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DRAINS is available in versions for 20, 50 and unlimited numbers of pipes or channels. The ILSAX
hydrology or rational method can be chosen. Optional ILSAX or rational method procedures, storage
routing models, GIS capabilities and unsteady flow hydraulics in overflow routes are available at extra
cost. Current prices are available from Bob Stack of Watercom Pty Ltd on (02) 6649 8005 or
[email protected].
1.1.2
Modelling Aspects
(a) Hydrology to Estimate Flows
The ILSAX hydrological model, illustrated in Figure 1.2, is the original model used to simulate the
operation of urban stormwater drainage systems in DRAINS. It comes from the ILSAX program
(O'Loughlin, 1993), described in Section 5.3.2. This model uses time-area calculations and Horton
infiltration procedures to calculate flow hydrographs from sub-catchments. The various sub-catchment
flows are combined and routed through a pipe and channel system. Calculations are performed at
specified times after the start of each storm, using time intervals of one minute or less. At each time step,
a hydraulic grade line analysis is performed throughout the drainage network, determining flowrates and
water levels.
Figure 1.2 Operation of the DRAINS Rainfall-Runoff Simulation Model Incorporating the ILSAX
Hydrological Model and Hydraulic Calculations
The design of a piped drainage system can be performed automatically, followed by an analysis, and
results can be checked, viewed and exported as CAD (computer aided drafting) files, GIS (geographical
information system) files and spreadsheet tables.
In addition to the ILSAX model, three other hydrological models are available as options in DRAINS:
(a)
Peak flowrates can be calculated by the rational method, traditionally used for calculating flowrates
for piped urban drainage design. Using the formula Q=C.I.A, it converts a statistical rainfall intensity
I to a flowrate Q using a runoff coefficient C and catchment area A (see Section 5.3.3). The rational
method’s main drawback that it does not calculate flow hydrographs, and it is gradually being
superseded by hydrograph-producing methods. DRAINS includes a search procedure that
determines the time duration that gives the greatest value of Q = C.I.A, thus resolving 'partial area'
problems.
DRAINS User Manual
1.2
November 2014