Rising Bed, Falling Freeboard: The Damodar Below Durgapur Barrage
Cross-sections surveyed in 1998 and again in 2023 along a 12 km reach of the Damodar in Paschim Bardhaman show that the lowest bed has risen by 1.6 m. Bed material is 78% sand and silt. The left flood embankment, built in 1959, was designed with 3.2 m of freeboard above the then low-water bed; the 2023 survey puts freeboard at 1.1 m. The reach overtopped at two places in 2017 and once in 2021, and 340 ha of paddy behind the embankment has been inundated four times in six years. Sand mining from the bed was banned in 2018 after a court order, and the district engineer reports that mining has since shifted to the banks, where fresh slumping is visible for several kilometres. The irrigation department has two proposals: raise and strengthen the embankment by 2.5 m at an estimated Rs 68 crore, or dredge the reach at about Rs 41 crore for every ten years of maintenance. A third, unstated option is watershed treatment in the coalfield catchments upstream.
1. From the survey figures, decide whether this reach is aggrading or degrading, and explain why the same discharge now produces a higher flood level.
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Verdict: the reach is aggrading. The bed rose 1.6 m in 25 years, an average of about 6.4 cm a year, and 78% sand-and-silt bed material is exactly what a supply-limited, depositional reach would hold.
Why the flood level rises for the same discharge:
- Stage is a function of discharge and channel geometry. A raised bed reduces the cross-sectional area of flow, so the same volume of water must stand higher to pass through.
- The reach becomes wider and shallower as it aggrades, raising hydraulic roughness and slowing velocity, which again lifts the water surface.
- The freeboard fell by 2.1 m (3.2 to 1.1 m) while the bed rose only 1.6 m. The extra 0.5 m is a warning: part of the loss is embankment settlement or a changed design discharge, not bed rise alone. A good answer notices that the two numbers do not match.
Where the sediment comes from: sheet and gully erosion in the coalfield and plateau catchments upstream, tributary inflow, and now bank slumping. The 2018 ban is part of the cause, not just the cure: by pushing extraction onto the banks it converted a stable, vegetated bank line into an active sediment source.
2. The executive engineer argues that dredging is the cheaper option at Rs 41 crore. Assess this claim against the sediment budget.
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First, size the problem. Volume of the 25-year rise = 12,000 m reach length x roughly 150 m active bed width x 1.6 m = about 2.9 million cubic metres, i.e. roughly 115,000 cubic metres a year.
Then price removal. At a realistic Rs 300 to 500 per cubic metre, keeping the channel at its 1998 level costs about Rs 3.5 to 5.8 crore a year, or Rs 35 to 58 crore per decade. The quoted Rs 41 crore per ten years therefore sits at the optimistic end and leaves almost no margin. If aggradation accelerates, the dredge refills before the decade is out.
Verdict on the claim: 'cheaper' is true only in the short run and only if dredging is repeated forever. Dredging is maintenance, not a cure, and it carries its own side-effects: a deep low-flow channel can steepen the local gradient, undercut banks and shift erosion downstream.
A fairer comparison: compare the Rs 68 crore embankment not with one dredging cycle but with ten cycles plus the residual flood risk. Neither number alone settles the question; the sediment budget does.
3. Recommend a strategy for the reach, and state what must be measured before the money is committed.
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Recommended package, in order of rupees-per-unit-of-risk-reduced:
- Reduce sediment at source - gully plugging, check dams and vegetative cover in the coalfield catchments. This is slow and invisible politically, but it is the only measure that lowers the annual 115,000 cubic metre inflow.
- Regulate sand extraction instead of banning it. A total ban pushed mining to the banks and added slump material. Controlled, monitored bed extraction in designated reaches removes sediment where it does most harm.
- Dredge selectively, not uniformly: the outfall and the two overtopped spots, not the whole 12 km.
- Raise the embankment only where freeboard is below about 1.5 m, with a funded maintenance line, because an unmaintained embankment becomes a breach waiting to happen.
- Floodplain zoning - stop new construction in the 340 ha that has flooded four times in six years.
Measure before committing: suspended and bed-load sediment at the barrage; bed levels on fixed cross-section pegs surveyed annually; a bank-line survey; and the volume actually removed by dredging. Without the sediment figure, the dredging contract cannot be checked.