USC FloodStudyUpdate Report - Flipbook - Page 254
Commercial and industrial areas will have slightly higher recommendations based on similar reasoning. XPRAFTS adopts a typical split catchment approach, with one subarea representing the pervious fraction, and
the other representing the impervious fraction (or EIA in this case). The implementation of this methodology
in the Upper South Creek XP-RAFTS models is outlined visually in Diagram 2.
Subarea 1
Subarea 2
Pervious
Fraction
Impervious
Fraction
(EIA)
100%
0%
Impervious
100%
Impervious
40%
60%
10%
90%
90%
10%
Diagram 2: Determining Impervious Fractions of a Subcatchment for the XP-RAFTS Model
3.3.4. Surface Roughness
The surface roughness of the two sub areas outlined above depends on the dominant land use type in
and grasses, while any developed areas with a pervious component should be assigned 0.025. The
Table 1. In subcatchments that consist of a mix of land use types, the dominant one should be chosen for
Table 1
Values for Subareas Depending on Land Use Type
Dominant Subcatchment
Land Use
XP-RAFTS Subarea 1 (Pervious)
XP-RAFTS Subarea 2 (Impervious)
Bush / Pasture / Grass
0.04
0.015
Urban Residential
0.025
0.015
Urban Industrial/ Commercial
0.025
0.015
Rural Residential
0.025
0.015
WMAwater Pty Ltd
118112: 201030_USC_Regional_Model_User_Guide_Final: 30 October 2020
4
