Pedder Bay, British Columbia-Wave Climate Study and Wave Protection Considerations

Final Report
( THE FIRST FEW PAGES ONLY ARE PRINTED. The COMPLETE VERSION IS AVAILABLE FROM FISHERIES AND OCEANS )

Prepared for: Government of Canada, Fisheries and Oceans

Prepared by: W.F. Baird & Associates Coastal Engineering Ltd.

Ottawa, Ontario

March 1991

TABLE OF CONTENTS

1.0 INTRODUCTION		1
2.0 SITE BATHYMETRY		2
3.0 WATER LEVELS		3
4.0 CURRENT DATA		5
5.0 WIND DATA		6
6.0 WIND-WAVE HINDCAST		11
7.0 PACIFIC OCEAN SWELL		17
8.0 RECORDED WAVE DATA		19
9.0 WAVE CONDITIONS AT THE PROPOSED SITE		20
10.0 SAMPLE BREAKWATER CROSS-SECTIONS		22
11.0 CONCLUSIONS AND RECOMMENDATION		24
REFERENCES APPENDIX A – WIND DATA SUMMARIES APPENDIX B – DEEP WATER WAVE HINDCAST SCATTER DIAGRAMS APPENDIX C – WAVE HINDCAST SCATTER DIAGRAMS FOR THE LOCAL CLIMATE AT THE SITE APPENDIX D – WAVE DIFFRACTION DIAGRAMS APPENDIX E – RECORDED WAVE DATA

 

1.0 INTRODUCTION

The Department of National Defence (D.N.D.) is considering the expansion of the existing facilities and the construction of a new jetty at Canadian Forces Ammunition Depot (CFAD) Rocky Point located on Pedder Bay. British Columbia. Pedder Bay is situated at the southern end of Vancouver Island and is open to the southeast to Juan de Fuca Strait, as shown in Figure 1. 1. Figure 1. 2 shows a closer view of the existing bathymetry and facilities at the site.

The site is directly exposed to waves generated locally within Juan de Fuca Strait, particularly by winds from the east and southeast directions. In addition, the site is also partially subjected to long period swells that propagate through the Juan de Fuca Strait from the Pacific Ocean and diffract around the Rocky Point headlands into Pedder Bay.

The overall objectives of this investigation were to define the wave climate at the proposed site and to complete an initial assessment of wave protection requirements. Specific tasks that were undertaken included:

Assessment of possible wave protection requirements for the proposed harbour.

The results of this investigation are presented in the following sections of this report.

2.0 SITE BATHYMETRY

The water depths within Pedder Bay and the nearby portions of Juan de Fuca Strait is described on a series of charts produced by the Canadian Hydrographic: Service , Fisheries and Oceans Canada,

The underwater topography (bathymetry) within Pedder Bay, as shown in Figure 1.2, is fairly regular with water depths in excess of 20 m at the entrance to the bay. Much of the bay. east of Watt Point, has water depths of approximately 5 to 10 m. Depths along the existing DND jetty range from 6.5 m to 9.5 m according to a 1982 survey by CHS. A small series of shoals and emergent rocks are located on the north side of Pedder Bay directly opposite the jetty. The navigable water is indicated by buoys maintained by the DND.

The bathymetry within Juan de Fuca Strait in the vicinity of Pedder Bay is relatively deep, dropping off to 80 m or greater in a distance of 1000 m from the entrance to the bay. A prominent topographic feature to the south of the bay is an extensive series of shoals called the Race Rocks. These rocks have a direct effect on waves generated from the south and on long period swells propagating down Juan de Fuca Strait.

A hydrographic survey has been carried out in the area east and north of Fossil Point by Klohn Leonoff Ltd. (1990) for DND. An initial comparison of the results of this survey with previous surveys shows little change to the bathymetry in this area.

 

3.0 WATER LEVELS

Water level recorders near Pedder Bay have been located at Victoria and at Sooke Harbors. The Victoria gauge has been operating since 1909 and the Sooke gauge has been partially operational since 1973. Tides In the waters between Vancouver Island and the mainland vary considerably due to the presence of numerous islands and the complex bathymetry of this region. The tides at Sooke are mixed. mainly semi-diurnal (two high and two low waters in one day) while at Victoria the tides are mixed, mainly diurnal (one high and one low water in a day).

The mean water level at Pedder Bay Is estimated by Fisheries and Oceans Canada at 1.8 m above Chart Datum. Table 3.1 represents the tidal heights and extremes for Pedder Bay, Sooke and Victoria.

 

Table 3.1

Tidal Heights and Extremes

Height Above Chart Datum (metres)RECORDED

	HHW	LLW	Mean	LLW Mean	HHW Mean	Highest	Lowest
Pedder Bay	3.3	0.2	2.5	0.6	1.8	N.A.	N.A.
Sooke	3.6	0.3	2.8	0.8	1.9	3.8	0.2
Victoria	3.3	0.2	2.5	0.7	1.9	3.7	-0.5

Note: HHW = Higher High Water

LLW = Lower Low Water

Based on the above values, it is recommended that a maximum water level of +3.8 m be utilized for the design of any harbour structures. The mean water level of +1.8 m was utilized for all refraction and diffraction analyses.

Conversations with representatives of CHS have indicated that Geodetic Datum has not been extended to the Pedder Bay area: however, the recent survey by Klohn Leonoff Ltd.

(1990) shows a conversion between Geodetic and Chart Datum. This conversion is given

as: Elevation (Geodetic) = Elevation (Chart Datum) -1.878 m

4.0??CURRENT DATA

The tides flowing into and out of Juan de Fuca Strait create significant currents within the Strait. The magnitude of these currents depends on the size of the tide and may be considerably altered by the meteorological conditions.

Currents were measured four miles south of the Race Rocks in the centre of Juan de Fuca Strait, as shown in Figure 1. 1. On a typical large tide. the maximum tidal currents are approximately 1.5 m/s and 2.0 m/s for the flood (rising) and ebb (falling) tides, respectively.

The peak tidal currents within Pedder Bay Itself are not expected to be large enough. to significantly affect the piloting of vessels into a new facility due to the limited tidal volume contained within the Bay. Currents will, however, have some effect on the refraction of waves into Pedder Bay at certain times, as discussed in Section 6.2.

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