This method was devised “from a study aimed specifically at small catchments (i.e.<25km²).  Efforts were made to include a greater proportion of commonly found catchments which are relatively permeable, lowland and partly urbanised.

Note that although a site of 25km² would be regarded as a large area for a new development it is a small area by river catchment standards.  Also, because of the wide variety of covering vegetation, land use, soil types, natural and formal drainage etc. it is more difficult to determine runoff accurately than on a fully sewered urban catchment.

Input Variables

Return Period

The program generates results for a range of return periods and the specified RP entered here.

Area

Catchment Area which is adjusted to km2 for use in the equation below.

SAAR

Average annual rainfall in mm (1941-1970) from FSR figure II.3.1 or equivalent.

Soil

Soil index of the catchment from FSR figure I.4.18 or Wallingford Procedure Volume 3.  Soil classes 1 to 5 have Soil Index values of 0.15, 0.3, 0.4, 0.45 and 0.5 respectively.

Urban

Proportion of area urbanised expressed as a decimal 0.000.

Region Number

Region number of the catchment based on FSR Figure I.2.4.  The region can also be picked using the region picker.

Growth Curve

Double click in the box to open the Growth Curve Editor.

Output Variables

QBAR rural

Calculated using the following formula (equation yields m³/s):

QBAR urban, Adjustment for urbanisation

From IH report 124 equations 7.2 to 7.4

CIRIA Book 14, 3.2.2. Equations yield m³/s.

Urbanised catchment growth factors for return periods not exceeding 50 years

CIRIA Book 14, 3.2.3

Calculate “equivalent reduced variate y from the following table based on CIRIA Book 14 Table 3.1.

Using the y value from the above table obtain the growth factor from the following table based on CIRIA Book 14 Table 3.2.

Region Britain is grouped into 10 regions for the determination of growth factors.  FSR Figure I..2.4. and CIRIA Book 14, Figure 3.7.

Urbanised catchment growth factors for return periods exceeding 50 years.

CIRIA Book 14 Table 3.2.4.

Find the rural and urban growth factors for the 50 year return period and compute their ratio G50.

Compute RMAF as the ratio of the rural and urban QBAR.

For the required return period t compute the ratio of the urban and rural growth factors from the following equation:

The rural growth factor is read from the following table based on CIRIA Book 14 table 3.3.

Multiply the rural growth factor by G_t to yield the urban growth factor.

Finally the urban t year flood is the product of the urban growth factor and QBAR_urban.

Mean Annual Flood Data

A matrix of return periods and flood flows is produced for all regions and return periods of 2 years to 1000 years.

Irish Growth Curves

The following growth curves are used for Ireland taken from the paper "Comment on Estimation of Greenfield Runoff Rates" - A.M.Crawley & C.Cunnane - National Hydrology Seminar 2003 (The one year value is take from the Greater Dublin Strategic Drainage Study, Volume 2.)

The following growth curve for the Greater Dublin region is defined below:

The one year value is take from the Greater Dublin Strategic Drainage Study, Volume 2.  The remaining values are derived (to two decimal places) from "An Investigation of the Flood Studies Report ungauged catchment method for Mid-Eastern Ireland and Dublin", Michael Bruen - Equation 7.