By Norman A. Latona, Jr.
Southeastern Pond Management

In a previous article, we discussed issues related to determining site suitability for the construction of a hillside or watershed pond. Included in this discussion there topics such as ground topography, water supply and soil suitability. As we pointed out, a qualified design engineer will be valuable in this process.

Although construction of new ponds take place in virtually every month of the year, fewer delays associated with weather may be experienced in the drier months. Commencing a construction project in May or June.should allow adequate time to complete the weather dependent phase of construction with the fewest interruptions. Keep in mind, fish may be introduced starting in October/November; the pond should be roughly 10% full prior to the introduction of fish (this will be discussed in detail in a later article).

The crucial phase of pond construction involves several steps including site preparation, i.e., clearing the construction zone of trees and debris, shaping and deepening of the edges and building the dam. The construction of the darn will be the topic of this discussion.

Earthen pond darns represent a most critical detail to successful pond construction. Although the technical specifications for the construction of pond darns vary, basic procedure remains essentially the same. For our purposes, the construction of the dam will be broken into the following components: dam design/placement, dam construction, spillway installation and finish grading.

Dam Design/Placement: The general location and design of the earthen dam will be determined by land topography, pond size and watershed area. However, other specifics of the dam, such as the width at the top, the back side slope and the height above the water (called, freeboard) are also important. These are issues that should be decided prior to construction. For example, depending on vehicle traffic on the dam it may be useful to add several feet to the top width. In addition, maintaining the back of the dam with a tractor and mower is made much easier by constructing a gentle slope (up to 5:1) in lieu of the steeper slopes commonly constructed in an effort to save cost.

Dam Construction: Proper construction of an earthen dam involves the installation of a clay core. The portion of the dam visible to the eye represents only part of the dam. The most critical part of a dam is located below the surface and is called the core.

The dam core ideally consists of impermeable soil with a high clay content. Proper installation of the dam core is essential to creating a watertight barrier, preventing seepage which might lead to serious problems in the future. Typically, a core trench is excavated beneath the entire area which will ultimately support the pond dam. The depth of the core trench vanes according to site soil conditions. The core trench is backfilled with high quality clay material. This material is tightly compacted in order to insure its ability to resist water penetration. Once the trench is completely filled, additional clay material is compacted to form the interior of the 'above grounds portion of the dam. Generally, the "above ground" portion of the dam core extends at least as high as the anticipated full pool water level; above this point, as well as on the back side of the dam, lesser material may be used.

Ideally, material suitable for the construction of the dam core is available on-site; the cost of importing clay to your construction site may be quite high. Often, clay availability can be determined prior to construction through the use of a soil auger or other earth excavating equipment.

Spillway Installation; Spillways are designed to channel excess water safely away Tom a pond during "high waters conditions. The number, size and type of spillways required for a pond depend largely on the ratio of watershed area to pond surface area. Typically, the greater the ratio of watershed to surface area, the larger the spillway requirement.

Specifically, most modern ponds are designed with two types of spillways, mechanical and emergency. Mechanical spillways are designed to remove "normal" flows of water such as those that might be experienced during a 1- or 2-inch rain. Traditional mechanical spillways include stand pipes located within the pond basin, often topped with trash racks, as well as siphon system arrangements which are designed to pull water from well below the surface when the water level exceeds a set elevation. Emergency spillways are designed to remove 'flood waters" from a pond basin. In cases where the inflow of water exceeds the capacity of the mechanical spillway, emergency spillways protect the dam from becoming overrun with heavy water flows. As such, emergency spillways are typically set at a higher elevation than mechanical spillways; water exits the mechanical spillway to its limit, then begins to flow across the emergency spillway. Often, emergency spillways are lined with rock, concrete or some other material which prevents soil erosion.

Finish Grading: The degree to which you choose to "finish" the dam depends primarily on use We touched on the subject of vehicle traffic in terms of dam width. Vehicle traffic may also necessitate the construction of a substantial roadway across the top of the dam. Otherwise, a pond dam should be finished with diligence. Due to the nature and shape of most pond dams, soil erosion should be considered a significant threat to the integrity of the structure. Great care should be given to fine grading, seeding and mulching all areas of exposed soil within the dam. Topsoil may be added to the surface on the top and back side of the dam to facilitate the growth of newly introduced grass seed. In some cases, erosion flats or other erosion-control measures may be required. In all cases, stabilizing the soil on and around your dam is an effort that should be accomplished immediately. Call Norman at (205) 664-5596

Next issue: Part 3 or our 4-part series on pond construction will look at making your pond fish and fisherman friendly. Questions?