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Pierce County regulates per PCC 18E.10.140 – Appendix A, Mapping Sources, the following flood hazard areas:

A. Potential Flood Hazard Areas.

1. Potential flood hazard areas, as depicted on the Critical Areas Atlas-Flood Hazard Area Map, include:

a. Detailed Study Areas. (See Figure 18E.70-1 in Chapter 18E.120 PCC.)

(1) FEMA Flood Insurance Rate Map numbered AE zones and VE zones.

(2) Areas within 150 feet horizontal distance from the base flood elevation established for the mapped A and V zones.

(3) Areas within the UGA that are 2 feet, or in rural areas 5 feet, of vertical height from the base flood elevation established for the mapped A and V zones.

b. Unstudied Areas. FEMA Flood Insurance Rate Map unnumbered A zones and shaded X zones and areas within 150 feet horizontal distance from the mapped areas of the mapped A and shaded X zones. (See Figure 18E.70-2 in Chapter 18E.120 PCC.)

c. Natural Waters/Watercourse. Areas within 65 feet horizontal distance from the ordinary high water mark of an identified natural watercourse. (See Figure 18E.70-3 in Chapter 18E.120 PCC.)

d. Groundwater Flooding Areas. Areas within 300 feet horizontal distance from a mapped groundwater flooding area. (See Figure 18E.70-4 in Chapter 18E.120 PCC.)

e. Potholes. Areas not identified as a mapped flood hazard area, but within 10 feet of vertical relief from the bottom of an identified pothole or within 2 feet of vertical relief of a potential surface water spillway or other type of outlet. (See Figures 18E.70-5 and 18E.70-6 in Chapter 18E.120 PCC.) Potholes may be identified by Pierce County topographic mapping, field survey, or site inspections.

f. Channel Migration Zones (CMZ). Channel Migration Zones shall apply only to those watercourses listed below in PCC 18E.70.020.B.4. In those areas where detailed CMZ studies have been completed and accepted by Pierce County, additional horizontal and vertical review threshold criteria (i.e., 150' horizontal and 2' or 5' vertical) shall not apply. (See Figure 18E.70-7 in Chapter 18E.120 PCC.)

2. The Critical Areas Atlas-Flood Hazard Areas Maps (referenced in PCC 18E.10.140 – Appendix A, Mapping Sources) may not show all potential flood hazard areas that may be necessary for a specific site analysis. The Department may make interpretations, where needed, as to the approximate location of the boundaries of potential flood hazard areas. Where the Flood Insurance Study, FIRM, and floodway maps do not provide adequate, best, or most recent information, flood information that is more accurate or detailed may be used. When there is a conflict between the elevations and the mapped potential flood hazard area boundaries, the elevations shall govern. Development determined to be above the BFE but shown on the FEMA map as Zone A or V will require a letter of map change or elevation certificate.

3. Where there is insufficient information shown on the Critical Areas Atlas-Flood Hazard Areas Maps, the Department may require the applicant to verify that the site is out of the flood hazard area using the flood hazard area review procedures set forth in PCC 18E.70.030.

B. Floodway. A floodway is an extremely hazardous area due to the depth and/or velocity of floodwaters which carry debris, potential projectiles, and have erosion potential. (See Figure 18E.70-8 in Chapter 18E.120 PCC.) The following areas are regulated by Pierce County as floodways:

1. Regulatory Floodway. Regulatory floodway designated by flood hazard area maps.

2. Deep and/or Fast Flowing Water Areas. Areas of deep and/or fast flowing water shall be regulated as a floodway. Based on the criteria set forth in PCC 18E.70.030 D., the Department shall make the determination after review and approval of the applicant's analysis of whether the project site falls within the floodway area based on deep and/or fast flowing waters. (See Figure 18E.70-9 in Chapter 18E.120 PCC.)

3. Potholes and Shaded X Zones. That portion of a pothole and shaded X zone area that is 3 feet or greater in water depth in a 100-year flood event shall be regulated as a floodway. (See Figure 18E.70-10 in Chapter 18E.120 PCC.)

4. Channel Migration Zones (CMZs).

a. Channel migration zones identified at severe risk of migration in an adopted geomorphic study shall be regulated as floodways, and shall apply only to those watercourses listed below

(1) South Prairie Creek;

(2) Carbon River;

(3) Puyallup River;

(4) White River (including W. fork White River);

(5) Greenwater River;

(6) Nisqually River; and

(7) Mashel River.

b. Channel Migration Zones on regulated watercourses (listed in subsection 4.a. above) will be regulated when CMZ studies are completed, accepted and adopted by Pierce County, except for the Puyallup River downstream of the confluence with the White River, where the default CMZ shall be the regulated FEMA floodway area.

C. Flood Fringe. All areas subject to inundation by the base flood, but outside the limits of the floodway and the limits of the channel migration zone as set forth in PCC 18E.70.020 B. Those portions of the A and shaded X zones not defined as floodway, and that portion of a pothole and FEMA shaded X zone area that is between 0 feet (base flood elevation) and 3 feet in depth shall be regulated as a flood fringe.

D. Coastal Flood Hazard Areas. Areas that are adjacent to Puget Sound marine waters where the wave velocity combined with high water can cause damage. These areas are designated as "A," "AE," "V," or "VE" zones as defined by FEMA and depicted on the FEMA maps or other maps adopted by Pierce County.

E. Other Areas of Special Flood Hazard.

1. Groundwater Flooding Areas. Groundwater flooding areas are those areas identified by Pierce County and shown on flood hazard maps and are subject to flood inundation from subsurface waters that result from a fluctuation of the groundwater table. Groundwater flooding areas shall be regulated as a floodway or flood fringe pothole.

2. Natural Waters/Watercourse. Natural waters/watercourse as identified on Pierce County topographic, planimetric or orthophoto maps, WDNR stream classification maps, USGS quadrangle maps, or other source maps that are not identified as a flood hazard area on the FEMA maps. That portion of the natural watercourse located within 65 feet horizontal distance from the ordinary high water mark shall be regulated as a floodway or flood fringe.

(Ord. 2021-90s § 2, 2021; Ord. 2017-2s § 2 (part), 2017; Ord. 2014-18s § 2 (part), 2014; Ord. 2008-16s2 § 3 (part), 2008; Ord. 2006-103s § 2 (part), 2006; Ord. 2004-57s § 2 (part), 2004)