The Laurenson Routing Procedure used in XPRafts has the following advantages:
- It offers a 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.
The hydrological data requirements for XPRafts are:
- catchment area
- degree of urbanisation
- loss rates
- observed or design rainfall
These data are used to compute the storage delay coefficient for each of the sub-catchments and hence to develop the non-linear runoff hydrograph. A default exponent is adopted, although the user may override this value with either a different non-linear exponent or a rating table of flow vs an exponent to define different degrees of catchment non-linear response.
Each sub-catchment is divided into 10 sub-areas. Each of the sub-areas is treated as a cascading non-linear storage obeying the relationship S=BavQ^(n+1), where n by default is set to -0.285 and B is computed from observed catchment event data or specified in terms of the catchment parameters. The rainfall is applied to each sub-area, an excess computed and the excess converted into an instantaneous inflow. This instantaneous flow is then routed through the sub-area storages to develop an individual sub-catchment outlet hydrograph.
Any local Intensity-Frequency-Duration information may be used to generate the hydrographs. Rainfall input can be of two types, either Design Rainfall or Historic Events. Design rainfall may be entered as a dimensionless temporal pattern with an average rainfall intensity, or in Australia may be extracted directly from Australian Rainfall and Runoff (Institution of Engineers, Australia, 1987).
Historical events may be entered by the user in either fixed or variable time steps, allowing long lengths of record to be defined relatively easily. Alternatively the rainfall data may be read from an external rainfall file in either of two ASCII text formats. They are the Hydsys Hydrographs file format or the XPX Format File.
The rainfall excess may be computed using either of the following methods:
- Initial and Continuing Loss Model: The initial depth of rainfall which is lost is specified along with a continuing rate of loss. For example 15 mm initial loss plus 2.5 mm/h of any further rainfall.
- Initial/Proportional: The initial depth of rainfall, that is lost, is specified along with a proportion of any further rain that will be lost. For example, 15 mm initial loss and 0.6 times any further rainfall.
- ARBM Process Summary (ARBM) Loss method: Infiltration parameters to suit Philip's Infiltration Moduleusing comprehensive ARBM algorithms are used to simulate catchment infiltration and subsequent rainfall excess for a particular rainfall sequence and catchment antecedent conditions. Data describing such things as the sorptivity, hydraulic conductivity, upper and lower soil storage capacities, soil moisture redistribution, groundwater runoff and catchment drying are required. Many of these data may be found from field measurements and this model allows for more realistic modelling of catchment response to storms, especially those with multiple bursts. A proportion of the outflow from the ARBM loss method may be redirected as base flow in a given reach.
Up to 10 storm events may be analysed in the same run and the results displayed on screen to determine quickly the critical duration for each location in the drainage system. Simulation runs of any length, from minutes to years, may be accommodated.
Gauged data may be entered by the user or read directly from an external file and compared to the computed hydrograph to assist in the calibration and verification of the drainage network simulation.