Foreshore Features and Tidal Flats

Zealand or Japan. The process of beach lithifica-tion and beach rock formalithifica-tion is highly contro-versial with different mechanisms of cementation responsible at different locations: Some authors argue that cementation of the beach sediment takes place subsurface in the area of water table excur-sion between the neap low and high tide levels.

Geomorphologists emphasize that beachrock rath-er is the product of cementation above the surface and occurs in the supratidal spray belt, which than may explain its different height level in regions with larger waves or greater exposure.

Beside waves, other important agents in form-ing and shapform-ing a beach are tides, currents, wind and in higher latitudes coastal ice. Beaches and beach features are dynamic coastal landforms and may change from day to day, week to week, season to season, and year to year.

5.2 Foreshore Features and Tidal

along the coast, tidal ranges may be amplified and may reach 15 m or more, as in the Fundy Bay, eastern Canada. Tidal currents in those areas are often very rapid and can approach up to 25 km/h leaving behind forms from ripples to mega-ripples, sandbars and ridge/runnel features, tidal deltas, creeks and channels (5.26–5.29). Some of the best known tidal flats occur along the coastlines of the North Sea (the German and Dutch Wadden Sea, the Wash in south-eastern England); the Bay of Mont St. Michel in France; the Gulf of California;

the Bay of Fundy in Nova Scotia or along King Sound in north-western Australia. Here, the tidal flats are dominated by siliclastic sediments such as quartz sand and quartz silt. Carbonate domi-nated tidal flats occur in mid- to low latitude warmer climate zones – famous examples are Andros Island of the Bahamas Islands and Shark Bay in Western Australia, UNESCO listed world heritage regions.

Muddy tidal flats are of a dark colour due to high organic content. The sedimentation of these fine particles occurs by settling of suspension load during low current velocity and tidal slack water (now to nil current velocities) in the short time between the incoming and outgoing tidal water flow.

Mud tidal flats are home to a rich diversity of different vegetation types (mangroves, salt marsh, algal mats) and to myriads of organisms (molluscs, crustacea, polychaetes, resident and nektonic fishes, birds) which have to cope with environmental conditions at the triple junction between land, sea and atmosphere. Benthic or sessile biota of tidal flats has to be adapted to saltwater conditions during tidal flooding, but may be exposed to freshwater (rain) during low tide. Surface temperatures may reach more than +50°C in summer and in some regions lower than -30°C in winter. With the ebb and flow of tides conditions can change within hours and some-times within minutes. Therefore, many organ-isms of the benthos are burrowing types and live within the sediment as a strategy to encounter an environment with continuing similar condi-tions, thereby constantly bioturbating the sedi-ment. The biogenic sediment structures of these organisms often are diagnostic for a specific tidal level.

Over time, the vegetation of the higher tidal flats (e.g. the halophyte Salicornia sp.) fixes and

Fig. 5.12 Tidal flats exposed during low water with a patch of Salicornia sp. in the semi-arid environment of eastern Patagonia (Argentina). (Photo credit: D. Kelletat).

Fig. 5.14 Temperate sandy tidal flats (here: north coast of Germany) are often decorated with billions of tiny sand mounds – excre-ments of the sandworm Arenicola sp., a species diagnostic for low-tidal sand flats (Photo credit: D. Kelletat).

Fig. 5.13 In tropical regions, sandy tidal flats may be inhibited by numerous crabs (e.g. Dotilla sp.) who distribute small sand pills radial to all sides around small vertical burrows in the sand. The higher the tide range, the deeper the crabs have to burrow to reach the water level in the sediment to allow breathing with their gills.

(Photo credit: A. Scheffers).

Fig. 5.15 Ripple fields (in the foreground) and larger sand bars (along the horizon) on a sandy tidal flat of the German North Sea coast.

(Photo credit: D. Kelletat).

Fig. 5.17 Another form of mega-ripples, here very regular made by tidal currents along the south island of New Zealand. (Photo credit: D. Kelletat).

Fig. 5.16 Mega ripples on a sandy tidal flat of southern New Zealand. (Photo credit: D. Kelletat).

accumulates sediments that are deposited dur-ing high tide events or storms. By this process, parts of the tidal flat or the landward margin may grow above the normal high water level, and more plants settle and stabilize the ground.

If sea level is stable, after hundreds of years this new salt marsh above the mean high water mark can grow above the highest storm lev-els and remains a terrestrial environment with new vegetation and soil development (Fig. 5.23).

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Fig. 5.19 A sandy tidal flat near Darwin in northern Australia shows different salt concentrations (white colours) on the sur-face due to evaporation. The dark fingering pattern is the rising tide water along small creeks. Oblique aerial photograph (Photo credit: A. Scheffers).

Fig. 5.20 Sand bars of different forms are typical for sandy tidal flats with strong currents as in the nearshore of the northern Netherlands´ coastline, here 25 km wide at 53° 30´N and 6° 33´E.

(Image credit: © Google Earth 2010).

Fig. 5.21 Tidal flats with a system of tidal creeks behind barrier islands of The Netherlands at 53° 12´N and 5° 03´E. The barrier island is nearly 20 km long. (Image credit: © Google Earth 2010).

Fig. 5.22 Tidal mud flats in a tropical environment with numer-ous tidal creeks (western Mexico at 24° 47´N and 108°W, around 9 km wide). (Image credit: © Google Earth 2010).

Fig. 5.18 Sand bars like mega-ripples migrating towards the beach from the shallow foreshore. The Netherlands at 51° 05´N and 2° 31´E, width is about 7 km. (Image credit: © Google Earth 2010).

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5.22 5.21

Fig. 5.25a Ripple pattern in the sandy foreshore of eastern Canada at 42°N and 70°W. The mega-ripple section is about 1.2 km wide, the wavelength of single ripples about 50m. (Image credit: © Google Earth 2010).

Fig. 5.25b Ripple pattern in the lower intra-tidal zone. Western France at 48° 40´N and 3° 35´W. Image shows a section 4 km wide.

(Image credit: © Google Earth 2010).

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Fig. 5.26 Tidal currents move sands and constantly transform these deposits (eastern Canada at 42° 39´N and 70° 44´W, 5 km wide). (Image credit: © Google Earth 2010).

Fig. 5.27 A tidal delta of the US east coast at 39° 30´N and 74° 20´W. Scene is 9 km wide. (Image credit: © Google Earth 2010).

Fig. 5.28 Tidal channels between islands in a Bahamian island chain at 23° 50´N and 76° 14´ with the deposition of the fine sedi-ments as ebb-tidal deltas and flood-tidal deltas at both sides of the island chain. (Image credit: © Google Earth 2010).

Fig. 5.29 Tidal delta in a large lagoon of the north coast of the Mexican peninsula of Yucatan at 18° 45´N and 91° 27´W. Scene in the image is about 22 km wide. (Image credit: © Google Earth 2010).

Fig. 5.23 A cross-cut in marshlands show the changing pattern of light coloured sands from storm floods and dark bands of soil development. North coast of Germany (Photo credit: A. Scheffers).

Fig. 5.24 Foreshore sand patterns from tidal currents in Shark Bay, Western Australia, at 26°S and 114°E in a 10 km wide section.

(Image credit: © Google Earth 2010).

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