XPRafts is a comprehensive software program to simulate runoff hydrographs at defined points throughout a watershed based on a set of catchment characteristics and specific rainfall events. As shown in the following diagram, the watershed can be subdivided into a number of sub-catchments from which runoff hydrographs are produced and routed through any configuration of network storages, channels, and pipes to determine flood mitigation options, drainage strategies, or hydraulic design data. 

XPRafts is suitable for application on catchments ranging from rural to fully urbanised. There are no specific limitations on catchment size. The software has been successfully used for catchments in excess of 20,000 km2 including on-site detention. XPRafts is capable of analysing watersheds including natural waterways, formalised channels, pipes, retarding and retention basins, and any combination of these.

XPRafts can be used to evaluate the effects of floods on major storage dams and the effect of a dam break on watershed. It can also simulate the attenuating effects of channel and floodplain storage on catchment runoff and assist the formulation of drainage strategies on developed or developing catchments. XPRafts can also be used to facilitate flood forecasting and subsequent flood plain management activities. The model allows rapid designs of networks with retarding/retention basins, giving flexibility in sizing outlets and emergency spillways to meet optimum requirements.

XPRafts can be used as either an event-based model (design storms) or continuous simulation model (historic time series rainfall data including the areal distribution over watershed). When operating in either mode, XPRafts has the option to utilise a water balance model generating continuous excess rainfall. This water balance model is a modified version of the Australian Representative Basins Model (Black & Aitken 1977, Goyen 1981, Goyen 2000).

In summary, XPRafts may be used for any of the following tasks:

  • Evaluating catchment runoff peaks and volumes.
  • Sizing of hydraulic elements in a drainage system, including reservoirs and retarding basins, pipes, channels, floodways, and river training works.
  • Examining of drainage and flood mitigation strategies.
  • Assessing the effects of various catchment changes, or urbanization, on runoff peaks and volumes.
  • Predicting flows for a flood warning system.
  • Estimating sewer flows.
  • Generating hydrograph flows for hydraulic modeling in other software packages.

Origin of XPRafts

The Rainfall/Runoff Routing Model described in this tutorial originated in 1974 in response to the need of analysing a complex drainage system associated with a major development in Darwin, including a new town for about 150,000 persons.

The program was originally developed jointly by Willing & Partners Pty Ltd and the Snowy Mountains Engineering Corporation (SMEC) (Goyen & Aitken 1976), and was named the Regional Stormwater Drainage Model (RSWM).

The concept of the RSWM model followed intensive research in the early 1970s into existing methods and computing rainfall/runoff models available in Australia and overseas including Great Britain, France and the United States of America.

A number of technical investigations were undertaken, including local research, a study by Mr. A.P. Aitken of SMEC (Aitken 1973), and a research project by the Australian Water Resources Council. Prior to the model being developed, a number of research activities concluded that no appropriate model consistent with Australian conditions and data was available. This was particularly evident for urbanised and developing catchments.

The basic aim in the development of RSWM included the following:

  • Provide a deterministic model capable of handling any conceivable drainage or river system including natural and artificial storages.
  • Limit data input requirements to be consistent with the availability of data and the required accuracy of results.
  • Allow rapid engineering and economic assessment of alternative solutions to flooding and drainage problems. Since 1974, the RSWM model has been applied to studies throughout New South Wales, the Northern Territory, the Australian Capital Territory, Victoria, Papua New Guinea, and Indonesia.

Watershed studies have ranged from rural to fully urban, with catchment areas ranging from less than 0.1 ha to several thousand square kilometers. The model has been improved on a semi-continuous basis since 1974 and continues to be updated as results of ongoing research are incorporated into the model structure. In the early 1980’s, the program was renamed the Runoff Analysis and Flow Training Simulation program (RAFTS). This major shift reflected a detailed urban analysis during the period between 1983 and 1997.

The significant changes during the early 1980’s to the XPRafts program are the following:

  • Separate consideration and routing of impervious and pervious sub-areas within a sub-catchment. This procedure improved urban runoff parameter calibration.
  • Separate routing of pipes and channels in a floodway environment.
  • Major enhancements in the retarding basin module to include hydraulically interconnected basins, on-line, and off-line basins with reverse flow considerations.
  • Substantial rewriting to make it compatible with micro-computer technology in terms of storage requirements and execution time.
  • Significant enhancements to the urban drainage capabilities.
  • Improvements to the very large river basin simulation capabilities.

In the 1980’s, the custodianship of the program's development was transferred from the general R&D of Willing and Partners Pty Ltd Consulting Engineers to XP Solutions (now Innovyze), a dedicated development group acting independently of the consulting organisation.

Since 1997, the enhancements have continued and included: 

  • Water Sensitive Urban Design (WSUD) including advanced on-site detention and retention analysis, roof water tank consideration, and advancements in Flood Forecasting facilities.
  • Advances in sub-catchment analysis to provide improved scaling process between sub-catchments of different sizes.
  • Automatic simulation of Probable Maximum Precipitation (PMP).
  • Major rewriting of the program to be fully compatible with Microsoft Windows and work in full 32-bit operating system.

Graphical Interface

The graphical EXPERT environment (XP) is a friendly, graphic based environment utilized by a range of software developed by Innovyze. It encompasses data entry, run-time graphics and post-processing of results in graphical form, and user definable tables.

In XPRaftsthe EXPERT shell acts as interpreter between the user and the model in the classical style of an embedded expert system. The environment incorporates both pre- and post-processors that use the expert system knowledge of experienced users to filter input data and to create and interpret a valid and reasonable model of the system being simulated.

The EXPERT environment of XPRafts allows engineers to devote more time to gaining an understanding of the problem rather than spending significant effort on the mechanical tasks of entering and checking data, getting a model to run, and interpreting outputs.

XPRafts allows you to work with CAD and GIS drawings to create scaled views of the drainage basin being considered. A detailed base map may be used and drainage networks may be created as a layer on top of the map. Base maps may be imported from third-party CAD and GIS packages.

Technical Overview

XPRafts is a non-linear runoff routing model used extensively throughout Australia and the Asia Pacific region. XPRafts works very well on catchments ranging from a few square metres to thousands of square kilometres, for both urban and rural nature. 

XPRafts can model up to 10,000 different nodes and each node can have any size sub-catchment attached, as well as a storage basin. Additionally, multiple on-site detention or retention structures, within a sub-catchment, can be included in the analysis.

XPRafts uses the Laurenson non-linear runoff routing procedure to develop a sub-catchment stormwater runoff hydrograph from either an actual event (recorded rainfall time series) or design storm using Intensity-Frequency-Duration (IFD) data together with dimensionless storm temporal patterns as well as standard AR&R 1987 data. From the 2009 version onwards, XPRafts simulates PMP.

Three loss models may be employed to generate excess rainfall. They are the following:

  • Initial/Continuing loss
  • Initial/Proportional loss
  • ARBM full water balance model

A reservoir routing model allows routing of inflow hydrographs through a user-defined storage using the level pool routing procedure and allows modelling of hydraulically interconnected basins and on-site detention facilities.

Three levels of hydraulic routing are available in XPRafts:

  • Simple hydrograph lagging in pipes and channels
  • Muskingum method
  • Muskingum-Cunge procedure to route hydrographs through channel or river reaches

The hydrographs may be transferred to the XPSWMM/XPStorm hydraulic simulation program for detailed hydraulic analysis

Hydrograph Generation: The Laurenson runoff routing procedure is used in XPRafts for the following reasons:

  • It offers a flexible model to simulate both rural and urban catchments.
  • It allows for non-linear response from catchments over a large range of event magnitudes.
  • It considers time-area and sub-catchment shape.
  • It offers an efficient mathematical procedure for developing both rural, urban, and mixed runoff hydrographs at any sub-catchment outlet.

Data requirements for XPRafts consist of:

  • Catchment area
  • Slope
  • Degree of urbanisation (derived from the nominated fraction impervious area)
  • Losses (observed or design) 
  • Rainfall data

These parameters are used to compute the storage delay coefficient for each sub-catchment to develop the non-linear runoff hydrograph. A default exponent is adopted, although you may specify this value with either a different non-linear exponent or rating table of flow vs. exponent to define different degrees of catchment non-linear response.

By default, each sub-catchment is divided into 10 equal sub-areas as shown in the following figure. 

Each sub-area is treated as a cascading non-linear storage following the relationship:


where n by default is set to – 0.285 and b is computed from observed catchment event data or specified in terms of catchment parameters.

The rainfall is applied in each sub-area, and the rainfall excess is computed and converted into an instantaneous inflow. This instantaneous flow is then routed through the sub-area storage to develop individual sub-catchment outlet hydrograph. The following diagram shows the hydrograph generation.


Rainfall: Any local Intensity-Frequency-Duration (IFD) information may be used to generate hydrographs. Rainfall input can be of two types, either Design Rainfall or Historic Rainfall. Design rainfall may be entered as a dimensionless temporal pattern with average rainfall intensity. In Australia, design rainfall patterns may be generated from AR&R 1987, with intensity information derived from Volume 2 of AR&R 1987. In this way, the appropriate intensity for the given ARI and duration is computed automatically. The zone of different regions of Australia may be entered and the appropriate temporal pattern may also be automatically identified from the built-in standard temporal patterns from AR&R 1987.

Historical events may be entered either in fixed or variable time steps allowing long periods of recorded data to be defined efficiently. Alternatively, rainfall data can be read from an external rainfall file in ASCII text format, either in HYDSYS or XPX formats.

Loss Models: The rainfall excess can be computed using any of the following methods:

    • Initial/Continuing: The initial depth of rainfall loss is specified along with a continuing rate of loss. For example, 15 mm initial loss plus 2.5 mm/hr of any further rainfall.
    • Initial/Proportional: The initial depth of rainfall loss is specified along with a proportion of any further rain that will be lost. For example, 15 mm initial and 0.6 times any further rainfall.
    • ARBM Loss method: Infiltration parameters to suit the Philip’s infiltration equation using comprehensive ARBM algorithms are used to simulate catchment infiltration and subsequent rainfall excess for a particular rainfall sequence and catchment antecedent conditions. 

The following diagram represents the ARBM Loss model:

The ARBM model requires data including sorptivity, hydraulic conductivity, upper and lower soil storage capacities, soil moisture redistribution, groundwater runoff, and catchment drying. These parameters may be determined from field measurements and is considered to provide a more realistic catchment response to storms, especially with multiple bursts. More details may be found in the Help file where you can find typical values for catchments. A proportion of outflows from the ARBM loss method may be redirected as base flow in a given reach.

Storms: Up to 10 storm events may be analysed in the same run and the results may be displayed on the screen and reviewed to determine the critical duration for each location in the drainage system. The following figure shows the stocked storms dialog:


Simulation runs of any period of time, from minutes to years, may be accommodated. Weighting of different Rainfall Stations in individual sub-catchments is provided.

Gauged Data: Gauged Data may be entered or read directly from an external file and compared to the computed hydrograph to assist the calibration and verification of the drainage network simulation.

Hydraulics: Hydrographs, that have been developed at individual nodes, may be transported through the drainage system in three ways:

    • Translation (Lagging): You may specify length of travel time from one node to the next and the hydrograph is translated on the time base by this length of time with no attenuation of peak flow. Appropriate values may be derived by estimating the velocity of flow and consequently the wave celerity, and knowing the length of travel.
    • Pipe Flow: A pipe may be specified (or sized) to carry flows. Any flows that exceed the capacity of the pipe will travel via the surface to either of two destination nodes. The travel time in this pipe may either be computed or set to a fixed number of minutes.
    • Channel Routing: A Channel/Stream may be defined using either compound trapezoidal channel or HEC2 style arbitrary sections. The cross-section may be imported directly from an existing HEC2 file. The Muskingum-Cunge method is used to route the flow through the channel with the consequent attenuation of the peak flow and delay of the hydrograph peak. Alternatively, the detailed channel data simple Muskingum K & X parameters may be utilised.

Any node may have a diversion link defined in addition to the normal link that will divert some or all of the flow and delay of the hydrograph peak.

The hydrographs generated in XPRafts can be transferred to the XPSWMM/XPStorm  hydrodynamic models. Hydrographs may also be read into other XPRafts models. For detailed description of XPSWMM/XPStorm, see separate XPSWMM/XPStorm technical descriptions on www.innovyze.com.

Storage Basins (On-Site Detention, Ponds, Dams, and so on): Any nodes in XPRafts may be defined as a storage node. This storage can be as small as few cubic meters or as large as a few millions cubic meters. On-line and off-line storages can be simulated and hydraulically interconnected.

Puls’ level pool routing technique is used to route the inflow hydrograph through the nominated storages. A stage/storage relationship is defined for each storage.

Different outlet structures can be modeled including:

    • Circular pipe culverts
    • Rectangular box culverts
    • Broad crested weirs
    • Sharp crested weirs
    • Ogee weirs
    • Erodible weirs
    • Multi-level weirs
    • High level outlets
    • Rating curve outlets
    • Evaporation
    • Infiltration

Optimization methods are also available to help design the basin. You may optimize the basin for a maximum discharge or for a maximum allowable storage.

Importing Data: Data may be imported from an ASCII text file in the XPX file format. This format allows you to create new data and objects as well as update and add to existing XPRafts networks. This facility may be used to import information from GIS’s, FIS’s CAD packages and other databases. Additionally, data may be directly imported from any ODBC compliant database including Excel, DFF Dbase, and so on. Plan drawings may be imported from any CAD or GIS packages to be used as a scaled base map. Formats accepted include BMPs, Shape files, JPEG, DWG, DXF, and so on.

Output: XPRafts provides results and data in various forms. All graphical displays may be output to printers, plotters, and to DXF files.
Graphical Output: XPRafts provides graphs of rainfall, rainfall excess, and hydrographs including total and local components of hydrographs. Stage history and storage history are also available for any pond or basin in the drainage system. The graphs of multiple locations may be displayed, printed, or results exported to a comma delimited ASCII text file for use in spreadsheets or databases.

Tabular Reports: Comprehensive tabular reports can be generated for both hydrology and hydraulic results and data. In addition, an ASCII text output file is available with detailed information on both hydrology and hydraulic calculations.

Accessing Help

The Help menu and Documentation are available for users and are easy to access. To access the Help information while using XPRafts, you can either select from the Help menu or click the Help icon located on your toolbar. Some dialogs provide Help options to specific information for that topic. If you click the Help icon, the pointer turns into a question mark, then click the area you require more information about. If this is not active, press F1 while the dialog is open. This provides relevant help information for all items in that dialog. Printed documentation is also available from Innovyze.

Contact Us

If you do experience difficulties when using XPRaftsplease contact our technical support team:

Contact Us

To improve your skills and increase productivity, Innovyze encourages you to attend the training workshops organised on a regular basis. These workshops provide comprehensive information to help you get the most out of Innovyze XP products. Information regarding these workshops is regularly posted on the website. Alternatively, you can contact Innovyze using the email address or phone number above. Innovyze can also tailor workshops to meet your particular requirements, as well as organise in-house training options.