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Stormwater Management
Stormwater management is the broadest of the water quality disciplines, and in some ways the one that ties the others together. Where a WQMP addresses long-term pollutant treatment and a SWPPP addresses construction-phase erosion, stormwater management is the underlying engineering that determines how water moves through and off a site at all the detention, retention, infiltration and conveyance systems that make sure runoff doesn’t overwhelm downstream infrastructure, flood a neighboring property, or exceed what the receiving storm drain system was designed to handle.
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Every other stormwater document eventually depends on this layer of design. A WQMP’s treatment BMPs have to fit within a hydraulic system that’s actually been sized correctly. A SWPPP’s temporary drainage measures have to work with, not against, how the site will ultimately convey water once construction is finished. Stormwater management is the calculation and system design that makes both of those documents technically defensible rather than just administratively complete.
What stormwater management actually involves
At its core, this work answers a hydrology and hydraulics question: given a design storm of a specified intensity and duration, how much runoff will this site generate, and where does it need to go without causing a problem. That starts with hydrologic modeling calculating peak flows and runoff volumes for pre- and post-development conditions, using the specific methodology a jurisdiction requires. From there, the engineering shifts to hydraulic design: sizing detention or retention basins, underground storage systems, infiltration facilities, and the pipes, channels or swales that convey water between them and to an approved point of discharge.
Many jurisdictions require post-development runoff to match or improve on pre-development conditions meaning the site can’t simply pave over permeable ground and send the same volume of water downstream faster than it used to arrive. That requirement is usually what drives the need for detention or retention infrastructure in the first place, sized specifically to hold back the difference between pre- and post-project peak flows.
Detention, retention, and infiltration different tools for different sites
These terms get used loosely, but they solve different problems. Detention systems temporarily hold runoff and release it slowly, reducing peak flow without necessarily reducing total volume. Retention systems hold water longer, sometimes permanently, often relying on evaporation or infiltration to dispose of it rather than a downstream release. Infiltration systems return water to the ground directly, which can satisfy both flow control and water quality goals at once, but only where soils and groundwater depth actually support it. Choosing between them is a site-specific decision soil permeability, available space, groundwater depth and downstream capacity all narrow the realistic options before design work goes further.
How this connects to the rest of the project
Stormwater management sits at the intersection of grading, drainage, and water quality design in a way none of the other documents quite do. The system has to be sized around finished grades set during precise grading, has to physically fit within the drainage layout already established, and often shares infrastructure directly with WQMP treatment measures a single detention basin, for example, might serve both flow control and water quality functions if designed correctly from the start. Treating stormwater management as a separate calculation performed after grading and WQMP work is finalized is one of the more common sources of late-stage redesign we see on projects that started elsewhere.
Agency review in California
Stormwater management design is reviewed against a jurisdiction’s specific hydrology manual and design storm criteria, which vary meaningfully across the state a coastal city with an aging storm drain system may impose stricter detention requirements than an inland jurisdiction with newer infrastructure. Some agencies require formal downstream capacity analysis showing that a proposed discharge won’t overwhelm existing storm drains; others focus review more narrowly on the site’s own detention sizing. Regional flood control districts sometimes add a separate layer of review on top of city requirements, particularly for larger sites or those near a mapped floodplain.
When you need this
Any project large enough to meaningfully change how much runoff leaves a site new construction, significant additions, and most commercial or multi-unit residential development needs stormwater management design as part of the civil package. Smaller residential projects may need only a simplified version of this analysis, but sites near capacity-constrained storm drains, floodplains, or with limited infiltration capacity often need more detailed hydraulic modeling regardless of overall project size.
- Common Questions
Frequently asked questions
Stormwater management addresses the hydraulic engineering how much runoff a site generates and how it's conveyed, detained or infiltrated. A WQMP addresses pollutant treatment specifically. The two are closely related and often share infrastructure, but they answer different technical questions and are reviewed against different criteria.
Detention isn't about whether water currently drains away it's about whether the rate and volume of runoff leaving the site after development matches what the downstream system and jurisdiction allow. Most agencies require post-development flows to match or improve on pre-development conditions, regardless of how the site currently performs.
It comes down to site conditions soil permeability, groundwater depth, available space, and downstream capacity. Infiltration is often preferred where soils support it because it addresses both flow control and water quality, but it isn't feasible everywhere, and the site itself usually narrows the realistic options quickly.
Yes. The system has to fit within finished grades and the drainage layout already established, and is most efficient to design alongside grading rather than as a separate calculation performed afterward.
No. Each jurisdiction sets its own hydrology manual and design storm criteria, and some regional flood control districts add additional review on top of city requirements. We confirm the applicable standard for the specific jurisdiction before beginning design.
Where we start
We begin with pre- and post-development hydrologic modeling using the methodology the jurisdiction requires, then size a conveyance and storage system that fits the site’s grading, soils, and downstream constraints. Send us the project address and site plan, and we’ll confirm the applicable design storm criteria and hydrology requirements for your jurisdiction, along with a fee and schedule in writing.
2102 Business Center Drive, Suite 130, Irvine, CA 92612