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HomeMy WebLinkAboutRS_Non-Motorized_Vessel_Wave_Evaluation_20170412_v2 Page 1 of 13 Introduction The Watershed Company is providing permit services to SECO Development for recreational boat docks at the Hotel at Southport. The proposed docks will be used primarily by non-motorized watercraft. To address concerns related to “increased boat traffic”, erosion along nearby Bird Island, and bulkhead stability along Gene Coulon Memorial Beach Park, the City of Renton originally requested a wave analysis. However, given the generally mild wave climate and the smaller vessels that typically use the docks, a numerical model wave analysis is not required. A wave study was performed in 2015 by CHE (now Mott MacDonald) that has been applied for this memo. The City of Renton has requested instead, “… a lesser report that provides expert defensible analysis demonstrating the insignificance of the wave study.” This memo addresses this need with the following analysis: 1. Estimates the waves that could be generated by the vessels likely to use the proposed docks; and, 2. Compares the estimated wave heights to waves that the project site presently experiences. Project Location The project area consists of a 608-foot long by 210-foot wide boat basin at the southern end of Lake Washington (see Figure 1). The boat basin runs parallel to the shoreline, which faces the northwest (see Figure 1). Boeing’s Renton plant borders the project area to the southwest; Gene Coulon Memorial Beach Park borders the project area to the northeast. A concrete block bulkhead runs along the landward side of the basin, while the opposite side is open to the lake (see Figure 2). An existing wharf runs along the southwestern side of the boat basin (see Figure 2). Bird Island and a 160-foot section of shoreline protected by a wooden bulkhead define the northeastern side of the boat basin. Bottom elevations in the boat basin range from -4.2 to +0.74 feet NAVD88 based on Survey 11810 by the National Oceanic and Atmospheric Administration (NOAA, April 1 to November 24, 2008). The High Water and Low Water elevations are +18.60 and +16.75 feet NAVD88, respectively (City of Seattle, 2014). Subject Memorandum - Recreational Dock Facility Passing Vessel Wave Review To Kenny Booth, The Watershed Company From Christopher M. Day, Senior Coastal Engineer Our reference 379538 Office Edmonds, WA Date December 19, 2016 Your reference SECO Development, Hotel at Southport Bulkhead Repair; Pile, Dolphin, and Finger Removal; New Ramp and Float Installation Technical notes Memorandum - Recreational Dock Facility Passing Vessel Wave Review Memorandum Page 2 of 13 Figure 1: Project Location. Page 3 of 13 Figure 2: Proposed Site Plan. N Page 4 of 13 Winds and Wave Exposure General Wind statistics at the project site are based on observed wind data at Renton Municipal Airport (CHE, 2015). The prevailing winds are from the south and the north-northwest (see Figure 3). The average sustained wind speed offshore is roughly 11 mph. However, given the site’s location, winds that generate incoming waves are limited to those from the west, west-northwest, northwest, north-northwest, and north. These occur about 35% of the time. The average sustained wind speed from these direction bands is roughly 14 mph, with a corresponding direction of 338° (NNW). The open fetch along this direction is approximately 1.9 miles. Average Conditions The average wave given a 14 mph wind from 338° was estimated using www.cress.nl. This online package offers several methods to evaluate fetch-limited waves, three of which are applicable to the project site – Donelan’s Method, the Wilson (1965) Formula, and Kahma and Calkoen’s (1992) Formulae. Given the assumed wind velocity and fetch length, the wave height based on these methods ranges from 0.6 to 0.7 feet, with a corresponding wave period of 1.6 to 1.8 seconds. Extreme Conditions Extreme wave conditions in Lake Washington were evaluated by CHE (2015) using the Simulating Waves Nearshore (SWAN 40.72ABCDE) model and the wind record at Renton Municipal Airport. Estimated waves are summarized in Table 1, at a point 770 feet from the basin’s inner bulkhead. Table 1: Extreme Wave Statistics Offshore, 47.5051471°N, 122.2072605°W (CHE, 2015) Return Period (years) Significant Wave Height (feet) Peak Wave Period (seconds) Wave Direction (degrees) Wind Speed* (mph) Wind Direction (degrees) 10 1.9 2.8 323 36.4 350° 50 2.4 3.2 320 44.6 350° 100 2.5 3.2 319 47.6 350° Note: *Wind speed is a sustained 2-minute average. Types of Vessels The assumed vessel types appear in Table 2. It is important to note that motorized vessels periodically use the basin at the project site. In addition, Coast Guard vessels around 84 feet in length frequently pass the basin offshore during Seafair and other times when boating traffic on Lake Washington is high. The motorized vessels in Table 2 are based on the watercraft that appear in GoogleEarth photos of the project site. Page 5 of 13 Figure 3: Renton Municipal Airport Wind Rose 1984-2013 (CHE, 2015). Page 6 of 13 Table 2: Vessel Types to be Considered in Analysis Vessel Type Length (feet) Width (feet) Moulded Depth (feet) Draft (feet) Speed (knots) Non-Motorized Vessels Likely to be Rented out to Hotel, Residence, and Office Patrons: Canoe 14 3.1 1.75 0.50 5.8 Canoe 20 3.1 1.75 0.50 6.9 Kayak 12 2.0 1.08 0.42 5.4 Kayak 23 2.3 1.25 0.42 7.4 Row Boat 12 4.5 1.83 0.42 4.0 Row Boat 15 4.6 1.83 0.42 4.0 Wind Surfer Boards 6 1.6 0.30 0.11 20 Wind Surfer Boards 9 2.5 0.35 0.12 20 Paddle Boards 11 2.7 0.40 0.15 5.2 Paddle Boards 13 2.5 0.45 0.16 5.2 Motorized Vessels that Presently Use Basin Based on Aerial Photographs*: Cabin cruiser 50 14.5 7.4 4.3 3.0 Fishing Boat 20 9.0 4.2 3.1 3.0 Fishing Boat 29 10.0 5.0 2.8 3.0 Miscellaneous*: Jet Ski 11.1 3.9 1.5 0.6 3.0 *NOTE: SECO does not plan to specifically prohibit motorized vessels from accessing the floats; however, there are no plans to rent motorized vessels. Page 7 of 13 Vessel speeds for motorized vessels are based the following: 1. KCC 12.44.090 – “Speed regulations within one hundred yards of shore on Lake Washington and Lake Sammamish. Except as otherwise specifically provided for in this chapter it is unlawful for any person to operate any watercraft or vessel at a speed in excess of eight miles per hour [7.0 knots] within one hundred yards of any shoreline, pier, restricted area or shore installation on Lake Washington or Lake Sammamish.” 2. The project owner’s proposed vessel speed restriction of 3 knots for motorized vessels within the established boat moorage area. To provide a basis for comparison, the assumed speeds of the motorized vessels presently using the basin is 3 knots. For the non-motorized vessels, this analysis assumes that users will not be able to estimate their speeds. Instead, the assumed speeds are based on the following:  Canoes and kayaks – https://paddling.com/learn/kayakorcanoewhichoneisbestforyou/.  Row boats – https://en.wikipedia.org/wiki/Rowing.  Wind surfer boards – Cribb (2010), www.windsurf.co.uk.  Paddle boards – http://supvelocity.com/whichpaddleboardisthefastestaracesupspeedcomparison/. Vessel speeds based on the sources above are generally conservative (high), especially for wind surfer boards. This analysis also includes jet skis, given that motorized vessels are not prohibited from using the proposed facilities. However, the project owner has designed the floats with the intent of supporting non-motorized vessels only and will not be renting Jet Skis. The assumed speed for Jet Skis is 3 knots based on the project owner’s proposed speed restriction. Vessel Routes Assumed vessel routes appear in Figure 4. The Watershed Company notes that the large existing wood deck in the middle of the site is too tall to be used by the vessels in Table 2. Accordingly, there are no routes to or from this dock. Vessel routes to and from the other docks are as follows:  Dock 1 – Occasional use by motorized vessels only.  Dock 2 – Regular use by non-motorized vessels and Jet Skis only.  Dock 3 – Regular use by non-motorized vessels and Jet Skis only. Page 8 of 13 Figure 4: Assumed Vessel Routes. Page 9 of 13 Waves Generated by Vessels Using the Proposed Facility Methods Empirical models developed by Bhowmik, Demissie & Guo (1982) and a Permanent International Association of Navigation Congresses (PIANC, 1987) working group were used to estimate vessel wake. The equations for these models are given below. These models consider wave parameters (significant wave height and wave period), site parameters (water depth and distance between vessel and critical area), and vessel parameters (length, beam, draft, and speed). Bottom grade elevations and water levels vary in the boat basin. This analysis assumes a typical bottom grade elevation of +1.8 feet NAVD88 and a mid-range water level around +17.8 feet NAVD88, resulting in a depth of 16 feet. Equation 1 gives the empirical model developed by Bhowmik, Demissie & Guo (1982): (1) where Hm is the vessel wave height measured (feet), V is the speed (feet/second), D is the draft (feet), and g is gravity (32.17 feet/second2). Equations 2-4 gives the empirical model developed by PIANC (1987): (2) where F = V / (gD)0.5 (3) A" = K D / Le (4) and coefficient d is the water depth (feet), S is the distance from the side of the vessel (feet), V is the vessel speed (feet/second), g is gravity, D is the vessel draft (feet), K is constant ranging from 1.5 to 4, and Le is the entrance length of the vessel (feet), defined as the length between the bow and “the point where the parallel middle body begins”. For most of the vessels in question, Le is the distance from the bow to the widest section of the vessel and is roughly 1/3 of the vessel length. Vessel wake periods are given by the following equation based on Sorenson (1997): T = 2 (0.816 V) / g (5) where V is the vessel speed (feet/second) and g is gravity. Motorized Vessels Presently Using the Basin Estimated wake characteristics appear in Table 3. These estimates assume that motorized vessels only use Dock 1, with distances of 500 feet between the sailing lines and the shoreline of Gene Coulon Park (see Figure 4). Given the motorized vessels presently using the basin, vessel wakes results range from 0.1 to 0.4 Page 10 of 13 feet along the vessel route. Near the park shoreline, vessel wake heights based on the PIANC formula (Equations 4-6) are negligible. Vessels Expected to Use the Proposed Facility Estimated wake characteristics for the small vessels using the proposed facility appear in Table 4. These estimates assume minimum distances of 30 feet between the sailing lines and the shoreline of Gene Coulon Park (see Figure 4, red line). For canoes, kayaks, row boats, and even Jet Skis, wake heights adjacent to the vessel are 0.7 feet or less. Near the shoreline of Gene Coulon Park, vessel wake heights based on the PIANC formula (Equations 2-4) are 0.3 feet or less. For paddle boards and wind surfers, wake characteristics are not estimated because neither vessel type has a keel, rendering Equations 1-5 inapplicable. Given that paddle boards and wind surfers are generally smaller or of similar size to the other non-motorized vessels, it is assumed that the wakes will be of similar or lesser size. Evaluation of Results Given the results in Tables 3 and 4, vessel wakes are no higher than the typical wind waves. Vessel- generated waves near the shoreline of Gene Coulon Park will generally fall with the acceptable marina wave heights for a small-craft harbor, which are 0.5 feet for head seas and 0.25 feet for beam seas (Maryland DNR, 1993: ASCE, 2015). Comparisons between the waves generated by the non-motorized vessels, the typical wind-waves, and the small-craft harbor standards appear in Table 5. Based on these results, the effects of vessel-generated waves on the adjacent shoreline will be insignificant. Given that typical wind waves and motorized vessel wakes will be the larger waves in the marina, non- motorized vessel wakes are considered insignificant. Conclusions Vessel wakes were calculated for motorized vessels presently using the basin and non-motorized vessels likely to use the proposed facility. The resulting wave heights of non-motorized vessels were compared to wave heights for motorized vessels, typical wind waves, and acceptable waves in small craft harbors. Non- motorized vessel wave heights were generally smaller than wind waves under average conditions. Overall, non-motorized vessel wakes will be small compared to other waves within the basin and can thus be considered insignificant. Page 11 of 13 Table 3: Vessel Wake Estimates for Motorized Vessels that Presently Use Basin Vessel Parameters Vessel Wake Height (feet) PIANC (1987) Vessel Type Length (feet) Width (feet) Moulded Depth (feet) Draft (feet) Speed (knots) 2 feet from side of vessel Gene Coulon Park Shoreline* Bhowmik, Demissie and Guo (1982) Wake Period (seconds) Cabin cruiser 50 14.5 7.4 4.3 3 0.08 0.01 0.25 0.81 Fishing Boat 20 9 4.2 3.1 3 0.15 0.02 0.21 0.81 Fishing Boat 29 10 5 2.8 3 0.09 0.01 0.20 0.81 NOTE: *Roughly 500 feet from the vessel sailing line, with S = (500 feet) – (Width / 2). Table 4: Vessel Wake Estimates for Non-Motorized Vessels using the Proposed Facility Vessel Parameters Vessel Wake Height (feet) PIANC (1987) Vessel Type Length (feet) Width (feet) Moulded Depth (feet) Draft (feet) Speed (knots) 2 feet from side of vessel Gene Coulon Park Shoreline* Bhowmik, Demissie and Guo (1982) Wake Period (seconds) Non-Motorized Vessels Likely to be Rented out to Hotel, Residence, and Office Patrons: Canoe 14 3.1 1.75 0.5 5.8 0.47 0.20 0.16 1.56 Canoe 20 3.1 1.75 0.5 6.9 0.66 0.28 0.19 1.86 Kayak 12 2 1.08 0.42 5.4 0.35 0.14 0.14 1.45 Kayak 23 2.3 1.25 0.42 7.4 0.64 0.27 0.19 1.99 Row Boat 12 4.5 1.83 0.42 4 0.10 0.04 0.10 1.08 Row Boat 15 4.6 1.83 0.42 4 0.08 0.04 0.10 1.08 Miscellaneous: Jet Ski 11.1 3.9 1.5 0.6 3 0.04 0.02 0.09 0.81 NOTE: *Roughly 30 feet from the vessel sailing line, with S = (30 feet) – (Width / 2). Page 12 of 13 Table 5: Wave Height Comparisons Wave Height (feet) Criteria Near Side of Vessel Shoreline of Gene Coulon Park Non-motorized Vessel Wakes 0.08 to 0.7 0.02 to 0.3 Motorized Vessels Presently Using the Basin 0.09 to 0.15 Negligible Typical Wind Wave 0.7 to 0.6 Acceptable Marina Wave Heights 0.5 (head) to 0.25 (beam) References American Society of Civil Engineers (2015). Planning and Design Guidelines for Small Craft Harbors (MOP 50) (ASEC Manual and Reports on Engineering Practice), American Society of Civil Engineers, Reston, VA. Bhowmik, N. G., Demissie, M., and Guo, C. Y. (1982). “Waves generated by river traffic and wind on the Illinois and Mississippi Rivers,” Report UILIWRC-82-167, Illinois State Water Survey, Champaign, IL. Bhowmik, N. G., Soong, T. W., Reichelt, W. F., and Seddik, N. M. L. (1991). “Waves generated by recreational traffic on the Upper Mississippi River system,” Research Report 117, Department of Energy and Natural Resources, Illinois State Water Survey, Champaign, IL. Board Size Chart – Windsurf. Retrieved 21 November, 2016, from http://www.surfertoday.com/board-size-chart/windsurf. Bottin, R. R. Jr., McCormick, J. W., Chasten, M. A. (1993). “Maryland Guidebook for Marina Owners and operators on Alternatives Available for the Protection of Small Craft against Vessel-generated Waves.” Maryland Department of Natural Resources Boating Administration, Annapolis, MA. City of Seattle (2014). City of Seattle Elevations & Datums, http://www.seattle.gov/util/cs/groups/public/@spu/@engineering/documents/webcontent/01_029210. pdf. Coast and Harbor Engineering, A Division of Hatch Mott MacDonald. (2015). Lake Washington Wave Climate Classification – Phase I. (Technical Report). Prepared for City of Seattle. Cribb, Andy (2010). Technique by Guy Cribb, Intuition Speed / Slalom Series, www.windsurf.co.uk. Donelan, M A, Skafel, M, Graber, H, Liu, P, Schwab, D and Venkatesh, S (1992). On the growth rate of wind-generated waves. Atmos-Ocean, vol 30, pp 457-478. Page 13 of 13 Kahma, K K and Calkoen, C J (1992). “Reconciling discrepancies in the observed growth of wind generated waves”. J Phys Oceanogr, vol 22, pp 1389–1405. Kayak or canoe which one is best for you. Retrieved 21 November, 2016, from https://paddling.com/learn/kayakorcanoewhichoneisbestforyou/ KKC 12.44.090. “Speed regulations within one hundred yards of shore on Lake Washington and Lake Sammamish. Except as otherwise specifically provided for in this chapter it is unlawful for any person to operate any watercraft or vessel at a speed in excess of eight miles per hour [7.0 knots] within one hundred yards of any shoreline, pier, restricted area or shore installation on Lake Washington or Lake Sammamish.” 2016. National Oceanographic and Atmospheric Administration (2007). Descriptive Report, Hydrographic Survey, RA-10-01-08, H11810, National Centers for Environmental Information, Boulder, CO. Netherlands Ministry of Infrastructure and Environment (2008). CRESS (Coastal and River Engineering Support System), http://www.cress.nl/. Permanent International Association of Navigation Congresses. (1987). “Guidelines for the design and construction of flexible revetments incorporating geotextiles for inland waterways,” Working Group 4 of the Permanent Technical Committee, Brussels. Rowing (2016). Rowing. Retrieved 21 November, 2016, from https://en.wikipedia.org/wiki/Rowing. Sorensen, R. M. (1997). “Prediction of vessel-generated waves with reference to vessels common to the upper Mississippi River System.” ENV Report 4. U.S. Army Engineer District, Rock Island, Rock Island, IL. Tan, S. W. (2012). “Predicting Boat-Generated Wave Heights: A Quantitative Analysis through Video Observations of Vessel Wakes (No. USNA-TSPR-409).” NAVAL ACADEMY ANNAPOLIS MD. Which paddle board is the fastest. Retrieved 21 November, 2016, from http://supvelocity.com/whichpaddleboardisthefastestaracesupspeedcomparison/. Wilson, B W (1965). Numerical prediction of ocean waves in the North Atlantic for December 1959. Deutsche Hydrographische Zeitschrift, vol 18, no 3, pp 114–130.