Physical Study of Geobag Stability for Erosion Protection Applications in Rivers
Creators: White, Kendra 
Publisher: name: University of Alberta Library
Year: 2022

<p>Geobags, or geotextile sand filled containers, have emerged as a cost-effective solution to providing large scale erosion protection for riverbanks and riverbeds in Bangladesh, China, and India. Geobags are currently designed using a formula intended for rock-based riprap that does not consider the flexibility and deformability of geobags. Diving explorations in the Brahmaputra River indicate that the geobags are oversized. The limited knowledge on the impact of geobag characteristics and bed configurations on the stability of geobag revetment structures in river applications provides an opportunity to improve the design formulas for geobags. In riverbed sediment studies, stability is often characterized by a critical Shields parameter. To date, there is only one study that has estimated a critical Shields parameter for geobags. Critical Shields parameters rely on critical bed shear stress estimates and are inherently statistical; more data is therefore required. Bed shear stress is challenging to measure directly, especially in complex field conditions, and therefore is often estimated indirectly. There are limited studies that evaluate the applicability of various bed shear stress estimation methods for complex flow conditions, such as flow around geobags. <br>
This study aimed to address these knowledge gaps and improve our understanding of the stability of geobags used in river erosion protection structures. Laboratory experiments were conducted to facilitate this study. First,  various bed shear stress estimation methods were compared for a single geobag placed on a rough bed. It was found that the turbulent kinetic energy method, using newly calibrated proportionality constants, provided the best estimates for bed shear stress. Second, the stability of a single geobag with varying fill ratios, bag materials and shapes were studied for both a flat and sloped bed. The results determined that fill ratio was a critical factor in geobag stability, where geobags with higher fill ratios and more flexible bag materials were most stable. A shape factor was proposed which improves the correlation between the critical Shields parameter and boundary Reynolds number when applied to the critical Shields parameters. Third, the stability of a scaled geobag revetment structure, made up of uniform geobags, was evaluated for both a flat and sloped bed. Different geobag sizes and fill ratios were tested and critical Shields parameters were estimated for each bed configuration. The critical Shields parameters ranged between 0.09 and 0.22. Geobags with lower mass and higher fill ratios tended to be more stable than geobags with higher mass and lower fill ratios. Pre-existing design formulas were modified to include the angle of flexibility and geobag fill ratio to improve their applicability for geobags. A modified Pilarczyk formula was recommended for geobags because it includes a critical Shields parameter and provides more reasonable bag sizes for extremely large flow depths, compared to the modified U.S. Army Corps of Engineers formula.</p>