An evaluation of flooding risks associated with giant reed (Arundo donax)

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 increased abundance of the invasive plant Arundo donax in a watercourse 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 had a direct effect on flooded area, causing increases up to 10% above baseline conditions, which represent cross sections with no A. donax present. Storm size also affected A. donax's impact on the size of the flooded area. Using the maximum value for Manning's n for A. donax in the simulations resulted in an increase in the flooded area from 10% to 19% compared to simulations in which there was no A. donax present. 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.