All parameter values for the natural catchment are entered here.
System
| Contributing Area [ha]: | The area of the natural catchment must be reduced to an effective area (which is only relevant for the STORM procedure). The size of this (effective) area should be at least ten times the total size of the urban area. The effective area must be multiplied by a reduction factor to get the runoff contributing area. The reduction factor can be estimated from recommended values for runoff coefficients of pervious surfaces: 0.05 for flat regions, 0.1 for regions with mild slope and 0.2 for steep regions. |
| Initial Loss [mm]: | Values from literature (CANOE, 2005) are: |
•slope J < 0.5 %: 12 mm
•slope J between 0.5 and 3 %:2+ 4·( 3 – J %) mm
•slope J > 3 %:2 mm
| Reservoir Time Constant [min]: | can be estimated e.g. by following formula (CANOE, 2005): |
k: |
reservoir time constant [min] |
A: |
catchment area [ha] |
J: |
slope [-] |
Lmax: |
longest flow path [m] |
REBEKA 2 estimates the runoff from natural catchments very roughly. It is far from being an exact runoff simulation of natural catchments (e.g. infiltration is only indirectly modeled by initial and constant percentage loss). Therefore the calculations and results should be interpreted carefully.