The design of soil erosion control works often involves the use of two approaches, the:
- Revised Universal Soil Loss Equation (RUSLE) to estimate potential soil loss; and
- Rational Method to estimate peak flows.
Both these methods require the use of design rainfall intensities but for different purposes. For the RUSLE we can use the 2 year, 6 hour rainfall to estimate the rainfall erosivity factor (R) but we shouldn’t be using this rainfall to estimate peak flows. The correct design rainfall to estimate peak flows depends on the catchment size. There are examples where there is confusion about the use of design rainfall for erosion control works.
For example, there is potential for confusion when following guidelines published by the Victorian Environmental Protection Authority (EPA, 2004, p27):
“Design and install appropriate erosion and sediment run-off control measures appropriate to site conditions to handle a one-in-two-year storm event (two-year ARI with intensity of six hours), for temporary structures…” (emphasis added).
The use of the 2-year 6-hour event to calculate peak flows or runoff volumes should be considered less than best practice. In most cases, both the volumes and peak flows estimated using this approach will be exceeded much more frequently than every two years which means erosion control works will fail more often than the intended frequency.
In summary, the 2-year, 6-hour rainfall intensity is appropriate to use to estimate the R factor in the RUSLE but for the estimation of design peak flows, three factors need to be chosen to be appropriate for the particular site and project:
- the Average Recurrence Interval,
- rainfall intensity and
- rainfall duration.
Just using the 2-year, 6-hour rainfall to guide design decisions will often result in sediment control works that perform poorly.
See the paper below for more details.