STRAK OF GEORGlA J map ofthe Fraser Rivet sohd bas denote no change.

- 38 - dunes ofthe backshore (Fig. 2.34). Logs and other debris Man-Made Structures commonly collect at the leading edge of the high-tide line. In many instances man has used dikes and fill to redaim The ~runlmechanisms of erosion and deposition, the matshland (as in the municipalities of Richmond and combined with the accompanyingpnxesses of redisuibu- Delta at the mouth ofthe Fnser River) and has altered the tion, provide a gradual modification of the coastline. natural restructuring of the delta by building jemes and Often these changes are in conflict with the requirements training walls. ofman, who endeavors to modify them by building, what Despite their extremely small slopes, tidal flats are far he considers, appropriate structuru~esin the nearshore from featureless. In addition to the constantly shifting zone. Such coastal engineenng structures serve threemain dendritic drainage channels and distributaries that cut p~ses:jetties, digwalls, and bdwaters aid navi- across the flats, the redismbution of surface sediments by ganon and provide protected moorage for boats; seawalls waves, tidal currents, and river 00w creates a variety of and groynes reduce erosion of the coast. miping sand ripples. These may have spunmidcrests Jetties are seaward projecting walls constructed at the (wave formed) or asymmewical crests (current formed). mouths of rivers or at the entrances to lagoons or tidal Sand-based megaripples 50-100 m long and up to 2 m estuaries, to stabilize the positions of navigable channels high are common on the Fraser River tidal flats, and are and to prevent shoaling through littoral drift. As with the especially well formed between Iona and North Arm jet- Steveston, Iona, and North Arm jetties at the mouth of ties and along the northern side of Steveston Jetty (Fig. the Fraser River (Fig. 2,34),jetties tend also to direct and 2.35). confine the flow, and thereby enhance the abllity of a channel to self-scour. (Training walls like those on the Squarmsh River sem a similar function.) Jetties help also to protect channel entrances from waves and allow land access to oE&ore dobgfacilities (e.g. theTsawwassenFerryCauseway).Toavoidsedimen- tation problems associated with the longshore driit of sand, most jetties extend beyond the breaker zone and act as barriers to the natural littoral drift induced by the action of waves. This results in a seaward advance of the shoreline. On the downdrift side, on the other hand, the . .... , shoreline retreats as the longshore current removes shore material to compensate for the loss of sediment load trapped upstream of the barrier (Fig. 2-36),Although

Fic.2.36. Effmofjmyonalongshoretranspartofxduncnr.&pi- tion mun upsueam and mion downsueam of jmy. such problems are not severe in the Strait of Georgia or other protected waters because of the comparatively low- energy waves, this is not the case on exposed coasts, for example California, where considerable erosion of beaches and property adjacent to jetties has occurred. Breakwaters (Fig. 2.37) are artiiicial harbors built to shield boats from the waves. They are usually attached to the shore at one or both ends, with a narrow opening for cinity of boat trac, and extend beyond the breaker zone. In re- spacings gions of persistent longshore drift, the entrances com- monly require periodic dredging to remove accumulated

- 39 - sands. It was once thought that detached breakwaters, shoreward slope. The height of the seawall is usually built parallel to the shore and not attached to it, would enough to prevent possible damage from overtopping by allow the littoral drift to continue ununpeded and avoid storm-wave run-up at high tide. Although a seawall the usual problems of sediment accumulation. However, provides protection for the land immediately shoreward, it was soon found that these breakwaters diminish the it affords no protection to the beach in front ofthe wall or focal wave energy and the ability of waves to transport to adjacent portions of the coastline. In fact, when a sand along the coast. As a consequence, the coastline in seawall is built on an initially receding shoreline, the the shadow zone behind the breakwater advances seaward erosion processes simply shift further along the coast. The until it forms a tombolo, and cuts off the passage behind only sure defense of a coast it appears, is a wide, high the breakwater (Fig. 2.37). beach. Seawalls constructed of a variety of natural or mm- A groyne (or groin) is a barrier that projects directly made materials are built roughly parallel to the shoreline seaward from the shore. Its primary purpose is to trap a to prevent direct wave erosion, or to inhibit slumping of portion ofthe littoral drift to add to an exisring beach that sea cliffs. Examples include the seawall along Ross Bay would otherwise be eroded. As with jetties, sediment near Victoria and the Stanley Park seawall of Vancouver. accumulates on the updrift side of the groyne whereas on Experience has shown that vertical seawalls undergo unac- the downdrift side the flow is starved of beach material ceptably high rates of erosion at their bases, SO most and erosion may take place (Fig. 2.38). &oat launching present day structures of this type are built with a ramps built by owners of waterfront property produce similar effects to gropes and can inadvertently cause erosion of a neighbor's beach further along the shoreline. Asidc from the usual attempts by British Columbia residents to stem erosion of a beach front, there have been organized and elaborate schemes. One involved the west- to-northwest sector of the Paint Grey cliffs, whose con- tinuing erosion into Cbc Strait of Georgia concerns nu- merous agencies. During the sunmer of 1974, a 1000-in long sand-fill, plus cobble-core berm and groyne system was constructed along Towers Beach, University of Brit- ish Columbia, to prevent marine erosion along the base of thc cliffs. The berm partially protected the cliff base from weathering during the following winter but by February had failed along a 450-rn scction, due to storm-generated west-to-northwest wavcs (McLean 1975). Moreover, the groynes proved ineffective in retaining the sand fill, and over 5400 m3 of sand were moved eastward by the littoral drift. With a useful lifespan of less than 2 yr the man-made berm had done little to limit the average recession rate for the cliffs of 15.2 cmivr.

BREAKWATER-

FIG,2.37. Thcc types of hreakuraters: (A) Oak Bay at eastern end ofJuan dc Fuca Strait, (B) English Bay in Burrard Inlet, and (C) Santa Monica on Californiacoast.Arrows show dircccioe ofprevailing waves. At detached breakwatcr at SantaMonka, littoral proresses have ledto growthof spitin leeof breakwater. In time, spit becomes a tombolo.

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