The soil characteristics of a site can have a big impact on the runoff, particularly large rural catchments. The Flood Studies Report used a theoretical model to classify soils according to their hydrological performance. The factors used were the soil types, depth to impermeable layers, how often the soils are waterlogged and the slope. These factors are considered most important in determining the soils likely response to rainfall.
Winter Rainfall Acceptance Potential
This new classification was named the Winter Rainfall Acceptance Potential (WRAP) and a map was produced showing the distribution of the 5 WRAP classes across the whole of the UK. From this map the WRAP class of soil can be determined at any location in the UK. This map is reproduced in the CivilWeb Ultimate Guide. The CivilWeb Rainfall & Runoff spreadsheet also includes WRAP information for 250 towns and cities across the UK.
WRAP Classification System
The WRAP classes are derived from the below table;
Each WRAP class has a Standard Percentage Runoff (SPR) value, which are shown below.
Class 1 – 0.15
Class 2 – 0.30
Class 3 – 0.40
Class 4 – 0.45
Class 5 – 0.50
Where a large catchment contains multiple soil types, the average value across the site is used.
The WRAP value is used in a number of different runoff calculation methods including IH 124, ADAS 345, FSSR6 and the Wallingford Procedure.
In the following years an attempt was made to update and improve on the WRAP soil index as soil data became more comprehensive. This led to the development of the Hydrology of Soil Types (HOST) project. Where applicable, the HOST values now supersede the older WRAP values as they are more detailed and more accurate. The CivilWeb Rainfall & Runoff Calculator allows the user to use either the default WRAP values or they can override these with the more accurate HOST values if known.
Related Spreadsheets from CivilWeb;
Runoff Calculator Spreadsheet
This spreadsheet calculates the design runoff flow for a site in accordance with the a number of different methods including the Wallingford Procedure.
Attenuation Design Spreadsheet
This spreadsheet calculates the requirements for a attenuation system and assists the user to design a suitable system.
Soakaway Design Spreadsheet
This spreadsheet calculates the requirements for a soakaway system and assists the user to design a suitable system.
Full Drainage Design Suite
Full drainage design suite (50% Discount) including 7 spreadsheets;
- Colebrook White Pipe Design
- Manning Pipe Design
- Manning Open Channel Design
- Linear Drainage Design
- Runoff Calculator
- Attenuation Design
- Soakaway Design