Low Level Drains
A low level outlet conduit or drain is required for emptying or lowering the water level in a reservoir, lake or pond in case of emergency and for inspection and maintenance of the dam.
Typical Types of Low Level Drains
- valve located in the spillway riser
- a conduit through the dam with a valve at either the upstream or downstream end of the conduit (upstream control is greatly preferable)
- a siphon system (often used to retrofit existing dams)
- a gate, valve, or stoplogs located in a drain control structure
- stop logs across a spillway
- any combination of the above
Uses of Drains
The following situations make up the primary uses of low level drains:
Should serious problems ever occur to threaten the immediate safety of the dam, drains may be used to lower the lake level to reduce the likelihood of dam failure. (Also see Emergency Action Plan (EAP) (PDF, 1.2MB) guidance documents for more information on dam emergency planning.) Examples of such emergencies include:
- clogging of the spillway which may lead to high lake levels and eventual dam overtopping development of deep slides or cracks in the dam
- severe seepage through the dam which may lead to a piping failure of the dam
- partial or total collapse of the spillway system
- landslide around reservoir rim
- slope protection repair
- spillway repairs
- repair and/or installation of docks and other structures along the shoreline
- dredging the lake application of aquatic herbicide
- Winter Drawdown
Some dam owners prefer to lower the lake level during the winter months to:
- reduce ice damage to structures along the shoreline.
- provide additional flood storage for upcoming spring rains perform several repair items during this winter drawdown period.
- periodic fluctuations in the lake level also discourage muskrat and beaver habitation along the shoreline. Muskrat burrows in earthen dams can lead to costly repairs. Beaver dams and debris can plug spillways.
Common Drain Maintenance Problems
Some common problems often associated with the maintenance and operation of low level drains include the following:
- deteriorated and bent control stems and stem guides
- deteriorated and separated conduit joints
- leaky and rusted control valves and sluice gates
- deteriorated ladders and platforms in control structures
- deteriorated control structures
- clogging of the drain conduit inlet with sediment and debris
- inaccessibility of the control mechanism to operate the drain
- seepage along the drain conduit
- erosion and undermining of the conduit discharge area
- development of instability of earthen sections resulting in slides along the upstream slope of the dam and the shoreline caused by lowering the lake level too quickly
Operation and Maintenance Recommendations
- All gates, valves, stems and other mechanisms should be lubricated according to the manufacturer's specifications. If you do not have a copy of the specifications and the manufacturing company cannot be determined, then a local valve distributor or your engineer may be able to provide assistance.
- The low level drain should be operated at least twice a year to prevent the inlet from clogging with sediment and debris, and to keep all movable parts working easily. Most manufacturers recommend that gates and valves be operated at least four times per year. Frequent operation will help to ensure that the drain will be operable when it is needed. You should contact DEC Environmental Permits in your region of the state to confirm if there are environmental issues with a significant release of water or sediment. All valves and gates should be fully opened and closed at least twice to help prevent sediment buildup and to obtain a proper seal. If the gate gets stuck in a partially opened position, gradually work the gate in each direction until it becomes fully operational. Do not apply excessive torque as this could bend or break the control stem, or damage the valve or gate seat. With the drain fully open, inspect the outlet area for flow amounts, leaks, erosion and anything unusual.
- All visible portions of the low level drain system should be inspected at least annually, preferably during the periodic operation of the drain. Look for and make note of any cracks, rusted and deteriorated parts, leaks, bent control stems, separated conduit joints or unusual observations.
- Low level drain control valves and gates should always be placed upstream of the centerline of the dam. This allows the drain conduit to remain depressurized except during use, therefore reducing the likelihood of seepage through the conduit joints causing internal erosion of the surrounding earth fill.
- For accessibility ease, the drain control platform should be located on shore or be provided with a bridge or other structure. This becomes very important during emergency situations if high pool levels exist.
- Vandalism can be a problem at any dam. If a lake drain is operated by a crank, wheel or other similar mechanism, locking with a chain or other device, or removal and off-site storage of the operator, may be beneficial. Fences or other such installations may also help to ward off vandals.
- Rapid draw down of the impoundment can result in a build-up of hydrostatic pressures in the upstream slope of the dam which can lead to slope failure. Lowering the water level slowly allows these pressures to dissipate.
Refer to the NYSDEC publication entitled "Guidelines for Design of Dams" (PDF, 139 KB) for low level drain capacity and design requirements.
For Questions or Comments Contact:
New York State Department of Environmental Conservation
Bureau of Flood Protection and Dam Safety, Dam Safety Section
625 Broadway, 4th Floor
Albany, NY 12233-3504
Telephone: (518) 402-8185
- NYSDEC publication, "Guidance For Developing An Emergency Action Plan" (PDF, 1.2 MB)
- NYSDEC publication entitled "Owners Guidance Manual for the Inspection and Maintenance of Dams in NYS"