Controls on Boulder Incipient Motion in an “Auto-naturalized” Constructed Step-pool River, Carmel River, California

Abstract:

River restoration commonly includes large boulder placement. Boulders are used for steps, vanes, hydraulic control, bank protection, and habitat complexity. Boulder transport away from constructed steps, vanes, and banks is typically considered a sign of project impairment or failure because natural boulder movement is relatively infrequent. Given sparse information about boulder mobility, it is challenging to select the appropriate boulder size and stacking architecture to achieve a desired hydrogeomorphic function and stability. We provide a study of boulder incipient motion that can foster boulder selection and hydraulic modeling of boulder stability.

A new 800 m long reach of the Carmel River (California) was constructed when San Clemente Dam was removed in 2015. Hundreds of large boulders used to construct 53 steps in the channel were scattered to new locations in high flows of 2017. This “auto-naturalized” river reach provides a natural laboratory for boulder mobility studies. We analyzed the causes of incipient motion and distance traveled for 226 randomly selected large boulders (0.5 m to 1.7 m) impacted by a flood event in winter of 2019. Channel width, water depth and isolation from neighboring boulders were the main variables controlling individual large boulder incipient motion during a 10-yr peak flow event in 2019. There is weak statistical evidence that a combination of shear stress and the presence of boulders located laterally downstream of the subject boulder controlled the distances the boulders moved. Frequentist statistics and AIC model comparison determined that boulder size, boulder shape, boulder roundness, and local thalweg slope were not good predictors of large boulder incipient motion or distance transported. Average dimensionless critical shear value for the four largest mobilized boulders (1.5 m to 1.6 m) was 0.014. In summary, individual boulder incipient motion and distance transported are difficult to predict. Boulder incipient motion is less related to size than we anticipated.