Turbid Seepage at an Earthfill Dam Toe
An 18 m high homogeneous earthfill dam has a downstream toe drain and a toe filter. Last year at full reservoir level (FRL), the measured toe seepage was 3 L/s and clear. This week, with the reservoir at FRL for three days after heavy rain, the seepage has risen to 12 L/s and is slightly turbid. A 15 m long wet patch has appeared on the downstream slope at about 4 m above the toe, and one piezometer in the downstream shell has risen by 1.2 m in 24 hours. The toe drain outlet is partly blocked by floating debris. No boiling or sinkhole is visible. The dam is 40 km from the divisional headquarters, and a village of 600 people lies 3 km downstream. The sub-divisional engineer proposes to plug the toe drain outlet with sandbags to ‘stop the loss of water’ and to wait for the next inspection after the rain.
1. Classify the situation using dam safety criteria and justify your classification.
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This should be classified as a dam-safety emergency, not a routine seepage observation. Three indicators point to possible internal erosion: seepage has increased fourfold, it has become turbid, and a piezometer in the downstream shell has risen sharply. Turbidity is the critical red flag because it suggests that fines are being washed out of the embankment. The wet patch 4 m above the toe indicates that the seepage path is not confined to the toe drain and may be emerging higher in the downstream slope. The absence of visible boiling or a sinkhole does not make the situation safe; piping can develop rapidly.
The correct response is to alert the dam safety organisation and the district disaster management authority, inform the collector and downstream village, and activate the emergency action plan. Waiting for the next inspection after the rain is not acceptable.
2. List the immediate actions in the first six hours and identify the proposed action that must be avoided.
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In the first six hours: alert the superiors and dam safety cell; start a continuous watch; measure seepage flow and turbidity at short intervals; read all piezometers; record reservoir level and rainfall; restrict downstream access and prepare evacuation if monitoring worsens; clear the toe drain outlet carefully without endangering personnel; and lower the reservoir in a controlled way if the spillway and downstream conditions permit. Keep heavy vehicles off the crest and downstream slope.
The action to avoid is plugging the toe drain outlet with sandbags. The drain is providing a controlled exit for seepage. Blocking it raises pore pressure within the embankment and can force water to erode a new path, converting a visible seepage problem into a sudden breach. Any relief measure must be designed as a filter, not as a plug.
3. What instrumentation and records should be maintained until the reservoir is lowered?
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Maintain a timed log of seepage flow and turbidity at each outlet, piezometer readings, reservoir level, rainfall and any change in the wet patch. Record crack widths, sinkholes, boils or new seepage points on the downstream slope. Take dated photographs and a sketch showing locations. Log every phone call, instruction and action with time. Continue readings at least hourly during the emergency and reduce frequency only after the reservoir is lowered and the readings stabilise. Keep the records signed and hand them to the dam safety team for the post-event review.