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Rate of Erosion in the Tanzawa Mountains, Central JapanAuthor(s): Masaou TanakaSource: Geografiska Annaler. Series A, Physical Geography, Vol. 58, No. 3, Case Studies ofRapid Mass Movements in Different Climates (1976), pp. 155-163Published by: Blackwell Publishing on behalf of the Swedish Society for Anthropology and GeographyStable URL: http://www.jstor.org/stable/520928 .Accessed: 24/05/2011 03:06

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RATE OF EROSION IN THE TANZAWA MOUNTAINS,CENTRAL JAPAN

MASAOU TANAKA

Department of Geography, University of Tokyo, Hongo, Tokyo

ABSTRACT. The Kanto Earthquake of 1923 triggeredwidespread debris-avalanching in the Tanzawa Moun-tains, located about 50 kms southwest of Tokyo,where since then small debris avalanches have fre-quently taken place at times of heavy rainfalls.

The author observed erosion processes and mea-sured rates of erosion from March, 1970, to Septem-ber, 1973, on a south eastward facing steep slope.The rate of erosion was estimated to be on the orderof 103 m3/km2 per year.

IntroductionIn Japan torrential rainfalls and violent earth-quakes have frequently triggered debris ava-lanches and landslides on mountain slopes andhillsides, which have killed a large number ofpeople and destroyed houses, farms and forests.Some catastrophic landslides changed land-

forms remarkably in a short time (Machida,1966). However, landslides and debris ava-lanches of an extraordinarily arge scale areconsidered to have occurred at such longintervals as 100 years or more (Yoshikawa,1974), while many small debris avalancheshave taken place at times of heavy rainfallsby typhoons or "Bai-u" every year. The quan-tity of debris removed by small debris ava-lanches was not large, but their scars haveremained uncovered by vegetation for a longtime and continue to supply debris. In such

temperate humid zones as Japan, debris-ava-lanching is one of the most active erosionalagencies on mountain slopes, and soil erosionin their scars is also a dominant erosionprocess.

Physiographic setting of the TanzawaMountainsThe Kanto Earthquake of 1923 triggered wide-spread debris-avalanching in the TanzawaMountains, located about 50 km southwest of

Tokyo, where since then small debris ava-lanches have frequently taken place at timesof heavy rainfalls. Numerous scars from debris-avalanching have remained on slopes untilnow as shown in Fig. 1.

The mountains are about 1,000 m above sea-

level. Slopes are dissected by many gullies andare generally as steep as 30 to 40 degrees ormore, becoming more gentle upslope. Themountains are mainly composed of Miocenetuffaceous rocks, intruded by Miocene quartz-diorite in the central part. Surface rocks areshattered by many faults, joints and cracks,and are remarkably weathered. Summits andgentle upper parts of slopes are covered byvolcanic ash 2 to 3 m thick erupted by Mt.Fuji, which is apt to cause debris-avalanching.Slopes are almost reforested with cryptomeriaand cypress, and primary broad-leaved forestsremain only on steep valley walls.

Daily precipitation of more than 100 mmhas been recorded at times of typhoons or"Bai-u" in summer (Fig. 2). In winter, rain-falls is very rare, but snow often falls inJanuary and February and accumulates severalcentimeters deep on upper parts of slopes forabout a month.

Rate of erosion in a debris-avalanche scarThe author observed erosion processes andmeasured rates of erosion from March 1970 toSeptember 1973 in and around a debris-ava-lance scar in the Tanzawa Mountains.

Higonosawa scar, located on a southeast-ward-facing steep slope, is dendritic in patternand divided into four subscars, A, B, C and D,as shown in Figs. 3 and 4. The B-, C- and D-scars are inferred to have collapsed during theearthquake in 1923, as they are marked on atopographic map surveyed in 1929 and wasteis recognized on aerial photographs taken in

GEOGRAFISKA ANNALER * 58 A (1976) * 3 155

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! scar of debris avalanche N 0 20km TOKYO',"sample area

6 rainfall station OK\0 1 2 3km Mt.Fui

Fig. 1. Distribution of debris-avalanche scars in the Tanzawa Mountains, plotted from aerial photographstaken in 1946.

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Fig. 3. View of the Higonosawa scar. A: A-scar. B:B-scar. C: C-scar. D: D-scar. V: V-shaped gully.

pes when snow melted, and volcanic ash layerscollapsed along marginal cliffs by frost actionin winter. Depositional areas of the scars asshown in Fig. 4 were gradually filled nearly upto adjacent slope surfaces with debris by theend of June each year. Debris was scarcelywashed away by rainfalls during that period.At times of torrential rainfalls by "Bai-u" andtyphoons in July to September, marginal cliffscomposed of volcanic ash layers collapsed into

large blocks, and blocks and debris weretransported down to the main stream throughgullies in a form of mudflow. Basal rocks whichwere exposed again on the bottoms of scarswere not trenched at all, and debris was rarelyfound even in gullies or at the junction of themain stream, by the end of September. Such aprocess of erosion in the scars was repeatedlyobserved every year. Consequently, the upper

Fig. 4. Topography of the Higonosawa scar in No-vember, 1970.

_vegetated araccumulation

158

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RATE OF EROSION IN THE TANZAWA MOUNTAINS, CENTRAL JAPAN

Sept. Oct. Nov. Dec. Feb.22 22 26 24 3

/ - -

/

//

,,/1

, / ?j

.,.

- i/

I /

,.''/

100

* ab

- cx d

1970- 71-- 1971 - 72- 1972- 73I /

200 Days 300 400

Fig. 5. Cumulative area of collapsed surfaces of thepainted rocks. Marginal cliffs rapidly retreated im-

S1 : debris,_ \ base rock

cm

Fig. 6. Change of the

scar floor.

zuu JUu c n

transverse profile S1-:

1/ 1970| Jun.

6

mediately after painting and rates of collapse weregreater in winter than in summer.

marginal cliff receded about 4 m upslope inthe C-scar and collapsed 1X2X 1 m3 in vol-ume in the D-scar during the observationperiod about three and a half years. Erosionaland topographic changes were not as great inthe A-scar surrounded by low marginal cliffs,and even less in the vegetated B-scar.

In short, debris was produced by collapseof marginal cliffs in winter, accumulated tem-

Jun. porarily in the scars and was washed by22 torrential rains in summer. Such a process

occurred periodically from year to year.( Assuming that the vegetated slopes around

i the Higonosawa scar have maintained initial2Y- landforms before debris-avalanching, the origi-

nal slope of the ruptured area was restored byenveloping it with adjacent stable slopes. Then,

usingthe

mapof the scar

surveyedin Novem-

Sept. ber, 1970, (Fig. 4), the total amount of erosion,22 including debris-avalanching in 1923, during

the period from 1923 to 1970 [C] was obtainedas shown in Table 1. From results of observa-

Dec. tion, debris fillings of the scars at the end of13 June, 1970, are considered to have been pro-

duced in the preceding one year and theirgross amount can be estimated by differencesbetween accumulation surfaces in June and

n basal surfaces in November of the scars (Fig.1' of the 4). Thus, the annual net amount of erosion in

the scars [E] was obtained from the estimatedGEOGRAFISKA ANNALER . 58 A (1976) . 3

Jun.

10060/0

Jy.22

80-

Apr. May3 1

Au .Sept.

60-

40-

20

U-

0 500

--- ?t--- ",-- - - - - - - -- -- ------

159

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M. TANAKA

Table 1 Estimation of erosion rate in the Higonosawa scarA: Subscar. B: Area. C: Total amount of debris eroded from 1923 to 1970. D: Mean depth (D = C/B).E: Amount of debris annually eroded. F: Amount of debris avalanches in 1923 (F = C-47 x E). G: Rateof erosion per unit area (G = E/B).

A :B :C :D :E : F : G:m2 : m : ms/year :m3 : 10-2 m3/m2'.year

A : 394 : 370 : 0.9 : - : - : -B : 1238 : 3580 : 2.9 : 46 : 1420 : 3.7C : 950 : 1720 : 1.8 : 20 : 780 : 2.1

gross amount of debris, inferring their porosityto be 34 % (Machida, 1966; Yoshikawa, 1974).The amount of debris avalanching in 1923 [F]was estimated by the formula shown in Table1, assuming that the rate of erosion in the scarshas been constant since 1923. The estimatedamount of debris avalanching in 1923 is con-sidered reasonable as compared with those inrecent years, and the rate of erosion afterdebris-avalanching in the scars [G] is on theorder of 10-2 m3/m2 . year.

Estimation of rate of erosion by variousprocessesTo estimate of erosion by various processeson ruptured and stable slopes of the TanzawaMountains, two sample areas, A and B, werechosen (Fig. 1), where the orientation errorfor photogrammetry was least. Scars of debris-avalanching in the areas were plotted separatelyon a map of 1: 5,000 by a photogrammetricmethod using the Autograph A8 and A7 fromaerial photographs taken in 1946 and 1971(Fig. 7, 8), whereas scars less than 5 m widewere not plotted because the orientation errorwas 5 m and it was too hard to measure area ofsuch small scars.

The scars in the areas are classified as

follows. "New scars" which can be recognizedonly on photographs taken in 1971 have beenformed since 1946. Scars recognized both onphotographs taken in 1946 and 1971 are called"continuously naked scars", and those only onphotographs taken in 1946 "vegetated scars".Their areas are summarized in Table 2.

It is considered that slopes are eroded bysuch processes as catastrophic debris avalan-ches, continuous debris fall in naked and vege-tated scars and soil erosion on vegetated slopes.If each rate of erosion by the above-mentioned

processes perunit area is

known,the volume

[DV] eroded during the period from t1 to t2can be calculated from the following formula;

t2 t2 t2DV=Ka E Sai + Kb J Sb.dt + Kc f Sc.dt +

i= ti ti ti

t2

+Kd J Sd.dtti

(1)

where Ka, Kb, Kc and Kd are mean rates oferosion by debris-avalanching, by debris fallin naked and vegetated scars and by soil erosionon vegetated slopes, respectively, and Sai isarea of debris-avalanching in the i-th year andSb, Sc and Sd are areas of naked and vegetatedscars and vegetated slopes which change withtime.

Mean rates of various erosion processes inthe Tanzawa Mountains were assumed asfollows. Scars of debris-avalanching in thesample areas were measured mostly 1 to 3 mdeep by photogrammetry and those deeperthan 10 m were not found. Moreover, it wasreported in Japan that most scars of debris-avalanching were 1 to 2 m deep (Machida,1966). The author, therefore, assumed meandepth of scars of debris-avalanching to be 2 min this region. Mean rate of debris fall in nakedscars was estimated at 3 X 10-2 m3/m2 . yearfrom the result mentioned in the preceding

paragraph. Judging from Kawaguchi's review(1951) of rates of soil erosion not associatedwith debris-avalanching on mountain slopes invarious regions of Japan, the author assumedmean rate of soil erosion on vegetated slopesto be 10-4 m3/m2 . year. As mean rate of rockfall in vegetated scars is inferred higher thanthat of soil erosion on vegetated slopes andlower than that of rock fall in naked scars, itwas estimated to be 10-3 m3/m2 *.year.

When all new scars are assumed to havebeen formed by debris-avalanching in 1970,total amount of debris eroded

by variousGEOGRAFISKA ANNALER - 58 A (1976) . 360

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RATE OF EROSION IN THE TANZAWA MOUNTAINS, CENTRAL JAPAN

Fig. 7. Distribution of scars of debris avalanches in the sample area A in 1946 and 1971. Symbols, see Fig. 8.

processes in a year from 1970 to 1971 iscalculated to be 38,020 m3 in area A and44,240 m3 in area B, which are the maximumestimate of erosion rates. If new scars hadbeen formed before 1970 and debris-avalan-ching had not occured in 1970, total amount

of debris eroded during the same period wouldbe 1,920 m3 and 1,430 m3 in areas A and B,respectively, which are minimum estimates oferosion rates.

Secondly, considering the transition fromnaked scars to vegetated ones with time, rates

Table 2 Area of scars of debris-avalanching n the sample areasA: Sample area A (3.065 x 106 m2). B: Sample area B (4.208 x 106 m2). C: Scars in 1971. D: Scars in 1946.NS: New scars. CS: Continuously naked scars. VS: Vegetated scars. TA: Total area of scars. MA: Meanarea of scars RS: Ratio of scars to the sample area. R: Reduction rate of naked scars.

:N :TA : MA :RS: Rm : m2 % : /year

NS : 38 : 18,320 : 482 : 0.60A :C/D: CS : 36: 26,930: 748: 0.88 : 0.037

VS : 213 : 300,870 : 1412 : 9.82

NS : 48 : 21,730 : 452 : 0.51B :C/D :CS : 10: 4,830: 483: 0.11 :0.039

VS : 105 :227,080

: 2163 : 5.40

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M. TANAKA

v New scar

9 Continuously naked scarQ Vegetated scar

0 100 500 800 ml l l l l l I , I *I i l I I

Fig. 8. Distribution f scars of debris avalanches n the sample area B in 1946 and 1971.

of erosion were estimated. As many scars re-cognized on photographs taken in 1946 can notbe found on those in 1971, they must havebeen gradually recovered by vegetation during

the above period. If the transition of scars hadproceeded at a constant rate, its rate [R] asshown in Table 2 could be calculated by thefollowing formula;

Table 3 Estimation of erosion rate in the sample area

S: Sample area. N: Number of example cases. i: Year when debris-avalanching occurred. Saj: Area of debris avalanchesat each accident. RS: Ratio of Sai to the sample area. a: Volume eroded by debris avalanches. b: Volume eroded bydebris fall in naked scars. c: Volume eroded by debris fall in vegetated scars. d: Volume eroded by soil erosion on vegetatedslopes. DV: Total volume of erosion. RE: Rate of erosion per unit area.

S N: Period :i : Sa :RS a b c d :DV :RE.0m2 % m3 m 33 m33 m3 : m3 km2year

1 : 1970-71 : 1970 : 18,320 0.60: 36,640 810 300 270 38,020 12,4002 : do. :- : 0 0 : 0 1,360 300 270 1,930 6303 : 1946-71 : every year: 1,410 : 0.05: 70,500 132,350 3,420 6,860 : 213,100 : 2,780

A : 4: do. : '46,'49,'52,:55,'58,'61, : 3,920 : 0.13: 70,560 146,780 3,910 6,800 228,050 2,980

: '64,'67,'70: 5 : do. : '46,'51,'56,: 8,230 : 0.27: 82,300 151,540 3,970 6,800 : 244,610 : 3,190

:61,'66: 6 : do. : '46,'56,'66 : 13,720 : 0.45: 82,320 151,540 3,980 8,130 : 245,970 : 3,210

: : do. : 1958 : 35,300 : 1.15: 70,600 139,900 3,870 7,070 221,440 2,890

: : 1970-71 : 1970 : 21,730 : 0.52: 43,460 150 230 400 44,240 10,510B :2 :do. - : 0: 0 800 230 400: 1,430: 340

: : 1946-71 : '46,'51, 56,: 10,470 : 0.25: 104,700 324,350 5,260 9,860 : 444,170 4,220: '61,'66

16dGoGRFSAAN R. 58:(,'19756)' ?

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RATE OF EROSION IN THE TANZAWA MOUNTAINS, CENTRAL JAPAN

R-= Sv/[25 x (Sv + Sc)] (2)where Sv and Sc are areas of vegetated andcontinuously naked scars, respectively. If newscars formed after 1946 have also been vege-

tated atthe same

rate,area of new scars

recognized in 1971 [Sn] can be expressed bythe following formula;

1971Sn= E Sa [l-R (1971-i)] (3)

i =1946Because it is unknown when and how ex-

tensively debris-avalanching occurred in thisregion during the above period, the authorcalculated mean rates of erosion, assuming thatdebris-avalanching occured every year, every3 years, every 5 years, every 10 years and once

in 1959, and that the total area of debris-avalanching in each year of its occurrence[Sai] was same through the period, and thatthere was no vegetated scars in 1946 (Table 3).Judging from the record of daily rainfall inYokohama, about 40 km east of the surveyedarea, it is considered most probable among theabove cases that torrential rainfalls have takenplace and triggered debris-avalanching at aninterval of about five years during the aboveperiod in the Tanzawa Mountains (Fig. 2).

Debris accumulating in these 25 years from

P to P' on the river floor of the Kanazawain area B (Fig. 1) is about 50,000 m3 in volume,equivalent to a debris supply of 500 m3/km2*year from mountain slopes. Because a con-siderable part of debris eroded from mountainslopes is transported away into the main valley,the rate of erosion estimated from collapse ofmountain slopes is generally higher than thatdeduced from accumulation in valleys. There-fore, the most reasonable estimate of erosionrate by various processes in tre surveyed areais of the order of 103 m3/km2 *year.

ConsiderationThe rate of erosion estimated above corre-sponds to that of the highest class evaluatedfrom sediment delivery rates to reservoirs in

Japan (Yoshikawa, 1974). Such a higher rateof erosion in the Tanzawa Mountains is prob-ably due to frequent occurrence of debris-avalanching.

Scars of debris-avalanching inferred to havebeen triggered by the earthquake of 1923 areremarkably larger in scale and more extensivein distribution than those recently formed, asshown in Table 2, and amount of debris re-moved by this accident is estimated to be ofthe order of 104 to 105 m3/km2. Whereas debrisavalanches of such an extraordinarily largescale seem to have taken place at an intervalof 100 years or more, small debris avalancheshave occurred more frequently and, therefore,are considered to be one of the continuouslyacting erosion

processes togetherwith debris

fall in naked scars and soil erosion on vege-tated slopes. It is concluded from the aboveestimation of erosion rates that, in the TanzawaMountaines, continuously acting erosion pro-cesses compare in order-of-magnitude estimatewith such catastrophic ones as debris avalan-ches of an extraordinarily large scale in a longtime. If the above-estimated rate of erosionis applied to erosion processes in geologicaltimes, mountains might have been erodedenough to attain several hundreds meters of

relief in several hundred thousands of years.

Mr. Masaou Tanaka, Department of Geo-graphy, University of Tokyo, Hongo, Tokyo,Japan.

ReferencesKawaguchi, T., 1951: Studies of soil erosion on moun-

tain areas. (1) Statistical studies by the formerdata. Rept. Govern. Forst Experirn. Stat., 61, 1-44.

Machida, H., 1966: Rapid erosional development ofmountain slopes and valleys caused by large land-slides in Japan. Geogr. Dept. Tokyo Metropol.Univ., 1, 55-78.

Yoshikawa, T., 1974: Denudation and tectonic move-ment in contemporary Japan. Bull. Dept. Geogr.Univ. Tokyo, 6, 1-14.

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