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<br />Flood Profile Reference Table
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<br />Water Suriace Profile
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<br />Distance in Feet X 100 Above Confluence
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<br />SAMPLE WATER SURFACE
<br />PROFILE (STEP 2)
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<br />Blockage may be artificially accounted for in the hydraulic calculations by
<br />simulating: '
<br />. an increase in the width of bridge piers;
<br />. a raising of the streambed elevation; or
<br />. a reducfion in the waterway opening by a percentage,
<br />Once all of the hydraulic variables have been considered and a backwater
<br />analysis has been completed. floodwater suriace elevations are portrayed in
<br />the floodplain report in two ways. First. flood profiles graphically show the
<br />relationship. in profile or side view. of the water suriace elevations to the
<br />channel bottom at a particular point along the stream, Second. a reference
<br />table shows elevations at each floodplain cross-section used in the study.
<br />The flood elevations presented in the profiles and the reference table
<br />should be calculated to 8' precision of :to,5 foot for the 10-,50-. 1 DO-, and
<br />500-year flood events and referenced to 1929 Mean Sea Leve/IMSL) datum,
<br />The flood elevations and outlines for an actual flood event may vary from
<br />these figures.
<br />It is difficult to evaluate all the variables which will affect hydraulic
<br />calculations. Variables include scouring of the channel due to high veioclties.
<br />sedimentation, variations in channel characteristics between cross-sections.
<br />amounts of debris accumulation, and limitations in field surveys and mapping
<br />techniques. Despite all of these engineering limitations. a detailed hydrologic
<br />and hydraulic investigation is still the mosf reliable method for determining
<br />flood elevations.
<br />
<br />Floodplain Delineations and Datum Consistency
<br />The next step of the floodplain study is the delineation of the flood limits on
<br />the topographic maps. Delineation requires the depicfion in plan or overhead
<br />view of the information which is shown in vertical view in the flood water sur-
<br />face elevation tables and profiles. The plan view provides a clearer picture of
<br />the extenf of actual flooding at specific locations along the stream,
<br />Maintaining consistency is very important to developing a technically and
<br />legaily acceptable regulafory document which can be incorporated into
<br />floodplain regulations and/or zoning maps. Problems may be eliminated
<br />through improved reporting and interpretation procedures.
<br />The procedure which the CWCS recommends for delineation of the 100-
<br />year floodplain on topographic maps is as follows:
<br />1) using the hydraulic analysis, the flood elevations at each cross-section
<br />are tabulated;
<br />2) the flood profiles are, drawn;
<br />3) using channel centerline stationing from the water suriace profiles. the
<br />centerline stations of the whole numbered flood elevations which cor-
<br />respond to the base co'ntour interval elevations of the topographic
<br />maps are tabulafed to produce the plotting fable;
<br />4) the location of all flood contours in the channel is marked at the appro-
<br />priate station along the <;enterline on the base maps;
<br />5) the flood contours,are extended by drawing "wiggly." or reference, lines
<br />perpendicular to the,direction of flow until they intercept their corres-
<br />ponding ground contours; this point is the edge of fhe floodplain, (The
<br />flood contours should parallel the cross-sections. If the cross-sections
<br />were incorrectly located. however. fhe flood contours may cross them,);
<br />6) flooded area boundaries are drawn by connecting the ends of each
<br />"wiggly" or reference line; and
<br />7) the flood delineation should be checked for reasonability and consls,
<br />tency with the cross-section data (top width of the floodplain), ,
<br />This is a simplified method which has many advanfages over the more com.
<br />mon practice of drawing the flood boundaries by connecting end points of
<br />the flood boundary width at each cross-section, The most important advan-
<br />tage is the degree of confidence gained in the interpretation of the 1 DO-year
<br />flood boundary. The flood contours become permanent documentation of the
<br />engineer's judgment in inferpreting the flood hazards.
<br />
<br />This method is also applicable fo approximate floodplain mapping where a
<br />water suriace profile can be drawn using an assumed depfh, Once the depth
<br />has been assumed, it is easy to show in plan view which areas would be
<br />flooded and which would,not.
<br />The delineafion ofttiefloodplain will need to be re-evaluated if development
<br />activities or other factors cause a sufficient change in the floodplain to alter
<br />the threat. When reevaluation is necessary. it is important that previously
<br />published information be reviewed and adequately correlated with the new
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