A Study of Ocean Waves at Fisherman s Wharf, Victoria Harbour

Transcription

1 A Study of Ocean Wave at Fiherman Wharf, Victoria Harbour David B. Fiel, Jianhua Jiang and Dave Billenne ASL Environmental Science Inc., 1986 Mill Rd., Sidney, BC, V8L 5Y3, Canada, Phone Number (250) and Paper preented at the 10 th International Workhop on Wave Hindcating and Forecating, Turtle Bay, Hawaii Nov , 2007 A equence of very trong wind event on the outh coat of Britih Columbia in December 2006 reulted in ome wave damage to the Fiherman Wharf facility in Victoria Harbour. A hindcat wave modeling tudy wa carried out uing the SWAN model to provide quantitative wave information for input to conideration of a breakwater or wave attenuation ytem built adjacent to the Victoria Harbour facility. The wave tudy addreed the regional wave condition in eatern Juan de Fuca Strait to provide open boundary input to a high reolution (10 m horizontal grid) of Victoria Harbour. The high reolution wave model provided computed wind wave generation, current-wave interaction, refraction, diffraction a well a attenuation due to the Fiherman Wharf floating dock. The model wa calibrated and verified for two period of large wave event meaured in March 1976 with wave data available from two temporary Fiherie and Ocean Canada wave tation operated imultaneouly in Victoria Harbour. The reult from thee two cae howed good agreement with independent obervation at the innermot wave tation located to the wet of Fiherman Wharf. Two cae of very large wave event, which occurred in March 1977 and December 2006, were then imulated uing the model. In the firt cae, obervation, with a maximum H of 0.9 m, were available near the Harbour entrance. At Fiherman Wharf, the H wave height were alway le than 0.40 m on it eaward edge. For the econd cae of December 12 15, 2006 very large wind peed peaking over 40 knot in Victoria Harbour generated very large wave. The wave model predicted maximum H wave height of 0.46 m early on December 15 at the eaward edge of Fiherman Wharf. 1

2 INTRODUCTION In the econd week of December 2006, a equence of powerful North Pacific Ocean torm croed Vancouver Iland on the outh coat of Britih Columbia, Canada. The intenification of the torm wind, in travering Juan de Fuca Strait, to wind peed of up to 68 knot, which reulted in widepread power outage due to tree falling acro power line, and home and vehicle being damaged by fallen tree. In addition, large wave in Victoria Harbour reulted in damage to ome of the dock at the Fiherman Wharf facility (Figure 1) in Victoria Harbour. Figure 1: Photograph of Fiherman' Wharf, Victoria BC, Canada. In thi paper, we preent the reult of a hindcat wave modeling tudy of Victoria Harbour (ee Figure 2). The tudy involved a review of hitorical wind and wave data et available over the pat 50 year and the implementation and operation of a high reolution numerical wave model (SWAN) to imulate wave at Fiherman Wharf under extreme wind and wave. The purpoe of the tudy wa to provide quantitative wave information a input to conideration of intalling wave protection device at Fiherman Wharf. HISTORICAL WIND AND WAVE DATA Data Review and Selection Hitorical meaurement of wind and wave over the pat 50 year were obtained and reviewed. 2

3 Figure 2: The tudy area of Victoria Harbour (inet), eatern Juan de Fuca Strait and the outhern water around Vancouver Iland BC, Canada. The key data et included: - wind and wave height obervation at Race Rock and Trial Iland and other coatal lighthoue (Environment Canada) - wind meaurement in Victoria Harbour (Greater Victoria Harbour Authority and Environment Canada) - long-tanding weather tation data et (e.g. Victoria-Gonzale, Equimalt Harbour) from Environment Canada and - long-term wind and wave data et from the U.S. NOAA (U.S. National Weather Service and the National Data Buoy Center) The location of the weather and wave tation are hown in Figure 3 and Figure 6. Analyi of key long term tatitic (including the mean/maximum value and directional ditribution) were computed and tabulated by month for wind peed and direction; and wave height, wave period and wave direction (where available). 3

4 Figure 3: The location of ome of the weather and wave tation in eatern Juan de Fuca Strait. Location of the wind and wave tation in Victoria Harbour are hown in Figure 6. Wind Climatology Maximum hourly wind peed attain value of 40 knot (74 km/hr) or more in the fall and winter month (October to March) a hown in Figure 4. The directional ditribution of wind reveal that the larget oberved wind (of 40 knot or more) are nearly alway from the outhwet wind direction, a hown in Figure 5. Strong wind in eatern Juan de Fuca Strait reult from three different large cale weather pattern: - Pacific Ocean cyclonic weather ytem that approach the B.C. coat from the wet with trong weterly wind, epecially in the fall and winter month. - Arctic outflow wind in the late fall and winter reulting from high preure cold continental air mae flowing from the northeat and eat to acro the Strait of Georgia to Vancouver Iland. - Summertime heating over land and inland water, reulting in a trong ea-breeze circulation characterized by weterly wind flowing through Juan de Fuca Strait. Gap Wind: The inflowing air from the wet i funneled through Juan de Fuca Strait which repreent a gap between the mountain range (Overland and Walter, 1981). Gap wind intenification occur for both the fall and winter Pacific Ocean weather ytem and the ummertime ea-breeze wind. 4

5 Figure 4: Maximum and mean hourly wind peed at location near Victoria Harbour. Figure 5: The directional ditribution of wind meaured at variou location near Victoria Harbour for each eaon. Wave Climatology The larget wave event occur from November to March. In Victoria Harbour, the available wave data are limited to two waverider buoy, MEDS004 and MEDS095, 5

6 operated by the Canadian Department of Fiherie and Ocean from 1975 to The maximum oberved Significant Wave Height (H) were 0.93 m Dec at ite 4, and 0.61 m on March 18, 1976 at ite 5. In nearby Equimalt Harbour, the maximum wave height meaured over a two year wave record wa 0.90 m H in December. The larget wave event have peak period of typically 3 to 5 econd. In eatern Juan de Fuca Strait, wave exceeding 1.9 m from November to March, were meaured with a peak value of 3.1 m in Feb The typical peak period of 5 to 14 aociated with the larget monthly wave. WAVE MODEL AND METHODS Regional Modeling The regional wave condition in eatern Juan de Fuca Strait were modeled uing SWAN to provide open boundary input to the high reolution model of Victoria Harbour. The model domain area i hown in Figure 2. SWAN provide the capability to model wind generation of wave, current-wave interaction, wave refraction, diffraction and reflection procee a well a hoaling effect. The model grid ize wa 50 m with regional wind forcing and wave forcing on the wetern boundary provided by???? High Reolution Wave Model of Victoria Harbour A numerical nearhore pectral wave tranformation model (SWAN) wa developed and implemented for thi tudy over the model domain (Figure 6) having an area of 2260 m by 2210 m in total ize with a grid ize of 10 m. In the model implementation currentwave interaction wa included uing tidal current and height, a computed from tidal analyi component in thi region. Wave Energy Tranmiion Through Floating Dock Wave energy can be tranmitted and reflected againt the floating dock which are about 0.3 m below water urface. Thi proce wa imulated in SWAN uing the obtacle command baed on: - each floating dock i repreented by an obtacle, - defined by a traight line with pecified wave tranmiion and reflection coefficient. - the wave tranmiion coefficient, trcoef, i etimated baed on wave energy flux for linear wave theory. 6

7 Figure 6: The model domain for the high reolution SWAN wave model of Victoria Harbour. Alo hown are the location of the wind and wave meaurement ite ued in the preent tudy. WAVE MODEL RESULTS Wave Model Calibration and Verification The model wa calibrated and verified for two period of large wave event meaured in March 1976 with wave data available from two temporary Fiherie and Ocean Canada wave tation operated imultaneouly in Victoria Harbour. The reult from thee two cae howed good agreement with independent obervation at the innermot wave tation located to the wet of Fiherman Wharf a hown in Figure 7. 7

8 Figure 7: Comparion of modeled and oberved ignificant wave height and peak period for the model calibration and verification cae. Modeling for Extreme Wind and Wave Condition Two cae of very large wave event, which occurred in March 1977 and December 2006, were imulated uing the model. For the firt very large wind/wave event, of March 1977, the obervation are available near the harbour entrance at the waverider buoy MEDS004. The oberved maximum H at MEDS004 i up to 0.9 m, and model with the data. H wave height are in very good agreement At Fiherman Wharf, the H wave height were alway le than 0.4 m on it eaward edge and le than 0.3 m within the Wharf area. The large reduction of H from 0.9 to < 0.4 m reult from the large wave paing to the north and wet of Fiherman Wharf. The econd very large wind/wave event modeled, occurred during December 12 15, Thi period wa characterized by very trong wind, with peed peaking over 40 knot in Victoria Harbour (and nearly 70 knot at Race Rock) and ditinctly different wind direction varying from E to SW to W, generated very large wave. The wave model predicted maximum H wave height of 0.46 m early on December 15 at the eaward edge of Fiherman Wharf and H wave height peaking at 0.32 to 0.4 m within Fiherman Wharf (Figure 8). Note the importance of the locally generated wind wave aociated with wind from the eat (Dec. 12) and the wet (Dec. 15) a compared to the more common outhweterly wind (Dec. 13). The larger wind fetche of the Dec. 12 and Dec. 15 event reult in larger wave height and horter wave period. 8

9 Figure 8: The high reolution wave model reult from Dec 12-15, 2006 at the Wet, Middle and Eatern ide of Fiherman' Wharf, Victoria Harbour. Alo hown are the local wind peed and direction at the time of the maximum wave height. The model prediction of wave direction and height at the time of the very large wave of early Dec. 15, 2006 are hown for the large domain of Juan de Fuca Strait and for Victoria Harbour. SUMMARY AND CONCLUSIONS The numerical wave model demontrate the enitivity of the wave at Fiherman Wharf to the local wind direction along the outh hore of the Harbour. For wind from the outhwet or more to the outh, the wave at the Wharf are due to local wind blowing off the horeline and remain mall at < 0.3 m H even under very trong wind. 9

10 Figure 9: Detail of wave model reult for Juan de Fuca Strait at 0400 Dec. 15, However for wind originating from the WSW W direction, the wind fetch i larger and the wave height at Fiherman Wharf can be larger by about50%. Eaterly wind, although le frequent and not a large in magnitude a outhwet to wet wind, have larger wind fetche and can reult in ignificant wave height of 0.4 m a demontrated from the model imulation for the eaterly wind event of Dec , Over all, the extreme WSW W wind event, uch a the one occurred on December 15, 2006, are expected to generate the larget wave at Fiherman Wharf with a typical peak wave period of 3 4 econd and ignificant wave height of up to 0.5 m. The wave height on the eaward edge of Fiherman Wharf are nearly uniform with thoe on the wetern ide lightly larger than at eatern ide. Within the Wharf area, the wave height in the middle are le than thoe at the wetern and eatern ide with the wave height at the eatern ide at intermediate level. REFERENCES Overland, J.E. and B.A. Walter, Jr., Gap Wind in the Strait of Juan de Fuca. Monthly Weather Review. Vol. 103, p

11 Figure 10: Detail of wave model reult for Victoria Harbour at 0400 Dec. 15,