Surface water storage volume estimation
The surface water storage volume design tool estimates the total surface water storage volume you are likely to need across your whole site with a fixed limiting discharge rate.
Background
The surface water storage volume design tool facilitates easy calculation of total site surface water storage volumes to assist developers, drainage engineers and those assessing planning applications, particularly at conceptual/outline planning stage. The tool uses a simple hydrological and hydraulic model to determine the total storage required on a site based on rainfall runoff from the site and an allowable discharge rate for surface water from the site.
This tool replaced the previous uksuds relationship-based tool in March 2025.
When to use this tool?
Development sites will need to provide surface water storage in order to meet allowable discharge rate criteria required to secure planning approval. The tool can be used at outline/conceptual design stages to estimate the total storage volume required on the site which can then be distributed across the site to maximise Interception, amenity, biodiversity and water quality benefits.
How to use the tool?
The surface water storage volume design tool is a simple online tool. The user simply needs to enter a postcode or click on an interactive map to allow location-specific data such as soil category and hydrological characteristics to be used by the tool (which can also be user edited). The user then needs to enter the site area and storage design information and choose which method to use to set the limiting discharge rate from the site. The user must then upload rainfall data from the Flood Estimation Handbook Web Service before running the model.
What are the tool outputs?
The surface water storage volume estimation tool gives estimates of the required total volumes of storage needed on a site along with the peak flow rate from the site and maximum water depth in the storage for a range of return periods. A report of all input data and results can be saved to form part of a drainage strategy or planning application.
FAQs
Long Term Storage is the term given to the volume of temporary storage which needs to be provided for the additional volume of surface water runoff that is generated by the development that is greater than the volume of greenfield runoff. The greenfield runoff volume is calculated using the 1:100 year 6 hour event. This volume is the amount that can be discharged at the 1:100 year greenfield runoff rate.
The additional runoff volume should be discharged from the site at a flow rate less than 2l/s/ha for this event. As critical duration events for the design of the site storage system will be much longer than 6 hours, the Long Term Storage volume is not calculated using the 1:100 year 6 hour event, but needs to be assessed using the critical duration event.
The surface water storage volume design tool estimates an attenuation storage volume required on the site. The estimate is based on the difference in the rainfall runoff flow being routed to the storage and the smaller allowable rate of flow out of the storage.
FEH22 rainfall and a climate change allowance factor is required to determine the design rainfall depth. Runoff takes place from contributing paved, roof and grass / vegetated surfaces with the percentage of runoff being lower and the speed of routing being slower for the grass / vegetated surface areas.
The allowable flow rate out of the storage is based on a calculation of the discharge flow rate from the site, which will depend on whether the site is greenfield or previously developed. The design flow rate is increased if the flow rate is too small to design for without causing a blockage risk to the throttle. The throttle from the storage is represented by an orifice with the diameter calculated to deliver the design flow rate through the orifice at the maximum depth of the storage.
The storage can be set to various shapes and porosities. The model is then run for a range of storm durations from 15 minutes to 48 hours to arrive at the design water depth for the design return period based on the critical duration event.
