Stormwater 101: Pond Dam Safety

June 4, 2015
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The Montgomery County Department of Environmental Protection (DEP) is responsible for the inspection of all stormwater maintenance facilities in the County to ensure they are properly maintained and repaired as necessary. This includes stormwater ponds/lakes and their associated dams.  Maintaining the ponds and dams in safe condition is the responsibility of every dam owner. Follow DEP guest blogger Audra Lew as she shows us the basics of dam safety inspections.

The Maryland Department of the Environment (MDE) requires all dams in Maryland be inspected by a professional engineer once per year.   In addition to inspections, high hazard dams are monitored during major storm events, such as during a hurricane or tropical storm.  During these types of events, DEP monitors the water levels in the pond and the stability of the dam to ensure it is working properly.  DEP does not do this alone; we also work with the Office of Emergency Management and Homeland Security (OEMHS)  during major storm events to protect downstream residents and businesses.

 

View of large stormwater pond from on top of a dam

A view of Crabbs Branch Regional Stormwater Pond from atop the dam.

 

The focus of this blog is the Crabbs Branch Regional Stormwater Pond dam in Rockville. I followed County Engineer Gene Gopenko and the County’s dam inspection contractors around on a beautiful spring morning to see what is involved in dam safety inspection.

The headwaters (beginning of a stream) of Crabbs Branch start in a highly impervious (lots of hard surfaces such as roadways and structures) commercial area just upstream. The Crabbs Branch Regional Stormwater Pond was constructed to control the runoff from these hard surfaces. Crabbs Branch runs through the pond and is a tributary to the Rock Creek Watershed.

 

Image of dam at Crabbs Branch

A view of Crabbs Branch Regional Stormwater Pond dam.

 

We started the inspection at the outfall of the dam, downstream of the pond. This is the area where water exits the facility via a concrete box culvert that directs water downstream from the dam’s control structure (riser) during both normal and high water rain events. On the day of the inspection, the water was about ankle deep as we traveled through the box culvert that runs under the road.  Inside the culvert, Gene and the county’s inspectors looked for cracks, crevices or other deficiencies in the concrete structure. The structures we entered are not safe for the general public.  Only trained professionals should enter these structures.

 

Image of dam inspectors standing in water near a large culvert

A bus travels on the road above the dam as inspectors evaluate the outfall culvert (defined above).

 

Once we made it through the box culvert, inspectors examined the inside of the riser structure. The riser allows water to flow out of the pond at a regulated rate.  It also has structures in it to stop large debris and trash from entering the creek.  I had never stood inside of one of these large riser structures before. It was huge– it felt like standing near a waterfall! Inside the structure, inspectors were again looking at all of the concrete as well as the metal structures within.  If cracking or rust had been discovered, repair work would have been ordered.

 

An inspector examines pipes in a riser structure

Inside view of riser structure.

 

A view from above the riser structure

Outside view of riser structure.

 

At the downstream end of the culvert, the water exits to the stream via a concrete channel. Concrete baffles were constructed to protect the stream channel from erosion by slowing the water. Erosion occurs when large volumes of fast moving water eat away at the banks of a stream or river.

 

An image of concrete blocks that slow the flow of water once it exits the culvert

Concrete baffles at the outfall of the dam slow water before it enters the stream to help prevent erosion.

 

Once out of the riser structure and back through the box culvert, we stayed on the downstream side of the dam to inspect the embankment. This is also called the downstream slope of the dam. The contractors and Gene were looking for any animal burrows, sinkholes or eroded areas that could cause larger issues. Maintaining properly stabilized side slopes is an important aspect of a safe, functioning dam.

 

Image of inspectors standing along a grassy embankment

In addition to checking for stability of the slope, inspectors also checked monitoring wells for depth of groundwater inside the dam.

 

Once we were finished on the downstream side, we crossed over the road to the upstream bank of the dam. There is a large section of riprap (piles of large stone) on this side to protect the upstream slope from wave erosion.

 

Image of the outside of the riser and piles of rocks on the dam embankment

An outside view of the riser and the rip rap for back stabilization.

 

The intake portion of the riser structure is also located here (the area mentioned above, that felt like standing near a waterfall).  This is where the water pours in slowly when the pond is at normal level.  During storm events, the water pours in faster and with greater volume. There is also a large trash rack to keep debris from falling in and jamming up this area. The trash rack is regularly inspected and cleaned. The inspectors checked all portions of the upside slope of the dam to look for any erosion or animal burrows.

 

Image of Crabbs Branch Stormwater Pond

Crabbs Branch Regional Stormwater Pond from top of the dam.

 

If needed, DEP orders repairs to any components of pond structures that require emergency attention. Otherwise routine repairs are made on dams (and all stormwater facilities) throughout the year. Funding for the inspections, maintenance and retrofits of these regional ponds is made possible by the Water Quality Protection Charge paid by all property owners in Montgomery County.

We hope you have enjoyed getting to know a bit more about just one of the many services DEP’s Stormwater Facility Maintenance Section provides. Next time you are near a pond, see if you can identify some of its features. Just remember to look from afar, staying out of the ponds and lakes and their associated features. Control structures and water conveyance systems like the riser and large culverts pictured here are confined spaces, and should not be entered by the general public. Professionals who enter the structures for inspection and repair are trained and they utilize air quality monitoring systems for the enclosed spaces that may contain noxious gases and fall protection equipment. Do not enter any of these structures.

Guest blog by Audra Lew, Planning Specialist for DEP Stormwater Facility Maintenance Program



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