Use of Strategic Placement of Dredged Sediments to Support Horseshoe Island in the Atchafalaya River, Louisiana: A Preliminary Ecological Survey

Abstract

This technical note describes an Engineering With Nature (EWN) project being conducted in the Atchafalaya River, LA. The current work presents a wetland classification, vegetation survey map, preliminary ecological inventory, and soil descriptions for an island receiving strategically placed dredged sediments (i.e., Horseshoe Bend Island) located within the Atchafalaya River. The practice of strategically placing dredged sediments upriver of a naturallyoccurring island was conducted with the intent of aiding the islands growth to produce greater environmental benefits than otherwise would be present using more conventional placement practices. Results provide background information regarding ecosystem classification and mapping, floral and faunal composition of the island, and background data supporting future research efforts. Opportunities for additional research are also presented. BACKGROUND: Horseshoe Bend is a segment of the Atchafalaya River and Bayous Chene, Boeuf, and Black Federal navigation channel located in the lower Atchafalaya River approximately 20 miles south of Morgan City, LA (Figure 1). Although dredging of the navigation channel received original authorization in 1910, active maintenance dredging of Horseshoe Bend was not required until 1990 because natural channel depths exceeded authorized dimensions. Beginning in 1990 and proceeding thru 2013, 17 maintenance dredging events within this segment occurred at a return interval of 1 to 3 years, with each event resulting in the removal of between 0.5 and 2.7 million cubic yards of shoal material. During the 1990s, placement of shoal material dredged from Horseshoe Bend occurred at eight wetland development sites located along the river's banklines adjacent to the channel. Capacity of these placement sites was nearly exhausted by 1999. Thus, to meet the anticipated disposal requirements for future channel maintenance, the US Army Corps of Engineers (USACE) New Orleans District (MVN) evaluated three placement alternatives: (1) convert the wetland development sites into upland disposal areas; (2) open water placement of dredged material via a long-distance pipeline into the open waters of the Atchafalaya Bay; and (3) mounding of material at mid-river open water placement sites within a 350-acre (142 ha) area immediately adjacent to the navigation channel and upriver of a small naturally forming island (Figure 2). Environmental and cost concerns precluded the selection of alternatives (1) and (2). The third alternative was selected on a trial basis to investigate the impacts of mid-river placement on shoaling trends downriver of the site.

Strategic placement for beneficial use of dredged material

“…The strategic placement project reduced demand on limited dredging resources by selecting a least-cost option for dredged material management while simultaneously providing a sustainable solution that produced environmental benefits. These benefits are quantified by Berkowitz et al (2014). The island formation had an added benefit to the navigation program ( Figure 26).…”

“…Dredge volumes for each event ranged from 0.5 to 1.8 Mcy. (Details of these placement events are provided in Berkowitz et al [2014].) The predominant practice during the 1990s was conversion of nearchannel open water placement areas into wetland through direct placement of the dredged material.…”

“…Evidence indicates that the island (now called Horseshoe Bend Island) formation is significantly enhanced by the strategic dredged sediment placements. Criteria used to determine success are provided in Berkowitz et al (2014) and are based on acres of habitat created as well as habitat diversity. The Island continues to grow as additional sediment is placed upstream.…”

“…As the Island grew, vegetation was established through natural processes, and birds now use the Island as a breeding ground. The 86-acre ecosystem that developed from the sand bar over the past decade has been meticulously documented in Berkowitz et al (2014).…”

“…Some island development was expected but not to the extent that occurred. Details of the island evolution are provided in Berkowitz et al (2014) and Kim and Suedel (2015).…”