Check Dam in a Loess Wadi Near Revivim
A small drainage basin near Revivim in the western Negev has a catchment of 6.4 km². Mean annual rainfall is 95 mm, but storms are short and intense: a recent event delivered 40 mm in two hours. The basin is underlain by loess and chalk, with slopes of 8–15% and sparse dwarf-shrub cover. Flash floods reach the road crossing within 25 minutes. A regional water authority proposes a 4 m high check dam (liman) at elevation 320 m to store floodwater for livestock and to reduce siltation of a downstream reservoir. The dam is expected to trap 12,000 m³ per event, but engineers warn that the reservoir already loses capacity. Farmers downstream rely on occasional flood spreads; ecologists note that sediment pulses maintain downstream shrub patches. No environmental impact assessment has been completed. The authority must decide whether to approve the dam, modify its design, or reject it, and what monitoring to require.
1. Explain the geomorphic processes that make this basin prone to flash floods and sediment delivery, and link them to the proposed dam.
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Two linked process sets dominate. First, infiltration-excess overland flow: loess surfaces seal and crust under intense rain, so a 40 mm storm in two hours exceeds infiltration capacity. Slopes of 8–15% and sparse shrubs concentrate water into rills, gullies and the channel. Second, sediment supply: sheetwash lifts silt and clay, gully walls and channel banks collapse, and the chalk–loess contact supplies coarser material. The result is a short lag time - about 25 minutes - and a flash flood with high suspended-sediment concentration.
The dam would intercept part of this cascade. It reduces peak discharge and traps coarse bedload, but it does not stop the hillslope processes generating sediment. Because the storm is short and intense, effectiveness depends on whether the structure stores surface water or recharges the shallow aquifer. A 4 m wall with 12,000 m³ capacity may fill quickly if sediment delivery is high, so it cannot be assumed to save all floodwater. The geomorphic reasoning points to a trade-off: lower downstream flood peak and less silt reaching the reservoir, but sediment starvation and possible channel incision below the dam.
2. Evaluate the claim that the check dam will both save water and reduce siltation of the downstream reservoir. Use a sediment-budget and downstream-effects argument.
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The claim is partly true but too simple.
Water saving. An open 12,000 m³ pond in the Negev loses a large share to evaporation, perhaps 2–2.5 m per year. A shallow surface pond therefore stores less useful water than its capacity suggests. If the dam is designed as a subsurface liman, evaporation is much lower and the saving is more realistic.
Siltation. The dam traps sediment, so less sediment reaches the downstream reservoir - good for reservoir capacity. But the check dam itself fills. With a 6.4 km² catchment and a 12,000 m³ trap, a few large events could deliver enough sediment to reduce storage significantly. Downstream, sediment-starved flows may erode channel beds and banks; flood-spread farmers lose the silt that maintains soil fertility; shrub patches that depend on occasional sediment and water pulses may decline.
Thus the dam may reduce reservoir siltation in the short term while transferring the sediment problem upstream and creating new ecological costs. A sediment budget is needed: measure sediment input, trap efficiency and downstream transport before approval. The correct judgement is conditional, not a simple yes.
3. Recommend two design or monitoring modifications and justify them in relation to the geomorphic processes you identified.
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Recommendation 1: redesign as a permeable or subsurface check dam with a controlled spillway and sediment bypass. A solid wall traps both water and sediment and creates a high evaporation loss. A permeable structure slows flow and promotes aquifer recharge while allowing finer sediment to pass. A spillway protects the structure in extreme events, and a sediment bypass or periodic dredging prevents rapid loss of capacity. Set a management trigger, for example dredge when 60% of design capacity is filled.
Recommendation 2: establish a before–after monitoring programme. Install rain gauges, water-level loggers upstream and downstream, turbidity and sediment samplers, channel cross-sections and vegetation transects. Monitor for at least five years. This is justified because the main uncertainties are sediment delivery, evaporation loss and downstream response - exactly the processes that determine whether the dam works. Monitoring also allows adaptive management: if downstream incision or loss of flood-spread benefits appears, the authority can modify releases or remove the structure. Require an environmental impact assessment and a downstream release rule as conditions of approval.