Mechanical Spillways and Improving Pond Habitat
A Master Class
A well-constructed pond will be enjoyed for generations.
This is the third and final segment of the pond building series. This session will cover installing spillways and how to use natural physics to manage the pond’s water habitat.
For the sake of discussion, we will be using PVC pipe for the spillways. Also, these basic concepts are to be included in the installation of the various spillway designs.
1. All pipe going through the dam or pond band need at least three anti-seep collars.
2. All pipes and anti-seep collars need to be installed with small 6” layers of unbroken, concentric and compacted clay.
3. All PVC pipe needs to be heavy grade pipes with an approved rating of approximately 160 PSI and glued connections.
4. All pipe ditches need to be dug with V-shapes of about 60°, to allow the pipe and anti-seep collars to be installed and to be compacted back into the dam or pond bank.
5. All pipes leaving the pond need anti-swirl structures and trash guards. (The trash guard should restrict fish and needs cleaned periodically.)
We will be covering three different pipe spillway designs.
Design A – The Shotgun Design.
The PVC pipe is installed near the top of the dam at the permanent pool level and slightly slopes through the dam to allow water to flow on the back side of the dam.
Pros –
1. Less labor required to install
2. Lower cost due to less labor and materials
3. Easier to replace or repair
4. Water pressure lower as it moves through the spillway pipe.
Cons –
1. Requires the pipe to extend down beyond the back side of the dam
2. Due to the slight slope of the pipe through the dam, water moves slower and tends to raise the pond water level faster during a storm.
3. After the pipe exits the back side of the dam, additional pipe is needed to slope steeply down the back side, thus the water gains speed and force, causing a back-swirl action as the water leaves the pipe, causing an undercutting at the dam base.
4. Serious erosion problems at the outlet can occur and erode the base of the dam if the pipe isn’t installed far enough down-stream. (See below)
5. Tends to need repaired or replaced more often.
This is a 'Shotgun Spillway' with poor design. The water coming through the spillway is eroding the dam and will eventually lead to dam failure. Additional pipe was needed to move the water past the dam. No anti-seep collars were installed so there is additional erosion around the pipe.
This is another shotgun spillway. Additional materials have been placed below the spillway to delay, but not eliminate, erosion of the dam. Plus requires extra time mowing.
Design B – Sloping Diagonal Spillway
This design allows the PVC spillway pipe to be installed at the shoreline or to extend out into the pond at the permanent pool level. But the pipe is installed at diagonal angle through the dam, exiting beyond the base of the dam.
Pros –
1. Due to the diagonal design, the water flows faster, carrying away more water.
2. This design helps to reduce pond overflow.
3. The sloping diagonal design transfers water down-stream from the base of the dam.
Cons –
1. It costs more to cut the V into the dam to install the pipe and collars. More compaction is also needed. Time x Labor x Equipment = $$$
2. More pipe is required, costing more.
3. High water pressure and speed are moving through the pipe, causing more cutting water action on the outside of the spillway pipe. Therefore, the collars even more necessary.
4. Additional anti-seep collars might be needed depending upon the pond’s surface area.
5. More expensive to repair, but generally has a longer life time.
This diagram illustrates the Sloping Diagonal Spillway. Be sure to extend the pipe beyond the dam to avoid erosion.
Design C – Tower spillway with bottom drain
This design is installed during the initial construction of the dam and the dam key at the deepest point in the pond.
Pros –
1. The pipe allows water to exit the pond during construction.
2. The tower spillway standpipe can be installed after the pond construction is completed.
3. If a gate valve is installed on the intake side of the spillway pipe, the owner will have the ability to lower or drain the pond. This feature is especially useful for fish farmers.
4. Drawing down the pool level in the winter can be used to control aquatic weeds.
5. The tower spillway can be adapted to increase the oxygen level in the pond. More on this later.
Cons –
1. Large initial cost for pipe, material, and labor.
2. Care must be taken during construction to prevent damage to the pipe.
3. Since there are more pipes and joints, there is a greater possibility of pipes being damaged or disconnected.
4. The bottom flow pipe is considerably longer, increasing costs.
5. The pond needs to be drained to make repairs and the dam may also need to be excavated which can be expensive.
6. Bottom gate valves may leak or be corroded shut.
7. Brush guards and screens will be needed on the intake side of the gate valve. Note: we have a dry hydrant on our lake, however no predator or fish screen was installed during construction. When the fire department has conducted drills, the pumper truck has pulled fish through the pipe to the water truck. “It’s a great way to get a catfish!”
8. Since the pipe is on the bottom of the pond, downward water pressure will be greater, therefore anti-seep collars are very important.
After the pond is constructed, the tower stand pipe is installed. If a permanent inside gate valve is to be used, a ‘T’ connection is made when the tower stand pipe is installed. Please pour a cement pad 4” thick to rest the connection and install a long steel post through the concrete, along the tower standpipe. If a gate valve is used place the steel post between the standpipe and gate valve.
If a gate valve is to be used, the PVC pipe can be reduced to a 4” pipe before the gate valve. Then connect a long rod from the top of the gate valve along the steel pipe to above the pond’s surface level. This rod will allow the pond to drain from the surface by turning the top gate valve handle. The rod only needs to be attached to the steel post once near the bottom and near the top for support, same as the tower standpipe.
A Tower Spillway is complex but gives you a clean pond with excellent fish production and weed control.
This expanded cross-section shows the thermal layers and water flow of the tower spillway.
This design also increases the oxygenation of your pond.
Using some basic physics, my dad discovered how to increase the oxygen level of the pond by increasing the volume area of the top 2 thermal layers. Simply by placing a larger diameter PVC pipe over the tower standpipe and keeping the larger pipe 2 feet above the bottom of the pond, the pond runoff will draw from the bottom thermal layer, which is the lowest level of O2 PPM. Therefore you don’t need electricity to run aeriation pumps.
The concept of the tower standpipe is the same except the standpipe isn’t attached to the steel post. If the standpipe is 6” in diameter a 10” – 12” cover pipe is placed over top of the tower standpipe. The larger pipe should be 2 – 4” above the tower. This can be achieved by drilling 4 holes 2– 4” from the top end at a 90° angle to each other. Install some rods through the holes to form an ‘X’ to rest on top of the tower stand pipe. (See diagram) The top cover pipe should be 2 feet shorter than the length of the tower standpipe. The cover pipe is attached to the steel post to anchor the spillway pipe.[Attaching the cover pipe to the steel support pole keeps the tower spillway pipe from falling over.]
If you plan on drawing the water down for Winter for aquatic weed control, the pond depth should be 12’ deep, since a draw down is 4 feet. In Ohio, ponds need to be at least 8 feet deep to prevent winter protection for the fish
Remember an anti-swirl device should be on the tower standpipe and the brush guard needs to be attached to the top of the cover pipe. The top pipe is open for heavy rains to allow flow through.
By increasing the top two thermal layers with higher O2 PPM levels, you can increase the amount of fish from 300 pounds per acre to 400 – 500 pounds/acre. However, realize you need to be concerned about depleting O2 levels during drought condition with no flow-thru water and snow-covered ice. You may have to provide temporary aeration.
Some contractors (there is one in Holmes County, Ohio) has a variation of this design. Remember simple is sometimes the best.
The spillway design you choose will be based upon your objectives and your pocketbook. However, please think long-term when making your decision.
This is the result of a poorly constructed pond. Poor construction. Poor site selection. Poor watershed. Poor dam construction. It was a waste of time and money.
Planning your pond, careful selection of site, evaluation of the watershed, and constructing the correct dam/spillway will give you a pond that will last for many, many years.
Enjoy!
You can contact me with questions or comments at: ragivens@gmail.com
Bob
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