Vegetation in the Floodway and the Flume Study
By Tom Griggs, Senior Restoration Ecologist
Riparian vegetation (trees, shrubs, grasses and other herbs) is not only important as the habitat where most species occur, but riparian vegetation also supports an ecologically healthy migration corridor along the river for all wildlife. This wildlife function of the Central Valley rivers and floodways is especially important, as we move into the unpredictable climate change era. In addition, recent research has revealed that riparian vegetation can help with solving floodway maintenance challenges and reduce costs while also providing quality wildlife habitat.
Some people look at riparian vegetation and think that it is a problem for the effective conveyance of flood waters – the trees and shrubs plug-up the floodway and slow down the flow. This could be a problem as anything that slows the flood flows could cause over-topping of the levee, or worse, cause the levee to fail. However, experience from visiting the floodway immediately after a flood recedes can be instructive. Early in my tenure as the manager of the Cosumnes River Preserve my neighbor informed me that after a flood the apparently impenetrable patch of blackberry can be found lying on the soil surface, being pressed down by the weight of the flood waters. A few weeks later I was standing on the banks of the Sacramento River listening to a landowner explain how the blackberry had caused the observed bank erosion. How could these two disparate interpretations exist in the minds of rational people? Namely, that the same species, blackberry, could be flattened by a flood or it was the cause of bank erosion. River Partners’ own on the ground experience was that narrowstemmed native plants were flexible and laid down under even shallow flooding. However we did not have the opportunity to test this until the Flume study at UC Davis.
To gain some insight into how native shrubs and vines behave during a flood, River Partners worked with DWR staff, UC Davis engineering professor Lev Kavvas and his team, and the US EPA to develop a study that measured the responses of four riparian shrubs – California blackberry, rose, sandbar willow, and mulefat – to varying flow velocities and depths. We utilized the large flume at the J. Amorocho Hydraulics Laboratory at UC Davis.
The “Flume” is a structure that imitates river flows at various flood stages. It is a rectangular box about 80 feet long, 6 feet deep and 5 feet wide. Large pumps move water down the flume at predetermined velocities and depths. Velocity sensors measure the speed of the flowing water at different depths and locations in the water profile. Large fruit boxes were planted with individuals of the four plant species, and these were placed into the flume. Different flow velocities and predetermined depths were aimed over and through the plants in the fruit boxes and the velocity of the water was measured over the plants, through their canopies, and under the plants at the soil surface.
Results showed that flow velocity of the water behaved in basically the same manner with all four of the plant species. The plant canopies bent over in response to the flowing water, bending more completely under the higher velocities and greater depths. Water flow slowed as it entered the plant canopy, and slowed even more dramatically at the soil surface under the plant. Meanwhile flow velocity increased as it moved over the tops of the plant canopies. Based upon these findings, these four species of native plants could protect the floodway from soil erosion by forming a protective layer while allowing flood water to easily pass over them.
These findings have great meaning for floodway managers. The most important is that flow velocities increased over the tops of the plants, contrary to many people’s speculation. Plants do not “hinder” or “clog” flood waters. This is due to the flexible nature of the plant stems that bend under flowing water, becoming streamline and dampening the turbulence in the water column. In addition, as the plants bend and the water slows under the plants at the soil surface, soil erosion is greatly minimized. [see figure 1, above] These effects of the flexiblestemmed plants can be used to manage flood conveyance and soil erosion. For example, intentionally planting rose on the side of a levee, or sandbar willow at the base of a levee, can protect the surface of the levee from erosion under high flows without slowing the design flow velocity in the floodway.
The Flume study has shown us that these four species of flexible-stemmed riparian plants hold great promise for solving floodway maintenance challenges and reducing costs while also providing quality wildlife habitat.
Using Vegetation for Floodplain Management in Restoration Sites