One Dam, Two Sites on the Ranganga: Storage versus Foundation
Site selection for a flood-control dam on the Ranganga, a right-bank tributary of the Ghaghara in Uttar Pradesh. The river rises at 1,250 m in the outer Himalaya, drains 1,840 km², and after a 30 km course through boulder-studded Bhabar it emerges near Kalyanpur, where the bed gradient falls from 12 m/km to 1.4 m/km and the channel braids across a 1.2 km wide bed of sand and gravel. Downstream, 46 villages on the 12 km wide Tarai plain were flooded in 2019 and 2022; in 2022 the flood level rose 1.8 m above the danger mark at the district town. Two sites are proposed. Site A, near Kalyanpur, has a 2.1 km wide valley, 18 m of alluvium above bedrock, and 640 ha of double-cropped land would be submerged. Site B, 7 km upstream, is a 180 m wide gorge cut through a sandstone ridge, with bedrock exposed at 3 m depth; no arable land is drowned, but the reservoir would be only 9 km long and 40 m deep at full supply level. The state can fund one dam only, and it must be commissioned within six years.
1. Recommend Site A or Site B for the single dam, justifying your choice on foundation conditions, storage, submergence and the six-year deadline.
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Step 1 - Foundation. Site A sits on 18 m of alluvium. A dam there needs a deep cut-off trench or an upstream impervious blanket, and it remains exposed to under-seepage and piping; the 2.1 km valley also means a long earthen embankment plus a separate spillway. Site B has bedrock at 3 m and a 180 m gorge, so a short concrete gravity section with a chute spillway on rock is possible. Foundation safety and construction time both favour B.
Step 2 - Storage. A 9 km long, 40 m deep reservoir is small (of the order of 100 million m³) for a 1,840 km² monsoon-fed catchment. It will shave flood peaks but cannot store a very large flood. Site A, in a wide valley, would give much greater storage.
Step 3 - Submergence and time. Site A drowns 640 ha of double-cropped land, requiring rehabilitation, compensation and likely litigation - exactly what delays projects. Site B drowns no farmland.
Judgement: Choose Site B, but conditionally - grout the jointed sandstone, provide a low-level sediment sluice, verify that full supply level can be raised economically to recover storage, and use the money saved on submergence to build embankments and a bypass channel at Kalyanpur. Storage at B alone will not give the same protection as A; the package must make up for it.
2. The river at Site B cuts through a sandstone ridge instead of flowing around it. Explain how this is possible and what it implies for the dam foundation and for leakage from the reservoir.
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How it happens. The gorge is a water gap produced by antecedent drainage (or by superimposition from an older cover of alluvium): the river's course is older than the ridge, and as the ridge slowly rose the river kept incising at the same rate, so it maintained its line instead of being diverted. A second possibility is that the sandstone is intensely jointed, and the river exploited a zone of weakness.
What it means for the foundation. The rock at 3 m is competent - better than 18 m of alluvium - but the same joints that let the river through also let water through. The design must include curtain grouting below the dam axis and water-pressure tests in boreholes to measure permeability. If the rock is a jointed aquifer, water can pass under or around the abutments.
What it means for the reservoir. The flanks of a gorge reservoir are often permeable; a reservoir sitting on jointed sandstone can lose water into the hillsides, and in limestone terrain could develop solution channels. Check for springs downstream of the abutments after first filling.
Examination point: the gorge is evidence of a long, graded river cutting into rising ground; it does not mean the rock is impermeable. Shallow rock is an advantage only after grouting is designed for it.
3. Interpret the 1.2 km braided bed at Kalyanpur for the sediment regime, and estimate how it affects the useful life of a reservoir at Site B.
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Reading the braid plain. Braiding over sand and gravel, at the point where gradient drops from 12 m/km to 1.4 m/km, records a sudden loss of transporting power at the mountain front - a depositional zone dominated by bed load. Narrow, shifting channels between bars also indicate a highly variable discharge: a violent monsoon peak and a low dry-season flow. Sediment is therefore coarse and abundant, not merely suspended silt.
Estimate. Take a sediment yield of about 2,000 m³/km²/year (reasonable for a Himalayan-fed catchment; the range is 1,500–2,500). Yield = 1,840 × 2,000 ≈ 3.7 million m³/year. If gross storage is about 100 million m³ and dead storage about a third of it, dead storage fills in roughly 9–12 years, and serious loss of live storage begins within 25–30 years. A small gorge reservoir has a high trap efficiency (about 90%), so it silts faster than a large one.
Consequence for the decision. Sediment turns the case into a package, not a single structure: check dams and afforestation in the Bhabar recharge zone, a low-level sluice for sluicing sediment during the early monsoon, and a re-survey of the capacity every five years. Without these, Site B's advantage over A is partly cancelled.