This is the 2nd of the 3-unit series on constructing a pond. I will be covering the basic elements of constructing a safe and long-term pond.
First, please go online to Shawnee County Conservation District in Kansas. Download the article How to Properly Build a Pond. www.scc.district.com This site provides a great overview of the basic requirements needed to correctly construct a dam. I will be giving additional information and/or reasons for each of the elements.
I have reviewed many of the pond construction sites and YouTube videos. In most of these, the basic elements are mentioned but their importance is not explained. You need to know if the pond constructor understands each of these elements or is he skipping some elements and trying to add other, expensive, construction items.
This is another reason why I recommend you work with your local Soil and Water District, whose employees are trained in proper pond construction. The SWD staff will supervise and make sure each element is correctly installed.
This is an diversion source, bringing additional water to the pond. The pond was constructed to contain water running off a large hillside which would not naturally run into the pond area.
1. Clean away vegetation – leaving vegetation causes a number of problems. The soil will not compact when mixed with vegetation and, if the vegetation is buried in the dam, as the vegetation decays it provides areas for water to deposit, causing the dam or pond bottom to leak like a sieve. Leaving vegetation can also cause water quality problems.
This organically rich soil should never be used to retain water in the dam or on the pond bottom. Use it on the 'dry side' of the dam.
2. Stockpile topsoil – please save all topsoil and store it away from the dam construction area. Several well-placed topsoil piles can reduce the amount of finish grading needing done. Remember that topsoil doesn’t make good compaction material in the dam and should be reserved to be placed as ‘icing on the cake’ to reestablish the vegetation around the pond site.
3. Core trench – read the online description about the core trench. Failure to construct a core trench is a major cause of pond leaks and washouts. The key principles of constructing the core trench is to:
a. dig the trench at least 3 feet below the impenetrable clay layer;
b. dig it back into the banks until it connects with the clay layer, creating a continuous impenetrable layer in the dam;
c. build the core trench by compacting 3 – 6” layers of impenetrable clay, continuously layering until reaching the top of the dam. This is known as the ‘KEY’ of the dam. Use a dozer, preferably with a sheep foot roller;
d. the core trench made of compacted clay layers will be at least 6 ft wide and should be wider for large ponds or lakes;
e. as stated in (c), the core key layer should be constructed to the top of the dam and then covered with top soil.
This is a sheep foot roller used to compact soil tightly on the surface.
4. Compacted fill – as the core key goes upwards both sides of the key need to have compacted soil added. The pond side is known as the wet side and the backside is called the dry side. The wet side should be compacted with 6” of clay material whereas the dry side can be compacted with non-clay soil. The finished slope should be 3:1. Too steep of a slope (2:1 or 1:1) will eventually erode to a 3:1 or a 4:1. (Note: a 3:1 slope means if a small child falls into the pond, the water will only be 3 feet deep when 9 ft from shore and 4 feet deep when 12 feet from shore. This is a good safety feature. Additionally, weed control is better to manage with a 3:1 slope.)
This is greater than a 3:1 slope. You are able to drive up and down it (with difficulty if it is muddy or snowy) but you will not be able to mow the grass easily.
5. Principle spillway – I agree with the SCC information linked at the beginning of this article and will be adding additional information on spillways in Part 3 of Pond Management. (Note: In 1972, Marlington High School’s VoAg land laboratory in Alliance, OH was the first time PVC pipe described by the SCC article was authorized to be used as a trail spillway system. The life expectancy was estimated to be 20 years, however, nearly 50 years later, the spillways are still working well.)
Here is an emergency spillway from 'across the pond'. Notice it is above the normal water level but below the top of the dam.
6. Rubber gasket joint – straight male/female joints were used in 1972. When backfilling the spillway trench, one joint was knocked loose and had to be dug up and repaired. Using gasket joints is better. The initial cost is higher but it is cheaper than making repairs later.
7. Anti-seep collars – the two most important elements of the dam are the core key and the anti-seep collars. All pipes running out of the pond, either through the dam or through the undisturbed pond banks, need anti-seep collars. The purpose of the collars is to counteract the magnetic charge created on the outside of the pipe as water moves through the pipe. This charge pulls water along the outside of the pipe, creating weakness in the dam. Each outlet pipe should have a minimum of 3 anti-seep collars installed. One collar needs to be installed on the pipe before it enters the core key of the dam, eliminating washouts around the pipe. Don’t waste your time installing collars on the dry side of the dam.
The collars should extend about 18” on all sides of the pipe and be at a 90° angle to the pipe. The 90° angle stops and backs up the water towards the wet side of the dam. If the water pressure somehow builds up and allows water to flow over the collar, its magnetic pull is reduced and shouldn’t have enough flow to bypass the 2nd and 3rd collars. (If you are still a non-believer in the use of anti-seep collars, pipes don’t wash out when first installed, but rather a few years down the road. Pouring concrete around the pipe on the wet side of the dam is only a bandaid. Water pressure builds and works its way around the concrete, following the pipe through the dam. It’s not IF the blow-out will happen but WHEN it will happen.)
A good anti-seep collar can be constructed with PVC pipe of various diameters and rubber mats, even livestock mats. Cut a circular hole in the center of the mat, grease the pipe and pull the collar over the pipe. Use something like Flexseal® to fill the gap between the pipe and mat. When installing the pipe through the core key, dig down below the pipe so the mat is at 90° completely around the pipe. Then pack the clay around the pipe and the collar. A power-tamper is helpful. Do this to all pipe/collar installations. One of my 10th grade students came up the with idea of using a semi-tire rubber mud flap. I have also used rubber mining/conveyor belting. Concrete poured collars have a tendency to leak due to temperature changed.
This easy to make anti-seep collar was constructed with 4" PVC pipe and mine belting (32"x32"). It is sealed with FlexSeal ®. The collars are also available for purchase online.
8. Canopy inlet – this canopy eliminates draining water from whirlpooling which can reduce water flow up to 50%. Even a flat board or a metal plate bolted to the top of the pipe will reduce whirlpooling.
9. Trash rack - this is always a good item to install to prevent things going down the spillway pipe and getting stuck, clogging the pipe. Remember the trash rack will need cleaned out after storms.
10. Permanent pool – this is the normal pond level. In Ohio, ponds need to be 8 ft deep to keep from freezing out. Extra deep ponds hold more water but this doesn’t increase fish populations. If the pond is to drawn down 3 or 4 ft for winter weed control, the pond needs to be 12 feet deep to sustain the fish. (12ft – 4ft draw down = 8 ft remaining to avoid freeze out.)
11. Emergency spillway – ‘ES’. The ES needs to be constructed on virgin soil – never over the dam where soil has been moved. The ES should be constructed at the halfway point between the permanent pool level and the top of the dam. The ES overflow outlet needs to be directed far enough downstream that the water doesn’t wash out the backside of the dam. Also, make sure the ES flows out onto your property and not your neighbors.
The spillway is lined over virgin soil (not disturbed during construction of the pond) and stone. It empties into a 3 acre area covered with vegetation.
12. Detention pool/Free-board level – this is the distance between the normal pool level and the level of the ES. Try to maintain at least 3 feet between the normal pool level and the top of the dam. If a one-acre surface pond raises 18” during a heavy storm, the pond can hold 500,000 gallons of water before going out the ES and another 500,000 before washing over the dam. That is a lot of water.
13. Clay bottom – clay needs to line the entire bottom of the pond. It is much more cost effective to do this during pond construction than coming back and fixing the leaks later. If you want a beach, sand can be added in those areas before the pond fills.
14. Vegetation – planting grass and covering the sloping pond banks with straw as soon as possible when construction is completed is important to prevent soil erosion. Kentucky 31 fescue is the standard grass seed used in eastern US. However, if you plan on pasturing animals around the pond, use a non-fescue mix. Don’t plant trees and shrubs on the dam because the roots will grow through the dam towards the water, causing leaks.
15. Grazing livestock – livestock grazing around the pond will break down the pond bank causing the banks to erode. Water tanks are ideal for quality water for livestock. If you need the pond for livestock water, you can construct an all-season pad along the pond edge by digging an area 2 feet below the normal pool level, pour 4” of concrete, extending about 6 feet out into the pond. Fence the concrete pad out into the pond and along the pond where the livestock will be grazing. This will provide clean, non-muddy water.
16. Wetlands – all wetlands are the governmental sacred cows – don’t mess with them. Some environmentalist will turn you into the local authorities if a wetland is disturbed during pond construction, you could be fined and even receive jail time. This is another reason to work with your local soil and water district. The cost and time for a governmental impact study is out of sight and the agents know your area well, helping you avoid government pitfalls.
This wetland area is below the pond and to the side of the emergency spillway. Wetlands are determined by soil type, type of plants growing, and water levels.
Please Note: A wetland as defined by the federal government is a separate ecosystem until itself compared to a pond or lake habitat.
Our next segment will cover spillways and some inventive ways to use nature to improve the pond’s habitat.
Have a safe and fun summer!
Bob
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