日本の豊かな景観をかたちづくる山と川、そして滝...
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145°0'0"E
140°0'0"E
140°0'0"E
135°0'0"E
135°0'0"E
130°0'0"E
130°0'0"E
125°0'0"E
45°0'0"N
40°0'0"N
40°0'0"N
35°0'0"N
35°0'0"N
30°0'0"N
30°0'0"N
25°0'0"Nknickzone frequency 0.1 /km 0
日本列島、
滝だらけ。
使用データ:北海道地図GISMAP Terrain 50 m ▶ Hayakawa, Y.S., Oguchi, T. (2014) Spatial correspondence of knickzones and stream confluences along bedrock rivers in Japan: Implications for hydraulic formation of knickzones. Geografiska
Annaler: Series A, Physical Geography, 96 (1), 9–19. doi:10.1111/geoa.12024 ▶ Hayakawa, Y.S., Oguchi, T. (2009) GIS analysis of fluvial knickzone distribution in Japanese mountain watersheds. Geomorphology, 111, 27–37. doi:10.1016/j.geomorph.2007.11.016 ▶ Hayakawa, Y.S., Oguchi, T. (2006) DEM-based identification of fluvial knickzones and its application to Japanese mountain rivers. Geomorphology, 78, 90–106. doi:10.1016/j.geomorph.2006.01.018
早川裕弌
東京大学空間情報科学研究センター
日本の豊かな景観をかたちづくる山と川、そして滝。日本列島の至る所に存在する滝(遷急区間)の位置を、数値地形情報から自動抽出し、地図上に分布密度とともに示した。いったい、どういったところに滝は多いのか?硬い岩に多くできそうな気もするが、実はそうでない。空間解析からは、地質的条件よりもむしろ、地形的・水文的条件が支配的であると判明した。
河床遷急区間(knickzone: 河川縦断面形において部分的に急勾配で、侵食が活発な区間。代表的には滝)は、岩盤河川地形において重要な地形要素であるが、その広範囲にわたる分布はあまり知られていない。ここでは、50 m DEM(デジタル標高モデル)の河床勾配計測により、河床遷急区間を定量的に抽出し、また河川沿いの存在頻度(/km)から補間したカーネル密度分布を示した。 遷急区間は河川の上流域、河床勾配が急になっている部分、また主要河川どうしの合流点付近により高密度に存在し、多くの遷急区間が河川の水理的な作用により形成されていることを示唆する。一方、岩質や断層など、地質的要因の遷急区間形成に対する影響は相対的に限られていることがわかった。
changes in drainage area along the streams. Thelevel of possible discharge increase is quantifiedusing the drainage area ratio, AR:
AAA
R = 1
2
(3)
where A1 is the drainage area at a point along thestream course and A2 is that at the adjacent point80-m upstream. Thus, AR represents the increase indrainage area between the two adjacent points, andan anomalously large AR represents a confluence ofa large tributary between the two points. To iden-tify such major confluences, a critical value of AR ischosen to be 1.74, 1 standard deviation (0.71) away
from the mean (1.03) for all the 818 663 samplingpoints. Although such a threshold based on meanand standard deviation may vary regionally, thisvalue is still effective in detecting marked increasesin drainage area throughout the entire study areabased on a consistent standard. The points exceed-ing the threshold (7903) are less than 0.9% of thetotal 818 663 points, and the mean reach lengthbetween adjacent points is c. 8.6 km (equivalent to0.12 km–1 as a frequency). The identified majorconfluences also occur in downstream reaches, i.e.fourth-order streams based on Strahler’s (1952)method, with approximately one confluence in a10 km reach (Fig. 3). Multiple major confluencesare often found along a stream, and the order of a
0
500
1000
1500
2000
2500
Plutonic rocks Metamorphic rocks
Sedimentary rocks
0.00
0.05
0.10
0.15
0.20
0.25
320 800 1,280 1,760
Gd with d of
–5.0
0.0
5.0
0 5 10 15 20 25 30
Threshold for knickzone: Rd = 1.42×10–5
(Artificial dam)
Distance from river head (km)
Rd
(x10
–5 m
–1)
Gd
(m m
–1)
Ele
vatio
n (m
)
y = –8×10–5x + 0.2
0.05
0.10
0.15
0.20
0 500 1000 1500 2000
Gd
(m/m
)
d (m)
d
E1
E2
d/2 d/2
Gd = d
a
b
d
c
e
(E1 – E2)
Fig. 2. Longitudinal profile and distribution of stream gradient and knickzones along a river, an example from central-north Japan (theJoganji River). (a) Longitudinal profile. (b) Schematic illustration for computing stream gradient Gd at the measure scale d. (c) VariousGd values along the river, with d of 320, 800, 1280 and 1760 m. The arrow marks the location of a prominent knickzone described in(d). (d) Scale-dependent change in Gd at the prominent knickzone. The negative slope of linear fit (8 × 10–5 in this case), representingthe scale-dependent decrease in stream gradient, is defined as Rd. (e) Distribution of Rd along the river. The threshold value(1.42 × 10–5) is shown as a horizontal line, and reaches having Rd larger than this threshold are identified as knickzones, but reachesof artificial dams and/or those with less than 10-m elevation drop are excluded.
YUICHI S. HAYAKAWA AND TAKASHI OGUCHI
© 2013 Swedish Society for Anthropology and Geography4