16.15.320 Floodplain and floodway fringe development standards and alterations

16.15.320 Floodplain and floodway fringe development standards and alterations.

In addition to other requirements of this chapter, the following standards apply to development proposals and alterations on sites within the floodplain and floodway fringe:

(1) Development proposals and alterations shall not reduce the effective base flood storage volume of the floodplain. A development proposal shall provide compensatory storage if grading or other activity displaces any effective flood storage volume. Compensatory storage shall:

(a) Provide equivalent volume at equivalent elevations to that being displaced;

(b) Hydraulically connect to the source of flooding;

(c) Provide compensatory storage in the same construction season as when the displacement of flood storage volume occurs and before the flood season begins on September 30th for that year; and

(d) Occur on the site. The Floodplain Administrator may approve equivalent compensatory storage off the site if legal arrangements, acceptable to the Department, are made to assure that the effective compensatory storage volume will be preserved over time;

(2) A registered professional engineer shall design and certify all elevated construction and submit the design to the City prior to construction;

(3) Subdivisions, short subdivisions, and commercial development proposal binding site plans shall meet the following requirements:

(a) New building lots shall include 5,000 square feet or more of buildable land outside the floodplain. Structures and other development shall be located in this area;

(b) All utilities and facilities such as sewer, gas, electrical and water systems are consistent with this section;

(d) A development proposal shall provide adequate drainage in accordance with the stormwater manuals adopted in CMC Title 13; and

(e) The face of the recorded subdivision, short subdivision, or binding site plan shall include the following for all lots:

(i) Building setback areas restricting structures to designated buildable areas;

(ii) Base flood data and sources and flood hazard notes including, but not limited to, base flood elevations, required flood protection elevations, the boundaries of the floodplain and the zero-rise floodway, if determined; and channel migration zone boundaries, if determined; and

(iii) Include the following notice:

Lots and structures located within flood hazard areas may be inaccessible by emergency vehicles during flood events. Residents and property owners should take appropriate advance precautions.

(4) If an existing lot does not have sufficient buildable area outside of the floodplain (5,000 square feet), the following shall apply:

(a) Structures must be located to the non-floodplain area to the extent practicable;

(b) If the lot has no buildable area outside of the floodplain, all structures and development must be sited to have the least impact on the floodplain and riparian habitat;

(c) All structures must be set back at least 15 feet from the RBZ and shall be sited as close to the 100-year floodplain boundary as possible;

(d) Creation of new impervious surfaces shall not exceed 10 percent of the surface area of the portion of the lot in the floodplain unless mitigation is provided to make the impervious area ineffective;

(e) Roads and access driveways to the site are not permitted in the floodplain, except if they are the sole and least impactful access to the site;

(f) No new stream crossings over Type S, F, or Np streams are allowed to provide access to flood-prone sites;

(5) New development or improvements to the following publicly owned infrastructure in the floodplain shall meet the following standards:

(a) Public trails and nonmotorized travel ways shall be allowed in floodplains subject to the following:

(i) Trails are not allowed in CMZ or floodways;

(ii) Floodplain storage compensation requirements of subsection (1) of this section are met;

(iii) The facility is elevated above the base flood elevation, floodproofed, or signed for closure to protect public safety during flood events;

(iv) Low impact design measures are applied to the extent practicable to make the impervious surface ineffective; and

(v) Removal of native vegetation is minimized and compliant with other City standards.

(b) Public road improvements at stream crossings or in floodplains shall be allowed in all floodplain zones subject to the following:

(i) Encroachment into the floodway does not exceed existing conditions;

(ii) Fill and encroachment into the floodplain are minimized to the extent practicable; and

(iii) Floodplain storage compensation requirements of subsection (1) of this section are met.

(i) Facilities are not allowed in the floodway or CMZ;

(ii) Fill and encroachment into the floodplain are minimized to the extent practicable;

(iii) Floodplain storage compensation requirements of subsection (1) of this section are met; and

(iv) Removal of native vegetation is minimized and compliant with other City standards;

(6) Public and private utilities shall meet the following standards:

(a) Dry floodproof new and replacement utilities including, but not limited to, sewage treatment and storage facilities, to, or elevate above, the flood protection elevation;

(b) Locate new on-site sewage disposal systems outside the floodplain. When there is insufficient soil or area outside the floodplain, new on-site sewage disposal systems are allowed only in the floodway fringe. Locate on-site sewage disposal systems in the floodway fringe to avoid:

(i) Impairment to the system during flooding;

(ii) Contamination from the system during flooding;

(iii) Design all new and replacement water supply systems to minimize or eliminate infiltration of floodwaters into the system;

(iv) Aboveground utility transmission lines, except for electric transmission lines, are allowed only for the transport of nonhazardous substances; and

(v) Bury underground utility transmission lines transporting hazardous substances at a minimum depth of four feet below the maximum depth of scour for the base flood, as predicted by a civil engineer, and achieve sufficient negative buoyancy so that any potential for flotation or upward migration is eliminated;

(7) Critical facilities are only allowed within the floodway fringe, when a feasible alternative site is not available and the following standards are met:

(a) Elevate the lowest floor to the 500-year floodplain elevation or three or more feet above the base flood elevation, whichever is higher;

(b) Dry floodproof and seal structures to ensure that hazardous substances are not displaced by or released into floodwaters;

(c) Elevate access routes to or above the base flood elevation from the critical facility to the nearest maintained public street or roadway. (Ord. 12-20 § 2 (Exh. A); Ord. 06-17 § 3 (Exh. A))