![]() (f) The base maps must also be accompanied by all field survey notes/computations, drawings, etc. DFF floodway studies must reflect all transitions as referenced above as well. (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. (d) Location and elevations of roadways, drainage facilities, water supply lines, and sanitary sewer facilities. (c) Size, location, elevation and spatial arrangement of all proposed structures on the site. (b) Elevations and dimensions of existing structures, fill, and compensatory storage areas. (a) Elevations, ground contours and spot elevations, reported in vertical datum NAVD 1988 (or most recent vertical datum adopted by Pierce County). 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.Ģ. 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 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. ![]() 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:ġ. A scaled survey base map stamped by a licensed professional land surveyor registered in the State of Washington. FLOOD STUDY CONTENT AND REQUIRED INFORMATION.Three copies of the completed floodplain/floodway analysis study report and the modeling digital files shall be submitted. Deep and/or Fast Flowing (DFF) Floodways.ġ. 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.Ģ. 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. 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.ī. 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.ġ. 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.Ģ. Transitions shall take into account obstructions to flow such as road approach grades, bridges, piers, culverts, or other restrictions. DFF areas are determined by using Figure 18E.70-9 in Chapter 18E.120 PCC. Encroachment cannot occur within these areas. Life safety and protection to improved properties are compromised if encroached upon. The Deep and/or Fast Flowing (DFF) water floodways are hazardous areas and conditions of the floodplain for both people and habitable structures. 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. FLOODWAY DETERMINATION.Pierce County recognizes three distinct floodways. 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. 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.įlood hazard studies must conform to FEMA regulations described in Part 65 of 44 Code of Federal Regulations (CFR). This Appendix describes the flood hazard analyses and studies as required by Title 18E.70 PCC, Flood Hazard Area Chapter. This appendix is included in your selections.
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