Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 35
Narellan Creek Catchment
Flood Study
3.2.3
Stream Routing
The sub-catchment area, percentage impervious and lag factor that are input into the
WBNM model are used by the model to simulate the transformation of rainfall into runoff
for each subcatchment. In addition to local subcatchment runoff, most sub-catchments will
also carry flow from upstream catchments along the main watercourses.
WBNM incorporates a non-linear routing calculation to account for routing of flows along
watercourses within each subcatchment. A stream lag adjustment factor of 1.00 is
recommended for natural channels. However, this factor can be adjusted for individual
subcatchments to reflect alternate channel types that might route flows more efficiently
(e.g., concrete channels).
Each watercourse within the catchment was reviewed and the most common channel type
within each subcatchment was identified, and the lag adjustment factor shown in Table 6
were assigned to each subcatchment. For urbanised sections of the catchment with no
formal watercourse, most of the flow would be carried via stormwater pipes and/or along
the gutter. This was considered to be analogous to a concrete lined channel.
Table 6
Adopted stream lag factors
Watercourse
Description
Lag Factor
Natural
1.00
Excavated
0.50
Concrete lined
0.33
Urban/gutter
0.33
The stream lag factors that were adopted for each subcatchment are provided in
Appendix C.
3.2.4
Rainfall Loss Model
During a typical rainfall event, not all the rain falling on a catchment is converted to runoff.
Some of the rainfall may be intercepted and stored by vegetation, some may be stored in
small depressions, and some may infiltrate into the underlying soils.
To account for rainfall
