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.

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.

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:

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:

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.

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:

Q_ss    =    Q_s(A_ss/A_g)^0.86

where

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

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

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

A_g    =    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.

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.

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.

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.