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HomeMy WebLinkAbout2000-02-07 City Council (9)City of Palo Alto City Manager’s Report TO: FROM: HONORABLE CITY COUNCIL CITY MANAGER 1 DEPARTMENT: PUBLIC WORKS DATE:FEBRUARY 7, 2000 CMR:124:00 SUBJECT:STATUS REPORT ON SANTA CLARA VALLEY WATER DISTRICT’S MATADERO/BARRON CREEK FLOOD CONTROL PROJECT Staff from the Santa Clara Valley Water District (District) and their engineering consultant will make a presentation to Council describing the proposed Matadero/Barron Creek Flood Control Project. Following the presentation, Council will have the opportunity to ask questions and provide feedback to District staff, however no Council action is required. BACKGROUND In October 1997, staff provided information to Council on a design error in the District’s Matadero/Barron Creek Flood Control Project (CMR:419:97). As a result of the error, additional remedial flood control work is required to provide one percent flood protection for residents and businesses along the two creeks. The one percent flood has a one percent chance of occurring in any given year, and is sometimes referred to as the 100-year flood, because it is the flood that would be equaled or exceeded on an average of one time every one hundred years, measured over a long time period. Since 1997, the District has been studying the causes and extent of the creek capacity deficiencies and exploring alternative solutions. In March 1999, the District retained Schaaf and Wheeler, a water resources engineering consultant, to perform hydrologic and hydraulic analyses of the creeks and to devise a project to provide increased flood protection on Matadero and Barron Creeks. The District has also hosted a number of neighborhood meetings, published project newsletters, and met individually with key stakeholders in order to keep local residents informed and to solicit their input on the scope of work for a remedial project (see list of outreach activities, Attachment A). DISCUSSION Upon completion of the hydrologic and hydraulic analyses, District staff and the consultant developed a set of project alternatives. The alternatives, which include components such as increased flood wall heights, channel widening, bridge modifications, and upstream detention, will be described as part of the District staff’s CMR:124:00 ’Page 1 of 4 presentation to Council and are outlined in the attached project summary (Attachment B). A copy of the January 24, 2000 Informational Memorandum to the District Board regarding this project is also attached (Attachment C). The District presented the project alternatives to local residents at a neighborhood meeting on September 22, 1999 and met with local environmental interest groups to seek their input on December 13 and 16, 1999. Taking into account costs, constructability, environmental impacts, and public input, District staff has tentatively identified a preferred project alternative. District staff has requested the opportunity to present the project alternatives to Council in order to receive feedback on the proposed flood control work. After receiving additional input from Council and members of the public (a third neighborhood meeting is scheduled for February 17, 2000), District staff will finalize its recommended project alternative and prepare an environmental impact report for review and adoption by the District Board in Summer 2000. The District staff’s current preferred project alternative consists of the following components: Downstream of Highway 101, construction of a 40-foot-wide concrete-lined bypass channel along the edge of the Municipal Service Center (MSC) to convey~ excess flows during a one percent flood event. Upstream of Highway 101, reconstruction of the Louis Road bridge one to two feet higher than the existing bridge and raising of floodwalls along the creek channel, by one to three feet, as necessary to meet the Federal Emergency Management Agency’s criteria for one percent flood protection. Staff has noted the following potential impacts of the project on Palo Alto residents and facilities, which would need to be studied, and mitigated if necessary, by the District staff during preparation of the project environmental assessment: General: noise; dust; ’construction traffic. New concrete channel adjacent to MSC: loss of portion of existing vegetative buffer along MSC perimeter; temporary loss of use of storage areas during construction; impact to aesthetics. Replacement of Louis Road bridge: temporary traffic detours (including bu~ and bicycle traffic) during construction; adjustment of utility lines; shortening of sight distances along Louis Road; modified drainage and driveway access for adjacent residences. Raised flood walls: impact to aesthetics; reduced visibility into creek corridor; impairment to local storm drainage into creek (higher flood walls will allow higher water levels to build up in the creek during large storms; when the creek is high, storm drains will not flow until water backs up in the pipes and/or gutters to a level as least as high as the creek). CMR:124:00 Page 2 of 4 RESOURCE IMPACT The Matadero/Barron Creek Flood Control Project has no fiscal impacts to the City. Design and construction of the project will be funded by the District. City staff will continue to coordinate with District staff during the design and construction of the project in order to minimize impacts to City facilities, residents, and businesses. POLICY IMPLICATIONS This report does not contain changes to existing City policies. TIMELINE District staff will submit an engineer’s report and an environmental impact report for the Matadero/Barron Creek Flood Control Project to the District Board for review and approval in Summer 2000. The District will retain a design consultant in Spring 2000 to prepare project construction plans and specifications. Construction is scheduled to take place during 2001 and 2002, with a projected completion date of Fall 2002. ENVIRONMENTAL REVIEW An environmental assessment is not required for this report. ATTACHMENTS Attachment A: Attachment B: Attachment C: List of the District’s proj ect-related public outreach activities Matadero/Barron Creek Flood Control Project Summary January 24, 2000 Informational Memorandum to the District Board PREPARED BY: Joe Teresi, Senior Engineer DEPARTMENT HEAD: CITY MANAGER APPROVAL: : ~OLENN S. R.OBE~RTS ]Director of fiubjJ~)Works l / / J~E FLEM~G / /City Manager// Greg Zlotnick, Santa Clara Valley Water District Board member Stan Williams, Santa Clara Valley Water District Kay Whitlock, Santa Clara Valley Water District Dave Chesterman, Santa Clara Valley Water District Dave Hook, Santa Clara Valley Water District Palo Alto Chamber of Commerce CMR: 124:00 Page 3 of 4 Steve Blanton, Peninsula West Valley Realtors Association Will Beckett, Barron Park Association Kristen Johnson, Ventura Neighborhood Association Cole H. Richmond, Chestnut-Wilton Homeowners Association Debbie Mytels, Midtown Residents Association Michael Gagliasso, Middlefield Road Residents Association Robert Debs, Midtown West Residents Association CMR: 124:00 Page 4 of 4 ATTACHMENT A MATADERO/BARRON CREEKS REMEDIATION PROJECT Summary of Public Outreach Activities Public meetings: Permit Strategy workshop with regulatory agencies Technical workshop with interested public Small group meetings with various community environmental groups Stanford meetings for upstream flow detention opportunities Project status briefing with the City of Palo Alto Newsletters about project status Rev. 1/6/00 6/20, and 9/22/99, 3rd meeting is scheduled on 2/17/2000. 7/20/99 8/24/99 12/13, and 12/16/99 9/13, 11/3, and 12/10/99 12/15/99 Sent to affected residents in late May and August, 1999, 3rd newsletters will be sent prior to the 3rd public meeting. Fact sheet Web site postings Sent to affected residents in late November, 1999. Monthly project progress updates, newsletters, and fact sheet. ATTACHMENT B Santa Clara Valley Water District Matadero/Barron Creeks Long-Term Remediation Revised Project Summary Purpose of this Study In 1997, the Santa Clara Valley Water District completed a series of flood control improvements on Matadero and Barton Creeks in Palo Alto. Although these improvements increased flood protection, they did not achieve the design objectives of adequately conveying the 100-year flood flows along Matadero Creek. Analyses made during the preparation of a Letter of Map Revision request to FEMA, as well as a storm event in early February 1998 underscored the need for additional flood protection. Very high flows in Matadero and Barton Creeks occurred during the 1998 storm (estimated to be close to the one percent event) and SCVWD crews performed manual emergency operations to prevent creek overflows downstream of E1 Camino Real. High water marks along Matadero Creek indicated the lack of conveyance capacity between E1 Camino Real and Ross Road. A high water mark on Matadero Creek at Highway 101 also raised concerns about backwater effects from the Palo Alto Flood Basin (PAFB) that could exacerbate flooding along the reaches from Highway 101 upstream to Louis Road. In the spring of 1998 the SCVWD decided to undertake a long-term remediation project to bring Matadero and Barron Creeks’ flood protection to Federal Emergency Management Agency (FEMA) national standards. The engineering planning study for this work began in March 1999. The scope of work developed for the planning study included a comprehensive evaluation of the entire hydrologic/hydraulic system’ for these two creeks. An analysis of the Palo Alto Flood Basin. was conducted as part of this scope of work, to not only update studies conducted over 20 years ago, but also to incorporate the PAFB’s present-day physical and operational characteristics into the evaluation of channel hydraulics in the lower reaches of Matadero Creek. Schaaf & Wheeler Revised Project Summary Matadero/Barron Creek Long-Term Remediation 1 January 11, 2000 Study Findings The results of the hydrologic, hydraulic and related analyses are as follows: 1) 2) 3) 4) 5) Based on an additional 25 years of streamflow data, the design one percent flowrate on Matadero Creek has been reduced by 12 percent; the design one percent flowrate on Barron Creek has been reduced by 3 percent. The Barron Creek channel will meet the national FEMA standards for one percent flood protection when full diversion to Matadero Creek is restored. The Matadero Creek channel does not meet FEMA standards for the one percent flood protection. There is restricted flow in the Matadero Channel due to several bridge crossings in the reaches upstream of Highway 101 and heavy vegetation and silt deposits in the channel reach downstream of Highway 101. The tidal behavior of San Francisco Bay during winter storm events exacerbates backwater problems within the lower reaches of Matadero and Barron Creeks. However, changes to flood basin operations are not warranied. There has been minimal change in the storage capacity of the Palo Alto Flood Basin over the last 25 years. Development and Evaluation of Solution Alternatives A combination of flood control components will likely be required to provide the desired protection to Matadero Creek. Potential components have been developed and evaluated as part of this engineering study. These are summarized below. Matadero Creek Downstream of Highway 101 The reach of Matadero Creek between Highway 101 and the Palo Alto Flood Basin does not have the capacity to effectively pass flood flows to the PAFB. The channel area has diminished over time due to significant silt deposits and resultant vegetative growth along the creek banks. This results in serious backwater effects during high flow periods that pose a flooding threat to residents as far upstream as Ross Road. To alleviate the backwater pressure, three alternatives were evaluated. They are labeled as "D/S" to denote "Downstream" of Highway 101 : D/S Alternative 1 Clearing the riparian vegetation and sediment deposits to widen the earthen channel section to match conditions in the early 1970’s. This alternative would include establishing and Schaaf & Wheeler Revised Project Supnmary Matadero/Barron Creek Long-Term Remediation 2 Janua~. 11, 2000 D/S Alternative 2 D/S Alternative 3 following strict maintenance guidelines to prevent future buildup of sediment deposits along the cleared channel. Constructing a concrete-lined bypass along the edge of the City of Palo Alto’s Municipal Services Center (MSC). The bypass would convey approximately half of the one percent flood. Construction of a sediment basin just downstream of E. Bayshore Road and periodic sediment removal from the existing channel would be required to maintain the conveyance capacity of the existing channel at its present level. Constructing a box culvert underneath E. Bayshore Road between Matadero Creek and the Animal Services Center. The culvert would convey approximately half of the one percent flow into the Palo Alto Flood Basin just south of the Animal Services Center. Construction of this culvert would result in significant traffic disruption, necessary relocation of several utilities, and raising the roadway elevation of E. Bayshore Road, affecting access to the MSC. Maintaining this facility would be very difficult and expensive. Preferred Flood Protection Plan for Downstream of Highway 101 Conceptual plans, preliminary mitigation requirements, and estimated costs were developed for each of these alternatives. These are summarized in Table 1. D/S Alternative 1 is the most costly alternative, due tO the anticipated mitigation requirement of nearly 10 acres. D/S Alternative 3 requires the least amount of mitigation, but would be very costly to build as well as maintain in the future. D/S Alternative 2 is the least costly of the three alternatives, reflecting both moderate mitigation requirements and moderate construction costs. This alternative is the preferred option for the flood protection plan. Table 1. Summary of Mitigation Requirements and Cost Estimates for Downstream of Highway 101 Alternatives Downstream Alternatives 1 - Enhanced Riparian Corridor 2 - Concrete Bypass 3 - Box Culvert Under E. Bayshore Required Mitigation Riparian 9.3 ac Marsh 0.8 ac Riparian 3.2 ac Marsh 0.5 ac Riparian 1.1 Available On-Site Mitigation ! .63 ac 0.9 ac 0.01 ac Land Costs for Remaining Mitigation* $16,900,000 $ 3,200,000 Estimated Mitigation Costs $5~,000 $2~,000 $100,000 Estimated Construction Costs $4,2~,000 $11,800,000 Total Estimated Costs $19,000,000 $10,000,000 $15,000,000 Schaaf & Wheeler Revised Project Summary Matadero/Barron Creek Long-Term Remediation 3 January 11, 2000 I Marsh 0.5 ac I 1 1 t I * For conservative purposes, it is assumed that off-site mitigation land will have to be purchased by the District at a cost of $2M per acre. Matadero Creek Upstream of Highway 101 Several solution components were developed for addressing one percent flood protection along Matadero Creek upstream of Highway 101. Raising floodwalls along the creek and at bridge crossings The February 1998 storm event resulted in Matadero Creek channel flows that nearly reached the top of the existing floodwalls along the channel and at some of the bridge crossings. To meet FEMA national standards, four (4) feet of freeboard must be provided at bridge crossings and 100 feet upstream and downstream of these crossings. Three (3) feet of freeboard are required at all other locations. Based on current conditions, the floodwall height requirements at the Greer and Louis Road crossings could be as high as 7 to 8 feet as measured from street sidewalks. Modifying the channel shape to a U-frame The Matadero Creek channel is presently trapezoidal in shape from Alma Street down to Highway 101. Modifying the channel from a trapezoidal cross-section to a U-frame section would increase the flow area and thus lower the water surface elevation during peak flows. This would reduce floodwall height requirements along the creek. In the Matadero channel, this modification would be a costly and risky undertaking due to the presence of a buried 60kV line that runs adjacent to and, at certain locations, less than 5 feet from the edge of channel. A sanitary sewer line also runs parallel to the creek channel in certain reaches. Modifying the channel cross-section would require significant dewatering as well as potential handling and disposal of contaminated soils. Modifying one or more bridge crossings Modifying some of the bridge crossings was originally suggested as a means to minimize floodwall heights at the crossings. Moreover, structural investigations conducted as part of the engineering analysis determined that the existing Louis Road Bridge could suffer significant damage from excessive uplift forces caused by pressure flows and subsequent high water levels at the bridge headwalls. Raising the floodwalls on this existing structure would further jeopardize its structural stability. Since this bridge must be replaced with a redesigned structure, it would be beneficial to raise the bridge as much as is practicable, as this would improve hydraulics and reduce the required floodwall height. Schaaf & Wheeler Revised Project Summary Matadero/Barron Creek Long-Term Remediation 4 January 11, 2000 Reducing the one percent flow volume in Matadero Creek An off-stream storage basin location was identified during the engineering study near the confluence of Deer and Matadero Creeks, at a site bounded by Page Mill Road, Foothill Expressway and Coyote Hill Road. This land is owned by Stanford University and is presently zoned for Academic Reserve. A storage basin could be created on approximately 20 acres of this parcel in a manner that would preserve the natural conditions at the site and perhaps ~even enhance the habitat for certain endangered species. During high flow events, the basin would temporarily detain a portion of the flows from both Deer Creek and Matadero Creek in order to lower the peak volume of the flood coursing downstream. After the peak of the flood would pass, the stored water would slowly be discharged back to the creek. Creating this off-stream detention basin would reduce required floodwall heights by approximately one (1) foot in the downstream portions of the creek. The District staff has met with Stanford officials several times to discuss the use of this land for such a purpose. Stanford has no plans to develop this land at the present time, but has declined the District’s request to implement this flood control option and enhance the habitat for local species. The District has the option of condemning the land for use-as a flood control measure; however, a fair market value would have to be paid for the land as part of this process. At an estimated value of $2M per acre, the cost of the land would be approximately $40M. Three alternatives that combine various flood protection components were developed for Matadero Creek upstream of Highway 101. The flood protection components were originally combined in a manner that would focus the work either within the channel (to minimize floodwall and bridge modification work) or outside of the channel (to minimize the risks and costs associated with in-channel construction). The three alternatives are designated as "U/S" to denote "Upstream" of Highway 101: U/S Alternative 1 (Minimizing In-Channel Work) Backwater relief improvements downstream of Highway 101 Modifications to the Louis Road Bridge crossing Raising floodwalls as necessary U/S Alternative 2 (Minimizing Floodwall Work) Backwater relief improvements downstream of Highway 101 Schaaf & Wheeler Revised Project Summary Matadero/Barron Creek Long-Term Remediation 5 January 11, 2000 ® Modifying channel to a U-frame from Greer Rd. to Alma Street. Raising floodwalls as necessary U/S Alternative 3 (Minimize Channel and Floodwall Work) ¯Backwater relief improvements downstream of Highway 101 ¯Reduce in-channel flow with off-stream storage basin on Stanford land Raising floodwalls as necessary In each of the three alternatives, it is assumed that the same backwater relief improvements will be implemented downstream of Highway 101. The second component in each alternative provides varying improvement in the form of additional channel capacity (U/S Alt. 1 and U/S Alt. 2) or reduced channel flow (U/S Alt. 3). The improvements gained from the first two components are then used to determine the necessary height of the floodwalls along the creek and bridge crossings in order to meet the FEMA national standards. Table 2 presents a comparison of the existing and required floodwall heights at the bridge crossings on Matadero Creek for the three alternatives. Undertaking the costly construction work to modify the channel from trapezoidal to U-frame from Louis Road to Alma Street (U/S Alt. 2) provides only a minor change in required floodwall heights as compared to U/S Alt. 1. Developing the off-stream storage basin on Stanford land (U/S Alt. 3) only lowers the required floodwalls at Greer and Louis by 0.8 to 1.0 feet below that required in U/S Alt. 2. During the process of obtaining a Conditional Letter of Map Revision (CLOMR) from FEMA, which is required prior to proceeding with the design of flood protection improvements, SCVWD will conduct a risk-based analysis to better determine the site- specific freeboard to pass the one percent discharge with a high degree of confidence. Based on these results, the District will make every effort to negotiate some relaxation of the freeboard requirements with FEMA, and thus lower the wall height requirements at Greer and Louis Roads. Schaaf & Wheeler Revised Project Summary Matadero/Barron Creek Long-Term Remediation 6 January 11, 2000 Table 2. Required Floodwall Heights at Matadero Creek Bridge Crossings to Meet National Standard of Flood Protection BRIDGE Greer Louis Ross Middlefield Cowper Waverley Bryant Above Alma Existing 4.0 3.0 2.6 2.6 2.6 2.0 n/a rda Required Floodwall Height Above Bridge U/S Alt. 1 7.1 7.0 3.4 no change no change 3.5 N/a N/a U/S Alt. 2 7.1 6.4 no change no change no change no change n/a n/a Sidewalk (feet) U/S Alt. 3 6.3 5.4 no change no change no change no change n/a n/a Table 3 compares preliminary estimated costs for the flood protection improvements that would be implemented upstream of Highway 101 for the three proposed alternatives. The estimated costs for U/S Alternative 1 include bridge replacement at Louis Road and the associated costs of utility relocation, roadway conforms, and impacts to drainage/access on adjacent properties. The estimated costs for U/S Alternative 2 account for the construction work that would have to be undertaken to modify the existing channel to a U-frame. The costs for U/S Alternative 3 include an estimated $40M for the fair market value of the off-stream storage basin site at Foothill Expressway and Page Mill Road. Table 3. Preliminary Cost Estimates for Flood Control Improvement Alternatives on Matadero Creek Upstream of Highway 101 Preliminary Cost Estimates U/S Alternative 1 $6,000,000 U/S Alternative 2 U/S Alternative 3 $ 30,000,000 $ 48,000,000 Schaaf & Wheeler Revised Project Summary Matadero/Barron Creek Long-Term Remediation 7 January 11, 2000 Based on preliminary cost estimates, U/S Alternative 1 is the preferred alternative for achieving one percent flood protection along Matadero Creek. Summary The engineering analysis conducted to date has determined that additional flood protection improvements are required on Matadero Creek both upstream and downstream of Highway 101. Based on the development and evaluation of several alternatives, the preferred plan includes the following: 1) 2) Downstream of Highway 101: Construct a concrete-lined bypass along the edge of the City’s Municipal Services Center to convey approximately 1,400 cfs during a one percent event. Upstream of Highway 101: Replace the Louis Road Bridge and raise it by 1 foot; modify the trapezoidal channel just upstream and downstream of Louis Road with a U-frame channel to improve conveyance capacity; and raise floodwalls as necessary along the creek channel and at various bridge crossings to meet the FEMA criteria for one percent flood protection. Schaaf & Wheeler Revised Project Summary Matadero/Barron Creek Long-Term Remediation 8 January 11, 2000 Santa Clarayalley Water District TO:Start Williams SUBJECT:Matadero/Barron Creek Remediation Project - - Sums Report on Alternatives MEMORANDUM FROM:Dave Chesterman DATE:1/24/00 Staff informed the Board on October 7, 1997 that the recently completed improvements on Matadero Creek did not meet the project goal of providing protection from the design flood flow of the 100-year stornl event. Staff was directed to proceed with interim remedial measures, which have be~n implemented every rainy season since then. Staff was also directed to bring proposed long-term remedial measures back to the Board for consideration and approval. On March 17 1999, Schaaf & Wheeler was retained to assist in the project planning study. Since then, there have been public meetings on June 10 and September 22 1999. On July 20,. we organized a Maudero/Barron Creeks field trip and permit strategy workshop with members of re~latory agencies. A Technical Workshop was held on August 24 1999. Small group meetings were held with various community environmental groups on December 13 and 16, 1999. Fact Sheets, Newsletters and web-site monthly progress updates have also been used to communicate with the public during the planning process. Staff have developed and evaluated several alternatives for the project. A third public meeting will be held on Thursday, February 17 to present the alternatives and solicit public comments prior to completing the draft Engineer’s Report and environmental documents. The alternatives will also be presented to the Palo Alto City Council on February 7, 2000. The attached sutus report (Attachment 1) includes a discussion of alternatives considered. Mitigation requirements and enhancement opportunities are also mentioned. A brief summary of the alternatives under consideration is also included in a recent public information brochure shown in Attachment 2. Following the third public meeting, the Engineers Report will be completed in May 2000. This will include a recommendation by staff for a preferred alternative. Following preparation of the Final Engineers Report and draft environmental document, the draft environmental document will be routed to the Board for approval to circulate for public comment. This is scheduled for August 2000. David J. Chesterman Manager Project Development Group Attachments Cc: Joe Teresi (City of Palo Alto), D. Chesterman, D. Hook, E. Tsou, File © © ®®® ® ®® Z z Ld z 0 ®® ®@ @ ® district started workingoi~ the March 1999 to achieve the .ation’al level ~f pr~te~ction by the e~d of 2002. SOLUTION ’Long Term Flood Prat~btion Remediatlon Project in Based on hydrologic and hydraulic findings ot: the Matadero and Barton creek channels and results fi’om investigations of the Palo Alto Flood Basin operations, Barton Creek, with diversion into Matadero, has sufficient flow capacity against the 100-year flood. Matadero Creek does not meet the standard as a result of restricted flow in the channel due to several bridge crossings and heavy vegetation and Silt deposits in an area of the channel downstream of Bayshore Freeway. COMPONENTS Matadero Creek Downstream of Highway 101 Just downstream of Bayshore Freeway, flood flows cannot pass effectively through a heavily vegetated stretch of channel to enter the Palo Alto Flood Basin. This results in serious backwater effects upstream of the Bayshore Freeway that poses a flooding threat to residents as far upstream as Ross Road. Possible solutions to alleviate the downstream channel blockage include ¯Clearing the riparian vegetation and widening the earthen channel section "Constructing a rectangular concrete-lined channel parallel to the main channel and along the boundary with the City’s Corp Yard "Building a large box culvert under the City’s Corp Yard parking lot, building a concrete lined channel along East Baysho;’e Road, or building a large box culvert under Easy Bayshore Road Matadero Creek Upstream of Highway 101 There are several solution components that would improve flood protection along Matadero Creek between Highway 101 and Alma Street. The four main components are ¯ Raising floodwalls ~ Modifying the channel shape to a U-frame (Vertical Sides) ~ Performing a bridge modification at Louis Road ~’ Reducing the flow in the creek channel by creating an upstream flood storage area Matadero & Barton Creeks Site Map Santa (iara Valley Water Distrid: 6 inlmlze ¯ _ -Floo~a~il Downstr~ifl Louis Rd. B~i~g~"~difi~:~ti~n ~ajor U-Frame Channel Work (kom Louis Raise Fl~dwalls as Necessa~ Alternative #3 - Minimize Channel and Floodwall Work ¯ Downstream Backwater Relief ¯ Reduce Flow with Upstream Flood Storage Area on Stanford Land ¯ Raise Floodwalls as necessary Environmental Impacts Preliminary comparisons of the environmental impacts of each alternative reveal that they range from "no impact" to "potentially significant." Each alternative has at least two impacts that are considered to be potentially significant, indicating that some detailed environmental analysis and mitigation would be required. Regulatory agencies generally require mitigation, or the development of replacement habitat to offset impact, when a habitat is altered. Each recommended alternative includes downstream backwater relief that will be achieved by incorporating one of the three downstream solution components. To clear the channel, riparian vegetation removal will likely require mitigation at a 3:1 ratio. This means three acres of riparian vegetation will have to be replaced for every acre that is removed. The Corp Yard option appears to have the least environmental impact of the three. Floodwalls The required heights of floodwalls at each bridge crossing between Greer Road and Alma Street vary with each recommended alternative, but could be as high as 6 to 7 feet at Greer and Louis Roads. Estimated floodwall heights reflect 4 feet of freeboard, as required by FEMA at all bridge crossings. However, the water district will investigate obtaining some relaxation of that requirement, which could make a significant difference at the bridges closest to Highway 101 - Greer Road and Louis Road. The water district ;;’i!! a!so investigat~ bridge, designs that wil! be ~estheticc,!]7 acceptable to the community. Engineering Considerations Alternatives were evaluated by the following preliminary criteria: cost, construction schedule, neighborhood disruption, and wall heights. The ratings ranged from "Acceptable" to "Some concerns" for those that had more significant impacts, to "Tough going" for the greatest impacts. The analysis reflected a mix of all three ratings for each of the alternatives; therefore, none of the alternatives stand out yet as the most desirable alternative to pursue. Hook at 408/265-2607 x235F. "