Runoff

Published: 28/05/26, Updated:

Version: 1.0.0

This calculation is currently under review and has not yet been verified for production use — print reports will include a DRAFT watermark. Please test it with your own data and use the Report a Problem button under the Results tab below to file a ticket if you find any errors or unexpected results. Your feedback helps us verify and release it sooner. Thank you!

Calculation estimates the design surface water runoff flow rate for a catchment using four established UK empirical methods: Wallingford (HR Wallingford Procedure), IH124 (Institute of Hydrology Report 124), ADAS (Agricultural Development and Advisory Service), and FSSR6 (Flood Studies Supplementary Report 6). An optional storage attenuation calculation is available for the Wallingford method.

Figure 1: Wallingford Procedure — Schematic Overview

Methods

Wallingford — Uses FSR rainfall data, rational-method percent runoff, and storm duration analysis to produce a design hydrograph.

IH124 — Flood frequency approach based on Q̄ (mean annual flood) and regional growth factors from WRAP and SAAR statistics.

ADAS — Rural catchment method using time of concentration, Q̄ for a 75-year event, and regional growth factors.

FSSR6 — Flood Studies report method using Q̄ (1-year baseline) and regional growth factors indexed to location.

Limitations

Methods are applicable to UK catchments only.

Storage calculation is only available for the Wallingford method.

WRAP for IH124 is expressed as a ratio (0–1), whereas Wallingford, ADAS, and FSSR6 use percentage (0–100%).

Climate change allowance is applied as a simple percentage uplift to the design flow.

Input Variable	Description	Limits
A_Ha	Catchment area	0 < A_Ha ≤ 1000
Region	Location region of catchment	England & Wales, Scotland & NI
City	Closest city to the catchment	England & Wales, Scotland & NI selected towns
M5-60	5 minute 60 year rainfall intensity	0 < M5-60 ≤ 1000
r_ratio	Ratio of M5-60 to M5 two day rainfall	0 < r_ratio ≤ 1
SAAR	Standard average annual rainfall	0 < SAAR ≤ 5000
WRAP	Winter rain acceptance potential	0 < WRAP ≤ 100
PIMP	Percentage of impervious area	0 ≤ PIMP ≤ 100
Cᵣ	Routing coefficient	For UK: Cᵣ = 1.3
D_mins	Storm duration	0 < D_mins ≤ 1440
T	Return period	T ∈ {1,2,5,10,20,25,30,50,100,200,500,1000}
season	Season type	Summer, Winter
CC	Climate change allowance	0 ≤ CC ≤ 40

Result Variable	Description
M5-D	Wallingford M5-D rainfall intensity
J0	Wallingford J0 coefficient
J1	Wallingford J1 coefficient
J2	Wallingford J2 coefficient
C_r	Wallingford Cr runoff coefficient
M_T-D	Wallingford M T-D rainfall intensity
i	Wallingford average rain intensity
AAF	Wallingford area reduction AAF
i_design	Wallingford design rainfall intensity
UCWI	Wallingford UCWI urban catchment wetness index
PR	Wallingford percent runoff
Q_design	Wallingford total design runoff

Input Variable	Description	Limits
Storage	Storage calculations	True, False
tank	Type of storage tank	Rectangular, Circular, Piped
L	Rectangular tank length	0 < L ≤ 200
W	Rectangular tank width	0 < W ≤ 50
H	Rectangular tank height	0 < H ≤ 20
V_r,rect	Rectangular tank void ratio	0 < V_r,rect ≤ 100
D_tank	Circular tank diameter	0 < D_tank ≤ 50
H	Circular tank height	0 < H ≤ 20
V_r,circ	Circular tank void ratio	0 < V_r,circ ≤ 100
D_pipe	Piped storage pipe diameter	0 < D_pipe ≤ 5
L_pipe	Piped storage pipe length	0 < L_pipe ≤ 1000
Q_out	Tank outflow rate	0 < Q_out ≤ 5000

Result Variable	Description
V_net	Net storage volume
V_req	Maximum storage volume required (critical storm duration)
H_max	Maximum tank water level
Tank_status	Flood status of the storage tank during the simulation period

Input Variable	Description	Limits
A_sqm	IH124 catchment area	0 < A_sqm
City	Closest city to the catchment	England & Wales, Scotland & NI selected towns
SAAR	Standard average annual rainfall	0 < SAAR ≤ 5000
WRAP	Winter rain acceptance potential	0 < WRAP ≤ 1.0
T	Design return period	T ∈ {1,2,5,10,20,25,30,50,100,200,500,1000}
L_idx	IH124 location index	1 ≤ L_idx ≤ 11
CC	Climate change allowance	0 ≤ CC ≤ 100

Result Variable	Description
Q_{bar 1y}	IH124 1-year Q̄ estimate
GF	IH124 growth factor
Q_design	IH124 design flow

Input Variable	Description	Limits
A	Catchment Area	0 < A ≤ 1000
City	Closest city to the catchment	UK selected towns
Location Index	Area Location Index	1 ≤ Location Index ≤ 11
SAAR	Standard Average Annual Rainfall	0 < SAAR ≤ 5000
WRAP	Winter Rain Acceptance Potential	0 < WRAP ≤ 100
L	Catchment Length	0 < L ≤ 100000
H	Catchment Average Height	0 < H ≤ 5000
T	Design Return Period	T ∈ {1,2,5,10,20,25,30,50,100,200,500,1000}
CC	Climate Change Allowance	0 ≤ CC ≤ 100

Result Variable	Description
Tc	Time of Concentration
Q̄_75y	ADAS Q̄ for 75-year baseline
GF_75y	ADAS Growth Factor for 75-year event
Q_design	ADAS Design Flow

Input Variable	Description	Limits
A_m²	Catchment area	0 < A_m² ≤ 1,000,000
City	Closest city to the catchment	UK selected towns
Location Index	Area location index	1 ≤ Location Index ≤ 174
SAAR	Standard average annual rainfall	0 < SAAR ≤ 5,000
WRAP	Winter rain acceptance potential	0 < WRAP ≤ 100
T	Return period	T ∈ {1,2,5,10,20,25,30,50,100,200,500,1000}
CC	Climate change allowance	0 ≤ CC ≤ 40

Result Variable	Description
Q_1y	FSSR6 Q̄ (1-year baseline)
GF	FSSR6 growth factor
Q_design	FSSR6 design flow

Standards

None

References

HR Wallingford (1981). Design and Analysis of Urban Storm Drainage: The Wallingford Procedure. DoE/NWC Standing Technical Committee Report No. 28.

Institute of Hydrology (1994). IH Report 124 — Flood Estimation for Small Catchments. Institute of Hydrology, Wallingford.

ADAS (1982). Land Drainage Design Guide. Agricultural Development and Advisory Service.

NERC (1977). Flood Studies Supplementary Report No. 6 (FSSR6). Natural Environment Research Council.

Release Date	Version	Description
April 2025	1.0.0	Initial release.

Project Information

Location Map

Click a county to zoom in and select a city

Calculation Method

Wallingford

Method

Wallingford — Location & Catchment

England & Wales

Location Region

City / Town

Select to auto-fill SAAR, WRAP, M5-60, r-ratio

Catchment Area [ha]

M5-60 [mm/hr]

SAAR [mm]

WRAP [%]

r-ratio (M5-60 / M5-2day)

PIMP — Impervious Area [%]

Routing Coefficient Cr

UK standard: Cr = 1.3

Wallingford — Storm & Return Period

Storm Duration [min]

Return Period [years]

Winter

Season

Climate Change Allowance [%]

Storage Attenuation (optional)

Disabled