Preliminary Assessment of Potential Impacts to Dungeness Crabs from Disposal of Dredged Materials from the Columbia River

Abstract

Dredging of the Columbia River navigation channel has raised concerns about dredging-related impacts on Dungeness crabs (Cancer magister). The overall objectives of this effort are to synthesize what is known about disposal effects on Dungeness crabs (Phase 1) and to offer approaches to quantify the effects, including approaches to gain a population-level perspective on any effects found in subsequent studies (Phase 2). This report documents Phase 1, which included 1) development of a conceptual model to integrate knowledge about crab biology and the physical processes occurring during disposal, 2) application of physics-based numerical modeling of the disposal event to understand the physical forces and processes to which a crab might be exposed during disposal, 3) conduct of a vulnerability analysis to identify the potential mechanisms by which crabs may be injured, and 4) recommendations of topics and approaches for future studies to assess the potential population-level effects of disposal on Dungeness crabs. The conceptual model first recognizes that disposal of dredged materials is a physically dynamic process with three aspects: 1) convective descent and bottom encounter, 2) dynamic collapse and spreading, and 3) mounding. Numerical modeling was used to assess the magnitude of the potentially relevant forces and extent of mounding in single disposal events. The modeling outcomes show that predicted impact pressure, shear stress, and mound depth are greatly reduced by discharge in deep water and somewhat reduced at longer discharge duration. The analysis of numerical modeling results and vulnerabilities indicate that the vulnerability of crabs to compression forces under any of the disposal scenarios is low. Modeling results and other information suggest that crabs may be vulnerable to injury from surge currents under some but not all disposal scenarios. For the shallow-water, short-duration disposal scenarios, the shear stress and surge currents estimated from the modeling and observed in the field at Palos Verdes appear to be sufficiently high to mobilize and transport bottom sediment and at least juvenile crabs. The surge currents estimated from modeling the deep water scenarios are not sufficient to mobilize sediment greater than 1 mm or juvenile crabs. The effects of such movement, if any, are not known and can be reduced by behavioral responses by crabs. There is evidence that burial by dredged materials can affect crab respiration and survival, but confounding factors in previous experiments preclude conclusions about thresholds and extent of effects. We recommend that future studies focus on burial effects during shallow water, short duration disposal scenarios and take into account the potential for behavioral responses to mitigate any effects.iii During disposal, the characteristics of the dredged materials (e.g., size distribution, cohesiveness), the water column (e.g., depth, stratification, currents), and the bottom (e.g., slope, grain-size distribution) interact to determine the characteristic...

Laboratory Assessment of Potential Impacts to Dungeness Crabs from Disposal of Dredged Material from the Columbia River

“…Modeling by Pearson et al (2006b) predicted maximum surge current velocities to be 3.3 m/s for the dredge Essayons and 4.1 m/s for the dredge Sugar Island when disposing their load at water depths of 45 ft.…”

“…The next step of the Phase I assessment was to conduct numerical modeling to obtain the range of compression force, shear force, and extent of mounding predicted to occur for a MCR material disposal event. Pearson et al (2006b) used the STFATE dredged material disposal model to estimate the magnitude of the three main parameters thought to have the potential to affect Dungeness crabs: 1) pressure developed by the convective descent, 2) the horizontal shear stress generated during dynamic collapse, and 3) the depth of burial following settling of the material. A matrix of disposal conditions was developed for the two dredges most likely to be used in the Lower Columbia River dredging operations, the Essayons and the Sugar Island, resulting in 36 test scenarios modeled in STFATE as follows:…”

“…synthesize existing knowledge about disposal effects on Dungeness crabs and to offer approaches to quantify these effects (Phase I, Pearson et al 2006b); 2. conduct laboratory studies to quantify effects of burial and surge on crab survival (Phase II, this report); 3. use previous results with a demographic model to predict losses to the actual MCR crab populations (Phase III, possible future research).…”

“…Phase I also included the numerical modeling of the disposal process using the Short-Term Fate (STFATE) dredged material disposal model developed by the Corps' Engineer Research and Development Center in Vicksburg, Mississippi, which provided information on the magnitude of vertical and horizontal forces and burial depth expected to be encountered by crabs during a variety of disposal scenarios. The results of the STFATE modeling were evaluated together with published information on crab biology to identify areas of greatest crab vulnerability (Pearson et al 2006b). The Phase I study concluded that crabs were more likely to be vulnerable to injury by tumbling in a surge current or by burial, and less vulnerable to the compression (vertical) forces exerted during a disposal event.…”

“…The Phase I study concluded that crabs were more likely to be vulnerable to injury by tumbling in a surge current or by burial, and less vulnerable to the compression (vertical) forces exerted during a disposal event. The recommended priority for future studies was simulation of burial depths and surge currents generated during a short-duration disposal event in shallow water, with experiments designed to include aspects of crab behavior that could change the vulnerability to a disposal event (Pearson et al 2006b). We review Phase I findings in more detail in Section 2.1.…”