8.4.5: Interrupted coasts: spits
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A spit is a pointed tongue extending into the sea. Spits develop where the longshore transport capacity is diminished due to coastline interruptions (river, estuary, end of island). The direction of a spit usually is a continuation of the shoreline from which sediment is supplied. An example of a spit is shown in Fig. 8.27. Waves coming pre-dominantly from the southwest cause a sand transport toward the north along the western slope of the island. As the water becomes deeper at the north end of the island, the waves no longer break, the sediment transport decreases (hence a negative transport gradient), the sand settles and the spit gradually builds out as an extension of the coastline.
The spit of Block Island is a good example of a spit formed at the end of a beach, where the longshore current loses its transport capacity. For the same reasons, spits can also form near the entrances of harbours or estuaries or where a river mouth interrupts an otherwise straight coast. Let us consider the situation of a modest river flowing into the open sea (see Fig. 8.28). The undisturbed longshore transport rate is \(S\ m^3/yr\). If the river mouth is sufficiently wide and deep to strongly reduce the longshore transport rate (hence a negative transport gradient), coastal material is deposited on the updrift side of the entrance, narrowing the mouth. At the downdrift side the waves regain their capacity to transport sediment (capacity is \(S\ m^3/yr\), whereas only a fraction thereof bypasses the river mouth. The positive transport gradient results in erosion on the downdrift side. The result is a slow displacement of the river mouth in the direction of the longshore transport; a spit develops and grows over time.
Landward of the spit, the river flows more or less parallel to the coast for some dis- tance. Because of the growing of the spit, the length of the river and as a result the water levels in the river behind the spit increase, eventually forcing a breakthrough somewhere on the updrift side of the spit. This is a periodic process for a fully natural river mouth with breakthroughs of the slender spits occurring during periods of large river discharges (e.g. during a wet season).
Spits may also develop where coastlines with sufficient longshore sediment transport are interrupted by lagoons or bays instead of by a river. If the longshore transport rate is large enough, a spit can also develop in combination with a delta (see for instance Fig. 8.32). Spits may eventually close off a lagoon or bay acting as a ‘bay barrier’ (Fig. 8.29a). Figure 8.29b shows a barrier spit that separates a tidal basin from the marine environment; tidal action keeps the entrance open. Breaches in long barriers can lead to barrier island formation.
The coastline interruptions may not only locally diminish the wave-induced longshore transport capacity, but act as a source (e.g. a river, see Sect. 8.4.6) or sink (e.g. a tidal basin) of sediment as well. Tidal basins (bays, lagoons, estuaries) are known to act as a sink for longshore sediment transport, thereby depriving adjacent coastlines of sediment. Tidal basins and their interaction with adjacent coasts are discussed in Ch. 9.