Implications for the Removal of Invasive Species in Canyon de Chelly NationalMonument

摘要

As part of a study to investigate the causes of channel narrowing and incision inCanyon de Chelly National Monument, the effects of Tamarisk and Russian-olive onstreambank stability were evaluated. Root tensile strengths and distributions instreambanks were measured and used in combination in a root-reinforcement modelto estimate the additional cohesion provided to layers of each streambank. Relationsbetween tensile strength and diameter for Tamarisk and Russian-olive were similar toother woody, riparian species. However, where the roots of other riparian specieshave roots concentrated in the top 1m of the soil profile, roots of Tamarisk andRussian-olive extend throughout the bank profile (to 4.3 m). The additional cohesionprovided by the roots in each 0.1m layer ranged from 0 to 6.9 kPa for Tamarisk andfrom 0 to 14.2 kPa for Russian-olive. Average root-reinforcement values over theentire bank profile were 2.5 and 3.2 kPa for Tamarisk and Russian-olive, respectively.That is, a cohesionless bank acquires the properties of a weakly cohesive bank due tothe presence of roots, providing an average 2.8 kPa of cohesion.The implications of vegetation removal on bank stability and failure frequency wereevaluated in two incised reaches by modeling bank-toe erosion and bank stabilitywith and without vegetation. Banks in the modeled sections are 4.3 and 3.9 m high,respectively and are dominated by sandy materials except near the toe where finegraineddeposits provide between 2.2 and 4.0 kPa of cohesion. The effects of a seriesof 1.0 and 1.5 m-deep flows on bank-toe erosion, pore-pressure distributions andbank stability were evaluated. Results of iterative modeling runs show that the effectsof root reinforcement provided by Tamarisk and Russian-olive have a significantimpact on bank-stability and bank-failure frequency. Removal of riparian stands ofTamarisk and Russian-olive will lead to bank instabilities and accelerated widening inCanyon de Chelly National Monument.