|Storm hydrograph affected by different barriers.|
I know England well, I was born and schooled there. I have walked the ancient hedgerows of western England. Guess what - wherever there are hedgerows soil has stacked up behind them creating areas for infiltration and reduction in severity of the flood hydrograph (i.e. flood water is reduced and released at a slower rate). Where the hedgerows have been removed there is no such slow down and we see the result in the red sediment laden streams and dirty near land sea water.
In the US at the Neal Smith National Wildlife Refuge (NSNWR) located in Prairie City, Iowa, experiments are being conducted to see how deep deep rooted prairie grasses when planted in strips on arable land will amongst other attributes reduce runoff and improve water recycling.
Those of us who work with vetiver grass have known for years that when planted as a contour hedgerow vetiver will significantly reduce rainfall runoff, will positively influence flood hydrographs by reducing, and slowing the release of potential flood water from upper watersheds, and will almost block all eroded soil from moving down the slope, and at the same time will reduce actual erosion by reducing the concentration and velocity of runoff.
So lets have a look at some of science behind this phenomena when it comes to vetiver barriers:
Storm Hydrograph - The 1991 experiment by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) subjected vetiver hedgerows to induced storm events comparing vetiver with lemon grass, stone barriers, and bare soil. Note the vetiver and lemon grass were less than a year old, and therefore were not as dense as mature hedges. Even so vetiver produced the most reduced runoff rates. It was a pity that ICRISAT did not repeat this experiment with older more dense hedges. Note the increased effectiveness on the shallower slope. One can conclude that its just as important to use vetiver barriers on flat land as it is on steeper land if flood water from upper catchment areas is to be slowed down. Field experiments by Paul Truong on the Darling Downs, Queensland are examples where vetiver proved to be very effective when floods occurred, but failed to attract the attention of policy makers.
|1.5 m terrace riser created by backwater sediment|