Bank retreat at a Brahmaputra village bend
At a village on the south bank of the Brahmaputra in Dibrugarh district, a 2.4 km embankment built in 2008 protects 380 households and 210 ha of paddy land. Survey stakes show that since 2015 the bank has retreated 12–18 m each year at the village bend, while a reach 3 km downstream has widened by about 90 m. The bed is fine sand with a median grain size of 0.18 mm, and suspended sediment concentration at the bend rises sharply from June to August. During the 2020 flood, water crossed the embankment for nine days and 40 households moved to a road embankment. Villagers want to extend the embankment upstream and build a concrete spur into the channel. A junior engineer warns that the spur may shift erosion to the opposite bank, where a ferry ghat and a primary school stand.
1. Explain the dominant geomorphic process at the village bend and why the 2008 embankment has not stopped bank retreat. Refer to the sediment and flood evidence.
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The village is on an outer bank, where secondary currents and high stream power attack the toe. The bed is fine sand (0.18 mm), which is non-cohesive: once the toe is saturated, grains are easily entrained. The June–August rise in suspended sediment shows that the river is actively reworking the channel at this season, not merely overtopping. The embankment protects the upper bank but leaves the toe exposed to scour. It may also concentrate flow in the channel by preventing sediment from being deposited on the floodplain. In 2020 the design flood was exceeded, so the bank became saturated and failed by mass movement as well as toe erosion. Retreat of 12–18 m per year is therefore consistent with toe scour plus slab failure of the saturated upper bank, not with overtopping alone.
2. Evaluate the proposed concrete spur against alternatives, and recommend a course of action. Name one monitoring indicator that should trigger a review.
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A solid spur deflects the thalweg away from the village, but it transfers energy downstream or across the channel. Because a ferry ghat and a primary school are on the opposite bank, that third-party risk is serious, and the downstream widening already suggests the channel is adjusting. A better package would attack the actual failure mechanism: (i) toe protection with a launching apron or revetment, not only an upper-bank wall; (ii) bioengineering with deep-rooted grasses and bamboo behind the toe protection; (iii) a setback and phased relocation of the most exposed households. If a spur is unavoidable, use a permeable or angled design and model opposite-bank effects first. Monitor bank-line position on permanent cross-sections after each monsoon; a trigger of more than 5 m retreat in one season, or measured toe scour below the apron, should prompt review.