A modelling system for improved discharge simulation in small urbanized catchments
Different system characteristics of urban and natural catchments resulted in the development of two model categories: urban pollution load models and rainfall-runoff models for natural catchments. However, it can be shown, and the trend of modelling “integrated systems” underlines this, that the systems cannot be looked at separately in all cases, since dependencies between the systems are obvious and interactions occur. For example, after heavy rainfall, natural areas can contribute considerably to the flow in the urban sewer system. On the other hand, the discharge from wastewater treatment plants and sewer overflows in urban areas contributes flow to the natural receiving water bodies and hence causes alteration of the flow dynamics in the natural system.
Most modelling systems do not take into account interactions between the natural and the urban system, and if they do, it is usually done in a simplified manner. This is often the case in modelling systems for natural catchment hydrology where streamflow from urban areas is modelled using higher runoff coefficients for the calculation of effective rainfall, and faster cascades or lower Manning n values for flow concentration.
In very large catchments, the influence of urbanization on streamflow in the receiving water body is usually small. However, the impacts become noticeable in small catchments where the proportion of discharge from urban areas in the receiving water body is large.
The purpose of the work presented was to develop a comprehensive modelling system that is able to model the influence of urban drainage system discharge on river flow dynamics in small urbanized catchments on a detailed and interactive basis.
Two readily available models, one hydrological catchment model and one urban pollution load model, where combined under one software shell to achieve the stated purpose.
The hydrological GIS- and grid based catchment model considers all important aspects of the hydrological cycle to simulate runoff from rural areas and in the receiving river. Most of its process simulation algorithms are based on conceptual approaches with a physical basis.
The urban pollution load model comprises a conceptual rainfall runoff model based on linear reservoir cascades. It handles both combined and separate sewer systems and offers an option to consider backwater effects in sewer systems on a simplified basis. The model is based on the concept of hydrological response units (HRU), with each HRU being geo-referenced in the GIS.
The integration of the two models allows for an interactive simulation of flow in rural and urban catchments, meaning that flow from natural catchments that contributes to flow in urban catchments and vice versa, can be computed. Furthermore, the effects of urbanization such as diminished groundwater recharge underneath urban settlements, or changes in the yearly water balance can be simulated. Some capabilities of the modelling system will be illustrated shortly by means of an example.
The integrated modelling system has been applied to the Bieber catchment with a size of 39 km2, being located some kilometers east of Frankfurt (Germany). This catchment contains two urban settlements, each connected to a separate wastewater treatment facility. The urban settlement area makes up 29% of the total catchment, each having a portion of about 51% paved area.
The results of a 12 day simulation period are shown in Figure 1. The solid line shows the discharge from both rural and urban areas at the catchment outlet. The total discharge can be separated into several discharge components. The dashed line shows the runoff from both urban areas, in this case the influxes to the wastewater treatment plants (WWTP) are shown. It can be seen, that maximum influx to the WWTP is limited to 0.5 m3/s. The dotted line shows the runoff from the rural catchment. Not shown in Figure 2 is the discharge from the sewer overflows, which also contributes to the total discharge (solid line) in the receiving stream.
Adding all flows from the urban and natural system results in the total discharge. However as can be seen from the displacement of the hydrograph under dry weather flow conditions (e.g. 6.12.1992), translation and retention effects in the receiving water are taken into account. This means that the position of WWTP within the rural catchment is of mayor importance for the simulation result.
The modelling system focuses on the simulation of highly anthropogenic deteriorated small catchments, where discharge in the receiving stream is partly or mainly due to effluents of the WWTP. It is capable of computing runoff from both urban and rural systems in a detailed manner with consideration of interaction of the subsystems. Hence, it can be modelled how the subsystems mutually influence each other. Eventually, discharges from the different systems can be superimposed at any grid point and transported to the catchment outlet. If desired, results can be displayed and validated at any location within either the urban or the rural system.
