FLOOD06423 - Laserfiche WebLink

Home Browse Search

6 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS If no solution is possible, a message, "NO SOLUTION TYPE FIVE FLOW." is printed. A type 6 flow computation is attempted wheth- er or not a type 5 flow solution is obtained. Type 6 Flow Type 6 flow occurs when a culvert flows full under R- high head. Six steps are re- quired to obtain the headwater elevation. 2 29nL.. td' 1. The term. 4/3' 18 compu e 1n R o which n is the roughness coefficient of the harrel and R is the hydraulic radius of the . 0 full barrel. 2. The value of C is taken from columns 62- 65 of card 8. 3. A correction factor, kr is determined from polynomial equations within the program using values determined in steps 1 and 2. The equations approximate the curves of figure 17 in Bodhaine (1968). 4. The term, ~, is computed. o Q 5. JC;JD is divided by kf' and the result o is used to obtain a value of hI! D from the polynomial equation of the bottom curve in figure 17, Bodhaine (1968). 6. hi is computed by multiplying the value of hj! D by D. PROCEDURES Field Data A computer program for a complete stage- discharge relation imposes a few constraints not necessary when only one peak discharge is determined~ Therefore, the field-party chief should be familiar with all the require- ments for computation. An approach section is necessary ~ If most of the flows will put the culvert under a high head with ponded approach conditions. a sim- ple trapezoidal section approximating the ap- proach channel can be defined in the field notes~ However, the stage- discharge relation for low-head flows may not be very reliable~ Be certain to extend the approach section higher than the maximum headwater elevation required, which is about 2~ 0 D +- z. If J.'ield conditions make this impractical, assume vertical ends up to the required elevation. Measure pertinent culvert geometry care- fully. Note size and spacing of corrugations, and radius of rounding of corrugated-metal pipes, pipe- arches, and multiplate arches. Describe entrance and getaway conditions so that an appraisal can be made of the discharg~ coefficient and the possibility of tail.water control. [)etermine elevations along the road- way where overflow might occur~ Use a -sur- vey datum low enough to avoid negative ele- vations, although the computer can handle them~ Roughness Coefficients Roughness coefficients must be selected for the approach section below and above designated elevations~ The elevations are to the nearest foot and must be 1 foot, or a multiple of 1 foot, apart~ The value of n is constant below the lower elevation and above the higher elevation and varies linearly with depth between the two elevations~ Roughness coefficients are selected in this manner for each subarea. If both n's are the same, there is no variation with depth. Select roughness coefficients for the culvert barrel. Bodhaine (968) furnishes guidelines on pages jO and 11. - Coefficients of Discharge Coefficients of discharge, C, for flow types 1-6 were defined by laboratory study and are discussed in detail by Bodhaine (1968), pages 37-45. The coefficients vary from 0.39 to 0.98, and they have been found to be a function of the degree of channel con- traction and the geometry of the culvert en- trance. For certain entranc-e geometries the dis- charge coefficient is obtained by multiplying a base coefficient by an adjustment factor such as k or k ~ If this procedure results r w in a discharge coefficient greater than O. 98, a coefficient of O. 98 is used automatically by the computer as the limiting value; The listing of discharge coefficients in Bodhaine (1968). has been put into categories ~