Hydraulic Design

Dependant factors

The amount of rainfall a surface water drainage system will need to deal with depends on the following:

• The length or duration of storm.
• The return period or frequency between storms.
• Geographical location of site.

The length or duration of storm

Rainfall intensity varies inversely with duration. For example, the shorter the storm generally the more intense the rain will be. Conversely, the longer the storm the less intense the rainfall will be.

Storm return periods

The intensity of a storm does not only depend upon its duration but also on how often it will occur. A storm that occurs every week will be considerably less intense than one that happens once in ten years or once in a hundred years. By selecting a longer return period the designer can reduce the risk of flooding occurring.

Geographical location

The location of the development can have a significant effect on rainfall intensity. Generally the western side of the UK experiences higher and longer periods of rainfall but not always the intensity.

Infiltration & attenuation

When infiltration is being considered a number of important issues need to be addressed:

• Field tests must be conducted to determine the suitability of the ground and establish an infiltration rate as recommended in BRE Digest 365
• The soakaway must be designed to half empty in 24 hours or less
• They must be located a minimum of 5m away from a building
• Ground water table must be at least 1m below the bottom of the soakaway
• A non-woven needle punched geotextile filter fabric should be used to encapsulate cells to allow free flow of water.

The storage capacity of attenuation and infiltration systems can be determined from the following equation:

Inflow (m³/min) x duration (min) - outflow (m³/min) x duration (min) = Storage Capacity

Design factors checklist

• The calculation method to be used - BRE (infiltration only) or CIRIA (infiltration or attenuation).
• The geographical location of the site (for selection of local rainfall statistics).
• The storm profiles to be used (return period in years).
• The catchment areas discharging into the stormwater system (roofs and other hard surface areas).
• Soil infiltration rate (derived from porosity test conducted on the site) and the ground water table level.
• Allowable outflow rate in l/s (for attenuation systems).
• Safety Factor to be applied which is normally agreed with the local authority, environment agency or water company (dependent on consequence of flooding).