Coastal and Estuarine Environment Department

Field measurements of the effect of vegetation on beach profile change in the backshore

Field measurements of beach profile and vegetation were conducted once a month during a period from June in 1995 to May in 1999 along three transects in the region from a backshore to a foredune near HORS (Figures 1, 2 and 3). The interval of the transects was 20 m. At P-116m and P-120m along each transect, fences composed of bamboo slates were located; its height was about 1 m and the porosity was about 30 %. In the investigation area, winds blew mainly from the north in winter and from the south in summer.

In the topography measurement, the beach profiles were measured from P-20m to P-115m every 5 m. In the vegetation measurement, species, height and cover area of vegetation within 1 m by 1 m square were observed from P-70m to P-110m every 10 m.

From spring to autumn, the area landward from P-65m was occupied by creeping beach grasses including Carex kobomugi (Japanese Sedge, Asiatic Sand Sedge, Figure 4), Calystegia soldanella (Sea Bindweed) and Elymus mollis (American Dunegrass). Figure 5 shows the seasonal change of the spatially averaged vegetation height Iv, which clearly shows that vegetation grew from spring to summer and died from autumn to winter.

Figure1 Investigation areaの画像

Figure1 Investigation area

Figure2 Beach profile along the survey line II.の画像

 

Figure2 Beach profile along the survey line II.

Figure3 Aerial view of the investigation area.の画像

Figure3 Aerial view of the investigation area.

Figure4 Carex kobomugi (Japanese Sedge, Asiatic Sand Sedge)の画像

Figure4 Carex kobomugi (Japanese Sedge, Asiatic Sand Sedge)

The investigation area experienced sediment deposits during the 4-year measurement period, which were probably caused by the predominant onshore winds. The time series of the elevations at P-70m and P-110m show that the accumulation is continual (Figure 6).

Figure6 Time series of the change in elevation from the profile.の画像

Figure6 Time series of the change
in elevation from the profile.

The cross-shore component of aeolian sand transport rate was estimated from the beach profile changes on the basis of a mass conservation equation, which is expressed as Eq. (1).

(1) where q is the cross-shore component of aeolian sand transport rate per unit length in the alongshore direction, x is the cross-shore distance, g ?is the bulk density of sediment (= 1650 kg/m3), z is the elevation, and t is the time.

The estimation of the cross-shore component of aeolian sand transport rate with Eq. (1) is based on an assumption that the beach profile changes were induced only by the gradient of the cross-shore component of the aeolian sand transport, and not influenced by the gradient of the alongshore component. Furthermore, at the landward boundary, the cross-shore component of aeolian sand transport rate was assumed to be zero owing to the two fences located at P-116m and P-120m.

The obtained cross-shore distributions of the cross-shore components of aeolian sand transport rates were normalized by the transport rates at P-62.5m, which was almost the seaward limit of vegetation and also landward limit of wave run-up. Then, the cross-shore distributions of the nondimensional cross-shore components of aeolian sand transport rates q' in the area landward of P-62.5m were approximated with quadratic curves where q' = 0 at P-117.5m and q' = 1 at P-62.5m; the curves are expressed as

(2) where a is a coefficient (1/m2), and x is the cross-shore distance (m). A positive value of a indicates that the cross-shore component of aeolian sand transport rate rapidly decreases seaward of P-90m, whereas a negative value of a indicates that the cross-shore component of aeolian sand transport rate decreases landward of P-90m (Figure 7). In Figure 7, the solid and broken lines show the values estimated with Eq. (2) and the solid and open circles show those estimated with Eq. (1).

Figure7 Cross-shore distributions of the nirmalized cross-shore component of aeolian sand transport rate with different a values.の画像

Figure7 Cross-shore distributions of the nirmalized cross-shore component of aeolian sand transport rate with different a values.

The value of a has a strong correlation with Iv (Figure 8), and the relationship is expressed as

(3) The result indicates that the cross-shore component of aeolian sand transport rate with vegetation rapidly decreased landward of the seaward limit of vegetation, whereas the transport rate without vegetation decreased near the foot of a foredune. When vegetation grew and wind blew landward, vegetation reduced the bed shear stress due to wind and trapped sediments transported landward from a foreshore. As a result, the landward aeolian sand transport rate rapidly decreased landward of the seaward limit of vegetation. When vegetation died, on the other hand, sediments were not trapped by vegetation, and hence the aeolian sand transport rate did not decrease from the seaward limit of vegetation. Because the wind velocity near the surface was reduced in the vicinity of the foredune, the aeolian sand transport rate decreased near the foot of a foredune.


Figure8 Relationship between Iv and a.の画像

Figure8 Relationship between Iv and a.