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HomeMy WebLinkAboutAgenda Report - November 5, 2008 K-02aAGENDA ITEM KOL06 &0� CITY OF LODI COUNCIL COMMUNICATION TM AGENDA TITLE: Receive Report on Mokelumne River Erosion Near Pigs Lake MEETING DATE: November5,2008 PREPARED B Y Interim Parks and Recreation Director RECOMMENDEDACTION: Receive presentation regarding the condition of the Mokelumne River bank erosion and its potential effect on Pigs Lake. BACKGROUND INFORMATION: On February 6, 2008, staff provided informationto Council related to the status of the Mokelumne River bank in the general vicinity of Pigs Lake within the Lodi Lake Nature Area. Civil Engineering firm of Kjeldsen, Sinnock & Nuedeck was contracted to provide, as best possible, a detailed report on the effects cf erosion of the river bank in the vicinity of Pigs Lake. On August 13,2008, staff received Mr. Sinnock's Preliminary Reportthat outlines the initial findings and recommendations regarding the erosion that is occurring on the south bank of the Mokelumne River adjacent to Pig Lake in the Lodi Lake Nature Area. FISCAL IMPACT The fiscal impacts are varied and complex. Temporary measures range in cost from $10,000 to $25,000, excluding permits and monitoring, which are an additional $25,000. Permanent repair costs may be greaterthan $2 million. If the embankment fails, this passive recreation area may be lost permanently as the cost of permitting and reclamation will be significantly higher. FUNDING AVAILABLE: None James M. Rodems Interim Parks and Recreation Director APPROVED: Blai�King, City Manager KJELDSEN,SINNOCK& NEUDECK,INC. CIVIL ENGINEERSAND LAND SURVEYORS STEPHEN K. SINNOCK CHRISTOPHER H. NEUDECK KENNETH L. KJELDSEN RETIRED 711 NORTH PERSHING AVENUE POST OFFICE BOX 844 STOCKTON. CALIFORNIA95201-0844 TELEPHONE (209) 946-0268 FAX (209) 946-0296 E-MAILksn@ksninc.com 2088-0010 Mr. Steve Dutra Parks & Recreation Department City of Lodi 125N. Stockton Street Lodi, CA 95240 August 13,2008 Re: Lodi Lake Nature Area Erosion Along Mokelumne River, Preliminary Report Dear Mr. Dutra, Kjeldsen, Sinnock& Neudeck, Inc. submits herewith our Preliminary Report that outlines the initial findings and recommendations regarding the erosion that is occurring on the south bank of the Mokelumne River, adjacent to "Pig Lake" in the Lodi Lake Nature Area. This letter report has been prepared pursuant to the scope of work described in our proposal to the City of Lodi dated October 17,2007. The purpose of this letter report is to communicate to the City of Lodi our preliminary findings and recommendations relative to 1) the probable causes of the erosion, 2) permitting requirements for construction of repairs, 3) options to repair erosion sites, 4) preliminary quantity estimates for repair options, and 5) preliminary cost estimates for repair options. I have included as attachments to this letter report, the following exhibits: Exhibit A - Aerial Map and River Bank Cross -Sections Exhibit B - Photographs of Active Erosion Sites Exhibit C - Annotated Photographs Exhibit D - Typical Repair Section Probable Cause Of Erosion There are several conditions occurring in this segment of the Mokelumne River that may be contributing to the aggressive erosion of this particular section of riverbank. Based upon our site inspections and preliminary evaluation of site conditions, there appear to be three mechanisms contributing to the active erosion of the riverbank, 1) hydraulic erosion on the lower slope, 2) wave erosion on the upper slope, and 3) bank instability due to the annual draining of Lodi Lake. First, there is hydraulic erosion occurring below the waterline that has undermined and washed out a previous repair project that was completed in 1994. The erosion area that is the subject of this report is located on the outside bend in the river, where the flow velocities will be Page 2 August 13,2008 the greatest. The 1994 repairs consisted of stacking a grid of tree roots and logs along the eroded bank and cabling them to existing live trees along the riverbank. The remnants of the trees and logs installed during the 1994repair project are scattered throughout the river bottom and the lower slopes of the bank as shown in the attached photos in Exhibit B. The remnant trees and logs may be aggravating the hydraulic erosive action of the river in this area, as they could be creating eddies and turbulence that are contributing to the erosion of the lower portions of the riverbank. Second, there appears to be wave wash erosion occurring higher up on the slope of the riverbank, at the summer water level when Lodi Lake is full. The waves are generated by both boats and wind. This erosion is clearly defined and localized at the top of the waterside slope, and is causing severe undercutting of the embankment crown. The wave wash was observed during the August inspection when watercraft passed the site. Third, the erosion of the riverbank may be aggravated by the annual draining of Lodi Lake. The rapid drawdown of the lake in the fall could be contributing to the "erosion" of the riverbank slope by not allowing adequate time for the saturated riverbank to drain. When soils experience a rapid drawdown, the moisture that was trapped in the soils at the higher water levels are not provided adequate time to drain. This trapped moisture destabilizes the soils and on steep slopes can result in the sloughing of the cohesionless sands on the upper portions of the riverbank. Permitting Issues Related To Repairs The permitting requirements will be primarily driven by the type of repairs made, and the impacts that the project might have on the environment. As with all such projects, the repair project will have to comply with the requirements of the California Environmental Quality Act (CEQA). An initial study will need to be performed to address all of the impacts associated with the project. That process will require a qualified Biologist to inventory the species present and determine how the proposed project might impact those species. Once the impacts are determined, the project scope might be modified to avoid or minimize impacts to the environment. The results of these investigations and modifications to the project will result in the CEQA determination and provide the basis for the permit applications to the regulatory agencies listed below. State Department of Fish & Game U.S. Army Corps of Engineers NOAA Marine Fisheries US Fish & Wildlife Service Central Valley Regional Water Quality Control Board State Reclamation Board Other Local, State and Federal agencies as necessary Our office had various discussions with the regulatory agencies and biological consultants that are familiar with current regulatory and environmental issues in the area of the Page 3 August 13,2008 project. It was made very clear in the conversations that due to the backlog of permit applications, the State and Federal agencies will not perform a site inspection to provide a preliminary assessment of impacts until a permit application is submitted by the project proponent. The project is in a biologically sensitive area where threatened and endangered species are likely to be present, and will require careful consultation and consideration during the development of the scope of work for this project. It should be noted that the permit application process requires substantial detail in order to be processed so it actually occurs after the project design and environmental issues are addressed. Estimating the costs associated with securing environmental documents and permits were not included in our scope of work. Erosion Repair Options This location presents several challenges in terms of implementing a repair project. The primary obstacles are that access to the repair site is very restrictive and that the top of the riverbank adjacent to the eroded area is extremely narrow and will likely not support large construction equipment. Access to the work locations is by way of narrow foot paths through the Nature Area. The repair options presented below could be performed by either waterborne or land based equipment, however the costs for both will be higher due to the access limitations. Emergency Protective Measures Due to the length of time that may be necessary to secure permit approvals for a major project, it would be prudent for the City to investigate the possibility of placing plastic sheeting or riprap on the upper slope of the riverbank, to stabilize and protect it against further erosion,during the months when Lodi Lake is full. The City should consult with the Department of Fish and Game and the Federal regulatory agencies to determine if the placement of riprap on the upper slope could be performed under an emergency notification. Another temporary option would be to limit boat speeds and wakes in that area. Repair the Entire 500 Foot Eroding Bank Placing clean quarry stone riprap along the entire reach of eroded levee would be considered the conventional repair for this location. Exhibit D illustrates a possible scenario for a repair cross section at the narrowest point between Pig Lake and the Mokelumne River. The embankment crown has been widened to 30 feet to allow for an area where plantings of various types of native vegetation could occur, as is usually required by the permitting agencies to mitigate project impacts. Another option for repairing the eroding bank could include driving sheet piles to form a bulkhead along the eroded area, and then backfilling the upper section as necessary. Various configurations could be used to make this type of repair more user and environment friendly. Flow Diversion Structure Page 4 August 13,2008 This site might lend itself to the installation of a water diversion flow structure that would divert the high velocity flows away from the banks and force it down the center of the river. This option would require that the hydrology of the project be studied in detail in order to determine the feasibility and impacts associated with that type of concept. Possible advantages of this concept is that it might be less invasive environmentally and could require less material to construct. Potential disadvantages are the cost of the analysis and the possibility that the eroding bank might require rebuilding and armoring in addition to installation of flow diversion structures. Preliminary Quantity & Cost Estimates For Repair Options KSN performed field surveys of the site to determine the typical dimensions of the project. A copy of an aerial photograph of the project site showingthe location of the 13 surveyed cross-sections of the eroded bank is attached to this report as Exhibit A. Photographs showing the existing condition of the riverbank taken at the cross-section locations is attached as Exhibits B). The cost for riprap is based on unit prices developed from discussions and site inspections by an experienced local contractor. The unit price for riprap "in place" is estimated to be approximately $75 per ton. Obviously, market conditions could change that estimate due to material demand, fuel prices, and changes in site conditions. Please note that these construction costs do not include the costs to secure the necessary environmental documents and permits. Temporary Emergency Protective Measures Several options exist to temporarily slow the erosion on the upper bank caused by wind and wave action when the lake is full. These measures could be implemented either by a contractor or City of Lodi staff, and would only be temporary in nature. It should be noted that placement or work on the channel bank could require that a Notification of Streambed Alteration permit be filed with the Department of Fish & Game. The first option would be to implement speed limits on boats in the damaged section of the river. This would be a relatively inexpensive option and would require notifications, buoys and possibly the costs of enforcement. We have limited experience with the costs associated with these actions, but assume that the City of Lodi is already enforcing speed limits in other areas of the lake so additional costs would be nominal. The second option would be to place plastic sheeting or geotextile fabric, and sandbags over the upper bank. This is a cost effective and easily installed measure. Plastic sheeting costs range from approximately $60 - $100 for 100 x 20 foot rolls. Estimated need is 500 feet so materials are estimated at about $500. These materials could be installed by City of Lodi employees. The installed cost of this temporary option is estimated at $10,000. Page 5 August 13,2008 The third, and most expensive option, would be to hand place riprap in the eroded areas at the "lake full" waterline. This option does have drawbacks due to access issues, high labor costs and the potential for site preparation work. Due to the soil types at the eroded sites, there is potential for the rock to slip down the slope. Estimated costs are based on using $75 per ton unit price for rock, and filling a band approximately 3 feet high and 500 feet long to a depth of 18 inches, or 660 lbs per foot. Estimated cost for this item is $25,000. Permanent Repairs As stated above, permanent repair options include placement of riprap on the entire eroded area, driving sheet piles or developing a hydraulic flow diversion structure in the channel. We are providing costs for armoring and sheet pile options. The flow diversion structure would require hiring a hydraulic consultant to determine the feasibility of such an option. Based on our survey data, the preliminary estimates indicate that 15,500 tons of material will be required to construct the structure illustrated in the attached Repair Exhibit (Exhibit D). The material would consist of a combination of rock fill and riprap. The total cost, based on $75 pre ton "in place" would be $1,116,000. Driving sheet piling along the eroded stretch is another option. Recent project costs, where there was good access, have been running about $40 per square foot, installed. Due to the access issues, and that the pile driving equipment might need to be marine based, we have doubled the square foot costs for installed sheet piles. Using 40 foot sheets for the entire reach of eroded area result in an area of 20,000 square feet. At a unit cost of $80 per square foot, the total estimate is $1,600,000. Please note that these estimates are preliminary and are intended to give the City an idea as to the magnitude of the costs associated with each repair option. The costs seem very high for the relatively simple work that is being performed, but with escalating costs for materials and the potential for double and triple handling of the materials, we feel they are a conservative estimate given the limited information that is available on the project at this time. If you should have any questions or require further information, please contact Bill Darsie or me at (209) 946-0268. Sincerely, KJE SEN, S K & NEUDECK, INC. L1 /Stephen K. Sinnock w/enclosures EXHIBIT B EXHIBIT D %A y �f � rd�'� 44 �� � 44 3 0' .� / `rN O 40 2 40 � o RIPRAP 1 36 � 36 ROCK FILL 8' 32 � 0 32 o a 0 28 28 o a o O fl 1.5 24 24 GEOTEXTILE FABRIC 20 ' ` 20 0 20 40 60 80 TYPICAL REPAIR SECTION K K ELpSEN LODI LAKE MOKELUMNE Design Sole Date S S�N N OC K Post Office aox Baa LODI, CALIFORNIA wc� MAY 2008 711 w Pershing Avenue Drawn I" = 10' Sheet Number N N E U D E C K §Officep' 20)g946� 268 LEVEE BANK EROSION CONTROL SAM Original Drawing Scale of 1 INC. Consulting Engineers Foxes: tl209} 946-0296 C V tJ l IT Check D W Project File No. I and Land Surveyors E-mail: KSN@ksninc.com REPAIR G An" 1 D I I SKS I 2 ct File o.