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SWMM Urban Catchment Peak Runoff & Infiltration Calculator

The EPA Storm Water Management Model (SWMM) represents urban catchments as non-linear reservoirs partitioned into impervious and pervious subareas.

Total drainage basin surface area in hectares (1 ha = 10,000 m²).

Percentage of surface covered with impervious pavements and roofs.

Peak rainfall intensity for the design storm event.

Ultimate saturated infiltration rate of native pervious soil.

Initial abstraction depth trapped in surface puddles and vegetation.

Calculated Result
1.369 m³/s

SWMM Peak Runoff Discharge

Total Catchment Peak Runoff

1368.9 L/s (1.369 m³/s)

Impervious Subcatchment Runoff

757.8 L/s (55.4%)

Pervious Subcatchment Runoff

611.1 L/s

Effective Composite Runoff Fraction (C)

0.79

Catchment Area

12.5 ha (45% impervious)

Calculation Breakdown

  1. Subcatchment Area PartitioningImpervious Area: 5.63 ha, Pervious Area: 6.88 ha
  2. Horton & Depression Storage AbstractionPervious Rainfall Excess: i - f_c - d_perv = 50 - 15 - 3 = 32 mm/hr
  3. Non-linear Reservoir Peak DischargeQ_imp = 757.8 L/s, Q_perv = 611.1 L/s => Total Q = 1368.9 L/s (1.369 m³/s)

Catchment Runoff Discharge (L/s)

Interactive visualization based on your current inputs

Discharge (L/s)
0.03416821.0k1.4kImpervious FlowPervious FlowTotal Peak FlowSubcatchment SourceDischarge (L/s)

What Is the SWMM Urban Catchment Peak Runoff & Infiltration Calculator?

The SWMM Urban Catchment Peak Runoff Calculator models hydrological rainfall-runoff partitioning for municipal stormwater catchments.

How Does the SWMM Urban Catchment Peak Runoff & Infiltration Calculator Work?

It partitions catchment area by impervious percentage, applies depression storage and Horton infiltration abstractions, and sums net peak flows.

SWMM Urban Catchment Peak Runoff & Infiltration Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

Q_{tot} = Q_{imp} + Q_{perv}, \quad Q_{perv} = A_{perv} \cdot \max(0, i - f_c - d_p)

EPA SWMM hydrological conceptual formulation abstracting infiltration and depression losses from gross rainfall.

How to Use the SWMM Urban Catchment Peak Runoff & Infiltration Calculator

  1. Input drainage catchment area in hectares.
  2. Specify percentage impervious cover.
  3. Enter design rainfall intensity and soil infiltration capacity.

Step-by-Step Example Calculation

12.5-Hectare Mixed Urban Catchment

Input Values:

catchmentAreaHectares:12.5
catchmentImperviousPercent:45
rainfallIntensityMmHr:50
hortonInfiltrationCapacityMmHr:15
depressionStoragePerviousMm:3
Worked Steps: Generates 1,363.9 L/s (1.364 m³/s) peak runoff with an effective composite runoff coefficient of 0.79.

Understanding Your Result

Total Catchment Peak Runoff: Maximum discharge rate in L/s and m³/s.

Impervious vs Pervious Breakdown: Respective volumetric contributions.

Composite Runoff Fraction: Empirical C factor reflecting overall watershed response.

Factors That Affect the Result

  • Higher impervious percentage drastically escalates peak runoff volume and response speed.
  • Soils with high infiltration (e.g. sands) eliminate pervious runoff during moderate rainfall.

When Should You Use This Calculator?

  • Storm sewer master plans, culvert sizing, detention basin inflow hydrographs, and LID mitigation studies.

Assumptions & Limitations

  • Assumes steady peak rainfall intensity and fully primed saturated soil state.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Based on EPA Storm Water Management Model (SWMM 5.2) Reference Manual Volume I.

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