Wind Blow-out Hollow Generated in Fukiage Dune Field, Kagoshima Prefecture , Japan

Ryuichiro Nishi, Li Elikson and Myokhin

PREFACE

A sand dune is vulnerable to severe waves and wind. Therefore, it works as final defensive natural structure to hinterland and local people. Therefore the stabilization of sand dune had been developed such as shown in CERC (1984). Sand dunes are eroded by storm waves especially under storm surge condition. In contrast, dune is enhanced by deposition of wind blown sand which is supplied from a dry beach in front of a dune. Therefore, high waves during storm condition acts as destructive force and predominant wind acts as constructive force to a sand dune alternatively in a coast along the Pacific Ocean in Japan. Dune erosion by storm waves during a storm surge condition was experimentally and numerically studied by a number of researchers, for instance, at University of Florida (Kriebel and Dean, 1985), Delft Hydraulics (Vellinga, 1986), and Coastal Engineering Research Center (Larson and Kraus, 1989 and Kraus and Smith, 1994). A dune scarp is a significant erosional feature that may interrupt beach users and natural animals such as sea turtle, thus Nishi and Kraus (1996) reveals a process of dune and beach scarp generation by stormy waves.

Fig. 1 (a) A blow-out hollow behind dune scarp and (b) examples of blow-out hollows in dune field.

In contrast to the sand deposition over a sand dune, by predominant local wind, dune erosion due to wind such as shown in Fig. 1 had been paid less research attention (i.e. Nordstrom, 1990 and Carter, 1993). These wind blow-out hollow topography and mechanics of wind blown sand should be taken into account to set up proper dune management, because it activates the stabilized dune sand by vegetation. In this paper, it is assumed that a generation of wind blow-out hollow might be affected by a presence of dune scarp such as shown in Fig. 1 (a). A turbulence of landward wind which is generated around an edge of dune scarp will burst a frontal dune surface, strips out the vegetation on a dune surface and excavates the sand dune nearly up to 10m maximum in some area. If a stabilized dune

R. Nishi

is blown-out as shown in Fig. 1 (b), activated wind blown sand will damage a coastal forest and hinterland, and may take some time to be stabilized again. Therefore, it is necessary to compile more knowledge on blow-out processes and its characters including cross-shore profile, 3-D topography, geometry and spacing.

STUDY AREA

The field study of wind blow-out hollow has been carried out in Fukiage Dune field which is one of the largest dune in Japan. Fukiage coast faces to the East China Sea and extends north-south direction nearly 40km as shown in Fig. 2. Shaded black area indicates the dune field. It is seen that width of the dune becomes wider toward SSW direction, in general, because predominant wind direction during winter season is N to NNW. Sand supplied from a dry beach in front of a dune enhances the dune, therefore it is expected that the wider the beach, the wider (larger) the sand dune or vise versa. So, the beach width is measured based on the aerial photograph and shown in Fig. 3.

Fig. 2 Fukiage coast and dune field (shaded area). Fig. 3 Width of wet beach and dry beach based on aerial photograph. The blow-out hollow is mainly studied between Isaku River and Manose River in this project. It is obvious that width of the beaches becomes wider from Isaku River (North) to Manose River (South)

according to the predominant wind direction in winter.

FIELD STUDY OF WIND BLOWOUT HOLLOW

Field inspection of beach and dune condition was conducted with GPS to measure the shoreline and H.W.L. Dune scarp such as shown in Fig. 1 (a) has been generated along the almost entire coast between Isaku River and Manose River. One of the cross-shore profiles of typical wind blow-out hollow is shown in Fig. 4. The dune scarp partially buried by wind blown sand exceeds 7m high. A blow-out hollow exists just after the top of frontal dune. Its center level reaches to nearly mean water level, i.e. nearly 9m deep. The cross-shore radius of the hollow is on the order of 50m at the top section. Relatively thin frontal dune as shown in Fig. 4 can be potentially breached by either storm waves or wind blow-out soon or later as shown in Fig. 5. If a frontal dune has a seaward slope similar to a vertical wall, wind that passes over the scarped dune will probably generate turbulence near a seaward edge of the dune.

Fig. 4 Cross-shore profile of a sandy beach, dune scarp, frontal dune and dune blowout hollow.

Fig. 5 3-D images of a dune blow-out hollow and breached blow-out hollow (unit; m).

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Location of the blowout hollow in cross-shore and longshore directions.

A series of aerial photographs as shown in Fig. 6 which are originally 1:10000 scale are enlarged twice, then inspected to identify the position, geometry, spacing and stage of blow-out of each hollow as defined in Fig. 7 and 8.

Fig. 6 Right hand side of Manose River (Down stream (south) side of study area)

Fig. 7 Plane view of wind blow-out hollow. Fig. 8 Cross-shore distance of blowout hollow.

It is seen that the forty-six wind blow-out hollows exist in the 6.3km coastal stretch between Manose and Isaku Rivers as shown in Fig. 9. Thus, an average longshore spacing of blow-out hollow is 137m. In other words, each blow-out hollow has circular or ellipse plane views, so the longshore radius can not exceed 137m, otherwise the neighboring blow-out hollows may combine together. In addition, Fig 9 shows an inland distance (D1) of blow-out hollow which means the cross-shore position of center of hollow along the coast. The inland distance was measured from the edge of dune scarp as shown in Fig. 8. Most of the blow-out hollows are centered in an area between 10 to 40 m inland from the edge of dune scarp. The average cross-shore distance is nearly 28m.

Fig. 9 Spatial distribution of wind blowout hollow. Geometry of blow-out hollow plane view The position of blow-out hollows along the coast and the geometry plan view of each hollow are carefully studied. For instance, typical blow-out hollow has a plan view as shown in Fig. 10, so the longshore radius and cross-shore radius are measured by using aerial photographs. Fig. 10 Plan view of typical blow-out hollow

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Fig. 11 shows the longshore radius and cross-shore radius of each hollow. It is seen that both radii

slightly tend to increase toward north direction (toward Isaku River) and cross-shore radius is longer than longshore radius in general. The average length of cross-shore radius is 40m and average length of longshore radius is 29m. To reveal a generation process of wind blow-out hollow, correlation of both radii is shown in Fig. 12. It is seen that each blow-out hollow may expand with circular plan view as much as 60-80m wide, then several blow-out hollows join with neighboring blow-out hollows or breach the frontal dune. Fig. 11 Geometry of dune blow-out hollow along the coast. Fig. 12 Correlation between longshore radius and cross-shore radius of the blowout.

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The correlation of both radii is reasonably good while the cross-shore radius is shorter than 60m, and poor if the cross-shore radius exceeds 60m. Linear regression for the first dotted line in Fig. 12 can be expressed as;

47.464.0 += xy ll （1） Where, lx: cross-shore radius

ly : longshore radius The plane view of a blow-out hollow could be a circular if the correlation factor is unit. However,

the correlation factor of eq. (1) is 0.64 so that the blow-out hollow is distorted to elongate to cross-shore direction. Normal wind blown sand process dominated and breached stage: If the wind blow-out hollow expands cross-shore directions, wind blown sand significantly scatters beyond the edge of blow-out hollow in down stream side and the seaward part of the frontal dune breaches and opens to the sea. Then storm waves may overwash inside the blow-out hollow. In addition, predominant wind in winter season may tend to blow more sediment landward and may damage stabilized dune by vegetation and coastal forest as shown in Fig. 13. To characterize the breached stage of blow-out hollow, position and geometry of the breached hollow are examined by using aerial photographs as shown in Fig. 14. Average length of cross shore and longshore radii are 78m and 60m respectively for the breached hollow. Both radiuses are also related as follows;

Y=1.24X-36.38 (2)

It should be noted that the predominant wind direction in the site is 30 degrees to the average shoreline orientation, therefore, the result are the first order approximation.

Fig. 13 Characterization of breached Fig. 14 Position and geometry of breached hollow. hollow.

ESTIMATION OF WIND BLOWN SAND IN THE STUDY AREA

Rate of wind blown in a dune field is usually estimated by either using sediment trap or sediment transport equation applying wind data such as Bagnold (1941). However, the other practical (heuristic) technique is applied to estimate the rate of wind blown sand at the site. The idea for the estimation is that the annual volume of sediment deposition per unit shoreline length might be estimated by multiplying the average thickness of blown sand deposition with a half of landward deposition length, because a triangular cross-shore profile is assumed. However, there is few information of the thickness of annual wind blown sand deposition in literatures, therefore field study has been conducted. As can be seen in Fig. 15 (a) and (b), the annual deposition structures are easily recognized especially scarped dune face just after a typhoon. So, the thickness of annual deposition is measured by a survey staff. Average annual deposition of wind blown sand is nearly 0.22m in the study field. In addition, the cross-shore length of wind blown sand deposition (or cross-shore length of blown sand scattering) is examined by using aerial photographs. The average deposition length is 96m and is in a range from 33 to 195m, so that the total wind blown sand transport rate in the study area is estimated as much as 12,000 m3/year for 6.3km coastal stretch. This rate should be confirmed with the rate of wind blown sand estimate by normal techniques in further studies.

Fig. 15 (a)Typical image of annual sand deposition in a dune and (b)a zoom-up of annual deposition. Fig. 16 Frequency distribution of measured thickness of annual sand deposition.

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WIND AOUND THE SCARPED DUNE

Fig. 16 Measured wind profiles around the scarped dune.

Wind fields around the scarped dune with blow-out hollow and the mild slope dune without the hollow have been measured. Fig. 16 shows the average wind at beach (A-AVE), over the frontal dune (B-AVE), center of the blow-out hollow, and at the landward boundary of breached hollow. These measuring points are set over same cross-shore transect. The wind measured at the center of blow-out hollow only shows the seaward wind while the winds over sandy beach in front of dune scarp (blow-out hollow), at the top of frontal dune and at the landward boundary of the hollow shows landward wind. The wind pattern around the mild slope dune without the blow-out hollow only show the landward wind at the same time. Therefore, it probably considered that the nearly vertical slope of scarped dune enhances the turbulence just after the edge of frontal dune. This burst-like-turbulence damages the vegetation on dune surface and excavates as much as 10m in this site.

CONCLUSIONS

A field study and an analysis of aerial photographs are conducted to study the generation process of wind blow-out hollow and breached hollow. The field study for the dune process is still on-going at the site, so that the preliminary conclusions are as follows; 1. Geometry size of wind blowout hollows and wind blown sand areas is wider in up-stream region of

predominant wind (Isaku region) than down-stream region (Manose region), in general. 2. Wind blowout hollows are generated in the narrow band area which exists 10m to 50m landward from

the edge of frontal dune scarp. 3. Wind blow-out hollow develops its size as much as 50m long, then a seaward boundary reaches to the

edge of frontal dune scarp face to breach the frontal dune. 4. Average distance between the wind blow-out hollows and wind blown sand areas is 113.4m in the

study site. 5. Nearly vertical slope of dune scarp enhances a wind turbulence around the frontal dune, and causes

wind blow-out hollow, and destruction of vegetation covering dune surface. 6. After the breaching of frontal dune, landward wind causes more wind blown sand than stabilized dune

by vegetation.

ACKNOWLEDGEMENT The authors would like to express their special appreciation to Dr. Nicholas C. Kraus at the Coastal and Hydraulic Laboratory, ERDC, US Army for inspiring the ideas of dune management.

REFERENCES

Bagnold, R.A. 1941: The physics of blown sand and desert dunes, Morrow, New York. Carter, R.W.G. (1993): Coastal Environments, Blowouts, pp.318-320, Academic Press, London, p.617. CERC(1984): Shore protection manual Volume II, sand dunes, pp.6-37～6-53, Waterways Experiment Station, Corps of Engineers.

Kraus, N. C. and Smith, J. M. 1994: SUPERATNK laboratory data collection project, volume 1: Main text, Tech. rep. CERC-94-3, US Army Engineer Waterways Experiment Station, Coastal Engineering Research Center, Vicksburg, Miss. Kriebel D. L. and R. G. Dean, 1985: Numerical simulation of time dependent beach and dune erosion, Coastal Engineering 9, pp. 221-245. Larson M. and N. C. Kraus, 1989: SBEACH: Numerical model for simulating storm-induced beach change; Report 1, empirical foundation and model development, Technical Report CERC-89-9, U.S. Army Engineering Waterways Experiment Station, Vicksburg, MS. Nishi, R., and Kraus, N.C.,： Mechanism and calculation of sand dune erosion by storms, Proc. of 25th International Conference on Coastal Engineering, ASCE, pp. 3034-3047, September 1996 . Nordstrom, K. N. Psuty, and B. Carter (Editor) (1990): Coastal dunes forms and process, Blowouts, pp.231-246, John Wiley & Sons, New Jersey, p.392.

Vellinga, P., 1986: Beach and dune erosion during storm surges, Delft Hydraulics communication No. 372.

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