FLOOD05059 - Laserfiche WebLink

Home Browse Search

as the exit section. The exit section is moved vertically so that the low point on the new section is the same as the low poi,nt on the road section for a bridge and the same as the, outlet invert for a culvert. Step 2.-The head loss or headwater elevation is found by assuming head losses beginning with zero loss and continuing in small increments. For each assumed loss, WSP2 finds the flow through the bridge opening or culvert(s), calculates the flow over the road, and adds these flows. The final head loss is the assumed loss at which the summed flows agree within 0.1 foot of head. The different procedures used to compute flow for a given head loss at a bridge opening and a culvert are described under the headings "BPR bridge loss analysis," "Culvert loss analysis," and "Contracted opening bridge loss analysis." Step 3.-After the headwater elevation is determined, energy is balanced from the upstream face of the bridge or culvert to the approach section. In order to do this, a velocity head must be calculated and added to the headwater elevation to get an ener;1y grade line elevation at the upstream face of the bridge or culvert. WSP2 manufac- tures another section at the upstream face with the same shape as the approach valley section. The approach section is moved vertically so that the low point on the new section is the same as the low point on the road section for a bridge, and the same as the inlet invert for a culvert. Using this new section, WSP2 finds the area, by seg- ment, at the headwater elevation and com- putes a weighted velocity head. Once this velocity head is found, the water surface profile at the approach section is deter- mined. The length to the approach section is the channel length on the reach card for the approach section. Flow over embankment analysis The flow rate over a road is found from a weir equation. Due to the irregular shape (across the valley) of most road surfaces, it is impossible to assume a common geo- metric shape and develop a specialized weir equation. Therefore, a modification of the rectangular weir equation Q = CLh'/' is used. The modification is the substitution of A (area) for Lh which yields the equation 0= CAh'/'. BPR bridge loss analysis WSP2 uses a ratio of conveyances (M) to predict losses in the area of a bridge (see BPR Manual). To obtain this ratio, divide the conveyance of the approach section for a width equal to the bridge opening width at the bridge tailwater elevation by the total approach section conveyance. The BPR Manual projects bridge abut- ments in the upstream direction to define the portion of the approach section that will be used for the numerator of the convey- ance ratio. This is valid only if the channel in the vicinity of the bridge is straight. Most channels are not straight in the vicinity of bridges. Therefore, a "workable" technique had to be developed for WSP2. The pro- gram uses the station for the lowest eleva- tion on the approach section as the center of the bridge opening width. If this extends the width beyond one bank but not the other, WSP2 piaces the edge of the bridge opening at the bank station and extends the width from that point. In other words, WSP2 uses all the channel before any part of the flood plain, Once the ratio of conveyances is found, the loss coefficients (K) are obtained from equations derived for each curve in figures 6, 7, and 10 in the BPR Manual. Loss for flow eccentricity is ignored. The equations represent the curves very accurately, with the exception of the pier equations. The pier curves are represented by linear equa- tions and are not very precise below a ratio of pier area to total bridge area of about 0.02. The pier curves are shown in figure 7 in the BPR Manual. They extend to a pier K value of 0.4, which is the maximum allowed in the computer program. The use of the curves in figure 6 in the BPR Manual should be as follows. The bot- tom curve (No.1) is for all spillthrough 5