Banner Photo Credit: Illustrated by Steve Holme, see PDF for image reference.
Understanding the physical processes active at shore zone project sites is a critical part of preliminary site assessments and project design.
Astronomical tides caused by the gravitational interactions between the sun, the moon and the earth result in the periodic rise and fall of ocean water levels. Water levels vary periodically over time in response to changes in the positions of the sun and the moon relative to the earth. Since the moon is closest, its gravitational force has the greatest impact on tides. The primary tidal constituents attributable to the moon orbiting around a rotating earth are the diurnal cycle, with one high and one low water stand per lunar day (24.83 hours), and the semi-diurnal cycle, with two high and low water stands per lunar day. Complicating factors leading to longer-period oscillations in tidal heights include the relative alignments of the sun, earth and moon, eccentricities in the various orbits, and the tilt of the earth’s rotational axis.
In addition to these periodic changes over time, tidally-forced changes in water levels vary considerably with location. Spatial variability in tidal characteristics is driven by several factors, such as the effects of water depth on tidal wave propagation in the open ocean, the blocking effects of continents on large-scale water movements and the Coriolis force associated with the earth’s rotation. The effects of water depth on tidal wave propagation are particularly pronounced in coastal waters, where complex bathymetry can lead to large differences in the tidal characteristics from one location to another, even when the distance of separation is relatively small.
Figure 4 shows the typical tidal fluctuations in water levels at several locations in British Columbia. In general, tides in British Columbia coastal waters are classified as mixed, mainly semi-diurnal, with two high and two low water stands of unequal heights each day. One exception is the Victoria area, where the tides are mixed, mainly diurnal; at times the water level fluctuations are such that there is essentially only one high water and one low water stand each day.
The tidal fluctuations in water levels are by necessity associated with large-scale mass movements of water in the form of long-period waves. These fluxes create tidal currents that vary on the same or similar time scales as the tidal water levels. In open ocean waters, tidal current speeds can be quite low, even when the tidal range is relatively high. However, the complex topography and bathymetry of British Columbia’s coast creates many “pinch points” where large volumes of water move through narrow or shallow channels on each tidal cycle.
Some examples of tidal passes that are routinely navigated by large vessels include First Narrows and Second Narrows in Burrard Inlet; Active Pass, Gabriola Passage and Porlier Pass in the southern Gulf Islands; and the many smaller channels located in the waters between northern Vancouver Island and the mainland coast. Peak current speeds in these passes typically reach 2.5 m/s. In extreme cases such as Seymour Narrows, peak tidal current speeds can reach 7.5 m/s on a large tide. Slack water periods, when current speeds fall to essentially zero, can last for as little as ten minutes per tidal exchange.
The spatial variability in tidal currents is more pronounced than the variability in tidal fluctuations in water levels. In addition to variability on the horizontal plane, ocean currents in general vary over the depth of the water column, and may not be strongest at the water surface. Flow direction may also change with depth, particularly within stratified estuaries such as the lower Fraser River during the summer months.
In addition to tides, large-scale oceanic or regional climate patterns and processes can affect marine water levels and ocean circulation patterns. Large-scale climate patterns affecting coastal British Columbia waters include the El Nino Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Although these climate patterns are not predictable in the same way as tides, they tend to persist for time periods on the order of months or years, have smaller effects in local waters, and show less spatial variability. Similarly, the seasonal changes in the dominant large-scale wind patterns in the waters of the northeastern Pacific Ocean drive smaller changes in water levels and ocean circulation patterns along coastal BC.
At regional and local scales, inputs of fresh water from rivers and streams can create significant non-tidal currents and changes to the vertical structure of the ocean current field. Internal waves may occur at a density interface within the water column. Density-driven currents can also occur in BC waters in response to offshore processes. Although they may be relatively infrequent and not associated with high current speeds, intrusions of high-density bottom water can have significant impacts on water quality parameters, such as dissolved oxygen levels in the bottoms of deep coastal fjords such as Saanich Inlet and Indian Arm.
Episodic storm events are often the largest contributor to the dynamic forces that shape the shore zone and impact project design. Storm winds created direct impact forces on coastal projects, inject spray into the backshore and upland areas, drive the formation of storm waves and ocean currents, and contribute to local increases in water levels, or storm surges. The drops in atmospheric pressure that occur as storm systems pass through a region are also major contributors to storm surges.
Available information from analysis of long-term water level records at British Columbian tide gauge reference stations indicates that measured storm surges during extreme storm events are generally less than about 1.5 m and that spatial variability is quite low. However, the available water level data may not accurately represent storm surge characteristics in shallow coastal waters.
Surface waves are driven by storm winds blowing over large areas of open water for prolonged time periods. Storm waves create rotational water movements with locally-high velocities that decrease with depth below the water surface (Fig. 5). In deep water the wave-induced water movements do not reach the seabed, although wave effects can still be observed at significant water depths in areas exposed to long-period ocean waves.
As waves propagate towards the shore, they undergo a number of transformation processes. These processes affect the direction and speed of wave travel, wavelength, waveform, wave height and the nature of the wave-induced orbital velocities (Fig. 6).
The wave-induced velocities at the seabed can be significant in shallow waters, often reaching or exceeding 1 m/s prior to wave breaking. The associated accelerations are also high, which leads to strong forces on the seabed and on structures exposed to wave action. The greatest wave-induced forces are often associated with localized effects of wave breaking.
In addition to oscillatory currents, storm waves can create wave drift currents in the direction of wave travel and longshore currents within the surf zone. Wave set-up acts to increase the mean water level, and wave runup with potential overtopping of coastal structures can be significant. Spatial variability in wave conditions is a function of the level of offshore wave exposure combined with the strong influence of nearshore bathymetry on wave transformation and breaking processes. Accordingly, this variability is high and necessitates site-specific analyses.
In areas where the seabed and beach consist of sedimentary materials, forces exerted by ocean currents and storm waves can mobilize and transport the shore zone sediments, whether they are clay sized particles or cobbles. Although the action of ocean currents, particularly in high-velocity tidal passes, cannot be neglected, shore zone sediment transport processes are generally dominated by the effects of storm waves.
Close to shore, waves breaking at an angle to the shoreline create a longshore current which can move beach sediments (Fig. 7). Sediment movement also occurs in a direction perpendicular to the shoreline, as a consequence of asymmetries in the waveform. Seasonal variability in storm wave conditions can create seasonal changes to the shape of the beach and nearshore profile as illustrated in Figure 8. Longshore and cross-shore sediment transport processes are driven by storm waves and therefore are highly episodic; larger materials may remain essentially immobile for long periods of time until mobilized by extreme storm events.
Although most pronounced closer to shore, sediment transport processes cannot be ignored in deeper waters. Mobile subaqueous dune features have been observed on the Boundary Pass seabed at water depths of up to 310 m and sediment movement due to storm wave action can be significant at water depths exceeding 50 m.
The mobilization and transport of sediments in marine environments is dependent upon the availability, or supply, of transportable materials. In the shore zone, sources of sediments include rivers, streams, and the erosion of coastal lands (e.g. bluffs). Marine sediment sources such as tidal flats and shallow subtidal sediment deposits may also be significant. Finer materials such as silts and clays may be transported large distances from their original source by ocean currents.
Sediment transport leads to changes in the shore zone’s shape, or morphology. Alterations to the seabed through the placement of materials or structures can impact sediment transport processes, and lead to either predicted or unanticipated changes to local or regional morphology. Two examples of changes to shore zone morphology in response to placement of coastal structures are illustrated in Figures 9 and the banner photo for this page. Note that the observed changes to shore zone morphology can extend well beyond the boundaries of the constructed project.