6.8 Floodplain Flow Distribution at a Bridge

When weir flow is computed, the flow distribution in the adjacent cross sections (2 and 3) to the bridge should be reviewed and compared to the flow passing over the bridge at sections BD and BU. Ideally, the weir flow passing over the roadway on each side of the bridge should approximate the discharge in the respective floodplain for sections 2 and 3. Figure 6.20 illustrates the profile results from a bridge model that will be used in the next section to provide an example of this review. The standard bridge tables available in HEC-RAS are used here to inspect the flow distribution. An incorrect flow distribution may not result in significant profile errors, but it can cause inaccuracies in bridge scour computations. Chapter 13 discusses bridge scour.

For the computations shown below, the weir flow was 16,399 ft³/s for the largest flood (shown in bold). Therefore, one would expect that a total discharge somewhat less than this flow would be present in the left and right floodplain sections immediately outside the bridge, because some of the weir flow across the roadway is passing between the channel bankline stations. For this example, these sections adjacent to the bridge are labeled 2.3 (downstream) and 2.4 (upstream). At these sections, the total discharge in the overbank areas is between 11,000 and 12,000 ft³/s (the sum of the bolded values for sections 2.3 and 2.4). The overbank discharge at sections BD and BU is about 25 percent higher.

If bridge scour analysis will take place for this bridge, some additional adjustment (decreasing overbank n to increase the overbank discharge) at sections 2.3 and 2.4 should be made to better approximate the proper flow distribution. Although this adjustment of n will not likely cause a significant impact to the flood elevations, a greatly unbalanced flow distribution can adversely impact bridge scour potential. For bridge scour computations, the proper discharge distribution is important. However, for a water surface profile analysis alone, the output does not seem unreasonable and would be acceptable for a final profile.

Bridge-Only Table - n = 0.04

River Sta	E.G. US, ft	Min El Prs, ft	BR Open Area, ft2	Prs O WS, ft	Q Total, ft3	Min Weir El, ft	Q Weir, ft3	Delta EG, ft
2.35	435.23	436.00	1362.14		15,000	440.01		1.32
2.35	444.73	436.00	1362.14		35,000	440.01	16,399	3.35

Six Bridge Sections Table - n = 0.04

River Sta	E.G. Elev, ft	W. S. Elev, ft	Crit. W.S., ft	Frctn Loss, ft	C&E Loss, ft	Top Width, ft	Q Left, ft3	Q Channel, ft3	Q Right, ft3	Vel Chnl, ft/s
2.5	435.60	435.32		0.10	0.26	533.33	5450.92	5702.92	3846.32	6.45
2.5	444.91	444.58		0.07	0.11	624.80	14621.21	9738.24	10640.55	7.71
2.4	435.23	434.08	427.31			546.86		15000.00		8.61
2.4	444.73	444.04	433.36			624.80	6913.17	23667.42	4419.41	8.06
2.35 BR U	435.57	432.08	429.82			83.16		15000.00		15.00
2.35 BR U	444.73	444.04	443.78			624.80	9653.08	18211.29	7135.63	9.86
2.35 BR D	435.05	429.82	429.82			78.65		15000.00		18.35
2.35 BR D	444.73	444.04	443.90			624.80	9737.80	18051.79	7210.41	10.11
2.3	433.91	432.00	427.71	0.33	0.76	528.72		15000.00		11.08
2.3	441.38	440.22	434.22	0.24	0.34	600.55	7133.59	23304.56	4561.85	10.51
2.2	432.81	432.43		0.50	0.00	541.15	5211.79	6126.74	3661.47	7.41
2.2	440.80	440.32		0.48	0.00	610.13	14245.44	10503.43	10251.13	9.13