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Agenda 02/26/2013 Item #16A152/26/2013 16.A.15. EXECUTIVE SUMMARY Recommendation to approve $350,000 in eligible costs for the FY13 Hideaway Beach erosion control structure project based on the January 16, 2013 Proportionality Analysis conducted by Coastal Engineering Consultants and reviewed by Collier County staff, make a finding that this item promotes tourism and approve necessary budget amendments. OBJECTIVE: Approve the eligible costs associated with Hideaway Beach FY 13 Erosion Control Project. CONSIDERATIONS: The Board of County Commissioners directed that a Proportionality Analysis be performed by the City of Marco Island through the Hideaway Beach District to determine the effectiveness of the proposed FY 13 Erosion Control structures in preventing and /or minimize shoaling inside the entrance channel of Collier Bay. The analysis was conducted by Coastal Engineering Consultants on January 16, 2013 and recommended a proportional public benefit of $730,000. Collier County staff reviewed this analysis with Coastal Engineering Consultants, the Hideaway District and the City of Marco Island and have agreed on a proportionally benefit of $350,000 for the following reasons: 1. The actual dredging of Collier Creek occurred in 2001 and 2012. This represents an eleven (11) year dredging cycle and not every six (6) years as noted in the report. 2. The single most significant event that occurred in this area in recent years is the disintegration of Coconut Island that served as a natural erosion barrier to this area. This occurred in 2006/07 and served to make the entire area more ephemeral. 3. Staff review is based on actual monitoring reports and physical data provided by Coastal Engineering Consultants for the past five (5) years. No modeling analysis was used in staff's analysis. 4. Monitoring reports indicated 1,300 Cubic Yards (CY's) per year filling in Collier Creek. 9,600 CY's of material was dredged from Collier Creek in 2012 and this permitted template will be used for future dredging events. This yields duration between dredging events without erosion control structures for Collier Creek of 7.4 years (9,600 CY's divided by 1,300 CY's per year). This serves to justify the shorter duration between dredging events since the destruction of Coconut Island. 5. Monitoring reports over the last ten (10) years indicate that 76% of the original fill volume is retained by the erosion control structures. This would increase the time between dredging events in Collier Creek if structures were in place to thirteen (13) years (7.4 years multiplied by 1.76). 6. Considering a 25 year project life, two (2) dredging events would be required if structures were in place and three dredging events would be required if no structures were installed. This saves one (1) dredging event if structures were installed. 7. The total cost of the 2012 dredging event was $336,000. This includes the construction, design and monitoring. It is realistic to expect that between $300,000 and $400,000 will be saved over the next 25 years in dredging cost if structures are installed. Staff is recommending $350,000 in savings as a result of the analysis. This complements the 2012 actual project cost of $336,000. Packet Page -1259- 2/26/2013 16.A.15. 8. Collier County staff, Coastal Engineering Consultants, the Hideaway District and the City of Marco have reviewed this recommendation and are in agreement with the revised analysis and costs. Thirty-eight (38) percent of the maintenance of these three (3) erosion control structures is proposed to be the responsibility of Collier County utilizing TDC funds. The balance of the maintenance costs will be by the Hideaway Beach Taxing District and the City of Marco Island. Collier County's maintenance share was determined by the ratio of the beneficial cost of $350,000 to the overall installation cost of $925,000. The Hideaway Beach District Board approved this agreement at their 2/15/2013 board meeting. The structures are assumed to have a useful life of 25 years. The cost split of eventual replacement of these structures would be based on performance monitoring data and the proportional effectiveness of the structures in minimizing shoaling. Tourism benefits from this project are as follows: 1. Marine tourism benefits to the extent that Collier Creek remains open to boaters. It is estimated that one -third to one -half of all boat traffic on Marco Island utilize Collier Creek for access to the Gulf. Esplanade Marina is located in Smokehouse Bay requiring access through Collier Bay and Collier Creek. 2. Tour operators from the Marco Island mainland utilize the Hideaway Beaches for shelling and beach excursions. 3. Public access is available to these beaches via boat. FISCAL IMPACT: Funding for the project under discussion is not presently budgeted. The proposed source of funding is Tourist Development Tax Beach Renourishment Fund (195). If the project is approved a budget amendment in the amount of $350,000 reducing reserves and establishing the project budget will be necessary. GROWTH MANAGEMENT IMPACT: No impact to the growth management plan would result from this Board action. ADVISORY COMMITTEE RECOMMENDATIONS: At the February 14, 2013 Coastal Advisory Committee (CAC) meeting this item was recommended for approval by a 5 to 1 vote. This item will be presented to the Tourist Development Council (TDC) on February 25, 2013 with results communicated to the BCC via written memorandum. LEGAL CONSIDERATIONS: This item has been reviewed by the County Attorney's Office, requires simple majority vote, and is legally sufficient for Board action. — RECOMMENDATION: Approve $350,000 in eligible costs for the FY13 Hideaway Beach erosion control structure project based on the January 16, 2013 Proportionality Analysis conducted by Coastal Engineering Consultants and reviewed by Collier County staff, make a finding that this item promotes Tourism and approve necessary budget amendments. Prepared By: J. Gary McAlpin, P.E., Coastal Zone Management, Natural Resources Department Packet Page -1260- 2/26/2013 16.A.15. Attachments: A) Coastal Engineering Consultants Proportionality Analysis dated January 16, 2013. Packet Page -1261- 2/26/2013 16.A.15. COLLIER COUNTY Board of County Commissioners Item Number: 16.16.A.16.A.15. Item Summary: Recommendation to approve $350,000 in eligible costs for the FY13 Hideaway Beach erosion control structure project based on the January 16, 2013 Proportionality Analysis conducted by Coastal Engineering Consultants and reviewed by Collier County staff, make a finding that this item promotes tourism and approve necessary budget amendments. Meeting Date: 2/12/2013 Prepared By Name: HambrightGail Title: Accountant,Coastal Zone Management 1/31/2013 11:34:15 AM Approved By Name: McAlpinGary Title: Director - Coastal Management Programs,Coastal Zon Date: 1/31/2013 11:47:49 AM Name: LorenzWilliam Title: Director - CDES Engineering Services, Comprehensive Date: 1/31/2013 4:31:22 PM Name: PuigJudy Title: Operations Analyst, GMD P &R Date: 2/1/2013 10:50:44 AM Name: LaPierreBarbara Title: Management/Budget Analyst,Transportation Administr Date: 2/1/2013 2:37:09 PM Name: MarcellaJeanne Title: Executive Secretary,Transportation Planning Date: 2/1/2013 3:27:51 PM Name: GreeneColleen Packet Page -1262- Title: Assistant County Attorney,County Attorney Date: 2/4/2013 8:48:34 AM Name: FinnEd Title: Senior Budget Analyst, OMB Date: 2/4/2013 3:59:06 PM Name: KlatzkowJeff Title: County Attorney Date: 2/4/2013 4:25:54 PM Name: OchsLeo Title: County Manager Date: 2/5/2013 9:28:57 AM Packet Page -1263- 2/26/2013 16.A.15. Attachment A Hideaway Beach Erosion Control Project 1 Introduction 2/26/2013 16.A.15. Proportionality Analysis The City of Marco Island's northern beach area has been experiencing significant erosion in the wake of recent storms including Tropical Storm Fay and coastal processes (winds, waves, tides and currents). The upland development and infrastructure are threatened by the continuing erosion. Based on these measured near -term erosion rates, it is expected the building foundations will soon be under direct wave attack as the remaining dune and wetland habitats and upland vegetated lawn areas will be lost to erosion. The Project is located along the City of Marco Island's northern beach area adjacent to the Big Marco River and Gulf of Mexico, Marco Island, Collier County, Sections 5, 6, and 7, Township 52S, Range 26E, Class III Waters. A location map is presented in Figure 1. Figure 1. Project Location Map. 1 January 16, 2013 Packet Page -1264- Attachment A 2/26/2013 16.A.15. Hideaway Beach Erosion Control Project Proportionality Analysis 2 Project Purpose To address this critical erosion area, the City through its consulting contract with Coastal Engineering Consultants and Coastal Technology Corporation has completed a detailed model study and design, has received the FDEP permit and USFWS Biological Opinion (draft), and is currently processing the last permit required from the USACE for installation of three erosion control structures (T- groins) and placement of up to 25,000 cubic yards of beach compatible sand. The sand sources include the previously utilized Nearshore Borrow Area (2010 project) and Collier Bay Entrance Channel (2001, 2005 and 2012 projects). A third sand source, the Capri-Big Marco Pass Borrow Area (2005 project) is also permitted and will be available for future renourishment events. The purpose of the beach fill and erosion control structures is to address significant erosion along this beach segment providing a mechanism to offset historical erosion, restore recreational beach, provide storm protection to the upland properties, provide and preserve sea turtle nesting sites, and restore coastal habitat for other threatened and endangered species. One public benefit realized by the Project is to reduce the maintenance dredging costs of the Collier Bay Entrance Channel which addresses the navigation needs for the boating community. Installation of the structures is expected to hold the sand placed along this critically eroded beach segment, thereby reducing Project related costs and future maintenance costs. This proportionality analysis has been completed in support of the City of Marco Island's Tourist Development Council's Grant Application in the amount of $925,000 to offset the materials and installation costs of the three (3) T -groin structures. Utilizing the consulting team's coastal processes analysis and detailed model study completed in support of the Project design and permitting, Collier Bay Entrance Channel maintenance history, and Project related monitoring surveys, the costs for maintenance dredging "without" and "with" the T- groins was estimated throughout the Project life which was defined as the expected life of the erosion control structures estimated to be 20 to 25 years. It is noted that no new data collection or numerical modeling was conducted due to the time constraint. 3 History of Management Activities (1997 to present) 1997 Two (2) temporary geotextile T- groins were constructed at South Point Three (3) temporary geotextile T- groins were constructed at Royal Marco Point 2001 • Survey (December) from Royal Marco Point to Collier Bay • Dredging of Collier Bay entrance (May through July); 24,100 cubic yards were removed and placed along the beaches extending up to the Royal Marco Point T- groin field • Two (2) additional temporary geotextile T- groins were constructed • Ville de Marco initiated seawall and dock repairs along Collier Creek (construction suspended due to contractor issues) 2 January 16, 2013 Packet Page -1265- Attachment A Hideaway Beach Erosion Control Project 2/26/2013 16.A.15. Proportionality Analysis 2002 -2003 • 500 -foot section of seawall and 10 finger piers along Collier Creek were replaced by Ville de Marco West (VDMW) in 2002 as result of channel scour. Concrete armor mat was installed along entire property fronting Collier Creek to protect finger piers and seawall from future scour. 220 -foot long steel sheet pile toe wall was installed along northwestern corner of property to protect seawall and observation pier from future scour. Sheet pile were 19 -foot long PZ -27 sections driven 17 feet into channel bottom. 2005 • Pre - construction survey (April) of Collier Bay and Hideaway Beach • Seven (7) temporary geotextile T- groins constructed were removed • 136,350 cy were placed along 2,800 ft of shoreline at South Point • 180,420 cy were placed along 3,000 ft of shoreline at Royal Marco Point • Five (5) T- groins were constructed at South Point • Five (5) T- groins were constructed at Royal Marco Point • Terminal jetty was constructed at the entrance to Collier Bay • Post - construction survey (December) of Collier Bay and Hideaway Beach 2006 • Monitoring surveys (June and December) of Collier Bay and Hideaway Beach 2007 • Monitoring survey (July) of Collier Bay and Hideaway Beach 2008 • Monitoring surveys (September) of Collier Bay and Hideaway Beach • Emergency revetment was constructed (September) between the South Point and Royal Marco Point T -groin fields 2010 • Survey (May) of Collier Bay and Hideaway's North Beach • Pre - construction survey of Collier Bay and Hideaway Beach • Approximately 122,000 cy were placed along 2,500 ft of shoreline • Six (6) T- groins were constructed • Post - construction survey (December) of Collier Bay and Hideaway Beach 2011 • Survey (April) of Collier Bay and Hideaway's North Beach • Tide, current and wave gauge deployment (March -April) • Monitoring surveys (August) of Collier Bay and Hideaway Beach 2012 • Collier Bay Entrance Channel was dredged (January), approximately 9,300 cubic yards (total) were excavated and placed in nearshore between H12 and H14 3 January 16, 2013 Packet Page -1266- Attachment A 2/26/2013 16.A.15. Hideaway Beach Erosion Control Project Proportionality Analysis • Monitoring surveys (July) of Collier Bay and Hideaway Beach • Bid Documents prepared for Hideaway Beach Erosion Control Project — Phase III to include 25,000 cubic yards between approximately H -12 and H -14 plus three new T- groins tapered to replicate the 1990 mean high water shoreline 2013 • Bids received for the Project. Apparent low bidder priced the three erosion control structures equal to $994,952 including mobilization, materials, installation, and demobilization along with 50% of the environmental protection bid item ($7,500). It is noted the environmental protection bid item was for the whole Project including both the dredging /filling operations and the structures. The bid tabulation sheet prepared by City Purchasing is included in Attachment 1. It is noted this bid is approximately 4% under the Engineer's Opinion of Cost for the structures (same bid items) equal to $1,036,370. 4 Terminal Jetty The existing terminal jetty located along the west side of the entrance to Collier Bay was constructed by the County in 2005. The jetty was placed within Sovereign Submerged Lands Easement No. 40548, BOT File No. 110337105. A 2001 to 2005 project components map is presented in Figure 2. 5 Collier Bay Entrance Channel 5.1 Introduction The Collier Bay Entrance Channel is located at the junction of the Big Marco River with the entrance channel to Collier Bay, locally known as Collier Creek. The dredge area was placed within a Sovereign Submerged Lands Easement No. 30622, BOT File No. 110222775. The design cut depth is to elevation -11.3 feet NAVD with a 1 -foot overdredge tolerance to -12.3 feet NAVD. The design cut width is 80 feet; however, due to the presence of the existing terminal jetty, between approximate Stations 2 +00 and 4 +50, the dredge cut width reduces by 10 feet (70 feet) to 38 feet (42 feet). Based upon field observations and review of the multiple monitoring surveys, the terminal jetty has filled to capacity by natural sand transport and bypassing is occurring into the channel resulting in a constriction of the Collier Creek waterway. This narrowing has increased the current velocities in the Creek resulting in significant scour along the eastern side of the channel adversely impacting the upland development on the opposite shoreline. The 2012 dredging project, FDEP Permit No. 0305112- 001 4C, was completed by the City (design and permitting) and County (construction and construction management) to temporarily alleviate this condition. 4 January 16, 2013 Packet Page -1267- Attachment A 2/26/2013 16.A.15. Hideaway Beach Erosion Control Project Proportionality Analysis Figure 2.2001— 2005 Project Components Map 5.2 Entrance Channel Maintenance History Details of the entrance channel maintenance projects are presented in Table 1. The volumes for each dredging event, costs for each construction project, and volumes within the dredge cut for various condition surveys are presented therein. Table 1. Collier Bav Entrance Channel Vnlume Chrannlnav Description Volume In Permitted Template (CI) 2001 Original Dredging 24,100 2005 Terminal Jetty Unreported April 2011 Condition Survey 9,900 2012 Maintenance D ing 9,300 July 2012 Condition Survey* > 6,000 * Limited number of surveyed channel cross sections With respect to computing the average dredge volume, it is noted that the terminal jetty was placed within the original dredge cut limits; no volume was recorded for the 2005 event that CEC's research was able to discern. To that end, the condition survey in 2011 and maintenance dredge project in 2012 yielded 9,600 cubic yards plus or minus 300 cubic yards in the permitted 5 January 16, 2013 Packet Page -1268- Attachment A 2/26/2013 16.A.15. Hideaway Beach Erosion Control Project Proportionality Analysis dredge cut. This indicates that once the channel infills to this approximate volume, no additional shoaling within the permitted dredge limits occurs. Rather, at this point the shoaling into the channel deflects the currents which then scour the sediment along the opposite shoreline as evidenced by the monitoring results described herein. Based on these data, the design maintenance dredge volume shall be set equal to 9,600 cubic yards. Based on the construction events (2000, 2005 and 2012), inlet maintenance has been completed on average every 6 years. However, based on the monitoring surveys and observed infilling / scouring trends, the entrance channel fills in more rapidly than every 6 years (as evidenced in the proceeding sections) suggesting the appropriate maintenance interval may be less than 6 years. Based on the coastal processes analysis and annual monitoring surveys /reporting, the maintenance dredge interval is primarily a function of episodic events which are the predominant coastal forcing function transporting sediment along the shoreline and into the inlet mouth. 6 Coastal Processes Assessment 6.1 Historic Shoreline and Volume Changes Bathymetric /topographic survey data and aerial photographs collected between 1990 and 2010 were used to analyze and compute historic shoreline changes. Figure 3 presents a summary of historic shoreline change rates at Mean High Water (MHW) within the Project area for four periods between 1990 and 2010. The rates indicate that the 20 -year trend (1990 -2010) within the Project area between approximately H -12 and H -14 was erosional with erosion rates ranging from 2 feet per year to 6 feet per year. Immediately outside the Project area the shoreline during this period was stable to accretional. Further, there is a significant shift in trends from the 1997- 2005 period when the shoreline within the Project area was stable to the most recent 2005 -2010 period when erosion rates ranging from 8 feet per year to 20 feet per year occurred. Bathymetric /topographic survey data collected in December 2005 and December 2010 were used to analyze and compute historic volumetric changes. Figure 4 presents a contour map of the changes with three (3) volume cells identified. Cell 1 approximately delineates the Project area experiencing erosion, Cell 2 represents the area updrift of the existing terminal jetty, and Cell 3 covers the entrance to Collier Creek and the creek interior. Seaward limits for Cells 1 and 2 were determined based upon comparison between the 2005 and 2010 profiles and defined as an approximate location of profile closure seaward of which cross -shore sediment movement did not occur. Based on the volumetric calculations, Cell 1 lost approximately 17,700 cubic yards between 2005 and 2010, Cell 2 gained approximately 4,900 cubic yards, and Cell 3 gained approximately 6,500 cubic yards. 6 January 16, 2013 Packet Page -1269- Attachment A Hideaway Beach Erosion Control Project 2/26/2013 16.A.15. Proportionality Analysis Figure 3. North Hideaway Beach Historic Shoreline Change Rates. Figure 4. Hideaway Beach North Segment Historic Volume Changes. 7 January 16, 2013 Packet Page -1270- Change, ft F 12i 10 u 9 8 7 t _4 H -1a 5 0 -r ® z L� . iryn 4 � 2 f cv 0.5 t 0.25 O a g� fj -0.25 Z H -12 \� -4 -5 -6 -8 -10 -12 Aerial Date: 2011 -15 n 414000 415000 Easting (NAD83, ft) Figure 4. Hideaway Beach North Segment Historic Volume Changes. 7 January 16, 2013 Packet Page -1270- Attachment A 2/26/2013 16.A.15. Hideaway Beach Erosion Control Project Proportionalitv Analysis 6.2 Sediment Budget The historic volumetric changes presented in the previous section were annualized and a local sediment budget was developed. It is presented in Figure 5. A negligible amount of sand was measured transporting from the existing T -groin field into Cell 1 (North Beach segment). The amount of sediment transported out of Cell 1 to the northeast was on the order of 3,500 cubic yards per year. The fillet formed updrift of the jetty at an average rate of 1,000 cubic yards per year attributed to the erosion from Cell 1. The net change in Cell 3 including the deposition into the channel and scouring on the opposite shoreline was approximately 1,300 cubic yards per year, again attributed to the erosion from Cell 1. A detailed coastal processes analysis was prepared on behalf of the City and submitted to the FDEP and USACE in support of the Project design and permitting (CEC, 2011). Figure 5. North Hideaway Beach Sediment Budget. The results of the sediment budget indicate the majority of the sediment transporting off the North Beach segment either deposits on the beach updrift of the jetty or infills the channel. 8 January 16, 2013 Packet Page -1271- Attachment A 2/26/2013 16.A.15. Hideaway Beach Erosion Control Project Proportionality Analysis 6.3 Current Conditions Examining the sediment budget for Cells 2 and 3 (Figure 5) and noting that the jetty fillet is at capacity under current conditions, the 1,000 cubic yards per year is likely bypassing the jetty and when combined with the 1,300 cubic yards per year, equates to a potential shoaling rate of approximately 2,300 cubic yards per year. Based on the design maintenance dredge volume of 9,600 cubic yards and the average annual shoaling rate, the design maintenance dredge interval equals approximately 4.2 years. Utilizing the limited number of inlet cross sections collected as part of the July 2012 monitoring survey provided by the City through their outside consultant, Humiston and Moore (H &M), it is estimated that over 6,000 cubic yards of sand is within the permitted dredge cut. This deposition occurred within 6 months of the 2012 maintenance event. It is noted that Tropical Storm Debby struck the Southwest Florida coastline during this timeframe impacting the Project area and contributing to this significant deposition rate compared to the five year average. Because the terminal jetty fillet remains filled to capacity, the significant majority of sand eroding off of North Beach was transported alongshore and deposited in the channel. 7 Numerical Modeling Study The main objective of the Numerical Modeling Study was to predict, using numerical model simulations, the sediment transport magnitude and directions, bathymetric changes, beach fill diffusion, and channel sedimentation rates for each alternative carried forward from the Plan Formulation Phase of the Project; and to compare the relative performance of the alternatives in order to select the recommended design. DHI's MIKE21 model was implemented in this Study. The model is state -of -the -art with features capable of simulating the processes driving morphologic changes in the Project area. MIKE21 is a fmite volume model based on unstructured mesh. Computational grids were created to include three (3) offshore (north, west, and south) and three (3) inland open boundaries. The morphology module, which computes both bed -load and suspended load transport of non - cohesive sediments, was activated and coupled with the flow module. The wave module was activated because wave action is an important process causing morphologic changes in the Project area. The computational grid used in the flow module was also specified in the wave module for simplicity. The coupling of flow, waves, and sediment transport was done dynamically. The primary forces behind processes within the Project area are wave action and tidal currents associated with the inlet system. Therefore, the flow, wave, and sediment transport modules were dynamically coupled within the model. The calibration and validation steps were performed for two independent time periods and utilized different input data. The consulting team concluded that the calibration and validation program was sufficient for utilization in modeling the defined beach and inlet management alternatives, specifically for comparative assessment and ranking of the performance of each alternative. As part of the alternatives analysis, a detailed modeling report was prepared on behalf of the City and submitted to the FDEP and USACE in support of the Project design and permitting (CEC, 2011). 9 January 16, 2013 Packet Page -1272- Attachment A Hideaway Beach Erosion Control Project 8 Alternatives Analysis 2/26/2013 16.A.15. Proportionality Analysis 8.1 Introduction The next step in the design and permit process was to model several alternatives and compare and contrast the results. The alternatives were simulated using the calibrated and validated MIKE21 model. These alternatives included but were not limited to: 1) No Action 2) Beach fill with T- groins (Figure 6). The T- groins are cited such that their T -heads are aligned with the 1990 shoreline position in an effort to restore the shoreline to pre- erosion control management implementation on Hideaway Beach as well as prior to the loss of Coconut Island which afforded protection to this shoreline segment. Further, according to the analysis of historic shoreline trends, North Beach was stable between 1990 and 1997. The toe of the fill is located approximately 20 feet landward of the steep river slope to avoid direct losses of the fill into the channel. Figure 6. Beach Fill with T- Groins Alternative Design Plan. 10 January 16, 2013 Packet Page -1273- Hideaway Beach Erosion Control Project 8.2 Model Results Attachment A 2/26/2013 16.A.15. Proportionality Analysis The No Action Alternative was chosen as a baseline alternative to which model results of the other alternatives were compared. It should be noted that due to model instabilities, approximately 10 months of the 1 -year simulation period were achieved. Figure 7 presents the comparison of morphologic elevations and volume change quantities within the beach fill between the two alternatives at the end of the 10 -month simulation. This comparison was created by subtracting the 10 -month modeled surfaces. The comparison indicates that for the No Action Alternative there was a loss of approximately 5,400 cubic yards of sand within the beach fill template; and that for the Beach Fill with T- Groins Alternative there was a loss of approximately 4,200 cubic yards of sand within the beach fill template. Based on the model results, Beach Fill with T- Groins Alternative reduced the losses from the beach fill by approximately 22 %. Figure 7. Analysis of Morphologic Changes and Volumetric Quantities. 11 January 16, 2013 Packet Page -1274- . 1 kr .111 11 .,11 4 ��r 1 — 11 a .i11 If �1/ Figure 7. Analysis of Morphologic Changes and Volumetric Quantities. 11 January 16, 2013 Packet Page -1274- Attachment A 2/26/2013 16.A.15. Hideaway Beach Erosion Control Project Proportionality Analysis Numerical modeling of the No Action Alternative resulted in deposition of approximately 1,800 cubic yards of sand within the permitted borrow area limits over a 10 -month simulation period, which is equivalent to a shoaling rate of approximately 2,200 cubic yards per year. Comparing this to the potential shoaling rate taken from measured results (Section 6.3), the model results are very realistic, and confirm the design maintenance dredge interval. 9 Proportionality Analysis 9.1 Benefits Analysis Two methods are examined to predict the reduction in the entrance channel infilling from installation of the three T- groins along the North Beach segment. 9.1.1 Numerical Modeling Approach First, the results of the numerical model study developed by the consulting team for the design and permitting were examined to compare the No Action Alternative to the Beach Fill with T- groins Alternative. The model results predicted the structures will reduce the beach fill losses by approximately 22 %. It is noted that due to the time constraint in preparing this analysis, a model run with beach fill only (i.e. no structures) was not achievable. It is suggested that the placement of beach fill without structures compared to the beach fill with structures would show similar results or quite possibly show that the structures have a higher percent retainage of fill. Thus the 22% is assumed to be the minimum benefit afforded by the structures for reducing the infilling into the entrance channel. 9.1.2 Empirical Approach Second, the performance of the existing T -groin field was reviewed to quantify the benefits the T- groins provide in retaining the sand within the fill template. Humiston and Moore (H &M) have completed long -term monitoring surveys and reports for the T -groin projects along Hideaway Beach for over a decade. As part of the post - construction report of the 2010 project, H &M (2011) reported that the T -groin field constructed in 2005 along Royal Marco Point retained approximately 76% of the original fill volume after four years and approximately 81% of the original fill volume after five years noting that 3,000 cubic yards had been added to this segment by virtue of the 2010 construction project. The current Project utilizes the same design principals as the 2005 and 2010 projects, and the performance of the T -groin field along North Beach is predicted to perform similar to the rest of the T -groin field. Thus the 76% is assumed to be the maximum benefit afforded by the structures for reducing the infilling into the entrance channel. R2 Dredge Cycles Table 2 presents a summary of dredge cycles and number of events computed for the Project life defined as the life of the structures which is equal to 25 years. The maintenance years are computed from 2012, the date of the last maintenance event. The 25 -year period begins in 2013, 12 January 16, 2013 Packet Page -1275- Attachment A 2/26/2013 16.A.15. Hideaway Beach Erosion Control Project Proportionality Analysis corresponding to the new T -groin installation, and ending in 2038. Based on the design maintenance dredge cycle of 4.2 years, the summary begins with the 4 -year cycle. Table 2.25 -Year Period Summary of Dredge Cvcles and Number of Maintenance Events Cycle Maintenance Years * Number of Events ** 4 -Year 2016, 2020, 2024, 2028, 2032, 2036 6 5 -Year 2017, 2022, 2027, 2032, 2037 5 6 -Year 2018, 2024, 2030, 2036 4 7 -Year 12019,2026,2033 3 8 -Year __L2020,2028,2036 3 9 -Year 12021,2030 2 * Measured from 2012 — date of last maintenance dredge event ** Within 25 -year project life (2013 -2038) Based on the two approaches, the benefits the structures will provide ranges from a 22% reduction to a 76% reduction in erosion off North Beach and alongshore into the channel. Applying these percent reduction rates, a four year dredge cycle (6 events) would extend to 4.9 years (5 events) and 7.0 years (3 events) respectively. This equates to a reduction of one to possibly three maintenance events within the period of analysis. Similarly, a five year dredge, cycle (5 events) would extend to 6.1 years (4 events) and 8.8 years (2 events) respectively. This equates to a reduction of one to possibly three maintenance events within the period of analysis. 9.3 Cost Savings The construction cost for the 2012 dredging project was $262,000. The design and permit costs were approximately $50,000. The construction administration and monitoring /reporting costs were $20,000. This equates to a total project cost of $332,000 in 2012 dollars. Future maintenance dredge costs are presumed to be slightly less than the 2012 project noting the design and permitting tasks will be able to take advantage of the extensive work completed to permit the project as well as the Erosion Control Project. It is assumed the maintenance event costs will be $315,000 in 2012 dollars. Applying the reduction in the number of maintenance events computed above, ranging from one to possibly three events, the cost savings from installation of the T- groins ranges from $315,000 to $945,000. The scenarios analyzed above include the likelihood of reducing the number of events by two within the 25 -year period of analysis. Therefore, the recommended proportionality analysis is a cost savings to the future maintenance dredge program of $630,000. It is noted that the City has borne the cost of the design and permitting for the Project, on the order of $300,000. There are three major components to the Project: beach fill, structures, and borrow areas. Using a simple assumption of equality, the assumed cost for the design and permitting of the structures is $100,000. Thus the proportional public benefit is recommended as $730,000. 13 January 16, 2013 Packet Page -1276- Attachment A 2/26/2013 16.A.15. Hideaway Beach Erosion Control Project Proportionality Analysis 10.0 Conclusion A Proportionality Analysis was completed in support of the City of Marco Island's TDC funding application for erosion control structures. The analysis was based on existing data, analyses, modeling results, and experience of the consulting team and City representatives. To date three maintenance projects have been completed by the County at the Collier Bay Entrance Channel. The dredge quantity ranged from 9,300 cubic yards to 24,100 cubic yards and one event included installation of the terminal jetty. Based on the coastal processes assessment, the design maintenance dredge volume and dredge interval are 9,600 cubic yards and 4.3 years respectively. Some of the results from the detailed numerical model study developed by the City's consulting team in support of the Project design and permitting were utilized in the analysis. The channel shoaling rate and thus the dredge interval were verified through comparisons with the measured data. The minimum benefits to be derived from installation of the structures were quantified through the modeling results which predicted a 22% reduction in the beach fill erosion rate when compared to the No Action Alternative. The maximum results were set based on the historic monitoring and reporting by the City's outside consultant (H &M) who demonstrated that the existing T -groin field as retained over 76% of the original beach fill volume after 4 to 5 years. Applying these benefits to the design dredge interval, it is predicted that the dredge cycle will increase from one to possibly four years; equating to a reduction in the number of maintenance events from one to possible three events within the Project life defined as 25 years. The reduction in the number of maintenance dredge events equates to cost savings. Based on the 2012 maintenance dredge event total project cost, the estimated savings for one event is $315,000 and for three events is $945,000. Using averages and including one -third of the design and permitting costs, the proportionality analysis yields a recommended total cost for the public benefit equal to $730,000. 11.0 References Coastal Engineering Consultants (CEC). 2011. Joint Coastal Permit Application, Attachment XI "Coastal Systems Assessment" and Attachment XII "Alternatives Analysis." Submitted to FDEP and USACE in support the Hideaway Beach Erosion Control Project. Humiston and Moore Engineers (H &M). 2011. Hideaway Beach Nourishment & T -Groin Erosion Control Project Phase II, One Month Post Construction Monitoring Summary Report 1, DEP Permit 0222764 - 006 -EM, March 2011. 14 January 16, 2013 Packet Page -1277-