When you hear the word “dam” you likely envision the large engineering structures such as Hoover Dam in the US, Three Gorges Dam in China, or the Oroville dam in northern California. These large dams allow the ability to produce hydroelectric power, store drinking water for domestic and irrigation use, and provide protection from flooding. While these structures are subject to very strict maintenance and inspection procedures, failures do occur, usually with catastrophic consequences to property and life. Not all dams are as large as these and generally, people may not realize many neighborhoods also utilize small dams, and these can fail for the same reasons as larger dams. Many commercial properties and homeowner associations, as well as municipalities, also have dams. Likely used for detention and retention systems to reduce the impacts to downstream properties from upstream development or simply to aid in the beneficial impacts to the environment.
This article summarizes the maintenance and issues pertinent to smaller scale dams.
As part of the requirements for developments and roadway projects, these structures provide stormwater/flood control systems to reduce the amount of runoff to near historic levels and to thus minimize the impact of the new development on the surrounding environs. These systems usually include retention or detention ponds, usually designed by engineers to specific criteria and regulations. The primary function of detention ponds is to reduce peak flows by storing runoff and then releasing that runoff at a controlled rate to downstream systems. Retention ponds perform a similar function but these either permanently store runoff or store runoff for extended periods of time.
In most cases when a new subdivision is built, a detention pond or retention pond can easily be excavated at the lowest part of the site. But there are cases when due to site constraints, a pond cannot be easily excavated, and in these situations, other configurations are required. In such cases, an earth embankment or dam is constructed to form a pond, and the required storage volume is provided on the surface. These dams are also subject to specific design criteria and usually, are constructed with emergency overflows or spillways. The State of Colorado Department of Natural Resources Division of Water Resources’ diagram below shows the general configuration of a low head dam.
Just as it is very important dams are properly designed and installed it is just as important that those responsible for dams routinely maintain and inspect those structures.
When a dam fails it can cause catastrophic damages. In some cases, fatalities occur at low head dams with spillways during normal operations. The following are reasons why dams can fail:
- Settlement of embankments
- The impounded water creates an open pathway, channel, or pipe through the dam
- Inadequate maintenance and upkeep
- Overtopping due to outlet structure blockage or inadequate capacity
- Erosion of the embankment material
- Poor compaction of embankment allowing seepage of water through the embankment
A pond serving a single neighborhood or multiple neighborhoods is typically owned by one HOA or more HOA, and those bodies are responsible for maintenance of that dam. In most instances, the HOA is aware of the maintenance requirements for a detention pond, but earthen dams and embankments add an additional level of maintenance requirements and HOA’s may not be as familiar with these. The following procedures are required as part of a regular inspection procedure to avoid dam failure:
- Inspection of the outlet structures (where water leaves the pond) are free of sediment, vegetation, and other clogging material
- Verify that trash racks are installed on all outlet structures
- Check the embankment/dam for signs of erosion, rodent holes and tree root damage
- Check the embankment for points of structural failure
- The pond should be observed frequently when to verify that the capacity is sufficient
- The dam should be check for seepage
- Safety – restrict access to the extent necessary to prevent deliberate damage
Other items that should be provided after larger rain events are visual observations of the spillways, erosion protection, rip-rap and other areas where water flow can scour or erode the earth. The maintenance of the pond should include cleaning and verification of the pond capacity after smaller events at least once a year to ensure that when the larger event occurs the pond is ready to receive those flows as it was originally designed.
Understanding the responsibility that comes with having a dam and stormwater pond on your property is crucial. Being diligent with maintenance and performing remedial work when required will reduce the odds of failure and damage to your site or downstream facilities.
*Please consult your HOA declarations so you understand and follow maintenance and repair responsibilities.