Tahirih C. Lackey

Predicting dredging-associated effects to coral reefs in Apra Harbor, Guam - Part 1: Sediment exposure modeling

Model studies were conducted to investigate the potential coral reef sediment exposure from dredging associated with proposed development of a deepwater wharf in Apra Harbor, Guam. The Particle Tracking Model (PTM) was applied to quantify the exposure of coral reefs to material suspended by the dredging operations at two alternative sites. Key PTM features include the flexible capability of continuous multiple releases of sediment parcels, control of parcel/substrate interaction, and the ability to efficiently track vast numbers of parcels. This flexibility has facilitated simulating the combined effects of sediment released from clamshell dredging and chiseling within Apra Harbor. Because the rate of material released into the water column by some of the processes is not well understood or known a priori, the modeling approach was to bracket parameters within reasonable ranges to produce a suite of potential results from multiple model runs. Sensitivity analysis to model parameters is used to select the appropriate parameter values for bracketing. Data analysis results include mapping the time series and the maximum values of sedimentation, suspended sediment concentration, and deposition rate. Data were used to quantify various exposure processes that affect coral species in Apra Harbor. The goal of this research is to develop a robust methodology for quantifying and bracketing exposure mechanisms to coral (or other receptors) from dredging operations. These exposure values were utilized in an ecological assessment to predict effects (coral reef impacts) from various dredging scenarios.

PTM modeling of dredged suspended sediment at proposed Polaris Point and Ship Repair Facility CVN Berthing Sites – Apra Harbor, Guam

Abstract : The U.S. Navy is studying alternatives for the construction of a new, deep-water wharf at Apra Harbor, Guam, to provide a berthing site for transient nuclear powered aircraft carriers. The U.S. Army Corps of Engineers, Engineer Research and Development Center was tasked with predicting the fate of resuspended dredged sediment during dredging operations for this port development. One byproduct of dredging is release of suspended material into the water column. This material temporarily increases turbidity during and after the dredging operation. Another byproduct is the deposition of suspended solids on surrounding coral. The Particle Tracking Model (PTM) was applied to evaluate exposure to coral for various dredging alternatives.