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HomeMy WebLinkAbout03/16/1999, SS1 - LAGUNA LAKE DREDGING council. V'JS Session 3/16/99 j acenba nepont 'S� CITY OF SAN LU I S OBISPO FROM: Michael D. McCluskey, Director of Public Works Prepared By: Barbara Lynch Civil Engineer SUBJECT: LAGUNA LAKE DREDGING CAO RECOMMENDATION 1. Confirm the Council's intent to maintain Laguna Lake as a viable recreational facility. 2. Direct staff to continue to pursue a revised annual dredging program reducing the scale of the project to minimize impacts to the Nature Preserve. 3. Direct staff to prepare necessary documents to allow a major dredging project to occur during the next drought. DISCUSSION The Laguna Lake Dredging Project was envisioned in the original Laguna Lake Management Plan adopted in 1982. (See Attachment A for detailed background.) In 1991, the City Council gave direction to proceed with environmental studies for a large scale dredging project. In 1993 the City adopted the Laguna Lake Park Master Plan which effectively eliminated the area identified for dredge spoils by the 1982 Plan. That area was identified as a nature preserve. In addition, the findings of the biological and vegetation studies, confirmed that designation and thus, it no longer makes sense to permanently place a very large quantity of dredge material on this area. Since the originally envisioned project is no longer feasible, the basic question arises: Should the City pursue some form of dredging project or let the lake become a marsh/meadow over time? Staff believes the lake serves a unique recreational need within the community. As such, we recommend a renewed commitment to maintaining the lake as a viable facility. Maintaining the Lake. If the lake is to be retained then it would appear doing a small scale annual program of dredging would be the most realistic approach. The original project (#3.below) anticipated when the City Council requested staff move forward with dredging of the lake is no longer a realistic project due to lack of available land within the park that could environmentally accept the dredge spoils. Of the three remaining options, staff feels Program #1 offers the best chance of success. (See Attachment B for a detailed discussion of Options.) Program Cost Effectiveness I Feasibility Duration 1. Dredge,hauling spoils $3,500,000 Good Probable 17 summers 2. Dredge in a drought,hauling spoils $5,000,000 Good Probable 1 summer 3. Dredge, leave spoils on park land $1,000,000 Good Unlikely 6 summers 4. Dredge wet,hauling spoils $12,000,000 Good Possible 1 summer Program #1 offers an extended project which would slowly dredge the bottom of the lake to the desired depth. A small area of 2-5 acres was identified that could be used successfully to place SSI-1 Council Agenda Report—Laguna Lake Dredging Page 2 dredge spoils yearly. After drying they would be hauled away. After 17 years the project would be complete. To keep the problem from reoccurring the City would still need to continue with its every three year dredging of the Prefurno Creek Arm project as this is the major source of sedimentation of the Lake. Regulatory Agencies and Environmental Permitting. Staff discussed the lake dredging project with representatives of the Army Corps of Engineers, California Regional Water Quality Control Board, Fish and Wildlife and National Marine Fisheries. While there will undoubtedly be mitigation requirements, depending on the City's approach, all of the agencies felt it was doable to dredge the lake. Disposal of the material is a separate issue for the agencies. The material can not be deposited in such a way that it will reenter the lake or other water way. Any water placed in the lake must also go through the approval process. Prepare for the next drought. Staff has been in contact with the various regulatory agencies that will be involved in"permitting"any of the projects the City proposes. They unanimously agreed that the easiest project to pursue from their perspective would be a major dredging project that would occur when the Lake was next dry (during the next drought). In actuality few permits would be required because water quality issues would be moot. Since this area is known to have periodic droughts, staff feels that the City should pursue this option and be ready to "jump on the project" should a drought arise. CONCURRENCES Public Works, Parks and Recreation and the Natural Resource Manager agree the Laguna Lake Park Master Plan should be the guiding document and large quantities of dredge material should not be placed on the Nature Preserve. A small area has been identified for drying dredge material before hauling. FISCAL IMPACT The cost of the recommended program is substantial: $3,500,000. However, what this really amounts to is slightly more than $200,000 per year for seventeen years. The different options spread the cost over a long period or result in high short term expenses. All but one option (program #3) assumes we can not permanently dispose of the dredged material on site. The project costs are very much interrelated with our ability or that of a contractor to dispose of the material. In estimating costs, we are assuming that disposal sites can be found within the County for wet dredge material. Currently $200,000 is identified for the 1999-00 budget (in the 97-99 financial plan) for equipment purchase and dredging. A request for $100,000 is being processed for dredging Prefumo Arm this summer. No funds will be actually available unless included in the 99-01 adopted budget. SSI-2 Council Agenda Report—Laguna Lake Dredging Page 3 ALTERNATIVES Do not maintain the lake. If the Council no longer supports dredging the lake the following options are available: Program Cost Effectiveness Feasibility 5. Continue dredging upstream(Prefumo Arm) $100,000/3 yrs Moderate Probable 6. Invest in increased sedimentation control upstream unknown Moderate Probable 7. Do nothing $0 None N/A Without a proactive approach to the sedimentation which arrives at the lake from upstream, the lake would continue to slowly decrease in depth, becoming a marshy area then a meadow. It is estimated that will take about 100 years to occur. This will reduce the parks' recreational value and eliminate certain types of habitat. ATTAC)Fl11IENTS A-Background B - Detailed discussion of Lake Management Options C- Summary of Geotechnical, Biological and Vegetation Reports \1955 momW"\groups\pve Nengr\projecrs\drainage\9110129un3 dredge\swdy smion.doc SSI-3 BACKGROUND In the late 1970's, due to drought, Laguna Lake dried up almost completely. This focused public attention on the lake and the fact that it had been slowly filling with sediment. In 1979 the City established a study committee which produced a proposed Laguna Lake Management Program. In 1982 the City Council adopted the Laguna Lake Management Program (lake program). Council direction at that time was to complete mechanical weed harvesting, remove sediment from Prefinno Arm and delta and install a log barrier at Madonna road to adjust the lake level. In 1991, staff returned to the City Council regarding management of the lake. At that time, Council directed staff to pursue the removal of the delta at the mouth of Prefumo Arm and begin the Environmental Impact Report(EIR) for dredging 5 feet of material from the entire water area of the lake. The City's adopted lake program identified four goals for the lake: • Wildlife Preservation • Recreation Enhancement • Shoreline Protection • Agricultural Preservation The lake program identified four activities to achieve these goals: 1. Reduce or eliminated existing aquatic weeds and provide ongoing weed control. This improves the open water wildlife habitat and reduces nuisance odors. Actions taken: a) Public Works completed weed harvesting for several years. b) The lake is now treated with something to kill the weeds on an as needed basis. 2. Prevent sediment from reaching the lake. This slows the rate at which the lake fills by providing a place for the majority of sediment to drop out before reaching the lake. Actions taken: a) The Prefumo Arm was dredged in 1983, 1990, 1995. b) In 1998, Public Works obtained a 10 year permit to dredge the amt once every 3 years. Dredging is planned for the summer of 1999. 3. Preserve lake characteristics important to wildlife,recreation and flood protection. Actions taken: a) Weir plates are adjusted twice a year to minimize the lake level in the winter and maximize it in the summer. b) Council adopted the Laguna Lake Park Master Plan in 1993. 4. Maintain 3 to 4 feet of lake depth at seasonal low water level. This increases the recreational value of the lake and reduces the weed growth. Actions taken: a) City Council directed staff to move forward with an EIR for dredging the lake. b) Community Development completed geotechnical, biological and botanical studies needed to prepare an environmental document on the dredging project. LAGUNA LAKE D lJ�?G4 AT I'ACI iA Public Works has been working with Parks and Recreation to try to achieve a dredging project which would be compatible with the park plan. Unfortunately, both Departments have come to the conclusion that an ongoing dredging project as originally envisioned will not be compatible with the park plan. The establishment of the Nature Preserve eliminated the area which was to be used for drying and disposal of dredged material. LAGUNA LAKE DRiEr I(,NC, LAKE MANAGEMENT PROGRAMS Staff is presenting three categories of management alternatives: a) dredging programs which would take several years to complete (long term), b) dredging programs which would be done in a single year(short term), and c)no lake dredging. Dredging could take up to 40 years to complete depending on the City's approach to the project. Dredging would be needed on an ongoing basis indefinitely to maintain the depth. , a) Long term lake dredgingproQrams: These two projects are ongoing annual summer dredging, using a portion of the Park for drying. Program 1. Dredge the lake,pumping the material to shore. Dry the dredge material on a 2 to 5 acre site. Haul the material to an alternate disposal site. There is about a 5 acre area where negative impacts to rare species and wildlife would be minimal if the site were temporarily covered with material. Parks and Recreation feel a smaller area, closer to 2 acres would be preferable. A 2 acre site is not an efficient size, being too small to allow continuous dredging during the summer. There would be periods where nothing would occur while the material dried enough to be removed or covered. Once the material was dry it would be disposed of off- site. Securing of necessary permits will be time consuming but once obtained, we assume would apply to the yearly operations. The extent of the return water clarification setup is not defined at this time. There would be some additional truck traffic and dirt on the roads to haul the material out. The impacts of trucks would be less noticed as it would be spread out over a long period. Program 3. Dredge the lake, pumping the material to shore. Dry and grade the dredged material on 25 acres of lake property. This project is the original proposal. This would require a revision to the Laguna Lake Park Master Plan to allow for the spreading and grading of material on the Nature Preserve. The material would be, on average, 10 feet deep. Extensive mitigation would be mandated by permitting agencies if they allowed it to occur at all. The dredging project,as originally envisioned,was to be accomplished by purchasing a small dredging machine which would be operated during the summer months by a City employee. The lake bottom would be lowered by 5 feet. The material would be pumped to the drying site along the lake. Water from the dredged material would be filtered and allowed to return to the lake. The solid material remaining would be spread on the park site and re-contoured using existing City owned equipment. The annual dredging operations were to be limited to a small area of the lake, governed by the size of the drying site (25 acres.) This would be a low cost but ongoing program. It was anticipated there would be a onetime cost of$100,000 to purchase equipment and the annual operational costs would be about$100,000. New costs associated with this option are unknown but would most likely be extensive due to payment for mitigation measures. Either a larger dredge than was originally envisioned or more than one small dredge would have to be working at the same time to fill the 25 acre LAGUNA LAKE DREDGING ATTACHN M4 site. The extent of the return water clarification setup is not defined at this time. This option appears least likely to succeed b) Short term lake dredging programs: These two projects require no drying areas on the park property and have the least impact on the park. Program 2. Dredge the lake during the next drought period. Dispose of the wet material off site without any drying. There is no known time frame of when this would occur although we do know that drought conditions are cyclical and we will most likely experience another drought within the next 20 years. The cost of this type of project will be significant and extensive hauling costs will be incurred. Costs would be less than under saturated conditions. There will be heavy truck traffic on the lake park roads, Madonna Road and possibly other City streets depending on the disposal site. We can expect mud to be dragged out on the trucks and end up on the City's streets. The park roads would need reconstruction after the work was completed. City streets may need reconstruction depending on their condition prior to the work being completed. Indications are from regulating agencies that the easiest conditions for obtaining clearances for dredging would be to assume the lake would be dry. We would obtain the permits in advance based on the assumption that the Lake would be dry, then plan to complete the project during the next drought. In this way, the work would not disturb any wildlife or habitat which relies on the water. Program 4. Dredge the lake with water in it. Dispose of the dredge slurry off site either by pumping the slurry to a nearby property or hauling it in trucks. Dredge slurry is approximately 10% solids, 90% liquid and therefore could be removed in either way. The amount of excavated material is estimated at over 500,000 cubic yards or enough to cover about eight soccer fields 2 feet deep. The cost to haul a near liquid material or install pipelines to pump the liquid will be high. A method would also have to be developed to return water to the lake or the lake would be dry long before the dredging was complete. It is very unlikely we could obtain a permit to dry out the lake. The dredging could be accomplished in one construction season. Securing of necessary permits will be complex and time consuming. There are two potential sources of water to maintain wet dredging operations. The first is Prefiimo Creek which fills the lake naturally. To use this, the dredging would have to be done in the winter when a live stream exists. Typically, the regulating agencies do not allow work in creeks or wetland areas during the winter season. The second source is treated wastewater. At the current level of treatment, the water still has significant nutrient levels and would contribute to plant growth in the lake. It is also not treated to a level which would allow for people to have contact with the lake water. It would cost an estimated $3 million to upgrade the City's Water Reclamation Facility to provide water which safe for body contact activities such as swimming or sailboarding. LAGUNA LAKE DREDGING ATTACHNSSI-4] If the material is hauled out wet, there will be heavy truck traffic on the lake park roads, Madonna Road and possibly other City streets depending on the disposal site. Wet disposal tends to leave a trail of muddy water in its wake. The park roads and possibly City streets, would need reconstruction after the work was completed. 1) No dredging of the lake: These three projects do not address the desire to increase the depth of the lake. Program 5. Continue with existing sedimentation controls. The City currently has permits to dredge the Prefinno Arm of the Lake every three years for the next ten years. This is the major source of sediment in the Lake. One strategy is to leave the Lake "as-is" but continue dredging the Prefumo Arm, concentrating on keeping sedimentation from getting worse. On-going costs would be limited to the three year schedule of dredging the Ann. Program 6. Alterative and/or increased lake management. This would involve looking at ways of controlling erosion above the lake, providing additional sedimentation control at the Prefumo Ann, alternate methods for algae and weed reduction and the possibility of raising the summer lake level. The rate of sedimentation of the lake should be confirmed including the effect on the lake of the dredging of the Prefumo Arm. This strategy may include 3a. above, dredging of Prefumo Arm. There would be initial study costs to investigate alternatives. The cost of implementing new strategies is unknown. Program 7. Do nothing. Laguna Lake would be allowed to continue to fill with sediment. Given the rate of sedimentation identified in the lake program, staff estimates the "lake" would be gone in 100 years. Laguna Lake, contrary to popular belief, does not decrease flooding to homes in the area. Once the lake begins to fill, the outlets from the Oceanaire area no longer drain. Eventually they backup into the streets, level with the surface of the lake. The depth of the lake does not make a difference on when this occurs, rather the level of the water when the rain starts is more of a controlling factor. The outlet of the lake at Madonna Road regulates how much water goes downstream. The remainder floods out across the park and in the adjacent street system. Based on the current pattern, the lake would eventually fill with sediment, radiating out from the Prefiirno Arm. Prefumo Creek would form a channel through a portion of the lake area to carry normal flows downstream. The remainder of the area would become marsh or possibly fill completely with sediment from other sources. Assuming the lake were to fill with sediment to near the natural water surface elevation, the area above the normal water level would continue to serve as the flood plain. In flood flow conditions, the creek would top its banks and flood out across the now marshy lake area. LAGUNA LAKE DREDGING ATTACH1 S&A Summary of Geotechnical, Biological and Vegetation Reports 1. The geotechnical report provided the following information: a) Dredging the lake and depositing the material on the site is feasible b) Samples indicated highly variable soils c) Samples yielded a range of 2% to 90% clay d) Expansion was 20 to 30%dry to wet e) Dredging can start at a distance of 50 feet from the shore at 2.5 foot depth and then moved out a distance of 75 feet from shore to dredge the full 5 feet f) No instability of shoreline structures should occur g) After compaction the material is expected to reduce to 30 to 40%of wet h) Topsoil may be necessary to allow landscaping of fill i) Cut and fill of existing soils was anticipated to create ponds j) No more than 1 foot of sediment at a time should be placed in the pond for drying k) Drying will take 4 to 6 weeks 1) An extensive drainage and silt removal system will be necessary to return the water to the lake. 2. The biological report provided the following information: a) Overgrowth of the open water by a dense cover of vegetation would in general decrease the value for most species of wildlife b) Lake serves as a connector habitat to the ocean and sewage treatment ponds c) Proximity of treatment ponds allows alternate refuge if lake is disrupted d) Lake area serves for foraging for a variety of wildlife e) Open lake area away from the shore is of particular value because it offers some protection from predators and human disturbance f) Water over 6 feet will typically deter the growth of emergents g) Some emergents are beneficial to wildlife - wildlife value drops considerably if they completely overgrow an area h) Mechanical removal and grazing currently keep the emergents at bay i) Wetland vegetation extends from the lake to the upland flood line j) The delta should remain intact and unaltered k) Grazing area above the lake and toward Foothill provide for hunting and habitat 1) There are several Species of Special Concern in the lake -regulated by F&G m) Dredging will disturb some,not disturb others and improve habitat for yet others n) Recommendations i) Dredge during the summer(5/1 - 10/31) ii) Avoid deposition of spoils in burrowing owl area iii) Avoid deposition of spoils in ground squirrel area iv) Maintain a buffer around burrowing owl area v) Maintain dense bulrushes around perimeter of lake vi) Maintain finger like configuration of northwest lake vii) Leave delta at the mouth of Prefiuno Arm LAGUNA TAKE D ATTACH=4) 3. The vegetation report provided the following information: a) Rare plants were identified b) Plants are expected to be on the endangered list in a forthcoming revision c) Documented flora exist at the park d) Plant habitat ranges from the mud at the edge of the lake to the serpentine grassland e) Wetlands include the Prefumo Arm, the lake and the areas which are seasonally flooded f) Most extensive riparian area is on the peninsula that extends from the eastern bank of the lake g) Several springs occur in the serpentine hillside on the eastern side of the park keeping the slope below wet year round h) Springs vegetation different from lake vegetation i) Threat of invasion by ornamental trees planted in park j) Plants can vary seasonally and on an annual basis depending on variations in precipitation and temperature possibly lying dormant for several years until the right conditions occur k) Presence of the rare plants places a considerable responsibility on the City to carefully manage the park 1) Park development should exclude the serpentine hillsides along the eastern side of the park-limited to foot traffic m) Any deposition of spoils should avoid rare plant areas as much as possible n) Dredge spoils will likely cause a marked increase in weed population as the material is unlikely to support the growth of native species o) Certain rare plants rely on the current water level. Changes in the water level can have a devastating effect p) Any project causing change in water level should include an in-depth study of the effect on rare plants LAGUNA:LAKE D 1 G ATT.aCI- "1�N