(a) Introduction

As an add-on you can expand this capability to include storage routing (RORB, RAFTS and WBNM) hydrological models, and unsteady flow hydraulic calculations in the Full Unsteady hydraulic model.

(b) Design and Analysis methods

You can use the various hydrological and hydraulic methods to design pipe systems automatically and by trial and error, and detention basins, open channels and other facilities by trial and error.  All methods involve simulation of flows passing through the drainage system that you establish in the Main Window. The DRAINS design method automatically determines sizes for pipes and pits while analysis can be used to check designed systems, and to model systems that are already constructed. Detention basins and open channels can be designed in this way.

• establish data bases for calculations (in the project menu) and provide a background image,

• draw a drainage network linking components, and enter data for these in property sheets,

• run the model in Design or Analysis modes,
  

• review and export results.

The base Horton (ILSAX) model has been in use in Australia since the 1980's, initially in the ILSAX program and then with the release of DRAINS.  It has been calibrated to gauged catchment data and uses Horton infiltration modelling and time-area routing.

The rational method can be applied in DRAINS, producing peak flowrates from IFD rainfall data rather than hydrographs patterns.  The extended rational method combines rational method loss modelling with the time-area method to generate hydrographs from rainfall patterns.

The three storage routing models perform calculations using methods from the non-linear storage runoff routing models, RORB, RAFTS and WBNM, which have been used in Australia since the 1970s. All have been calibrated to gauged catchment data.

These are all event models, producing flows from design storms that are usually intense enough to cause flooding.  DRAINS performs simulations over short to medium times (say three days or less) and does not have a continuous modelling capability.

A new hydrological model from Australian Rainfall and Runoff 2019, the initial loss-continuing loss model is available in DRAINS, but only on a provisional basis, as details of its operations are still incomplete.

(e) Hydraulic models

The Lite hydraulic model applies unsteady flow calculations to pipes and open channels.  Over flow routes are modelled using simple normal depth calculations.  The Full Unsteady hydraulic model provides more rigorous and accurate calculations for overflow routes, with greater allowance for storage effects. It can model complex situations that are difficult to simulate using the Lite hydraulic model. Differences between the models are set out here.

(f) Which model should you use?

The choice of hydrological model will depend on the task to be undertaken with the model, and by the likelihood of acceptance of the model by approval authorities.

All hydrological models except the rational model produce hydrographs, which are necessary for modelling unsteady flows, detention storages and complex networks. The Horton (ILSAX) and storage routing models (RORB, RAFTS and WBNM) are backed by testing programs in which their performance has been tested against gauged rainfall and runoff data.  The rational method models have not been extensively tested, but have been the most commonly-used models in many applications.  Some authorities consider them to be acceptable benchmarks.  The extended rational model included in DRAINS is an extension of the rational method that is widely employed in the United States.

The storage routing models are the accepted methods of modelling broad-scale urban catchments in Australia and can cope with the hydrological effects of urbanisation.  The various models produce different flow estimates due to (a) use of different rainfall data, notable I-F-D statistical relationships and temporal patterns, (b) models being derived for different purposes, scales of operation (pipe system sub-catchments compared to larger broad-area sub-catchments), and calibration to different data sets, and (c) modelling choices by users (some models allow users much more scope than others).

For routine applications such as on-site stormwater detention (OSD) calculations, designers probably should choose models accepted by approval authorities, while for more complex or critical applications, the more scientifically-proven and calibrated models will be the ones that can best model situations and be most easily justified.