Optimising land drainage with natural methods

Land drainage plays an important role in farming and forestry in Finland. Excess water must be led away in order for crops and trees to thrive. The best overall results can be achieved by using natural methods.

The drainage of farmland can be divided into arterial drainage and field drainage. Arterial drainage refers to main ditches, whereas field drainage comprises field ditches and subsurface drains.

Natural methods are used for arterial drainage, in particular, and for stormwater management in urban areas. In farming, the objective is to achieve efficient drainage while protecting water bodies from silting and excess nutrients. Diverse and meandering channels that resemble natural watercourses are favoured. They lead water away efficiently while retaining suspended solids better than a straight channel.

The channel can have two levels: a deeper channel and shallower flood plains on one or both sides of it. During a flood, the water rises onto the flood plains in a controlled manner. During a dry period, only the deeper channel contains water. It is narrower and meanders more than a traditional main ditch.

A two-stage channel has many advantages. The vegetation growing on the flood plain binds nutrients and solids and creates nice and shady conditions for aquatic and littoral organisms while also producing food for them. In the deeper part of the channel, water keeps flowing throughout the summer, which prevents silting and overgrowth with vegetation. The organisms living in this channel never end up high and dry. Two-stage channels promote biodiversity by offering many types of habitats and living conditions.

Other natural methods include re-meandering of channels, buffer zones and wetlands. Bottom weirs can also be used to create a sedimentation basin or pit in connection with the channel and to retain solids carried by the water.