Integrated urban drainage pilots

As noted in Chapter 16, there is a complex mix of responsibilities, institutional and funding arrangements that divide responsibilities between water companies, the Environment Agency, planning departments in local government, housing developers, householders and internal drainage boards. An integrated approach is seen as necessary to ensure that these different stakeholders communicate and work together to manage flood risk and water quality in the urban environment. The Government's 'Making Space for Water' programme therefore identified a need for 'integrated urban drainage management' (IUDM) as an important part of sustainable water management and development.

To develop new approaches to IUDM, Defra supported 15 pilot projects in 2007/08 which aimed to:

• understand the causes of flooding in urban areas and the best ways of managing urban drainage to reduce flooding;

Fig. 18.13 Mapping of areas naturally vulnerable to surface water flooding.

• examine the effectiveness of partnership working between various drainage systems currently and how this partnership can be improved to find solutions to flooding problems;

• test the effectiveness of new approaches to urban flood risk management, including: the use of hydraulic models, surface water management plans, SUDS and the managed routing of drainage exceedance flows.

By 2009, experience with the IUDM pilot studies already showed benefits in terms of partnership working, applications of a range of modelling tools and development of models and risk mapping of areas vulnerable to surface water flooding of use for development control departments and emergency planning. The IUDM pilots found that measures to reduce surface water flood risk and improve water quality can be more effective and cost beneficial when developed by stakeholders working in partnership than when acting individually. For new developments, drainage strategies were produced to safeguard downstream areas, protect the development and are adaptable to climate change.

The IUDM pilot studies also identified some of the problems that hinder integrated urban drainage management. Data and models were often found to be poor, incomplete or inaccessible. A number of methods to assess flood risk were trialled but without conclusive findings about which ones are best in which situations. The current fragmented institutional arrangements and responsibilities can make it difficult to coordinate and to fund integrated improvements to urban drainage management. Also, the IUDM pilots found that urban drainage issues are often endemic, being built into past infrastructure and planning decisions, and may be very hard to improve without town centre redevelopment, which may take years to realise.

Whilst some IUDM studies focus on very detailed analysis of surface and subsurface drainage networks, others adopt a more hydrological approach based on understanding the role of the urban drainage within a whole catchment context. One of the IUDM pilot studies took this approach to consider flooding problems, which have occurred on a regular basis in the Red House Farm Estate in Gosforth within the Ouseburn catchment to the north of Newcastle. Here, a Muskingum-Cunge flow routing model (see Chapter 14) was used to represent the propagation of flood events through the Ouseburn catchment. Rather than build detailed drainage system models of the catchment, surface drains and sewers, the behaviour of the urban drainage network was represented by hydrological inputs to the catchment routing model, in the form of unit hydrograph models that were calibrated to represent discharges from detailed urban drainage network models where necessary.

Based on these model results, it was determined that levels in the Ouseburn could interact with discharges from a CSO at Red House Farm to create conditions for flooding. One option for flood management in the area is to use SUDS methods such as flood storage ponds near to areas of new housing development within the catchment. One such pond was found to attenuate design flows as shown in Fig. 18.14 for a 25-year return period design event, which in this case reduced to approximately a 10-year peak flow.

During the IUDM pilot study, the public were involved via leaflet drops, press pieces and a drop-in workshop and site tour attended by the professional partners working on the project. Public concerns about sources of runoff, channel maintenance and the functioning of the SUD ponds were factored into the study so that they could be investigated explicitly. As a result of this, the IUDM study led to proposals for more flexible use of the SUDS to provide river flood storage as well as solely urban drainage attenuation. Whilst the viability of these suggestions is subject to further detailed study, the IUDM pilot showed that the integrated approach helped in identifying such approaches that join together urban drainage, planning and river maintenance, activities generally falling under the responsibility of different bodies.

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