20D.140.40 Frequently Flooded Areas.

20D.140.40-010 Classification and Rating of Frequently Flooded Areas.

To promote consistent application of the standards and requirements of this chapter, frequently flooded areas within the City of Redmond shall be rated or classified according to their characteristics, function and value, and/or their sensitivity to disturbance.

(1)    Frequently Flooded Areas Classifications. Frequently flooded areas shall be classified according to the criteria in this section.

(a)    Floodplain. The total area subject to inundation by the base flood (the flood that has a one percent chance of occurring in any given year).

(b)    Flood Fringe. The portion of the floodplain outside of the floodway which is generally covered by flood waters during the base flood; it is generally associated with standing water rather than rapidly flowing water.

(c)    FEMA Floodway. The channel of the stream and that portion of the adjoining floodplain which is necessary to contain and discharge the FEMA base flood flow without increasing the FEMA base flood elevation more than one foot.

(d)    Zero-Rise Floodway. The channel of the stream and that portion of the adjoining floodplain which is necessary to contain and discharge the base flood flow without increasing the base flood elevation. The zero-rise floodway will always include the FEMA floodway.

(2)    Classification of frequently flooded areas shall be determined by the Committee based on consideration of the following factors:

(a)    Maps adopted pursuant to this chapter including the frequently flooded areas map, which identifies the approximate location and extent of the 100-year floodplain. This map shall be used as a general guide only for the assistance of property owners and other interested parties; boundaries are generalized. The actual type, extent, and boundaries of frequently flooded areas shall be determined in the field by a qualified consultant according to the procedures, definitions, and criteria established by this chapter. In the event of any conflict between the critical area location and designation shown on the City’s map and the criteria or standards of this section, the criteria and standards shall prevail.

    The City will employ hydrologic models to define the extent of the zero-rise floodway. If the zero-rise floodway has not yet been defined for the property in question, the applicant will be responsible for modeling the base flood elevation and delineating the extent of the zero-rise floodway, consistent with the assumptions in the Bear Creek Basin Plan as adopted by the City. In the absence of a City hydrologic model, FEMA data will be acceptable;

(b)    Flood Insurance Rate Maps published by the Federal Emergency Management Agency;

(c)    Application of the criteria contained in these regulations; and

(d)    Consideration of the technical reports submitted by qualified consultants in connection with applications subject to these regulations. (Ord. 2259)

20D.140.40-020 Alteration of Frequently Flooded Areas.

Alteration of frequently flooded areas may only be permitted subject to the criteria in RCDG 20D.140.20-040 through 20D.140.20-050, 20D.140.30-030, 20D.140.40-030, 20D.140.50-020, and 20D.140.60-040. (Ord. 2259)

20D.140.40-030 Flood Hazard Areas – Development Standards.

(1)    Flood Hazard Areas Generally. For all new structures or substantial improvements, the applicant must provide certification by a qualified consultant of the actual as-built elevation of the lowest floor, including basement, and, if applicable, the actual as-built elevation to which the structure is flood-proofed. If the structure has a basement, this must be indicated.

(2)    The Flood Fringe Outside the Zero-Rise Floodway.

(a)    Except for downtown development along the Sammamish River in the 100-year floodplain from the Puget Sound Energy transmission line crossing to SR 520, development shall not reduce the effective base flood storage volume of the floodplain. Grading or other activity which would reduce the effective storage volume must be mitigated by creating compensatory storage on the site. Off-site compensatory storage may be permitted if binding legal arrangements assure that the effective compensatory storage volume will be preserved over time.

(b)    No structure shall be allowed which would be at risk due to stream bank destabilization including that associated with channel relocation or meandering.

(c)    All elevated construction must be designed and certified by a professional structural engineer registered in the State of Washington and must be approved by the City prior to construction.

(d)    Subdivisions, short subdivisions, binding site plans, site plan review, special development permits, and general development permits shall follow the following requirements:

(i)    New building lots shall contain 3,600 square feet or more of buildable land outside the zero rise floodway and building setback lines shall be shown on the face of the plat to restrict permanent structures to the area so defined;

(ii)    All utilities and facilities such as sewer, gas, electrical, telephone, cable communications and water systems shall be located and constructed consistent with subsection (2)(i) of this section;

(iii)    Base flood data and flood hazard notes shall be shown on the face of the recorded plat, including, but not limited to, the base flood elevation, required flood protection elevations, and the boundaries of the floodplain and the floodway, if determined; and

(iv)    The following note shall be recorded with the King County Department of Records and Elections for all affected lots:

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.

(e)    New residential construction and substantial improvement shall meet the following criteria:

(i)    The lowest floor shall be elevated to the flood protection elevation.

(ii)    Portions of the building that are below the flood protection elevation shall not be fully enclosed. The areas below the lowest floor shall be designed to automatically equalize hydrodynamic flood forces on exterior walls by allowing the entry and exit of floodwaters. Designs for meeting this requirement must meet or exceed the following minimum criteria:

(A)    Minimum of two openings on opposite walls having a total open area of not less than one square inch for every square foot of enclosed area subject to flooding shall be provided;

(B)    The bottom of all openings shall be no higher than one foot above grade.

(iii)    Openings may be equipped with screens, louvers, or other coverings or devices provided that they permit the unrestricted entry and exit of floodwaters.

(f)    New nonresidential construction and substantial improvement of any existing commercial, industrial, or other nonresidential structure shall meet the elevation requirements of residential construction.

(g)    All new construction and substantial improvements shall be anchored to prevent flotation, collapse, or lateral movement of the structure.

(h)    For all mobile and manufactured homes, all standards for flood hazard protection for conventional residential construction shall apply. All manufactured and mobile homes must be anchored and shall be installed using methods and practices that minimize flood damage. All new and replacement water supply systems shall be designed to minimize or eliminate infiltration of flood waters into the system.

(i)    Utilities shall meet the following criteria:

(i)    All new and replacement utilities, including sewage treatment facilities, shall be flood-proofed to, or elevated above, the flood protection elevation.

(ii)    New on-site sewage disposal systems shall be located outside the limits of the 100-year floodplain. The installation of new on-site sewage disposal systems in the floodplain is prohibited.

(iii)    Sewage and agricultural waste storage facilities shall be flood-proofed to the base flood elevation plus three feet.

(iv)    Above-ground utility transmission lines, other than electrical transmission lines, shall only be allowed for the transport of nonhazardous substances.

(v)    Buried utility transmission lines transporting hazardous substances (as defined by the Washington State Hazardous Waste Management Act in RCW 70.105.005) shall be buried at a minimum depth of four feet below the maximum depth of scour for the base flood predicted by a professional civil engineer licensed by the State of Washington and shall achieve sufficient negative buoyancy so that any potential for flotation or upward migration is eliminated.

(j)    Critical facilities may be allowed within the flood fringe of the floodplain. All such proposed uses shall be evaluated through a special development permit. Critical facilities constructed within the flood fringe shall have the lowest floor elevated to three or more feet above the base flood elevation. Flood-proofing and sealing measures must be taken to ensure that hazardous or toxic substances will not be displaced by or released into floodwaters. Access routes elevated to the flood protection elevation shall be provided to all critical facilities to the nearest maintained public street or roadway located outside of the floodplain.

(k)    The Committee shall review all development permits to determine that all necessary permits have been obtained as required by federal or state law, including Section 404 of the Federal Water Pollution Control Act Amendments of 1972, 33 U.S.C. 1334, as required by Section 60.3(a)(2) of 44 CFR.

(l)    Storage and containment of hazardous or dangerous chemicals, substances or materials, as those terms are determined by applicable state and federal regulations, shall be prohibited; provided, that existing uses involving storage, etc., shall conform to the flood protection elevation when applying for any permit.

(3)    Development in the Zero-Rise Floodway.

(a)    Activities allowed within the zero-rise floodway must conform to the requirements of this section, as well as the requirements that apply to the flood fringe outside the zero-rise floodway as identified in subsection (2) of this section.

(b)    No development activity shall reduce the effective storage volume of the floodplain.

(c)    No development, including permitted new construction or reconstruction, shall cause any increase in the zero-rise base flood elevation.

(d)    No temporary structures or storage of materials hazardous to public health, safety and welfare shall be permitted in the zero-rise floodway.

(e)    Construction of new residential or nonresidential structures is permitted in the zero-rise floodway only in the following circumstances:

(i)    The structure must be on a lot legally in existence at the time the ordinance codified in this chapter becomes effective;

(ii)    The structure must be on a lot that contains less than 3,600 square feet of buildable land outside the zero-rise floodway; and

(iii)    The structure must meet the construction standards set forth in subsections (2) and (3)(b), (3)(c), and (3)(d) of this section.

(f)    New lots that include part of the zero-rise floodway may be created only if the lots meet the requirements of subsection (2)(d) of this section and administrative rules, or are declared as nonbuilding lots on the face of the plat.

(g)    The following circumstances are presumed to produce no increase in base flood elevation and shall not require special studies to establish this fact:

(i)    Substantial improvement on existing residential structures outside the zero-rise floodway where the building footprint is not increased.

(ii)    Substantial improvement of an existing residential structure shall meet the requirements for new residential construction set forth in subsection (2)(e) of this section.

(h)    Reconstruction of an existing residential structure shall meet the requirements for new residential construction set forth in subsection (2)(e) of this section.

(i)    Utilities and roads are permitted in the zero-rise floodway only when no other location is practicable, or when mitigating measures achieve zero-rise floodway elevations, and shall meet the minimum criteria set forth in subsection (2)(i) of this section and the following requirements:

(i)    Construction of sewage treatment facilities shall be prohibited.

(ii)    Utility transmission lines transporting hazardous substances shall be buried at a minimum depth of four feet below the maximum depth of scour for the base flood as predicted by a professional civil engineer licensed by the State of Washington and shall achieve sufficient negative buoyancy so that any potential for flotation or upward migration is eliminated.

(j)    Critical facilities shall not be constructed in the zero-rise floodway.

(k)    Floodway Dependent Structures. Installations or structures that are floodway dependent may be located in the floodway; provided, that the development proposal receives approval from all other agencies with jurisdiction and meets all standards in RCDG 20D.140.20-040 and 20D.140.30-030. Such installations include but are not limited to:

(i)    Dams or diversions for water supply, flood control, hydroelectric production, irrigation or fisheries enhancement;

(ii)    Flood damage reduction facilities such as levees and pumping stations;

(iii)    Stream bank stabilization structures where no feasible alternative exists to protecting public or private property;

(iv)    Storm water conveyance facilities subject to the requirements of the development standards for streams and wetlands, and other relevant City of Redmond development standards;

(v)    Boat launches, docks and related recreation structures;

(vi)    Bridge piers and abutments; and

    (vii) Fisheries enhancement or stream restoration projects.

(l)    Development of the area located downstream of Redmond Way on Bear Creek may be allowed when (a) mitigating measures achieve zero-rise floodway elevations, or (b) when surface water elevations are not increased over one foot provided no significant unmitigated upstream, downstream, or on-site environmental impacts are created.

(4)    Development in the FEMA Floodway.

(a)    Construction or placement of new residential or nonresidential structures is prohibited within the FEMA floodway. Shoreline protective structures, bridges, roads, trails and railroads are permitted within the FEMA floodway.

(b)    No development subject to these regulations, including permitted new construction or reconstruction, shall cause any increase in the FEMA base flood elevation.

(c)    Substantial improvement of an existing residential structure located in the floodway must meet the requirements set out in WAC 173-158-070 as amended. Such substantial improvement is presumed to produce no increase in base flood elevation and shall not require special studies to establish this fact. (Ord. 2259)