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- . e e MEASUREMENT OF PEAK DISCHARGE AT CULVERTS BY INDIRECT METHODs 9 For type 1 flow, d, is assumed to occur at the point where the headwater intersects the mitered entrance. Photographs Obtain stereophotographs showing culvert details and all pertinent conditions upstream and downstream from the culvert. These pic- tures are extremely important and may often avert a return trip to the site if certain data are unintentionally omitted during the field survey. Good photographs are a required part of the data necessary before computations can be completely reviewed. General views of the relationship of the eul- vert to the approach channel, to crest-stage gages if they exist, and to the getaway condi- tions are useful. Take at least one closeup of the culvert entrance to show entrance detail, A level rod standing at the entrance furnishes a permanent record of culvert height and is a good reference for other details. Where road overflow occurs, include a view showing the entire overflow section. Special conditions Hydraulic characteristics of culverts in the field can be greatly different from closely con- trolled laboratory conditions. Before eoeffi- cients rrnd methods derived in the laboratory can be applied to field installations, eonsider any features that tend to destroy model- prototype similarity, Debris Examine drift found lodged at the inlet of a culvert after a rise and evaluate its effect. It is not uncommon for material to float above a culvert at the peak without causing obstruc- tion and then lodge at the bottom when the water subsides. However, if examination shows it to be well compacted in the culvert entrance and probably in the same position as during the peak, measure the obstructed area and deduct it from the total area. Sand and gravel found within a culvert barrel is often deposited after the extreme velocities of peak flow have passed; where this oceurs, use the full area of the culvert. Careful judgment, must, be exercised because, in many plaees, levels before Hull "fter a peak show virtually the s"me ;'1\"erl elevations even though high velocities occulTed, Where diseharge will be computed many times through a culvert tha t has a shifting bottom (natural bottom or deposited material), a cross section should be run after any seveTe flood, or at least once a year, and a record kept to evalu"te the effect. In certain areas ice and snow may present problems, Ice very often causes backwater p"rtly blocking the culvert entrance, Snow frequently causes the deposition of misleading high-water m"rks as it melts, Break in slope Sometimes culverts are installed with " break in bottom slope, At other times a break in slope will occur as a result of uneven settlin~ in soft fill material. Determine the elevation and location of the invert at the break. A break in slope frequently occurs where " culvert is lengthened during road reconstruc- tion. In rare cases the size and shape of the culvert may be changed at this time. Notural bottom Many culverts, espeeially small bridge-type structures and multiplate arehes, have natuml stream bottoms. The irregularity of the bottom may present difficulties in applying these datil to the formulas for certain types of flow, Com- pute slope using average bottom elevations, The determination of depth to the minimum elevation (definition of d) {n the cross section or to the average elevation has no effect in flow types 1, 2, and 3 so long as hlo d" "nd h, are measured at the same points. For flow types 5 and 6, use the "verage bottom eleva- tion to determine h" Because natural bottoms in culverts usually cause nonuniformity in cross-sectional areas, special treatment must be given when the culvert is flowing full. An example is in type 4, where the standard formula is not applicable and the routing method of computing dis- charge should be used,