Estimating the Erodibility of Ephemeral, Cohesive Streams that Receive Coal Bed Natural Gas Product Water in the Powder River Basin of Wyoming

In the Powder River Basin (PRB) of Wyoming, a primary method of Coal Bed Natural Gas (CBNG) product water disposal is to discharge the water into existing ephemeral and intermittent channels. The increased and continuous application of CBNG product water has the potential to significantly alter the morphology of stream channels with significant increases in erosion. Traditional methods for analyzing vertical (bed) stability fall into two primary categories: (1) maximum permissible velocity and (2) critical tractive force. Currently, incipient motion analyses are conducted using permissible velocity tables for relatively simple sand bed channels. This method does not account for the cohesive nature of soils in the PRB. In response, a study was completed to attempt to correlate cohesive soil properties to critical shear stress and the erodibility coefficient, determined from a submerged jet device, to aid in the prediction of a permissible flowrate using the critical tractive force method and simple field investigations. This paper describes the results of this investigation. Excess stress parameters (τ_c and k_d) and cohesive soil properties are presented for 25 ephemeral, cohesive channels in the PRB. The coefficients obtained from test results are used to determine the erodibility of cohesive soils. The rate of erosion, ε (in m/s), is proportional to the shear stress in excess of the critical shear stress expressed as ε = k_d(τ_e – τ_c), where k_d is the erodibility coefficient (m³/N-s), τ_c is the critical shear stress (Pa), and τ_e is the effective shear stress (Pa). Significant relationships between cohesive soil characteristics and τ_c were also developed for the study sites. These relationships can be used to derive τ_c for other sites in the PRB based simply on the laboratory testing of cohesive soil properties. This information is critical to agencies looking for a conservative methodology to protect cohesive channels against potential flow-induced instability given the variability of the field data and the ephemeral nature of the stream channels.