Turbid Seepage at a Thermal Plant Ash Dyke Before Monsoon

Increasing flow with turbidity is not normal clear seepage. It indicates initiation of internal erosion or piping, which is a high-risk condition for an earth dyke. The priority order must be life safety, reduction of the driving head, then controlled monitoring.

Within 12 hours I would:

1. Alert the plant head, dam safety cell and district administration; restrict access to the downstream seepage area and warn the village.
2. Lower the pond level by stopping or reducing ash slurry disposal and by decanting or pumping water where safely possible. Target at least 0.5 m below the current level.
3. Stop ash slurry disposal unless there is no alternative; if the alternative pond is full, use it only after a written risk acceptance and with continuous watch.
4. Place a graded filter blanket of sand and gravel over the seepage patch and toe. Do not seal the exit with clay or concrete, because that can raise pore pressure and push the erosion elsewhere.
5. Install piezometers and a seepage weir, and inspect hourly for new boils, cracks or settlement.

The village at 600 m and the 120 mm forecast make delay unacceptable. Continuing disposal at the current pond level would add both water and ash load while the dyke is already showing distress.

A minimum programme should cover pore pressure, seepage quantity and quality, and deformation.

Instruments and locations:

• standpipe piezometers at the crest, mid-slope and downstream toe in at least three sections along the 700 m dyke;
• a calibrated weir or bucket measurement at the toe drain and at the new seepage patch;
• turbidity measurement of seepage water;
• settlement markers on the crest and downstream slope;
• visual inspection points for boils, cracks and wet patches.

Readings should be taken every 2 hours during the emergency, then daily if stable. Record pond level with each set.

Trigger values should be set before the next reading: a seepage flow above 15 L/s, a rise of more than 25% in 24 hours, turbidity above 50 NTU or visibly increasing, a new boil, piezometric level above the downstream toe, or any crest settlement. Any trigger should mean stop disposal, lower the pond further and seek geotechnical advice.

The investigation should include test pits, gradation and permeability tests of the dyke and foundation soils, and a stability analysis using the measured pore pressures. The five-year-old review is not adequate for the current condition.

My starting position is not to allow 36 hours of ash slurry disposal at the current pond level. Turbid seepage is an internal erosion warning, and the forecast of 120 mm of rain in 48 hours will raise the pond and pore pressures. Adding slurry increases both hydraulic head and stored material.

I would allow only a very limited continuation if all of the following are met:

• the pond level has been lowered by at least 0.5 m and is stable;
• seepage flow and turbidity are stable or falling over at least four hourly readings;
• a filter blanket and toe weighting are in place;
• continuous manned watch and an emergency stop procedure are available;
• the alternative pond is being prepared in parallel;
• the district administration and dam safety cell have been informed.

Even then, I would limit disposal to the minimum time needed, not automatically 36 hours, and stop immediately if flow, turbidity or settlement increases. If the pond cannot be lowered, the correct decision is to stop disposal and accept the operational cost rather than risk a dyke failure above a village.