HomeMy WebLinkAboutRS_Non-Motorized_Vessel_Wave_Evaluation_20170412_v2
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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
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Figure 1: Project Location.
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Figure 2: Proposed Site Plan.
N
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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.
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Figure 3: Renton Municipal Airport Wind Rose 1984-2013 (CHE, 2015).
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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.
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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.
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Figure 4: Assumed Vessel Routes.
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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
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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.
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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).
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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)
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