マングローブ林の防災機能に関する研究(4)

誌名誌名琉球大学農学部学術報告 = The science bulletin of the College ofAgriculture, University of the Ryukyus

ISSNISSN 03704246

著者著者 佐藤, 一紘

巻/号巻/号 31号

掲載ページ掲載ページ p. 189-200

発行年月発行年月 1984年12月

農林水産省 農林水産技術会議事務局筑波産学連携支援センターTsukuba Business-Academia Cooperation Support Center, Agriculture, Forestry and Fisheries Research CouncilSecretariat

Studies on the protective Functions of the Mangrove

For℃st against Erosion and Destruction

( lV l The斗 protcctivど functionsof the mangrove forst

and a preliminary hydr‘aulic model experiment

。nthe root system of mangrove*

Khora sty/osa a認ainstlhc

fl＜パV、札 preliminaryhydraulic model cxpcrimenl w川 carriendout. Th巳

followings bee日meevident after cxaminin只 theexperimental rりsult日

ohlained

l f'hc Iのり1sv日tcmof R. sty/osa仁loselvinfluenced the flow O日 th《A

sh品［）ゼ andre日istibility。fit. 2. Th日 roり1system remarkably redu仁川ithe velocity in its 日xtcnt日nd

divided the flow in two and the extent of lhe influence was larger as

the depth of fl《JWbecame sh日llりW.

:l. The 1nfluいneeりfthe model remarned ves乱1the cross section about

2.:7l time日 thゼ len只thof the lhickness of the model in downflow dir引〉

I ion from the lower end of the model.

Introduction

189

Thぞ nwnifoslationsof th巳 functionsof man符rovefores仁川 theace日！引at1onりfthe

ー八 i川 rtof this I叶＞Oriwa日 presented 品1the 8

190 Sci. Bull. Coll. Ag-ric白 Univ.Hyukyus, No.31 (198~ ）

！）！（）只rndation.theる prけteclion of I he cけastlines and thゼ日tabilizationけ｛仁h孔nnel日 111the deゐ

I 1ι：1. hλve been po111tcdけuthy many n・日carchers.11It i日与凡Ii日factorylu consi〔lerth日tI hese

lllilll if川｛く：11onsresult from the buffer fu日ctiけnof 111凡口氏r〔JV

191

九八TO・fvlのd（イけfroot of Rhiw1りhorastylos11

thee日iU乱ry.it protects the hiink日必1in日iド1りsion川 itrcsi日l詰 aga111日1slrt:am flow. and preザ

vents ch社11nelsfrom shiftrng theぐりurse日りIthem

192 おく~i・Bull. Coll. Agric. Univ. Hyukyus. No. :31 ( HJ84)

Human works D工saster Action e

vt

os

re

qJr

no

azt

M

Natural phenomenon

Agricul七ure

工ndustries

wave

蜂 ー ャ Destruction 犠ケ＼？ '¥Great 一、、日rosion 繋h ・・B ・ earthquake

．，．ー 『吋 Flood録曲 Heavy rain _ dimensions

degree ll

I hese func、tion日 like a ferro仁川1crctcbreakw乱tcr，乱ndto plan such mangrove fりrestunder

the、generalconditions. In future. we must estimιate scientifically lhc role an〔iutilize

actively the protective functions of man♂rove fores!. In case of札口礼ttackof bi必 lid孔l

wave、thema口氏roveforest m品ybe destroyed‘ but the destructive ener必ymu日tbe highly

redu引：d.So such man只roveforest may bむ regarda日aself日 crificin符 forest.

Preliminary hydraulic expeliment on the root system

The invc日ti必1t1onon the characteristics of the 1001 syslem of Rhizoρhora日pp.anwn必

I he other m日日必rovesis乱ninteresting, important and basic problem for the cstimatiりn凡nd

ut ilizatiりnof the protecti v日 functiりnsof man只roveforest. The model experiment is usdul

for an understanding of the charact日risticsas the field observation is accompanied wilh

判V《~ral difficulties. Bui there is a problem of modelling of the root system. Indeed、h。wto model the root sy日ternりfa tr日Cor a stand will be recognized as problem in fulurc

（〕nthe root systemりfRhizof>hora stylosa in Okinawa. the relation日 betweenthe height

from the le’vel of th日 mudsurface around the base of main日！日m (z) and t h日 numberのi

prop roots (11) wer仔りxamrncd-° Two I日日rθssionゼquationswere applied司j〕礼rtλboveand

below a certain height (he）乱日りneequation was not日わlじ tobe appli仔dto any invest只ated

trees. The separate exponential funclio日日 wer日 well品pplicdabove or below h正 Acぜrtain

Si¥TO : Model of r Oりlof Rhizυ＇flhura stylosa.

hりi只ht(he) was仔xpres日cdas fol low目。

he= a・ hmax十え

where . h11111.r口 highests1〕routedpoint of prop root （正：m）‘

a. Ii= coefficients‘

The r C只ressionぜquatio川 hctween z and 11 were obt日Incd as the rりllowin只form日、

if 0 ~三 ：：： ;s; lu二d・I lサ／・

JI ニニ c . t 、

if he二三士豆 hmaぷ,d・ll .; I I .／’） /,( I I

月二二 c.イ

whet・e・ II口 treehci只ht(m）句

11 = number of pro1〕root、

z = hei符htfrom the base of main stem （じm).「、 d.j‘j ニぐりdfici計nts.

193

Thou只hthese equations have to be examined closely in future. it is considered that

the equations are useful for modelling of the root system of R.. stylosa as a characteristic

uf increa~五 i 11京日uddenlythe numberりfprop roots near mud surfac口、 isexpressed well.

A.Methods

l) Experimental flume

The‘expぞれ口氏、ntιdflume was the woりdenopen flume coated with paint and had the

dimension of SO cm d＜、日［〕 bySO cm wide by 12 m long. The water in a flume was circu-

lated through a return channゼ1.The flow rate of water was measured with a m日asurin符

triangular weir・andwas adjusted tけ therate corresponding lo a mean velocity of about l l

cm/sec.

2) lV!odd

The model w出 the、geometricmodel basゼdonly on the shape of the root system and

was not consideredυn the iコroudemod日1On the root system im日誌ined.th日 extentdistri-

but in符 proproots was 1 m high by 2 m wide by 2 m thick. 'The length ratio was decided

to be ・1 in consid日rationof the dimension of a channel. On the vertical distribution of the

numlwr of prop roots. ihc line shown in F；ほと wasdecided to l日 similarto some exam-

pies measured in the field. While the number of prop roots distributed at each height、

was con日id（マedto take th日 same value with the line in 1才i民.2、eachroot w品目 set on the

plywood roughly symmetrically with respect lo the stem. The material of the model w削

wooden bar of dianH、ter10 mm. Th巴 modelwas fixed by an adhesiv日 ona plywood of the

dimension of 10 mm thick by SOじmwideも by250 cm lo日仏 Thisplywood and mod日iwas

varnished and fixed on th日 bedof fl umc. Th日只apsat the ends of plywood with the bed

of flume were filled smoothly oil clay.

3) Meas日renwnt日

The flow vel＜〕（、itiPswere measured with a sm日IIpropeller-type flow meter with a

194 Sci. Bull. Coll. Agric. Univ. Hyukyus, No.3l ( 1984)

puisC' countt'r. ThC' I（）（、ationwhcrl' the vcloc・itywas mea日urcdwas expressぞdin three

dimensionalれ iordin汀tcswith tht、unitof

195

トき八1、（） : Modtd of root of Rhizo/;hora sty/osa.

oi

一

山

マ

一

一

三

＼

一

b

＠

／

／

＠

一

い

＠

心

一

yん絞れjHAf

一一

LV14鶴イ町議↓；

side right v 25

20

10

Fhu i

fj

（｝

（

円

｛

）

円｛］門凶

ωQ

lo I hγty1〕icalflt川’ IV('rt＇りわ肘 rvドdpartially ｜）じcau日ethe slight llH'i1!1de1sけffl引woc口irred

日目 lhe reel if iγal iり11のir iりwwas not t'll

196

Sci. Bull. Coll. Agric. Univ. Hyukyu日， No.31(198'1)

1., I v = 10.7 cm s問 一一一鴨O:''-i帽 1・

lυぷミ議委第盛向山一← エf ... 7'・・ 〉一一ぷλ.r.～＋－~~」＠ σモrMこ／’＇／／’

> l)lil ；一 (J ~） 0 一－1 一寸ト11・開ー“ 11.:) 0 一一一一一一時d

I IH

-25 1・2 I 2ち’ ・l50 cm

一25 ・12

Fi只ろ 11.I Chan努（？のfvclo('ity by JV!odcl at each depth

I a ・1 before sett in符口10dcl (b) after settin只model

Secondly‘孔hovethe extent of about l 0 cm from the bed. the velocityりfintermediate

flow between the mid-flow and the flow near i〕ythe side wall is fastest as the profiles at

X 出土ll an〔ix z 士2:3intersect and replace. llowevcr the vel〔1city。fflow al mid-flow is the lowest in comparison with other profiles and is faster abov日 theextent than bclc川V

礼ndshows a tendcncv to beじりmefaster as it comes near the wat日『日urface.ドromthis

matter. it seem日 tobe quite reasonable to consider that the flow礼bovethe extent distri-

buted root日 modell日dpresents the circumstances shifting to the condition 九九＇ithoutthe

model. In札口！Se日tmore deeper position. it is assumed th日lthe pre〕fileat x口 0al日ore悶

plac('s and each velocity of flow at x ニロ（）‘ X 1 土11‘X ご江主2:31日faster in the same or〔！日1

in upper layer.

Thirdly. at each x-coorclinatc the velocity is slower in the. extent distributed density

root日 asthe depth (h) becomes deeper and. in contrast with this、isfaster above the ex-

tent as the d日pthbecomes deeper. Thi日 m日tteris shown distinctly in the left side of Fi箆

11. 'l、hestrict comparison of each profil巴 isnot appropriate for the m日anvelocity in each

九八’i、0: Model of rりりtof Rhizoj;/wra sty/osa

197

へ「＼〉

[,j

Io

b＼〉

υ付

、Jれり

-25 0 +25 ト50 i 75 + 100 十125 →150 -25 十2÷25 ギ50 '1'75 1 !00 +125 十150y cm y erれ

hv＼〉

－

ワ6｝

（ （）わ

1.2

hv＼〉

。メ

｛）わ

25 0 十25 十50 i 75 十 100 +125 +150 ,-25 十2 十25 i 50 1・75 ←100 +125 +150 y cm y 口n

ドl符.5 i2) Chan日じ ofv日locilyby Model at each depth

ta I b日forf'.sdting model (b) after scttin日model

depth i日 different.But if it can be assumed that these experiments h孔vel〕eenc司Iried out

under an approxim日tcme札口 velocity、it is consid仔1edth乱lthe water c日nflow faster in

日pa仁りのflcs詩 resistanceabove the extent whゼll th日小：•pth is d日eperas the V日locityis

slow日rin the extent of root system model！仔d.The facts mentioned礼boves口氏広刊tth孔tthe

root system of R. stylosa must closely affect flow on the shapむ礼ndresistihility of it

Figure 5 show日 thecomparison uf the cりnditionof fl川 V in channel when the model

is set up札口dis日ot.The vclocitv at three kinds of the relative position‘z/h = 0.9、0.5() 2‘was drawn with the ratio白山1inおithe mean vdo仁ily.

Vvhen the modd is not日etup、theVゼlocityon each depth is the呂lowest只encrally

11('社rby the side wall. thou只hthe condition of flow is ［社irlydifferent at f乱chsection.

Thou氏hthe condition of flow日tx = 0 is not so different from it at xニニ十 11.the v日locily

at xココ0can be assumed to be slightly faster th乱れ ii品tx口十 l1.八leach coordinate of x

。、 ート l l 、→ 2.3, ドl以 1 ぉhows品 general tendency of the velocity to 1〕fslower as it be命com円 IH'礼rI〕ythe lwd. /¥s孔 wholetherゼ isdisorder by slight meander日， hut the concli・

198

Sci. Bull. Coll. J¥g-ric. Univ. Hyukyus, No.31 (198tl)

l l () 1lりrflow can bγi C必ardcdlo be genがIally日1mi！日rwith the flow in a lypic孔Iopen仁han・nel. ’rhc rnclin日tiりnform the mean velりcitvi日礼1most恰 ithina i礼丹氏じり［土0.2.

Wht、ntiH、modeli日setup、the、conditionof flow at c’乱chX…coordinate i~ remarkably

different in th（、 extentof model. Al x ＝十：：n.the velo仁川yat日achrelative position is II日‘fastc川、江ll〔Ion I he cのnlr礼rvは（ xロ 01t is lhe slowest. /¥t xヱ十日、 ihe velocilv礼1the re・

!alive pり日111011of z/h口けり isfasler th社nthe me日nvelocity （マト but‘althe relative po日j.

lionりrz/hニ＝ () 5 or 0.2. 11 i日 sluwerrou只hly./¥s compared lhc、conditionりfflυw at x →l 1 with each川 heral立只ivendepth. 1n case of hロ l0 cm. broken line日 ar1:I川、いwthe

Jinじり｛九／むロ l.O.i. c. llw velocity ale日chrelative pり日itionis slりwcrlh日nv. in c礼SCof h

15 cm. t如、 brokenline of z/h切りρiscl＜日irvabove the line of v／守口］（）‘ incase of h

2() Cl日、 1如、 broken line of z/h z り5al日。只el日lightly！］（汀ll the line of v／すコゴコ lO、ζand、in

t日日日けfh ニ 2ろ（＇Ill、lht' broken linesけfz/hヱエ 0.9and 0.5社 内 abov日 theline of v／守口］（）

/¥1 x =O可 lhψ 川 lidline show that lhe model has a日lronginfluence on the dee℃lcration of

thり ilow in 1hi日！）凡rt、1c. the mid flow. The solid line of z/h口 0.9じり日1e日日plo the line

り｛ v/v ニニ l .O n日 lhe （！日plhbecomes deeper In case けih 口 25Cl孔 this line is above the

line of v/vニ］.｛）.’！＇ hi日 lllilll台rsh（川’日 thatthe modぞireduces remarkablv lhc v日locitvin

thi日 extenland divides the fl＜〕win lwo and the extenl竹fthe influence is lar只eras the

d付1lhof flow becomes shallow

In the lowぞrcourse of flりwfrom thじ model.lhe dotted lines at xコニ -t2:3 show日町一

ducing tend即日cyand、。nthe contrary、ihe broken lines al xニ二十 ll and the誌のlidlineお al

x'=Oshc川川linc、rcas1n日lcndcncy出 thedeplh b閃りm日日 deゼper.This su符日開Isthat the

flo日 triesto m日inlarnit日 nri引nalcondition and the de只lee lo rdurn i日 largeras I he

d日pthbecomes deeper.’fhe influence 。fthe model remain日yet内i礼町ctiりn2.fl times as Ion只品目 Ihe t hicknes日 of‘ihe model downfluw from the lower end of the model. In the ex

tent。flhc modぜIor just lower courseりfflow from the model, the inじi1 nal ion I礼ngefrom th＜、 meanvel川、1tvis larger as the depth becomes sh孔llowゼIand、incase of hニ 10cm‘it

is f刊 Ill 十 0.:)4lo -0.71 in case of hエニ 15cm from ト0.113to 0.112. in ca刈ぱ hヱニ 20

仁川‘ fromト0. .39 to一0.4Gin仁社日eof h口 25cm、from+ 0.32 to -0.112. The facts discribed above礼reconsidered to cxpained well that、inthe mangrりve、

ふirest、thesedimentation is accelerated、the只rainsize‘of日cdirnentis relativ肝lyfi n官、 the

日巳di日1entonce deposited is hard lo be eroded、1hcpro符rad乱tionwilh the ma口紅rovefo1 esl

品目 the tot品ire日ultsof these matter出 ispromoted mりreand the bank in estuえiryl詰ドrolc~ctcd

in th肝日imilarmanner with the previous日purjelly.

I would lik日 to肘xpres日 th('deepest appreci礼tion to Dr. Kozo Sato、l〕rofessorof J.

watc Univc1sily、Mr.Hirohaiu T品kahashi.as日ociateprofessor of Iwate lJ niversity and

Mr Sadanori Aoki. professor of !¥kit乱 A日ricultural.Junior Colle、只C、iりrtheir 、＇aluable

advice and facililic、S日corded.

199

s八丁Cl: l'vl＜〕delof root of Nhiz1ゲ＞homstyli＇人W

References

I l clt、laCruz A. J¥. ( l 979) Th日 func、ti（）日日。rman只roves.rrnrn:op SpPC. Pulil. No. 10

: l 2::i-l :38

2) ！弓hikawa,M.、！〈日dり.T. 乱ndMatsuoka‘II. ( 19お：3).Theぱfectsand damλ只cof theじり出ー

tal prevention forest by the tsunami induced by the Nihonkai chubu jishin. Chizan.

VO I. 28、No.4. 4…lり（injζa pan附け

:r1 lくりki、ど (l ~J78) 日tudyon flyin日saltin Okinawa frけmthe vicwpけintof sea日hor日仁川1匂

ser V礼tiりILSげ.Bull。（：。iL i¥gr Univ. l¥yukyus N引. 25 : 1129 ::i::i・1 1¥) Sato. K. (197お）.Studies on the protc~clive functions of the mangrove fore日lag礼inst

erosion and de日truction( L ）出、lBull. Coll. Agr Univ. f

200 Sci. Bull. Coll. Agric. Univ. Hyukyu日， No.31(1984)

マング口一プ林のIVj災機能Jこ！期する1Ul)E

i;fj 4報 マングロープ林の防災機能の検討

と線系iζ｜却する予備水］理模型実験

ff,: Ii長 P 昼紘

要 約

ング仁］ーヅ村、か、 I'I r1; f~i Ji"I: 0） ，っとしぐら’｛1>.J ゐ •Jrではたしごいど〉と々えられる，やn 々の防災機lie

｛こ ω）い一ζ杉山、；し／よ ｜’｜夕、上irn，と人l1''ijO）＇／，＂＼・；：：＇.）との 11¥J℃＇！：.4ゐ災；l；と，マンクζl…ブ朴、(i)／）イJ、わりイ＼：：＇ ll}j災という rll1.'Y1• り•！W'I！し， jχl1r、した（

ヤコ二ヤマヒルギ（ Rh氏。＇／＇hornパtylo.川） (/).jj{,f~υ）ある似の十日＇l'U~lll いて，そのがWiこりえゐ：；！5~！·~：につし》

f'(ii/iil'~ ノjく芦H’：）~似合 ir った J 寸の~：，＇ i '.+!. おお上・cl欠υ）•J(/1 、i智！らかに ！；~ っ fごり

I. IJ~ コiミの；；！5 子~＇イr ’：三 IJ cい；：，， :1QilJll ：よ， H心.v~()) Jf:5 そよくばil'!i していゐ’j （から， tJ~,f, Iよ，そU)IJ.¥:l/L'l'U: , ~＂ i本〈／）｝削丈とと、強l< i~ ［ .JL i こ；；引~~： Jを＇J.Jミ乙内

2. 十日子？といlでi.J., 1/;frli が%しく品主今、し，｝，＇；jfJll に ifiii~jO!ijY/}Jff•J· ゐ｛'ii＼分ができ， i/;tれ ~·：，たさく . ))' Jど） v

:i. HUじの｜、ilitご！之、徐々 に）［：i)) i~i :I l ()):IJ¥fi!.': i二iU寸るが． ド i~dlllilこ，似ふの絞りii 0)2. :i ｛，＇~ ()) J二二／〉ぐも

+J~ ，1ミ（ノコ；；！；＇.＼！・1』 i.J伐〈｝、