Stormwater design moves rainfall safely off sites through gutters, pipes, and channels while meeting regulations and flood risk.

Scope (stormwater & drainage)

This page covers conveyance of rainfall runoff from roofs, pavements, and catchments through gutters, pipes, and channels, using Manning resistance for open reaches; it also situates detention, infiltration, and water-quality treatment as part of modern stormwater management. In South Africa, projects must align with NEMA and water-resource authorisations where applicable, municipal stormwater by-laws, and road authority manuals such as SANRAL for linear drainage. Use the approved hydrology, land-use plan, and outfall constraints—this page supports hydraulic checks, not a full stormwater management plan on its own.

Stormwater conveyance (concept)

  1. Obtain design peak flow per subcatchment (rational method, SWMM, or regional equation).
  2. Route flow through pits and pipes; check capacity and freeboard in channels.
  3. For open channels or partially full pipes modelled as open flow, use Manning with appropriate n and slope S.
  4. Add detention/WQ per policy; use drainage modelling software for networks, storage routing, and water-quality modules.

Hydraulics & Hydrology — Stormwater & Drainage Design

Move runoff safely off-site: collection, conveyance, detention, and protection of receiving waters.

Introduction

Stormwater systems limit flooding and erosion by collecting runoff and conveying it to legal outfalls. Open channels and many swales use the Manning equation to relate discharge to cross-section, roughness, and slope. Water-sensitive design adds detention, infiltration, and treatment before discharge to protect rivers and estuaries.

The calculator below gives Q from assumed A, R, n, and S—typical for a channel or a simplified pipe reach; full part-full pipe hydraulics often needs charts or software.

Diagram of stormwater runoff from urban surfaces to storm drains and receiving water
Urban stormwater path (roofs and pavements to drains). See licence on Wikimedia Commons
Diagram of pollutants washed into storm drains with runoff
Runoff and pollutant pathways to storm drains. See licence on Wikimedia Commons

What stormwater design produces

  • Pipe and channel sizes for design storms and allowable spread in roads.
  • Storage volumes and outlet hydraulics for detention basins and tanks.
  • Energy dissipation and erosion control at outfalls and vortex drops.
  • Documentation for permits and environmental management plans.

Standards (South Africa)

Follow the municipal stormwater policy and any catchment management framework. Roads: SANRAL drainage requirements where the works are on the national network. Water quality and receiving-water protection: NEMA and DWS guidance as applicable. Coordinate with the geotechnical report for infiltration and pavement drainage.

Manning (SI):   Q = (1/n) A R2/3 S1/2

Notation (common)

  • n — Manning roughness (concrete, grass, natural channel tables).
  • A — flow area (m²); R — hydraulic radius A/P (m).
  • S — friction or bed slope (energy grade for uniform flow).
  • Q — discharge (m³/s).

Examples in practice

  • Residential estate: trunk stormwater line and on-lot swales to wetland buffer.
  • Highway median: grated collection and discharge to lined channel.
  • Commercial roof: gutter sizing and downpipe connection to soakaway or network.
  • Industrial site: oil–grit separators and spill containment upstream of outfall.

Calculator — Manning discharge

Uniform open-channel or equivalent reach; n, S, R, and A must be positive. Uses Q = (1/n) A R2/3 S1/2.

Q = (1/n) A R2/3 S1/2

Discharge

Manning discharge Q = (1/n) A R2/3 S1/2 for a uniform open-channel section.

Key terms

Design storm
Rainfall intensity–duration–frequency for the selected return period and duration.
WQ treatment train
Sequence of screens, ponds, wetlands, or filters before outfall.

Software and stormwater modelling

Engineers model networks, storage, and sometimes water quality with event or continuous simulation. Examples (official sites):

  • EPA SWMM — urban runoff, drainage networks, and storage/treatment units.
  • Autodesk Civil 3D — catchments, pipe networks, and grading.
  • OpenFlows Storm — StormCAD and related stormwater system modelling (Bentley).
  • DHI MIKE — integrated urban and catchment tools where specified.
  • Autodesk InfoWorks ICM — integrated catchment and drainage network modelling.

No product endorsement—match software capabilities to the project's hydrology memo, WQ targets, and approval process. The Manning calculator is a single-section capacity check.

Diagram sources

Educational schematics. Files in Images/stormwater-drainage-design/ were downloaded from Wikimedia Commons into this repo (not copied from other topic folders). Confirm licence on each Commons file page before reuse.