With hurricane season approaching (and the 10th anniversary of Hurricane Irene just past), thoughts of floods and heavy rain management naturally occur to pond owners. Big runoff also involves high spring water and snowmelt too, of course. Spillway checkup time folks.
Discharging high pond water requires a properly functioning spillway, so let’s review the basic types of pond spillway and their pros and cons especially regarding floodwaters.
Most ponds benefit from having a continuous exchange of fresh water, that is, water flowing in, and out, via some type of spillway. This helps maintain the designed water level, keeps water cool and oxygen rich, creates circulation, and perhaps even flushes out leaves and floating debris. But no flooding please. Good properly sized and erosion-resistant spillways prevent flooding.
1) Earthen Spillways
This is the simplest discharge system, and usually means creating a stream-like channel at the designed pond water level so water leaves via an erosion-proof channel. Discharge at this level means pond water will rise no higher than the spillway, as long as the spillway is built big enough to handle maximum flood volumes. Sometimes a second spillway is built at a slightly higher level to discharge extreme flood runoff. Earthen spillways can be built using natural stone to create a natural stream look. Waterfalls can be built into the stream for landscape effect, and sometimes if useful a bridge will be installed over the stream for foot or vehicle traffic. Siting the spillway at the opposite end of the pond from inflow helps create flow/circulation/oxygen distribution across the pond, and can help flush out leaves and floating debris. The spillway needs periodic inspection to make sure it is not clogged with debris, vegetative growth, or dammed by beavers. In case of a dammed pond, make sure the spillway does not discharge down the back slope of the dam and cause erosion. The spillway should be big enough to handle flood water. You may be able to get runoff figures to help size the spillway from your county Natural Resources Conservation Service or state university extension service, or an engineering firm. By the way, earthen spillways are less expensive to build than the piped or concrete versions.
2) Pipe Spillway
A pipe spillway may be called for where high volume discharge calls for erosion/flood control that an earthen spillway may not guarantee. One of the best pipe systems is a vertical standpipe set to discharge at designed water level, drop water to roughly pond bottom level, and then flow out to daylight usually through the bottom of the dam. Standpipes may not be possible on level terrain unless the discharge pipe runs to daylight. Standpipes may allow you to install a drain at the bottom of the vertical riser, useful for pond maintenance. Again, like the natural spillway, the pipe needs to be big enough to handle flood water. For safety sake, an emergency spillway is often installed to handle extreme flood water. This is usually an earthen channel cut across the shore/dam, at a slightly higher level than the discharge pipe. Some people like a pipe system so they don’t need a bridge across the stream spillway for vehicles, and to perhaps install a drain. It may also be possible to add or remove extensions to the standpipe to vary the pond water level. Often the standpipe has a trash rack to keep it from plugging with debris, or a more elaborate baffle to prevent beaver damming.
Pipe spillways buried in the pond structure should have anti-seep collars on both the standpipe and discharge pipe.
3) In-Line Control Structure
In-line control structures can also be used for spillways. These are manufactured chutes similar to a standpipe, but where the pond water level is controlled by discharging water from the bottom of the pond. They are modeled on older versions of poured concrete drop boxes with wooden gate boards now called “logs.” (See photo.) The overflow is taken by pipe from near the bottom of the pond. The structure includes tracks for insertion of “stop logs”, which are used to set the pond water level. Water flows into the box at the bottom, rises to the desired level, and then overflows the logs down the opposite side of the box and out the discharge pipe.
There are a couple of interesting merits to this system. Using a control handle, it is easy to add or subtract gates, and thus control the pond water level. For instance, perhaps you want a lower pond level from autumn through spring, to reduce flood potential. But for summer, you raise the water level for storage purposes, cooler temperatures, more enjoyable pond level, etc.
In addition, you are taking overflow water off the bottom of the pond. Some pond owners feel this removes silty bottom water, rather than cleaner top-level water. The downside may be that taking out low-level water takes out the coolest water, and may add to summer pond temperatures. It also eliminates the debris flushing effect of a water level discharge via an earthen spillway or standpipe. On the plus side, since underwater discharge is silent, beavers which may be attracted to damming noisy flows, will not plug the pipe; and the pipe would be simpler to screen against beaver damming. Also, with the potential to remove many logs, this system can function as a pretty good drain. In-line structures are built with various inlet pipe and box dimensions, so determine your watershed and flood runoff potential to determine proper in-line structure size. In-line structures are popular for wild duck management too, so ponds can be created to attract migrating waterfowl in fall, then drained and planted with waterfowl grasses and reflooded for the next season.
Having an adequate emergency spillway, and removable stop logs, means good potential flood prevention.
Note: I did not mention horizontal pond water level culvert pipes. I think they are vulnerable to leakage and frost damage.
Here are two sources for information and ordering in line control structures and other piping:
Topics vary from month to month and provide great information when researching where you want to build a pond, or how to keep it clean. Other areas of interest are pond use for more than swimming, and how to keep fish happy in your pond.