USACE EMERGENCY RESPONSE SALTWATER SILL CONSTRUCTION (UPDATE):
The U.S. Army Corps of Engineers (USACE) completed construction of the saltwater sill to the augmented height of (-30 feet) from September 24 to October 15 (2023) with a 620-foot wide deep-draft channel (-55 feet). The diagram depicting the sill’s design as provided by the USACE is attached. The Crescent River Port Pilots Association and Federal Pilots and Docking Masters of Louisiana continue to report no problems with one-way ship traffic moving through the area of the saltwater sill (Mile 63.8 Above Head of Passes [AHP]) with minimal (manageable) impacts to ship traffic.
Weeks Marine removed its equipment from the immediate vicinity of the saltwater barrier on January 1, 2024, and on January 2, 2024, the U.S. Coast Guard released an MSIB that establishes safe transit conditions for shallow-draft vessels drafting 25 feet or less to transit over the saltwater sill favoring the western side of the buoys that outline the deep-draft notch. The sill design features the 620-foot-wide navigation notch (-55 feet) marked with four buoys U.S. Coast Guard buoys to identify the deep-draft channel. The MSIB limits all vessels drafting 25 feet or greater to one-way traffic from Mile 63.5 AHP to Mile 64 AHP and directs all deep-draft vessels “transiting this zone shall proactively make passing arrangements to avoid meeting or overtaking between MM 63.5 and 64.0.” The lower 2700 feet of the Alliance Anchorage will remain closed to all traffic, to help facilitate safe transits for the shallow-draft vessels between the right descending bank and the western buoys.
The USACE contracted Weeks Marine to construct the saltwater barrier in 2022 and twice in 2023. Weeks raised the level of the saltwater sill with their cutterhead dredge J.S. CHATRY, the dredge, pipeline, and equipment are no longer in the area. These transit arrangements are expected to remain in effect until significantly higher river stages wash the saltwater sill away, no official projections at this time, but it could be March or later this spring before normal transit conditions resume. The USACE reports that the sill seems to be remaining intact and that they will continue to monitor surveys of the sill structure for degradation.
The USACE last measured the saltwater wedge on January 15 at Mile 44.7 AHP confirming the higher river stages have pushed the saltwater wedge almost 20 miles downriver since the last measurement. The USACE also predicted that on January 22 the wedge would retreat down to Mile 11 AHP (Venice) – again due to the higher river stages.
This is the first time the sill’s construction has been required in back-to-back years as it was constructed during last year’s low water event also by Weeks Marine (October 2022). The hydrology of the Mississippi River is ever changing as 2019 was the first time the Bonnet Carré Spillway had been operated in back-to-back years (2018/2019) and then twice in the same year (2019) and briefly in 2020 (three years in a row).
The stage at Cairo (IL) at 1000 hours today was 23.57 feet with a 24-hour change of + 3.47 feet.
The National Weather Service’s (NWS) Extended Streamflow Prediction forecast the stages at Cairo will continue a slow steady rise to 33.0 feet on February 1 and will then begin a slow fall to 19.4 feet on February 22 (2024).
