Leaky dam Copyright Yorkshire Farming Wildlife Partnership

Level 2 interventions

The following flood management measures are classified as level 2. This means that they require a certain level of consultation and possibly consent of authorities (see Consent & approval page). They are a mix of low to medium cost and may need contractors’ help to install them.

Bunds and detention basins

Earth bunds work most efficiently when located across known runoff pathways which appear following heavy rainfall or when the soil is saturated. The creation of a bund will also mean the corresponding creation of a detention area where water is retained while being dispersed through a combination of infiltration, evaporation, and slow release by flow control (for example, small pipe, orifice plate or filter material). This can be carried on a small to large scale, depending on the size of the catchment area and the local soil conditions.

The reprofiling of the land can be designed so that the retention area is normally dry and can remain productive, as well as providing an opportunity for reclaiming soil and nutrients.

Alternatively, levels can be set to encourage the development of wetland habitat within the flood storage area by permanently retaining some water.

Example of a functioning earth bund
Example of an earth bund – “Copyright JBA consulting”

Swales

Swales are linear, shallow, vegetated drainage features that convey and store surface water and provide the opportunity for infiltration and water treatment by encouraging settlement.

They can be built in combination with bunded detention areas, or on their own to channel and redirect water flow that happens after heavy rain.

Easily incorporated into the landscape, the increased roughness of the vegetated channel helps to slow the flow of water. This can be reduced further by the introduction of check dams and berms across the swale.

Example of a functioning swale.
Example of a swale – “Copyright JBA consulting”

Sediment traps

Sediment traps can take many forms, but normally comprise an excavation located on a surface runoff pathway. Runoff enters the excavation and is detained there, allowing sediment to settle out before the runoff is discharged, usually via a gravel outlet. Sediment traps are unlikely to derive significant flooding benefits on their own. However, when used in conjunction with other runoff management features, they can help to control the release of sediment to the river network.

Scottish Environment  Protection Agency (SEPA), 2015

Example of a working sediment trap
“Copyright Yorkshire Dales Rivers Trust”

In-channel barriers

These can be constructed in streams and ditches. When whole trunks, secured into place with stakes and wires, are used they are often known as large woody dams. More engineered structures are also called leaky dams. The dams are set above normal stream level, so only flood flows are blocked. Water is stored within the channel behind constructed dams, reducing the downstream flood peak by slowing the flow.

Two successive functioning leaky dams across a watercourse.
Series of functioning leaky dams – “Copyright Yorkshire Farming & Wildlife Partnership”

Offline flood storage ponds (permanent structure)

Offline flood storage ponds are constructed adjacent to watercourses and – during periods of high flow – some of the river flow is diverted out and into the pond. By forcing some of the flow to travel through a storage pond, the route for the flow downstream is more tortuous, and therefore flood peaks downstream are slower to rise.

Active offline flood storage pond.

Blocking moorland drainage grips

Upland drainage has resulted in changes in water flow paths through and over blanket peatlands and has been reported to both increase and decrease flood peaks. The drainage slightly lowers the water table, providing extra water storage capacity during rainfall events. However, the ditches themselves speed up the removal of water from the land into streams and rivers.

Blocking of grips and moorland drainage channels converts drained moorland back to peatlands. Restoration of wet moorland protects the peat (a carbon sink) and reduces peat and soil erosion, which contributes to discoloured waters and a high sediment load in rivers.

Blocked grip
“Copyright Yorkshire Peat Partnership”