CAN FOREST MAKE A DIFFERENCE TO FLOODING?
Written by Jerry Vanclay
Long ago, a chap called Manning came up with a formula (now called Mannings formula) to estimate the speed and conveyance (volume/second) of water from the slope, depth (actually, cross-section and wetted perimeter, but depth is an approximate equivalent) and roughness.
A lot of effort has been devoted to calculating the (Manning’s n) coefficient for roughness – about 0.01 for smooth concrete, 0.03 for grassy stream banks, and about 0.1 for forested floodplains. Manning’s n is a divisor, so this tells us that water flow over smooth concrete is about 3 times faster than over grass, and about 10 times faster than through forest. (Examples of Manning’s n here https://pubs.usgs.gov/wsp/2339/report.pdf – see pics at end of report).
That’s relevant, because someone in The Channon recently asked me about tree-planting to mitigate floods. Let’s try with a thought experiment… So plant a nice thick belt of trees across the river, upstream of The Channon, so we reduce the flow of water to a third of what it was before. Trouble is that Terania Creek there is steep, falling 10m vertically in a 200m horizontal distance. So it doesn’t take long to accumulate a few extra cubic metres of water above our belt of trees – and in a steep narrow valley, that volume means a few decimetres increase in depth (and in the gradient of the water surface through our trees. Now Manning tells us that more depth and more slope mean faster water, so we soon lose the benefit of a belt of trees. Planting trees is still good, but a narrow belt won’t do much – in Terania Creek at The Channon we need to reforest the whole flood-prone area if we want to ease the floods. We can still have grassy pastures, but these need to be above the flood-prone area, with trees below the flood line.
Now consider Lismore, and imagine a belt of trees across the flood plain near Tuncester or near Woodlawn. A nice broad, dense belt of trees right across the floodplain, say up to the 14m contour. This could slow the water to a third of its previous flow, and water will be held back – but here the flood plain is wide and flat – you need to go at least 20km up Leycester Creek to reach the 10m contour, and 25 km up the Wilson’s River (above Eltham) to reach the 10m contour. So we can delay a lot of water on this floodplain with only a small increase in height, so belts of trees will work. Obviously, they should be as dense and broad as possible, and should ideally form a barrier right across the floodplain (so water doesn’t just go around), but every bit will help. The species doesn’t matter – it could be rainforest restoration, or commercial plantations (such as the pecans near Boat Harbour) - any trees are good [and interestingly for foresters, it is not about basal area, but about sum of diameters, because it is about intercepting the water, so lots of small trees are more effective than a few large trees].
Conversely, we want to keep the floodplain below Lismore clear, ideally smooth short grass (with no mounds with structures like houses and sheds). If we do that, we can slow the arrival of water into Lismore to a third of previously, and hasten the departure of water three-fold. Obviously, the floodplain below Lismore is very flat, to there are limits to wat ca be achieved, but it is kilometres wide, so a lot of water can go downstream with imperceptible increase in height.