What is the IH124 method?
The Institute of Hydrology carried out a number of studies on revising the runoff equations produced in the original Flood Studies Report (1975). IH124 was specifically produced to address the runoff from small catchments. (Institute of Hydrology, 1994).
Although shown to be slightly less accurate than more recent FEH based methods, it is still considered to be an acceptable approach for assessing greenfield runoff rates. The IH124 equation estimates Qbar with the following equation:
Qbarrural = 0.00108 x (0.01 x AREA)0.89 x SAAR1.17 x SPR2.17, m3/s
- Qbarrural is the mean annual flood flow from a rural catchment (approximately 2.3 year return period).
- AREA is the area of the catchment in ha.
- SAAR is the standard average annual rainfall for the period 1941 to 1970 in mm (SAAR 41-70). SAAR 61-90, which was analysed for FEH for rainfall from 1961 - 1990, is virtually the same and can also be used.
- SPR is Standard Percentage Runoff coefficient for the SOIL category.