Information provided by Mr Darcy Gilberd, PO Box 1078, Whangarei

Fixing leaking dams is not an easy job. Earth dams are simple and cheap to construct, and work well where there is plenty of depth of impervious material to hold the water in the dam. But they can produce problems.

When earth dams leak, you first have to find out where the water is escaping. If it’s through the front wall of the dam, you’ll maybe be able to see it coming through in dry weather. If it’s not leaking at the front wall, then it must be somewhere in the bottom of the holding area or around the edge.

Where to start?
  • First you must find out how deep the porous level is - or at what depth the impervious layer is.
  • Bore a hole with a hand-held (50mm) post-hole borer - outside the dam wall in the normal material that forms the base of the dam.
  • You can get these from hire centres.
  • It’s quite easy with auger extensions to bore down to 10m.
  • Every 0.5 to 1m, stop and pour water into the hole and see if it disappears.
  • Carry on drilling until you find the water starts to come into the hole and filling it to a constant depth.
  • Make yourself a long stick from 50x25mm timber with a bean or jam tin screwed on the side as a baler.
  • You may need a torch too to see down the hole! And make sure you don’t loose your specs, diary or dentures down the hole!
  • When you bale past the porous layer, you’ll see the water coming into the hole from the sides. This is usually about 1-2m deep.
  • So now you have found the depth at which the dam’s foundation must start for the front wall.
Making a new front wall (called the bund) 
  • Dig a trench about 0.5 - 1m wide down to the bottom of the porous layer.
  • Backfill this trench with watertight clay.
  • Tamp it firmly with a fence rammer or an appropriate roller from a hire centre.
  • Carry the trench out up the side of the dam above the expected height of the water.
  • This should fix the problem - it will take time and could be costly.
Fixing the dam base with clay
  • Consider whether it’s going to be worthwhile trying to fix it?
  • Empty the dam.
  • Cover the base of the dam with about 0.5 - 1m of impervious clay.
  • Tamp it in tight with a sheep-foot roller from a hire centre.
  • Note that if the dam goes dry, this clay may crack and leak. It usually takes up and seals again when re-saturated with water. But not always!
Fixing the dam base with plastic lining
  • Suitable for coral Islands or volcanic porous sand soils.
  • Line the whole ponding area with plastic sheet.
  • Contact an appropriate supplier of the material and get advice.
  • Pay special attention to sealing the joints - get a tradesman to do this.
  • Take the lining well up the sides of the dam and protect it from damage.
  • Pay special attention to the slope (batter) of the sides so soil etc does not fall in.
  • All pipes bringing water into the dam must not run down the sides and cause erosion.
  • Make entry pipes discharge about 1.5-2m onto the dam surface.
  • Fence all dams to prevent stock damage and pollution.
Consider cutting your losses and build another dam
  • This may be an option. For example if the source of the stream or water supply (say 50m further down the slope) is high enough to be useful, then build the dam below that.
  • Where a spring comes to the surface is a good indicator that the material below the spring is impervious and a dam built on that should be successful.
  • If building a dam across an existing creek, remove all the vegetation, as it may rot and allow leakage.
  • Remove all loose material from the streambed.
  • Check the material at the side of the stream to see if it’s impervious (use the water soak-away test).
A plughole in a dam
  • This is an important point and well worth doing.
  • It will allow you to drain the dam easily for cleaning and repairs.
  • Use a long-span concrete pipe.
  • Put it through the dam wall near the bottom.
  • Wash it as clean as possible and put some 2:1 concrete mix around the pipe.
  • It’s good to put a collar on the end of the pipe inside the dam - 300-400mm from the end. Make it stand out 50-75mm and grout it on with concrete.
  • Let this set before continuing to build the dam wall again.
  • Close the pipe off with a wooden or concrete disk.
  • Put the plug on the inside of the dam so it can be pushed out with a length of timber.
  • The plug can be replaced after the dam is empty.
Emptying by siphoning
  • Use 100mm yellow plastic drain coil or anything that is flexible and water tight.
  • Immerse it totally in the water so there is no air in the pipe.
  • Then quickly drag an end over the side of the dam and hold it below the water level in the dam.
  • You may have to have a few attempts.
Dams with rock foundations
 

If you are building a dam across a stream where the sides and floor are rock, then you’ll need to use concrete and this could easily grow to a large project. You would be very wise to seek engineering advice. A basic need here is to drill holes into the sides and base and grout in steel rods with cement for reinforcement.

It’s usual to build a flume to take water through the dam while it’s being built. Seal around this with clay to keep the water away from the working area. If it’s just a small flow, use a spun concrete pipe, which in future can be used as an outlet for emptying the dam. Use plenty of reinforcing in the dam - this is critical. And if it’s going to hold a lot of water, bend it inwards to take the pressure like the many of the hydro dams are.

Eels - beware
 

If part of a concrete dam is on soil foundation - beware as eels will burrow along the concrete wall and around the end. This happened to a farm dam in Clevedon on an all-clay site where eels dug around the end of a concrete dam and let it go.

However, they will stop burrowing when they meet a turn back upstream or downstream. So on the foundation at the end of the dam wall, put a right angle lip (at least 300mm long) on both the upstream and downstream side. This will fool them!

Seek advice
  • The people who excavate dams have a lot of experience - use them.
  • Every dam is different.
  • There are no short cuts to a successful dam.
  • Get it right first time rather than try to fix problems later.
Mr Gilberd is a retired engineer and well driller whose family have run a drilling and engineering business in the Auckland province for four generations. He took part of the business over from his father, and as well as servicing the farming industry, he worked on major engineering projects like the Waikato hydro dams and the Auckland harbour bridge. He has also done a lot of volunteer pioneering work in the Pacific islands helping people to find water and other resources.
 
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