Hampton Roads Sanitary District—Outfall Survey, Inspection, and Design

Client: Hampton Roads Sanitary District

Location: Virginia

Services: Marine Engineering

The HRSD entered into a contract with KCI to provide professional engineering services including underwater inspection services, surveying, structural design and cost estimating associated with HRSD treatment plant outfall structures. HSRD owns and operates nine wastewater treatment plants, each with its own outfall(s).

Survey and Inspection. During April and May 2000, an inspection of eight of the outfalls was completed by KCI in preparation for the implementation of an Outfall Inspection Program. Inspections of land facilities included the outfall at the Army Base Plant and two at the Virginia Initiative Plant (VIP). The inspections consisted of penetrations inside portions of the outfalls on land. The Army Base Plant penetration included approximately 665 LF of 48” diameter reinforced concrete pipeline. Inspections at the VIP plant included approximately 150 LF of the 48” emergency or controlled diversion outfall and the 50’ long 54” diameter short outfall, both constructed of reinforced concrete pipe. Inspections of the sub-aqueous outfalls were performed at the Atlantic, Boat Harbor, Chesapeake Elizabeth, Nansemond, VIP, Williamsburg, and York River facilities. Work included topographic surveys of the beach; hydrographic surveys of the outfalls; diving inspection of diffusers, tie downs, and emergency discharges; and field verifying GPS coordinates. Outfalls facilities ranged from 36” to 66” in diameter, and included HDEP, RCP, and PCCP construction. Findings and recommendations were documented in the July 2000 report, “Outfall Inspection Project.”

Boat Harbor and Williamsburg Plant. In 2001, HRSD added the design of repairs and construction administration of the Boat Harbor Plant and the Williamsburg Plant. One hundred feet of the Boat Harbor diffuser had been damaged by a vessel collision. KCI designed a replacement for the 54” diameter pre-stressed concrete cylinder pipe. Many of the Williamsburg Plant diffuser outlets were found to be silted over. KCI designed the partial replacement of the 525’ long diffuser including 72, three-inch diameter outlet pipes. In addition, a new warning buoy was installed around the outlet. KCI provided construction administration and inspection services on both of these repairs.

Atlantic Plant. In 2001, HRSD tasked KCI to provide additional investigation services and structural analysis for the Atlantic Plant outfall diffuser. The normally buried main barrel of the outfall diffuser was found to be exposed several feet. KCI’s follow-up hydrographic survey of about one square mile found a large depression area exposing the pipe. Detailed cross sections of pipe exposure were used to design repairs. Hydrographic survey data was also used in the structural analysis and risk analysis of the exposed pipe which determined the urgency for repairs for the Atlantic Plant outfall diffuser is 3-5 years.

Army Base Treatment Plant. Hampton Roads Sanitation District’s Army Base Treatment Plant is situated within the Norfolk International Terminals site on the west side of Norfolk, south of the US Naval Station. The existing outfall includes a land portion of 48” diameter reinforced concrete pipe and a sub-aqueous 2,000’ portion of 48” diameter flexible joint cast iron pipe that extends from the treatment plant westward discharging an average of 12 MGD of treated effluent into the Elizabeth River. HSRD contracted KCI to investigate and design a multi-port diffuser extension to meet Virginia Pollution Discharge Elimination System (VPDES) permit requirements. Work involved detailed design and cost investigation, construction administration, field engineering, and on-site construction inspection and certification. KCI conducted an internal inspection of the outfall, including diving on the offshore end of the existing outfall pipe; identifying the type of pipe and manufacturer; completing a detailed bathymetric survey of the river bottom in the vicinity of the outfall and inspecting/dimensioning the three concrete chambers along the outfall outside of the plant. KCI developed three alternatives capable of diluting effluent consistent with a concept developed previously by another consulting firm. In developing alternatives, engineers considered diffuser alignment, pipe materials, port and/or riser configuration, impacts to plant hydraulics, potential construction methods, and requirements for marker buoy or other warning/protective devices. Preliminary engineering included investigating the elevation of the 25-year storm; exploring various configurations; determining the head loss for these configurations; determining the diffusion of these configurations; and exploring various pipe and port materials. To confirm the hydraulic profile of the plant, KCI engineers physically observed both the effluent elevation and tide elevation at Manhole Box #1 on three separate dates to verify head loss caused by the existing outfall pipe. In addition, KCI engineers interviewed Army Base representatives, including the Plant Manager, regarding past maximum flow conditions. Site selection evaluations considered risk of collision by vessel traffic, dredging operations, sedimentation, and adequate depth of diffusion. In the case of the Army Plant Diffuser, major factors included: maximizing the distance from the shipping channel; minimizing the diffuser height relative to the local bottom; providing diffuser markings (warning buoy); and various pipeline materials. Key design factors included: roughness (for hydraulics), durability, longevity, cost, and ease of installation. Other design considerations included: methods of joining a new multi-port diffuser to the existing pipe; joint restraint to ensure the joints maintain integrity under the increased trust gravel bedding; and anchoring system including thrust restraint effluent containment under 100-year flood and 100-year tide conditions. Alternatives developed during concept design included a wye multi-port diffuser with two branches, a 215’ straight HDPE multi-port diffuser, and a 215’ straight HDPE multi-port diffuser with open ports at the spring line. Each alternative was modeled using the EPA’s Visual Plumes software to determine the respective dilution ratios and determine compliance with VPDES permit requirements. Cost estimates were prepared for each alternative. A report summarizing the inspections, analyses, and conceptual designs was prepared, including a comparison of the three alternatives, a recommended alternative, implementation plan including operations and maintenance expenditures, and schedule for project completion. After selection of a preferred alternative, KCI prepared final design and construction plans and cost estimates for the wye multi-port diffuser that contained 44 diffuser outlets. During construction, KCI provided project management, technical consultation, and resident inspection.

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