Rational Method Model property sheet
This window, accessed from Project → Hydrological Models → Add Rational Method Model, enables you to enter, view or edit a rainfall-runoff model using the rational method. Three types of rational method model are available, and the inputs required will differ with each model. The three models are:
(a) a general model that allows for impervious and pervious areas in sub-catchments, with the option of having separate runoff coefficients for minor and major flows;
This applies different runoff coefficients to pervious and impervious areas, if you wish to apply a single runoff coefficient without distinguishing between impervious and pervious areas, set the same runoff coefficient for both.
(b) a model based on urban drainage design procedures in the Australian Rainfall and Runoff 1987, which requires runoff coefficients for impervious and pervious areas, corresponding to a 10 year average recurrence interval;
The impervious area C10 value is always 0.9, while the pervious area value is
C'10 = 0.1 + (0.7 - 0.1) x (10I1 - 25) / (70 - 25)
= 0.1 + 0.0133 x (10I1 - 25)
Where 10I1 is the 10 year ARI, 1 hour duration rainfall intensity at the site.
Based on Table 14.13 in the 1987 version of ARR:
(c) a model that implements the relatively simple rational method procedures for pipes surface drainage systems from the Australia / New Zealand Standard AS/NZS 3500.3 which covers roof and property drainage. (This requires the choice of a soil, and of 10I1, a 10 year ARI, 60 minute rainfall intensity for the locality of the drainage system. The pervious area runoff coefficient can be calculated from this intensity. It has three types of land-use - roofs with a C of 1.0, impervious surfaces with a C of 0.9 and pervious areas with a variable C.)
This dialog box can be used to set up two or more rational method procedures, if required. Different rational method models can be employed in a single network, allowing for property and street drainage systems. While DRAINS allows you to create Horton (ILSAX) and rational method hydrological models, it does not allow you to mix these types of model in the various sub-catchments of a stormwater drainage system.