Though the general structure and location of the convergence zone has been described by Fredriksson (1993) and Swift et al. (1996), present availability of the Differential Global Positioning System (DGPS) enables more precise and reliable positioning of the convergence zone axis. Since this information is critical for siting the two opening anchors of the U-shaped containment configuration (see Figure 4), a re-surveying of the convergence zone track was highly desirable. Currents at the anticipated position of the submergence plane device and bathymetry of the area were also required to make siting and anchoring decisions.

Field trials of the latest submergence plane barrier, Bay Defender II (BDII), were necessary to thoroughly check this recently fabricated design. Since anchoring off Dover Point is difficult and crucial to the project, anchoring strategies also needed to be developed and field tested before the full convergence zone field test.

The culmination of this work was the field deployment of BDII as the apex of a containment configuration placed to intercept oil concentrated by the convergence zone. Lead-in conventional booms on both sides would each be 400 feet long. The exercise would be a cooperative effort involving the Piscataqua River Cooperative (the consortium of four Piscataqua terminals organized to combat oil spills), the NH Department of Environmental Services (DES), the U.S. Coast Guard (USCG) and the Portsmouth Naval Shipyard (PNSY).

Development of a pumpout system was scheduled for Winter 1999-2000 and Spring 2000. Thus the final task of the first six-month time period was to initiate information gathering with respect to commercially available systems and what the Coop and DES already have on hand. Next, calculations for pumpout specifications under possible spill scenarios and the evaluation of systems would be started.

WORK ACCOMPLISHED

Convergence Zone Survey

To add to our data base of convergence zone position, essential for siting BDII optimally, the convergence zone was mapped over the flood tide on August 16, 1999. The method of observation consisted of releasing a drogue off Sprague-Newington between navigation buoys C13 and N16 and recording drogue position using a small boat equipped with a DGPS. The drogue was fabricated to respond to surface current in the upper half meter. After release, the drogue quickly moved towards and locked-in to the convergence zone, as evidenced by floating debris. The drogue was recovered at the end of each run in the vicinity of the Dover Point bridge or the Hilton Park ramp. The drogue was then quickly transported back to the release area for the next run. Ten runs were completed over the duration of the flood.

Results shown in Figure 5 indicate considerably more scatter than previous tracks reported by Fredriksson (1993) and Swift et al. (1996). Three runs actually ended while moving towards the Upper Piscataqua - a path that had not been seen in many prior drogue experiments. The track of the last drogue release (red), which stayed along the axis of the main channel, is, however, typical of previous tracks recorded during the very last portion of the flood.

The scatter is explained by the very small salinity gradients that prevailed at the time; Summer 1999 was unusually dry, so freshwater input was extremely low. Evidence presented by Swift et al. (1996) indicates that density gradients have an important role in driving surface current convergence. Thus, the diminished density gradients reduced the converging surface current effect. The Figure 5 data set, therefore, represents an extreme in convergence zone variability.

In response to these observations, the mouth of the containment configuration was opened up by increasing the length of the lead-in booms over that originally proposed. The anchor points were sited optimally, so that the configuration would capture the majority of oil concentrated in the zone.

In addition, several surface current measurements were made off Dover Point in the general area in which BDII itself would actually be sited. Speeds ranged from 1.4 to 1.9 knots - all within the demonstrated operating range of BDII. It should be noted, however, that the surrounding areas can have current speeds up to 4 knots, so placement of BDII is critical.

Bathymetry Survey

Bathymetry in the vicinity of the Dover Point convergence zone was measured on August 30, 1999. A small boat was used that was equipped with a depth sounder and DGPS reading into a laptop computer. The time of each depth and position entry was also recorded. Data was acquired along a path consisting of closely spaced cross and along channel transects. Measurements were then corrected for tidal elevation to depth at mean low water.

Results, shown on Figure 6, are consistent with several observed features of flood tide flow. The convergence zone track, for example, closely parallels the "wall" off the South Berwick shore. There is visible upwelling and turbulence over the central shallow region. The main deepwater channel under the Dover Point bridge is heavily biased towards the Newington shore. On the Hilton Park side, flood currents under 2 knots in the region of 25 to 30 ft depths approximately double when the shallow extension is encountered under the bridge.

Depths in the area of where the anchors are expected to be placed are 30 to 40 ft. Thus scope for the usual lightweight, embedment anchors should be at least 200 ft.

Preliminary Field Trial of BDII

At the start of this project, BDII had recently been fabricated but had not yet been field tested. Based on the successful Bay Defender prototype shown in Figure 1, BDII incorporated more robust materials and construction and employed several innovations in assembly methods. It was, therefore, necessary to conduct a "shakedown" field test before the full Dover Point deployment.

The "shakedown" took place on August 12, 1999 off Sprague-Newington (see Figure 2), during the flood tide, using the Coop shore facilities, one Coop towboat and a Portsmouth Towing vessel. BDII was assembled on the shore at low water, and short lead-in conventional booms were attached to the two bow corners. The system was then towed from the beach at the onset of flood current. Towing was done end-first using one of the mid-end longitudinal tow points (as designed). When on-site, each lead-in boom was anchored "on the fly" using crown lines on 55 lb. Fortress anchors. Side anchors were then set and adjusted.

The set was successful resulting in a symmetric configuration with BDII at the apex as shown in Figure 7. Flood currents at the site, near the NH shore southeast of Sprague-Newington, averaged 1 1/2 knots giving a meaningful assessment of shape rendition. Recovery was in reverse order. To test if other towing methods were possible, BDII was towed back to shore by one of the lead-in booms. There were no severe problems encountered, but this method was generally found to be less handy than use of one of the mid-end longitudinal tow points.

Some minor modification were made to correct weaknesses noted during the shakedown experiment. Several fastening systems were upgraded employing more robust and reliable connections. The forward corners and the entire forward edge of the submergence plane fabric were strengthened. A bridle was made to control the angle of the forward end longitudinal during towing. A slight planing attitude was built-in to prevent diving while towing against high currents.

Preliminary Anchoring Trial

Anchoring a large containment configuration in the vicinity of Dover Point was a critical issue. Discussions were held with all concerned with the deployment logistics, and a field trial was completed. In a planning session with Coop representatives, the idea of using very heavy permanent anchors was rejected due to objections from fishing interests and vandalism. Instead, it was decided to use double 55 lb. Fortress anchors on each side of the opening. Each pair would use a common buoy with the two anchor lines extending up-current in a V-shape. Five to one scope would be used with a length of heavy chain at each anchor. Anchors would be placed so the mouth intercepted the convergence zone with the exact setting done using DGPS.

The field trial for this anchoring concept took place on September 23, 1999 during the flood tide. A containment configuration with 600 ft of conventional boom each side (1200 ft total) was set without BDII. In place of BDII, two towboats occupied the apex position representing the drag of the submergence plane barrier. The configuration was maintained for a sufficient length of time to confirm that no dragging was taking place.

A check of the final anchor buoy coordinates (using DGPS) revealed that the configuration was about 200 ft down current (towards the bridge) than intended. This was attributed to "sailing" of the anchors, possible surface sediment movement and anchor drift before being set hard into the fixed bottom. For the full system deployment, allowance for movement would, therefore, have to be made. Also, the deployment plan would provide for the prompt and hard setting of each anchor immediately after lowering.

Convergence Zone Field Test

The main goal of the Summer and Fall 1999 effort was to deploy BDII as the apex of a containment configuration in a position to contain oil concentrated by the convergence zone. The lead-in boom anchors would be set so that the mouth intercepts the average position of the convergence zone track. BDII would be sited in an area where its speed capacity would not be exceeded. Thus situated, the configuration would be effective in preventing slicks originating downriver, where oil transport operations take place, from entering Little Bay and Great Bay.

The full convergence zone deployment took place on October 13, 1999 as a cooperative effort among UNH, the Coop and Portsmouth Towing. Representatives from NH DES, the PNSY and the USCG were on hand and also took part. Towboats from the Coop and Portsmouth Towing participated, and there were a number of additional boats for observing the activities.

At low water, BDII was assembled on the cobble/mud beach just north of the Hilton Park boat ramp. The anchors were put in at low slack water using DGPS and employing the same gear tested in the preliminary trial (double 55 lb Fortress anchors at each end of the opening). The anchors were immediately set hard in the flood current direction. At the same time, an aluminum barge containing the conventional lead-in boom was towed to the site and anchored off the Hilton Park ramp.

Two 400 ft sections of river boom (6 inch skirt) were pulled out of the barge to serve as the lead-ins on each side. After attaching the lead-in boom sections to the anchor buoys, BDII was attached in the apex position.

As the flood commenced, it became apparent that BDII was not square to the current. It was angled slightly clockwise, so the local current approached from its left. Observing the action of current on the lead-in booms, it was clear that the entire configuration was so large that the current changes direction (turning towards the south) over the system's length. A correction was made by taking a small bight in the southeast lead-in boom at the anchor end thereby effectively shortening this (starboard) boom by 30 ft. BDII was then square to the current, while the lead-in boom loop was outside the configuration and did not interphere with the containment function. Demonstrating the ability to adjust the system without resetting anchors was important since, under emergency conditions, achieving perfectly placed anchors may be difficult.

The final configuration is shown during mid-flood conditions in the the Figure 8 photograph taken from the Dover Point bridge. The towboat seen making an inspection is 30 ft long. Current speed at BDII at this time averaged 1 1/2 knots, so positioning of the device was good. The system was allowed to remain set for an additional hour to verify that there was no anchor drag. Overall, participants were pleased with the successful deployment and the new capability to close off the upper system before Dover Point.

Several minor difficulties were encountered during recovery when gear was allowed to sag into the much higher currents near the bridge. There was no physical danger, however, and all equipment was recovered without damage within 1 1/2 hours.

Initiation of Pumpout Technology Investigation

Work has started on the next objective - developing and testing a system for recovering oil from the containment volume. This effort will continue through the end of the first year (August 2000). Hardware and machinery available commercially have been surveyed, and opinions are being solicited from experienced experts within the USCG and Ohmsett. Input is also being obtained from NH DES and the Coop regarding likely and worst case spill scenarios, as well as equipment already on hand. We anticipate purchasing and testing the most effective system before August 2000.

CONCLUSION

The results of the first six-month effort, culminating in the submergence plane deployment at the Dover Point interception site, are very positive. This has demonstrated the technical feasibility of solving a long standing problem - how to prevent spills originating in the lower system from entering the ecologically sensitive Little Bay and Great Bay regions (including the Great Bay Research Reserve).

This approach using a hybrid of conventional boom and BDII is also conveniently adaptable for protecting individual river tributaries should the need arise. Closing off rivers such as the Bellamy, Oyster, etc. at the mouth is entirely feasible. Unlike previous approaches using conventional boom only, oil is moved away from both shorelines to a central collection point.

Desemination of technical developments to other applications and locations has also been initiated. During a meeting held at UNH on November 18, 1999, the results of the Fall 1999 effort were communicated to oil response leaders within USCG Headquarters, the USCG Research and Development Center, the Atlantic Strike Team, Minerals Management Service and Ohmsett. The adaptability of submergence plane technology to other sites and applications was discussed. A possible demonstration at Cape May, New Jersey is now under consideration.

The progress made thus far justifies continuation of the program, particularly the development of a longer submergence barrier during the second year. The variability found in the convergence zone track provides added motivation for enlarging the sweep capacity of the submergence plane barrier.

ACKNOWLEDGEMENTS

This research was supported in part by government funds under a grant awarded by the Cooperative Institute for Coastal and Estuarine Environmental Technology (CICEET).

A special thanks is extended to Mr. Steve Root of Portsmouth Towing, as well as co-captains and crews, for their participation in the planning and carrying out of the deployment studies. Their practical experience, seamanship and superior boat-handling skills are much appreciated. The assistance of Mr. Jim Collins and the Piscataqua River Cooperative is also gratefully acknowledged.

The field work would not have been possible without the enthusiastic help of the dedicated graduate students in the University of New Hampshire's Master of Science in Ocean Engineering program. Mr. Ata Bilgili, in particular, is to be thanked for his role in conducting the convergence zone tracking and bathymetry surveys. Mr. Joe Santamaria of JPS Industries completed the fabric reinforcement, and we are grateful for his assistance.

The many helpful technical discussions with Mr. Kurt Hansen of the U.S. Coast Guard R&D Center, CDR Chris Doane and LCDR Rob Loesch of the U.S. Coast Guard, Mr. Jim Lane of Minerals Management Service and Mr. William Schmidt of Ohmsett are very much appreciated.

REFERENCES

Fredriksson, D.W. (1993) "Protection of Little Bay and Great Bay from Oil Spills", M.S. Thesis, Ocean Engineering, UNH, Durham, NH.

Steen, R. (1997) "A Flexible Oil Barrier for Fast Currents", M.S. Thesis, Ocean Engineering, UNH, Durham, NH.

Swift, M.R., D.W. Fredriksson and B. Celikkol (1996) "Structure of an Axial Convergence Zone", Estuarine, Coastal and Shelf Science, Vol 43., 109-122.

Swift, M.R., B. Celikkol, R. Steen, M. Ozyalvac and D. Michelin (1998) "Development of a Rapid Current Containment Boom: PHASE III, Report submitted to MMS, Volpe Center, Cambridge, MA.