Water quality based impact assessment of urban drainage impact in Europe - A Perspective Paper (original) (raw)
Ambient water quality impact assessments provide for a procedure to evaluate urban discharges with respect to the state of the aquatic environment. They are used to identify critical discharge points and they give a framework for the re-development of urban drainage infrastructure. This development has become necessary because the EU Water Framework directive promotes achieving good water quality (chemically and biologically) as well as good water quantity. Discontinuous wet-weather discharges are known to have various detrimental impacts on receiv-ing waters. Due to their high variability and their number they are difficult to monitor. Therefore, mathematical models are commonly used to specify the hydraulic impact and pollutant concentration in the recipient. These numerical results are then interpreted with respect to the (probable) abundance of aquatic species. For the last decade guidelines have been available addressing engineers who want to optimize the urban water management system without endangering water quality goals. The urban water and the natural systems vary greatly in size and complexity. Therefore, the models have to be chosen accordingly with the background that more complex models require more basic data and extended skills. In the end choosing and parameterizing the model often becomes decisive whether a wet-weather discharge is considered detrimental or not. In practice, economic constraints and misused models lead to contradicting results, expressing a subjective impact as-sessment. Therefore, currently available ambient water quality assessment guidelines were re-viewed to (i) identify a structure ensuring objectivity, (ii) determine properties supporting the deci-sion making, and (iii) pin-point knowledge gaps that hinder an efficient transition towards water quality based urban water management. To these aims seven European guidelines were structurally analysed and published cases studies were critically reviewed. Furthermore, the algorithms behind the three most comprehensive guidelines were compared against a generic decision support algorithm used in river rehabilitation. Because simplified models are an important part within the assessment, three models were applied to a didactical example. The review of literature and models revealed the following. • An efficient AWA requires a layered approach to include i) screening for problem identifi-cation, ii) quantitative analysis of the various stressors, iii) transparent pre-selection of alternatives and fitting models for the design of protection measures and iv) performance evaluation of measures to be implemented. Each step has to be accompanied by verifying data needs and required model complexity. • Complex ambient water quality assessments demand large investments and expert knowledge from both the regulator and operator. Transparency can only be achieved by clear documentation and a proper uncertainty assessment of the used model. • The information available to engineers about the response of the aquatic environment to wet-weather discharges is insufficient. In science even, many aspects linking chemical water quality or structural integrity to the abundance of species is missing. Effective and efficient planning, however, depends on implementing these links into the urban water management models. The presented results have to be seen in front of the wide spectrum of available guidelines. The more flexible and conceptual it was designed the more responsibility is granted to the planning engineer and decision maker.
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