Coping with Storm Water Exceeding the Capacity of Drainage System
Current climate change predictions indicate that severe weather events will become more frequent. As a result, the capacities of sewers, watercourses and other drainage systems will be exceeded on occasions. Periods of exceedance occur when the rate of surface water:
Underground conveyance cannot economically or sustainably be built large enough for the most extreme events and, as a result, there will be occasions when surface water will exceed the capacity of drains. This is especially problematic where the drain is a combined sewer and sewage flooding can result. When drainage is exceeded the exceedance, flow is conveyed above ground, and will travel along streets and paths, between and through buildings and across open space. Flooding of property can occur when this flow of water is not controlled.
The Department for Environment Food and Rural Affairs (DEFRA) document ‘Making Space for Water’ suggested that highways could be used to facilitate the management of extreme events. If highways and other urban features are to be used effectively to convey exceedance flow, then careful design will be essential. Relatively minor features such as kerb height, traffic calming, and property threshold details can affect flood risk significantly.
Limitations of Piped Networks
Traditionally, highway and urban drainage generally are designed to meet a specified level of service, known as the 'Target Level'. This level of service is normally expressed as a frequency of property flooding. A level of protection of 1 in 100 years might be defined for internal property flooding as a suitable Target Level for a new development.
This level of protection can be delivered using a traditional piped drainage network with full capacity achieved during a 1 in 2-year storm event and then checking the flood protection performance using a computer modelling software package such as FLOW or WinDes.
Unfortunately, existing piped drainage systems do not always achieve the same level of service for the following reasons:
Modern piped systems (referred to below as the Minor System) are designed to ‘not flood’ during a 1 in 30-year storm event. This is achieved by ‘backing up’ in the system that artificially raised the hydraulic gradient above the constructed gradient of the system. An existing system may flood onto the surface during much more frequent storm events.
Once the capacity of the piped system is exceeded, surface flooding will occur. The excess flow that appears on the surface is known as the exceedance flow.
The rainfall events that result in exceedance flow are known as extreme events. Exceedance flow will be conveyed on the ground by surface flood pathways. These may be roads, paths or depressions in the surface. The system of above ground flood pathways, including both open and culverted watercourses, is known as the major system.
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The connection between the minor and major systems is complex and can only be accurately represented by a computer simulation model of both systems.
Managing exceedance
For rainfall events with a return-period in excess of 30 years, surface flooding of open spaces such as landscaped areas or car parks is acceptable for short periods, but the layout and landscaping of the site should aim to route water away from any vulnerable property and avoid creating hazards to access and egress routes. No flooding of property should occur as a result of a one in 100-year storm event (including an appropriate allowance for climate change).
Above ground flood pathways during extreme events may on a daily basis serve as:
These pathways may be determined by site inspection, and where necessary confirmed by digital terrain models.
If property flooding is to be avoided, then the conveyance capacity of flood pathways should be designed to convey the whole of the exceedance flow. The conveyance capacity can be influenced by relatively minor detail such as kerb heights. Often the effective conveyance or routing of flood flows can be achieved by modifying the detail of a carriageway cross-section, for example by revising the detail of drop kerbs, or lowering the highway surface.
Surface pathways should be linked together in the same way as conventional drainage networks to effectively convey the exceedance flows off the highway. Flows should be prevented from accumulating at low spots except where temporary surface storage is incorporated into the drainage strategy.
Principles of Design
The main use of surface pathways is referred to as the primary function, with flood conveyance becoming the secondary function. In exceptional circumstances pathways may be defined where flood conveyance is the primary function.
The design should not compromise the primary function of the proposed pathway, ensuring that safety issues are not introduced by the secondary function. Surface flood channels for extreme events should: