Ryuichiro Nishi, Li Erikson and MyoKhin Kagoshima Univ., Lund Univ., Daiichi Institute of Technology

Transcription

1 Dune blowout hollow generated in Fukiage Dune, Kagoshima, Japan Ryuichiro Nishi, Li Erikson and MyoKhin Kagoshima Univ., Lund Univ., Daiichi Institute of Technology

2 Contents of this presentation; topography change of frontal dune No.1

3 Topography change of frontal dune No. 2 Dune scarp

4 Processes of Dune topography changes 1. Sand dune stabilized by vegetation and coastal forest 2. Dune scarp generation caused by storm surge and high waves 3. Wind blowout hollow generation and expansion, and destruction of vegetation surface 4. Breaching of wind blowout hollow and destruction of frontal dune 5. (Normal) Enhanced wind blown sand process Dune protect human beings and property against high waves, storm surge and tsunami! It should be properly stabilized!

5 Example of cross-shore profile of dune blowout hollow

6 Example; 3D topography of wind blowout hollow Unit (m)

7 Example 2; 3D topography of wind blown sand area Topography after a breaching of wind blowout hollow

8 Previous study on a dune Dune erosion -----Erosion due to storm surge and high waves Dune erosion ---- Dune scarp generation Wind blown sand ---- Annual and seasonal wind blown sand rate Stabilization of dune ----Vegetation, coastal forest and fence Topography change due to wind vortex (turbulence)

9 Overview of wind blown hollow *Growth of coastal forest Saburo Akagi (1991) Yoshiro Hatakeyama(1998) *Dune (wind blown sand) stabilization CERC(1984) : Shore protection manual *Character of wind blown sand Carter (1993) Nordstrom et al. (1990)

10 Study area (Fukiage dune, Kagoshima, Japan) Tokyo Kagoshima

11 Study area (Fukiage dune, Kagoshima, Japan) Kagoshima Dominant wind direction in winter Kagoshima Pref.; 2700km coastal line 600km long Manose R. Isaku R. Okinawa

12 Aerial photographs used for analysis Predominant wind in winter No. 1 Right hand side of Manose River (Down stream side) No. 2 Intermediate region No. 3 Left hand side of Isaku River (Up-stream side) Original aerial photo 1/10,000 Enlarged to 1/5,000 for analysis

13 Example of aerial photographs used for analysis Predominant wind in winter Predominant wind in winter

14 Position and geometry of wind blowout hollow Sea Sandy beach Dune field L 2 D 1 L 1 D 1 ; Cross-shore distance L 1 ; Cross-shore radius L 2 ; Longshore radius Dune scarp Wind blowout hollow

15 Cross-shore distance of wind blowout hollow; D 1 L 1 Cross-shore distance from the edge of dune scarp (m)

16 80 70 Spatial distribution of wind blowout hollow 風食孔の岸沖距離 (m) Cross-shore distance; D 1 (m) 浜崖頂部から風食孔までの距離 (m) Longshore 沿岸距離 distance ( m) (m)

17 Frequency diagram; landward distance of a hollow center from an edge of dune scarp 10 風食孔発生位置の頻度分布 頻度 ( 個所 ) Frequency Data: Data13_Count Model: Gauss Chi^2 = y ± xc ± w ± A ± Gaussian distribution ガウス分布フィット Cross-shore 浜崖頂部から distance from の内陸側距離 the edge of (m) dune scarp (m)

18 46 wind blown hollow in 6.3km coastal stretch 1 in every 137m Most of the hollow features are created in an area between 10 to 40 m from the edge of dune

19 Geometry of dune blowout hollow 160 Cross-shore radius & longshore radius (m) 風食孔長軸 短軸長さ ( m) 長軸長さ ( m) 短軸長さ ( m) 区間 : 万ノ瀬川 ~ 伊作川 沿岸距離 ( m) Longshore distance (m)

20 Average length of cross-shore radius = 40m Average length of longshore radius = 29m

21 Cross-shore 風食孔短軸長さ radius (m) ( m) Geometry of dune blowout hollow 風食孔の形状 ( 長軸長さと短軸長さの比較 ) Expansion of single blowout hollow Breaching Combination of neighboring blowout hollows Longshore 風食 radius 孔長 (m) 軸長さ ( m)

22 Geometry of blowout hollow; expansion stage y = 0.64x R = 0.84, SD = 5.81 Cross-shore 短軸長さ radius ( m) (m) Longshore radius 長軸長さ (m) (m)

23 D 1 ; Cross-shore distance L 1 ; Cross-shore quasi-radius L 2 ; Longshore quasi-radius Sandy beach Wind blown sand dominated area Dune field Sea L 2 D 1 L 1 (Breached) dune scarp

24 Stage III (Normal wind blown sand); Position of wind blown sand area 100 飛砂領域の岸沖距離 ( m) Cross-shore distance; D 1 (m) 岸沖方向距離 ( m) 沿岸距離 ( m) Longshore distance (m)

25 Geometry of wind blown sand dominated area L 1 & L 2 Cross-shore radius & longshore radius (m) 長軸 短軸長さ ( m) 長軸長さ ( 飛砂の卓越方向 ) 短軸長さ ( 沿岸方向 ) Longshore 沿岸 distance 距離 ( m) (m)

26 Average length of cross-shore radius = 78m Average length of longshore radius = 60m

27 Geometry of wind blown sand area; stage III 250 Cross-shore 短軸長さ radius ( m) (m) 飛砂発生域の長軸 短軸長さ Y=1.24X Longshore radius 長軸長さ (m) ( m)

28 Distance between neighboring wind blown sand areas & wind blowout hollows 風食 飛砂領域の発生間隔 ( m) 400 Distance 発生間隔 ( (m) m) Identification number 発生個所 No.

29 Frequency distribution of distance between wind blown sand areas & blowout hollows Frequency (No.) 頻度 ( 個所 ) 風食孔 飛砂領域の発生間隔頻度 Data: Data11_Count Model: Gauss Chi^2 = y ± xc ± w ± A ± Gaussian ガウス分布フィ distribution ット Distance (m) 発生間隔 ( m)

30 Average annual deposition rate per unit shoreline length & total wind blown sand rate Total wind blown sand transport rate in the study area is nearly 20,000 to 30,000 m 3 /year Annual deposition of wind blown sand is nearly 20cm Summer season Winter season

31 Wind observation Wind speed (m/s)

32 Wind numerical simulation Example of numerical result; wind velocity field

33 Major conclusions; 1. Geometry size of wind blowout hollows and wind blown sand areas is wider in upstream 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.

34 Major conclusions; 3. Wind blowout hollow develop it s size as much as 50m long, then a seaward boundary reaches to the edge of frontal dune scarp face to breach the frontal dune completely. 4. Average distance between the wind blowout hollows and wind blown sand areas is 113.4m in the study site.

35 Major conclusions; 5. Nearly vertical slope of dune scarp caused by storm surge and high waves enhances a wind turbulence behind the frontal dune, and causes wind blowout 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.

36 Ready for submission;

37 Time for Q&A Thank you very much for your kind attention!

38 Plan view of dune blowout Longshore distance (m) Cross-shore distance(m)

39 Dune profile without wind blowout hollow Isaku, Fukigae 10 8 E 6 l e v 4 a t i o 2 n (m) Distance offshore(m)

40 Wind blowout hollow which is cut at seaside edge

41 Spreading out of wind blown sand

42 Turbulence created behind an edge of frontal dune Snapshot behind an edge of frontal dune