Retention Ponds: Widely Used Stormwater Management Facilities

This article looks at dry ponds and wet ponds (ponds with permanent storage). In addition to defining both types of ponds, we cover their advantages and disadvantages, design criteria and maintenance. Learn more about these popular stormwater management facilities.

What is a retention pond?

Retention ponds are a type of stormwater management facility. They are designed to temporarily store stormwater before releasing it at a controlled rate into a watercourse. They are the most commonly used type of facility for managing stormwater in Quebec. They are generally built in low-density residential developments or in industrial or commercial areas.

There are two types of retention ponds: dry ponds and wet ponds, also called ponds with permanent storage.

What is a dry pond?

Dry ponds temporarily receive runoff during rainfall. They drain quickly (generally in less than 24 hours) and remain dry when there is no precipitation. Their main purpose is to reduce the amount of water in the storm sewer system during heavy rain to prevent downstream flooding.

What is a wet pond?

Wet ponds, or ponds with permanent storage, hold a volume of water between rainfall events and have additional storage volume that varies with the flow. They provide water quality control in addition to quantity control. Pollutants are removed mainly by settling, but also through various biological processes.

Advantages of retention ponds

Storing stormwater in retention ponds helps limit flooding and watercourse erosion. When well designed, they blend attractively into the urban landscape and can increase the value of neighbouring properties. They can also serve additional community functions, such as parks and playgrounds.

Some advantages are specific to each type of pond:

• Dry ponds stand out for their easy maintenance, good winter performance and low cost;
• Wet ponds remove various pollutants, whether in solid or dissolved form. They are more effective and require less space than constructed wetlands.

Maintenance is needed less often than for other types of stormwater management facilities.

Limitations of retention ponds

On the other hand, retention ponds:

• require a large site, particularly wet ponds;
• require regular sediment removal to prevent resuspension;
• can lead to the accumulation of debris and the growth of weeds or odours. They therefore require regular maintenance to prevent outlet clogging and stagnant water;
• sometimes need a pretreatment structure to ensure satisfactory water quality.

If poorly designed or maintained, wet ponds can:

• create conditions that degrade water quality and lead to nutrients and metals entering watercourses. Proper management is therefore needed to prevent eutrophication in these ponds, as well as the proliferation of mosquitoes and frogs;
• discharge warm water in summer, which limits their use when fish species sensitive to thermal shock are present in the receiving watercourse.

Wet ponds are also more expensive than dry ponds.

How to design a wet pond

The design of a wet pond must meet the following 10 criteria:

1. The tributary drainage area must be at least 5 hectares;
2. The pond volume must allow for the removal of 80% of suspended solids;
3. The pond volume must be able to hold a 100-year storm;
4. The retention time in the pond should ideally be 48 hours;
5. The sediment forebay must cover no more than 33% of the permanent pool area, have a minimum depth of 1 metre and a length-to-width ratio of 2:1;
6. The pond must have a length-to-width ratio between 3:1 and 5:1, which can be achieved with berms or other means to maximize the flow path and minimize the potential for short-circuiting;
7. The permanent pool depth must average between 1 and 2 metres;
8. The active storage depth must be between 1 and 2 metres;
9. Side slopes must be no steeper than 3:1;
10. Inlet and outlet pipes must have a minimum diameter of 450 mm and a slope greater than 1%. If quality control is provided, the outlet must have a minimum diameter of 75 mm, and the orifice must be adequately protected against clogging.

Inspecting and maintaining retention ponds

Maintaining dry ponds is fairly simple and mainly involves mowing, trimming shrubs, removing debris in spring and fall, and carrying out inspections.

For wet ponds, particular attention must be paid to algae growth, mosquito proliferation and uncontrolled regrowth of vegetation. Ice thickness must also be monitored in winter, weeds (aquatic and terrestrial) must be managed, and sediment removal must be planned. Water quality control also requires observations and, if needed, sampling and analysis.

During periods of heavy rain or drought, inspections of inlets and outlets should be planned, along with clearing outlets (debris and ice), repairing embankments and banks, monitoring low water levels and controlling algae growth.

Given the growing attention paid to stormwater management in the current context of climate change, the number of retention ponds in Quebec is expected to keep increasing in the coming years.

To learn more about other types of stormwater management facilities, read Nine Types of Stormwater Management Facilities Used in Quebec.

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References

• Website of Quebec’s Ministère de l’Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs (2021). Stormwater management section.
• MDDEP and MAMROT (2011). Guide de gestion des eaux pluviales : stratégies d’aménagement, principes de conception et pratiques de gestion optimales pour les réseaux de drainage urbain. Ministère du Développement durable, de l’Environnement et des Parcs and Ministère des Affaires municipales, des Régions et de l’Occupation du territoire, Québec, 386 p.
• Ville de Québec. Bassin avec retenue permanente, 5 p.