It is important that the material in the dam be tied to the soil in the foundation. A cutoff trench should run lengthwise along the dam. This cutoff or core trench should be backfilled with clay-type material compacted in layers. Failure to install a core trench may result in seepage and even loss of the dam.
The dam should be constructed of impervious moist material that is compacted in continuous horizontal layers as it is installed. Dams pushed up with a dozer and not compacted have a greater chance of failure.
The dam should be constructed with slopes that will not slump or slide. The steepness of the slope on the pond side of the dam should not exceed 3 feet horizontal to 1 foot vertical; and the steepness of the slope on the downstream side should not be greater than 2 1/2 feet horizontal to 1 foot vertical.
The recommended top width is 10 feet for a dam less than 20 feet in height. This width should be increased an extra 2 feet for each 5 feet of dam height over 20 feet.
All dams should have extra height or freeboard to prevent floodwater and waves from topping the dam. The minimum elevation of the top of a dam should be 1 foot higher than peak flow of water in the emergency spillway or 3 feet higher than the non-flowing elevation of the emergency spillway.