Based on ADAS 345 and originally intended for land drainage design. See ADAS reference book 345 (The design of field drainage pipe systems), ADAS.


The Peak Flood Flow Q0 for catchments under 30ha

The ADAS 345 document does not refer to any return period but yields a “Peak Flood Flow”. However, the ADAS 345 method is a simplified form of a more comprehensive ADAS method within report number 5 which is no longer in print. In Report Number 5 return periods were listed rather than the crop types shown in ADAS 345.  Crop types were intended to imply a selected risk category and therefore give a different return period depending on the value of the crop.  ADAS has confirmed that selecting “Grass” from the chart you are actually selecting the return period of 2 years.  Therefore growth curves can be applied to the 2 year return period event. This in confirmed in the Environment Agency Flood Estimation Guidelines.

A 2 year peak runoff may be converted to a Mean Annual Flood using Table 1 of  FSR No 2, 1977.  The Mean Annual Flood may then be converted to other return periods using the method described for IH 124.

Q0 = ST.F.A

where

ST: Soil type factor

A: Area in hectares

F: Number derived from a lookup process outlined below:

Catchment Characteristic C

This is then used along with the selected crop type and average annual rainfall to determine the image from the graph below:

This image has been copied from 'The design of field drainage pipe systems' ADAS Reference Book 345.


Input Variables

Area

Catchment area.

Length

The maximum length of catchment.

Average Slope

Average slope of catchment Height/Length

AAR

The average rainfall.

Soil Type

Determine the soil type factor ST.

Permeability class

St

Very slow

1

Slow to Moderate

0.8

Moderate

0.5

Very Rapid

0.1

(There are more details available from the above reference)

Paved Area

Percentage paved but not exceeding 10%

Return Period

Rainfall return period between 1 and 1000 years.

Region Number

Region number of the catchment based on FSR Figure I.2.4.


Output Variables

Q0

Peak flood flow for 2 year return period

Total Q

Peak flood flow including Paved area for 2 year return period