Urban Drainage

There are many challenges that face us as we enter the new millennium, not least the demand for new, sustainable housing and industrial development. Every developer knows that plans for each new development must make provision for the surface water runoff to be drained from the site.

One of the difficulties that needs to be resolved is the removal of surface water without causing flooding or pollution, and without sterilising valuable development land. New development can be designed to incorporate Sustainable Urban Drainage Systems (SUDS) to mitigate the effects of the change in land use.

Drainage and flooding The rainfall and subsequent flooding experienced in the UK over the past three years has brought forward discussions about drainage. Although the main cause of recent flooding has been the sheer volume of rainfall there are plenty of actions that can be taken to reduce problems in the future. Nearly two million homes and businesses lie in natural river and coastal floodplains in England and Wales. Except for those properties that line our coastline, the remainder all fall within natural river catchments and therefore contribute to the flows in the rivers.

All new developments require planning permission before they can go ahead and flooding risk is considered at this stage. Developments in floodplains are obviously at greatest risk and should be avoided if at all possible. Most development is now not be allowed to go ahead without the necessary flood mitigation systems built in.

Existing Sewerage Systems
The first sewers carried both foul and storm water (combined sewers), but it is more common now for the systems to be separated into two independant drainage systems. This is a mixed blessing, as storm water can be as polluting as foul sewage, and is normally discharged directly to a watercourse. The benefit is that, when the storm sewerage system is unable to cope, the overflow does not contain obvious gross pollution.

Tens of thousands of properties could be at risk of flooding from sewers in the next 10 years. As well as being a key issue for the owners and occupiers of those properties, it is one that has important implications for sewerage undertakers and the water industry regulator, Ofwat.

Where flooding, or discharge from overflows is a problem, hydraulic modelling is often used to understand the behaviour of the drainage system. The drainage system to be simulated is modelled from available records. These are amplified as necessary by site investigation and closed circuit television investigation.

In many cases, the solutions need not involve expensive major works aimed at increasing overall sewer capacity. The installation of anti-flooding devices, for example, or making better use of storage capacity in the system may prove just as effective.

We have carried out the detailed design of flood storage reservoirs for flood control in urbanised catchment areas. Studies have included the causes and prevention of flooding in urbanised areas, flood estimation, flood routing and detailed engineering design of storm sewerage systems.

Foul Drainage
During the 18th and 19th centuries, the Houses of Parliament were often suspended because of the stench of sewage from the River Thames. We have come a long way since then, and everyone now expects the clean and convenient disposal of domestic waste water -

'when I have pulled the chain, I don't normally expect to see it again'.

Foul drainage is often pumped, sometimes several times, between gravity sections. Sewage deteriorates rapidly, and can easily become septic, particularly in long pumping mains. Both gravity and pumped systems must be designed to achieve self-cleansing velocities to minimise siltation in the pipework. As a result, foul sewers are generally laid to steeper gradients than storm sewers, often leading to deep excavations.

As an alternative to passive gravity drainage systems, sealed vacuum collection systems have been installed. These can show significant cost advantages in the right locations.