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<br />6 <br /> <br />TECHNIQUES OF WATER-RESOUR{:ES INVESTIGATIONS <br /> <br />Flow under high heod <br /> <br />High-head flow will occur if the tailwater is <br />below the crown at the outlet and the head- <br />water-diameter ratio is equal to or greater than <br />1.5. This is an approximate criterion. The two <br />types of flow under this category are 5 and 6. <br />As shown in figure 2, part-full flow under a <br />high head is classified as type 5. The flow pat- <br />tern is similar to that downstream from a sluice <br />gate with rapid flow near the entrance, The <br />occurrence of type 5 flow requires a relatively <br />square entrance that will cause contraction of <br />the area of live flow to less than the area of the <br />culvert barrel. In addition, the combination of <br />barrel length, roughness, and bed slope must <br />be such that the contracted jet will not expand <br />to the full area of the barrel. If the water sur- <br />face of the expanding flow comes in contact <br />with the top of the culvert, type 6 flow will <br />occur, because the passage of air to the culvert <br />will be sealed off causinlr the culvert to flow full <br />throughout its length. Under these conditions, <br />the headwater surface drops, indicating a more <br />efficient use of the culvert barrel. <br />Within a certain range either type 5 or type <br />6 flow may occur, depending upon factors that <br />are very difficult to evaluate. For example, the <br />wave pattern superimposed on the water-surface <br />profile through the culvert can be important <br />in determining full or part-full flow. Within the <br />range of geometries tested, however, the flow type <br />generally can be predicted from a knowledge of <br />entrance geometry and length, culvert slope, <br />and roughness of the culvert barrel. <br /> <br />Type 5 flow <br /> <br />Type 5 flow is rapid at the inlet. The head- <br />water-diameter ratio exceeds 1.5 as shown on <br />figure 2, and the tailwater elevation is below <br />the crown at the outlet. The top edge of the <br />culvert entrance contracts the flow in a manner <br />similar to a sluice gate. The culvert harrel <br />flows partly full and at a depth less than <br />critical. The, discharge equation is <br /> <br />Q=CAo.J2g(h,-z). <br /> <br />Type 6 flow <br /> <br />In type 6 flow the culvert is full under <br />pressure with free outfall as shown in figure 2. <br /> <br />(10) <br /> <br />The headwater-diameter ratio exceeds 1.5 and <br />the tailwater does not submerge the culvert <br />outlet. The discharge equation between sections <br />1 and 3, neglecting V,'/2g and h"_,, is <br /> <br />Q=(,AO.Jii!i(h,:':'h,'::-h,~~,), (11) <br /> <br />A straightforward application of equation 11 <br />is hampered by thc necessity of dctermining h" <br />which varies from a point bclow the cent,er uf the <br />outlet to its top, even though the water surface is <br />at the top of the culvert, This variation in <br />piezometric head is a function of the Froude <br />number, This difficulty has been circumvented <br />by basin!'; the data analysis upon dimensionless <br />ratios of physical dimensions related to the <br />Froude number. These functional relationships <br />have been defined by laboratory experiment, <br />and their use is explained on page 31. <br /> <br />Field Data <br /> <br />Make a transit survey of floodmarks and <br />accurately measure the culvert geometry as <br />soon after the flood as possible. Use the <br />methods of surveying previously described; <br />read elevations of highwater marks, hubs, <br />reference marks, and culvert features to hun- <br />dredths of a foot and ground elevations t,o <br />tenths. Obtain high-water profiles as well as a <br />complete description of the culvert geometry, <br />Describe entrance and getaway condition~ <br />if not evident, from other data, Choose rough- <br />ness coefficients (values of n) for the culvert as <br />well as for the approach section, and obtain <br />stereophotographs documenting pertinent <br />features, Describe any unusual conditions at <br />the site. Appraise the possibility of entrance or <br />barrel obstruction at the time of the peak; <br />document evidence obtained from observer:s. <br />Determine the elevation of the low poiut o[ <br />the crown of the road over the culvert or make <br />note that there is a high fill, If there is II <br />possibility that water flowed across the road- <br />way, define a profile along the crown or hi!(h <br />point of the road. <br /> <br />High-water profiles <br /> <br />Obtain high-water profiles which adequately <br />define the headwater and tailwater elevations <br /> <br />e <br /> <br />. <br /> <br />e <br /> <br />, <br /> <br />e <br />