Dam safety and spillways: the basics for pond owners

A pond dam holds back water and protects whatever is downstream. Most pond problems that make the news come down to one failure mode: water overtopping the dam because the spillway could not pass a big storm. This guide covers the basics of dam safety and spillways so you can plan a pond that stands up. It is general information, not a design for your site.

What makes a dam safe

Sound pond dams share a few features:

• A spillway sized to the watershed. The spillway has to carry the runoff from a large storm without water rising over the dam crest.
• Adequate freeboard. Extra dam height above the design water level, as a safety margin.
• Seepage control. A compacted clay core or liner so water does not pipe through the embankment, which on low clay Ozark soils is a real concern.
• A settlement allowance. Extra height built in because a new dam settles over time.
• Stable slopes. A waterside slope no steeper than about 1 in 3 is common guidance for Missouri ponds.

These are the kinds of requirements set out in the USDA NRCS Conservation Practice Standard 378, which applies where the effective dam height is 35 feet or less and the product of storage times effective height is under 3,000.

Principal and emergency spillways

Many well built ponds use two spillways working together:

• The principal spillway, often a pipe through the dam, handles normal flow and small storms and sets the pond’s water level.
• The emergency (auxiliary) spillway, usually a wide vegetated channel routed around the end of the dam, safely passes the rare large storm that exceeds the principal spillway.

Sizing both to the pond’s watershed is where watershed sizing and dam safety meet. A large watershed behind a small spillway is the classic recipe for an overtopped dam. See our watershed sizing guide.

The Missouri height thresholds

Missouri regulates dams by height. Non-federal, non-agricultural dams under 25 feet are not regulated by the state dam safety program, dams 25 to 35 feet are inventoried but not permitted, and dams 35 feet and higher require a construction permit prepared by a Missouri registered professional engineer plus a safety permit to operate (Missouri DNR Dam and Reservoir Safety Program). Most farm ponds fall below 35 feet, but being unregulated is not the same as being safe by default. The design still has to be right. For the full permitting picture, see our Missouri pond permit guide.

Maintenance keeps a dam safe

A safe dam stays safe with upkeep: keeping woody growth off the embankment, controlling burrowing animals, keeping the emergency spillway clear and vegetated, and watching for wet spots that signal seepage. If your dam already shows problems, our dam construction and engineered repair service connects you with a contractor for planned, engineered repair. We do not handle emergencies; if a dam is actively failing, contact local emergency services and the Missouri DNR.

Bottom line

Size the spillway to the watershed, build in freeboard and seepage control, and maintain the dam. When you are ready to build or repair, we connect you with one licensed local contractor who designs to these standards.