Home Feature Issues with use of Trees on dam walls

Issues with use of Trees on dam walls

by editor arbor age

The commissioner on a recent Land Environment Court case referred to the use of trees on dam walls as an issue for development. This article reviews literature that explores issues regarding trees and dams.

Trees are a resource on farms as they create shade for livestock and habitat for wildlife. The retention of trees is desirable on farms unless they present risk of damage to people or property. Any tree poses a level of risk, however the location of a tree on a dam wall has the potential to increase this risk further.

It is recommended by various NSW government bodies to avoid planting trees or deep-rooted shrubs on dam walls as roots can cause seepage, or the collapse of the dam wall (Water NSW 2011 and LLS 2021).

The inevitability of tree removal can also be a reason to avoid planting trees along dam walls or embankments, as the removal of the trees roots allows for seepage of water (Omofunmi et al 3017). Tree roots also can attach opportunistically to dams and they can have waterlogged roots which may be weaker.

Compaction of the soil by loaders and trucks squeezes the oxygen from the soil thus depriving the tree of the oxygen it needs to survive (Sullivan 2019).

Trees like Salix sp Willow can be tolerant to saturated soil but are less dense and more likely to have root detachment in dam walls. The reason farmers may use these type of trees are they can be a secondary source of fodder.

From a dam engineering perspective, trees that are located on the crest of a dam have the potential to affect the outflow of water from the dam which creates additional stressed to the dam wall (Chanson and James 1999).

Trees uprooted by storm damage on a dam embankment increase the failure potential as there is a high probability the void formed in the uprooting of the tree will lead to erosion and create a spillway (Amegashitsi 2016).

Large roots from trees can block drainage systems in dams, lead to undesirable changes in the soil, or cause piping, that is, the change of the water flow which can lead to erosion (Laasonen 2013).

Piping in particular can be a serious issue that can lead to whole dam failure, and this risk increases with large roots or roots with mould cavities (Evans et al 2000). However, it is noted that the risk of any individual tree will vary by case (Laasonen 2013).

In the mining country there have been remedial cases where trees have been removed to ensure the dam wall is structurally intact.

Root mapping of trees along dam walls would be useful in determining the extent of the root penetration.

Sampling the structural root zones within the root mapping process for encroachments into the dam wall. Secondary issues arising from installing fencing or drainage installations where soil is disturbed and can allow for the opportunistic roots.

Finally, anyone who has walked along the dam wall would notice that debris can build up and inspection of the surface is hindered once vegetation is allowed to establish.

Dam friendly vegetation
The illustration describes how different parts of a dam will benefit from different vegetation types. For example, any vegetation on a dam wall should only include shallow rooted plants and a filter zone should include groundcover plants such as sedges and rushes. Trees in a shade zone can help reduce water temperature in warmer months, discourage the growth of water weeds and benefit fish and other wildlife living in the dam.

For more information get in touch with the team at Tree Contractors Association Australia by calling 1300 660 379 or via email – [email protected]

Related Articles