WaterSubject

Modern methodologies for planning and design of urban drainage systems are based on the idea of minimising the receiving water pollution. However, in order to implement this concept of ambient water quality standards in practice, there is a need for simple tools that can establish a clear cause––effect relation between the drainage measures and the impact on the receiving water ecosystem. REBEKA is a software tool that has been developed to provide such a tool for a particular type of receiving waters––small, alpine and pre-alpine streams. The software has been launched among Swiss practitioners in the summer of 2000 and it is already widely applied. In this paper we outline the background of the methodology.Traditionally, measures for the improvement of municipal drainage systems (e.g. detention basins) are planned and designed on the basis of emission restrictions. However, there is a lack of evidence that these efforts to protect the environment against intermittent discharges are successful. The main reason being that the design is based on simplified performance criteria (overflow reduction) and not on the actual effect on the environment. Emission based planning can therefore easily lead to ineffective solutions when aiming to improve the water quality in the receiving water.

This problem was recognised and investigated already two decades ago. The Swiss Pollution Control Association was among the first to propose and launch national regulations for planning urban drainage measures based on ambient water quality standards. However, practitioners had difficulties implementing those regulations, as simple planning tools were missing.

In contrast to the emission based regulations, ambient water quality standards cannot be based on simplified methods such as IDF-curves but require a direct relationship between the input to the system, i.e. sewage and rain runoff, and the resulting effect on the receiving water ecosystem. The tools needed to analyse the performance are numerical models that are capable of simulating the behaviour of the whole system over long time periods. The relevant parameters for the evaluation of the performance of the system are subsequently derived from statistical analysis. Commercial software presently available to solve this problem is usually based on complex deterministic models of the sewer system and the receiving waters. As a result of such model complexity, the time needed for model building, calibration and actual calculation is typically prohibitive for using such complex software for simple planning exercises.

The Swiss Federal Institute for Environmental Science and Technology (EAWAG) has recently developed the software tool REBEKA to fill that gap. REBEKA aims to establish a clear cause––effect relationship between the drainage measures and the resulting impact on the receiving water ecosystem for a particular type of receiving water––small, alpine and pre-alpine streams, while still maintaining the necessary simplicity of a planning tool. The underlying concepts have been successfully tested at two locations in Switzerland and the software has been launched among––mostly Swiss––practitioners.

REBEKA aims to establish the specific relations between urban drainage measures and the––predominately––alpine and pre-alpine streams of Switzerland. In surface waters with short hydraulic retention time, such as alpine streams (creeks and rivers), water quality is primarily affected by the effects and pollutants that cause short-term, acute impacts to the ecosystem. Prior investigations revealed that the detrimental impact from intermittent discharges in Swiss rivers is predominately due to the following effects:

• erosion of the river bottom material

• toxic concentrations of unionised ammonia.

However, rainfall as the elementary source of the investigated form of water pollution is irregular in occurrence, intensity and duration. It is clear that the resulting detrimental effect on the environment has also stochastic features. The resistance of the ecosystem as well as the public concern are related to both the frequency and the magnitude of the impact. A solution can be found by applying extreme event statistics for evaluating the receiving water quality impacts. Hence, REBEKA was designed to compute the frequency of erosion and toxic impacts per year based on a long time series of precipitation. The results of frequency analysis are subsequently compared against the maximum number of impact events acceptable from an environmental point of view.

The basic concept outlined above requires simulation of the systems dynamics over long time periods (here 10 years) but with a sufficiently small temporal scale for predicting acute receiving water impacts (length of a timestep was chosen as 10 min). Despite the computational effort required by this approach, REBEKA was designed as a simple planning tool, mostly for screening the overall situation. Hence, only the simplest possible models providing the results acceptable for the intended purpose have been implemented. The main objective of this simplification was to reduce the effort of model calibration. However, it is clear that rigid model structure simplification goes hand in hand with increasing model uncertainty.

In its basic intentions REBEKA bears many similarities to the well-established British software tool SIMPOL. Both tools are designed as simple planning instruments for practitioners. In the development of REBEKA special emphasis was put on user-friendliness, easy implementation and computational speed (one calculation takes only few seconds on a modern PC). REBEKA is distributed and maintained by the Swiss Federal Institute for Environmental Science and Technology (EAWAG).

Although the tool was developed specifically for alpine and pre-alpine streams, the general principles are not restricted to such situations. A derivative of REBEKA, that includes also the utilisation of dissolved oxygen in the stream, was already tested successfully for German stream sites.Urban drainage management involves all issues of drainage, treatment and disposal of rainwater and raw sewage from urban watersheds. It is clear that such a complex problem needs to be analysed and solved from an integrated point of view. The selection of optimal strategies for handling wastewater must take into account the variety of options available, the effect of each on the total system, the cost of implementation and also the risk of failure.

It is interesting to note that only very few legislation around the world follows the principles of defining ambient water quality criteria for prescribing a controlled environmental impact. Typically, it is only the volume and/or the number of CSOs that is to be kept below a certain prescribed value. Part of the reason for missing holistic environmental legislation is the lack of simple planning tools that are able to establish a clear cause–effect relationship between the measures and the impact on the receiving water. REBEKA has been developed to close this gap for a specific problem area, that is small, alpine streams. The impact on the receiving water system is here computed by means of long-term simulations and expressed by the number of critical situation with respect to acute toxicity (unionised ammonia) and erosion of the river bottom material. It is to be expected that the advance of simple modelling tools like this will be helpful to promote holistic environmental legislation as already seen in the EU Water Framework Directive.