Managing your reservoir during dry weather
Check for cracks and signs of damage
When an earth embankment dam dries out, it shrinks and when it becomes wet it enlarges. You may see cracks appear as the dam shrinks and then disappear as they heal. Cracking may occur more quickly if there is shallow topsoil.
Figure 1. Watch for cracks appearing and keep records over time showing how they change.
Cracks may open and close over any dry period. You should monitor them and make sure “self-healing” occurs when wetter weather returns.
You need to monitor and record the location, extent, depth and orientation of cracking or other damage. Takes photographs if able. Share the information with your Supervising Engineer and confirm it is within expected norms.
If the cracks remain over a long period of time, you should expect further dam erosion due to rainfall, improper drainage, or freeze-thaw weathering. You should take advice at an early stage from your Supervising Engineer to rectify the problem.
Figure 2. Measure and record the length, width and depth of cracks. Share the information with your Supervising Engineer.
Long periods of dry weather can cause more problems. If the entire embankment dries out too much, the inner impermeable core may crack and form a leakage path through the dam. Leakage through a dam could result in its fast, catastrophic failure, or the weakness may develop over a longer period leading to a dam breach.
When the rain returns
Dry weather cracks should self-heal when sufficient damp or wet weather returns. Check that they do.
Cracks which get bigger during wet weather is a weakness and should cause you concern. Contact your Supervising Engineer immediately for advice.
If heavy or prolonged rain returns suddenly:
- weakened vegetation roots may not provide a durable surface and erosion may occur
- surface water run-off may enter the cracks causing them to enlarge, or create subsurface flow paths
- water entering deeper cracks can lubricate the join between topsoil and subsoil and cause the surface to slide or slump
The first thing you may notice is horizontal cracking with ridges forming further down the slope.
Figure 3. Deep cracks can allow water to penetrate and cause the surface to slide. Look for signs of lumps and bumps downhill of cracks or other damage
Slumps and slides do not self-heal and you should contact your Supervising Engineer immediately for advice.
Inspect the dam’s upstream face
If the water level is low, it is an ideal time to inspect the dam’s upstream face and carry out any maintenance that is needed. Your Supervising Engineer may be interested in looking at the upstream face in low water level conditions, so let your Supervising Engineer know of the situation.
Figure 4. Use low water levels to inspect and maintain the upstream face.
Maintain good grass cover
The grass and topsoil provide a protective blanket to the embankment’s sub-structure and helps free drainage of surface water during wet weather.
Prolonged dry weather can reduce the effectiveness of the grass cover as it dries out. A dusty or sandy surface is easily eroded. Dry weather conditions may coincide with thunderstorms producing flash flooding.
You need to keep this surface in good condition at all times.
Figure 5. Well maintained grass on an earth embankment.
Figure 6. Well maintained grass and soil surface allows rainfall to drain away without damaging the embankment.
The effects of climate change
Prolonged dry weather has become a relatively common feature of the weather in the United Kingdom over recent years. Climate change predictions show we will probably experience droughts more often, and when it does rain, it is predicted to be heavier and more intense.
These extremes stress an earth embankment. You need to maintain your reservoir at all times to minimise the effects of extreme weather.
Find information about drought planning and management on our page Drought planning and management
You should plan and implement contingency measures to manage reservoir water storage and usage during prolonged dry weather.
Report an incident
If cracking or damage of the earth embankment is sufficient to prompt you to reduce the water level in the reservoir, or your engineer recommends you carry out a precautionary drawdown of the water level, you must report it to us on 03000 65 3000 or report an incident online.
Further advice
If you do not have a Supervising Engineer, you should contact a qualified civil engineer from one of the reservoir panels for advice.