Loading...
HomeMy WebLinkAbout1988-03-08; City Council; 9322; Presentation of draft report for preliminary phaseGIT' JF CARLSBAD - AGEND, JILL AB# DEPT 932 a. 03/08/88 MP TITLE: PRESENTATION OF DRAFT REPORT FOR THE PRELIMINARY ENGINEERING PHASE OF THE BATIQUITOS LAGOON ENHANCEMENT PROJECT DEPT. HD^yt^ CITY ATTY ^O5 r*iTV MfSD^i^L^i^l i i nn u n. ^*\ o§ o oo RECOMMENDED ACTION: Accept and file the Draft Preliminary Design Report for the preliminary engineering phase of the Batiquitos Lagoon Enhancement Proj ect. ITEM EXPLANATION: On April 14, 1987, the City Council adopted Resolution No. 9022 approving an agreement with the City of Los Angeles through its Board of Harbor Commissioners (Port of Los Angeles) for the preparation of preliminary engineering studies for the Batiquitos Lagoon Enhancement Project. Thereafter, on April 21, 1987, the City Council adopted Resolution No. 9028 approving a consultant agreement with the firm of CH2M Hill, Incorporated, for the preliminary engineering studies and related technical work for this project. In accordance with the terms of the City's agreement with CH2M Hill, an Interim Report was prepared by the consultants and submitted for City Council review on October 13, 1987. The City Council will receive at this meeting a formal presentation by the consultant group summarizing the Draft Preliminary Design Report for the project. The following study issues are addressed in detail in the report: Existing lagoon sediment characteristics, qualities, and quantities. Volume of dredged/excavated material by type and location for the three (3) enhancement alternatives. Dredged/excavated material disposal sites and methods. Dredging/excavation depths and boundaries including detailed grading plans for the three (3) alternatives. Tidal inlet design recommendation. Beach nourishment design recommendation. Hydraulic modeling including circulation, flushing, and water quality analysis. Lagoon sedimentation and sediment control management plan. Existing bridge placement and methods of protection. Page Two of Agenda Bill No. Avifaunal surveys completed to date. Engineering cost estimates of all project components. Attached is a copy of the Draft Preliminary Design Report's Executive Summary summarizing the study's findings. Copies of the larger Draft Preliminary Design Report and Technical Appendix have been previously distributed to the City Council. FISCAL IMPACT; The study for the preliminary engineering phase of the Batiquitos Lagoon Enhancement Project is funded entirely by the Port of Los Angeles as set forth in the City of Carlsbad/City of Los Angeles agreement . EXHIBITS ; 1. Executive Summary, Draft Preliminary Design Report, Batiquitos Lagoon Enhancement Project. 2 . Draft Preliminary Design Report and Technical Appendix for the Batiquitos Lagoon Enhancement Project on file in the Office of the City Clerk. Executive Summary Draft Preliminary Design Report - February 1988 BATIQUITOS LAGOON ENHANCEMENT PROJECT Prepared by CKMHIUL Tekmarine BATIQUITOS LAGOON ENHANCEMENT PROJECT DRAFT FINAL REPORT EXECUTIVE SUMMARY OVERALL STUDY OBJECTIVES The Batiquitos Lagoon Enhancement Project Preliminary Design Report represents a detailed engineering study and analysis to evaluate the feasibility of the engineering aspects and associated costs of the enhancement project. The enhance- ment project endeavors to fulfill the goals set forth in the California Coastal Conservancy Draft Batiquitos Lagoon Enhancement Plan: to restore tidal flushing by creating adequate tidal prism while conserving and enhancing existing wildlife habitat values and establishing new habitats. The draft enhancement plan was developed over a period of more than 2 years through a public process involving state, federal, and local public agencies; property owners; envi- ronmental and citizen groups; and interested individuals. This report reviews the methodology and findings of the following: o Existing lagoon sediment characteristics, qualities, and quantities o Volume of dredged/excavated material by type and location for three enhancement alternatives o Excavated/dredged material disposal methods o Excavated/dredged material disposal sites o Excavation/dredging depths and boundaries (detailed grading plans) for three alternatives o Tidal inlet design recommendation o Beach nourishment design recommendation o Hydraulic modeling (circulation and flushing) and water quality analysis within the lagoon for three alternatives o Lagoon sedimentation (SED-4 modeling results) o Sediment control management plan o Existing bridges relative to the lagoon enhance- ment project and methods for their protection DDHl/007 o Avifaunal surveys to date o Engineering cost estimates of project components SUMMARY OF FINDINGS LAGOON SEDIMENTS The sediments proposed to be removed from the lagoon are not hazardous, containing trace or lesser amounts of pollutants, and are well below threshold limit concentrations as defined by the California Administrative Code, Title 22. Therefore, the sediments may be disposed of by conventional land dis- posal methods. Sediments in the western half of the lagoon are composed predominantly of sands and are suitable for beach-front dis- posal, beach nourishment, and least tern nesting areas^ Sediments in the eastern half of the lagoon consist of elastic silts, fat clays, and sands. These sediments can be used as an impermeable capping or lining material, a non- structural fill, or, if mixed properly with other material, as structural fill. DREDGING/EXCAVATION AND DISPOSAL CONCEPTS The three enhancement alternatives require differing amounts of soil removal, as follows: o Alternative A: 3,096,000 yd3 o Alternative B: 2,632,000 yd3 o Alternative C: 2,347,000 yd3 Approximately 380,000 yd3 of the above quantities will be used as fill material within the lagoon boundaries to form least tern nesting sites and a freshwater marsh. The sand in the west and central basins is relatively easy to dispose of because it can be placed on adjacent and near beaches as nourishment. The material comprising most of the east basin is more difficult to dispose of because of its wet cohesive consistency, which creates transport and place- ment problems. The recommended dredging and disposal scheme is to remove an additional depth of sand in the central lagoon, thereby creating a "pit" that can accommodate the material from the east lagoon. A sand cap will then be placed over the relocated material. This method requires moving additional quantities of material locally, using efficient hydraulic dredge equipment. Even though this recommendation requires the dredging of nearly 2 million additional cubic yards for DDH1/007 Alternative A (over the quantity shown above), the estimated cost is lower than the most economical land disposal option. This is an efficient construction technique for in-lagoon sediment disposal that will not alter predicted habitats of the final enhancement project. In addition, it creates more beach nourishment material, reduces the total project con- struction time, and reduces offsite project impacts. As an option to the recommendation discussed above, the most preferred dry land disposal of the east basin material is in the Green Valley area just south of the eastern end of the lagoon. Even though the material would be placed on dry land, hydraulic dredging would be most attractive and cost- effective because the material could be easily conveyed the relatively short distance required of dredge pipe. If a disposal site is selected more than 3 miles away, soil removal in the east basin would best be handled using con- ventional grading equipment. The east basin could be diked off and dried, and San Marcos and Encinitas Creeks tempora- rily rerouted. The material would be hauled to a disposal site by truck. The nesting season of the California least tern was taken into account when determining the construction schedule to minimize effects to the bird's nesting and foraging habitats. Maintenance dredging will be required periodically. For Alternative A, approximately 25,000 yd3 of material could accumulate in the western portion of the lagoon each year from marine sources. Alternative B could accumulate 17,000 yd3, and Alternative C could accumulate 13,000 yd3. In addition, it is estimated that 3,500 yd3 of sediment will be caught in the east basin sediment trap. This study contains a maintenance program to ensure that the cumulative effects of sedimentation will not affect the long-term viability of the enhancement project. TIDAL INLET A comprehensive study has been made of various alternatives for a tidal inlet design. Particular attention has been given to the following: o History of Batiquitos Lagoon and the surrounding area o Longshore sediment transport o Characteristics of other Southern California coastal inlets DDH1/007 o Function and purpose of the inlet o Constructibility o Maintenance With respect to all these considerations, the preferred concept is a small armored channel configuration. The design emphasizes short, low profile jetties with the objective of nominal disruption to the longshore sediment transport. BEACH NOURISHMENT CONCEPT The beach nourishment operations are proposed to take advantage of a large amount of sandy material greater than 0.15 mm in median diameter available from the recommended planned dredging in the lagoon. Considering the recommended central basin disposal pit scheme, approximate total amounts of the sand in the beach nourishment size range are 3,180,000 yd3 under Alternative A, 2,600,000 yd3 for Alternative B, and 2,450,000 yd3 under Alternative C. Although some amounts of this sand will be used for construction of the least tern nesting sites, most is available for beach nourishment. As much as 60 percent of this material is finer than 0.20 mm and is subject to easy washout by wave action. Selection of nourishment sites was made where the presence of cobbles and reduced wave energy (caused by wave divergence) will make this material useful for beach nourishment. Two specific areas have been evaluated for beach nourish- ment: a 3,000-foot shoreline segment near Batiquitos Lagoon, and a 5,700- to 6,500-foot shoreline segment at the Encinas Creek outlet. Both of these sites provide excellent beach access and have provisions for a sizable public parking facility. The Batiquitos site is limited in size and can only accept about 114,000 yd3. The material would be placed on the surface of a regraded cobble berra to improve the amount of time it would remain at the site. A useful period at this site could be 3 years, although a variation from normal ocean conditions could cause substantial variations in duration, either shorter or longer. It should be noted that the proposed maintenance dredging of the inlet area would replenish the Batiquitos site on a continual basis. The natural loss of material from this beach will benefit the downcoast shoreline for a distance of approximately 2 miles, including Leucadia and the northern reach of Encinitas. DDH1/007 At the Encinas Creek site, sand placement totaling as much as 2.1 to 3.1 million yd3 will push the shoreline seaward • 485 to 640 feet. The length of time that nourished sand remains at this beach is variable. One possible scenario could be: 1. Half the width of beach would disappear under a severe low frequency occurring storm. 2. Sand would remain for approximately 15 years under average weather conditions. 3. Longer residency would be obtained if nourished periodically with maintenance dredging, or if unusually calm weather prevails. The beach fill at the Encinas Creek site will serve as a major source of littoral sediment for the shoreline at South Carlsbad State Beach. HYDRAULIC MODELING AND WATER QUALITY EVALUATION Current meters and tide gauges were placed at strategic locations inside Batiquitos Lagoon prior to removal of the natural cobble bar at the mouth of the lagoon in May 1987. Current and tide information obtained from these instru- ments, combined with profile mapping of the ocean bottom conditions, enabled the calibration of hydrodynamic (cir- culation and flushing) and water quality models comparable to actual conditions. The models show the alternatives are capable of achieving 85 to 90 percent of the potential tidal prism, indicating the design of the entrance channel is effective in allowing continuous tidal exchange. This estimate is consistent with previous tidal prism estimates including that computed by the Coastal Conservancy. The tidal prisms of the three alternatives fall into the range of 2.1 to 2.6 million yd3. This volume of water amounts to more than 50 percent of the total volume of water in the lagoon (at +5.0 feet mllw) for any of the three dredging plans. This continual exchange of ocean water will serve to reduce the potential impacts of long-term stagna- tion, as now occurs. The normal water quality of the lagoon should be similar to the ocean throughout the year except during major storm events, which would result in large volumes of fresh water from the uplands. However, the time needed to return to normal water quality would be relatively short throughout the basin. The habitat for fish and other aquatic life should be greatly improved over existing conditions because of improved tidal circulation and water quality DDH1/007 with more consistent levels of salinity, concentrations of nutrients, dissolved oxygen, and food chain organisms. SEDIMENTATION AND SEDIMENT MANAGEMENT Existing data indicate the inlet closure at Batiquitos Lagoon was not a result of filling by upland sediments and subsequent loss of effective tidal prism. Inlet closure appears to be a result of bridge constrictions near the mouth. The constrictions reduced the scouring ability of the tidal flows leading to sedimentation and closure of the inlet. When the inlet is closed, fine sediments that may normally move through the lagoon (to the sea) are instead trapped in it. Previous estimates of upland sediment delivery to the lagoon appear to be significantly higher that what is expected in the future. This result is due in part to overly conserva- tive estimating techniques and in part to the decreasing effects of development on sediment load. The use of sediment traps as a management technique may not be effective. Under low to moderate flood events, the sediment loads may not be large enough to be of concern. Under extreme flood events the sediment traps may be inef- fective. Sediment basin placement must be considered in relation to the potential for increasing flooding (San Marcos Creek) and interference with naturally existing sediment traps (Encinitas Creek). In both cases, there are strong arguments for placing such structures lagoonward from the bordering highways. For the small creeks entering the lagoon, management of sediment and erosion at construction sites should be sufficient. Construction of permanent sedi- ment traps should be considered only if the sedimentation problem clearly is not transient. The most serious sedimentation problem is the transport of sand through the inlet. Above mllw, the adjacent beach is currently composed of cobbles. However, sufficient sand for transport into the inlet appears to be present on the beach below low water. Beach nourishment by the implementation of the enhancement plan will also add to the supply. Two im- portant consequences are evident: the inlet is the most critical part of a lagoon enhancement design, and the inlet will have to be armored on its sides. EXISTING BRIDGES Existing information has been obtained on all five of the bridges crossing the lagoon. To accommodate the proposed dredging, all of the bridges will require either a form of structural modification or foundation protection. The western portion of Carlsbad Boulevard Bridge may possibly require DDH1/007 extensive footing modifications; however, that cannot be determined until the water level in the lagoon subsides and an investigation of the existing footings can be made. AVIFAUNAL SURVEYS Six avifaunal surveys have been completed to date. Addi- tional surveys will be conducted to complete a 1-year sampling period. Bird use fluctuates with seasonality, with waterfowl populations remaining high through winter and shorebirds decreasing after summer breeding. Water elevations also affect bird usage, as shorebird use continues to decline with high winter water levels (and inundation of habitat). The California least tern (federally listed endangered species) was noted during the breeding seasons and not observed during the winter season (when the bird is typically in Central America and Mexico). The state-designed endangered species/ Belding's savannah sparrow, was noted during all six surveys but fluctuated significantly according to season. Construction Costs and Schedule Budget level construction cost estimates and an anticipated construction schedule have been prepared. The total project costs for the various alternatives are summarized as follows: Central Basin Disposal Green Valley Disposal Alternative A $23,488,000 $34,848,000 Alternative B $20,465,000 $29,798,000 Alternative C $19,626,000 $26,119,000 The recommended central basin disposal construction scenario will require a construction contract of 1-1/2 years. Ninety percent of the entire project, including all dredging, will take place in & 5-1/2 month period during the first fall and winter season. The west and central basins will be complete and tidally influenced after this period and the west basin and Park and Ride least tern nesting sites will be finished. The east basin will have been dredged, but finish grading work will need to be done after the basin has had time to dry out somewhat. If the Green Vallfey or similar offsite disposal scenario is used, the total construction time will remain at about 1-1/2 years. Tito lagoon west of 1-5 will still be complete at 5-1/2 months, but the east lagoon dredging will be a slower process, attending over the entire contract length. DDHl/007