Flood-Damaged Box Culvert on a State Highway
A 25-year-old, three-span 2 m × 2 m RCC box culvert carries a state highway across a seasonal stream in north Bihar. It was designed for a 25-year return period flood of 40 m³/s. Catchment urbanisation has since raised the 25-year peak to about 55 m³/s. During the last monsoon the road remained submerged for 36 hours, upstream afflux reached 0.6 m, a 1.8 m deep scour hole formed near the second pier, and one barrel was found choked with silt and debris. Traffic is 8,000 PCU/day and the permanent diversion is 22 km. The department has Rs 4.5 crore and about five months before the next monsoon; land acquisition is not feasible and a nearby irrigation canal limits realignment. The executive engineer must decide whether to retain, rehabilitate or replace the culvert, and in what sequence.
1. Check the hydraulic adequacy of the existing culvert and decide whether it can be retained. Show the calculations you would use and state what field data you need.
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Step 1 - barrel capacity. Three barrels of 2 m × 2 m give 12 m². Treating the culvert as a submerged orifice with Cd ≈ 0.62 and head difference 2.2 m, Q ≈ 0.62 × 12 × √(2 × 9.81 × 2.2) ≈ 49 m³/s. A Manning check per barrel at the affluxed upstream level gives a similar order. The updated design flood is 55 m³/s, so the structure is inadequate.
Step 2 - overtopping. Use the broad-crested weir form Q = 1.7 L H^1.5 over the submerged road length. A 36-hour closure with 0.3 m head over a 60 m road length gives about 33 m³/s, confirming that a large part of the flood passed over the road.
Field data needed: fresh HFL marks, updated catchment and 25/50-year peaks, barrel condition (cover, chloride content, cracks), scour soundings, and the road profile. Retention is possible only if an additional vent of about 10–12 m² is provided and the road is raised by at least 0.8 m.
2. Rank the immediate pre-monsoon actions within the available budget and justify the order.
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Rank by risk reduction per rupee: (1) Clear both barrels and remove debris - low cost, restores roughly one-third capacity. (2) Protect the scoured pier - fill the 1.8 m hole with graded stone and place a launching apron/gabion; the pier is the collapse risk. (3) Repair cracks and apply a protective coating inside the barrels to slow chloride ingress. (4) Install a painted gauge board and an upstream rain gauge with a threshold warning so the road can be closed early. (5) Raise the low upstream approach by 0.5 m with granular fill to cut overtopping duration.
Do not start a new bridge this year: design, approval and land take cannot be completed in five months. Keep a contingency of about 15% and photograph the pre-monsoon condition for later contractor claims.
3. What permanent option and design changes would you recommend, and how would you justify the extra cost?
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Recommend a parallel twin-cell 3.0 m × 2.5 m box on the downstream side, designed for the 50-year peak with 1.0 m freeboard, plus a raised causeway profile. Justify with a life-cycle comparison: 36 hours of closure at 8,000 PCU/day imposes roughly Rs 60–80 lakh per event in vehicle operating and time costs, apart from accident and detour costs; two such events exceed the incremental capital cost.
Design checks: IRC SP 13 for scour, Lacey's regime for the waterway, rechecking the existing structure for the new afflux, and a sediment survey to fix the invert level. Add a 10% climate allowance to the design discharge. Provide a monitoring plan - gauge board, pier settlement pins, and inspection after every flood above a defined threshold.