The Restoration of Batiquitos Lagoon | The Restoration Story

Photo of one of the nesting islands

After a lengthy period of environmental documentation, design, and permitting, lagoon restoration began in March 1994 and was completed under the managing partnership of the City of Carlsbad and the Port. Work entailed a process of dredging and improving the infrastructure. The major construction process involved

dredging sand from the central basin for use in beach replenishment and creation of tern colonies (1994-1995)

placement of fine sediment materials dredged from the eastern basin into the central basin borrow pit and construction of the lagoon mouth jetties (1995-1996)

capping the central basin borrow pit, containing fine sediments, with sand dredged from the western basin.

Bulldozer beginning to open the mouth of the lagoon

On December 7, 1996, the restoration was completed and the mouth of the lagoon was opened to re-establish continuous tidal flushing. Afterward, minor additional work was done during early 1997 to

remove and recontour an access ramp in the eastern basin

plant pickleweed along parts of the northern shore of the eastern basin

remove an undesirable underwater ridge in the entrance channel of the lagoon

weed and repair erosion damage on tern colonies and prepare colonies for the 1997 nesting season

enhance the facing of the railroad bridge abutments

Innovative Elements of the Construction Project at Batiquitos Lagoon

Environmental Engineering

Bulldozer continuing to open the mouth of the lagoon

Engineering was key to the success of the restoration project. Engineering expertise was used in the environmental review phase to ensure that proposed habitat changes were feasible. Feasibility testing included hydrological modeling, sediment transport studies, and littoral drift modeling of the alternative restoration plans developed during environmental review. The environmentally superior alternative that was selected was also feasible and functional. During the permitting phase of the project, a physical model of the tidal inlet was constructed to test how the inlet would respond to cobbles, which are present on nearby beaches. The jetty length was adjusted based on the results of this study. Because the physical factors in the lagoon are so complex, national and international experts in coastal engineering reviewed the project design.

Construction

Initial opening of the lagoon to the Pacific Ocean

Although some elements of the project were fairly routine for an experienced contractor, all the project features combined with the many environmental restrictions required careful scheduling and sequencing of construction. The sequencing reflected a need to avoid certain types of construction during the bird nesting season, to avoid disturbing salt marsh, to cease beach nourishment during summer, and to control water levels. The project team reviewed contractor submittals for construction of the lagoon to ensure that resources were being protected. Other, more unconventional, construction methods were used in the restoration project. Dredges were custom-modified to yield a precise, specified acreage of very flat slopes within certain elevation bands in the eastern basin. Stable vertical positioning of the dredge was accomplished by a water level control/feed system that allowed a stable water level in the lagoon so that precision dredging could occur. This stable water level was accomplished by constructing an intake pipe that extended from the lagoon into the ocean. As the dredge moved water and sand to the beach nourishment areas, water was pumped into the lagoon to maintain the water level.

Contractor Payment

Expanded opening of the lagoon's mouth

Dredge contractors typically get paid for the quantity of material dredged, with a limitation on overdredging. Although this method of payment is acceptable for navigation channels, it does not provide the proper incentive where very flat slopes must be created. To achieve the desired habitat types at Batiquitos Lagoon, two payment schedules were given to the contractor. In the eastern basin, where some slopes had to be in excess of 100:1, the contractor was paid by the acres constructed within a 6-inch margin of error from the design. In the central basin, where a large pit was to be excavated and slopes typically had to be 10:1, the contractor was paid for quantity, with a larger tolerance. This provided an incentive to the contractor to achieve the design in all parts of the lagoon while allowing him to achieve a profit and make progress on the project.

Dredge Material Disposal

Lagoon mouth with scour protection in place

Contouring of the lagoon's eastern basin generated nearly 1 million cubic yards of silts, clays, and organic materials for disposal. Depositing this material on upland areas or offshore was determined to be infeasible. A plan was developed to dispose of the material in a deep pit dredged in the sandy material of the central basin. Sandy material dredged from the central basin was used to construct nesting sites and to nourish local beaches. After placing the fine sediments in the pit, the area was capped with sand taken from the western basin. This activity required care and patience; the process was drawn out by waiting periods because the bulking factors and settlement rates of dredged materials were not known. The process was complicated further by the need to cap the fine material in a way that would not disturb it and that would achieve the final design elevations.