Managing gully erosion with debris dams

Dams and erosion control

Debris dams are small control structures that are built in the stream beds of actively eroding gullies. They stabilise the gully floor and allowing tree planting to minimise further erosion on the gully sides. While vegetation is generally the main long-term tool for gully control, trees can be difficult to establish if water channels continually undermine the toes of the hill slopes.

Application

Debris dams are largely confined to the control of v-shaped gullies. They are normally built in series over time, with the base of the upstream debris dam level with the top of the debris dam below. Locating a suitable site to commence debris dam construction is an important part of ensuring their success. It is important that the site is able to give sufficient support to the sides of the dam.

An effective debris dam system will:

• manage grade control, by eliminating bed level fluctuations
• reduce water velocity by lowering the slope angle and increasing channel width
• raise and stabilse bed levels, thereby supporting the base of -adjacent gully slopes
• centralise water flow in the channel
• trap and hold sediment. This not only gives associated tree planting a better growing medium, but also helps reduce how much sediment is deposited downstream.

Case study – Onenui Station

The Station is on Te Māhia and has areas of highly erodible coastal land with deep v-shaped gullies prone to sediment loss whenever there was rain.

The Council worked with other agencies and the station owners to install a series of debris dams in June 2023 at one site on the station. A video of the installation is below.

It was pleasing to see that within weeks the debris was at such a level that planting of Poplar and Willow poles was possible to further stabilise this area.