Sediment Starvation Below a Dam
A 120-km river flows below a 60-m-tall dam completed 40 years ago. The reservoir traps about 90% of the bed load and a large share of suspended sand. Since closure, repeat cross-sections show channel incision of 1.5 m at 10 km below the dam, 0.8 m at 40 km, and 0.2 m at 100 km. The median bed grain size at 10 km coarsened from 18 mm to 32 mm. A highway bridge pier founded on piles with 4.0 m of embedment now has 2.1 m of pile exposed. Tributary sediment inputs are small, and the downstream channel bed is sand and gravel over resistant bedrock. Residents report bank slumping and lower summer flows. The highway agency must decide whether to protect the bridge now, request dam sediment management, or both, and it needs a monitoring plan that detects further incision before the pier is undermined.
1. Explain the downstream incision and coarsening using a sediment budget, and predict how the disturbance will move over time.
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The dam cuts off bed load, so the downstream reach has a sediment deficit. Transport capacity is set by discharge and slope; with little sediment supply, the river erodes its bed to satisfy the load, a process called clear-water scour. Incision is largest near the dam (1.5 m at 10 km) and decreases downstream (0.2 m at 100 km) because tributaries and bank input gradually restore some load and because the bed becomes resistant. The grain-size increase from 18 to 32 mm is consistent with winnowing of fines and armoring by coarse gravel. The erosion front-a knickpoint or degradation wave-will migrate downstream, but its rate will slow where bedrock or coarse armor is encountered. Bank slumping occurs because incision steepens and undercuts banks, and lower summer flows may reflect channel storage changes. The system will not simply stabilize at current depths.
2. Assess the bridge-pier risk and recommend immediate and long-term actions.
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The pier has lost slightly more than half its design embedment (2.1 of 4.0 m exposed), which reduces lateral capacity and skin friction and leaves little margin for a design flood or further scour. The channel at 10 km is still incising, and a flood could mobilize the armored layer. Immediate actions: install real-time scour monitoring such as sonar or tilt sensors, place riprap or a concrete collar around the pier, and restrict heavy loads if scour progresses. Long-term: evaluate a grade-control structure or bed sill downstream of the bridge, require sediment bypass or sluicing at the dam, and resurvey cross-sections after every major flood. Because the dam is the cause, a bridge-only fix may be repeatedly undermined; the agency should coordinate with dam operators and cost-share a sediment budget study.
3. Design a monitoring program that would distinguish ongoing incision from temporary scour and give at least three specific measurements.
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Use fixed cross-sections at 5, 10, 25, 50, and 100 km, surveyed quarterly and after floods, with permanent benchmarks outside the channel. Install bedload samplers or repeat particle-size counts to track armoring. At the bridge, use scour chains, sonar, and pier-mounted tilt sensors. Bank pins or repeat LiDAR detect retreat. Pressure transducers at upstream and downstream ends allow calculation of discharge and shear stress. A turbidity sensor can indicate sediment pulses from tributaries. To separate temporary flood scour from progressive degradation, compare pre- and post-flood bed elevations and look for a sustained downward trend at low flow. Set action thresholds: for example, 0.3 m additional pier exposure or a 0.2 m drop at the 10-km section triggers inspection. Report data to the dam operator annually.