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<br />horizontal widths shall be computed. The lOO-year <br />flood boundaries shall be delineated between these <br />cross sectional locations by transposing the water <br />surface elevations to a one or two foot contour <br />topographic base ~qp. These intermediate lr.:ations <br />:nust be correl ate.. to the reference 1 ineid the <br />iOO-Year flood profi~e as to their specific iocation <br />and elevation. <br /> <br />5. BlocKaQe. All culverts and bridges shall be considered <br />for the potential to become blocked by floating debris. In <br />determining if there is a potential for blockage, and <br />subsequent reduction in conveyance, old photographs, the <br />history of maintenance during high flows, watershed <br />characteristics such as erodibility of channel banks, <br />amount and type of vegetation along stream, and size and <br />character of the waterway shall be evaluated. Blockage <br />may be accounted for in computer runs by increasing width <br />of piers, raising streambed elevation or reducing waterway <br />opening by a percentage. Where the potential for blockage <br />can be shown, human intervention (e.g., snagging) shall be <br />considered only if such flood fighting activities are <br />specifically included in the community'c adopted emergency <br />response plan. <br /> <br />6. RouQhness Coefficients. Values for the roughness <br />coefficient at valley cross sections shall be based on <br />assumed worst case vegetation conditions along the stream <br />and floodplain. Past flood data, if available, shall be <br />used to verify the adopted roughness coefficients, taking <br />into consideration any alteration in the channel subsequent <br />to the floods. Photographs shall be taken of the study <br />reaches of the stream channel and floodplain to support <br />roughness coefficients used for hydraulic computations. <br /> <br />7. Consistency. A comparison of any proposed IOO-year flood <br />profile with all previously designated and approved <br />information shall be provided to the Board. Except where a <br />clearly identified change in flooding characteristics or <br />error in the existing data can be shown, the proposed <br />lOO-year flood elevations shall agree with those of other <br />COllt~IlOO.S. studies on the same stream. Elevations of cross <br />sections shall be computed to match within fO.5 foot of an <br />existing valid elevation; however, the final published <br />IOO-year flood water surface profile shall be shown to <br /> <br />-14- <br />