ARS | Publication request: An evaluation of flooding risks associated with giant reed (Arundo donax)

Submitted to: Journal of Freshwater Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 11, 2013
Publication Date: N/A

Interpretive Summary: We tested the hypothesis that giant reed increased the risk of flooding in rivers that it grows adjacent to. To do this we determined the roughness coefficient which measures how giant reed changes the flow of water. We then used this value in conjunction with a widely accepted model to estimate the change in the area flooded when different amounts of giant reed are present in the channel. Model predictions indicated that when the average value for the roughness coefficient was applied the flooded area increased by 10% for the case of a 2-year flood. Similar predictions using the greatest value for the roughness coefficient that we measured indicated that the area flooded increased by 19% over conditions where no giant reed was present. These results are the first to quantify the effects of giant reed on flooding risk for a watershed. They can be used to set priorities for projects aimed at giant reed management or removal.

Technical Abstract: The presence of vegetation in a channel may influence water movement in the channel and the surrounding land in several ways. One hypothesis is that the invasive plant Arundo donax contributes to increased flooding risk. To test this hypothesis, we determined Manning’s coefficient for A. donax and subsequently used it in the HEC-RAS model to estimate the impact that A. donax has on flooding risks at three sites within two moderately sized northern California rivers. The mean value for Manning’s coefficient for A. donax was 0.066, and it varied from 0.019 to 0.121 (N = 37). Model simulations showed that A. donax within a stream channel has a direct effect on flooded area, causing increases up to 10% above baseline conditions. Storm size also affected A. donax’s impact on the size of the flooded area. Using the maximum value for Manning’s the simulations resulted in an increase in the flooded area from 10% to 19%. Impacts of A. donax on other stream systems could differ, but they can be estimated using the value for Manning’s n for A. donax provided in this study. This information can be used in conjunction with natural resource conditions, such as soil type, to prioritize areas for A. donax management