Infiltration volume estimation
The infiltration volume design tool calculates the depth or base area of storage required to manage a specified rainfall return period based on the soil infiltration capacity and areas draining to the infiltration system. The tool also calculates the drain-down time to half empty the infiltration system.
Background
The infiltration volume design tool facilitates calculation of the depth or base area of storage required and the drain-down time to half empty for infiltration systems for new developments to assist developers, drainage engineers and those assessing planning applications. Various soakaway arrangements can be analysed. The tool applies the method defined in the report Infiltration Drainage (CIRIA report R156, 1996), which is re-stated in the SuDS Manual (CIRIA report C753, 2015), except for a 3D system with no infiltration from the base where the time to half empty is based on the method from BRE Digest 365 (2016).
When to use this tool?
Development sites will need to provide surface water storage in order to meet planning approval. If the site will discharge surface water to the ground, this tool provides infiltration storage volume estimates and the drain-down time to half empty the infiltration system based on the infiltration and site characteristics.
How to use the tool?
The infiltration volume design tool is a simple online tool. The user needs to enter the type of infiltration system and its geometry, the porosity of any material that is contained within the infiltration storage volume void, and the soil infiltration rates. The user must then enter the areas to be drained and upload rainfall data from the Flood Estimation Handbook Web Service before running the model.
What values will the tool give?
The infiltration volume design tool gives an estimate of the minimum height of the soakaway or base area of a plane infiltration system (where the side area is negligible to the base area) as well as the time it takes for the system to half empty for the design return period.
User can generate a report of all input data and results can be saved to form part of a drainage strategy or planning application.
FAQs
The concept of infiltration is aimed at trying to prevent any runoff taking place from sites when there are small rainfall events. The aim is to minimise the discharge of polluted runoff from entering streams and rivers, particularly in summer periods when they have low flows and the water is warm; conditions which are already stressful for flora and fauna. The emphasis is on achieving no runoff for small rainfall events which are less than 5mm. A small study was carried out by HR Wallingford on Interception (Supermarket representation for SuDS Guidance, Interception storage analysis (2014) using time series rainfall analysis to show how SuDS with little or no infiltration capability could deliver Interception. The three key points to note are:
- Evapotranspiration in summer is a key mechanism for preventing runoff using soil storage;
- Very low rates of infiltration can be effective in significantly enhancing prevention of runoff;
- An Interception criterion of 5mm of rainfall does not mean that rainfall from every event up to 5mm in depth will be prevented from runoff as antecedent conditions for some events will be particularly wet. A probabilistic approach is therefore needed for defining and assessing compliance.
Detailed time series runoff analysis need not be carried out if this is considered to be too complex, and simple rules of thumb can be used. These are based on showing that losses based on using evapotranspiration and marginal infiltration are greater than average daily rainfall (annual rainfall / 365) using summer conditions. Guidance on this is provided in the SuDS Manual.
Rainwater harvesting is another mechanism by which Interception can be delivered. See “How can Rainwater harvesting be used for surface water management?”
Infiltration should be used wherever possible, subject to the need to protect groundwater against polluted runoff. As all runoff is to some degree contaminated, protection is provided by ensuring that some treatment is provided by SuDS systems and that the base of all infiltration units should be at least 1.0m above the highest expected groundwater level. This is often difficult to establish, so groundwater levels should be measured through at least one winter period and a judgement made as to how much higher it might get in a really wet winter period.
Where there are important aquifers, or the runoff is particularly contaminated, there may be a need to prevent the use of infiltration.