Hydrology of Soil Types (HOST)

CivilWeb Spreadsheets > Drainage Design Spreadsheets > Runoff and Rainfall Intensity Calculator Spreadsheet > Hydrology of Soil Types (HOST)

The Hydrology of Soil Types (HOST) project was intended to improve on the accuracy of the original WRAP Soil Index. The WRAP system of soil classification developed as part of the Flood Studies Report has some significant limitations, particularly the small number of classes and the low resolution of the distribution maps. A chance to improve the system came in the 1980s with the completion of national soil survey maps.

HOST Project

The Hydrology of Soil Types project used these national soil maps to reclassify UK soils according to 29 different hydrological performance classes. The classification system is shown in the table below.

Where high resolution maps including the catchment are not available, the HOST class can be estimated using the above table.

Standard Percentage Runoff

The HOST project published a map of the UK showing the 29 HOST classes and assigned a Standard Percentage Runoff (SPR) value for each class, which can be used in place of the WRAP SPR value included in the Flood Studies Report. These results are shown here;

In many cases the catchment being considered will include more than different soil type. This is often the case when analyzing larger catchments. The CivilWeb Rainfall & Runoff Calculator spreadsheet includes a tool which allows the user to easily determine the average value for design. The user can enter the total area of each HOST soil type within the catchment and the spreadsheet will calculate the overall average SPR value which can be used for design.

Applications

This SPR value can be input wherever the WRAP Soil Index is included in the runoff design formula. This includes the Wallingford Procedure, IH 124 Method, ADAS 345 Method and FSSR 6 Method.

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