ARR 2019 Initial loss - Continuing loss (IL-CL) hydrological model

ARR 2019 Initial loss - Continuing loss (IL-CL) hydrological model

The IL-CL hydrology procedure in DRAINS is an alternative to Horton (ILSAX). Both methods are accepted in the ARR 2019 guidelines and discussed in Book 5 Chapter 3. The IL-CL model and its parameters are set out in Section 3.5.3 of ARR 2019.
ARR 2019 mentions several categories of area including:
  1. Total Area (TA),
  2. Total Impervious Area (TIA),
  3. Effective Impervious Area (EIA),
  4. Directly Connected Impervious Area (DCIA),
  5. Indirectly Connected Impervious Area (ICIA),
  6. Indirectly Connected Area (ICA) and
  7. Pervious Area (PA).
There is much discussion in the guidelines about how these various areas are related. Some key points are:
  1. Looking at GIS data you might assess Impervious Areas (those that appear to drain directly to the kerb and gutter) as DCIA. But the research suggests that not all of this area behaves as if it is directly connected. This led to the use of the term Effective Impervious Area. Section 3.4.2.3 suggests EIA = 70 to 80% of GIS DCIA (i.e. of apparent DCIA from GIS information). But this can trend towards 100% for small sub-catchments as the Total Impervious Area approaches 100% of Total Area.
  2. The ARR guidelines propose that the Impervious Area that is not EIA can be combined with the Pervious Area (PA) to form a category called Indirectly Connected Area (ICA). In DRAINS, this is also referred to as Remaining Area (RA).
  3. Additionally, Impervious Areas that are not DCIA may drain directly on to Pervious Areas (such as sheds or exposed patios without piped drainage - this makes them Indirectly Connected to the drainage system). These are classified as Indirectly Connected Impervious Areas (ICIA)
  4. As urban areas contain a higher percentage of these non-effective impervious areas within ICA (or RA), ARR recommends an Urban factor to be applied to Rural IL of 60% to 80%. Perhaps this is not surprising given that rural catchments are 100% pervious and the Remaining Area on urban catchments is partly impervious.
  5. For detailed urban models with varying catchment characteristics, it might be too general to specify a fixed Urban IL depth for all sub-catchments. A user could create multiple IL-CL hydrological models in DRAINS with varying Urban IL depths and specify which hydrological model applies to each individual sub-catchments, and additionally a Rural IL model for rural parts of the catchment.
To overcome the complexities of varying different Urban IL values between different sub-catchments, the IL-CL model in DRAINS uses three categories:
  1. Effective Impervious Area (EIA),
  2. Remaining Impervious Area (RIA) and
  3. Pervious Area (PA).
Splitting the Remaining Area (TA - EIA = ICA) into two categories (RIA and PA) allows a simpler treatment of Initial and Continuing Losses. By increasing RIA the total proportion of PA in the Remaining Area is reduced, effectively reducing the total available Rural IL applied to that Remaining Area. In DRAINS, runoff from the RIA is routed across the PA and this uses up some of the IL-CL capacity of the PA (in effect further reducing available losses for rain falling on the PA). In combination, method in DRAINS overcomes some of the difficulty of how to reduce rural catchment losses to suit urban catchments as mentioned by the researchers.
In the absence of better guidance from the researchers or Authorities, we suggest using rural catchment losses for the PA in an urban model.  
A further benefit to the clear definition of EIA and RIA allows modellers and reviewers to combine the two categories to calculate the Total Impervious Area of a catchment, rather than guessing how much of the Remaining Area is consists of impervious areas.
Below is a graphical representation of how to divide up a catchment for various hydrological models, how to specify their losses, and how the various parts of the sub-catchments interact with each other. The size of each box is not representative of the percentage or proportions of areas.

 

Parameters for IL-CL should be obtained from local available data. In the absence of available data, IL-CL values can be obtained from the ARR Data Hub.

In DRAINS, the parameters for the IL-CL model are entered in its property sheet:

 

and the property sheet of a sub-catchment will appear as:




    • Related Articles

    • Initial loss

      This is a hydrological loss that occurs at the start of a storm event. It is expressed in mm and is subtracted from the beginning of a hyetograph by adjusting the rainfall values. Initial losses are an important feature of IL-CL (initial loss - ...

    • Major/minor design for a small street drainage system at Gymea, NSW, using an IL-CL model and Lite hydraulic calculations

      The initial loss - continuing loss urban hydrological model recommended in ARR 2019 is applied in this model, along with the rainfall ensembles and median design procedure for ARR 2019. A DRAINS file for the example, named Gymea IL-CL Example - ...

    • Loss model

      In rainfall-runoff modelling, loss models are methods or procedures that determine the 'losses' from rainfall that do not contribute to runoff. They are abstractions from the rainfall, producing rainfall excess, equal to rainfall minus losses. The ...

    • Hydrological model dialog box

      From this window you can select the hydrological rainfall-runoff model to be used. The current default model selected for Designand Analysis runs is shown in the list box to the left. Clicking the Delete Default Model button deletes the highlighted ...

    • IL-CL Sub-Catchment property sheet

      This is used to enter data for the ARR 2019 IL-CL model. Different sheets are used for Horton (ILSAX) and the rational method, and other formats are used for storage routing model sub-catchments. At the top there is a text box for the sub-catchment ...