Infiltration volume estimation

The Infiltration volume estimation tool calculates the depth of storage required and the drain-down time to half empty for soakaways based on information on their plan area dimensions, fill porosity and infiltration rate. Various soakaway arrangements can be analysed. The tool applies the method defined originally in the report Infiltration Drainage (CIRIA report R156, 1996), which is re-stated in the SuDS Manual C753.

When to use this tool?

The Infiltration volume estimation tool enables the user to size soakaways where runoff from paved surfaces is to be infiltrated

How to use the tool?

The tool is available as an excel spreadsheet, which all registered users can download and saved locally for free. Once saved locally, the tool is only valid for 19 days. After that, you will need to download the spreadsheet again from this website.

Important note
The spreadsheet does not work on a Mac. To the best of our knowledge the spreadsheet also does not work in Excel 2016. However it does work in Excel 2010 (and earlier versions) and in Excel 365. When you open the spreadsheet it will ask you to ‘Enable Editing’ and ‘Enable Content’ before you can use it. You do not need a password to use the spreadsheet. 

What values will the tool give?

For a given geometry of a soakaway, a porosity value and infiltration rate, the tool gives you the minimum required depth for the soakaway and the time it takes to half empty. 

User can save and compare the results obtained.


How can Interception storage be provided when infiltration rates are low or non-existent?

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?”

When can infiltration be used?

Infiltration should be used wherever possible, subject to protection of 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.

Where there are important aquifers, or the runoff is particularly contaminated, there may be a need to prevent the use of infiltration, even where the infiltration rate is low. The SuDS Manual and other guidance should be followed where this situation applies.