If you have observed the power of flooding water rushing over a spillway, you likely related the memorable experience numerous times. Results can be damaging regardless of lake size. Spillways must be professionally designed and maintained to prevent serious consequences.
Our colleague, Mike Otto, has seen countless examples of these events during 40 plus years building ponds. In his book, Just Add Water, Mike explains most lakes have a primary spillway or drain pipe. If the pipe cannot drain all in-coming water, the lake will fill until reaching the emergency spillway. The emergency system usually is at the end of the dam, designed to release large volumes of water, and prevent lake levels from flowing over the top of the dam. As Mike says, “When these inevitable rains come, you better be prepared. It’s like a third string quarterback on the football team. He doesn’t play much, but when he does, he better be prepared to be the hero”.
Emergency spillways can be earthen, concrete, or rock structures. Carefully analyze the watershed and engineer them for specific situations–like the biggest rain you could ever get. If water goes over the dam, you’ll likely be constructing a new dam. Dirt spillways are cheapest to build, but experience maintenance issues. Expect erosion and occasional repairs. Keep them well vegetated. Mechanical types are made with concrete or rock and needed in urban areas. They may not be built at the end of the dam if water needs to be directed away from a valuable section of the property.
Spillways should release water far enough from the dam to keep swirling water from eroding the back side of the dam. Small terraces can be used to prevent this occurrence. Spillway size obviously depends on the drainage basin. Calculations can be made using topo maps and average annual rainfall data. Mike recommends memorizing this number: one-acre, one-inch deep is 27,000-gallons of water. By determining pond surface acreage and how much acreage naturally flows into the pond, you can estimate how much water you must manage with a spillway. He adds, you may be stunned by the number, but better now than after the dam washes away. Engineers typically design for a 100-year flood event. Learn the lay of land upstream. Conditions may bring slow movement to your site. On the other hand, a steep grade covered with rock, concrete, or rooftops will not soak into the ground and drain straight to you. Plan as if your pond is full consistently. Evaluate if spillway flow will cause damage as water rushes back to the creek. Adjacent photos illustrate potential effects. Again, keep dirt spillways vegetated to prevent erosion.
Mike reminds, “Water lines have some latitude, but there are always a few rules to remember when planning spillways. Some are common sense and some the sheriff may want to talk to you about, especially if you don’t follow that ‘common sense’ concept”. He emphasizes the following rules when making decisions about emergency spillways:
1) Water cannot cross land belonging to someone else. Direct spillways back into the creek, wash, or draw you plan to dam. All water should be back in the original creek channel before crossing the fence to the property downstream.
2) You must not build a lake or pond that moves water down a creek into your lake, then move it to another creek when it leaves your property.
3) Don’t flood the county road. Roads upstream must be water-free.
4) Your house must be built much higher than the water. It doesn’t matter how good your lake is or how well the dam is constructed. Without a good spillway, all could be lost.
Mike says he started digging in his backyard with a bucket and spoon and is still digging. By his teens, he was harvesting wheat from Texas to Canada. It wasn’t long before he traded wheat combines for bulldozers and learned how to build golf courses across the U.S. In 1979, he became a proud owner of his first dozer and started moving dirt coast to coast. His professional focus grew when meeting Pond Boss Bob Lusk in 1992. Since then, Mike has devoted his career to constructing recreational lakes. His book can be ordered at www.pondboss.com.