USC FloodStudyUpdate Report - Flipbook - Page 253
These areas are shown visually in Diagram 1.
Pervious Area
Directly Connected Area
House
Indirectly
Connected
Area
Directly
Connected Area
Path
Road
Total Impervious Area
Diagram 1: Delineation of Urban Impervious Areas
The Total Impervious Area (TIA) is the sum of the DCIA and ICIA.
While it is usually easy to apply reasonable rainfall losses and routing to the directly connected impervious
areas, the inclusion of the indirectly connected areas is not straightforward. This is because some of the
indirectly connected area is pervious and some is impervious. This is further complicated by the fact that
the impervious areas can flow through pervious areas and experience some loss into the soil before
reaching the drainage system. The fact that this infiltration occurs at a different location from the location
where the rain falls is not compatible with an initial loss/continuing loss framework that removes the losses
from the rainfall before it is applied to the model.
To account for this,
is a concept
identifying the amount of impervious area that acts as directly connected for total runoff purposes, including
consideration of both the DCIA and ICIA.
A detailed GIS analysis was undertaken for a typical residential street block from Oran Park and it was
found that the TIA was approximately 80%. Using the literature from Australian studies in ARR 2019, the
relatively large proportion of roof area to block area (with roof area being directly connected) for the typical
Oran Park development, the degree of directly connected surfaces is most likely quite high. An EIA/TIA
fraction of 75% is considered to be reasonable for new development in this area. This results in an EIA
fraction of 60% of the total urbanised residential area as follows:
WMAwater Pty Ltd
118112: 201030_USC_Regional_Model_User_Guide_Final: 30 October 2020
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