As any other computer model, the ILSAX model has its model parameters. The ILSAX model is conceptualised as shown in Figure 4.7 showing its model parameters. They can be divided into two main groups. The first group deals with the parameters responsible for the rainfall excess. The second group accounts for routing parameters of pervious and impervious areas, and drainage pipes and channels. These two groups are loosely termed in this thesis as hydrological and routing parameters respectively. The hydrological parameters are the pervious area depression storage (DSp), the impervious area depression
storage (DSi), the soil curve number (CN) and the antecedent moisture condition (AMC).
The parameters CN and AMC define the infiltration process of pervious areas. The routing parameters are the Manning’s friction coefficient of pipes (Np), the retardance coefficient
of pervious areas (Nr) and the choke factor (CF). Additionally, the gutter flow factor
(GUT) and two pit parameters (CAP3 and CAP4) for grade pit inlets were also considered. Although the sag pits can be modelled with ILSAX, they are not shown in Figure 4.7, since they were not present in this study catchments described in this thesis. Therefore, altogether six ILSAX routing parameters were considered in this study.
GUT can be estimated from hydraulic data of the gutters and the pit capacity parameters from published literature based on physical hydraulic modelling and hence, both these parameters are dependent on the physical characteristics of gutters and pits. Therefore, GUT and pit capacity parameters can be considered as data, but they should be carefully selected since they affect the output response.
The hydrological parameters are responsible for the runoff volume of the catchment but the routing parameters do not affect the runoff volume. Therefore, the runoff volume depends only on the hydrological parameters, while the peak discharge depends on both hydrological and routing parameters.
supplementary
area directly connected
impervious area pervious area infiltration
DS
pDS
ipit
inlet
kerb and gutter
conduit
overflow
CN
AMC
GUT
CF
CAP3
CAP4
NpN
rFigure 4.7: ILSAX Model Representation and Its Parameters
The hydrologic parameters define the rainfall excess and depend on specific catchment characteristics (e.g. soil type, percent imperviousness, and depression storage) and in some cases on rainfall characteristics. These parameters are sensitive to output responses such as runoff volume and peak of the hydrographs. Therefore, the hydrologic parameters should be calibrated for gauged catchments or estimated by some reliable method for ungauged catchments. The routing parameters describe flow routing in the catchment and the pipe/channel systems. They are often fairly constant or, at least, can be estimated or extracted from literature with less variability. These parameters are less sensitive to output responses compared to the hydrologic parameters. The sensitivity of both sets of parameters was carried out and discussed in detail in Chapter 6. The estimation of the ILSAX model parameters is discussed in Sections 4.3.1 to 4.3.3.
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4.3.1 Pervious and Impervious Area Depression Storage
Depression storage is a volume that must be filled prior to the occurrence of runoff on both pervious and impervious areas. It represents a loss or an initial abstraction caused by such phenomena as surface ponding, surface wetting, interception and evaporation. Depression storage of directly connected impervious areas may be derived from rainfall-runoff data by plotting runoff depth versus rainfall depth for storm events (Alley and Veenhuis, 1983, U.S. Environmental Protection Agency, 1992, and Boyd et al., 1993). This method will be discussed in detail and used in Chapter 7 for study catchments. However, there is no such method available to compute the pervious area depression storage and therefore, it is necessary to treat the pervious area depression storage as a calibration parameter if rainfall/runoff data are available for the catchment.
As will be discussed in Section 8.2, Kidd (1978a) developed a regional regression equation to compute the directly connected impervious area depression storage using data from European catchments. In this equation, the directly connected impervious area depression storage is expressed as a function of the catchment slope. However, the directly connected impervious area depression storage does not depend only on the catchment slope, but also on land-use type, physiographic condition of surfaces, etc. Due to these reasons, the applicability of the equation for Australian catchments is questionable. However, the U.S. Environmental Protection Agency (1992) suggested the use of this equation to compute the directly connected impervious area depression storage, in the absence of better information.
4.3.2 Infiltration Parameters
As explained in Section 4.2.1, the ILSAX model uses the Horton’s infiltration equation to compute the infiltration losses from pervious areas. Although it is one of the well-known infiltration equations available, there is little guidance to determine parameters fo, fc and k
for a particular catchment. Some guidance is available for estimating fc based on the soil
group (U.S. Environmental Protection Agency, 1992). The parameters fc and k depend on
the soil and vegetation. Ideally, these parameters should be estimated using results from field infiltrometer tests for several sites of the catchment. They can be estimated without any reference to a particular storm. However, the results from such infiltrometer tests are
not generally available for use in urban catchments. The parameter fo, on the other hand, is
storm dependent or should be known prior to the storm event. Hence, it is not practical to determine fo through field infiltrometer tests prior to the storm event. In the absence of
such infiltrometer measurements, these parameters have to be calibrated using rainfall- runoff data of storm events for gauged catchments. As stated in Section 4.2.1, it is possible to estimate fo from Table 4.2 using 5-day prior rainfall depth.
As stated in Section 4.2.1, in the ILSAX model, the user can input either the Horton infiltration parameters (fo, fc and k), and four AMC values or an infiltration curve from four
pre-defined curves (identified by integer or non-integer CN between 1-4) according to soil type. The latter method is preferred since it involved one curve number and one AMC value for calibration compared to four parameters in the former.
4.3.3 Other Parameters
Np and Nr can be obtained from the ILSAX user’s manual or other literature since they are
fairly standard values. Pit capacity parameters are available for few configurations of grade pits in the literature (The Institution of Engineers, 1987; O’Loughlin et al., 1992a; O’Loughlin, 1993, Pezzaniti et al., 1999). Pit capacity parameters for sag pits are difficult to find from the literature except for one sag pit configuration given in the ILSAX user manual. There is not much information about the values for CF in the literature except one value each has been suggested for grade and sag pits in the ILSAX user manual. Therefore, further hydraulic model studies should be conducted to define these parameters for different common configurations of all types of pits.
GUT can be computed from geometry of kerb and surface roughness of gutter using Equation 4.9. In ILSAX, GUT should be entered into the Pipe file as data. However, there is no guidance given in the ILSAX user manual to choose the value for GUT. After discussions with engineers in City/Shire councils in Victoria on typical gutters and their dimensions, the GUT factor was calculated by the author for these typical sections. These GUT factors are given in Table 4.3.
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Table 4.3: GUT Factor for Typical Gutter Section in Victoria
Gutter Section dG (mm) ZG Zp nG np GUT 1 150 8.0 40 0.012 0.014 10 2 150 8.0 30 0.012 0.014 11 3 150 7.5 40 0.012 0.025 4 4 150 33.0 40 0.012 0.025 12 Notes:
ZG is the gutter cross slopes (m/m), (Figure 4.5a),
ZP is the pavement cross slopes (m/m), (Figure 4.5a),
nG is the Manning roughness coefficient of gutter, (Figure 4.5a),
np is the Manning roughness coefficient of pavement,
dG is the greatest gutter depths (m), and
dP is the greatest pavement depths (m).