The 2005 hurricane season raised awareness of flood risk across south Louisiana. They also prompted a FEMA process of reviewing flood risk assessment and revising flood insurance rate maps. This, in turn, has resulted in higher anticipated flood levels and therefore the need for many people to build their homes higher than natural grade, either voluntarily or as required by flood damage prevention ordinances.
In many cases, property owners are choosing to achieve the desired elevation by creating a raised building pad, borrowing dirt from part of their property. As discussed later in this article, this method may or may not be a good way to elevate the home, but it is legal in most cases. The widespread adoption of moving earth on site for home construction is resulting in many properties being left with unsafe and unsightly borrows pits.
If you’d rather have a pond than an unsightly, unsafe pit, there are several things you can do in the digging process to achieve this result. First, though, you’ll need to be sure the ground will hold water. Soil with 20 percent or higher clay content holds water well. Most soils in south Louisiana have that much clay content, but they may contain sand lenses, which will allow water to pass through easily. Sandy areas will be discovered during the digging process; they should be dug out and filled over with clay. Another potential problem is tree roots. Dig them out and pack extra clay in the holes left by the roots.
Size, depth and side slope are important
When borrowing dirt for fill under a home, many people try to save real estate by excavating the maximum amount of soil from the smallest possible area. The resulting pits are small, very deep and have very steep sides. These pits are both unsafe and difficult to maintain. Follow these guidelines for safety and a pond with some beneficial use.
Side slopes: If livestock or a child or pet falls into a pond, they must be able to climb out easily. This means that vertical pond walls are not acceptable – not even in a few places. If you do not have room on your lot to slope the pond sides at a 3-to-1 slope (at the steepest), then you should not dig a pond or pit.
Size: When it comes to fish ponds, bigger is better. A one-acre pond will need far less intensive management that a one-tenth acre pond. Larger ponds can approach the level of self-sustaining ecosystem – like a small lake. Very small ponds can provide an attractive landscape feature and a bit of fishing, but they require careful management – they’re more like an aquarium than a lake.
Depth: The ideal depth for fish ponds is about five feet, or a little more. In most cases, areas over seven feet deep are little used by fish. Very deep areas can become totally devoid of dissolved oxygen. If a storm causes the oxygen-deficient water to mix with the surface layer, the rapid destratification can cause a fish kill. Too much shallow water can also be a problem. Large areas of water less than two feet deep are almost impossible to keep from becoming clogged with aquatic weeds.
Add water control features
In some ponds runoff control is needed. If rainwater moves across your property where the pond is located, you have two options: add levees around the pond or add water controls within the pond. If levees are added to keep the runoff out of the pond (or if the pond is situated in a dry site), then a source of fill water is needed. Without any method to add water, many ponds will get very low during dry weather, with reduces its capacity to keep fish alive. Most people choose to install a well or run a line from an existing well. Remember that well water has no dissolved oxygen content: adding lots of well water without aeration is a good way to kill your fish! A simple series of splash screens will solve this problem.
In-pond water controls include drainpipes and spillways. While watershed ponds in hilly areas need to have both, many ponds on flat terrain will need neither. Drainpipes are not an option for borrow ponds that are deeper than adjoining ditches. Ponds that catch runoff will need a spillway at the lower end of the property to divert overflow without causing erosion.
Add water quality features
If you have or need an area of the pond to be deeper than seven feet, aerating the deeper areas will solve most of the basic water-quality problems. Pumped-air diffusion or turbulent mixing, or both, can be used. Aeration will prevent stratification, improve nutrient cycling and prevent problems from periodic low dissolved oxygen levels. Fountain-type aerators don’t generate the most efficient mixing, but in small ponds they provide an attractive feature and adequate aeration.
Remember other potential pond applications
Although most people think of ponds in a fishing or landscape context, ponds may also serve as part of the heat-exchanging mechanism for water-source heat pumps, which are highly energy efficient.
Mounds of fill dirt diminish flood storage capacity
While raised homes in FEMA’s designated V-zones cannot be supported on fill material (ordinance requirements in V-zones), the majority of homes across south Louisiana are in A-zones, where the home can be supported on any type of elevated foundation – columns, piers, foundation walls, or mounds of compacted fill.
If a house is being built on fill in a special flood hazard area (the flood zone), the fill brought onto the site takes up space that formerly was available for storing flood water. Over time, continued filling in the flood plain will result in flood levels being higher for everyone. While placing fill in the floodplain is legal in most communities, if must be “permitted” and is often regulated or restricted. The property owner may be allowed to place fill material on the building site, but required to make an equal amount of room elsewhere for storage of floodwater. This is called compensatory storage. The pit would serve as compensatory storage if it wasn’t full of water and the volume of the pond above the natural water surface elevation may be counted toward the compensatory storage requirement.
Dirt that’s good for ponds may not be good for the building pad
Remember that for good pond-making – that is, if you want the pond to hold water – the soil should have a clay content of 20% or greater. For building, less clay is bette, especially if it is organic clayr. If a soil test shows more than 12 percent clay, the clay will be analyzed for its behavior when wet. This is because clay can turn to liquid, reduce the soil's bearing strength, and cause the soil to exert pressure on the foundation. A soil that seems to have a lot of gravel or sand in it could still contain 20 to 30 percent clay. If it does, it's going to act like clay, which can give your project poor drainage and plenty of problems. See the related article "What will your soil support" and the AgCenter publication "Building on Louisiana Soils" for more information on soils requirements for residential buildings.
Find out about your soil using the USDA Web Soil Service
The USDA has developed a very sophisticated Internet mapping tool that can be used to determine the composition of soil in an area of interest, and learn something about what uses that soil will support. Web Soil Survey (WSS) provides soil data and information produced by the National Cooperative Soil Survey, which is operated by the USDA Natural Resources Conservation Service (NRCS) and provides access to the largest natural resource information system in the world. Soil surveys can be used for general farm, local, and wider area planning. Onsite investigation is needed in some cases, such as soil quality assessments and certain conservation and engineering applications.
The Web Soil Survey can be accessed at http://websoilsurvey.nrcs.usda.gov
Audio clip: What's wrong with elevating on fill in the floodplain?
Pond information taken from a fact sheet by Glenn Thomas, Louisiana Sea Grant
For more information on making and maintaining ponds, go to the
LSU aquaculture website:
http://www.lsuagcenter.com/en/crops_livestock/aquaculture/
or the LSU Sea Grant aquaculture site:
http://www.seagrantfish.lsu.edu/aquaculture/index.html
or request publication #2573: Management of Recreational and Farm Ponds in Louisiana from your parish AgCenter office.