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t . . l l est point of the stream. From these elevations and the distances between the cross sections the stream slope may be calculated. . t c. Resistance. The hydraulic resis- tance coefficient of the natural channel must be evaluated in order to calculate preproject flow conditions. This resis- tance coefficient is usually taken to be the Manning n value. Various methods are available to evaluate resistance coefficients for natural streams, includ- ing comparisons with photographs of streams with known resistance values or tabular methods based on stream characteristics (16 to 18). Table 11, appendix D, provides Manning n values for selected natural channels. . l t . t f , , f . . . t d. Tailwater, Culvert performance is likely to be affected by the down- stream water surface elevation or tailwa- ter. Therefore, conditions which might promote high tail water elevations during flood events should be investigated. Downstream impoundments, obstructions, channel constrictions, tidal effects, and junctions with other watercourses should be investigated, based on field observations and maps, in order to eval- uate their impact on the resultant tail- water elevation. Lacking these condi- tions, tail water elevations should be based on water surface elevations in the natural channel. These elevations can be accurately determined from water sur- face elevation calculations or estimated using simplified approximations of water depth. For most culvert installations, an approximation is sufficient. . . . ~ . . . ~ . , . , I , t t . . ~ ~ " . . e. UDstream Storal!e. The storage capacity available upstream from a culvert may have an impact upon its design. Upstream storage capacity can be obtained from large scale contour maps of the upstream area, but a 2-foot (O.Sm) contour interval map is desirable. If such maps are not available, a number of cross sections should be obtained upstream of the proposed culvert. These sections must be referenced horizontally as well as vertically. The length of upstream ~ . , . t t l f I . t t . , . I channel to be cross-sectioned will depend on the headwater expected and the stream slope. The cross sections can be used to develop contour maps or the cross sectional areas can be used to compute storage. The topographic information should extend from the channel bed upward to an elevation equal to at least the design headwater elevation in the area upstream of the culvert. 4. Roadway Data. The proposed or existing roadway affects the culvert cost, hydraulic capacity, and alignment. Roadway profile and the roadway cross section information can be obtained from preliminary roadway drawings or from' standard details on roadway sections. When the culvert must be sized prior to the development of preliminary plans, a best estimate of the roadway section can be used, but the culvert design must be checked after the roadway plans are com- pleted. a. Cross Section. The roadway cross section normal to the centerline is typi- cally available from highway plans. However, the cross section needed by the culvert designer is the section III the stream crossing. This section may be skewed with reference to the roadway centerline. For a proposed culvert, the roadway plan, profile, and cross-sec- tional data should be combined as neces- sary to obtain this desired section. A schematic roadway plan and section with important elevations is shown in figure II-7. b. Culvert Len2th. Important dimen- sions and features of the culvert will become evident when the desired roadway cross section is measured or established. The dimensions are obtained by superimpos- ing the estimated culvert barrel on the roadway cross section and the streambed profile. (figure II-7) This superposition establishes the inlet and outlet invert elevations. These elevations and the resulting culvert length are approximate since the final culvert barrel size must still be determined. 21