Key Takeaways
- Paved surfaces significantly increase runoff compared to undeveloped land.
- Runoff calculations based on site size and rainfall data determine how drainage systems are sized.
- Retention and detention systems reduce peak flow into downstream drainage networks.
- Municipal drainage systems have limited capacity, which developments need to plan around.
- Stormwater management works best when it’s part of site planning from the start, not an afterthought.
In This Article
Undeveloped land absorbs a significant portion of rainfall naturally. Once that land is covered with buildings, roads and paved surfaces, the same rainfall has nowhere to go except downhill, and often into a drainage system that was never sized for the new volume. Stormwater management is how a development accounts for this change instead of creating flooding problems for itself or its neighbors.
Why New Development Changes How Water Moves
Paved surfaces and rooftops don’t absorb rainwater the way natural ground does, which means a developed site produces significantly more surface runoff during rain than the same site did before construction. Without proper planning, this additional runoff has to go somewhere, and it often ends up as flooding on-site, in neighboring properties, or overwhelming existing municipal drains that weren’t sized for it.
Calculating Runoff for a Site
Before designing a drainage system, engineers calculate expected runoff based on the site’s size, the percentage of surface that will be paved or built on, and local rainfall intensity data. This calculation determines how much water the drainage system actually needs to handle during a significant rain event, which is the basis for sizing every drain, channel and retention feature that follows.
Drainage Infrastructure Within a Development
A development’s internal drainage system, storm drains, channels, and culverts, needs to be sized and positioned based on the runoff calculation, not just placed wherever is convenient during construction. Proper grading across the site is equally important, directing water toward drainage infrastructure rather than allowing it to pool in low areas or flow toward buildings.
Retention and Detention Systems
Retention ponds and detention basins temporarily hold excess stormwater and release it gradually, reducing the peak flow that reaches downstream drainage systems. For larger developments, these systems are often what makes it possible to connect to existing municipal drainage without overwhelming it, since they smooth out the surge of runoff during heavy rainfall rather than sending it all downstream at once.
Connecting to Municipal Drainage Systems
Most developments eventually connect to a municipal drainage network, and that network has a limited capacity. A development that sends more runoff into the system than it can handle creates flooding problems beyond its own boundaries. This is why many approval processes require a stormwater management plan that demonstrates the development won’t overload existing municipal infrastructure.
Planning Stormwater Management From the Start
Stormwater management works best as a design consideration from the earliest site planning stage, not an add-on after the layout is finalized. Site grading, building placement and paved area all affect runoff, and accounting for this early makes the eventual drainage system simpler and more effective than retrofitting it onto a design that didn’t consider water flow from the start.
Frequently Asked Questions
Why does new construction increase flooding risk?
Paved surfaces and rooftops prevent rainwater from soaking into the ground the way it naturally would, increasing surface runoff that needs to be managed by a drainage system.
What is the difference between retention and detention systems?
Both temporarily hold stormwater to reduce peak flow, but they differ in how and when the water is released or reused, and the right choice depends on the specific site and local requirements.
Do all new developments need a stormwater management plan?
Requirements vary by project size and location, but most developments connecting to municipal drainage systems need to demonstrate their runoff won’t overload existing infrastructure.
