A model for the design and field evaluation of the structural stability of ballasted rootwads in streambanks

This research developed and evaluated a mathematical model, WADCALC, to predict the stability of rootwads that are embedded in streambanks and ballasted with rocks. WADCALC calculates factors of safety to indicate whether a rootwad will be stable or prone to failure, and the degree to which this stability is expected. In addition to calculating a factor of safety for the entire rootwad, WADCALC also predicts the stability of each individual component of a rootwad; i.e. the ballast rocks and each log.;Over 200 rootwads were installed in field applications as part of stream restoration or stabilization projects at 11 locations in Pennsylvania. These sites included a variety of stream classes, sizes, discharges and flow velocities. Using data from 127 sampled rootwads, the WADCALC factors of safety were compared to the actual stability of each rootwad and its components. Other rootwad sites throughout the Mid-Atlantic region were also assessed to help with the development of the model.;The Pennsylvania sites were monitored over a four year period for stability of the rootwads, including the flood events to which each was subjected. Flood stages and discharges were documented by local USGS recording gages at 5 of the sites, and with on-site staff gages at all sites. Monitoring also included observation of flow patterns around the rootwads to help evaluate the hydrodynamics affecting these structures. This work was consistent with other studies of rigid structures that protrude into stream flows and subsequently produce turbulence that are difficult to predict, and thus the need for additional research.;The majority of the rootwads remained stable, as was desired for their role to prevent streambank erosion. However, enough partial and complete failures were documented to provide a statistically significant measure of the validity of WADCALC. For both the entire rootwad structure and its individual log components, the critical factor of safety (between stability and instability) was unity, as set forth by the study's hypothesis. However, for individual ballast rock components, the critical factor of safety was 2.6. This higher value appears to be consistent with studies on turbulence in the vicinity of exposed boundary perturbations. As with other design models, e.g. reinforced concrete, such an increase above unity could be considered as a “load factor” to account for the variability of the rootwad materials and of the hydrodynamics. (Abstract shortened by UMI.).