Real Time Control of Urban Drainage Systems

Abstract

Since the last few decades, the focus of urban drainage technology has been greatly expanded. The urban drainage problem has changed from ‘simply’ draining the storm water, to disposing of it in an acceptable sanitary way. In solving this problem, the modern urban drainage engineer has to consider all pathways of the water in the urban area, i.e. the sewer system, the groundwater system and surface waters. Besides improved (computerized) design methods of storm water collection, transport and treatment facilities, various methods have been developed to upgrade the performance of urban drainage systems, i.e. to reduce flooding problems and to limit the pollution outflow to receiving waters. These methods may be divided into three broad categories: Source controls, i.e. measures taken to reduce the peaks and volume of surface runoff entering the sewer system. These are generally the most cost–effective methods for reduction of runoff in urban areas. Several measures can be considered to increase storm water infiltration, such as porous pavements, soak-away pits, seepage trenches, the use of cisterns to store water for e.g. garden watering, infiltration basins, etc. Although source controls may be very effective in reducing runoff volumes, the protection of groundwater quality must restrict their application to less polluted storm water originating from e.g. rooftops, backyards and residential streets. ‘Structural’ measures taken to enlarge the system capacity. Retention and detention are the most frequently employed methods of runoff control and can be used to achieve virtually any degree of runoff control, provided that the costs of such facilities are not prohibitive. The basic principle is to provide sufficient (in-line or off-line) storage in the drainage system in order to keep the water in the system until there is sufficient capacity to lead it to the treatment plant. The advantage of off-line storage tanks is that pollutants will be removed to a certain extent by settling of suspended solids. The pollution outflow to receiving waters can be further reduced by implementing improved overflow structures, 382such as the swirl concentrator, the high sided weir chamber, etc. The efficiencies of the various measures greatly depend on the hydraulic design and on local circumstances. ‘Non–structural’ measures, i.e. by improving the planning and operation of the system. Water managers generally recognize that a proper planning and maintenance is an absolute requirement for a good systems behaviour. However, in solving urban drainage problems the common approach is still to provide sufficient system capacity (given a certain design load), rather than investigating how the available capacity can be used in a more efficient way. This is not State–of–the–Art. The limited efficiency of this approach, which is caused by the lack of flexibility of the operation of the system, is being recognized by an increasing number of urban water managers. The awareness is growing that by using ‘proper’ system dimensions, that meet all the design constraints, does not automatically imply that the urban drainage system performs optimally for all rains that it is exposed to. The main reasons are: The design method itself is by definition inaccurate, due to schematizations and assumptions that are necessary for the computations; The planned and actual drainage conditions will differ due to urban development, maintenance work, sewer construction, system failures, etc. As a result, some sections will have more storage and/or discharge capacity compared to other sections of the system, meaning that some parts of the system may be overloaded while elsewhere in the system some capacity is still available; The temporal and spatial variability of the system loading will lead to an uneven use of the available capacities. A homogeneous design storm, on the basis of which the system has been designed, will never occur as a physical event. Real storms are distributed in time and space. Although they might not reach the depth of the design storm, local storms might result in sewer overflows, while elsewhere in the system storage capacity is still available. Theoretically, in an uncontrolled system, maximum use of all available storage and transport capacity will only be achieved when the entire system is loaded with a storm greater or equal to the design storm. By definition, for every other loading some capacity will remain unused. The effects of the system output on the environment are variable in time and space.