Detention PondsRegulations and Water Quality

Westmoreland Conservation District

Engineers Webinar

March 5, 2021

James W. Pillsbury MS PE

Detention ponds

• Regulations pertaining to Detention Ponds • Chapter 105 and detention ponds

• Wetland and stream impacts and detention ponds

• Meeting water quality requirements with Detention Ponds

Detention pond—the most common form of stormwater management BMP

Regulations and Detention Ponds

• Chapter 105 Dam Safety and Waterway Management

• A Dam Safety permit is required if a detention pond has an embankment more than 15 feet high as measured from the upstream toe of the dam at the original channel, to the top of the dam.

Yes, it’s more than 15 feet high…

This means that the maximum depth of water in the pond will be 13 feet or less.

You need to account for the Emergency Spillway and for Freeboard.

Note the relationship between the top of the riser and the emergency spillway and the top of the embankment!You want the water to always flow out the riser before it gets to the E. S. level.

You can excavate out behind the dam into original ground.

Beware of excavating into bad soil or rock… it may leak or cause a pollution problem.

Regulations and Detention Ponds

• A dam safety permit is required if the drainage area tributary to the dam is > 100 acres

Regulations and Detention Ponds

A Dam Safety permit is required if the impoundment stores more than 50 acre-feet of water

Regulations and Detention Ponds

• A Water Obstruction and Encroachment Permit (Joint Permit) is needed if construction of the pond will impact Wetlands or Regulated Waters of the Commonwealth

• Building a detention pond in an intermittent or perennial stream may require a WOEP/Joint Permit

• A permit is needed if you change the course, current, cross section, or quality of Waters of the Commonwealth.

• A permit is needed if your work encroaches on a floodway.

Wetlands: Saturated Soil, Water, Predominance of Wetland Vegetation

DCNR

My favorite wetland plant!

Phipps

How do you tell if a stream is…

• Perennial

• Intermittent

• Ephemeral (and mostly not subject to Chapter 105 regulations)

Poll Question:

Is the Monongahela River…

a) Perennial

b) Intermittent

c) Ephemeral

???

Environmental Assessment

• Some Chapter 105 permits require an Environmental Assessment

• Some Chapter 105 waivers require an Environmental Assessment.

Even if you are <100 acres and <15 feet high, if your pond is impacting wetlands or a stream, you may need this form.

The Environmental Assessment Form is part of a Joint Permit

Remember…

• No permit is needed to maintain a detention pond to its original condition. You can clean it out, remove cattails, etc.

News-Herald

Water Quality and Detention ponds

• Removal rates for various BMPs

• Calculation of pollutant removal

• Design for pollutant removal

Ponds naturally trap pollutants…

EPA

A dry detention pond traps very few pollutants

Calculating the pollutant removal effectiveness

• Use a DEP-approved method, either the Excel spreadsheets or something else.

Water quality for a wet pond/ retention basin

Water quality for a dry extended detention basin

Water Quality

• If your goal is to improve water quality by use of a detention pond, your best bet is to have a wet pond with a permanent pool and a forebay.

Many detention ponds are “wet” anyhow…

Design of a detention pond

• Forebay

• Permanent wet pool

• Underdrain

• Riser

• Emergency spillway

Schematic of a wet pond from the 2006 DEP manual.

Wet Ponds: detention pond in State College PA at a Lowe’s Store.

Rock berm sediment forebay in Massachusetts

Mass allows a 25% credit for TSS removal for a forebay

Colonial Construction Materials, Virginia

Gabion Basket forebay

A sediment forebay in PhiladelphiaPhila. Water Department

Penn Twp./ Chris Droste

Poll Question:

Have you designed a sediment forebay on a pond?

Y/N

Detention pond riser: the purpose is to control the outflow.

Pay attention to elevations—riser top, emergency spillway, orifices, underdrain, barrel

Line of sediment shows the pond is working

Managed Release Concept can be used in conjunction with a detention basin.

MRC Design Summary Sheet is to be filed with the Spreadsheets, one sheet per use of an MRC BMP

Further Water Quality benefits from using MRC

Must check the box.

Heavy clay soil.Worked in the winter.Infiltration here?

Is this pond infiltrating?What’s wrong with this riser?

Flow splitter manhole for treating water quality.

Montgomery County, Maryland

Stormwater ManagementIs Your Friend

• www.westmorelandconservation.org

• www.westmorelandstormwater.org

• jim@wcdpa.com