48 



Tide range and type can be of considerable importance in beach-dune 

 interactions. On beaches where tide ranges are high, more sand is uncovered 

 at low water, effectively increasing the size of the source area. Tide type is 

 also important because the wind velocity necessary to move sediment is 

 significantly increased when the sediment is moist or wet. Because of this, 

 more sediment is available in areas where diurnal tides occur, since a longer 

 period of time is available for the foreshore sand to dry out between high tide 

 stages. 



Effective transportation of sediment by wind occurs when the wind reaches 

 a threshold velocity that is determined not only by sediment characteristics, 

 but by moisture content, slope, radiation and energy balance factors, chemical 

 precipitates, and vegetation (Carter 1988). The threshold velocity is signifi- 

 cantly increased for moist material. Also, the seaward slope of the beach 

 surface increases threshold values in accordance with the steepness, because 

 the particles must move upslope to reach the dune area. Sand transport from 

 the beach to dune areas occurs only when wind blows in an onshore direction, 

 whereas sand transport from major inland sources occurs only when wind 

 blows in an offshore direction. Thus, the frequency of onshore and offshore 

 winds, their relation to local sources, and their velocity, are important factors 

 in dune development. 



Dune formation requires an open space inland of the coastline that can be 

 reached by windblown sand from the beach. A relatively high cliff or steep 

 slope at the coasdine can prevent dune formation by blocking the transport 

 path. In such cases, windblown material may accumulate at the base of the 

 cliff or slope, but this material will be periodically eroded during high water 

 levels, and it will develop into true dunes. 



The stability of dunes varies greatly and depends primarily on the amount 

 of vegetation cover. Dunes found in arid climates are usually not vegetated 

 and tend to be mobile. Coastal dunes in non-arid climates are likely to 

 support some vegetation cover and be more stable. However, unvegetated and 

 thinly vegetated dunes can occur in any climate. Colonization of dunes by 

 vegetation not only depends on whether an area has sufficient moisture, but 

 whether the grasses and other plants are salt-tolerant and can respond to 

 conditions of rapid sand accumulation. The types of dune grasses found in an 

 area show spatial variety according to the types of species in an area, and 

 rapid temporal successions are often present (Carter 1988; Chapman 1964; 

 Goldsmith 1985; Woodhouse 1978). 



Vegetation cover is a factor in the stability of dunes because the plants can 

 both decrease the mobility of sediments and increase the likelihood of deposi- 

 tion by changing local aerodynamic conditions. Many dune plants have long 

 roots, rhizomes, and runners that help hold sand in place (Figure 19). The 

 presence of dense vegetation, in turn, at normal wind speeds can displace the 

 aerodynamic boundary of the wind velocity profile upwards. This process 

 then provides a net downward momentum flux, which promotes sediment 

 trapping. 



Chapter 3 Variable Coastal Features 



