Stormwater Management Plan and Drainage Calculations
A stormwater management plan with runoff calculations, detention or infiltration design and erosion control, prepared to your local ordinance.
- Stamped by a PE licensed in your state
- Built to IBC, IRC and local amendments
- Plan-check comments answered
Handling the extra runoff your project creates
Every new roof, driveway, parking lot and building pad changes how rain behaves on a site. Water that used to soak into the ground now runs off faster and in greater volume. Local governments manage the effect on streets, streams and neighboring properties by requiring a stormwater management plan once a project adds enough impervious surface or disturbs enough land.
PE Engineer prepares stormwater management plans and reports for residential, commercial, industrial and agricultural projects. We calculate pre- and post-development runoff, design detention basins, underground storage, retention areas, infiltration trenches, dry wells and water quality features, and prepare the erosion and sediment control plan for construction. For sites that disturb one acre or more, we can prepare the Stormwater Pollution Prevention Plan required for construction permit coverage.
Every plan is designed to the ordinance, design manual and calculation methods your city, county or state requires, and is sealed by a Professional Engineer licensed in the state where the project is located.
What you receive
- Pre- and post-development runoff calculations
- Detention, retention or infiltration design
- Storm drain layout and pipe sizing
- Erosion and sediment control plan
- Stormwater report or SWPPP
- PE stamped plans and calculations
Stormwater features we design
We choose the stormwater approach that fits the site's soils, space, budget and the controls your jurisdiction accepts.
Detention basins and ponds
Dry basins and ponds that hold runoff temporarily and release it through an outlet structure at a controlled rate, sized to match pre-development peak flows.
Underground detention
Pipe, chamber and vault storage beneath parking lots and yards where surface space is limited, with inlets, outlet control and maintenance access designed in.
Retention and infiltration
Retention areas, infiltration trenches, dry wells and permeable pavement that return runoff to the ground, where soils, groundwater and local rules allow infiltration.
Water quality treatment
Bioretention, rain gardens, vegetated swales, filter strips and manufactured treatment devices that capture sediment and pollutants before water leaves the site.
Storm drain systems
Inlets, catch basins, pipes and outfalls sized for the design storm, with hydraulic grade checks and energy dissipation to prevent erosion at the outlet.
Construction erosion control
Silt fence, inlet protection, stabilized entrances, sediment traps and stabilization schedules that keep soil on site and in compliance during construction.
Your Stamped, Permit-Ready Plans Start With One Upload
Send your drawings or describe the project. A licensed engineer reviews it and sends a clear, fixed-fee proposal, often within hours.
- Free and no obligation
- Reviewed by a licensed PE
- Secure file upload
When a stormwater management plan is required
Stormwater requirements come from several levels of government, and a single project may trigger more than one. Most local stormwater ordinances set thresholds based on the amount of new or replaced impervious surface, the area of land disturbed, or the type of development. Above the threshold, the project must show that post-development runoff is controlled and, in many places, treated for water quality.
Federal Clean Water Act rules add a second layer. Construction activity that disturbs one acre or more, or that is part of a larger common plan of development that will disturb one acre or more, needs coverage under an NPDES construction stormwater permit. EPA issues the Construction General Permit in a limited number of areas, and most states run their own program with a state general permit. Either way, the permit requires a Stormwater Pollution Prevention Plan, known as a SWPPP.
Typical triggers include:
- New commercial buildings, warehouses and parking lots.
- Residential subdivisions and multi-lot developments.
- Large agricultural buildings, barns and equipment yards.
- Single homes on large lots in watersheds with strict local rules.
- Redevelopment that replaces existing pavement over a set threshold.
If your permit reviewer has asked for a stormwater report or drainage calculations, send us the comment. We prepare exactly what the jurisdiction’s checklist requires, and we respond to any follow-up comments and revise within the original scope at no extra charge.
How we prepare a stormwater management plan
A stormwater management plan starts with understanding how the site drains today. We identify drainage areas, flow paths, soil types and the point where water currently leaves the property, then calculate pre-development runoff for the design storms your jurisdiction specifies. Many agencies require analysis of several storm frequencies, such as a frequent water quality storm and larger flood control storms.
Next we model the proposed development with its new roofs, pavement and grading and calculate the increase in peak flow and volume. Common calculation methods include the Rational Method for small sites and the NRCS methods described in TR-55 for larger watersheds, along with local methods where a drainage manual requires them.
Then we design the controls:
- Size detention storage and the outlet structure so post-development peak flows do not exceed the allowable release rate.
- Size retention or infiltration features for the required volume where infiltration is allowed, based on soil infiltration rates.
- Select water quality treatment practices that meet the local standard for pollutant removal.
- Lay out storm drain inlets and pipes to carry flows to the controls and the outfall.
The final deliverable is a sealed plan set and a stormwater report with calculations, maps and an operation and maintenance plan, which many jurisdictions now require so the owner knows how to keep the system working.
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- Free and no obligation
- Reviewed by a licensed PE
- Secure file upload
Infiltration, soils and site constraints
Infiltration is often the most economical way to manage stormwater, because water returned to the ground does not need a large basin or outlet. It only works when the soil can absorb water at a reasonable rate and groundwater is deep enough. Sandy and gravelly soils often infiltrate well. Clay soils, shallow bedrock and high groundwater usually do not, and some jurisdictions prohibit infiltration near foundations, slopes, septic systems or contaminated soils.
Where infiltration is proposed, many agencies require field infiltration or percolation testing at the location and depth of the proposed feature. We specify the testing needed, work from the results, and apply the safety factors the local manual requires. When soils are poor, we design detention with a controlled release instead.
Space is the other common constraint. Small commercial lots may not have room for a surface pond, so we look at underground chambers, oversized pipes, permeable pavement or a combination. Each option has different costs for construction and maintenance, and we discuss those trade-offs early. The stormwater layout is drawn on the same base as the grading and drainage plan, so finished grades deliver water to each feature as designed.
Erosion control and SWPPPs during construction
Construction is when a site is most vulnerable. Bare soil washes into streets, storm drains and streams with every rain until the site is stabilized. Most jurisdictions require an erosion and sediment control plan for any project with significant earthwork, and the NPDES construction permit requires a full SWPPP for sites disturbing one acre or more.
Our erosion control plans show the practices needed at each phase of construction, such as:
- Perimeter controls like silt fence, compost socks or sediment barriers.
- Inlet protection for nearby storm drains.
- Stabilized construction entrances to keep mud off public roads.
- Sediment traps or basins for larger disturbed areas.
- Temporary and permanent seeding, mulching and slope stabilization.
A SWPPP adds the narrative and records the permit requires: site description, pollutant sources, control measures, inspection schedules and responsibilities. Inspections are typically required on a regular schedule and after significant storms, and the plan must be updated as the site changes. We prepare the SWPPP document and drawings, while the site operator is responsible for obtaining permit coverage, performing the required inspections and maintaining the controls. For industrial facilities that also need foundations or structures on the same site, our tank and skid foundation team can work from the same site plan.
You likely need this if
- New pavement or roofs exceed the local threshold
- The site disturbs one acre or more
- Your reviewer asked for a stormwater report
- You need a detention pond or underground storage
- The county requires an erosion control plan
- Runoff must be treated before it leaves the site
How it works, start to finish
From your first message to stamped drawings, you work directly with the engineering team.
- 1Share your projectSend the address, scope and any drawings, photos or plan-check comments. Two minutes is enough.
- 2Receive a fixed-fee proposalAn engineer reviews the scope and replies with a firm price and delivery date for your stormwater management drawings.
- 3Engineering and draftingWe run the calculations and prepare drawings to the codes your jurisdiction enforces, checked before sealing.
- 4Stamped set deliveredYou receive a sealed PDF set ready to submit, and we answer any plan-check comments within scope.
Regulations and methods we work to
Stormwater designs follow the ordinances, manuals and permits that apply to your site, including:
- Local stormwater management ordinances and drainage design manuals
- EPA NPDES Construction General Permit or your state’s construction stormwater general permit
- State stormwater and erosion control design standards where adopted
- NRCS TR-55 and the Rational Method for runoff calculations, as accepted locally
- International Building Code (IBC) and IRC site drainage provisions
How licensing works. Engineering drawings must be sealed by a Professional Engineer licensed in the state where your project is located. PE Engineer works with a network of state-licensed PEs, so every set is reviewed and stamped by an engineer with an active license in the state where your project is located. Their name and license number appear on the seal, exactly as your building department requires.
Related services
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Flood Elevation & FloodproofingElevated foundations, engineered flood openings, dry floodproofing design and FEMA floodproofing certificates for buildings in flood zones, sealed by a licensed PE.Explore More
Site & Plot PlansScaled site and plot plans showing property lines, setbacks, easements, lot coverage, utilities and drainage for residential and commercial permits.Explore MoreFrequently asked questions
What is a stormwater management plan?
A stormwater management plan shows how a development will control the increased runoff created by new roofs, pavement and grading. It includes runoff calculations, the design of detention, retention, infiltration or treatment features, storm drain layout and often an operation and maintenance plan, all prepared to the local ordinance.
When do I need a SWPPP?
A Stormwater Pollution Prevention Plan is required when construction disturbs one acre or more of land, or is part of a larger common plan of development that will disturb one acre or more, under the EPA Construction General Permit or the equivalent state permit. Some local agencies require one for smaller sites too.
What is the difference between detention and retention?
Detention holds runoff temporarily and releases it slowly through an outlet, so downstream peak flows do not increase. Retention holds water permanently or until it infiltrates or evaporates, with no regular surface outflow. The right choice depends on soils, groundwater, space and what your jurisdiction allows.
Do I need infiltration testing?
If the design relies on infiltration through dry wells, trenches, permeable pavement or retention areas, most agencies require field testing at the proposed location and depth. Testing confirms the soil can absorb water at the rate the design assumes. We specify the testing and design around the results.
Can you prepare stormwater plans for a single home?
Yes. Some jurisdictions require stormwater controls for single homes on large lots or in sensitive watersheds. These are often simple, such as dry wells, rain gardens or downspout dispersion, sized with straightforward calculations and shown on the grading plan. Most residential stormwater plans are delivered within a few business days.
Who maintains the stormwater system after construction?
The property owner usually does. Many jurisdictions require a recorded maintenance agreement and an operation and maintenance plan describing inspections and cleaning for basins, pipes and treatment devices. We prepare the maintenance plan with the design so the owner knows exactly what is needed.
Get your free stormwater management proposal
Tell us where the project is and what you need. Attach any drawings, photos or plan-check comments. A licensed engineer reviews every request.
- Upload your filesPlans, sketches, photos or comments
- Get a fixed-fee proposalUsually the same day
- Receive stamped drawingsSealed by a PE licensed in your state