Appendix I - Stormwater Management Plan

STEP 4 Determine the 10 year discharge coefficient (C;o) value from tables 4.5.3 and 4.5.4.

STEP 5 Multiply the C; value by the frequency factor (F,) to determine the coefficient of runoff
for the design storm (C,).

Cy=F,. Cro (4.4)

Note: In certain circumstances the resulting value of Cy will be greater than 1.0. In accordance with
the recommendations of ARR (1998), a limiting value of Cy = 1.0 should be adopted for urban
areas.

There is little evidence to support an allowance for either slope or soil type in fully developed (non
WSUD) urban areas. If there are significant local effects, and reliable data is available, then
adjustments for soil type may be incorporated within the calculations at the discretion of the
designer in consultation with the relevant local authority.

The relationships presented in Book 8 of ARR (1998) and adopted in this Manual apply to areas
that are essentially homogeneous, or where the pervious and impervious portions are so
intermixed that an average is appropriate. In cases where separable portions of a catchment are
significantly different, they should be divided into sub-catchments and different values of C applied.

Notwithstanding the above notes and limitations, it is the responsibility of the designer to ensure
each sub-catchment flow is determined using a suitable coefficient of discharge. The local
government may set specific C-values to be used within their area.

Table 4.5.1 — Fraction impervious vs. development category

 

 

 

 

 

 

 

 

 

 

Development category Fraction impervious (fj)

Central business district 1.00

Commercial, local business, neighbouring facilities, service industry, 0.90

general industry, home industry (Dave lopedl)

Significant paved areas e.g. roads and car parks 0.90

Urban residential — high density 0.70 to 0.90

Urban residential — low density (including roads) 0.45 to 0.85

Urban residential — low density (excluding roads) 0.40 to 0.75

Rural residential 0.10 to 0.20

Open space and parks etc. 0100? (Undew loped)

 

 

 

Notes (Table 4.5.1):

1. Designer should determine the actual fraction impervious for each development. Local governments may
specify default values.

2. Typically for urban residential high density developments:

townhouse type development f= 0.7
multi-unit dwellings > 20 dwellings per hectare f, = 0.85
high-rise residential development f=0.9

3. In urban residential low density areas f, will vary depending upon road width, allotment size, house size
and extent of paths, driveways etc.

4. Refer to Table 7.3.3 for the definition of development categories.

Queensland Urban Drainage Manual Provisional edition, 2013 4-10