A. Standards For Floodplains Without Established Base Flood Elevations (BFE):
Within the special flood hazard areas designated as zone A (also known as unnumbered A zones) and established in section 10-1-6, where no base flood elevation (BFE) data has been provided by FEMA, the following provisions, in addition to the provisions of section 10-1-8-2 and section 10-1-8-6, shall apply.
The BFE used in determining the flood protection elevation (FPE) shall be based on the following criteria:
1. When base flood elevation (BFE) data is available from other sources, all new construction and substantial improvements within such areas shall also comply with all applicable provisions of this chapter and shall be elevated or floodproofed in accordance with standards in section 10-1-8-6.
3. All subdivision, manufactured home park, and other development proposals shall provide base flood elevation (BFE) data if development is greater than five (5) acres or has more than fifty (50) lots/manufactured home sites. Such base flood elevation (BFE) data shall be adopted by reference in accordance with section 10-1-6 and utilized in implementing this chapter. The applicant/developer shall submit an application for a conditional letter of map revision (CLOMR) prior to preliminary plat approval and have obtained a letter of map revision (LOMR) prior to any building permits for structures being issued.
4. When base flood elevation (BFE) data is not available from a federal, state, or other source as outlined above, the lowest floor shall be elevated or floodproofed (non- residential) to two feet (2') above the highest adjacent grade (HAG) at the building site or to the flood protection elevation (FPE) whichever is higher, as defined in section 10-1-5. All other applicable provisions of section 10-1-8-6 shall also apply.
B. Standards For Rivervine Floodplains With Base Flood Elevations But Without Established Floodways:
Along rivers and streams where base flood elevations (BFE) data is provided by FEMA or is available from another source but floodway areas are not identified for a special flood hazard area on the FIRM or is in the FIS report, the following requirements shall apply to all development within such areas:
1. Standards of section 10-1-8-6.
2. Until a regulatory floodway or flood fringe area is designated, no encroachments, including fill, new construction, substantial improvements, or other developments shall be permitted unless certification with supporting technical data by a registered professional engineer is provided demonstrating that the cumulative effect of the proposed development, when combined with all other existing and anticipated development, will not increase the water surface elevation of the base flood at any point within the community.
C. Standards For Floodways And Flood Fringe Areas: Areas designated as floodways are located within special flood hazard areas established in section 10-1-6. The floodway is an extremely hazardous area due to the velocity of floodwaters which carry debris, potential projectile and erosion potential. The following provisions apply:
1. Encroachments, including fill, new construction, substantial improvements and other development, are prohibited unless an approved floodplain development permit is issued demonstrating that encroachments shall not result in any adverse impacts during the occurrence of the base flood.
2. Encroachments, including fill, new construction, substantial improvements and other development are prohibited unless certification by a registered professional hydraulic engineer is provided demonstrating that encroachments shall not result in any increase in flood levels during the occurrence of the base flood discharge; and uses within the floodway shall be restricted to those which are required by public necessity (for example, bridges, water pumps); recreational use (for example, paths); wildlife habitat improvements (for example, vegetation, nesting structures, pool/riffle improvements); and gravel extraction; provided that the use/encroachment meets the approval of the federal emergency management agency and national flood insurance program and does not jeopardize the city's participation in the national flood insurance program.
3. Division C1 of this section shall comply with all applicable flood hazard reduction provisions of this section 10-1-8.
4. The placement of any mobile homes, as defined in section 8-1-2 of this code, is prohibited.
5. All buildings shall be set back a minimum of one hundred feet (100') from the floodway line. Except that when the special flood hazard area boundary is one hundred feet (100') or less from the floodway line, the boundary line shall be the setback line.
6. No development is permitted within the twenty five foot (25') setback from all waterways called the riparian zone and riparian vegetation shall be maintained in its natural state for the protection and stabilization of the riverbank, and removal of trees or other vegetation is regulated in accordance with this chapter.
7. For all subdivisions along the Boise River or Dry Creek, prior to approval of the final plat written approval from flood control district 10 is required to protect access to the river for maintenance.
8. Compensating excavation in accordance with an engineered plan for orderly conveyance of floodwater, or equivalent mitigating measures may be performed in the floodway when certified by a registered professional engineer. Mitigation design shall include provisions to prevent relocation or diversion of flow paths from causing increased jeopardy to any off site property at any level of flooding from the one (1) year flood up to the base flood. Maintenance provisions for excavated areas prone to fill from sediment and other debris shall follow the requirements of section 9-3-2-5C of this code for private streets.
9. In the design of public and private parks and open space areas, fixed structures or equipment that would impede floodwaters shall not be permitted within the floodway.
10. Any non-floodway area surrounded by the floodway shall be subject to the conditions of divisions C1 through C9 of this section, unless such area is accessed by at least one (1) road with suitable structures or culverts to pass floodwaters. If a bridge is required, it shall be elevated one foot (1') above the base flood elevation, have scour resistant footings extending to five feet (5') below the bottom of the channel and be capable of supporting an H-15 load as defined by the american association of state highway and transportation officials (AASHTO).
11. A conditional letter of map revision (CLOMR) has been approved by FEMA. A letter of map revision (LOMR) must also be obtained within six (6) months of completion of the proposed encroachment.
D. Standards For Areas Of Shallow Flooding (Zone AO, AH, AR/AO, or AR/AH): Located within the special flood hazard areas established in section 10-1-6, are areas designated as shallow flooding areas. These areas have special flood hazards associated with base flood depths of one (1) to three (3) feet where a clearly defined channel does not exist and where the path of flooding is unpredictable and indeterminate. In addition to sections 10-1-8-2 and 10-1-8-6, all new construction and substantial improvements shall meet the following requirements:
1. The lowest floor shall be elevated at least as high as the depth number specified on the flood insurance rate map (FIRM), in feet, plus a freeboard of two feet (2'), above the highest adjacent grade; or at least three feet (3') above the highest adjacent grade if no depth number is specified.
2. Non-residential structures may, in lieu of elevation, be floodproofed to the same level as required in section 10-1-8-7D1 so that the structure, together with attendant utility and sanitary facilities, below that level shall be watertight with walls substantially impermeable to the passage of water and with structural components having the capability of resisting hydrostatic and hydrodynamic loads and effects of buoyancy. Certification is required in accordance with sections 10-1-8-2K and 10-1-8-6B.
3. Adequate drainage paths shall be provided around structures on slopes to guide floodwaters around and away from proposed structures.
E. Riparian Areas: Located within areas of special flood hazard, established in section 10-1-6, are areas designated as riparian areas. Since riparian areas provide critical flood management and fish and wildlife habitat, the following provisions apply:
1. Preservation or restoration of the inherent natural characteristics of the river and creeks within the floodplain;
2. Preservation of riparian vegetation and wildlife habitat, if any, along the stream bank and within the required minimum twenty five foot (25') setback or riparian zone;
3. No development or other than development by the city of Eagle or required for emergency access shall occur within the twenty five foot (25') riparian zone with the exception of approved stream stabilization work. The city council may approve access to property where no other primary access is available. Private pathways and staircases shall not lead into or through the riparian zone unless deemed necessary by the Eagle city council;
4. Plan and time frame shall be provided for restoration of riparian vegetation damaged as a result of the work done;
5. New or replacement planting and vegetation shall include plantings that are low growing and have dense root systems for the purpose of stabilizing stream banks and repairing damage previously done to riparian vegetation. Examples of such plantings include: red osier dogwood, common chokecherry, serviceberry, elderberry, river birch, skunk bush sumac, Bebb's willow, Drummond's willow, little wild rose, gooseberry, and honeysuckle.
F. Map And Narrative Required: All subdivision proposals and development permit applications shall provide a map and detailed narrative describing any abutting or on site manmade topographic features, current or proposed, that increase flood damage potential above the natural level, including road fills and other manmade embankments, prior stream channelization, existing and/or proposed landscaping features, and evident agricultural land leveling that has altered the floodplain from its original topography.
G. Fill Requirements: For fill placed in the floodplain:
1. Development within the area of special flood hazard shall result in no net loss in natural storage. Grading plans shall show that existing natural storage volume in the floodplain, as bounded by the existing surface topography and the base flood elevation surface, shall not be reduced from the current quantity. Postdevelopment storage volumes will be calculated from the postdevelopment flood elevation for the base flood event. Depressions which will be filled with groundwater and sections of the floodplain which are restricted from floodwater conveyance due to roads built above the base flood elevation shall not be considered when determining storage volumes. Development within the DDA, TDA or CEDA design review overlay areas shall not be subject to the provisions of this subsection.
2. The slope of fill adjacent to the floodway line, hereinafter called the floodway boundary slope, shall not be steeper than five horizontal to one vertical (5:1). Where a five to one (5:1) slope is not feasible due to lot dimensions, compensating measures to protect against floodway encroachment through erosion shall be constructed as designed and certified by a professional engineer, architect, or landscape architect.
3. The floodway boundary slope shall be maintained in groomed perennial turf or equivalent low ground cover vegetation not taller than twelve inches (12") to provide protection from erosion. Porous stonework or other durable surfacing may be used in lieu of ground cover vegetation for design enhancement on floodway boundary slopes.
H. Roadways And Manholes: Roadways and manholes within the floodplain shall be a minimum of one half foot (0.5') above base flood elevation at centerline and manhole ring respectively.
I. Flow Paths And Natural Storage Areas Maintained: New roads built above the base flood elevation shall not restrict conveyance of floodwater into sections of the floodplain that may be cut off by the proposed road. Culverts or bridges shall be provided under roads to maintain existing natural storage areas in the watershed to reduce flood damage.
J. Open Areas: Except for required berms, open areas such as parks, golf courses, greenbelt areas, parking lots, and the like, within the area of special flood hazard shall be designed and operated so that they may flood and maintain existing natural storage capacity for waters during the base flood event. Levees or berms shall not be used for flood control.
K. Approval Of Plans: Plans for development within the floodplain showing construction and postconstruction best management practices (BMPs) conforming to the catalog of stormwater best management practices for Idaho cities and counties shall be submitted for approval before any grading or other construction activities commence.
L. Flood Study: When a new flood study is provided by the developer:
1. The study shall be performed using the U.S. army corps of engineers hydrologic engineering center's HEC-2, HEC-RAS (river analysis system), or equivalent programs to calculate water surface elevations.
2. The study report shall include field survey data and relevant bench marks used as bases of calculations as cross sections, stream profile and plan showing cross section locations.
3. The study report shall attach HEC-2, HEC-RAS, or equivalent input and output reports as appendices.
4. Any floodway or floodplain modification shall not be effective until approved by FEMA.
5. The study shall include a reach of stream extending beyond the property boundaries of the development upstream and downstream not less than twice the average width of floodway on or adjacent to the development property, or as approved by the zoning administrator.
M. Critical Facilities: Critical facilities shall not be allowed within any SFHA, special flood hazard area, including areas designated zone shaded X: .2% chance of flooding in any given year.
N. Alteration Of Watercourses:
1. If a watercourse is to be altered or relocated, the following shall all be submitted by the permit applicant prior to issuance of a floodplain development permit:
a. Notify adjacent communities and the Idaho Department of Water Resources prior to any alteration or relocation of a watercourse and submit evidence of such notification to FEMA;
b. A description of the extent of watercourse alteration or relocation;
c. A professional engineer's certified report that assures the flood-carrying capacity within the altered or relocated portion of the watercourse is maintained and the effects to properties located both upstream and downstream;
d. A map showing the location of the proposed watercourse alteration or relocation;
e. The proposal is shown to be a permanent solution and creates a stable situation;
f. No increase to the one hundred (100) year floodplain upstream or downstream requires certification by a registered Idaho engineer;
g. The recreational use of the stream, including access along any and all public pedestrian/fisher's easements and the aesthetic beauty, shall not be obstructed or interfered with by the proposed work;
h. Fish habitat shall be maintained or improved as a result of the work proposed;
i. The proposed work shall not be in conflict with the local public interest, including, but not limited to, property values, fish and wildlife habitat, aquatic life, recreation and access to public lands and waters, aesthetic beauty of the stream and water quality;
j. The work proposed is for the protection of the public health, safety and/or welfare such as public schools, sewage treatment plant, water and sewer distribution lines and bridges providing particularly limited or sole access to areas of habitation;
k. An Idaho stream channel alteration permit approval from the Idaho department of water resources shall be provided by the applicant to the floodplain administrator.
(Ord. 700A, 3-11-2014; amd. Ord. 815, 7-23-2019)
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