Table of Contents
1. Introduction to Stormwater Drainage
Stormwater drainage systems are the backbone of urban infrastructure, designed to collect, convey, and discharge rainfall runoff from developed areas. As urbanization increases impervious surfaces, natural infiltration diminishes and runoff volumes and peak flows rise dramatically. A well-designed drainage system protects property, prevents flooding, maintains traffic safety, and minimises environmental impact.
Modern stormwater design integrates two fundamental components: hydrologic analysis โ determining how much runoff the system must handle โ and hydraulic design โ sizing the conveyance elements (pipes, channels, inlets) to carry that runoff. Increasingly, designers also incorporate water quality treatment and green infrastructure to meet regulatory requirements for stormwater management.
This guide presents a systematic approach to stormwater drainage design, covering rainfall analysis, runoff estimation, conveyance sizing, detention design, and low-impact development techniques. A worked example demonstrates the complete design process for a residential subdivision. Related resources include the Stormwater Runoff Calculator and Manning's Equation Calculator for automated design computations.
2. Hydrologic Analysis
Hydrologic analysis quantifies the runoff that a drainage system must convey. Three methods dominate practice: the Rational Method for small urban catchments, the SCS TR-55 (now NRCS) method for larger watersheds, and IDF-based approaches for design storm selection.
Rational Method
The Rational Method, first published by Emil Kuichling in 1889, remains the most widely used approach for drainage design in urban catchments under 80 hectares (200 acres). The fundamental equation is:
Where Q is the peak runoff rate (mยณ/s or ftยณ/s), C is the dimensionless runoff coefficient (0 to 1), I is the rainfall intensity (mm/h or in/h) for a duration equal to the time of concentration, and A is the catchment area (ha or acres). The method assumes that peak runoff occurs when the entire catchment contributes, which happens when the rainfall duration equals the time of concentration.
Limitations of the Rational Method: The Rational Method does not account for rainfall distribution, infiltration variability, or storage effects within the catchment. It is suitable only for small, uniformly developed areas. Composite C values for heterogeneous catchments are area-weighted. For larger watersheds or where hydrograph timing matters, use the SCS TR-55 method or a hydrologic model such as SWMM, HEC-HMS, or CivilFlow's Stormwater Runoff Calculator.
SCS TR-55 (NRCS Method)
The Soil Conservation Service TR-55 method (now NRCS) uses the Curve Number (CN) approach to estimate runoff volume from rainfall depth. The CN is a function of soil type, land use, and antecedent moisture condition. Runoff depth Qd = (P - Ia)ยฒ / (P - Ia + S), where P is rainfall depth, Ia = 0.2S is initial abstraction, and S = (25400/CN) - 254 (metric, in mm). The method produces a full runoff hydrograph using a dimensionless unit hydrograph, enabling storage design and detention routing.
Design Storm Frequency
Design storms are selected based on the criticality of the facility. Typical return periods: 2-year for residential street drainage, 5-year for major arterial roads, 10-year for stormwater trunk mains, and 100-year for flood protection of buildings. Many jurisdictions require dual-level design: minor system (5-10 year) for frequent events and major system (100-year overland flow) for extreme events. These concepts are covered further in the Hydraulics Learning Hub.
3. Rainfall Intensity-Duration-Frequency (IDF) Relationships
IDF curves relate rainfall intensity, duration, and return period (frequency). They are derived from statistical analysis of historical rainfall records and published by national meteorological agencies (NOAA Atlas 14 in the US, BoM IFD in Australia, Environment Agency in the UK). The general form of an IDF equation is:
Where I is intensity (mm/h), t is duration (min), and a, b, c are location-specific coefficients obtained from regression of local rainfall data. The critical duration for Rational Method design equals the time of concentration of the catchment. For a 10-minute duration, 10-year return period, typical intensities range from 80-120 mm/h in temperate climates to over 200 mm/h in tropical regions.
Designers must obtain local IDF data from the relevant authority. When site-specific IDF data is unavailable, regional IDF equations from nearby stations or synthetic IDF curves from published studies may be used. The Stormwater Runoff Calculator includes built-in IDF parameters for common US and international locations.
Always verify that the IDF curve applies to the design return period and rainfall duration range (typically 5 min to 24 h). Extrapolation beyond the range of observed data introduces significant uncertainty and should be validated against regional guidelines. See the Engineering Standards page for jurisdictional rainfall requirements.
4. Runoff Coefficient Determination
The runoff coefficient C represents the fraction of rainfall that becomes surface runoff. It depends on land use, soil type, slope, and antecedent moisture. For composite catchments, a weighted C value is computed by area.
Table 1: Runoff Coefficients for Various Land Uses
| Land Use | Soil Type / Slope | C (2-yr) | C (10-yr) | C (100-yr) |
|---|---|---|---|---|
| Pavement (asphalt/concrete) | Impervious | 0.90 | 0.93 | 0.96 |
| Roofs | Impervious | 0.85 | 0.88 | 0.92 |
| Lawns (sandy soil, flat < 2%) | Pervious / A | 0.10 | 0.15 | 0.22 |
| Lawns (clay soil, steep > 7%) | Pervious / D | 0.35 | 0.42 | 0.50 |
| Industrial / Commercial | 60-90% Impervious | 0.70 | 0.75 | 0.82 |
| Residential (suburban, 1/4 ac lots) | ~40% Impervious | 0.40 | 0.46 | 0.55 |
| Forest / Woodland | Pervious | 0.05 | 0.10 | 0.20 |
Values are for 24-hour antecedent moisture condition II (average). For AMC III (wet condition), increase by 10-20%. For flat slopes (less than 2%), decrease by 5-10%. These coefficients are used in the Rational Method via the Stormwater Runoff Calculator.
5. Time of Concentration Calculation
Time of concentration tc is the time required for runoff to travel from the hydraulically most distant point of the catchment to the design point. It governs the design rainfall duration for the Rational Method and directly affects peak runoff magnitude. Three widely used methods are presented below.
Kirpich Equation
Developed for small agricultural watersheds, the Kirpich formula remains common for initial estimates:
Where tc is in minutes, L is the flow length in metres, and S is the dimensionless slope (m/m). The equation assumes uniform slope and cleared conditions. For paved surfaces, multiply by 0.4; for concrete channels, multiply by 0.2.
Manning's Kinematic Wave
For overland flow on planar surfaces, the kinematic wave equation computes tc directly from surface roughness, length, slope, and rainfall intensity:
Where n is Manning's roughness for overland flow (0.011 for smooth pavement, 0.15 for turf, 0.40 for dense forest), L is flow length in metres, S is slope (m/m), and I is rainfall intensity (mm/h).
NRCS (SCS) Lag Method
The NRCS method relates tc to the watershed lag time tl = 0.6tc where tl is computed as:
Where L is the longest flow path in feet, S is the Curve Number retention parameter (inches), and Y is the average watershed slope (%). This method is preferred for NRCS TR-55 hydrology and integrates directly with CN-based runoff estimation.
In practice, tc is rarely less than 5 minutes for paved areas or 10 minutes for pervious areas. Many authorities enforce minimum tc values. The Stormwater Runoff Calculator implements all three methods and automatically selects the governing value.
6. Stormwater Pipe Sizing
Stormwater pipes are designed using Manning's equation for steady uniform flow in closed conduits flowing full (or partial flow for surcharge analysis). The governing equation for full-pipe capacity is:
Where Q is discharge (mยณ/s), n is Manning's roughness coefficient, A is cross-sectional area (mยฒ), R is hydraulic radius (A/P, where P is wetted perimeter in m), and S is the friction slope (m/m, typically equal to pipe gradient for uniform flow).
Table 2: Manning's n Values for Stormwater Pipes
| Pipe Material | Manning's n | Typical Application |
|---|---|---|
| PVC / HDPE (smooth) | 0.009 โ 0.011 | Storm drains, culverts |
| Concrete (centrifugally spun) | 0.011 โ 0.013 | Trunk stormwater mains |
| Concrete (cast-in-place) | 0.013 โ 0.017 | Box culverts, large pipes |
| Corrugated metal (CMP) | 0.020 โ 0.025 | Culverts, detention outlets |
| Ductile iron (cement lined) | 0.010 โ 0.012 | High-pressure stormwater |
| Vitrified clay (VCP) | 0.011 โ 0.015 | Sanitary/storm combo |
Minimum Velocity and Self-Cleansing: Stormwater pipes must maintain a minimum velocity of 0.75 m/s (2.5 ft/s) at design flow to prevent sediment deposition and a maximum velocity of 4.5 m/s (15 ft/s) to prevent erosion and scour. In practice, a minimum grade of 0.5% for small pipes (โค 300 mm) and 0.3% for larger pipes is typical, subject to local code requirements. The Manning's Equation Calculator can verify velocity conditions for any pipe size and slope.
For partial-flow analysis (pipes flowing less than full), the hydraulic elements chart provides proportional discharge and velocity ratios. At 80% full depth, a circular pipe conveys approximately 96% of its full-flow capacity while maintaining higher velocity. Surcharge analysis under extreme events considers the head available at upstream manholes. Advanced pipe network analysis requires hydraulic grade line (HGL) computation, which is implemented in the Hazen-Williams Calculator for pressure systems and Manning's equation for gravity systems.
7. Open Channel Design for Drainage
Open channels โ roadside ditches, swales, lined channels, and diversion channels โ are designed using Manning's equation for steady uniform flow. The same equation applies as for pipes, but with the cross-section defined by channel geometry (trapezoidal, triangular, rectangular, parabolic).
Key design parameters for channels: freeboard (typically 0.3 m minimum above design water surface), side slopes (2:1 or 3:1 H:V for grass-lined channels, 1:1 for lined channels), longitudinal slope (matching natural grade where possible), and erosion protection (lining, riprap, or turf reinforcement for velocities exceeding 1.5 m/s on erodible soils).
The critical design check is the Froude number Fr = V / โ(g ร D), where V is mean velocity, g is gravity, and D is hydraulic depth (A/T, top width). Subcritical flow (Fr < 1) is preferred for drainage channels to avoid hydraulic jumps and erosion. Supercritical flow (Fr > 1) is acceptable in lined channels with energy dissipation at outlets. The Manning's Equation Calculator provides channel sizing for common geometries.
8. Inlet and Catch Basin Design
Inlets are the points of entry for stormwater into the drainage system. Types include curb-opening inlets, grate inlets, combination inlets, and slotted drains. Inlet spacing and sizing must limit the width of flow on the roadway (spread) to acceptable levels โ typically 3 m (10 ft) for arterial roads and 6 m (20 ft) for local streets during the design storm.
Curb-opening inlet capacity depends on approach flow depth, weir length, and clogging factor. For unsubmerged conditions (depth less than opening height), flow behaves as a sharp-crested weir: Qi = Cw ร L ร d1.5, where Cw โ 1.66 (SI), L is opening length, and d is flow depth at the curb. For submerged conditions, orifice flow governs. The FHWA HEC-22 manual provides comprehensive design charts and equations for all inlet types.
Clogging Factors: Inlet capacity is significantly reduced by debris accumulation. Standard practice applies a clogging factor of 50% for grate inlets in sag locations and 25% for curb-opening inlets on grade. FHWA HEC-22 recommends a minimum of two inlets at all sag points to provide redundancy. The Weir Flow Calculator can be used to model curb-opening inlet capacity at various approach depths.
Grate inlets are sized by analysis of frontal flow interception and side flow interception. The interception efficiency depends on grate geometry, bar spacing, approach velocity, and cross-slope. Bicycle-safe grates with narrow bar spacing (max 32 mm parallel to traffic) are required in most jurisdictions. Catch basins (manholes with sump) provide sediment trapping and should be designed with a minimum 0.6 m sump depth and cleanout access.
9. Detention Basin Design
Detention basins temporarily store stormwater runoff and release it at controlled rates to mitigate the adverse effects of urbanization. The fundamental design objective is to limit post-development peak discharge to pre-development levels for a range of design storms (typically 2-year, 10-year, and 100-year events).
Storage volume is determined by hydrograph routing. The modified Puls method (storage-indication method) is the standard approach:
Where S is storage volume, Q is outflow, I is inflow, ฮt is the routing time step, and subscripts 1 and 2 indicate successive time steps. The storage-outflow relationship is derived from basin geometry and outlet control structure hydraulics.
Detention Basin Design Considerations: (1) Outlet control โ typically a combination of low-flow orifice for water quality capture and a high-flow overflow weir or riser for peak attenuation. (2) Freeboard โ minimum 0.3 m above the design water surface for emergency spillway. (3) Drawdown time โ most jurisdictions require full drawdown within 24-72 hours. (4) Maintenance access โ provide vehicle access for sediment removal and vegetation management. (5) Safety โ incorporate fencing, warning signs, or bench-terraced side slopes (max 4:1) for public safety.
The outlet configuration typically includes a primary orifice (sized for the allowable release rate) and a secondary overflow weir (handling extreme events). Low-flow orifices are sized using: Q = Cd ร A ร โ(2 ร g ร H), where Cd โ 0.62, A is orifice area, and H is head above orifice centreline. The Weir Flow Calculator aids in outlet structure design.
Preliminary detention volume can be estimated using simplified methods: the Rational Method hydrograph approach (Qpeak ร D ร 0.5 for approximate triangular hydrograph), or the NRCS TR-55 detention volume curves (Figure 6-1). Detailed routing requires a hydrologic model such as SWMM, PondPack, or the routing tools within CivilFlow's calculator suite.
10. Stormwater Quality
Stormwater runoff carries pollutants from urban surfaces โ sediment, nutrients, heavy metals, hydrocarbons, pathogens, and trash. Best Management Practices (BMPs) are designed to treat runoff before discharge to receiving waters. Common quality objectives: 80% total suspended solids (TSS) removal, 50% total phosphorus removal, and capture of the water quality volume (typically the first 25 mm or 1 inch of runoff, known as the "first flush").
Treatment BMPs include sedimentation basins, vegetated filter strips, grassed swales, bioretention cells, sand filters, and manufactured treatment devices. Low-impact development (LID) integrates these practices into the site layout to treat runoff at its source. LID principles emphasise preserving natural drainage patterns, minimising impervious surfaces, and distributed treatment rather than end-of-pipe solutions.
Water quality design volume is typically calculated as: Vwq = P ร A ร Rv, where P is the water quality rainfall depth (e.g., 25 mm), A is the contributing area, and Rv is the volumetric runoff coefficient (Rv = 0.05 + 0.009 ร I, where I is the percent impervious). This volume is routed through the treatment BMP at a defined surface loading rate (typically 50-200 mm/h for bioretention).
11. Green Infrastructure
Green infrastructure uses natural processes to manage stormwater at its source. Three widely adopted systems are rain gardens, permeable pavement, and bioswales.
Rain Gardens
Rain gardens are shallow, vegetated depressions that collect and infiltrate runoff from roofs, driveways, and lawns. Design depth is typically 150-300 mm with an engineered soil mix (sand:compost:topsoil, 50:30:20 by volume). The surface area is sized for the water quality volume: Agarden = Vwq / (dg ร ng), where dg is ponding depth and ng is the porosity of the soil media (โ0.4). Underdrains are provided when native soil infiltration rates are below 13 mm/h.
Permeable Pavement
Permeable pavement systems (permeable interlocking concrete pavers, porous asphalt, pervious concrete) allow rainfall to infiltrate through the surface into a stone reservoir below. The reservoir provides temporary storage and gradual infiltration into the subgrade or release via underdrains. Design parameters include surface infiltration rate (typically 250-1000 mm/h), reservoir void space (30-40% for clean stone), and structural capacity for traffic loading.
Bioswales
Bioswales are linear, vegetated conveyance systems that provide both treatment and runoff reduction. Unlike standard grass swales, bioswales have an engineered soil media, optional underdrain, and check dams for increased residence time. The longitudinal slope is typically 0.5-4% with check dams to prevent erosion. The water quality residence time should exceed 10 minutes for effective sedimentation and filtration.
Green infrastructure systems require regular maintenance: vegetation management, sediment removal, inlet cleaning, and replacement of soil media at 10-15 year intervals. The Civil Engineering Handbook provides detailed design guidelines for LID systems, and the Engineering Glossary defines key terminology used in stormwater quality management.
12. Drainage System Layout and Grading
System layout begins with delineating the watershed boundary and establishing drainage divides. The minor system (piped network) is laid out along streets and easements, with pipes at minimum cover (typically 0.9-1.2 m to allow for frost protection and utility crossings). Manholes are placed at changes in direction, grade, pipe size, and at junctions โ maximum spacing 120 m (400 ft) for pipes up to 900 mm, and 180 m for larger pipes.
Grading design establishes surface elevations that direct runoff to inlets and prevent ponding. Minimum grades: 1% for paved surfaces, 2% for grass areas, and cross-slopes of 2-4% on roadways to direct flow to curb inlets. Building finished floor elevations must be a minimum of 300 mm above the 100-year flood elevation and 150 mm above the adjacent gutter grade.
The major system (overland flow path) is a critical component often neglected in design. For storms exceeding the minor system capacity, runoff flows along streets and designated overland flow routes to a safe discharge point (creek, detention basin, or regional drainage channel). Buildings must be protected against the 100-year overland flow with adequate freeboard. The Hydraulics Learning Hub provides further guidance on dual-drainage system design and flood routing.
13. Worked Example
Hydrologic and Hydraulic Design of a Subdivision Drainage System
Given: A 6-hectare residential subdivision (40% impervious) in Atlanta, GA. Site slopes average 2%. Sandy clay loam soil (Hydrologic Soil Group C). Tc = 12 min. Design storm: 10-year ARI. IDF coefficients: a = 720, b = 12, c = 0.8 (I = a / (t + b)^c). Allowable post-development release rate: 0.12 mยณ/s.
Step 1 โ Runoff Coefficient: Impervious C10 = 0.88 (weighted pervious C10 = 0.35). Composite: C = 0.40 ร 0.88 + 0.60 ร 0.35 = 0.562.
Step 2 โ Rainfall Intensity: I = 720 / (12 + 12)^0.8 = 720 / 24^0.8 = 720 / 12.46 = 57.8 mm/h.
Step 3 โ Peak Runoff (Rational Method): Q = 0.562 ร 57.8 ร 60 / 360 = 0.562 ร 57.8 ร 0.1667 = 5.41 mยณ/s. Note: conversion factor 1 ha ร 1 mm/h = 0.00278 mยณ/s.
Step 4 โ Pre-development Runoff: Forested C10 = 0.15. Qpre = 0.15 ร 57.8 ร 0.1667 = 1.45 mยณ/s. Required storage to limit post-development to Qpre = 0.12 mยณ/s is based on allowable release rate (limited by downstream capacity). Required attenuation ratio = Qp / Qa โ 5.41 / 0.12 = 45.1.
Step 5 โ Detention Volume (Triangular Hydrograph Method): Approximate storage: Vs = 0.5 ร tc ร (Qpost - Qallow) ร 60. Vs = 0.5 ร 12 ร (5.41 - 0.12) ร 60 = 6 ร 5.29 ร 60 = 1,904 mยณ. For detailed routing, use the modified Puls method (Section 9).
Step 6 โ Trunk Main Sizing (Manning's Equation): Design Q = 5.41 mยณ/s. Use reinforced concrete pipe (n = 0.013) at slope S = 0.005 m/m. Try 1200 mm diameter (A = 1.131 mยฒ, R = 0.30 m): Qfull = (1/0.013) ร 1.131 ร 0.30^(2/3) ร 0.005^(1/2) = 76.9 ร 1.131 ร 0.448 ร 0.0707 = 2.76 mยณ/s โ undersized. Try 1500 mm diameter (A = 1.767 mยฒ, R = 0.375 m): Qfull = 76.9 ร 1.767 ร 0.520 ร 0.0707 = 4.98 mยณ/s โ still slightly under (partial flow). Try 1650 mm diameter: Qfull = 76.9 ร 2.138 ร 0.549 ร 0.0707 = 6.39 mยณ/s. Check velocity: V = Q / A = 5.41 / 2.138 = 2.53 m/s at design flow (within 0.75-4.5 m/s). Use 1650 mm RCP at 0.5% grade.
Step 7 โ Outlet Structure: Low-flow orifice for water quality control: Qwq = 0.12 mยณ/s. Using Cd = 0.62, H = 1.5 m: A = Q / (Cd ร โ(2gH)) = 0.12 / (0.62 ร โ(2 ร 9.81 ร 1.5)) = 0.12 / (0.62 ร 5.42) = 0.0357 mยฒ. Orifice diameter = โ(4A/ฯ) = โ(0.0455) = 0.213 m โ 200 mm. Provide emergency overflow weir for 100-year event. Verify all inputs using the Stormwater Runoff Calculator and Manning's Equation Calculator.
14. Frequently Asked Questions
What is the difference between minor and major stormwater systems?
The minor system (piped network, inlets, small channels) is designed for frequent storms โ typically the 2- to 10-year return period. The major system (overland flow paths, streets, large channels) conveys runoff from extreme events (50- to 100-year) when the minor system is surcharged. Dual-drainage design ensures that even when the piped system is overwhelmed, floodwater follows designated safe pathways away from buildings.
When should I use the Rational Method versus SCS TR-55?
Use the Rational Method for small urban catchments (under 80 ha) where only peak discharge is needed. Use SCS TR-55 for larger watersheds, when a full hydrograph is needed (required for detention routing), or when the authority requires NRCS-compliant hydrology. The Rational Method is simpler but gives only peak flow; TR-55 provides runoff volume and timing.
What is the minimum pipe slope for stormwater drainage?
The minimum slope must provide a self-cleansing velocity of at least 0.75 m/s at design flow. For small pipes (โค 300 mm), a minimum slope of 0.5% (1:200) is typical. For larger pipes, 0.3% (1:333) may be acceptable. Steeper slopes may be needed for flat sites where grades are limited. Local codes often specify minimum slopes and should be consulted.
How do I estimate time of concentration for an undeveloped catchment?
For undeveloped catchments, use the NRCS lag method (Section 5) which incorporates CN and slope. The Kirpich equation provides reasonable estimates for natural catchments under 80 ha. The kinematic wave equation is best for overland flow on uniform slopes. Always verify tc against locally observed values or guideline minimums (often 10-15 minutes for undeveloped sites).
What is the water quality volume and how is it computed?
The water quality volume (WQV) is the runoff volume from the first 25 mm (1 inch) of rainfall, intended to capture the pollutant-rich first flush. It is computed as Vwq = P ร A ร Rv, where Rv = 0.05 + 0.009 ร I (I = % impervious). The WQV is then treated through BMPs such as bioretention, sand filters, or wet ponds with a minimum 80% TSS removal efficiency.
How does permeable pavement differ from traditional pavement hydrologically?
Permeable pavement allows rainfall to infiltrate through the surface into a stone reservoir, reducing or eliminating runoff from the paved area. Traditional pavement generates nearly 100% runoff (C โ 0.90-0.95), whereas permeable pavement can reduce the runoff coefficient to 0.2-0.4 depending on subgrade infiltration rate and reservoir depth. This significantly reduces peak runoff and provides on-site treatment.
What is the difference between detention and retention basins?
Detention basins temporarily store runoff and release it at a controlled rate โ they drain completely between storms. Retention basins (wet ponds) maintain a permanent pool of water and provide water quality treatment through sedimentation and biological uptake. Detention basins primarily address flood control; retention basins address both flood control and water quality. Extended-detention basins combine both functions with a 24-48 hour drawdown for water quality.
What standards govern stormwater drainage design?
Key international standards include: ASCE/EPA Stormwater Management Guidelines, FHWA HEC-22 (Urban Drainage Design Manual), NRCS TR-55 (Urban Hydrology for Small Watersheds), AASHTO Drainage Manual, BS EN 12056 (Gravity Drainage Systems Inside Buildings), IS 1742 (Rainwater Roof Drainage), and IS 12386 (Code of Practice for Storm Water Drainage). Local municipal codes and stormwater management ordinances may have additional requirements. See the Engineering Standards page for jurisdiction-specific references.
References & Standards
- ASCE/EPA. Stormwater Management: Hydrologic and Hydraulic Design. ASCE, 2023.
- FHWA HEC-22. Urban Drainage Design Manual. 3rd ed., US DOT, 2022.
- NRCS TR-55. Urban Hydrology for Small Watersheds. USDA, 1986 (revised 2024).
- Kuichling, E. "The Relation between the Rainfall and the Discharge of Sewers." Transactions of ASCE, Vol. 20, 1889.
- AASHTO. Drainage Manual. 2nd ed., AASHTO, 2023.
- BS EN 12056. Gravity Drainage Systems Inside Buildings. BSI, 2000.
- IS 1742. Code of Practice for Rainwater Roof Drainage. BIS, 1987.
- IS 12386. Code of Practice for Storm Water Drainage. BIS, 1994.
- Civil Engineering Handbook โ Hydraulics and Drainage chapters.
- Engineering Formula Library โ Hydrologic and hydraulic formulas.
- Engineering Standards โ Stormwater design standards by jurisdiction.
- Engineering Glossary โ Hydraulics and drainage terminology.