Why understanding greenfield runoff is important to your SuDS strategy

Thursday 24th March, 2022

Sustainable Drainage Systems (SuDS) are important considerations for any new hard landscaping project and they bring many benefits, including providing a habitat to local wildlife, to a scheme. SuDS is probably best known for managing runoff volumes and flow rates from hard surfaces, reducing the impact of urbanisation on flooding. But in order to manage runoff effectively a developer must know their starting position, or greenfield runoff.

Usually, in the new development planning process, a local lead flood authority will request that a site be restricted to the greenfield runoff rate. This means that no additional water should enter the existing drainage systems as a result of land development. But to make sure this happens, we need to understand what greenfield runoff is, how it is calculated and how hard landscaping can affect the rate.

What is greenfield runoff?

When rainwater falls it can either infiltrate into the ground or remain on the surface and flow or ‘run’ in the direction of the slope. The proportion of water infiltrating into the ground depends on the properties of the ground itself, some soils can be quite permeable such as well-graded sand, other soils can be fairly impermeable, for example heavy clays.

Runoff is the portion of rainwater that flows over the ground in the direction of the slope and is generated when more water falls onto the ground than can infiltrate into it.

Greenfield runoff is the amount before any ground work begins on a new development and this is our base.

Calculating the rate of rainwater runoff

The runoff rate is the peak runoff expected in an area over a specific time period and is calculated using a number of regional factors such as soil types and annual rainfall. You can obtain the rate from the UK SuDS website.

Runoff rate is usually quoted as a flow rate per unit area of the site, for example:

5 litres(l) second(s) / hectare (Ha)

This means that if your site is 1,000m2 (or 0.1ha) then the runoff rate for your site is 5 x 0.1 = 0.5 l/s.

The greenfield runoff rate is the rate expected on an undeveloped site and is the base that needs to be maintained after site development. Depending on rainfall and the ground properties, the rate will vary from site to site but is typically between 2-7 l/s/Ha.

Why is water flow often restricted from a site to the greenfield run-off rate?

In undeveloped areas you can expect up to 95% of rainwater to permeate naturally into the ground and 5% runoff, this is reversed on developed areas where 5% permeates the ground and 95% is now runoff. Most sites connect to an existing drainage network which takes water runoff from the site and allows it to flow through the network of pipes to a wastewater treatment site or appropriate receiving water course. The problem is that existing network infrastructure has been designed to cater for a given amount of flow, and the network may not have additional capacity. Increasing capacity involves installing larger pipes which is disruptive, time-consuming and can be extremely costly.

When a site is developed, lots of hardstanding areas are introduced, such as carparks, roads, and building rooves . In general, these are usually impermeable and do not allow rainwater to infiltrate them. This means less water can infiltrate into the ground on the developed site, generating more runoff.

If the existing network is already accepting the greenfield runoff from adjacent undeveloped areas and is close to capacity, then when the site is developed the expected increase in runoff needs to be somehow restricted to the greenfield runoff rate. If this is not carefully managed it makes the development vulnerable to flooding.

Preventing flooding without installing cost prohibitive new pipes is why the local flood authority seek to restrict the runoff to the greenfield rate.

How can runoff be restricted?

 

Prevention is always better than cure and there are a number of sustainable drainage solution (SuDS) that can be used to reduce the runoff rates before storage and treatment solutions are required. Infiltration should be promoted across the site wherever possible, such as incorporating infiltration basins, soakaways, permeable pavements, raingardens and others.

If infiltration is not possible, then the flow from the site must be attenuated by incorporating storage structures, such as Type C permeable paving system, blue/green roofs, and even tanks.

How can I calculate the greenfield runoff rate?

 

The most commonly used methods for calculating greenfield runoff are IH124 (Institute of Hydrology, 1994) and FEH (Flood Estimation Handbook). These methods are derived from river catchment run-off data and have been extrapolated to site scales. UK suds hosts an online tool which allows for the easy calculation of the greenfield runoff rate.

The Marshalls Permeable Paving Design Guide can be downloaded online, and we have more information about SuDS and Marshalls permeable paving too.

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