Hydrology links rainfall on a catchment to runoff and peak flows for bridges, spillways, and storm drains.

Scope (rainfall & runoff)

This page covers catchment rainfall as the driver of surface runoff: intensity–duration–frequency (IDF) storms, simple rational-method peak flows, and the idea of more detailed rainfall–runoff models. In South Africa, design rainfall and regional methods are published by SAWS and DWS; roads and urban drainage often follow SANRAL and municipal by-laws. Use the approved IDF, catchment delineation, and loss model from the hydrology report—this page supports quick checks, not a substitute for a calibrated HEC-HMS or regional flood study.

Peak flow (concept)

  1. Delineate the catchment and area A; classify land use and impervious fraction.
  2. Select storm duration and return period; read rainfall intensity I from IDF curves (mm/h).
  3. Choose runoff coefficient C (or time–area / unit hydrograph in advanced methods).
  4. Apply Q = C I A / 360 (I in mm/h, A in ha → Q in m³/s); use catchment hydrology software for design storms, losses, and routing.

Hydraulics & Hydrology — Hydrology (Rainfall & Runoff)

Link rainfall over a catchment to peak flows and runoff volumes for drainage and flood design.

Introduction

Hydrology quantifies how precipitation becomes streamflow: losses to infiltration and evaporation, temporary storage on land and in channels, and routing to the catchment outlet. For small catchments, the rational method gives a peak discharge from rainfall intensity, area, and a runoff coefficient. Larger or regulated catchments use unit hydrographs, continuous simulation, or regional flood-frequency analysis.

The calculator below implements the common South African form Q = C I A / 360 with I in mm/h and A in hectares.

Diagram of the global water cycle: precipitation, runoff, evaporation, and storage
Water cycle (precipitation, runoff, storage). See licence on Wikimedia Commons
Schematic nonlinear reservoir model for rainfall-runoff
Nonlinear reservoir concept (rainfall–runoff modelling). See licence on Wikimedia Commons

What rainfall–runoff analysis produces

  • Design peak discharge for culverts, bridges, and stormwater infrastructure.
  • Runoff volume and hydrograph shape for detention basins and flood routing.
  • Inputs to 1D/2D hydraulic models (stage, extent, velocity).
  • Calibration targets when gauged streamflow or event data exist.

Standards and data (South Africa)

Design rainfall: use SAWS / DWS publications and regional IDF updates as specified by the client. Municipal stormwater and major roads: SANRAL and local by-laws. Align return period, climate change allowances, and loss rates with the approved hydrological memo.

Rational method (common form here):   Q = C I A / 360   with I (mm/h), A (ha), Q (m³/s).

Notation (common)

  • C — runoff coefficient (dimensionless; land cover and soil).
  • I — rainfall intensity for selected duration and return period (mm/h).
  • A — catchment area (ha in the calculator formula).
  • Q — peak discharge (m³/s) from the rational form above.
  • tc — time of concentration (often used to pick storm duration).

Examples in practice

  • Urban subdivision: peak flow to size trunk stormwater pipes.
  • Rural bridge: design flood from catchment rational method or regional equation.
  • Dam spillway: probable maximum flood or design storm hydrograph per dam safety rules.
  • Mine or industrial site: runoff coefficients for paved and rehabilitated areas.

Calculator — rational method

Enter C (0–1 typical), I (mm/h), and A (ha); all must be positive.

Q = C I A / 360   (I mm/h, A ha → Q m³/s)

Peak discharge

Rational-method peak flow Q = C I A / 360 with I (mm/h), A (ha), Q (m³/s).

Key terms

Runoff coefficient (C)
Relates rainfall excess to total rainfall; depends on land use, slope, and soil.
IDF curves
Intensity for a given duration and annual exceedance probability.

Software and catchment hydrology

Rainfall–runoff modelling uses design storms, loss models, and routing to the outlet. Examples (official sites):

  • HEC-HMS — watershed and hydrologic modelling (USACE).
  • EPA SWMM — urban runoff quantity and quality.
  • Autodesk Civil 3D — catchments, pipe networks, and hydraulics workflows.
  • DHI MIKE — MIKE SHE and related integrated hydrology where applicable.
  • HEC-RAS — hydrology links and hydraulic routing for river studies.

No product endorsement—verify rainfall data, subcatchments, and loss parameters against the project brief. The rational-method calculator is a single-value peak check, not a full design hydrograph.

Diagram sources

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