Appendix I - Stormwater Management Plan

(b) Design Steps

To determine the overland flow travel time the following steps should be applied:

e Where practical, inspect the catchment to determine the length of initial overland sheet flow, or
for new developments measure the length of overland flow from the design plans.

e Where it is not practical to inspect the catchment, determine the likely length of overland sheet
flow based on Table 4.6.4.

e Determine the sheet flow travel time using either the Friend’s equation (equation 4.5 —
preferred method) or the Kinematic Wave equation (equation 4.6).

e Determine or measure the remaining distance of assumed concentrated overland flow from the
end of the adopted sheet flow to the nearest kerb, channel, or pipe inlet.

e Determine the concentrated flow travel time using either Figure 4.8 or Manning’s equation.

Table 4.6.4 - Recommended maximum length of overland sheet flow

 

 

 

 

 

 

Surface condition Assumed maximum flow length (m)
Steep (say >10%) grassland (Horton’s n = 0.045) 20
Steep (say >10%) bushland (Horton’s n = 0.035) 50
Medium gradient (approx. 5%) bushland or grassland 100
Flat (0-1%) bushland or grassland 200

 

 

(c) Friend's equation/nomograph for overland sheet flow time (preferred method for
overland flow calculation)

The formula shown below and attributed to Friend (1954) may be used for determination of
overland sheet flow times. This was derived from previous work in the form of a nomograph for
shallow sheet flow over a plane surface (Figure 4.6). This assessment process is recommended in
preference to that presented in (d) below. It is noted that values for Horton’s ‘n’ are similar to those
for Manning's ‘n’ for similar surfaces.

 

60 40 30 20 10 6 4 3 2 1 5 10 20 50 100 200 500 1000
Time of travel over surface (min) Length of overland ‘sheet’ flow (m)

Figure 4.6 — Overland sheet flow times (shallow sheet flow only) (source: ARR, 1977)

Queensland Urban Drainage Manual Provisional edition, 2013 4-19