Pond Planning Part 2 (Laying out the Pond Site)
There is a lot to consider when planning for your pond installation. It is a good idea to thoroughly plan you pond installation prior to the machines arriving. I personally like to measure, get the levels, and paint the site to see for myself what the pond will look like. Often times a design on paper doesn’t survive this process. You will often find that the pond will be different in real life than you were thinking it would be on paper.
I had planned on (3) pond sites based on my design, however after surveying with my laser level, I have decided that (2) of them are not worth doing. In permaculture, there are only (4) pond types we should consider, and topography will determine their viability.
A saddle dam is very rare, but there are some sites where this would be viable. This works where you have (2) hill tops relatively close to each other so it creates almost a valley. A dam wall is installed at either end of the valley, and swales provide the catchment. I certainly do not have a saddle dam site.
A ridge dam or pond is installed on the flat area of a ridge before the slope drops off. These are fed with swales, and can be great for irrigation. I don’t have any ridge dam sites.
A valley dam is the most common type of pond that we install. It is typically the cheapest type of pond. A valley dam only requires the dam wall on one side, making it very economical. Unfortunately, no valley dams on my site either.
A contour dam can be installed on areas with less than an 8% slope. These ponds are set on contour, and because they are relatively flat, there are no large walls to install. I have one site on my property that I can install a contour dam or pond, and even it is not ideal. However, I think it is really important to have open water on my design site, that I am willing to spend extra time and money to develop the site. Open water is going to dramatically increase the diversity of my site!
For a surface runoff pond, ideally you have at least 15 times the catchment as the size of the pond. For example, if you had 15 acres of land that would eventually drain into your pond, you could theoretically have a 1 acre pond. Now this is a very crude way of looking at it, as you have to take rainfall and shade as well. For example, if you have a shady pond site and you receive 70 inches of rain a year, you may not need as much catchment, but if you are in a dry environment without shade, your pond would dry up with that catchment.
I have a 1700 square foot pond planned, with about 90,000 square feet of catchment. I have about 53 times the size of my pond in catchment. I have trees surrounding three sides, but they are still small on the north and south side, so it will take a while to get much shade. I do plan on planting some bamboo on the dam wall, which is on the south side, so this will shade the pond rather quickly. I will be installing drain pipe along the back of my house, where water collects, and moving this water around the corner, before emptying out into a dry river bed of stones, and then into the pond. This will help to add oxygen to the water before it enters the pond which will be important for the fish. I also have (2) swales that will feed the pond. It is really important that my pond does not dry out, because I am using sodium bentonite to seal it. Sodium bentonite will expand 15X when wet, but it will also shrink when dry, so if my pond dries up, it will leak.
After I figured out where my catchment was coming from, I painted out where the drain pipe would go. Then I found the contour line with my laser level to establish the pond edge of my dam wall. I then figured out how high would be practical for my dam wall. This height determines my shore line, by simply finding that level up slope. This upslope contour line ends up being my shore line. My pond site is a bit complicated in that it is a contour dam, but it has half of a valley dam. The west side of my pond site has the same level as my shore line, so that contour line just had to be extended around. This expands my pond without having to build a dam wall on the west side.
My dam wall is on the south side and the east side, while the north and west side is the shore line. It is important that the top of the dam wall is at least 6 feet wide for stability. Also, the slope on the interior of the dam wall must be no steeper than 3 to 1, and outside of the dam wall should be no steeper than 2 to 1. The key way which is a compacted clay core, for me it will be my soil mixed with sodium bentonite, should be at least 3 feet wide, and it should go below the deepest part of your dam wall to prevent under the surface erosion. This is extremely important. A keyway must always be installed on a dam wall.
It is very important to plan overflows and spillways. Without these, you will have water spilling over your dam wall causing erosion, and eventually failure of the dam. I am planning a standup pipe that will be a double sleeve overflow pipe. This will keep the water level constant, but it will also pull stale water from the bottom of the pond. This will help to oxygenate the pond. I will also have an emergency spillway in the event of a large storm. The emergency spillway will exit on the east side of the pond near the shore line if the pond gets 6 inches over the freeboard. It is important for a pond that you have at least 1 foot of freeboard, which is the wall space above the water level. Larger ponds may need more freeboard.
Once your pond is complete, it is important to vegetate your dam walls quickly to prevent erosion. I am planning to plant clover and bamboo. I recently spoke with an engineer that specializes in ponds, and he was not too keen on bamboo. He thought that might be a problem. Geoff Lawton on the other hand uses bamboo often. The engineer said grass was proper. I’m not sure if he realized that bamboo is a grass, and it is shallow rooted. The roots are very fibrous, and it actually stabilizes dam walls.