Driving down flooding risks
Norman Flemming, Hyder Consulting's lead consultant for the Glasgow drainage project, reports on the stormwater masterplan for reducing floods in the area
When the entire average month’s rainfall fell on Glasgow in just ten hours in July 2002, it caused widespread flooding in the city’s East End. A situation that initially threatened to become a political football has instead turned into a major co-operative project to develop a long-term drainage strategy focused on development and regeneration needs.
Glasgow is not unlike any city in the UK. As urbanisation has progressed, watercourses have come to be regarded as safety hazards or inconveniences. Around the country, they have become hidden behind fences or walls or routed in culverts or concrete channels. Where they do emerge into public view, the strange magnetic force that seems to make them a dumping ground for shopping trolleys and various unwanted objects takes effect.
In Glasgow, the 2002 floods brought the conflict between economic development and the protection of the environment sharply into focus. The sight of submerged cars and more than 500 homes flooded in some of the poorest areas of Scotland triggered political interest and some lively debate between Scottish Water and Glasgow City Council. From initial investigations, it became clear there was significant hydraulic interaction between sewers and watercourses, both via combined sewer overflows (CSOs) and overland flow.
It also became clear a joint approach would be required to identify the right solution. While the desire to reduce flooding risk is a primary driver in the strategic drainage study and plan that is continuing, it also addresses development constraints, water quality and habitat
enhancement in an area targeted for regeneration. Responsibility for driving the study and strategy lies with a powerful steering group led by Scottish Water and comprising Scottish Environment Protection Agency (SEPA), Glasgow City Council, Scottish Enterprise and Scottish Water Solutions (SWS). Lead consultant for the project is Hyder Consulting, which completed Stage 1 of the project in April 2004, and is due to
finish Stage 2 in June 2005.
The project area has been defined as the WwTW catchments of Daldowie, Dalmarnock, Dalmuir and Shieldhall. This takes in a combined population of about 1.1M and the six local authorities of Glasgow City, East Renfrewshire, East Dunbartonshire, North Lanarkshire, West Dunbartonshire and South Lanarkshire.
The main focus for Stage 1 of the study has been the East End of Glasgow within the Dalmarnock WwTW catchment, where drainage problems and development constraints are particularly concentrated. For Stage 2, the methodology developed for the East End is being applied on a wider basis to areas of Glasgow that suffer from similar problems. The key objectives of the project are:
Many of Glasgow’s drainage problems are traceable to the increase in impermeable cover since the drainage system was first built. As a result, water quality is generally fair to poor – mainly class B (fair) to C (poor) in the freshwater sections of the River Clyde, the White Cart Water and the North Calder Water within the greater Glasgow conurbation. Class C or D (seriously polluted) water quality predominates in the East End. The degradation can be mainly attributed to CSOs, cross-connections and untreated urban run-off. Base flows can be low because the combined sewer system intercepts the majority of surface water.
Appreciable lengths of Glasgow’s watercourses have been culverted, varying between 75% closure in Dalmarnock and Dalmuir and 30% closure in Shieldhall. These man-made structures affect the flow characteristics of the watercourse, increasing the velocity of the flow and reducing the opportunity for attenuation. Accumulations of debris and siltation also increase flood risk.
Flood patterns for the July 2002 flooding in the East End indicated there was flow interaction between the sewers and watercourses, and an integrated model of the Dalmarnock catchment has been built by MWH. The integrated hydraulic model allows the level of interaction between the sewers and watercourses to be quantified for the first time. Modelling work for the other three catchments is being carried out under the drainage area plan (DAP) process by consultants MWH, Ewan Associates and Hyder Consulting.
For Stage 2, Hyder has amalgamated each set of DAP models into one large model for each catchment. The hydraulic models are built using InfoWorks CS and exceed 20,000 nodes each, challenging the computer hardware and software during computation runs.
Stage 2 is examining water quality issues at two levels of detail. The River Clyde is the core watercourse through Glasgow and therefore a detailed understanding is needed of the influence of continuous discharges from the four existing WwTWs and intermittent discharges for the numerous CSOs. To that end, Hyder is constructing water quality models of the Clyde river and estuary using Mike11 and Delft3D systems. The modelling work is being carried out in different phases:
n phase 1 – a coarse modelling exercise to provide a preliminary assessment of design options and provide a strategy and planning tool,
n phase 2 – a more rigorous modelling exercise (involving data collection, river surveys and model verification) to provide a detailed water quality assessment for design options.
Modelling and survey work is being carried out in conjunction with SEPA and will provide, for the first time, a complete water quality model of the River Clyde. The water quality issues of the watercourses contributing to the River Clyde also need to be understood. Hyder has therefore also carried out an urban pollution management (UPM) initial planning study, concentrating on the sewer network, WwTWs inputs and minor watercourses. The study is to determine the most appropriate modelling tools throughout the catchments, compatible with the use of Mike11/Delft3D for the main rivers.
Simulations carried out using the amalgamated catchment models are being used to assess deficiencies in the network. This allowed outline remedial measures to be identified to deal with water quantity and water quality deficiencies. Having the complete Glasgow system modelled allows cross-catchments solutions to be assessed.
Options are being tested both for their ability to minimise hydraulic deficiencies and also by their effect on the River Clyde, as determined by the water quality models. The strategic planning process has also identified opportunities for adopting a more sustainable approach to drainage, rather than using only traditional hard engineering solutions. Soft engineering practices and greater consideration to how watercourses are managed has provided a chance to restore the down-graded stretches of burns and rivers that run through the city.
The study has identified opportunities to address capacity problems in the city’s watercourses. The use of on-line and off-line attenuation ponds has been considered along with daylighting of buried watercourses. These measures provide the additional benefit of providing amenity and habitat improvements and can increase the value of adjacent properties and brownfield sites.
Stage 1 identified a total of 23 areas where attenuation ponds could be potentially sited, as well as 30 sections of watercourses for deculverting. In Stage 2 many more such areas were identified throughout the rest of the city. According to the city plan, the green space network accounts for more than 20% of the city’s total area, providing significant scope for remedial measures to be introduced. It is important to recognise these on-line and off-line ponds are not sustainable urban drainage systems (SUDS) and would only be used to attenuate flows in watercourses. These facilities would not be used to attenuate or treat urban run-off directly, but they are a component in the overall stormwater masterplan.
Although SUDS are not flood prevention measures, they have a
valuable role to play in reducing the rate of run-off and controlling
pollutants from urban developments. Introducing SUDS can achieve multiple outcomes – effective stormwater drainage, water quality improvements in run-off, aquatic habitat creation and protection, stormwater recycling and amenity value.
All catchments have significant areas of green space and brownfield sites that could be used to accommodate SUDS. These were sele
cted using a hierarchical approach:
Hydraulic modelling has indicated that while individually they might provide little benefit, by grouping potential sites the cumulative effect could lead to a considerable reduction in spills and flood volumes. The introduction of SUDS, daylighting of watercourses and the creation of on-line ponds for attenuating flows in streams and rivers, will all provide the opportunity to create new wildlife habitats.
Hyder is utilising this approach to produce the surface water management plan for the Cardowan Link sewer sub-catchment, to support the hard engineering conveyance solution, currently identified to unlock constraints presently imposed on proposed new development. This work seeks to achieve a better balance between the surface water utilities and maximise the opportunities to progressively remove surface water from the local sewerage system to the local watercourses. The intention is the results will be a ‘surface water strategy’, which will have application elsewhere.
Flooding is often caused by insufficient conveyance of surface flows and blockages or constrictions in flood paths. Any strategy or planning for surface water therefore must include an understanding of the flooding performance of the catchment and its land form. To this end, Hyder is undertaking pilot studies into overland flood routing, using recent developments in LIDAR surveys and modelling software. The intention of the work is to replicate known flood locations and mechanisms. The effects of increased rainfall intensity due to climate change are also being examined. Following the Stage 1 work concentrated in the East End area, the study team has moved on to the remaining two stages, which take in the other areas within the defined catchment. The growing set of data and recommendations have to be considered not only in the context of improving drainage in the affected areas but as part of the strategic development and regeneration plans for the city.
The forward picture
Stage 1 identified eight potential corridors of opportunity in the East End for combined drainage and city regeneration. In Stage 2 the ongoing results and conclusions continue to be fed in to the joint steering group for review and development of future plans.
The longer term outcome of the modelling and strategy project is likely to cost several hundred million pounds of capital expenditure, spread over ten-15 years. Some will come from Scottish Water, but the co-operative approach with the city council and other bodies means other funding sources will contribute to an integrated plan.
The city is also looking at insisting drainage investment is included in all private sector development plans. It is all part of a vision to take the lessons learned from the 2002 floods and create an infrastructure that is geared to a city focused on sustainable development.
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