A stormwater management plan (SWMP) is part of the civil engineering drawings and describes how a site manages rainfall runoff during and after construction. It addresses water quantity (preventing flooding), water quality (removing pollutants), and erosion control (preventing sediment from leaving the site). These plans are required by the EPA's Clean Water Act, state NPDES (National Pollutant Discharge Elimination System) permits, and local stormwater ordinances.
Step 1: Understand the Drainage Area Map
The drainage area map divides the site into sub-watersheds, areas that drain to a common collection point. Each area is labeled (e.g., DA-1, DA-2) with its total area and percentage of impervious coverage.
- Watershed boundaries, shown as dashed or colored lines following ridgelines and high points
- Flow arrows, indicate drainage direction based on grading contours
- Collection points, where runoff concentrates: inlets, swales, basins, outfalls
- Off-site drainage, runoff entering the site from adjacent properties must be accounted for
- Time of concentration (Tc), the time for runoff to travel from the most remote point to the outlet
- Curve Number (CN), soil/cover condition index used in the SCS runoff method (higher = more runoff)
Step 2: Read Pre vs. Post-Development Conditions
The stormwater report compares runoff before and after development. Construction increases impervious surfaces (roofs, pavement), which increases runoff volume and peak flow rates.
Pre-Development
Existing ground cover, natural infiltration rates, existing drainage patterns, undeveloped curve numbers
Post-Development
Proposed impervious area, modified drainage patterns, increased runoff volume, need for stormwater BMPs
Design Storms
1-year, 2-year, 10-year, 25-year, 100-year storms, each with specific rainfall intensity and duration
Regulatory Standard
Post-development peak flow must not exceed pre-development peak flow for required design storms
Key Principle
The goal is "no adverse impact", development should not increase flooding risk to downstream properties. BMPs are sized to store or infiltrate the difference between post-development and pre-development runoff.
Step 3: Identify BMPs (Best Management Practices)
BMPs are engineered features that manage stormwater quantity and/or quality. The type of BMP depends on the site's soils, space constraints, regulatory requirements, and maintenance capabilities.
Detention Basin (Dry)
Quantity ControlAn excavated or bermed basin that temporarily stores runoff during storms and slowly releases it through a controlled outlet. Basin is dry between storm events.
Sizing: Sized to attenuate post-development peak flow to pre-development levels for specified design storms
Maintenance: Mowing, outlet clearing, sediment removal every 5–10 years
Retention Pond (Wet)
Quality + QuantityA permanent pool of water that receives and treats stormwater through sedimentation, biological uptake, and extended detention above the permanent pool.
Sizing: Permanent pool volume typically equals mean storm runoff volume; extended detention above pool for quantity control
Maintenance: Aquatic plant management, sediment removal, inlet/outlet maintenance
Bioswale
Quality + ConveyanceA vegetated channel designed to slow runoff, filter pollutants through soil and vegetation, and convey stormwater. Typically trapezoidal or parabolic cross-section.
Sizing: Designed for water quality volume at shallow depth (typically 6"–12") with flow velocity under 1 fps
Maintenance: Mowing, re-planting, sediment removal from forebay, underdrain inspection
Rain Garden / Bioretention
Quality + InfiltrationA shallow planted depression with engineered soil media that filters stormwater through soil layers. Underdrain connects to storm system when infiltration is insufficient.
Sizing: Typically sized for water quality volume, first 1" of rainfall from contributing impervious area
Maintenance: Plant care, mulch replacement, soil amendment, underdrain flushing
Permeable Pavement
Quality + InfiltrationPorous asphalt, pervious concrete, or permeable interlocking pavers over an aggregate storage reservoir that allows rainfall to infiltrate through the surface.
Sizing: Reservoir depth sized for water quality volume and/or quantity detention; infiltration rate must exceed design rainfall intensity
Maintenance: Vacuum sweeping 2–4 times per year, joint material replacement (pavers), avoid sand application
Underground Detention
Quantity ControlSubsurface chambers, pipes, or vaults that temporarily store runoff below grade. Used where surface area is constrained (urban sites, parking areas).
Sizing: Sized to match surface detention volume requirements; controlled outlet restricts release rate
Maintenance: Sediment removal, structural inspection, outlet clearing, requires confined space entry
Green Roof
Quality + RetentionVegetated roof assembly that retains rainfall in soil media and plants, reducing and delaying runoff from the building footprint.
Sizing: Extensive (2"–6" media) retains 0.5"–1.5" of rainfall; intensive (6"+ media) retains more
Maintenance: Plant care, irrigation during establishment, drain inspection, media replacement
Step 4: Check Pipe Sizing
Storm pipes are sized using Manning's equation based on the calculated peak flow from each drainage area. Pipe sizes, slopes, and materials are shown on the storm drainage plan and profile sheets.
Use
Building downspout connections, parking lot leads
Use
Small drainage area collectors, parking lot mains
Use
Site collector pipes, moderate drainage areas
Use
Major site mains, trunk lines
Use
Large trunk lines, outfall pipes
Use
Major outfall pipes, regional systems
Step 5: Review Outfall Locations
Outfalls are the points where stormwater exits the site. Each outfall must comply with NPDES permit requirements for flow rate, water quality, and discharge limitations.
- Outfall location, typically discharges to an existing creek, ditch, storm sewer, or regional system
- Energy dissipation, riprap, stilling basins, or level spreaders to prevent downstream erosion at discharge point
- Water quality treatment, runoff must be treated (through BMPs) before reaching the outfall
- Flow restriction, outlet control structures limit discharge rate to pre-development levels
- Permit requirements, NPDES permit may require monitoring, sampling, or specific pollutant limits at outfalls
Step 6: Verify Erosion Control Measures
The SWPPP (Stormwater Pollution Prevention Plan) shows erosion and sediment control measures required during construction. These are temporary measures that prevent sediment from leaving the site while permanent BMPs are being built.
Silt Fence
Perimeter ControlGeotextile fabric supported by posts, installed along the downslope perimeter of disturbed areas to filter sediment from sheet flow.
Placement: Along contours at base of slopes, around stockpiles, along waterways
Construction Entrance
Tracking ControlStabilized pad of crushed stone at all site access points to remove mud from vehicle tires before entering public roads.
Placement: Every point where construction traffic enters/exits public roads
Inlet Protection
Inlet ControlFilter fabric, gravel bags, or manufactured devices placed around storm drain inlets to prevent sediment from entering the storm system.
Placement: All active storm drain inlets within or downstream of disturbed areas
Sediment Basin
Basin ControlTemporary basin that detains sediment-laden runoff long enough for particles to settle. Usually converted to permanent BMP after construction.
Placement: Low points receiving runoff from disturbed areas > 10 acres typically
Erosion Control Blanket (ECB)
Surface StabilizationBiodegradable or synthetic blanket staked over seeded slopes to prevent erosion and promote vegetation establishment.
Placement: Slopes steeper than 3H:1V, channels, swale bottoms
Check Dams
Channel ControlSmall temporary dams of stone, logs, or sandbags placed in drainage channels to slow flow velocity and trap sediment.
Placement: Temporary channels, swales, concentrated flow paths
Temporary Seeding
Surface StabilizationFast-growing grass seed applied to disturbed areas that will remain inactive for 14+ days. Provides vegetative cover quickly.
Placement: All disturbed areas not actively being worked for 14+ days
Sources
EPA, NPDES Stormwater Program
ASCE/EWRI, Urban Stormwater Management Manual
USDA NRCS, TR-55: Urban Hydrology for Small Watersheds
State and Local Stormwater Management Regulations
