Post-Construction Stormwater Management

The stormwater pollution Best Management Practices (BMPs) implemented during development and construction are aimed at stopping immediate pollution. After construction, the long-term management of stormwater and snow melt runoff is just as crucial in safeguarding our valuable water resources

New construction causes the land to be altered—there are now more impervious surfaces such as roofs, parking lots, and roads. Consequently, less rainfall is able to infiltrate the ground naturally, which causes the volume and speed of runoff to rise. This runoff also needs somewhere to flow, and if not properly controlled, will transport pollutants from our developed areas directly into the storm drains and eventually into our local waterways.

Consequently, the EPA, State, and local regulators require a "Long-term Stormwater Management Plan" (LTSMP) after the construction phase is completed. This includes plans and systems designed to manage stormwater runoff and prevent pollution for the long term. These plans are sometimes called “Post-Construction Stormwater Plans.” 

What is Post-Construction Stormwater Management

Post-construction stormwater management involves the permanent systems and practices put in place after construction is complete to control both the quality and quantity of stormwater runoff. It involves long-term planning—considering how rainwater is going to be managed on this site for years into the future

Why is a Long-term Stormwater Management Plan significant?

A long-term or post construction stormwater management plan is important to prevent:

• Water Quality Impairment: Runoff from developed sites can transport pollutants such as oil, grease, pesticides, fertilizers, pet waste, and heavy metals straight into our streams, rivers, and lakes, killing aquatic animals and possibly making water unsafe to recreate or even drink.

• Increased Flooding: More impervious surfaces mean more runoff reaching drainage systems sooner, increasing the likelihood of downstream localized flooding.

• Erosion and Sedimentation: High-velocity runoff can lead to stream bank erosion and excessive sedimentation, harming aquatic ecosystems and impacting water clarity.

• Reduced Groundwater Recharge: When rainwater doesn't percolate into the soil, it lowers the quantity of water that replenishes our groundwater aquifers—very important sources of water, particularly in the Mountain and Southwest regions.

What Does a Post-Construction Stormwater Management Plan Include?

A well-planned post-construction strategy incorporates a collection of Best Management Practices (BMPs) to manage the quantity and quality of runoff. 

Non-Structural BMPs: These are management strategies and practices that aim to prevent pollution at the source. A few examples are:

• Education on the proper disposal of trash and debris

• Proper storage and minimization of the application of fertilizer and pesticides

• Refrain from hosing off driveways and sidewalks

• Stop washing vehicles on non pervious surfaces

• Install water-efficient landscaping

• Regular clean storm drain grates

• Vacuum storm drains when dirt and debris has collected.

Structural BMPs: These are physical systems that are carefully designed to treat or regulate runoff. Some common examples are:

• Rain Gardens and Bioretention Areas: These vegetated depressions filter pollutants and permit water to percolate into the earth.

• Infiltration Trenches and Basins: Carefully engineered depressions to intercept runoff, allowing it to slowly infiltrate the ground.

• Retention and Detention Ponds: Basins that temporarily hold runoff, giving sediments time to settle and water a chance to be slowly discharged. Retention ponds also offer some degree of water quality treatment.

• Permeable Pavements: Pavements that allow water to percolate through them, reducing runoff volume.

• Underground Detention/Infiltration Systems: These are underground storage facilities (typically using pipes, vaults, or gravel beds) designed to temporarily hold runoff and release it gradually or permit it to infiltrate. Convenient where surface area is limited.

• Oil/Water Separators: These are specifically intended to catch oil and grease in stormwater runoff, often used in industrial or commercial applications where petroleum contamination is a potential issue.

Long-Term Inspections and Maintenance:

It should be understood that the use of a post-construction stormwater management plan is not a one-time event. Rather, it is a long-term operation and maintenance commitment.Like any infrastructure, these BMPs need to be periodically inspected, cleaned, and maintained in order to function as intended. Maintenance neglect will lead to system failure and ultimately water quality problems.