Simulating the Potential Impact of Borrow Site Excavation on Sediment Transport Along the Gulfstream Pipeline, Petit Bois Pass, Alabama

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A numerical modeling analysis was completed to estimate potential seafloor erosion impacts to the Gulfstream and BP pipeline from dredging at the proposed Petit Bois East and West (PBE and PBW) borrow sites. A suite of numerical models was employed in this analysis, including the synoptic-scale hydrodynamic model ADCIRC (ADvanced CIRCulation Model for Ocean and Estuarine Waters) and the local-scale coastal processes modeling package CMS (Coastal Modeling System). The different models relied on various sources of data to specify required inputs such as bathymetry, shorelines, wave spectra, hydrodynamic boundary conditions, and sediment characteristics. Other data sources including tide data and the results of bathymetric change analysis were used to calibrate and corroborate model performance. Three separate model simulations were run for the same 12-month period for 1) existing bathymetric conditions and 2) proposed PBE and PBW borrow sites including a 305-meter (1000-ft) buffer area along the BP pipeline. By running the same time period for both pre- and post-dredging bathymetric conditions, a direct comparison of bathymetric change could be made highlighting the change that occurs as a result of dredging at the borrow sites. In addition to the 12-month simulation, a single high-return-period tropical storm was simulated to gauge what additional impacts to the pipeline may result from relatively infrequent large storm events. Littoral transport in the region of the barrier islands of Mississippi Sound is heavily influenced by storm activity relative to average conditions, so the storm simulation was considered a necessary component of the study.

Because model results for the proposed excavation plan of PBW indicated there was a credible risk of long-term erosion impacts to the seafloor covering the pipeline, the original excavation plan was modified so that no sand would be removed from PBW sub-areas PBW1 and PBW2 that lie outside the 1000 ft pipeline buffer. Morphological model results from Hurricane Katrina simulations accounting for reduced sub-area excavation illustrates that maximum seafloor erosion along the pathway of the pipeline within the borrow site is reduced by an order of magnitude. Based on these borrow site analysis results, it was recommended that the dredging plan for PBW be altered so that there is no dredging of sub-areas PBW1 and PBW2. The modification greatly reduces seafloor erosion risks to the pipeline during extreme storm events.

Mississippi-Alabama Barriers, Mississippi, Alabama, Numerical Modeling, Borrow Site Assessment