Traditional BMP

• Key features of a dry detention basin include:
• The dam, or embankment, impounds water to reduce downstream flooding.
• A sediment forebay slows incoming stormwater runoff and allows sediment to settle out.
• The outlet control structure (OCS) contains a low flow opening, or orifice, that controls the flow of water out of the basin. In the event of heavy rain, the OCS also includes a high-flow opening that allows water to bypass the basin and prevent the embankment from overtopping.
• The bottom of the dry detention basin is gradually sloped towards the outlet to allow water to drain out of the basin and prevent standing water. Vegetation (typically turf grass) helps provide some water quality benefit, stabilizes soil in the dry detention basin, improves aesthetics, and can also provide recreation opportunities.
• Riprap is installed at the forebay to slow down runoff and filter sediment, debris, and other solids from runoff.
• Filter stone is typically installed in front of an OCS to filter sediment, debris, and other solids from runoff.

Dry detention basins are designed to completely drain following a storm event and are normally dry between events. If your basin retains water for more than 48 hours after the last rain event, there may be a problem. 

Why should we maintain our dry detention basin?

Failure to maintain a dry detention basin can lead to structural failure and expensive repairs.

Without proper maintenance, dry detention basins will not provide their intended flood control and water quality functions. Poorly maintained or failing dry detention basins can pose a risk to public health and safety.

Failure to maintain dry detention basins can also lead to nuisance conditions including mosquito habitat, odors, and undesirable aesthetics (e.g., stagnant water, overgrowth of vegetation, trash accumulation, etc.).

When should we maintain our dry detention basin?

Most maintenance problems with BMP are less costly to correct when they are caught early. Dry detention basins should be inspected at least quarterly and after large storm events (i.e., greater than 1 inch of rainfall). Common maintenance indicators for dry detention basins include:

• Heavy sediment buildup
• Trash, litter, and debris accumulation
• Clogged inlet and outlet structures
• Broken, cracked, or separated structures
• Stagnant water (dry detention basins should be dry between storms)
• Poor vegetation health or no vegetation at all
• Areas of erosion, scour, or bare soil
• Odors and poor appearance
• Trees or shrubs growing on the embankment or near OCS
• Animal burrows
• Presence of wetland vegetation, such as cattails

How do we maintain our dry detention basin?

A well-designed maintenance program includes both “routine” and “non-routine” maintenance tasks. Routine maintenance includes regularly scheduled tasks that are necessary to maintain the function and desired appearance of a dry detention basin. This includes:

• Performing regular inspections
• Mowing vegetation in and around the basin
• Prevent small trees and shrubs from growing
• Removing minor sediment deposits from the forebay
• Removing trash and debris
• Clean/replace riprap around forebay
• Clean/replace filter stone around OCS

Non-routine maintenance consists of more difficult or complex maintenance tasks that are typically performed on an as-needed basis after a regular inspection identifies a problem. This can include:

• Repair and stabilization of eroded areas
• Removal or excavation of accumulated sediment
• Repair or replacement of structures
• Identification and removal of undesirable vegetation
• Identification and repair of structural deficiencies in embankment
• Repair of animal damage and animal removal
• Review joints and replace grout as necessary

When should I hire a professional?

Drastic changes in dry detention basin performance or appearance are signs that it’s time to call a professional. Regular inspections will help identify changes early on before they become serious (and expensive!) problems. Involving a professional in some of the more routine and regularly scheduled inspections and minor repairs will help prevent serious issues from developing over time. Professionals should be consulted for the following:

For vegetation needs call:

• A professional landscaper
• A horticulturist or other plant professional

For maintenance and repair needs call:

• A licensed contractor
• A professional engineer
• A business certified in BMP maintenance

What safety considerations are involved with maintaining a dry detention basin?

Important safety considerations for dry detention basins include: • Steep embankments: These can make it difficult to climb out of the basin and can be hazardous to maintenance staff.

Inlet and outlet structures

• Outlet structures that are open and unprotected by trash/ safety racks pose a danger.
• Outlet structures should always be equipped with these features, and they should be in proper working order.
• Only professionals with specialized training and permits may enter confined spaces and structures.

General safety tips

• Avoid performing maintenance activities in wet weather.
• Maintenance staff must wear appropriate safety gear.
• Maintain lighting, signage, and fencing, where appropriate, to discourage public access.