uyen tap Cdng trinh Hpi nghi Khoa hpc
ffhpc Thuy khi Todn quoc, Phan Thiet, 2008 215
Nguyen ciru kha nang giam song cua rung ngap man Viet Nam
Ho Viet Hiing Dai hoe Thuy loi
Tom tdt: Mdt trong cdc bien phdp bao vc bd bidn vd de bicn Id Irong rirng ngap man chdn sdng. Khi cciy ngap mim phdt trien tdt se tieu hau nimg lirgng sdng. lam giiim chicu cao .wng. too thdnh nhirng hdng riio xanh bao ve bd. tdng kha nimg idng ^dgng phii xa vd tgo nen nhiing vimg ddt bdi ven bien. Bdo cdo ndv trinh bav kit qua nghien ciru khd nang tieu hao ndng luong sdng. tinh todn he sd glum sdng cua rirng ngap mim bdng cdc thi nghiennmd hinh vd so sdnh vdi cdc sd lieu do dgc thuc dia cua nhiivg nghicn ciru trude ddy. lu do kien nghi mot sd gidi phdp bao ve de biin bdng cdv chdn sdng.
Abstract: One of the measures in protection of coastal dykes and littoral Is wave fender mangrove forest. When salt-marsh plant develops well there will eonsume
wave energy, alleviate wave height, constitute green hedges protected shore. Increase deposition capability oj alluvium and create coastal alluvial regions. This Report presents study result of wave energetic loss capability by mangrove forest calculates wave step down ratio by model experiments and compares with measurement data on the field of researched in the past, since then propose a number of solution to protect coastal dykes by wave fender tree.
1. Vai tro ciia r u n g ngap man trong viec bao ve vung ven bien Viet nam
Tir nhirng nam dau the ky XX, nhdn dan cac viing ven bicn phia Bac nude ta da trdng mot sd loai cay ngap man nhu trang va bdn chua de chan sdng, bao ve de bicn va viing cira song. Mac dii thdi ky dd dc chira dugc be tdng hod va kc da nhu bdy gid nhung nhd cd rirng ngap man (RNM) ma nhicu doan dc khdng bj vd khi cd bao cap 6 ^ 8 .
Mot sd vi dy ve khd nang cua RNM trong viec giam thieu tac hai cua sdng bao, bao ve de bien cd the thong ke dudi day. Nam 2000, con bao sd 4 (Wukong) vdi sue gid cdp 10 dd bp vao huyen Thach Ha, tinh Ha Tmh, nhd cdc ddi RNM trdng d 9 xa viing nude Ig nen he thing dc sdng Nghcn khdng b\ hu bdng. N I U khdng trdng RNM chan sdng thi de Ddng Mdn da bj va va thi xa Hd TTnh da bj ngap sdu, thiet hai do can bao nay gay ra sc rat nang nl. Thang 7 nam 1996, khi con bao sd 2 (Fiankie) vdi sire gid 103 - I 17km/s do bd vdo huyen Thai Thuy (Thai Binh) nhd cd dai RNM bao ve nen dc biln va nhieu bd dam khdng bj hu hdng, trong liic dd huycn Tiln Hai do pha phan Idn RNM nen cac bd dam deu bj xdi Id hoac bi vd. Nam 2005, viing ven biln huyen Thai Thuy tuy khdng nam trong tam bao sd 7 (Damrey) nhung sdng cao o sdng Trd Ly da lam sat lo ban 650m de noi khdng cd RNM a thdn Tan Bdi, xa Thai Dd trong liic phdn Idn tuyln de cd RNM d xa nay khdng bi xat Id vi tham cay day dac da lam giam dang kl cudng do sdng. 0 Thai Thuy, cd 10,5km de bien dugc bao vc bdi RNM hdu nhu khdng phai sua chira, lu bd bang nam kc tir khi RNM trudng thanh, khcp tan.
Ngodi ra, RNM cdn cd lac dyng trong vice bao ve diil boi, chdng xdi Id, ban che xam nhap man. Re cay ngap man chdng chit, dac biet Id nhirng qudn the thyc vat lien phong mgc day dac cd tac dyng idm giam van tdc ddng chdy tao dilu kicn cho tram tich bdi ty nhanh hon d cac vimg cua sdng ven biln. Chiing vira ngan chan cd hicu qua boat ddng cong pha
bd biln cua sdng, dong thdi la vai can him cho tram lich lang dgng. Vi du nhu, hang nam' vime cua sdng Hong tai Ba Lat tiln ra bien 60--70m, mot so xa d tinh Tien giang, Ben tre \ dat boi ra biln 25-^30m, Tra vinh, Sdc trang 15--30in, Bac lieu, Ca mau 30^40m. 6 viing:
cua sdng Idn nhu be thing song Hong, sdng Cuu Long, phu sa thuong ngung dgng tren
\bn<2. sdnu va ngoai cua sdng tao nen nhirng hdn dao ndi. Neu dieu kien thuan Igi thi ehi sau mgnhdi'"gian, cac loai cdy ngap man tien phong se den cu tru tao mdi trudng cho nhung loai cay d^ln sau va ddt bdi dugc ndng ddn Icn, nhu Cdn Ngan, Cdn Lu d Nam Dinh, Cdn : Vanh d Thai Binh. Cdn Ngoai va Cdn Trong d Tay Nam mui Ca Mau.
Hon the niia. RNM con cd lac dyng tich cue ddi vdi nidi trudng sinh thai, gdp phdn dilu boa khi hau. Vi du nhu RNM Cdn gid, TP Ho Chi Minh dugc xem nhu la phdi xanh cua thanh phd.
2. Mot so ket qua nghien cuu he so giam song
Cac lac gia Schiereck & Booij (Irong "Wave transmission in mangrove forests") da dua ra kit qua nghicn ciru he so giam sdng (K) khi truyen qua rirng ngap man, he sd nay la mot ham so cua he so sire can (CO- Trong dd K = H/HQ, vdi Ho la chicu cao sdng cd y nghia tai nude sau, H, la chicu cao sdng cd y nghia tinh loan sau khi truyen qua dai rimg rgng lOOni. Hinh 1 dudi ddy minh hga ket qua linh loan he sd giam sdng cho dai rirng rgng
100m; chieu cao sdng cd y nghia lls=0.4*d, trong dd d la do sau nude. Ba mat do rirng khac nhau da dugc xem xet: Rirng thua (ung vdi dudng cong Ircn cung), rirng day (irng vdi dudng net cham dudi ciing) va rirng vdi mat do trung binh (dudng cong cdn lai).
OJ
0.1
0.4
OJ
•VAK
•AvinJ
"DCNSt • •
. - " •
^ - • ^ ....••
OJ 1 I J WATEROCfTH (m)
Hinh 1: He sd gidm sdng eho ddi rirng rdng 100m, H, = 0,4*d
Tir nghien cuu cua minh cac tac gid da cd nhirng nhdn xet sau: Bg re cua cac cay duoc, dude ddi, man trang, man quan (mdm) cd tac dyng giam sdng Idn ban nhilu so vdi than canh, dieu nay ddng nghia vdi viec hieu qua giam sdng se gidm khi do sau nude tang;
Chu ky song co anh huong nhd tdi su triet giam sdng; Vdi viing nude cd chilu sau nhd ban Im, song t.eu tan khoang 50% trong rimg ngap man vdi bl wng lOOm; Vdi cac vung nude sau hon, song t.eu tdn khoang 20 . 30%, Vi vay tac dung gidm sdng cua cac cay ngap man trong thai ky nuac dang se bi han che ban.
Yoshihiro Mazda, Phan Nguyen Hdng vd cdng su trong bai bao "Vai trd chdn sdng va
bao \'c bo bien cua riing ngap man ddng bang Bdc Bg Vict Nam" da kit luan rdng, rimg
^ Viet Hiins -.j.^
trang trdng d khu vyc ven biln xa Thuy Hai huyen Thai Thuy, linh Thai Binh cd tac dung iang kc trong vice giam tac dgng cua sdng lirng cd chu ky sdng tir 5 - 8 g.dy va do dd ddng vai tro quan trgng trong viec bao ve bd bien. Dai rimg ngap man 6 ludi vdi chieu rgng 1,5km cd the giam dg cao sdng tu Im o ngoai khoi cdn 0,05m khi vao tdi bd. Do mat do cay ngap man phan bd dcu trong loan bg do sdu vimg nude nen hieu qua guim sdng hdu nhu khdng ddi cho du do sdu myc nude cd tang len. Cac tac gia cho rdng cdn liln hanh nhilu nghien ciru ban nua de dinh lugng sy phy thuoc cua hieu qua giam sdng vao do sdu mire nude, chu ky sdng, do cao sdng, lodi ngap man va khoang each giira cac cay ngap man.
Trong bdi bdo "Sy gidm sdng irong rung ngap man cd loai bdn im the", Yoshihiro Mazda va cdng sy da xdc duih he sd giam sdng tren mdi don vi chilu dai theo hudng truyin song nhu sau
AH I
r = (1) H Ax
trong dd: H - chieu cao sdng Idn lai mdi tram ven bien bat ky; AH - lugng giam chicu cao sdng d tram ven bd; Ax - khoang each lir tram ven bd doe true x, iheo hirdng sdng truyen.
Klii nghien cuu dgc bd bicn xa Vinh Quang, huyen Ticn Lang, thanh phd Hai Phdng cac lac gia thay rang: He sd suy giam cua sdng bicn a vimg nay thay ddi Idn theo giai doan iricu do cau true thdng dung dien hinh ciia cay ban, Trong khu vyc cd RNM, he so r giao dgng lir 0,001 den 0,004 khi do cao sdng lang lir 0,12m den 0,16m, Dac diem nay ngugc \di lac dpng do ma sat day. 6 vung nude ndng, vi re hd hdp cua cay ban thon ddn hudng Icn ircn, tac dgng ciia lye keo ciia nhirng re nay ddi vdi vice gidm sdng giam nhicu khi myc nude tang, do vay he sd suy giam sdng giam dang ke, Mai khac, khi myc iiirdc dat den do cao ciia tan la va canh ram rap cua nhung cay ngap man nay thi he sd suy giam sdng lai lang theo myc nude tang, Hon nira, tai myc nude cao nay thi he sd suy giam phu thugc Idn vao chicu cao song. Thyc tc nay cho thay tan la cdy, ibam cay ngap man ram rap cd tac dpng Idn den sy phdn tan ndng lugng sdng, dac biet trong cac con bao va sieti bao.
Phan Nguyen Hdng va cdng sy khi nghien ciru sdng truyen qua cac dai rirng ban chua thudn loai d khu vyc Cdng Rdc, xa Vinh Quang (huyen Tien lang, Hai phdng) trdng nam 1995, cd chilu rgng vdo thdi dicm nghien ciru (2005) khoang 920ni, chicu cao trung binh ciia cdy la 8^9m, dudng kinh Id 15^18cm, Thdi gian do sdng tir l l h l 5 ' den 15h45' ngdy 17/8/2005, Kit qua do do cao sdng cho thdy: Luc trieu cao nhat kel hop vai gid diing budng vao RNM, do cao sdng trung binh tai Nung nude nong each rimg 180m la 0,43m, sau chan Kriig bdn cd do rgng 920m thi do cao song da giam xudng cdn 6,8cm. Do cao va he so suy giam do cao qua mdi doan rirng la: Sau 100m dau do cao con 0,39m he sd suy giam la 3 T/'o, sau 300ni cdn 0,19m vd he so la 57%, sau 920m cdn 6,8cm la 84%,
Rimg trang d Bang La, Dd Son - Hai Phdng dugc trong lir nam 1997, den 1999 trdng xen ban chua vdi mat do thua, Tai thai dilm nghien ciiu, chieu cao trung binh cua trang la 1,72m (5 tuoi) va 1.95ni (6 tudi), ban chua la 3,8ni, chieu rgng dai rimg khoang 350m, Thai gian do .sdng tir I Ihl 5' din 15h45' ngdy 17/8/2005. Ket qua tinh loan va phan lich cho thay:
Liic trilu cao nhdt kit hgp vdi gid dung hudng, do cao sdng trung binh lai vimg nuoc nong each rimg 150m Id 0,42m, sau chan rirng trang cd dg rgng 650m thi dd cao song da giam xudng chi cdn 2,5cni, Dd cao va he sd suy giam dd cao sdng trung binh qua moi doan rung la: Sau 150m ddu do cao c3n 0,195m va he^d suy giam la 53,7%, sau 250m con 9,4cm la 77,7%, sau 350m cdn 5,4cm la 87,4%, sau 450m cdn 4,9cm la 88,4%, sau 550m eon 4,3cm la 89,9%, sau 650m cdn 5cm la 94,1%.
218 Nghien ciru khd ndng gidm sdng cua rirng ngdp mdn VietNam
Tac gid Vii Doan Thai ciing chgn rirng trang (lai Bang La) va rung Ban (tai Vinh Quang) Hdi Phdng de nghien ciru, do dac sdng va tinh todn he sd suy giam do cao sdng trong bao. Ba ccm bao: sd 2 (ngay 31/7/2005), sd 6 (ngdy 18/9/2005), va so 7 (ngay 27 va 28/9/2005) dd bg vao viing ven bien Hdi Phdng, vdi do cao sdng Idn nh<4t ngoai khai tuang irng la 4,26 m, 3,4 m va 4,18 m, myc nude cao nhdt tuong ung la 4,26 m, 4,0 m va 4,18 m (Tram Hdn Dau), Kit qua do dac va linh todn tai hai khu vuc tren cho thay: RNM da lam giam dang kl do cao sdng trong bao, Trong con bdo sd 7 (28/9/2005), ddi vai ning trang cd do rdng 650m a Bang La, rirng ban X H- 9 tudi cd do rgng 920m vd 650m, do cao sdng sau rimg giam tu 77 ^ 88%. Miic do giam sdng trong bao khi qua rung vdo bd phu thugc vao kieu cau tnic - loai rirng ngap man vd hudng sdng truyen. Ddi vdi rimg trang, he sd giam sdng cao han so vdi rung ban (riing trang giam tu 80 -H 88%. riing bdn gidm tir 77 -^ 81 %).
3. Nghien ciiu anh h u o n g ciia R N M toi chieu cao song b a n g thi n g h i e m mo hinh
3.1 Thi nghiem md hinh
De phuc vu cho cdng tac thiet ke md hinh, cac chuyen di thyc dia tai ba tinh Hai Phdng, Nam Dmh vd Thai Binh da dugc thyc bicn. Cac thanh vicn trong nhdm nghien ciru da lam vice vdi Chi cue dc dicu va Phdng chdng hit bao cua hai tinh Hdi Phdng va Nam Dinh, di khdo sat cdc tuyen de bien cua ba tinh vd mot sd khu vuc cd riing ngap man, tiln hanh do dac vd Idy sd lieu ve cac dac trung cua rimg ngap man, bai tiarde de va thu thap s6 lieu ve cdc mat cat ngang ciia dc.
• Sau dd, cac thi nghicm da dugc thuc hicn trong Phdng thi nghiem Thuy luc tdng hgp ciia Trudng Dai hge Thuy Igi theo sa dd nhu d hinh 2 dudi day. Thi nghiem dugc tiln hanh tren mo hinh chinh thai, long the. Vi dnh hudng cua trgng lye la chu ylu nen md hinh dugc thiet ke theo lieu chuan Frond, vdi ty le 1/20, Thi nghiem dugc thyc hien vdi hai md hinh bai cd chieu dai khdc nhau va hai md hinh riing vdi mat do khdc nhau tren cac bai dd. Cy the nhu sau:
Md hinh de: De dugc chi tao bdng gd theo 2 md hinh thi nghiem khac nhau, md hinh thii nhat cd chieu cao 50cm, md hinh thir hai cao 75cin; mai phia biln m=3; mai phia dong m=2; be rdng dinh de 25cni; khdng cd CO, khdng cd tudng dinh.
- Md hinh bai: Bai dugc chi tao tir vira xi mang, thco 2 md hinh doc Idp tuang irng vdi cac md hinh dc. Mo hinh 1 ddi 6m ggi la bdi ngdn, md hinh 2 ddi I Im ggi la bai ddi - cac tir hdi ngdn va hdi hdi d day dugc diing dl phan biet cac trudng hgp Ihl n^\Mcm {khdng phdi Id cdc khdi niem vd bai), moi md hinh dIu gdm 2 phdn chinh la phdn bai \a phan chuyen tiip:
o Bai ngan: phdn bai ddi 3m, chilu cao tinh tir day mang 22,5em, do doc bang 0 (bdl gdn nhu ndm ngang); phdn chuyin tiip dai 3m cd chilu cao bien doi dan tu 22,5cm den 0 (do doc m =0,075)
o Bai ddi phdn bai dai 6m, chilu cao tinh tir day mang 22,5cm, do ddc bang 0; phan chuyen ticp dai 5m cd chilu cao biln ddi ddn tir 22 5cm din 0 (dg ddc m =0,045)
- May tao sdng ddt each md hinh 45m vl phia ben phai. Hudng truyin sdng duac chi dmh bang miii ten tren hinh.
{nTiet Hiing 219
Ddu do sdng sd 1 (WHMl) dat d viing nude sau irudc bai (ngoai viing chuyen tiip). Cac dau do sdng cdn lai dugc bd tri vdi khoang each thich hgp luy theo thi nghicm vdi bai ddi hay ngdn va se dugc trinh bay ciing vdi ket qua do sdng.
SOanviTScai
WHM4 VI/HM3 WH(u12
n a n
Hinh 2: Bo tri tdng the cdc thi nghiem - Md hinh rimg: cd hai md hinh vdi cac mat do cay khac nhau;
o Riing day: cac cdy chdn sdng dugc trdng theo hang vdi khoang each giira cac cay la 20cm, cac hang cdch nhau 10cm tren sudt be rgng mang va trai dai tir vi tri each chan de 20cm tdi phdn chuyen tiep ra nude sau.
• ' o /?«-«^ r/iM'fl: khoang each giira cac cdy va cdc hdng la 20x20cm.
Trong pham vi cua nghien ciru nay, nhdm md la mdt each dinh tinh va budc ddu dinh luong qua trinh tieu tan nang lugng sdng qua cac loai rung ngap mari khac nhau, loa, cay
u!,c chon d l thi nghiem la cdnh ngdu vd. hinh thuc thdn canh va tan la tuong doi phu hgp ho v? e'md binh b'oa rung cdc cdy hg bdn cd do mdi khoang 4 - 5 tudi, 0 day, mgt so g . thilt da duoc dp dung nhu: co. viing trude bai la viing nuac sau (chieu cao song giam khong dang k l khoang 2 ^ 5%, khi truyin tir may tao sdng tdi truoc vung chuyen tep vao
^ ) t d i hdng (m = oy, trong md, thi nghiem myc "uactrung binh k h n g doi kho^n^^^
phong chinh xdc dugc khu vuc cd sue can cao d gdn day. Tuy nhien ^o J ^ c nua^ th ghiem duac chgn thudng la cao (tuong irng vdi trudng hgp nguy h - - " - . l^c te - n nude ddng do bao va thuy tr.lu) nen tuong tac giiia phan than canh va tan la voi song dong vai trd quan trgng trong viec lieu tan nang lugng song.
- Thi nghiem dugc tiln hanh vdi cdc trudng hap sau: 2 - f l^'^^b^-^^^.^^JJ ' ^ ^ mo hinh rirng ngap man; 3 chilu sau nude tinh trong mang (40cm, 45cm, 50cn^^ 5 eh e^
cao sdng H. (5cm lOcm, 12,5cm, 15cm, 17,5cm). Mot chu ky song ^;^l^^'^2!^2^^^^^
IaT„.= l 8s tuang iing vdi T„=8s ngoa, thyc te), Khi do, '^h/eu dai song o nua sau t o g md hinh xdp xi U - 5,05m (tuang irng vdi L s ^ 100m ngoai thyc te), Chieu cao song dugc tai cac ddu do la Hg.
3.2 Ket qud thi nghiem
Bang 1 va 2 dudi day tdng hop kit qua cac thi nghiem ^-ng ^ ^ ^ a c ^ J ^ ^ ^ J | ^ man trude de irng vdi md hinh bai ngdn va bai dai. He trye oa^ ^ ~ bit ddu (X=0) tafddu ba. (hit phdn chuyen tiep tir nuoc sau), ^ ^ ^ ^ ^ ^ ^ ^ f ^ ^ ^ ^ . a / t a o Chan de. Ddu do WHM1 dat each diem cudi cua phan chuyen uep^^ ^ ^ ^ ^^ ^^ ^^^,^ ^.^
song. Trong cac thi nghiem chieu cao cua bai tinh tu ^ay "i^ng _ ^2 5cm- 27,5cm.
nude thuc t l tren bai trong cac trudng hgp thi nghiem lan lugt la i /, -
220
Nghien citv khd ndng gidm sdns cua rirng ngdp mdn VietNam_
Thi nghifm
so X X/Ls
5 6 7 8
12 13 14 15
Bdng 1
Ch.sau nude(cm)
40 40 40 40 45 45 45 45
Ket qua thi nghiem md hinh bdi
Ho
(em)
10 12.5
15 17.5
10 12.5
15 17.5
Hs(WHMl)
(m)
-5
-0,096 0,119 0,136 0,148 0,097 0.121 0,141 0,155
Hs(WHM2)
(m) 0.2 0.04 0,087 0,097 0,108 0,113 0.089 0.112 0.131 0.146
ngdn. cd rirng (Ls ~ 5,05m)
Hs(WHM3)
(m) 1.2 0.237 0.059 0.067 0.073 0,078 0.071 0.085 0.094 O.I
Hs(WHM4)
(m) 2.2 0.435
0.03 0.033 0.036 0.038 0.038 0.044 0.051 0.053
Hs(>VHM5)
On)
3 0.594 0.012 0,038 0.041 0.044 0.038 0.046 0.053 0.058 Bdng 2 Ket qua thi nghiem md hinh bdi ddi, cd rimg (Ls ~ 5,05m)
Thi nghiem
SO
Ch.sau nuoc
(cm)
Ho Hs(WHMl)
(em) (m)
Hs(WHM2)
(m)
Hs(WHM3) (m)
Hs(VVHM4)
(m)
Hs(WHM5)
(m)
X
2.5
X/Ls 0.198 0.495 0.791 1.187
40
0.096 0.055
0.037 0.0240.021
40
12.5 0.II8 0.06]
0.041 0.027 0.02340
15 0,135 0,065
0.0430.029 0.025
40
17.5 0.148
0.0720.046
0.033 0.0340 0.047
0.039
0.0240.017 0.014
45 0.048 0.036
0.022 0.0240.013
45 10 45
45
12.5 15 45 17.5 II 50
12
13 1415
50 50 50 50 40
12.5 15 17.5
0.096 0.067
0.038 0.0240.021
0.12 0.078 0.046
0.028 0.0230,14 0.081 0.052 0.036 0.026
0,155 0.088 0.055
0.0350.029
0.048 0.036 0.029 0.017 0.013
0.096 0.069 0.056
0.0320.021
0.119 0.141 _O.I58 0.049
0.087
0.068
0.04 0.0270.104 0.115 0.043
0.072 0.077 0.035
0.05 0.056 0.032
0.032
_p,036
0.027
UdfViet Hiing
22117
f —
18 19 20 21 22 23 24 25 26 27 28 29 30
40 40 40 40 45 45 45 45 45 50 50 50 50 50
10 12.5
15 17.5
5 10 12.5
15 17.5
5 10 12.5
15 17.5
0.097 0.119 0.137 0.149 0.049 0.097 0.12 0.14 0.155 0.049 0.096 0.12 0.141 0.159
0.071 0.078 0.084 0.089 0.045 0.086 0,099 0.106 O.III 0.047 0.094 0.116 0.124 0.132
0,056 0,062 0.065 0.069 0.035 0.064 0.072 0.078 0.082 0.038 0.072 0.087 0.099 0.107
0.045 0.049 0.051 0.054 0,03 0,058 0.068 0.073 0.075 0.037 0.076 0.085 0.097 0.104
0,04 0,047 0,052 0,056 0,026 0,046 0.054 0,055 0,06 0.027 0,052 0,065 0.073 0.079
Tir cac ket qud thi nghiem thay rang, trong trudng hgp khdng cd rirng, chieu cao sdng tai vi tri gdn chan de (iing vdi bai dai) cd the giam tir 10 den 40% so vdi chieu cao sdng ban ddu tai nude sdu. The nhung khi ed rirng ngap man vdi mat do thua, chieu cao sdng cd the giam 35^55%, vdi rirng cd mat do day, sdng gidm tir 70% tdi ban 80%. Dieu nay cho thdy hieu qud gidm sdng ciia mot dai rimg ngap man cd chieu ddi chi vao cd mot con sdng khi bao. Kit qua thi nghiem ciia trudng hgp do sdu nude tinh bang 40cm trong hinh 3 dudi ddy CO the minh hoa ro net dieu nay.
r *
4. Xac dinh he so giam song
He so suy gidm sdng tren loan bg chilu ddi bdi theo phuang truyin s6ng.dugc xac dinh theo cdng thirc:
//c, -H., R =
sj_H
(2)
51
- Trong dd: Hg, - chilu cao sdng tai viing nude sdu (WHMl); Hs, - chieu cao sdng tai cuoi bai (WHM5).
Kit gua tinh loan he sd suy giam chilu cao sdng R dugc long hgp trong bang 3 dudi day, He sd gidm sdng trong trudng hgp nude sdu, chilu cao sdng nhd va khdng cd rimg la tucmg ddi be, mot sd con sdng hdu nhu khdng mat nang lugng tren suit chieu dai bai. The tiliung, vdi su cd mat ciia RNM vdi chilu dai thich hop, he sd gidm sdng dugc tang len dang kl, cd thi dat tren 80% (xem hinh 4).
So sanh vdi nhirng nghien ciru da dugc thong ke trong myc 2 cd the nhan xet rang, nhirng kit qua thi nghiem trong bao cao nay kha phii hgp ydi nhirng ket qua nghien cuu thuc dja cua cac tac gia Yoshihiro Mazda, Phan Nguyen Hong, Vu Doan Thai ' ^ ' / ° ^ ^ ^ ^ 5hilu cao sdng truyin qua rirng bdn chua va rirng trang d Hai Phdng. Tir bang 3 ™y/.^"|:
jSi^vdi rimg thua he sd giam sdng giao dgng tir 0,45 - 0,60 va khoang 0,70 = 0,82 doi v
222
Nghien ciiv khd nans sidm sdns ciia rims ngdp man VietNajn
rirng day, Theo Phan Nguyen Hong he sd suy gidm do cao sdng khi di qua 300m rirng ban chua la 57% vd sau 250m riing trang la 77,7%. Theo Vu Doan Thai he sd suy giam song cua rimg trang tir 80 = 88%), rimg bdn la 77 -^ ST/o. Nhu vay la thi nghiem da md phdng tuong doi chinh xac kha nang giam sdng cua cac cay ngdp man ho bdn trong viing ven bien nude ta.
1 0 -
0 8 - 0.7 - 0 6 - o i 0.5 - X
0.3 H 0.2 , 0.1 ,
Trifcrng
1
QL hp'p bai
A N H E ( H dai, CO r i
• ^ ^
»
t
MY 0) - (X/Ls)
g 20x20, rwrirc sau 40cm
1 1 i
Ho=12 5cm
- - » - - Ho=10cm Ho=15cm - . , . - - Ho=17 5cm
" • «
~~ • - 1
^ - - - -
^~~~^
_ .^- .---«
0.0 0.2 0.4 0.6 0 8 1.0 1 2 1.4
0 9 . 0 8 • 0 7 , 0 6 - o 0.5 .
i
^ 0 4 , 0.3 , 0.2 , 0.1 , 0.0 ,
Tripcmg
•
QL hp'p bai
\
\
\
A N H g ( H dai, CO n
s
a ^ \
i/Ho) - (X/Ls)
m g 20x10, nirifc sau 40cm
* Ho=10cm(&tp 1) - • - - Ho=12.5cm(E«p.2)
l-to=15cm(Exp.31 - - - Ho=17.5cni(Exp.4) - , » - , H o = 5 c n i ( & p 5)
•n
•- . ~ —_
...
—
0.0 02 0.4 0.6 O.B 1 0 1 2 1 4 X A J
Hmh 3: Tdc dung gidm chieu cao sdng cua RNM, tnrang hap nuac sdu 40cm.
Bdng 3: He sd gidm sdng theo ket qud thi nghiem
Exp.
No.
I 2 3 4 5 6 7
Rirng day k/c (cm)
20x10
20x10
T
Chieu sdu nude
(cm) 40 40 40 40 40 45 45
Ho (cm)
5 10 12.5
15 17.5
5 10
R
-
0.699 0.785 0.805 0.815 0.800 0,722 0,785
Exp.
No.
16 17 18 19 20 21 22
Rirng thua k/c (cm)
20x20
20x20
1
Chieu sau nude
(cm) 40 40 40 40 40 45 45
Ho (cm)
5 10 12.5
15 17.5
5 10
R
-
0.442
0.587
0.604
0.623
0.625
0,471
0.524'^Viet Hdng 223
t LX- ho' JTi ^IT"
^ 7 3 ^
nr"
15
20x10
45 45 45 50 50 50 50 50
12.5 15 17.5
5 10 12.5 15 17.5
0.809 0,817 0.811 0.726 0,778 0.769 0,776 0.771
23 24 25 26 27 28 29 30
20x20
45 45 45 50 50 50 50 50
12,5 15 17,5
5 10 12,5 15 17.5
0.551 0.607 0.612 0.450 0.454 0.457 0.482 0.506
« Rung day n Rung thua i Khdng co ru'ng 0 9
0.8 07 0 6 0.5 0.4 0.3 02 0 1 0 0
* 1 * • I •
a '
S
Q a 1
^ 1 D
a
°n
a
9
A
a i
'
1 1
A 1
'L - _ A _ _ j j
^^-- - r * - -
A I 1
0 05 0 1 0 15 Chieu cao song nipo'C sau Hs (m)
Hinh 4: Mdt quan he giira He sd giam sdng vd Chidu cao sdng trong cdc thi nghiem cd vd khdng ed rirng ngdp man. bdi ddi.
5. Mot so kicn nghi trong vice su dung rung ngap man bao ve de bicn
Trong rirng phdng hg chdn sdng bao ve de cac vung ven biln nude la dd trd thanh van dl cap thilt. Tuy theo vi tri, dia hinh va tinh chdt ciia ddt de chgn loai cdy ngdp man va trIng ddi RNM cho phu hgp, cy the la:
- Chd cira sdng, nlu long sdng hep ddi cay it nhdt rgng 30m, mat do ddy; chd rgng ban chilu ngang ciia ddi rimg tren dudi 50m, tranh tinh trang dai cay trong rgng qua lam cho long song bj thu hep hoac dm ddng, nhung dai rimg hep qua cung khdng cd tac dyng.
- Ddi vdi de biln, trong cac khu vuc cd thi trdng dugc RNM thi chilu rdng cua dai RNM phdi tir 120m trd len dl dam bao hicu qud gidm sdng .
- Nhdm dat bieu qua trong viec gidm sdng, chilu rgng cua dai RNM tinh lir bien vao, toi thilu phdi Idn hon 2 lan chieu ddi budc sdng.
- Cac cdy trdng so le nhau, theo timg hang each nhau 2m, trong mdi bang cac cdy each nhau tir 2^4in.
- Cac loai cay trdng cdn thich hgp vdi tirng vung d.a ly, Ddi vdi ^ungven biln Nam Bg, bien phdp tdt nhdt la trdng xen dude, dung hoac trang vai cac loai cay mgc ty
224 Nghien ciru khd ndng gidm sons ciia rims nsdp mdn VietNam nhien. Doi vdi cac tinh Hai Phdng, Thai Binh, Nam Dinh cd the trdng ban chua va trang. Cac loai cay ngdp man cd the trdng xen ke de tao thanh hai tang khdc nhau.
Tdng I la tdng cdy cao cd thi trdng bdn chua, tang II la tang cdy thap trdng trang.
Ngoai vi?c trdng va bdo ve cac dai rung ngdp man, can trdng cd chiu man de phii mai de nhu cd ga, qud nd, rau reu... hoac mdt sd cdy than cd cd nhieu gai nhu trinli nii, ca gai vd mot sd lodi day leo chiu man.
Ngoai ra, chinh phu nen quy dinh vanh dai RNM la mot thanh phan quan trgng trong he thing de bicn va danh mot phdn kinh phi thich hgp trong tu bd de dieu de phuc hoi hoac
trdng mdi vd bdo ve Idu ddi vanh dai ndy. 3 Tdm lai, RNM cd vai ird lo Idn trong viec gidm sdng bao ve de bien va vung ven bien,
vi vay nd cdn dugc bao ve, khdi phuc vd phat tricn. 0 cac khu vuc cd the trong dugc RNM nen xem xet giam nhc cdc dieu kicn bicn khi thict kc dc bien. RNM gdp phdn Idm gidm chi phi cho duy tu, bao dudng dc bicn ma do an loan van dam bao.
Tai lieu tham khao
[I] "Smh thdi hge rirng ngdp man ", Nguyen Hoang Tri, Nha xuat ban Nong nghiep, Ha noi - 1999.
[2] "Rirng ngdp man Viet Nani\ Phan Nguyen Hong, Nha xuat ban Nong nghiep. Ha noi - 1999.
[3] "Lich sir nghicn ciru ddt ngdp nude ven bien ]'iet Nani\ Phan Nguyen Hong, 2004.
[4] "Vai trd ciia he sinh thai rirng ngdp man vd rgn san hd trong viec gidm nhe thien tai vd cat thicn cage sdng d vimg ven bicn", Phan Nguyen Hong va cong sy, 2007.
[5] "Gido trinh Sdng gid ", Vii Thanh Ca, Trudng Dai hoc Thuy Lgi, Ha noi - 2005.
[6] "Vai trd chdn sdng vd bdo ve bd bien cua rirng ngdp man a ddng bdng Bdc Bg Viet Nam", Yoshihiro Mazda, Phan Nguyen Hong va cong sy, 1997.
[7] "Sirgidm .sdng trong rung ngdp man cd lodi hdn uu the", Yoshihiro Mazda va cong sy, 2006, [8] "Budc ddu nghien cim khd ndng chdn sdng, bdo ve bd bien trong bdo qua mot so kieu cdu tnic
rung ngdp man trdng ven bicn Hdi Phdng", Vu Doan Thai, 2007.
[9] "Tieu chudn nganh 14 TCN 130-2002: Hudmg ddn thiet hide bien", Bg NN&PTNT, 2002. , \ [10] "Wave transmission In mangrove forests", Schiereck & Booij, 1995. - ' [ I I ] "Introduction to bed, bank and shore protection", Gerrit J, Schiereck, Delft University - 2001.
[12] "Surface waves propagation in mangrove forests and induced suspended sediment concentration", Stanislaw Massel, 2007.
[13] "IVave attenuation In mangrove wetlands", De Vos, 2004.
