can be made to mitigate the damage from flooding. In some cases, it may be financially feasible to raise or elevate a structure so that the lowest floor is above the base flood elevation. There are also methods for dry flood-proofing structures so that all portions of the building below the base flood elevation are made watertight. Certain properties may also have options of physically raising structure openings such as window wells or constructing berms around the foundation. 

Several documents on flood protection are on file at the Boise Public Library including "Homeowner's Guide To Retrofitting (Six Ways To Protect Your House From Flooding)."

Thank you for maintaining an awareness of the potential hazard from flooding in our community. Please feel free to contact the PDS Staff at 608-7100 should you need information or assistance with floodplain issues.