Steven E. Pells

Water wave attenuation due to opposing wind

A laboratory investigation of the attenuation of mechanically generated waves by an opposing wind has been completed. Wave attenuation was quantified by measurements of the decline in surface variance. These measurements show higher effective levels of monochromatic wave attenuation than predicted by air-side measurements: approximately an order of magnitude higher than measurements by Young & Sobey (1985) and, a factor of 3 higher than those of Donelan (1999) for waves in a JONSWAP spectrum. Furthermore, they show that theoretical estimates currently underestimate the attenuation rates by a factor of at least 3. This study has shown that the magnitude of wave attenuation rates due to opposing winds is approximately 2.5 times greater than the magnitude of wave growth rates for comparable wind forcing. At high wave steepnesses, detailed analysis suggests that air-side processes alone are not sufficient to induce the observed levels of attenuation. Rather, it appears that energy fluxes from the wave field due to the interaction between the wave-induced currents and other subsurface motions play a significant role once the mean wave steepness exceeds a critical value. A systematic relationship between the energy flux from the wave field and mean wave steepness was observed. The combination of opposing wind and wind-induced water-side motions is far more effective in attenuating waves than has previously been envisaged.

Rock Mass Erodibility

AbstractErosion of rock masses by water typically involves unraveling of blocks of rock along existing defects in a manner that is not represented by analytical solutions for sediment transport or ...

The influence of geological conditions on erosion of unlined spillways in rock

Erosion in 33 unlined spillways in rock has been studied for dams in Australia, South Africa and the USA. Geological factors which influence the amount of erosion have been identified using published and project data and spillway site inspections by the authors. These are the orientation, persistence, spacing and nature of rock defects including bedding partings, joints, foliation and shears. The presence of kinematically viable blocks which can be detached and the persistence of the basal defect for these blocks are the most important factors. Where spillways discharge on to a natural slope the presence of valley stress relief features, such as sheet joints parallel to the slope, or kinematically viable blocks, often with open sub-vertical defects, can lead to significant erosion even with small spillway discharges. The mechanism can be one of slope instability rather than erosion as water pressure destabilizes the slope. A rock mass characterization index, the ‘Rock Mass Erodibility Index’ (RMEI), which considers spillway flow conditions and erosion mechanisms, has been developed. It can be used as a guide to spillway erosion and, when coupled with stream power for spillway flows, provides a method for preliminary assessments of likely amounts of spillway erosion.

Application of Dupuit’s Equation in SWMM to Simulate Baseflow

AbstractThis article demonstrates a procedure for simulation of baseflow in the EPA Storm Water Management Model (SWMM) using the SWMM groundwater flow editor. The procedure is based on simulation of baseflow by usage of the one-dimensional (1D) Dupuit analytical solution to groundwater flow. The analytical basis for the procedure is presented, and a solution within SWMM is validated.