Calculating the right design
Changes in the way we travel, shop and spend our leisure time have resulted in the creation of increasingly large paved areas. These include car parks in out-of-town shopping centres and supermarkets, park-and-ride car parks, and standing areas for aircraft at airports. In addition, the trend towards pedestrian precincts in urban centres has also led to an increase in large paved areas in towns and cities.The British Oil Spill Control Association (BOSCA) was formed in 1981 by a small number of expert companies active in the sector. During heavy storms, such areas generate large volumes of surface water run-off. This provides an engineering challenge since current drainage standards were developed for much smaller areas (under 600m²) and it is uncertain how well they represent larger ones. As a result, drainage systems for large paved areas may either be over-designed and unnecessarily expensive, or they may fail to perform satisfactorily and lead to an unacceptable level of surface flooding.
The aims of the project were to:
Researchers at HR Wallingford have now published simple design guidance on flow rates and water depths on impermeable pavements, knowing site characteristics such as the type of surface, catchment length/slope and rainfall conditions. The new guidance is based on more detailed factors such as: the critical storm intensity and duration for the particular geographical location of the catchment, the frequency of occurrence (or return period) of the event and the characteristics of the catchment. These are concepts that have been used in the determination of run-off from pervious catchments and in more complex urban drainage design methods, and are equally applicable to impervious surfaces.
Specifically, the new design method takes account of the following parameters:
- the duration and frequency of occurrence of storms that might occur in different parts of the country,
- time-varying rainfall intensity based on the 50% summer storm profile as recommended in the Flood Estimation Handbook,
- depths of water during a storm,
- the duration water is temporarily ponded on the paved surface,
- the type of surface material,
- the size, layout and gradients of the paved areas.
The guidance is presented as a series of tables of maximum flow depth and flow rates for:
- asphalt and concrete pavements,
- different UK geographical locations,
- different catchment lengths, ranging from 10-100m,
- slopes varying between 1:150-1:50.
The design procedure to determine the maximum runoff rates and water depths on paved catchments involves the following steps:
- choice of the required return period,
- determination of the catchment characteristics (length, slope and nature of surface),
- determination of the catchment geographical location,
- use of tables for the determination of design unit flow rate and associated water depths on pavements.