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:
  • identify acceptable design criteria for draining large paved areas (e.g. maximum acceptable water depths and duration of ponding),
  • develop a procedure for estimating runoff rates and water depths on pavements,
  • prepare design guidance for applying the new procedure. Current UK practice for calculating the design rate of flow from an impermeable area assumes a constant rainfall intensity of, for example, 50mm/h and 100% runoff from the surface. The flow rate is then used to determine the size of the site drainage system needed to collect and convey the surface runoff to the point of discharge. This design procedure may be adequate for small paved areas around buildings. It is, however, not satisfactory for large surfaces since it takes no account of the geographical location of the site, the duration and design period of the storm, or the time taken by the runoff to flow across the paved surface and enter the drainage system. It also provides no information about two factors that are of interest to the owners and users of the paved areas: the maximum depth of temporary ponding that will occur during the design storm and the length of time for which that depth will persist.

    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 design method was developed using results from a newly developed numerical model of the surface runoff process based on kinematic wave theory. This model computes the variation of water depth along a catchment length for time-varying rainfall. The numerical solution method used to solve the momentum, continuity and resistance equations was the method of characteristics, with a single forward characteristic. The model was calibrated against extensive field measurements. The study team identified a suitable field test site at Heathrow Airport, where (after fulfilling strict security and safety requirements) they set up rain gauges, ultrasonic probes to measure runoff depth and data logging equipment. The site consisted of an 83m-long brushed concrete pavement, with a gradient of 1:120, draining to a large capacity slotted system. It proved difficult to find a second test site, representative of car parks: it was necessary to have a single gradient so accurate measurements could be taken of very shallow water depths – in the order of a few millimetres. Car parks tend to have uneven gradients and many are in constant use, so installing instrumentation long-term was inconvenient. To overcome these problems, a special test surface was built at Wallingford: an outdoor pavement representing a section of an asphalt car park with a gradient of 1:60.

    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 tables also give the storm durations that produce maximum water depth, as well as the period during a storm for which the water depth is likely to exceed 75% of maximum depth. This provides important information on safety or inconvenience for users of paved areas.

    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.
    The design guidance on the Hydraulic Design of Paved Areas is available on HR Wallingford’s website at A report with full details of the study is also available for purchase from HR Wallingford

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