Chapter 18E.70
FLOOD HAZARD AREAS Revised 6/15 Revised 5/17 Revised 12/17 Ord. 2017-12s

18E.70.010    Purpose.

18E.70.020    Flood Hazard Areas. Revised 5/17

18E.70.030    Flood Hazard Area Review Procedures. Revised 6/15 Revised 5/17 Amended Ord. 2017-12s

18E.70.040    Flood Hazard Area Standards. Revised 5/17 Revised 12/17 Ord. 2017-12s

18E.70.050    Appendices. Revised 5/17

A.    Floodplain/Floodway Analysis.

18E.70.010 Purpose.

The purpose of this Chapter is to promote the public health, safety, and general welfare of the citizens of Pierce County. Under this Chapter, development is protected from the impacts of flood hazards by establishment of minimum standards for sites which contain or are adjacent to identified flood hazard areas. The standards contained in this Chapter are intended to minimize public and private losses due to flood conditions in flood hazard areas and provide criteria necessary for regulated activities located within flood hazard areas in Pierce County. The following statements describe the purpose of this Chapter:

A.    Protect human life and health;

B.    Minimize expenditure of public money and costly flood control projects;

C.    Minimize the need for rescue and relief efforts associated with flooding;

D.    Minimize prolonged business interruptions;

E.    Minimize damage to public infrastructure, facilities and utilities;

F.    Minimize damage to critical fish and wildlife habitat areas;

G.    Minimize net loss of ecological functions of floodplains;

H.    Ensure that potential buyers are notified that property is in a flood hazard area;

I.    Ensure that those who occupy flood hazard areas assume responsibility for their actions; and

J.    Qualify Pierce County for participation in the National Flood Insurance Program, thereby giving the citizens of Pierce County the opportunity to purchase flood insurance with particular emphasis to those in flood hazard areas.

(Ord. 2008-16s2 § 3 (part), 2008; Ord. 2004-57s § 2 (part), 2004)

18E.70.020 Flood Hazard Areas. Revised 5/17

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 and Floodway Map numbered A zones and V 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 unshaded X zones and areas within 150 feet horizontal distance from the mapped areas of the mapped A and B 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 Figure 18E.70-5 and Figure 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. 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)

18E.70.030 Flood Hazard Area Review Procedures. Revised 6/15 Revised 5/17 Amended Ord. 2017-12s

A.    General Requirements.

1.    The Pierce County Critical Areas Atlas-Flood Hazard Area Map provides an indication of where potential flood hazard areas are located within the County. The actual presence or location of a flood hazard area shall be determined using the procedures and criteria contained in this Chapter.

2.    The Department will complete a review of the Flood Hazard Area Maps, and other source documents, for any development proposal to determine whether the proposed project area for a regulated activity falls within a potential flood hazard area. When there is a conflict between the elevations and the mapped 100- or 500-year floodplain or floodway boundaries, the elevations shall govern.

3.    When the Department's maps or sources indicate that the proposed project area for a regulated activity is or may be located within a potential flood hazard area, the Department shall require a flood boundary delineation survey as outlined in subsection B. below, and may require a flood study as outlined in subsection C. below, a deep and/or fast flowing water analysis as outlined in subsection D. below, and/or a zero-rise analysis as outlined in subsection E. below, except for coastal flood hazard areas which shall not be required to submit a flood study, deep and/or fast flowing water analysis, or a zero-rise analysis.

4.    Any proposed development located within a flood hazard area shall comply with the flood hazard area standards set forth in PCC 18E.70.040.

5.    The Department shall utilize an expedited review and approval process in communities recovering from County, State or Federally declared disasters. This process shall be developed administratively and shall be documented in the Department's Disaster Recovery Standards (DRS). The purpose of these standards is to assist property owners recovering from declared disasters including, but not limited to, floods. The DRS shall establish processes for allowing recovery related to sediment and debris removal, expedited building permit review for repair and reconstruction of damaged structures, and other recovery procedures deemed necessary by the Department. A copy of the Department's DRS shall be available to the public upon request.

6.    A FEMA Letter of Map Amendment (LOMA) or Letter of Map Revision (LOMR) shall not be submitted to FEMA until review and approval has been granted by Pierce County. Pierce County shall not recognize any LOMA or LOMR as an amendment to the Department's Flood Hazard maps unless prior approval has been granted by the County Public Works Department.

7.    Unless otherwise stated in this Chapter, the critical area protective measure provisions contained in PCC 18E.10.080 shall apply.

B.    Flood Boundary Delineation Survey.

1.    If the Department's maps or sources indicate that the proposed project area for a regulated activity is located within a potential flood hazard area, then a flood boundary delineation survey that delineates the horizontal and vertical limits of the base flood elevation shall be submitted to the Department.

a.    In areas where a base flood elevation has not been previously determined, it will be necessary to conduct a flood study pursuant to subsection C. below in order to produce the flood boundary delineation survey.

b.    A base flood elevation that has been established through a detailed flood study accepted by Pierce County may be used in lieu of conducting a flood study.

c.    The floodplain limits for a natural watercourse as set forth in PCC 18E.70.020 E.2. shall be established at 65 feet horizontal distance from the ordinary high water mark.

2.    The requirement to submit a flood boundary delineation survey may be waived at the Department's discretion, when the Department can determine, using contour elevations, base flood data, orthophotos, and parcel data, that the extent of the regulated activity is clearly above the base flood elevation.

3.    The flood boundary delineation survey shall be prepared, signed, and dated by a professional land surveyor.

4.    The Department shall review the flood boundary delineation survey to evaluate if the proposed development is located within a flood hazard area.

5.    If the proposed development lies within the flood hazard area, the limits of the floodway, as well as the base flood elevation, shall be shown on the flood boundary delineation survey.

C.    Flood Study.

1.    A flood study shall be conducted when the Department's maps or sources indicate that the proposed project area for a regulated activity is or may be located within a potential flood hazard area where base flood elevation data is not available through the Flood Insurance Study or other authoritative sources, or when an established base flood elevation is contested. An engineering analysis to determine the base flood elevation shall be required by Pierce County. Base flood elevations shall be determined using the detailed methods established in PCC 18E.70.050 – Appendix A. Alternative methods may be approved by the County.

2.    The flood study shall be prepared under the responsible charge of and signed and dated by a professional engineer.

3.    Once the Department has reviewed and approved the flood study, the applicant shall be required to provide a flood boundary delineation survey, utilizing the newly established base flood elevation, as outlined in subsection C. above.

4.    Flood studies shall not be required for coastal flood hazard areas. New coastal flood studies may, however, be accepted by the County that show a different base flood elevation if prepared by a professional engineer and based on FEMA coastal guidance. An applicant may request the use of an approved coastal study. The County may allow use of a previously approved coastal study when the conditions on the sites are sufficiently similar and the sites lie in proximity to each other.

D.    Deep and/or Fast Flowing Water Analysis.

1.    When the Department determines that a proposed project area for a regulated activity is located within a flood hazard area, a deep and/or fast flowing water analysis based on Figure 18E.70-9 in Chapter 18E.120 PCC and PCC 18E.70.050 – Appendix A shall be required to determine the floodway limits.

2.    The floodway limits and flood fringe limits identified in the deep and/or fast flowing water analysis shall be depicted on the flood boundary delineation survey, as outlined in subsection B. above.

3.    The deep and/or fast flowing water analysis shall be prepared under the responsible charge of and signed and dated by a professional engineer.

4.    Deep and/or fast flowing water analysis shall not be required for coastal flood hazard areas.

E.    Zero-Rise Analysis.

1.    When the Department concludes that a proposed project area for a regulated activity is located within a flood hazard area, a zero-rise analysis shall be required to determine that no increase in base flood elevation, displacement of flood volume, or flow conveyance reduction will occur as a result of the development.

2.    The zero-rise analysis shall be conducted utilizing HEC-RAS modeling methodology or other alternative methodology approved by the County (see PCC 18E.70.050 – Appendix A). The analysis shall show that no rise (0.01 feet or less) has occurred as a result of the proposed development except that development proposed to occur in a FEMA mapped floodway shall show no rise in excess of 0.00 feet. The proposed development may need to be reduced or specially engineered (such as utilizing piers or pilings) to achieve zero-rise.

3.    The zero-rise analysis shall be prepared under the responsible charge of and signed and dated by a professional engineer.

4.    The zero-rise analysis shall be documented on the Zero-Rise Analysis Form, as set forth in PCC 18E.70.050 – Appendix A, and shall be attached to the flood hazard area permit.

5.    Zero-rise analysis shall not be required for coastal flood hazard areas.

6.    The requirement to submit a zero rise analysis may be waived at the Department's discretion for the following types of projects:

a.    Structures elevated by pier or pilings, or where no fill is placed in the flood hazard area.

b.    Placement of instream structures for the purpose of fish habitat enhancement, stream restoration, and monitoring, where it is readily apparent that such placement will not negatively impact adjacent properties or heighten flood risk.

c.    In the flood fringe where there is ineffective flow, as defined by the latest edition of FEMA's "Guidelines and Specifications for Study Contractors", and as determined by a Pierce County certified floodplain manager.

d.    In the flood fringe when at least 25 percent additional live compensatory storage is provided than is required in this Chapter and the fill/development is achieved in a manner that does not adversely affect performance standards for flood water conveyance as specified by a Pierce County certified floodplain manager. Hydraulic analysis for conveyance may be required.

(Ord. 2017-2s § 2 (part), 2017; Ord. 2015-25s § 2 (part), 2015; Ord. 2014-18s § 2 (part), 2014; Ord. 2008-16s2 § 3 (part), 2008; Ord. 2006-103s § 2 (part), 2006; Ord. 2004-57s § 2 (part), 2004)

18E.70.040 Flood Hazard Area Standards. Revised 5/17 Revised 12/17 Ord. 2017-12s

A.    General.

1.    New construction done by or for Pierce County, such as bridges, roads, flood control works, revetments, retaining walls, drainage structures, sewer or water lines, parks, or other structures necessary to promote the public's health, safety, and welfare shall be allowed in a flood hazard area when:

a.    The project is prepared under the responsible charge of and is signed and dated by a registered professional engineer in the State of Washington. The project shall be designed so the project does not result in any increase in flood levels during the occurrence of the base flood discharge (zero-rise) nor obstruct the floodway nor cause an adverse impact to critical fish or wildlife habitat on adjacent, cross-channel, or upstream or downstream properties; and

b.    The improvements utilize appropriate flood hazard protection standards.

2.    A Federal Emergency Management Agency (FEMA) elevation certificate shall be required for new construction, additions affixed to the side of a structure, and substantial improvements located within flood hazard areas. The most current version of the FEMA elevation certificate must be completed by a professional land surveyor, currently licensed in the State of Washington, and kept on file with the Pierce County Planning and Land Services Department.

3.    Agricultural chemicals, fertilizers, pesticides, and similar hazardous materials that may contaminate surface and groundwater in the event of flood inundation shall not be stored in agricultural accessory structures except as otherwise provided within PCC 18E.70.040 B. and C. below1.

4.    Plat notes for the Puyallup, Carbon, Nisqually, Greenwater, and White Rivers and Creeks and for other Flood Hazard Areas as well, shall be placed on the face of any final plat (includes commercial, industrial, single family, and multi-family residential), short plat, large lot, or binding site plan documents which lie in these areas. The plat notes shall be per PCC 18E.10.140 – Appendix B, Title and Plat Notification/Plat Notes.

B.    Floodways. Any development, encroachment, filling, clearing, grading, new construction, and substantial improvement shall be prohibited within the floodway, except as follows:

1.    When located in a floodway only designated as a floodway because it is in a Channel Migration Zone and when in conformance with PCC 18E.20.040, Nonconforming Uses.

2.    Structures that do not require a building permit and that do not have any associated fill.

3.    Agricultural activities as follows:

a.    Agricultural activities that do not require the installation of structures and that do not have any associated fill;

b.    Agricultural structures that do not require a building permit and have no associated fill;

c.    The storage and manufacture of compost outside the FEMA mapped floodway on land meeting the definition of farm or agricultural land pursuant to RCW 84.34.020; provided that the compost is manufactured from feedstock produced on-site. The composting operation shall meet the requirements of WAC 173-350-220 and the maximum volume of composting material on site at any one-time shall be limited to 25 cubic yards. Compost and composting material meeting the requirements of this subsection shall not be considered fill; and

d.    The import and application of compost for purposes of soil amendment on land meeting the definition of farm or agricultural land pursuant to RCW 84.34.020 and located outside the FEMA mapped floodway. The maximum amount of compost imported to a site shall not exceed 20 cubic yards per acre per year. Compost imported to a site shall be applied within 180 days and not more than one year's supply of compost shall be present on site. The import and application of compost consistent with the requirements of this subsection shall not be considered fill.

4.    In the event that construction, reconstruction, or replacement in a floodway is allowed, the construction must comply with the standards of PCC 18E.70.040 C.

5.    Repairs of an existing structure within the footprint of the existing structure provided the repairs are not substantial improvements.

6.    Repair, reconstruction, and replacement of substantially damaged residential structures in accordance with WAC 173-158-076, as amended, when recommended by the Department of Ecology and constructed to the standards set forth within PCC 18E.70.040 C.

7.    Repair, reconstruction, replacement, and improvements to existing farmhouses pursuant to WAC 173-158-075, as amended, within an Agricultural Resource Lands (ARL) or Rural Farm (RF) zone when in compliance with the following standards:

a.    The new farmhouse is a replacement for an existing farmhouse on the same farm site;

b.    There is no potential safe building site for a replacement farmhouse on the same farm site outside the designated floodway or the location requires close proximity to other structures in the farm operation in order to maintain the integrity and operational viability of the farm; in no case shall a replacement be located into an area with higher flood hazards in terms of depths, velocities and erosion;

c.    Repairs, reconstruction, or improvements to a farmhouse shall not increase the total square footage of encroachment of the existing farmhouse;

d.    A replacement farmhouse shall not exceed the total square footage of encroachment of the structure it is replacing;

e.    A farmhouse being replaced shall be removed, in its entirety, including foundation, from the floodway within ninety days after occupancy of a new farmhouse;

f.    For substantial improvements, and replacement farmhouses, the construction shall comply with the standards set forth in PCC 18E.70.040 C.;

g.    New and replacement water supply systems are designed to eliminate or minimize infiltration of flood waters into the system;

h.    New and replacement sanitary sewerage systems are designed and located to eliminate or minimize infiltration of flood water into the system and discharge from the system into the flood waters; and

i.    All other utilities and connections to public utilities are designed, constructed, and located to eliminate or minimize flood damage.

8.    Repairs, reconstruction, replacement, or improvements to existing non-residential agricultural structures as follows:

a.    The non-residential agricultural structure is a replacement for an existing structure on the same farm site and the use of the structure must be limited to agricultural purposes only;

b.    There is no potential safe building site for a replacement non-residential agricultural structure on the same farm site outside the designated floodway or the location requires close proximity to other structures in the farm operation in order to maintain the integrity and operational viability of the farm; in no case shall a replacement be located into an area with higher flood hazards in terms of depths, velocities and erosion;

c.    The agricultural structure must be built, repaired, or reconstructed with flood-resistant materials for the exterior and interior building components and elements (i.e., foundation, wall framing, exterior and interior finishes, flooring, etc.) for all parts of the building below the BFE;

d.    The agricultural structure must be adequately anchored to prevent flotation, collapse, or lateral movement of the structure. All of the building's structural components must be capable of resisting specific flood-related forces including hydrostatic, buoyancy, hydrodynamic and debris impact forces;

e.    The agricultural structure must meet the NFIP openings requirement. NFIP requires that enclosure walls or foundations walls, subject to the 1 percent recurrence interval flood, contain openings that will permit the automatic entry and exit of floodwaters;

f.    Any mechanical, electrical, or other utility equipment must be located at or above the lowest floor per PCC 18E.70.040 C. or floodproofed so that they are contained within a watertight, floodproofed enclosure that is capable of resisting damage during flood conditions;

g.    An agricultural structure being replaced shall be removed, in its entirety, including foundation, from the floodway within 90 days of final inspection of the structure by Pierce County;

h.    New and replacement water supply systems are designed to eliminate or minimize infiltration of flood waters into the system;

i.    New and replacement onsite sewerage systems are designed and located to eliminate or minimize infiltration of flood water into the system and discharge from the system into the flood waters;

j.    All other utilities and connections to public utilities are designed, constructed, and located to eliminate or minimize flood damage;

k.    Storage of agricultural chemicals, fertilizers, pesticides, and similar hazardous materials shall be permitted only where no other on-site storage alternative outside the floodplain exists and the building permit is accompanied by a written description of how on-site storage procedures will prevent the release of agricultural chemicals during a flood event; and

l.    The repair, reconstruction, replacement, or improvement of the non-residential agricultural structures shall not exceed the square footage of the structure being repaired or replaced except where through an approved zero-rise analysis the applicant has shown it will not result in an increase in flood elevations.

9.    Construction of new non-residential agricultural structures are allowed in the Lower Puyallup River area, downstream of River Mile 6, under the criteria set forth in PCC 18E.70.040 B.8.

10.    Park and recreational uses and facilities that do not require the installation of structures and that do not have any associated fill.

11.    For existing park and recreational property uses, existing park and recreation structures and facilities may be substantially improved provided they are located outside of the FEMA mapped floodway and comply with the following conditions:

a.    The repair, reconstruction, replacement, and improvement of the existing structure and facility complies with the elevation requirements set forth in PCC 18E.70.040 C.

b.    The repair, reconstruction, replacement, and improvement of the existing structure and facility shall not increase the total square footage of encroachment of the existing structure and facility.

c.    Any fill material used to elevate the structure(s) shall provide compensatory storage per PCC 18E.70.040 C.4.a. Backfill material within a stem wall crawlspace foundation does not apply to the compensatory storage requirement.

d.    The existing access to the site shall not be required to be elevated to 1.0 feet above the BFE, as outlined in PCC 18E.70.040 C.3.

e.    The repair, reconstruction, replacement, and improvement of the existing structure and facility will not result in an increase in flood elevations.

12.    Individual recreational vehicles which are licensed and ready for highway use, not permanently attached to the site, and occupy the site for fewer than 180 consecutive days. Vehicles may return to the same site after demonstrating road readiness.

13.    Habitat enhancement/stream restoration activities are permitted subject to the provisions outlined in PCC 18E.70.040 D.

14.    Rehabilitation, reconstruction, or an upper story addition to an existing structure that does not exceed the limits for a substantial improvement.

15.    Private bridges may be allowed to cross the floodway provided that the structure meets the requirements contained in PCC 18E.70.030 and the following:

a.    The lowest structural member of a private bridge proposed to cross the floodway portion of any of the rivers listed in PCC 18E.70.020 B.4. shall be a minimum of 6 feet above the base flood elevation.

b.    The lowest structural member of a private bridge proposed to cross the floodway portion of any other watercourse shall be a minimum of 1 foot above the base flood elevation.

C.    Flood Fringe Areas. All activities allowed in PCC 18E.70.040 B. shall be permitted in a flood fringe area. Any other proposed development, encroachments, filling, clearing or grading, new construction, and substantial improvements are prohibited in a flood fringe area except as follows:

1.    Structures that do not require a building permit and that do not have any associated fill are permitted.

2.    All other regulated activities shall only be allowed when the proposed development is located on an existing lot of record that was created prior to March 1, 2005. Applicants shall demonstrate there are no other feasible alternatives that would allow the proposed development to occur completely outside the flood hazard area. At a minimum, the following shall be demonstrated:

a.    The development cannot be located outside the flood hazard area due to topographic constraints of the parcel or size and/or location of the parcel in relation to the limits of the flood hazard area and a building setback variance has been reviewed, analyzed, and rejected as a feasible alternative to encroachment into the flood hazard area; and

b.    The proposed development shall not cause an adverse impact to adjacent, cross-channel, or upstream or downstream properties.

3.    Access.

a.    Access including, but not limited to, roads, bridges, driveways, emergency vehicle access, and other access routes and easements, where allowed, shall be constructed and armored based on the standards in PCC 18E.70.040 C.4. and elevated a minimum of 1 foot above the base flood elevation. Provided, however, that the requirement to elevate the access may be waived in whole or part by the PALS Department whenever it is determined that the roadway to which the access connects is located at an elevation that would make the roadway impassible during the base flood and no plan for elevating the roadway exists.

b.    Parking lots shall be elevated to a minimum of one-half foot below the base flood elevation.

4.    Grading and Filling. When development is permitted under this subsection, it shall be designed to a zero-rise standard as set forth in PCC 18E.70.030 E. and PCC 18E.70.050 – Appendix A. Any filling, grading, or clearing associated with the permitted development shall not increase flood hazards, water velocities, or flood elevations. In addition to meeting the requirements for zero-rise, all permitted development must also meet the following requirements:

a.    Compensatory Storage. New excavated storage volume shall be equivalent to the flood storage capacity eliminated by filling or grading within the flood fringe. Equivalent shall mean that the storage removed shall be replaced by equal live storage volume between corresponding 1-foot contour intervals that are hydraulically connected to the floodplain through their entire depth (refer to Figure 18E.70-13 in Chapter 18E.120 PCC). The compensatory storage requirement may be waived for pre-existing lots where compensatory storage is not available and filling or grading is limited to either elevating the interior building crawl space (see Figure 18E.70-14) or providing positive drainage from the structure in accordance with IBC standards.

The requirement for compensatory storage may be waived at the discretion of a Pierce County Certified Floodplain Manager in flood hazard areas when all of the following conditions exist:

(1)    The flood hazard area is the result of emergent groundwater;

(2)    Groundwater flooding is isolated in porous soils; and

(3)    No surface flow component contributes to flooding.

b.    Flow Conveyance. New excavated conveyance areas shall be equivalent to existing conveyance within the flood fringe. Equivalent shall mean a mechanism for transporting water from one point to another using an open channel system.

c.    Erosion Protection. Development shall be protected from flow velocities greater than 2 feet per second through the use of bio-engineering methods or, when bio-engineering methods have been deemed insufficient to protect development, then hard armoring may be utilized. All erosion protection shall extend 1 to 3 feet, depending on development requirements, above the base flood elevation and shall be covered with topsoil and planted with native vegetation. (See Figure 18E.70-14 in Chapter 18E.120 PCC.)

5.    Critical Facilities.

a.    New construction, additions affixed to the side of an existing structure, and substantial improvement of hazardous facilities and special occupancy structures are prohibited.

b.    New construction of an essential facility, reconstruction of an existing essential facility, or additions to an existing essential facility that exceed the threshold for substantial improvement shall be permitted when no feasible alternative site is available outside the flood hazard area. Such regulated activities are subject to the following:

(1)    Essential facilities with a crawlspace elevated by fill shall have the lowest floor and any utilities and ductwork elevated a minimum of 3 feet above base flood elevation. (See Figure 18E.70-14 in Chapter 18E.120 PCC.)

(2)    Essential facilities elevated by piers or pilings shall have the bottom of the lowest horizontal structural member and any utilities and ductwork elevated a minimum of 3 feet above the base flood elevation and must be designed by a professional structural engineer. (See Figure 18E.70-15 in Chapter 18E.120 PCC.)

(3)    Essential facilities shall be armored based on the standards in PCC 18E.70.040 C.4. above. Flood resistant materials, construction methods and practices shall be used in construction of such facilities.

(4)    Adequate containment and sealing measures must be taken to insure that toxic or explosive substances will not be displaced or released into floodwaters.

6.    Structures. Single-family, two-family, multi-family, mobile/manufactured homes, commercial, and industrial structures, etc., except for critical facilities as set forth in PCC 18E.70.040 C.5. above, shall be allowed subject to the following standards:

a.    New construction, additions affixed to the side of an existing structure, and substantial improvement of any structure shall have the lowest floor elevated a minimum of 1 foot above base flood elevation when constructed with a crawlspace or a minimum of 2 feet above the base flood elevation when constructed using a slab on grade construction, except as provided in PCC 18E.70.040 C.6.c. and d. below. (See Figure 18E.70-14 in Chapter 18E.120 PCC.) Flood resistant materials, construction methods and practices shall be used.

b.    New construction, additions affixed to the side of an existing structure, and substantial improvement of any structure elevated by piers or pilings shall have the bottom of the lowest horizontal structural member elevated a minimum of 2 feet above the base flood elevation, except as provided in subsections 18E.70.040 C.6.c. and d. below, and must be designed by a professional structural engineer. Areas below the lowest horizontal structural member shall not be enclosed and shall remain free of obstructions. Flood resistant materials, construction methods and practices shall be used. Fully enclosed areas below the lowest horizontal structural member are prohibited. Screens, louvers, or other coverings or devices are allowed if they do not restrict the entry and exit of flood waters. Skirting for mobile homes is allowed where the material is not rigid and is easily collapsed or removed by floodwaters.

c.    New construction of minor residential detached accessory structures and substantial improvement of minor residential detached accessory structures including, but not limited to, storage buildings, detached garages, sheds, and small pole buildings, together with attendant utility facilities may as an alternative to the provisions of PCC 18E.70.040 C.6.a. and b. be flood proofed when compliant with the following:

(1)    The structure must have a low potential for structural flood damage and shall not exceed 576 square feet in size;

(2)    Be designed and oriented to allow the free passage of floodwaters through the structure in a manner affording minimum flood damage;

(3)    Not be used for human habitation;

(4)    Shall not have internal plumbing nor be connected to sanitary sewage/on-site septic facility;

(5)    Include adequate hydrostatic flood openings;

(6)    Use flood resistant materials below the flood protection elevation (minimum elevation required per foundation type as set forth in PCC 18E.70.040 C.6.a. and b. above);

(7)    Must offer minimum resistance to the flow of floodwater;

(8)    Must be anchored to prevent flotation, collapse or lateral movement; and

(9)    All utilities must comply with the standards set forth in subsection 18E.70.040 C.9.e.

d.    New commercial, industrial, or other non-residential structures and substantial improvements of such structures shall elevate the lowest floor to the flood protection elevation (minimum elevation required per foundation type as set forth in PCC 18E.70.040 C.6.a. and b. above) or meet the following standards:

(1)    Dry flood-proof the structure to the flood protection elevation to meet the following standards:

(a)    The applicant shall provide certification by a civil or structural engineer that the dry flood-proofing methods are adequate to withstand the flood depths, pressures, velocities, impacts, uplift forces, and other factors associated with the base flood. After construction, the engineer shall certify that the permitted work conforms to the approved plans and specifications; and

(b)    Approved building permits for dry flood-proofed non-residential structures shall contain a statement notifying applicants that flood insurance premiums are based upon rates for structures that are 1 foot below the BFE;

(2)    Use materials and methods that are resistant to and minimize flood damage;

(3)    Design and construct the crawlspace below the dry flood-proofed floor to automatically equalize hydrostatic and hydrodynamic flood forces on exterior walls by allowing for the entry and exit of floodwaters as set forth in subsection 18E.70.040 C.9.b.; and

(4)    Dry flood proof all electrical, heating, ventilation, plumbing, air conditioning equipment, and other utility and service facilities to, or elevated above, the flood protection elevation.

e.    A written notice regarding the use of flood proofing, in a form approved by the Department of Planning and Land Services, shall be recorded with the title of the property. The notice shall:

(1)    Disclose that the structure is located in a flood hazard area and portions of the structure are below the base flood elevation;

(2)    Disclose that flood proofing has been utilized and detailed instructions of how to operate and maintain all flood-proofing design features;

(3)    Include a diagram identifying the flood proofed spaces; and

(4)    Disclose that the use of flood proofing in lieu of elevating a structure to the flood protection elevation may result in increased flood insurance premiums.

f.    The property owner shall be responsible for notifying future owners of the use of flood proofing and associated risks and limitations.

7.    Agricultural Accessory Structures.

a.    Storage of agricultural chemicals, fertilizers, pesticides, and similar hazardous materials shall be permitted only where no other on-site storage alternative outside the floodplain exists and the building permit is accompanied by a written description of how on-site storage procedures will prevent the release of agricultural chemicals during a flood event.

b.    The lowest floor in an agricultural accessory structure shall be located at the base flood elevation or higher provided that the structure is designed to automatically equalize hydrostatic flood forces on exterior walls by allowing for the entry and exit of floodwaters. Designs for meeting this requirement must either be certified by a professional engineer in the State of Washington or must meet or exceed the following minimum criteria:

(1)    A minimum of two openings having a total net area of not less than one square inch for every square foot of enclosed area subject to flooding shall be provided;

(2)    The bottom of all openings shall be no higher than 1 foot above grade; and

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

8.    Livestock Flood Sanctuaries. Livestock flood sanctuaries pursuant to RCW 86.16.190, as amended, shall be allowed when in compliance with the following minimum criteria:

a.    The livestock sanctuary is accessory to a farming operation located within an Agricultural Resource Lands (ARL) or Rural Farm (RF) zone and located on the same site as the farming operation;

b.    The keeping of livestock is a component of the farming operation;

c.    There is no other suitable holding area on the site outside the floodplain to which the livestock have access;

d.    The livestock flood sanctuary complies with the compensatory storage requirements set forth within PCC 18E.70.040 C.4.a.;

e.    The livestock sanctuary will not result in an increase in flood elevations;

f.    The livestock sanctuary is located in the area least subject to risk from floodwaters; and

g.    Structures shall not be erected or placed upon a livestock flood sanctuary. A notice in a form approved by the Planning and Land Services Department shall be recorded with the title indicating the limits of the livestock flood sanctuary and acknowledging the prohibition of structures.

9.    Construction Standards.

a.    Construction of a basement is prohibited.

b.    Crawl spaces shall meet Pierce County Building Code requirements. Crawl spaces shall either be backfilled with clean earth material and have a finished grade within the crawl space that is at least 1 foot above the base flood elevation and the following:

(1)    The building must be designed and adequately anchored to resist floatation, collapse, and lateral movement of the structure resulting from hydrodynamic and hydrostatic loads, including the effects of buoyance.

(2)    The height of the crawlspace, measured from the interior grade of the crawlspace to the next higher floor must not exceed 5 feet at any point.

(3)    Provide a minimum of two openings on each of two opposite side walls in the direction of flow, with each of these walls having a total open area of not less than one square inch for every square foot of enclosed area subject to flooding;

(4)    Design the bottom of all openings to be no higher than 1 foot above grade, the higher of the interior or exterior grade; and

(5)    Screens, louvers, or other coverings or devices are allowed if they do not restrict entry and exit of flood waters.

c.    Flood proofing in lieu of elevating the structure is prohibited, except for non-residential structures and residential accessory structures as set forth in PCC 18E.70.040 C.6.c and d.

d.    All single-family, two-family, multi-family, mobile/manufactured homes, commercial, and industrial structures shall be placed on standard concrete stemwall/footing foundations or piles, piers, or column foundations as designed in accordance with Figures 18E.70-14 or 18E.70-15 in Chapter 18E.120 PCC. Structures shall also be anchored to the foundation to resist floatation, collapse, and lateral movement.

e.    Electrical, heating, ventilation, plumbing, air-conditioning equipment, other service facilities, and associated ductwork shall be elevated to at least the minimum required lowest floor elevation; however, the Department may approve a lesser minimum distance above base flood elevation provided that the systems are designed to prevent floodwater from entering or accumulating within the components. (See Figure 18E.70-15 in Chapter 18E.120 PCC.)

10.    Sewage Disposal and Potable Water Installation.

a.    New and replacement public water sources (i.e., wells and water supply lines) and public sanitary sewage conveyance systems are allowed. These systems shall be designed to withstand scour resulting from flow velocity, minimize or eliminate infiltration of floodwaters into the systems, and minimize or eliminate discharge from the systems into floodwaters.

b.    All replacement wells and replacement on-site sewage system (OSS) shall be designed to minimize or eliminate impairment to them or contamination from/to them during flooding (i.e., infiltration of floodwaters into or discharge out of the systems). They shall not be located in pothole or no-outlet floodplains.

c.    All new individual wells and new on-site sewage system (OSS) shall be prohibited. Conveyance systems from a structure to a well or OSS located outside of the flood hazard area shall be allowed provided these systems are designed to meet the standards in PCC18E.70.040 C.3. above.

D.    Alteration of Watercourses. Any alteration of a watercourse shall comply with the following standards:

1.    Pierce County will notify adjacent communities and the Washington State Department of Ecology prior to any alteration or relocation of a watercourse proposed by the applicant and submit evidence of such notification to the Federal Insurance Administration.

2.    Pierce County shall require that maintenance be provided within the altered or relocated portion of said watercourse, so that the flood-carrying capacity is not diminished. Therefore, if the maintenance program calls for future cutting of planted native vegetation used in performing the alteration, the system shall be oversized at the time of construction to compensate for said vegetation growth or any other natural factor that may need future maintenance.

3.    The project engineer shall design the watercourse alteration so the activity does not increase the water surface elevation (zero-rise); decrease the capacity, storage, and conveyance of the watercourse; nor cause an adverse impact to adjacent, cross-channel, or upstream or downstream properties. Pierce County has the discretion to determine if potential impacts may be insignificant or not applicable.

E.    Coastal Flood Hazard Areas and Coastal Flood Fringe Areas. Any proposed development, encroachments, filling, clearing, grading, new construction, and substantial improvements within a coastal flood hazard area or coastal flood fringe area shall be subject to the following standards:

1.    Access.

a.    Roads, bridges, driveways, emergency vehicle access, and access routes and easements, where allowed, shall be constructed and armored based on the standards in PCC 18E.70.040 C.3. and elevated a minimum of 1 foot above the base flood elevation.

b.    Parking lots shall be elevated to a minimum of one-half foot above the base flood elevation.

c.    Private bridges may be allowed to cross Puget Sound marine waters subject to all Federal, State and local requirements.

2.    Grading and Filling. All permitted development must also meet the following requirements:

a.    The use of fill for structural support of buildings is prohibited.

b.    Man-made alteration of sandbars or spits shall be prohibited.

3.    Structures. Single-family, two-family, multi-family, mobile/manufactured homes, commercial, and industrial structures, shall be allowed subject to the following standards:

a.    New construction, additions affixed to the side of an existing structure, and substantial improvement of any structure with a crawlspace may be located only landward of a line 2 feet above the base flood elevation. (See Figure 18E.70-16 in Chapter 18E.120 PCC.) Flood resistant materials, construction methods and practices shall be used.

b.    New construction, additions affixed to the side of an existing structure, and substantial improvement of any structure located seaward of a line 2 feet above the base flood elevation may be allowed when elevated by piers or pilings provided:

(1)    The bottom of the lowest horizontal structural member is elevated a minimum of 2 feet above the base flood elevation.

(2)    The structure must be designed by a professional structural engineer.

(3)    Electrical, heating, ventilation, plumbing, air-conditioning equipment, and other service facilities and associated ductwork shall be elevated a minimum of 2 feet above base flood elevation. (See Figure 18E.70-16 in Chapter 18E.120 PCC.)

(4)    Areas below the lowest horizontal structural member shall not be enclosed and shall remain free of obstructions.

(5)    Structures shall be anchored to the foundation to resist floatation, collapse, and lateral movement due to the effects of wind and water loads acting simultaneously on all building components. Flood resistant materials, construction methods and practices shall be used. Water loading values used shall be those associated with the base flood. Wind loading values used shall be those required by the Pierce County Building Code.

(6)    All new construction shall be located landward of a contour line delineated at elevation 9.5 feet, NAVD 1988. (Note: The 9.5 feet elevation approximates mean high tide, where mean high tide varies from 8.0 to 9.25 NAVD 1988 along Pierce County Puget Sound Marine Waters.)

c.    New construction, additions affixed to the side of an existing structure, and substantial improvement of any structure located in the coastal flood fringe area may be allowed when elevated on a stem wall foundation provided (Figure 18E.70-16):

(1)    The footing of the stemwall is placed on existing subgrade and per Figure 18E.70-14.

(2)    The crawlspace is backfilled to an elevation that is 2 feet above the base flood elevation.

d.    New construction of minor residential accessory boat storage structures and substantial improvement of minor residential accessory boat storage structures located outside of a V zone may, as an alternative to the provisions of PCC 18E.70.040 E.3.b., be flood proofed when compliant with the following:

(1)    The structure must have a low potential for structural flood damage and shall not exceed 576 square feet in size;

(2)    Be designed and oriented to allow the free passage of floodwaters through the structure in a manner affording minimum flood damage;

(3)    Not be used for human habitation;

(4)    Shall not have internal plumbing nor be connected to sanitary sewage/on-site septic facilities;

(5)    Include adequate hydrostatic flood openings;

(6)    Use flood resistant materials below the flood protection elevation;

(7)    Must offer minimum resistance to the flow of floodwater;

(8)    Must be anchored to prevent flotation, collapse or lateral movement;

(9)    All utilities must comply with the standards set forth in subsection 18E.70.040 C.9.e.;

(10)    New construction shall be located landward of a contour line delineated at elevation 10.5 feet, NAVD 1988;

(11)    Substantial improvement of existing structures, including those located over water, shall ensure that the bottom of the lowest horizontal structural member is located above elevation 10.5 feet, NAVD 1988, or the pre-substantial improvement elevation, whichever is greater;

(12)    No expansion of a building footprint over water shall occur;

(13)    The structure is compliant with the requirements of Title 20 PCC; and

(14)    Boat storage structures and similar storage structures which exceed any of the standards set forth in subsections (1) through (11) above, shall be subject to the requirements set forth in PCC 18E.70.040.E.3.a. and b.

e.    Adequate containment and sealing measures must be taken to insure that toxic or explosive substances will not be displaced or released into Puget Sound marine waters.

f.    Rehabilitation, reconstruction, or an upper story addition to an existing structure that does not exceed the limits for a substantial improvement shall be allowed.

4.    Sewage Disposal and Potable Water Installation.

a.    New and replacement public water sources (i.e., wells and water supply lines) and public sanitary sewage conveyance systems are allowed. These systems shall be designed to withstand scour resulting from flow velocity, minimize or eliminate infiltration of floodwaters into the systems, and minimize or eliminate discharge from the systems into floodwaters.

b.    All new or replacement individual wells and on-site sewage system (OSS) shall be subject to the Tacoma-Pierce County Health Department regulations.

(Ord. 2017-2s § 2 (part), 2017; Ord. 2016-52 § 1 (part), 2016; Ord. 2014-18s § 2 (part), 2014; Ord. 2008-16s2 § 3 (part), 2008; Ord. 2006-103s § 2 (part), 2006; Ord. 2004-57s § 2 (part), 2004)

18E.70.050 Appendices. Revised 5/17

A.    Floodplain/Floodway Analysis.

18E.70.050 – Appendix A
Floodplain/Floodway Analysis

This Appendix describes the flood hazard analyses and studies as required by Title 18E.70 PCC, Flood Hazard Area Chapter. Flood hazard studies establish the base flood elevation and delineate floodplain and/or floodway(s) when a proposed project contains or is adjacent to a river, stream, lake, or closed depression.

Flood hazard studies must conform to FEMA regulations described in Part 65 of 44 Code of Federal Regulations (CFR). In addition, the following information must be provided and procedures performed for flood hazard studies used under Chapter 18E.70 PCC to examine development proposals or improvements within a floodplain.

I. FLOODWAY DETERMINATION

Pierce County recognizes three distinct floodways. The FEMA Floodway describes the limit to which encroachment into the natural conveyance channel can cause 1 foot or less rise in water surface elevation. The Deep and/or Fast Flowing (DFF) water floodways are hazardous areas and conditions of the floodplain for both people and habitable structures. Life safety and protection to improved properties are compromised if encroached upon. Encroachment cannot occur within these areas. DFF areas are determined by using Figure 18E.70-9 in Chapter 18E.120 PCC. The Channel Migration Zone Floodways apply only to regulated watercourses per PCC 18E.70.020 where a detailed CMZ study has been completed by Pierce County.

A.    FEMA Floodways.

1.    FEMA Floodways are determined through the procedures outlined in the FEMA publication Guidelines and Specifications for Study Contractors using the 1-foot maximum allowable rise criteria.

2.    Transitions shall take into account obstructions to flow such as road approach grades, bridges, piers, culverts, or other restrictions. General guidelines for transitions may be found in HEC-RAS, Water Surface Profiles-Users Manual, Appendix IV, Application of HEC-RAS Bridge Routines, published by the Hydrologic Engineering Center, Davis, California.

B.    Deep and/or Fast Flowing (DFF) Floodways.

1.    DFF floodways are generally assumed to include the entire 100-year floodplain until Pierce County approves a detailed floodway analysis that defines areas of DFF within the entire floodplain area based on the criteria.

2.    The hydraulic model must adequately be calibrated to known or recorded stage elevations of past flood events with a computed recurrence frequency intervals for the 100-year flood recurrence interval. This is to ensure model accuracy.

II. FLOOD STUDY

CONTENT AND REQUIRED INFORMATION

Three copies of the completed floodplain/floodway analysis study report and the modeling digital files shall be submitted. The report submittal must be stamped by a licensed professional civil engineer and include the following information in addition to that required for the drainage plan of a proposed project:

A.    Floodplain/Floodway Map.

1.    A scaled survey base map stamped by a licensed professional land surveyor registered in the State of Washington. The map must accurately locate the proposed development with respect to the floodplain and floodway, the channel of the subject stream, river, and/or pothole location, and the existing improvements within the subject study area. It must also supply all pertinent information such as the nature of the proposed project, legal description of the property on which the project would be located, fill quantity, limits and elevation, the building floor elevations, and use of compensatory storage.

2.    The map must show elevation contours at a minimum of 2-foot vertical intervals and shall comply with survey and map guidelines published in the FEMA publication Guidelines and Specifications for Study Contractors. The map must show the following:

a.    Elevations, ground contours and spot elevations, reported in vertical datum NAVD 1988 (or most recent vertical datum adopted by Pierce County).

b.    Elevations and dimensions of existing structures, fill, and compensatory storage areas.

c.    Size, location, elevation and spatial arrangement of all proposed structures on the site.

d.    Location and elevations of roadways, drainage facilities, water supply lines, and sanitary sewer facilities.

e.    Areas of DFF must clearly be shown and plotted on the map sheet depicting the bounded area of the floodway on both sides of the study channel, through the subject site. DFF floodway studies must reflect all transitions as referenced above as well.

f.    The base maps must also be accompanied by all field survey notes/computations, drawings, etc. for each cross-section with water surface elevation at the time the cross-section field survey was done.

B.    Study Report.

1.    Soil maps, groundcover maps, and photographs.

2.    A narrative report containing purpose of the study and description of the study area, data collection, methodology for both the hydrology and hydraulics, detailed discussion on the input parameters used, modeling results, and conclusions.

3.    A floodplain/floodway analysis must include calculations and all computer analysis input and output information, supporting graphical illustrations as well as the following additional information:

a.    Scaled cross-sections showing the current/existing conditions of the river/stream channel, the floodplain adjoining each side of the channel, the computed floodway, the cross-sectional area to be occupied by any proposed development and all historic high water information.

b.    Profiles showing the bottom of the channel, the top of both left and right banks and computed base flood water surface elevations for the 10-, 25-, 50- and 100-year events.

c.    Plans and specifications of any flood protection for structures, construction areas, filling, dredging, channel improvements, storage of materials, water supply, and sanitary facilities within the floodplain.

d.    Complete printout of input and output data of the model that was used for the analysis. Liberal use of comments and written discussion will assist considerably in understanding the model logic and minimize misinterpretations and/or questions.

e.    A map showing the graphical/plotted location and limits of the computed floodway and/or floodplain. All mapping must conform to the County's accepted horizontal/vertical datum standards.

f.    Three copies of ready-to-run digital files of both the hydrologic and hydraulic model and its input and output files used in the study. Data shall be submitted on a disk in standard ASCII format, ready to use on an IBM-compatible personal computer and in the applicable software application (i.e., HEC-RAS, HSPF, SBUH, etc.).

g.    A section on the flood flow including computer modeling and/or calculations (see below for additional requirements on flood flow determinations).

h.    Aerial photographs of the site including pre-Feb. 1996 and post-Feb. 1996 photos of the site.

i.    All field survey notes/computations, maps, and drawings for each cross-section with water surface elevation at the time of the cross-section field survey.

C.    Computer Modeling Information. Floodway/floodplain studies submitted to Pierce County for review must include output summary tables and include the following (but not limited to) items:

1.    Cross-section(s) identification number.

2.    Range of flows being examined.

3.    Computed water surface elevation at each cross-section.

4.    Energy grade line at each cross-section.

5.    Graphical plots of the channel cross-sections with computed water surface elevations for all model runs including calibrated model runs.

6.    All model input and output printouts.

7.    Graphical plots of the model output data that shows the points and segments along each cross-section where Deep and/or Fast Flowing water occurs. This shall include cross-section plots of depth and velocity in one-unit increments. The plots shall also be accompanied with a table listing the station distance (right and left bank), flow rate, area, hydraulic depth, velocity, and whether each point is floodway.

8.    A plan sheet clearly showing the graphical representation of the bounded area of the floodway based on DFF criteria through the entire study site and reach. Note that identified "islands" or "pockets" within the middle of the bounded floodway area are generally considered as part of the floodway, unless otherwise approved by Pierce County.

9.    Discussion on the starting water surface elevation for the hydraulic model.

III. DETERMINING FLOOD FLOWS

The three techniques used to identify the flows used in a flood study depend on whether gage data is available, whether a basin plan has been adopted, or a detailed flood study has been done and approved for use by Pierce County. The first technique is for basins with adopted basin plan areas. The second technique is used if a gaging station exists on the stream. The third technique is used on un-gaged catchments or those with an insufficient length of record. In all cases (and at minimum) the engineer shall be responsible for assuring that the hydrologic methods used are technically reasonable, conservative, conform to the FEMA publication Guidelines and Specifications for Study Contractors, and are acceptable by FEMA and Pierce County.

A.    Flood Flows from Adopted Basin Plan Information. For those areas where Pierce County has adopted basin plans with future conditions flow modeling, flood flows may be calculated using information from the basin plan. The hydrologic model used in the basin plan shall be updated to include the latest changes in zoning or any additional information regarding the basin which has been acquired since the adoption of the basin plan.

B.    Flood Flows from Stream Gage Data. Calculating flood flows from stream gage data uses the Log-Pearson Type III distribution method as described in the Guidelines for Determining Flood Flow Frequency, Bulletin 17B of the Hydrology Committee, United States Water Resources Council (revised September 1981).

1.    This methodology may only be used if data from a gaging station in the basin is available for a period of at least 10 years.

2.    If the difference in the drainage area on the stream at the study site and the drainage area to a gaging station on the stream at a different location in the same basin is less than or equal to 50 percent, the flow at the study site shall be determined by transferring the calculated flow at the gage to the study site using a drainage area ratio raised to the 0.86 power, as in the following equation:

Qss    =    Qs(Ass/Ag)0.86

where

Qss    =    estimated flow for the given return frequency on the stream at the study site.

Qs    =    flow for the given return frequency on the stream at the gage site.

Ass    =    drainage area tributary to the stream at the study site.

Ag    =    drainage area tributary to the stream at the gage site.

3.    If the difference in the drainage area at the study site and the drainage area at a gaging station in the basin is more than 50 percent and a basin plan has not been prepared, a continuous model shall be used as described below to determine the flood flows at the study site.

4.    In all cases where dams or reservoirs, floodplain development, or land use upstream may have altered the storage capacity or runoff characteristics of the basin so as to affect the validity of this technique, a continuous model shall be used to determine flood flows at the study site.

5.    This methodology can only be used for areas outside of the urban growth boundary. Future flow values must be used for rural areas when an adopted basin plan provides such flow values. Within the urban growth boundary, future flows from current adopted basin plans must be used. Where adopted basin plans are not existent, then continuous flow simulation modeling must be used to determine future conditions.

6.    Flows for major river systems within the urban growth boundary do not need to be computed using continuous simulation modeling. Regression analysis using the referenced Bulletin 17B may be used instead to determine flow magnitudes where an adopted or formally approved and accepted flood study done by or for the County (purposes of regulatory or best information) is non-existent.

C.    Flood Flows from a Calibrated Continuous Model. Flood flows may be calculated by utilizing a continuous flow simulation model such as HSPF or other equivalent continuous flow simulation model, as approved by the County. Where flood elevation or stream gaging data are available, the model shall be calibrated to the known gage data. Otherwise, County accepted and approved regional parameters may be used.

IV. IDENTIFYING FLOOD ELEVATIONS,

PROFILES and FLOODWAYS (Hydraulic Model)

A.    Reconnaissance. The applicant's project engineer is responsible for the collection of all existing data with regard to flooding in the study area. This shall include a literature search of all published reports in the study area and adjacent communities and an information search to obtain all unpublished information on flooding in the immediate and adjacent areas from Federal, State, and local units of government. This search shall include specific information on past flooding in the area, drainage structures such as bridges and culverts that affect flooding in the area, available topographic maps, available community maps, photographs of past flood events, and general flooding problems within the community. Documented discussions with nearby property owners should also be done to obtain a witness account of the flooding extent. A field reconnaissance shall be made by the applicant's project engineer to identify hydraulic conditions of the study area, including type and number of structures, locations of cross-sections, and other parameters including the roughness values necessary for the hydraulic analysis.

B.    Base Data. Channel cross-sections used in the hydraulic analysis shall be current/existing (unless otherwise approved by the County) at the time the study is performed and shall be obtained by field survey. Topographic information obtained from aerial photographs/mapping may be used in combination with surveyed channel cross-sections in the hydraulic analysis. The elevation datum of all information used in the hydraulic analysis shall be verified. All information shall be referenced directly to NAVD 1988 unless otherwise approved by Pierce County.

C.    Methodology. Flood studies and analysis (including Deep and/or Fast Flowing floodways and Zero-Rise Analysis) shall be calculated using the U.S. Army Corps of Engineers HEC-RAS computer model (or subsequent revision) unless otherwise approved by Pierce County.

D.    Adequacy of the Hydraulic Model. Pierce County considers the following (but not limited to) factors when determining the adequacy of the hydraulic model for use in the floodway/floodplain model:

1.    Cross-section downstream starting location and spacing.

2.    Differences in energy grade line (significant differences in the energy grade line from cross-section to cross-section are an indication that cross-sections should be more closely spaced or that other inaccuracies exist in the hydraulic model.)

3.    Methods and results for analyzing the hydraulics of structures such as bridges and culverts.

4.    Lack of flow continuity.

5.    Use of a gradually varied flow model. In certain cases, rapidly varied flow techniques may need to be used in combination with a gradually varied flow model such as weir flow over a levee, flow through a spillway of a dam, or special application of bridge flow (pressure flow if bridge superstructure is shown to be submerged for the study event).

6.    Mannings "n" value.

7.    Calibration of hydraulic model to known and/or observed flow stage elevations including past flood events.

8.    Special applications. In some cases, steady state-one dimensional hydraulic models may not be sufficient for preparing the floodplain/floodway analysis. This may occur where sediment transport, two-dimensional flow, or other unique hydraulic circumstances affect the accuracy of the model. In these cases, the project engineer must propose and obtain Pierce County approval of alternative models for establishing the water surface elevations.

9.    All reported error and/or warning messages by the model must be properly and adequately addressed and/or resolved and included in the report for review verification.

V. ZERO-RISE ANALYSIS (ZRA)

A.    Zero-rise analysis (ZRA) is required where encroachment within the flood fringe area is allowed and approved by Pierce County. The ZRA must show that the proposed development encroachment in the flood fringe area will not show a measurable rise in the base flood elevation (i.e., less than 0.01 foot), resulting from a comparison of existing conditions and proposed conditions. This is directly attributable to development in the floodplain but not attributable to manipulation of mathematical variables such as roughness factors, coefficients, discharge, and other hydraulic parameters.

B.    In addition to those items listed in A. above, the following shall be included in a ZRA:

1.    Floodway boundaries (based on zero-rise) are to follow the stream lines and reasonably balance the rights of property owners on either side of the floodway. Use of the automatic equal conveyance encroachment option in the model will be considered equitable.

2.    The ZRA must include a sufficient number of cross-sections in order to accurately model the subject fill and compensatory storage areas of the site. In all cases, cross-sections shall be located downstream, through the subject site and upstream of the site at a very minimum. They shall also be located where changes in channel and the fill material characteristics occur, such as slope, shape, and roughness. The sections shall also be located perpendicular to the flow path in the channel and the outside overbank areas. Pierce County shall review and approve the proposed number and location of cross-sections. All cross-sections and surveys shall be prepared and certified by a professional land surveyor or registered professional engineer in the State of Washington.

3.    Difference between two profiles water surface elevation at the cross-section (e.g., difference between existing and encroached water surface). The model must report 0.01 foot or less an allowable change in the water surface elevation. This must be shown in the profile graphical plot as well.

4.    Difference between profiles of the energy grade line at the cross-section. The model must report 0.01 foot or less. This is the allowable change in the energy grade line. This must be shown in the profile graphical plot as well.

5.    Where the encroachment is in a floodway shown on the FEMA FIRM, the analysis shall show no rise measured to 0.00 feet.

C.    Conveyance Capacity.

1.    The ZRA must also show that the proposed development encroachment in the flood fringe area will not show a measurable decrease (less than 0.1 CFS) in the conveyance capacity of the channel, resulting from a comparison of existing conditions and proposed conditions, for each of the cross-sections. This is also directly attributable to development in the floodplain but not attributable to manipulation of mathematical variables such as roughness factors, coefficients, discharge, and other hydraulic parameters.

2.    The analysis must provide calculations of the reduction in conveyance caused by the proposed development encroachment, assuming no change in the water surface elevation, and using the roughness coefficient value(s) appropriate for the proposed development.

3.    The analysis must then provide calculations for the increase in conveyance of the proposed compensatory measure, using the roughness coefficient value(s) appropriate for the proposed development.

4.    Include a comparison analysis and discussion from No. 2 and 3 above. The comparison must adequately show that the conveyance capacity has not measurably decreased between the existing condition and proposed development condition.

Floodplain/Floodway Zero-Rise Certification

This is to certify that I am a duly qualified professional engineer licensed to practice in the State of Washington.

This is to further certify it is my professional opinion that the attached floodplain/floodway Zero-Rise Analysis conclusively shows that the proposed development of:

___________________________________________

__________________

(Name of Development)

Parcel Number

will not increase the 100-yr base flood elevation(s) and widths nor reduce the conveyance capacity of the floodplain/floodway and its associated channel to the ____________________

(Name of River, Stream, Pothole or other Watercourse)

Supporting Data

Base Flood Elevation (Pre-Development) = __________________ FT (NAVD 1988)

Base Flood Elevation (Post-Development) = _________________ FT (NAVD 1988)

Conveyance Capacity (Pre-Development) = _________________ CFS

Conveyance Capacity (Post-Development = _________________CFS

with compensatory storage)

Fill Added to Floodplain (At or Below BFE) = _________________ C.Y.

Compensatory Excavation from Floodplain = __________________ C.Y.

__________________________________________

Signature

_____________

Date

_________________________________

Title

__________________________________________

Firm Name

__________________________________________________________________________

Address

__________________________________________________________________________

City

State

ZIP Code

Seal, Signature, and Date

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


1

Agricultural accessory structures mean a non-residential structure such as, but not limited to, sheds and silos.