carried seaward along the rill fans out as it reaches a flat portion of the beach much the same as strean-borne sediments fan out into a valley. Rill marks are apparently produced by accumulations of percolated water that seep out ofthe beach at low tide. Sand domes are small mounds in an otherwise level beach raised up by alr trapped within the sand. The air in this case is forced out as the wave swash percolates into initially dry sand. Small holes rcscm- bling those made by sandhoppers are produced when air escapcs through wet sand in the swash zonc. Other types of ripples commonly seen on beaches include oscillatory ripples, formed by the back and forth motions under waves, and current ripples, produced hy uiidirectional flow such as longshore currents, rip cur- rents, or tidal currents. The former have symmetrical crests and rounded troughs oriented parallel to the shoreline (that is, at right angles to the drection of the waves), whereas the latter are asymmetrical with gentle up-current slopes that give way to steeper slopes on the downstream side of the flow (Fig. 2.23). Current ripples are typically oriented at an angle to the shorchnc as cur- : rents tend to align themselves parallel to the coast; where rip currents lead out to sea, however, ripples tend in the F~G.2.24. Oscillatory ripple marks at 90 m depth off west coat direction of the coastline Both types of ripples arc often Vancomrr Islad, photographed from Pixa IVsubmcrsiblc, June 1977. P4pplcs 30 cm high. about 30-60 cm apart, and oriented parallel to coast. Coarse carbonate shcU hash is concentrated in troughs, and finer carbonates uxtcnd up thc flank and across crests. Small current ripples on flanh at right angles ro the horr ripples. Rattish is about 30 un long. (From Yorath et 31. 1979) Sand ripples above the high-water mark are gena- ated by coastal winds. The orientation of these patterns depends on the prevailing wind direction so ripple crests may run at various angles to the shorelinc. Wind-gener- ated ripples are usually asymmetric with gently rising windward sidcs and stecp-sidcd leeward sides. Length scales range from a few centimctres to many metres (as for dunes). Frequently there arc ripples on low isolated mounds that are separated by relatively flat areas of sand FIG 2 23 Oscillatory ripples wrsus current ripplcs (Fig. 2.25). ,Miniature wind-shadow ridges a few cen- timetres long arc often formed in the lee of driftwood, exposed at low tide on expansive tidal flats such as the pebblcs, shells, or other objects with the ridge axis aligned Fraser River delta where, because of their large size (100 parallel to the prevailing wind direction. m or more from crest to crest and up to 1 m high), they have become known as megaripples. These sand waves are not fixed, but migrate slowly in the direction of the pre- wiling flow, both up and down current. On a recent underwater geological survey on the continental shelf off the northwest coast of Vancouvcr Island on the Ptrces N submersible, geologists further observed extensive oscillatory ripples in depths of 100 m that were oriented parallel to the coast. These ripples of carbonate shell hash and volcanic sand and pebblcs had spacings of about 100 cm and heights of over 15 cm, with rounded crests and near-flat troughs (Fig. 2.24). Calcula- tions have shown that these deep ripp caused by long, storm-generated swells southwest with heights of 10-15 m. Smaller ripples cross- ways on top of the ripples probably are produced by strong tidal currents that flow parallel to Vancouver Is- Fiti 2.26 \\'ind-induced sand rippler above higher high-warrr lcvclar land. Similar kinds ofripples have been observed at depths Cox Bay, wurh ofTofino, WcIr cobn Vanmuvcr Idand. Piccc ofmod ar to 125 m on the Oregon continental shelf. luwer ryhr is nbour 15 cm 16 in.) long il'hom hv the authnr) - 31 . ...C ’’ ’ > FIG.2.26. Prominent spits in the eastern sector of Juan& Fuca Strait (A) 1. Whiffi Spit; 2. Esquimalt Lagoon (Cobourg) Spit; 3. Cordova Spit; 4. Sidney Spit; 5. Spencer Spit; 6. Spit (Marrowstone Is.); 7. Gibson Spit; 8. Dungeness Spit; 9. Ediz Hook; 10. Smith Island Spit; with expanded view of Wi Spit (B). Frc. 2.26. Expanded view ofDungeness Spit(C),and Sidney Spit(D). Spits and Cusps 14,000 yr ago when glaciers began retreating from Juan A spit (or hook) is a beach with one end joined to the de Fuca Strait and local sea level was roughly 30 rn lower shore and the other end free where it terminates in a hook than it is today. As the ice melted and sea level rose, the or recurve (Fig. 2.26). The spit elongates in the direction Elwha River, 13 km west of Port Angeles, cut through the of longshore sediment drift and can be an alongshore glacial deposits and carried sand and gravel to the Strait. extension ofan existing beach or may be aligned across the These sediments were then transported eastward by the direction of the prevailing waves. Spits are most common prevailing littoral current set up by the action of up-strait on irregular coasts where they often grow across bay winds and waves, and the river delta grew to the east (Fig. mouths and the entrances to rivers and extend them in the 2.28a). Sea level continued to rise and additional sedi- direction of the littorsdrift. In this way, spits provide an ments were added to the alongshore drift through the effective mechanism to straighten out existing bumps in erosion of adjacent sea cliffs. This led to the creation of the coastline. Some of the more striking examples of spits small spits to the east of the delta, the forerunners of the (as well as those in Fig. 2.26) include Rose Spit and Sand present day hook (Fig. 2.28b). Growth of Ediz Hook Spit in the Queen Charlotte Islands, Ediz Hook and the began in earnest once sea level had approximately reached Smith Island Spit (Fig. 2.27) in the eastern sector of Juan its present level a few thousand years ago. With the con- de Fuca Strait, Goose Spit (PI. 5) near Comox, and Re- tinued deposition of material from the Elwha River and becca Spit on Quadra Island. Sequim Bay southeast of the erosion ofsea cliffs, the spit grew eastward as a natural Dungeness illustrates an embayment that has been almost extension of the shoreline, which itself turns abruptly totally cut off from the sea by the growth of two spits at its southeastward at the base of the spit (Fig. 2.28~).At the mouth. same time the sea cliffs were eroding southward, the Ediz Hook (Fig. 2.28) is a textbook example of how westerly base of the spit was migrating southward also. spits evolve and what effects man can inadvertently have Periodically, moreover, waves would breach the spit and on their stability. Formation of this spit started about carry sand to its inner side by “overtopping,” a process - 32 - 33 - a deep-water port.” The suggested solution? First, build a troughs. Cusps attain maximum development during the rock revetment (retaining wall) 5 m above the low-tide transition from winter to summer beach profiles, and are level and 2 m thick on top along the northern side of the often destroyed when the transition is reversed. The rea- spit. Then nourish the seaward edge6of the spit with son for their remarkably even spacing is only now becom- sufficient beach sand (about 83,000 m3) to raisc the beach ing understood. According to the latest notions, beach profile enough to prevent initial undermining or scour of cusps are formed through the action of an alongshore the revetment toe. And finally, truck in an additional pattern of persistent nearshore circulation cells. These 10,000 m3/yr to maintain a stablc beach profile. The costs current cells are in turn generated by the interaction of for this effort runs into the millions and thcre would be no incoming surface waves with another type of wave known guarantee of success. Only time would tell if man can as an edge wave, essentially a low, visibly undetectable rectify his interference in the natural processes of beach oscillation of the sea surface with crests and troughs per- formation. pendicular to the shoreline rather than parallel as with Beach cusps are crescent-shaped shoreline features of ordinary waves. Simply put, the uniformly spaced troughs seaward protruding ridges or mounds of sand or gravel and crests of the edge wave modify the incoming breakers separated by small rounded embaymcnts. These may be along the beach in such a way that the wave swash con- isolated formations, but more commonly occur in a series verges to form cusp ridges and diverges to form cusp with fairly uniform spacing along the shore (Fig. 2.29). troughs at regular intervals. Edge waves will be included in the discussion of rip currents in Chapter 8. Sea Cliffs to Tidal Flats Many localities along the west coast are bordered by vertical or near vertical sea cliffs that often plummet abruptly beneath the level of wave action without the hint of a backshore. Within the fiords and interconnecting channels of the sheltered inshore water are precipitous bluffs tens of metres high, where massive glaciers once gouged their way seaward through existing valleys. Ero- sion of these cliffs has generally been minimal because of the resistant nature of the rock and the low energies of the waves.