Formulating Dynamic Pressure Characteristics at Flat Plunge Pool Bottom and Inside Rock Joints

Accurate prediction of pressure caused by plunging jets of high spillways is essential for assessing the bed rock scour initiation and expansion. The hydrodynamic pressure characteristics at plunge pools’ bottom and inside rock joints are assessed based on the experimental results and the analysis of extensive available library experimental data. A new empirical model of mean dynamic pressure coefficient, the $C_P$, and pressure fluctuation, the $C_P^{\prime }$, for core and developed jets at stagnation point are presented. The effects of jet break-up ratio ($L/L_b$), the jet’s fall height $L$ to the jet break-up length $L_b$ ratio, on $C_P$ and $C_P^{\prime }$ at the pool bottom and inside the rock joints are assessed. The maximum $C_P$ at pool bottom and inside rock joint occurs in $L/L_b\le 1.2$ range while the maximum $C_P^{\prime }$ occurs within $0.85<L/L_b<1.2$ range at pool bottom. Transient dynamic pressures inside the rock joint generates a stress field inside the rock joint, which highly depends on the joint length, and it is characterized by the stress intensity factor. The effects of joint length on pressure filed and the stress intensity factor are assessed to determine the fluid resistance due to joint elongation and to formulate the stress intensity factor. A new data set of dynamic pressure measurements inside rock joints at different lengths is presented. Experimental results indicate that the stress intensity factor inside the rock joint reaches its maximum values when the joint length reaches 4 times the initial formed rock joint length in rock mass.

Determining the pressure characteristics is essential when assessing the rock joint stability and the scour geometry at plunge pools. In this study, the hydrodynamic pressure characteristics at the bottom of plunge pools and inside rock joints are investigated, and a new empirical model of mean dynamic pressure coefficient, $C_P$, and pressure fluctuation, $C_P^{\prime }$, for the core and developed jets is presented. The effects of joint length and the jet break-up ratio ($L/L_b$) on the pressure field and the stress intensity factor are assessed. The stress intensity factor inside the rock joint is modified to accurately assess the hydraulic fracturing of the rock joint and scour geometry development.