Make fences more resistant to flood damage

So, you want to build a fence in a flood-prone area. I bet you want it to survive floods with minimal repairs too. A conventional fence probably won’t cut it, but some design tweaks may just get you there.

Typically, the degree to which a fence can survive floods, particularly when floodwaters are loaded with debris, will depend on a range of factors, such as: fence alignment; strength of strainers and posts; fence type; number and placement of wires; and fence height.

Let’s have a look at each of these.

Fence alignment in relation to flood flows. In general, fences should be installed considering the terrain (i.e. contours) of the floodplain. As a minimum, the fence alignment should be no less than 45° to the flood flow path. However, as the risk of flooding increases, the fence alignment should be installed near-parallel to the direction of flood flows.

Strength of strainer (end) assemblies. During floods, debris can place additional tension on fences with many kilometers damaged or lost through end assembly failures (Wallace 2011). The key elements to consider when constructing strainer assemblies in flood prone areas are:

• the depth of the strainer post. In flood-prone areas, longer strainer posts (e.g. 2.7 m) should be used with approximately half set into the ground.

• the method of setting the strainer post. Staton and O’Sullivan (2006) state that a driven post will be 1.5 times more secure than a post that has been placed in an over-sized hole with the earth rammed back around it.

Strength of in-line posts (to resist over-turning). In addition to stronger strainer assemblies, in-line posts can also be strengthened to resist over-turning during flood events by considering factors such as the:

• type of post used. For floodplain fencing, the stronger posts (e.g. concrete, steel) are generally preferred.

• depth of posts. As for strainer posts, in-line posts exposed to floods should be longer and set deeper into the ground than standard fence posts.

• post spacing adopted. It is preferable to reduce post spacing, particularly in sections of fence that are more likely to encounter flood debris.

Fence type. Avoid mesh fencing and barbed wire in flood prone areas.

Number of wires. Keeping the number of wires to a minimum helps to limit the load carried by a fence during a flood. For this reason, electric fences which use fewer wires than other conventional fences are often the best choice in flood-prone areas.

Placement of wire on the posts. Fencing wire should be placed on the paddock or downstream side of posts so they pop their staples and drop rather than breaking.

Fence height. Fences should be constructed with the least vertical height possible to give adequate livestock control. This is generally between 1.2 m and 1.4 m.

For more information, see the Guidelines for riparian fencing in flood‐prone areas - https://www.water.vic.gov.au/__data/assets/pdf_file/0040/669595/guidelines-for-riparian-fencing-in-flood-prone-areas.pdf

Have you got any tips for building flood-resistant fencing?

References:

• Staton, J. and O'Sullivan, J. (2006). Stock and Waterways: A Manager's Guide. Canberra: Land and Water Australia.

• Wallace, L. (2011). Finding Flood-proof Fencing Solutions. In Farming Ahead February 2011 No. 229.