Author:Mike Timperley, Melanie Skeen, Rajika Jayaratne
Source:Auckland Regional Council
In order to assess the environmental effects of stormwater discharges, information about the nature and quantity of the contaminants that they contain is required. Stormwater quantity and quality can be highly variable and challenging and resource-intensive to monitor. A pragmatic solution is to model contaminant discharges based on information derived from observations of stormwater quality.
The Contaminant Load Model (CLM) is a spreadsheet-based model which has been developed to enable estimation of stormwater contaminant loads on an annual basis. The model is very simple in principle - the area of a particular land use (source) within the area being studied (the catchment) is multiplied by the quantity of contaminants discharged from that land use (source yield) to provide an annual load from that source. The loads from each source within the catchment are then added together to provide an annual contaminant load for the catchment of interest.
The CLM incorporates six urban land use types (sources), these being roofs (divided into nine different types of material); roads (divided into six different vehicles/day categories); paved surfaces, other than roads and roadside footpaths (divided into residential, commercial and industrial); urban grasslands and trees (divided into three different slope categories); urban streams; and construction sites (considered to be 100% bare earth for the purposes of estimating contaminant loads). Although it has been developed for urban stormwater discharges, the CLM also incorporates five rural land uses, each subdivided into three categories, to enable mixed land use catchments around the fringes of the Auckland urban area to be modelled.
Source yields from the different source areas are provided for Total Suspended Solids (TSS), zinc (Zn), copper (Cu) and Total Petroleum Hydrocarbons (TPH).
A further consideration in modelling contaminant loads is any stormwater management or treatment that is implemented to reduce the loads of contaminants discharged to receiving water bodies. These are generically represented in the model by load reduction factors (LRF), and can include a range of options from source control to stormwater treatment devices.
The first version of the CLM was made available for general use in January 2006 and a modified version currently in use was issued in May 2006. A new version, Version 2, has been made available with the release of this report (CLM Users Manual) and the CLM Development Report. The new version includes improved model parameter values and the option for users to enter their own load reduction factors (these were fixed at default values in the previous versions of the model). This users manual provides guidance on how to use the CLM model. It explains the required and optional model inputs, the model outputs, how to use the outputs, how to apply the model to complex sites and the factors affecting the accuracy of the calculated loads. The development of the model is described in an accompanying report (Timperley et al, 2010, TR2010/004).
Auckland Regional Council technical report, TR2010/003