Natural Flood Management (NFM) Knowledge System: Part 3 – The Effect of Land Drainage on Flood Risk and Farming Practice

Janice Blanc, Scott Arthur, Grant Wright

Research output: Book/ReportCommissioned report


Executive summary
Land drainage is typically classified as either surface or subsurface and is widespread throughout developed countries. Substantial drainage has been undertaken during various periods in history and it is estimated that within the United Kingdom 60.9% of agricultural land is drained. In Scotland there was a dramatic increase in drainage after the Second World War, mostly due to the need to increase food production aided by a rapid development in mechanised installation; increased drainage was also evident during a period of agricultural intensification in the 1960s and 1970s. In Scotland the aim has often been to lower the water table to encourage vegetation cover more suitable for livestock grazing. Whilst drainage was common for grazing land, extensive land drainage was also undertaken in upland regions for commercial forestry operations. It should not be doubted that land drainage has shaped the way society has grown and developed.

While offering benefits that may improve yield, agricultural and forestry drainage have altered the rate of water runoff and increased peak flows during heavy rainfall and can result in diffuse pollution. It can therefore have a significant impact on the landscape, biodiversity and downstream hydrological processes.

Land Drainage is now recognised as having an impact on peak flows however, the extent of any potential changes is uncertain and likely to be site specific. Reviews of drained sites indicate a variety of responses. A number of studies found that field drainage could increase or decrease peak drain flows by as much as two to three times; the behaviour appeared to depend on soil type, antecedent conditions and rainfall event. Fundamentally, the key factor is the relative importance given to two processes; increasing flood flows due to the ability of drains to carry water faster than subsurface flow through the soil and reduced flood flows due to an increase in soil storage capacity created by lowering the water table. Which of these processes exerts the greatest influence will depend on various factors including: drainage density and geometry, hydraulic conductivity, drain and surface roughness, topography, event size, and antecedent conditions. Although not conclusive, authoritative studies have linked land drainage derived increased flood risk to dry catchments and arterial network geometry.

Drain blocking, commonly undertaken by installing a series of permanent dams in a drain, can be used to help restore a site to its pre-drained condition. However, a number of studies report that while drain blocking of peatland has benefits for the ecosystem, the impact on peak flows and flood volumes is not clear. Controlling the volume of flow through an existing drainage network
in a manner which allows peak flow control while also maintaining water table levels appropriate for agriculture offers an alternative to permanent blocking. This review has found that there may be an opportunity to meet the needs of agriculture whilst managing diffuse pollution and flood risk by deploying real-
time control as a method of dynamically controlling land drainage.
Original languageEnglish
PublisherCREW - Centre of Expertise for Waters
Commissioning bodyCentre of Expertise for Waters
Publication statusPublished - 2012


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