SIM ƯLATIO N O F C H A N G E S OF RED RIVER S Y S T E M P R O F I L E ƯS1NG HEC-6 M O D EL
(Da r i v e r r e a c h fr o m Hoa B i n h to T h a o - D a c o n A u e n c e a n d Red r i v e r r e a c h f r o m T h a o - D a c o n A u e n c e t o Ha N o i)
N g u y e n T h i N g a
D e p a rtm e n t ofHyd.ro Meteorology a n d Occanoỉogy Collcge o f Science, V N U
A b s t r n c t . ỉ loa B in h re s e rv o ir was b u ilt in ỉ)a r iv c r to c a r r y o u t tw o top ìm p o rta n t d u tic s t h a t are g c n c ra tio n e le c tr ic ity and flo o d p re v e n tio n for ìow ]ands. S in cc o p o ra tio n , Hoa B in h re s c rv o ir has b cen b r in g in g a bo ut v c ry big uscíuỉnesses, b u t also has bcen c a u s in g u n fa v o u ra b lo ch a n g e s , such as d e p o s itio n in th e ro s e rv o ir, local scour in lo w e r o f th o dam a n d (lis s e m in a te d scour to w a rd s lovvcr. K s tim a tio n , s im u la tio n and p re d ic tio n th c s c rh a n g e s to li m i t dam ages a lw a y s a re p re ssin g and necessary p ro b le m s . M a th o m a tic a l m odels a rc e ffe c tiv e and e co n o m ir to o ls to solvp these p ro b lo in s .
I1EC-G is a o n o -đ im iỉn s io n a l m ovable b o u n d a ry opon c h a n n e l flo w n u m tíric a l m odcl dosigned to s im u la te and p r iid ic t changcs in r iv o r p r o íìlc s r o K u ltin g fro m scour a ru l/o r d o p o s ilio n o v c r m o d c ra tc tim c p crio d s.
T liis p a p c r s tu d ie s and a p p ỉio s 11KC-G lo s im u la tc c h a n g e s o f D a r iv o r p ro lìlc (reach from ỉ loa B inh t.o Thao-Da ronílue n cc) and Red riv e r p ro íílc (rea ch from T h a o -D u co n ílu e n ce to l l a N o i). T h o r c s u lts (>f m o d e l c a lib r a tio n and v e rific a tio n show th rtt IIE C -6 m odel vvith th o selected so t o f o p tim a l p a ra m e te rs can use to s im u lu to changes o f Red r iv c r system p ro íìlc in th o ĩu tu r o w ith accoptahỉo accuracy. T h e rc s u lts o f Red r iv c r bed chang!! s im u la tio n u s in g the s o i o f solccteđ p a ra m e to rs ; in it ia l c o n d itio n is Red r iv e r bcul, w h ic h s u rv e y e d in 1992; u p s tro a m c o n c iitio n s a rc ty p ic a l d isch a rg e proccss (avcĩragc ty p o o f periocl 1988 1998) a i h y d ro lo g ic a l s ta tio n s : Hoa B in h , Y e n B a i, V u Q u a n g , T h u o n g C a t and đ o w n s tre a m c o n d itio n s is ty p ic a l 8tag<‘ proccss (a v o ra g o typcĩ o f p e rio d
1988 1908) a t H a n o i h y d ro lo g ic a l s ta tio n show th a t:
Dcĩíìp scour p h o nom enon s r c s u ltin g fro m r e g u la tin g o ffí‘c:t C)f ỉ lo a B in h ro s o rv o ir h a p p o n almost. o n ly in Da r iv o r and tra n s m it o n ly to T r u n g ỉ la s ta tio n .
Deep scour speẹđ rodur.es g ra c iu a lly . Thcíroĩorc, Da r iv tĩr bcuỉ w ill bceome g ra d u a lly s t a b ilit y (to ro o s ta b lis h th o new b a la n cc State).
1. I n t r o d u c t i o n
Red r iv e r n e tv v o rk , th e second b ig g e s t r iv e r n e tw o r k in V ie tn a m , has been p la y in g a s ig n iíic a n t ro le in th e s o c io -e c o n o m ic a l đ e v e lo p m e n t o f th e c o u n tr y . Red r iv e r ris e s fro m a h e ig h t o f 2 0 0 0 m A M S L in V a n N a m ( C h in a ) . I t h a s a to t a l le n g th o f 1126 k m . T h e b a s in o c c u p ie s t o ta l a re a o f 165794 k r r r , in w h ic h 8 2 6 3 0 k m2 lie s w it h in in V ie tn a m . Recỉ r iv e r a cts as w a te rw a y s a n d flo o d c o n v e y a n c e to p ro te c t th e m o st N o r th e r n p a r t o f V ie tn a m fro m flo o d in g a n d in u n d a t io n . R ed r i v e r is c re a te d
68
S i r n u ỉ a t i o n o f c h a n g e s o f R e d r i v c r s ystcm .
69
fro m th r e e m a in t r ib u t a r ie s : D a, T h a o a n d L o r iv e r s . T h a o r iv e r o r ig in a te s fro m D a i L v L a k e in V a n N a m o f C h in a , vvhere i t is c a lle d N g u y e n r iv e r . F lo w in g d o w n fro m th e a ỉt it u d e o f 2000 m A M S L in a N o r th w e s t-S o u th e a s t d ir e c tio n , i t e n te rs V ie tn a m - C h in a b o r d e r a t L a o c a i p r o v in c e a n d is knovvn as T h a o r iv e r . I t has a to ta l lo n g th o f 9 0 2 k m in w h ic h 332 k m v v ith in in V ie tn a m . F ro m Y en B a i to V ie t T r i, its w iđ th s r a n g e fr o m 100 * 150 m in a ve ra g e . R iv e r b a n k e le v a tio n s v a ry belovv th e v a lu e s o f 20 m ; th e a v e ra g e d e p th is a ro u n d 5 m c o rre s p o n d in g w it h th e b a n k - fu ll d is c h a rg e . T h e b e d s lo p e is a p p r o x im a te ly 1%U. T o ta l c a tc h m e n t a re a o f T h a o r iv e r in s id e V ie tn a m is 12100 k n r . D a r iv e r ris e s fro m th e h ig h ly m o u n ta in o u s a rea in th e sa m e v i c i n i t y o f T h a o r i v e r ’s source. R u n n in g in a N o rth w e s t-S o u th e a s t (iir e c tio n to H o a B in h , i t th e n c h a n g e s th e flo w in g d ir e c tio n to S o u th - N o r th , a n d jo in ts T h a o r i v e r a t T r u n g H a , 12 k m u p s tre a m fro m V ie t T r i to w n . T h e to ta l le n g th fro m b o rd e r t o V ie t T r i is a b o u t 5 7 0 k m w it h th e c a tc h m e n t a re a o f 2 6 8 0 0 k m 2. I t is th e m o s t im p o r t a n t t r i b u t a r y o f Red r iv e r n e tvvo rk a n d n o r m a lly c o n tr ib u te s up to 50% t o t a l in f lo w fo r R ed r iv e r . L o r iv e r also o r ig in a te s fro m V a n N a m m o u n ta in s a rea lo c a te d i n C h in a . Its t o t a l le n g th is 470 k m ; th e c a tc h m e n t a re a is 136900 k m 2.
In V ie tn a m t e r r i t o r y , th e b e d slo p e o f L o r iv e r is 2 6%0. I n th e d o v v n s tre a m course fro m T u y en Q u a n g to V ie t T r i, it s v v id th is a b o u t 200 m a t flo w d e p th ra g in g fro m 1.5 to 3 m . T h e t o t a l b a s in o f R ed r iv e r n e tw o r k c o n s id e re d a t Son T a y is a p p r o x im a te ly 7 0 7 0 0 k m 2, o c c u p y in g 45% o f N o r th e r n p a r t a re a o f V ie tn a m . Red r iv e r c o n s id e re d fr o m V ie t T r i, flo w s th r o u g h im p o r ta n t u r b a n c e n te rs , Son T a y p ro v in c e a n d H a n o i C a p i t a l w h e re th e p o p u la tio n s a re th e h ig h e s t c o m p a rin g th r o u g h o u t o u t t h e n a tio n .
H oa B in h r e s e r v o ir , lo c a te d o n D a r iv e r , 50 km fa r fro m T h a o -D a c o n ilu e n c e , was p u t in t o o p e r a tio n p a r t ia lly in 1990 a n d f u l ly i n 1994. T h e e ffe c ts o f th e c o n s tru c tio n h a v e b e e n seen o b v io u s ly in D a rea ch a fte r o f c lo s u re . P re v io u s ly , th e re vvere som e re s e a rc h e s p r e d ic tin g th e im p a c t o f H oa B in h re s e rv o ir o n d o vvnstream m o rp h o lo g ic a l c o n c iitio n s in d e s ig n s ta g c s . E ach o f th e p re v io u s s u d ie s has m e r it p o in ts a n d l im i t a t i o n s d u e to th e d e fic ie n c y in d a ta fo r v e r tic a tio n a n d c o m p u ta tio n a l m e th o d s .
In r e c e n t y e a rs , d a ta a n d o b s e rv a tio n s o b ta in e d by D e p a r tm e n t o f M a n a g e m e n t & F lo o đ C o n tr o l a n d R e se a rch I n s t it u t e o f W a te r R esources sh o w t h a t th e d e g ra d a tio n o f D a r iv e r b e đ h a s been p r o p a g a tin g to th e d o w n s tre a m . B a n k erosions a n d a v u ls io n s d ư r in g re s e n t y e a rs in d o w n s tre a m h a v e been s e rio u s ly o c c u rrin g a t a la r m in g ra te s . T h e riv e rb e d s in o u te r b a n k a re b e in g deepened ra p id ly , th e flo w p a th a p p ro a c h in g th e r iv e r b a n k c r e a tin g s e rio u s a v u ls io n s in bo th flood a n d t r a n s i t i o n a l s ta g e s. T h is s itu a tio n th re a te n s th e s a fe ty o f n a tio n a l d ik e system a n d v v ill c a u s e b ig d a m a g e s i f th e re is no in - t im e a c tio n to p re v e n t th e erosion.
T h is re seach a p p lie s H E C-6 m o d e l to s im u la te a n d p r e d ic t c h a n g e s o f Da r iv e r p r o file s (re a c h fr o m H o a B in h to T h a o -D a c o n flu e n c e ) a n d Red r i v e r p r o file s (re a c h fro m T h a o -D a c o n ílu e n c e to H a N o i).
2. Theoretical basis o f m o v a b l e boundary calculation of IIEC-6
HEC-6 is a one-dimensional movable boundary open channel flow numerical
m o d e l. I t is đ e s ig n e d b y U S A H y d ro lo g ic a l E n g in e e rin g C e n te r to s im u la te a n d p r e d ic t ch a ng e s in r iv e r p ro íĩle s r e s u ltin g fro m s c o u r a n d /o r d e p o s itio n o v e r m o d e ra te tim e p e rio d s ( ty p ic a lly m o n th s o r y e a rs , a lt h r o u g h a p p lic a t io n s to s in g le flo o d e v e n ts a re p o s s ib le ). H E C-6 used in th is re s e a rc h is H E C- 6 v e rs io n 4.1 p u b lis h e d in 1993.
In H E C-6 m ocỉel, a c o n tin u o u s flo w re c o rd s is p a r t itio n e d i n t o a s e rie s o f s te a d y flo w s o f v a r ia b le d is c h a rg e s a n d d u ra tio n s . F o r each flo w , a w a te r s u rfa c e p r o íile is c a ỉc u la te d th e r e b y p r o v id in g e n e rg y slope, v e lo c ity , d e p th , etc. a t each cross s e c tio n . P o te n tia l s e d im e n t t r a n s p o r t ra te s a re th e n c o m p u te d a t each s e c tio n . T h e se ra te s , c o m b in e d w it h th e d u r a tio n o f th e flo w , p e r m it a v o lu m e t r ic a c c o u n tin g o f s e d im e n t w it h in each re a c h T h e a m o u n t o f
SCOU1*
o r d e p o s itio n a t e a c h s e c tio n is th e n c o m p u te d a n d th e cro s s s e c tio n a d ju s te d a c c o rd in g ly . T h e c o m p u ta tio n s th e n proceed to th e n e x t flo w in th e sequence a n d th e c y c le is re p e a te d b e g in n in g vvith th e u p d a te g e o m e try . T h e s e d im e n t c a lc u la tio n s a re p e rfo rm e d b y g r a in size fr a tio nthereby allowing the sim ulation of hydraulic sorting and amoríng Features of HEC-
6 in c lu d e : c a p a b ility to a n a ly z e n e tw o rk s o f s t*e a m s , c h a n n e l d r e d g in g , v a rio u s levee a n d e n c ro a c h m e n t a lte r n a tiv e s , a n d to use s e v e ra l m e th o d s fo r c o m p u ta tio n o f s e d im e n t tr a n s p o r t ra te s .
2.1 T h e o r e t i c a l l i a s i s f o r H y d r a u l i c C a l c u l a t i o n s
T h e h y d r a u lic p a r a m e te rs needs to c a lc u la te s e d im e n t t r a n s p o r t p o te n tia l a re v e lo c ity , d e p th , w id th a n d e n e rg y s lo p e -a ll o f w h ic h a re o b ta in e d fr o m vvater s u rfa c e p r o file c a lc u la tio n s . W a te r s u ría c e p ro fíle s a re c a lc u ỉa te d u s in g th e s ta n c ỉa rd -s te p m e th o d to s o lve th e c o n t in u it v e q u a tio n a n d o n e -đ im e n s io n a l e n e rg y e q u a tio n ( E q u a tio n 2.1) a n d th e h y d r a u lic
p a ra m e te rs a re c a lc u la te d a t each cross s e c tio n fo r su cce ssive d is c h a r g e . F ig u re 2.1 show s a r e p re s e n ta tio n o f th e te r m s in th e e n e rg y e q u a tio n .
ws, = w s , + ^ ỉ - + he. (2.1)
2g 2g
ws?
_
°
^ -1' -Ị” l ĩ s
x — C roó* U n« 1
Wo4»' S«ftoc« 1 * ' v * 9
ws,
0 o t u » T >
Figure 2.1
S i m u l a t i o n o f c h a r iỊ Ịc s o f R e d r i v c r systcm .
71
vvhere: g = a c c e le ra tio n o f g r a v ity ; h c = e n e rg y loss; V j, V , = a v e ra g e v e lo c itie s (to ta l d is c h a rg e - r t o t a l flo w a re a ) a t ends o f re a c h ; W S j, W S2 = w a te r s u rĩa c e e le v a tio n s a t e n d s o f re a c h a n d a | t a , = v e lo c ity d is t r ih u ta tio n c o e ffic ie n ts fo r flo w a t c n d s o f re a c h .
T h e e n e rg y loss te r m , h,., in e q u a tio n 2 . 1 is com posed o f í r ic t io n loss, hf, a n d fo rm losses, h ot as shovvn in e q u a tio n 2.2. O n ly c o n s tra c tio n a n d e x p a n s io n losses in th e g e o m e tric fo rm loss te r m .
h e = h i + h 0 . (2.2)
To a p p r o x im a t e th e tra n s v e rs e d is t r ib u tio n o f flo w , th e r iv e r is d iv id e d in to s tr ip s h a v in g s im ila r h y d r a u lic p ro p e rtie s in th e d ir e c tio n o f flo w . E ach cross s e c tio n is s u b d iv iđ e d in to p o rtio n s th a t a re r e íe rre d to as s u b s e c tio n s . F r ic tio n , h f>
loss is c a lc u la te d as shovvn b e lo w :
h í (2.3)
in w h ic h :
NSS k
; -
zJ 1
( ả2 + Ạ j) . r 2 + r , "
2
l1 49 2
->
J
" ,
LV2
(2.4)
w h e re : A j,
Á ,
= d o w n s tre a m a n d u p s tre a m a rea, re s p e c tiv e ly , o f th e flo w n o r m a l to th e cro s s s e c tio n ; N S S = to ta l n u m b e r o f s u b s c c tio n s across each cro ss s e c tio n ;Kị
= le n g th * w e ig h te d s u b s e c tio n co nveyance; 1^ = le n g th o f th e j lh s t r ip b e tw e e n s u b s e c tio n s ; n = M a n n in g 's ro u g h n e s s c o e ffic ie n t; Q = w a te r d is c h a rg e a n d H ;. R2 = d o w n s tre a m «ancỉ u p s tr e a m h y d r a u lic r a d iu s , re s p e c tiv e lv .E n e rg y losses d u e to c o n s tra c tio n s an d e x p a n s io n s a re c o m p u te d b y th e fo llo w in g e q u a tio n :
2
h o = C ,.
a2VỈ q,v,
2g
(2.5)
w te re : C( = lo»ss c o e ffic ie n t fo r e x p a n s io n o r c o n s tra c tiơ n .
2.2 T h e o r e t i c a l B a s i s f o r S e d ir n e n t C a l c u l a t i o n s
S e d im e n t tr a n s p o r t ra te s a re c a ic u la te d fo r cach flo w in th e h y d r o g r a p h fo r e a ;h g r a in s iz e . T h e t r a n s p o r t p o te n tia l is c a lc u la te d fo r each g r a in s iz e cla ss i n th e beJ as t h o u g h t h a t a re c o m p ris e d 1 0 0 ° o o f th e bed m a te r ia l. T r a n s p o r t p o te n tia l is
th e n m u lt ip lie d b y th e ír a c tio n o f each size class p re s e n t in th e b e d a t t h a t tim e to y ie ld th e tr a n s p o r t c a p a c ity fo r t h a t size class. T h e s e ÍY a c tio n s o fte n c h a n g e s ig n if ic a n t ly d u r in g a tim e s te p , th e re fo re an ite r a c tio n te c h n iq u e is used to p e r m it th e s e c h a n g e s to c ffe c t th e tr a n s p o r t c a p a c ity .
T h e s e d im e n t t r a n s p o r t íu n c tio n fo r becỉ m a te r ia l lo a d is s e le c te d by u s e r.
T r a n s p o r t íu n c tio n s a v a ila b le in th e p ro g ra m a re th fo llo w in g : T o f f a le t i’s (1 9 6 6 ) tr a n s p o r t íu n c tio n
M a d d e n 's (1 9 6 3 ) m o d iíic a tio n o f L a u rs e n 's (1 9 5 8 ) r e la t io n s h ip Y a n g 's (1 9 7 3 ) s tr e a m povver fo r sands
D u b o y s ' t r a n s p o r t fu n c tio n (V a n o n i 1975) A c k e rs -V V h ite (1 9 7 3 ) tr a n s p o r t fu n c tio n C o lb y (1 9 6 4 ) tr a n s p o r t fu n c tio n
T o f f a le t i (1 9 6 6 ) a n d S c h o k lits c h (1930) c o m b in a tio n M e y e r- P e te r a n d M u lle r (1948)
T o f f a le t i a n d M e y e r-P e te r a n d M u lle r to m b in a tio n
M a d d e n 's (1 9 8 5 , u n p u b lis h e d ) m o d iíic a tio n o f L a u r s e n 's (1958) r e la t io n s h ip
M o d if ic a tio n b y A r ia t h u r a i a n d K ro n e (1 9 7 6 ) o f P a r th e n a iđ e s ' (1965) m e th o d fo r s c o u r a n d K ro n e 's (1962) m e th o d fo r d e p o s itio n o f co h e s iv e s e d im e n ts .
C o p e la n d 's (1 9 9 0 ) m o d iíìc a tio n o f L a u rs e n 's r e la t io n s h ip (C o p e la n d a n d T h o m a s 1989)
U s e r s p e c ific a tio n o f tr a n s p o r t c o e ffic ie n ts based u p o n o b s e rv e d d a ta .
T h is re s e a rc h is c h o s e n Yang*s s e d im e n t tr a n s p o r t f u n c t io n to c a lc u la te th e u n m e a s u re d s e d im e n t in p u t fo r H E C-6 m o d e l. T h is is h is u n i t s tre a m p o w e r e q u a tio n (Y a n g , 1 973), n a m e ly :
w h e re : Ct8 = t o t a l s a n d c o n c e n tra tio n (in pp m by w e ig h t);co = t e r m in a l fa ll v e lo c ity ; d = m e d ia n s ie v e d ia m e te r o f s e d im e n t p a rtic le s ; V = k in e m a tic v is c o s ity ; g = g r a v it a t io n a l a c c e le ra tio n ;
v s
= u n it s tre a m povver a n dV crS
= c r it ic a l u n it s tre a m p o w e r r e q u ir e d a t in c ip ie n t m o tio n .V (0
( 2 . 6 )
S i r n u l a t i o n o f c h a n g c s o f R ed r i v c r systcm . 73
T h e b a s is fo r a d ju s tin g bed e le v n tio n s fo r s c o u r o r d e p o s itio n ( s im u la tin g v e r tic a l m o v e m e n t o f th e bed) is th o c o n tin u ity e q u a tio n fo r s e d im e n t m a te r ia l ( E x n e r e q u a tio n ):
2 .
, B r •cĩ
0(2.7)
w h e re :B u = w id t h o f m o v a b le bed; t = tim e ; G = ave rag e s e đ im e n t d is c h a rg e ( f t Vsec) r a te d u r in g t im e s te p A t ; X = đ is ta n c e a lo n g th e c h a n n e l a n d Y s = d e p th o f s e d im e n t in c o n tr o l v o lu m e .
E q u a tio n s 2 . 8 a n d 2 .9 re p re s e n ts E x n e r e q u a tio n e x p re s s e d in f i n it e d iffre n c e fo rm fo r p o in t p u s in g th e te rm s s h o w n in F ig u re 2.2
G , ~ G . Bsp(Ysp- Y sp) G , - G u t
0.5fLt + L„) r
At=
0.
sp Y ---—
(0.5)B 5P
G ạ - G , + L u
(2.8)
(2.9)
w here:B *p = w id th o f m o v a b le bed a t p o in t P; GU1 G d = s e d im e n t loacis a t th e u p s tr e a m a n d d o w n s tre a m c ro s s s e c tio n s , re s p e c tiv e ly ; L u, L d = u p s tre a m a n d d o w n s tre a m re a c h le n g th s , re s p e c tiv e ly , betvveen cross se ctio n s; Y sp,Y jp = d e p th o f s e d im e n t b e fo re a n d a f t e r tim e step, re s p e c tiv e ly , a t p o in t P; 0.5
= th e "v o lu m e s h a p e fa c to r"
w h ic h w e ig h ts th e u p s tr e a m a n d d o w n s tre a m re a ch le n g th s a n d At = c o m p u ta tio n a l tim e step.
3. Simulation of c h a n g e s o f red river system proílles u s in g HEC-6 model 3.1 Collectecl d a t a
R esearch r i v e r re a c h is D a r iv e r re a c h (fro m H oa B in h to T h a o -D a c o n flu e n c e ) a n d Red r iv e r re a c h ( fr o m T h a o -D a c o n flu e n c e to H a n o i). T h e fo llo w in g d a ta has been c o lle c te d to s im u la t e ch a n g e s in th c p ro íĩle o f th is r iv e r re a c h :
• G e o m e tric d a t a :
47 cro s s s e c tio n s (fro m 1 to 47) in Da r iv e r re a c h ( fr o m H o a B in h to T h a o - Da c o n A u e n c e ) so u n d e d in 1992.
17 cross s e c tio n s (fro m 48 to 64) in R ed r iv e r re a c h fr o m T h a o -D a c o n ílu e n c e to H a n o i so u n d e d in 1992.
10 cro ss s e c tio n s in T h a o r iv e r (fro m Y e n B a i to T h a o -D a c o n flu e n c e ) so u n d e d in 1992
6 cross s e c tio n s in L o r iv e r (fro m V u Q u a n g to V ie t T r i) s o u n d e d in 1992
2 cross s e c tio n s in D u o n g r iv e r so u n d e d in 1992
T ha lvveg p ro íìle s o f Red r iv e r (fro m H oa B in h to T h a o -D a co nA uence) so u n d e d in 1992, 1994, 1997
•
H ydrological d a t a :
- F lo w d a ta : d a i ly a v e ra g e w a te r d is c h a rg e s in y e a rs fro m 1988 to 1998 a t H a N o i a n d Son T a y s ta tio n s in Red r iv e r , T h u o n g C a t s ta tio n in D u o n g r iv e r , Y en B a i s ta tio n in T h a o r iv e r , H oa B in h s ta tio n in D a r iv e r a n d V u Q u a n g s ta tio n in L o r iv e r .
- W a te r le v e l: d a ily a v e ra g e vvater le v e l in 1993 y e a r a t S on T a y s ta tio n a n d in y e a rs fro m 1988 to 1998 a t H a n o i s ta tio n .
- W a te r te m p e r a tu re : m o n th ly a v e ra g e vvater te m p e r a tu r e o f p e r io d fro m 1988 to 1998 a t G s ta tio n s : H o a B in h , Y e n B a i, , V u Q u a n g , S on T a y , H a n o i, T h u o n g C a t, s ta tio n s .
•
S e d im e n t d a t a :
- D a ily a v e ra g e s u s p e n d e d s e d im e n t c o n c e n tra tio n s a n d d is c h a rg e s y e a rs fro m 19 88 -1 9 96 a t 7 s ta tio n s : H o a B in h , Y e n B a i, V u Q u a n g , Son T a y , H a n o i, T h u o n g
Cat.
• M o n th ly a v e ra g e g r a in size c o m p o s itio n o f su s p e n d e d s e d im e n t a t 7 s ta tio n s : H oa B in h (64-7 0 , 7 2 -75 , 7 7 -7 8 ), Y e n B a i (64-7 0 , 7 2 -75 , 77), V u Q u a n g (6 5 -7 0 , 72-75, 7 7 -7 8 ) , Son T a y (6 5 -7 0 , 7 2 -75 , 7 7 -7 8 ), H a N o i (6 5 -7 0 , 7 2 -75 , 7 7 -7 8 ), T h u o n g C a t (6 5 -7 0 , 74, 77).
- G r a in size g r a d a tio n c u rv e o f su sp en d e d s e d im e n t a n d ta b le o f g r a in c o m p o s itio n p e rc e n ta g e s c o rre s p o n d in g to g r a in d ia m e te rs a t cro s s s e c tio n s : 2 in Lo r iv e r ; 10 a n d 37 in D a r iv e r ; 1 in T h a o r iv e r ; 54, 63, 70 in Red r iv e r a n d 2 in D u o n g r iv e r (s u rv e y e d in O c to b e r 1996).
- G r a in size g r a d a tio n c u rv e o f b e d s e d im e n t a n d ta b le o f g r a in c o m p o s itio n p e rc e n ta g e s (%) c o rre s p o n d in g to g r a in d ia m e te rs (m m ) s u rv e y e d i n M a y 1996 a t cross s e c tio n s 63, 65, 67 in R ed r iv e r a n d cross s e c tio n 4 in. D u o n g r iv e r (d o vvn stre a m o f T h u o n g C a t s ta tio n ) .
S i m u l a t i o n o f c h n n g e s o f R e d r i v e r systern. 75
3.2 C o in p iita lio n a l scheine
D e p e n d in g o n th e d a ta a v a ila b ilit y , h y d ro lo g ic s ta tio n s a n d p r in c ip le s used in H E C-6 m o đ e l, th e w h o le re s e a rc h r i v e r n e tvvo rk is s c h e n iă tic a lix e d in íìg u re 3.1.
U p s tre a m b o u n d a rie s used in th e m o d e l a re H o a B in h , Y e n B a i, V u Q u a n g a n d T h u o n g C a t s ta tio n s . D o v v n s tre a m b o u n đ a r ie is H a n o i s ta tio n . C o m p u ta tio n a l schem e fo r th e r iv e r n e tv v o rk c o n s is ts o f 1 m a in r iv e r a n d 3 t r ib u t a r ie s vvith to ta l 82 cross s e c tio n s . M a in r iv e r o f th e c o m p u ta tio n a l s c h e m e is Red r iv e r re a ch w it h d o v v n s tre a m b o u n d a ry is H a n o i h y d r o lo g ic a l s ta io n and u p s tre a m is H o a B in h h y d ro lo g ic a l s ta tio n . T h e f i r s t t r ib u t a r y ( t r ib u t a r y o f d iv is io n w a te r) o f th e c o m p u ta tio n a l s c h e m e is D u o n g r iv e r re a c h w it h u p s tr e a m b o u n d a ry is flo w d iv is io n a l p o in t betvveen Red
r iv e r a n d D u ô n g r iv e r a n d
Fiẽ u re 3.1.
C o m p u ta tio n a l schem e o f Red dovvnstrea m b o u n d a r y is T h u o n g r iv e r n e tw o rk (re se a rch reach)C a t h y d ro lo g ic a l s ta tio n . T h e seconti
tr ib u t a r y o f th e c o m p u ta tio n a l s c h e m e is L o r iv e r re a c h w it h d o w n s tre a m b o u n d a ry is ju n c tio n o f R ed r iv e r a n d L o r iv e r a n d u p s tre a m b o u n d a ry is V u Q u a n g h y d ro lo g ic a l s ta tio n . T h e t h i r d t r ib u t a r y o f th e c o m p u ta tio n a l sch e m e is T h a o r iv e r reach vvith u p s tr e a m b o u n d a ry is V u Q u a n g h y d ro lo g ic a l s tá tio n a n d d o vvnstrea m b o u n d a rv is T h a o -D a c o n flu e n c e .
3.3 I n p u t d a t a
In p u t d a tn file o f H E C-6 is o rg a n iz e d in to 3 g ro u p s : th e f i r s t g ro u p is g e o m e tric d a ta , th e fo llo w in g g ro u p is s e d im e n t d a ta a n d th e la s t g ro u p is h y đ ro lo g ic a l d a ta . G e o m e tric d a ta is a rra n g e d in fo llo w in g o rd e r: th e p o s itio n o f local ju n c tio n a n d d iv e r s io n p o in ts in th e r iv e r n e tv v o rk , th e p o s itio n o f cross sections in th e r i v e r n e tv v o rk , th e g e o m e trv o f cross s e c tio n s (c o o rd in a te p o in ts o f crcss s e ctio n s a n d th e d is ta n c e s b e tw ee n cross s e c tio n s ), th e v a lu e s o f M a n n in g 's rou gh n e ss n in c ro s s s e c tio n s (in c lu d e M a n n in g ^ n v a lu e s o f th e m a in c h a n n e l and o v e rb a n k a re a s ) a n d c o n v e y a n c e lim it s o f c h a n n e l a t cro ss s e c tio n s . S e d im e n t d a ta
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in c lu d e s f lu id a n d s e d im e n t p ro p e rtie s , th e in flo w in g s e d im e n t lo a d d a ta , th e g r a d a tio n o f m a te r ia l in th e s tre a m bed, th e tr a n s p o r t c a p a c ity r e la tio n s h ip a n d u n it vveights o f d e p o s ite d m a te r ia l. H y d ro lo g ic a l d a ta in c lu d e s vva te r d is c h a rg e s , te m p e ra tu re s , d o w n s tre a m w a te r s u rĩa c e e le v a tio n s a n d flo w d u r a t io n .
3 .4 I n i t i a l a n d b o u n d a r y c o n d i t i o n s
I n i t i a l c o n d itio n is cro ss s e c tio n g e o m e try o f r iv e r s e g m e n ts i n c o m p u ta tio n a l schem e a t th e tim e t h a t c o m p u ta tio n is s ta rte d . M e a s u re d g e o m e tric d a ta o f 82 cross s e c tio n in c o m p u ta tio n a l schem e in 1992 is u se d as th e i n i t i a l c o n d itio n . T h is d a ta is re p re s e n te d in c o o rd in a te p o in t fo rm (s ta tio n s , e lv a tio n s ) o f p o s itio n s in cross s e c tio n s in th e o r d e r fro m th e le f t to th e r ig h t (d o v v n s tre a m ). T h e e le v a tio n s m ay be p o s itiv e , zero o r n e g a tiv e . T h e cross se ctio n s a re a rra n g e d fr o m d o w n s tre a m to u p s tre a m , s t a r tin g fro m cro ss s e c tio n s o f th e m a in r iv e r , th e n to cross s e c tio n s o f th e t r ib u t a r y r iv e r .
I n a r iv e r s y s te m th e r e are th re e ty p e s o f b o u n d a rie s : u p s tr e a m , d o v v n s tre a m and in t e r n a l. T h e u p s tre a m a n d d o vvn stre a m b o u n d a rie s a re a t th e cross s e c tio n s th a t a re m o s t u p s tre a m a n d m o s t d o vvn stre a m , re s p e c tiv e ly , o n a s tre a m s e g m e n t.
T h e re a re th re e ty p e s o f in t e r n a l b o u n d a rie s : a lo c a l in f lo w , a t r ib u t e r y ju n c t io n , an d a h y d r a u lic c o n tr o l p o in t. T h e re a re also th re e b o u n d a ry c o n d itio n s t h a t c a n be p re s c rib e d by H E C -6: w a te r d is c h a rg e , s e d im e n t d is c h a rg e , a n d s u rfa c e e le v a tio n (sta g e ). T h e w a te r a n d s e d im e n t d is c h a rg e s m u s t be d e íìn e d a t each u p s tre a m b o u n d a ry a n d a t each lo c a l inílo vv p o in t. S ta g e m u s t be p re s c ib e d a t th e d o vvnstrea m b o u n d a ry o f th e p r im a r y s tre a m s e g m e n t; a n d i t c a n be p re c rib e d a t h y d r a u lic c o n tr o l p o in ts . In s tu d y r iv e r n e tvvo rk th e u p s tre a m b o u n d a ry c o n d itio n s in c lu d e w a te r d is c h a rg e , s e d im e n t d is c h a rg e a n d d a ily a v e ra g e te m p e r a tu r e a t H oa B in h , Y e n B a i, V u Q u a n g a n d T h u o n g C a t s ta tio n s . T h e d o v v n s tre a m b o u n d a ry c o n d itio n is d a ily a v e ra g e s ta g e (vvater s u ría c e e le v a tio n ) a t H a n o i s ta tio n . T he c o m p u ta tio n a l tim e s te p is o n e d ay.
3 .5 M o d e l c a l i b r a t i o n
H E C-6 m o d e l s im u la te s cha ng e s o f r iv e r p ro file s c o m b in e d u s in g th re e m odels:
h ycừ a ư lics m o d e l, s e d im e n t tr a n s p o r t m o d el and m o rp h o lo g ic m o d c l. H y d r a u lic s m odel is used to c o m p u te w a te r s u ría c e p r o íìle a t each t im e step. S e d im e n t tr a n s p o r t m o d e ỉ is used to c o m p u te s e d im e n t tr a n s p o r t d is c h a rg e a cross each cross s e c tio n in c o m p u ta tio n a l sch e m a a t each tim e step. M o r p h o lo g ic a l m o d e l is u se d to c o m p u te ch a ng e s o f r iv e r bed e le v a tio n s fo r s c o u r o r d e p o s itio n . P a ra m e te rs needed c a lib r a tio n in H E C-6 a re m a in ly p a ra m e te rs o f h y d r a u lic s m o d e l, t h a t are M a r m in g ’s ro u g h n e s s c o e ffic ie n ts o f r iv e r bed, le f t a n d r ig h t o v e r b a n k s .
T h e s tu d y used h v d ro lo g ic a l a n d s e d im e n t tr a n s p o r t d a ta in 1993 to c a lib r a te h y d r a u lic a n d s e d im e n t tr a n s p o r t m o d e l.
S i m u l a t i o n o f c h a n g e s o f R e d rivcr systcm. 77
T h e c a lib r a t io n p ro ce ss o f h y d r a u lic m o d e l is c a r r ie d o u t as fo llo w s :
*
Preliminary selection a set of roughness coefficients at all
C )fcross sections in
c o m p u t a t i o n a l s c h c m e . T h is p r elim in a ry sele c tio n is c a rr ie d o u t o n experience:s e l e c t i o n r o ư g h n e s s c o e f fic ie n ts fronì 0.1 to 0 .1 5 for o v e r b a n k part a n d from 0 .0 2 2 to 0 .0 4 2 for r iv er bed part.
- U sin g
H E C -6model, in turn smulation hydraulcs with the selected set of
h y d r a u lic s p a r a m e t e r s in co n d itio n th at th e ch a n n el is r eg a r d e d fixed c h a n n el.In p u t d a t a o f h y d r a u lic s are d a ily a v e ra g e w a te r d is c h a r g e o f Hoa B inh , Y en Bai, Vu Q u a n g , T h u o n g C at s t a t i o n s an d daily a v e ra g e s t a g e ( w a te r s u r ía c e e le v a tio n ) at Ha noi s t a t i o n in 1993. R e s u l t s o f h vd ra u lics s im u la t io n a re th e s t a g e at all o f cross s e c t io n s in t h e c o m p u t a t io n a l s c h e m e at each t im e step .
- T a k e S o n T a y
s ta tio n
in R e driver (section
54) tomake control
s e c tio n .From
hyciraulics s i m u l a t i o n r e s u lt s w ith th e s e le c te d s e t of p a r a m e t e r s , e x tr a ctio n and d r a w in g 021 g r a p h c o m p u te d d a ily a v e ra g e s ta g e Ị)rocess a t Son T a y s ta tio n 1993.C o m p a r is o n it vvith m e a s u r e đ d aily a v e ra g e s t a g e p r o c ess at Son Tay s t a t io n 1993 and e s t i m a t i o n a g r e e le v e l by N a sh norm.
After a lot o f try and error t im e s with many different couples o f river bed and over bank r o u g h n e s s coeĩricients, th e stud y selected a couple of river bed and over bank roughn ess co efficien ts th a t r esu lts in the best hvdraulics s im u la t io n result: river bed roughn ess c o efficien t is 0.029, left and over bank r o u g h n ess c o eííicien ts is both equal 0.10. W ith t h is set o f h yd rau lic param eters, effective level o f th e m odel com pu ted on Nash norm is th e h ig h est (98.9%) and the forms o f c o m p u te d and m easu red water suface e le v a t io n process graph a g r ee highly each oth er (F ig u re 3.2).
Stage (m)
Kigurc 3.2
Compiitcd and meastiriđ dail V slage proccss al Son Tav station ỉn 1993
A ccu ra cy o f s e d i m e n t tra n sp o rt s im u la t io n o f riv er n e tw o r k d e p e n d s on a c cu ra cy o f h y d r a u lic s i m u l a t i o n r e s u lt s , on a ccu racy o f d e f in it io n p a r a m e t e r s o f s e d i m e n t t r a n s p o r t m o đ el and s e le c t e d s e d im e n t t r a n s p o r t fu n c tio n . T h e s t u d y d e ĩin e d c a r e f u lly p a r a m e t e r s o f s e d im e n t tr a n s p o r t m o d el b a s in g o n e s e lf on c o lle c te d d a t a , s e l e c t e d Y an g's s e d im e n t tr a n s p o r t fu n c tio n a n d ran s e d i m e n t t r a n s p o r t m od el u s i n g t h e s e t of h y d ra u lic p a r a m e t e r s t h a t s e le c t e d vvhen c a lib r a t io n h y d r a u lic m o d e l. T h e s im u la t io n r e s u lt s are d a ily s e d i m e n t d is c h a r g e s a c ro ss e a ch c r o s s s e c t i o n in c o m p u ta tio n a l s c h e m e in 1993. F ro m t h e s e r e s u lt s, t h e stuciy e x t r a c t e d d a ily s c d i m e n t d is c h a r g e process d a t a a c r o s s c r o s s s e c t io n of S o n T a y s t a t io n , d r e w c o n ìb i n a t iv e lv c o m p u te d a n d m e a s u r e d d a il y s e d i m e n t d is c h a r g e p ro c ess g r a p h s at S o n T a y s t a t io n in 1993 on t h e s a m e c o o r d in a t e s y s t e m , a n d e stim a te cl e f f e c t iv e le v e l o f s e d im e n t tr a n s p o r t m od el on N a s h n orm . T h e s e r e s u l t s s h o w e d t h a t c o m p u t e d a n d m e a s u r e d s e d im e n t d is c h a r g e s a g r e e r a t h e r ea ch o t h e r (F ig u r e 3.3), e ff e c t iv e le v e l c o m p u te d o n N a s h norm r e a c h s 0 .7 6 .
Sedim cnt d isch arg c R (kg /s)
F ig u re 3.3. C o m p u te d a n d m easured d a ily s e d im e n t d is c h a rg e process g ra p h s a t Son T a y s ta tio n in 1993
3.6 M o c ỉc l v c r i f i c a t i o n
M o rp h o lo g ic a l m o d e l is s im u la t e d b a se d on c o m p u t a t io n a l s c h e m c e s t a b lis h e d for h y d r a u lic m o d el w i t h u lt im a t e h y d ra u lic a n d s e d i m e n t p a r a m e t e r s . R iver thalvvegs m e a s u r e đ in 1 9 9 4 o f reach from Iioa B in h to T r u n g H a. a n d in 1997 o f reach from ỉ lo a B in h to Ma Noi are u s e d to verifv t h e m o d el. I n p u t d a ta is d aily a v e r a g e d is c h a r g e from 1 9 9 2 to 1997. T h e r c s u lt s inclu de: t o t a l s e d i m e n t d is c h a r g e
S i m l i l a t i o n o f c h a n g c s o f R e d rivcr s ỵ s t c m . 79
inílovv and o u t flo w e a c h river reach for each g r a in tv p e a t e a c h t im e s t e p , q u a n t it y o f s c o u r or d e p o s it io n s e d im e n t on ea ch river reach a t e a c h t i m e s t e p , w a t e r s u r fa c e a n d river ỉ)ecỉ e l e v a t i o n s a t c ro ss s e c tio n s o f c o m p u t a t io n a l s c h e m e a t ea ch t im e s t e p . totnl s e d i m e n t d is c h a r g e from tr ib u ta r ie s inflow m a i n r iv e r and o u tflo w m ain riv er , total sco u r and d ep o sitio n s e d i m e n t q u a n tity over s t u d y r iv er reach .
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Figure 3.5.
C o m p u ted and m ea su red river t h a lw e g of reach from H oa B inh to Ha Noiin 1997
R e s u l t s o f c o m p a r is o n betvveen c o m p u t e d a n d m e a s u r e d th alvvegs o f D a river r e a c h from H oa B in h to T r u n g H a in 1994 a n d Red river reach from Hoa B i n h to Ha N o i in 1997 a re s h o w n i n F i g u r e s 3.4 a n d 3 .5. T h e s e fig u r e s shovv t h a t c o m p u t e d r iv e r t h a l w e g a g r e e r a t h e r vvith m e a s u r e d river th a lw e g . E f f e c t iv e le v e l of th e m o d el c o m p u t e d o n N a s h n o r m o f 1994 r e a c h s 0 .9 3 a n d o f 1 9 9 7 r e a c h s 0 .8 7 .
3 .7 S i m u l a t i o n c h a n g e s o f R e d r i v e r b e d to 2 0 32
T h e r e s u l t s o f m o d e l c a lib r a t io n an d v e r if ic a t io n sh o w t h a t H E C -6 m o d e l w ith t h e s e t o f s e l e c t e d o p t i m a l p a r a m e t e r s c a n u s e to s i m u l a t e c h a n g e s o f R ed r iv e r bed in t h e fu tu r e .
W ith in t i n i a l c o n d it io n is m e a s u r e d river bed p r o íìle in 1 992, t h e
Symbol
ic w a t e r d i s c h a r g e p r o c e s s e s ( a v e r a g e o f 11 y e a r s fom 1988 to 1998) a t Hoa B in h , Y en B a i, V u Q u a n g , T h u o n g C a t s t a t i o n s are t h e u p s t r e a m b o u n d a r i e s and th e s y m b o lic s t a g e p r o c e s s e s ( a v e r a g e o f 11 y e a r s fom 1988 to 1998) a t H a N o i s t a t i o n is th e dovvn stream b o u n d a r y , t h e s t u d y s i m u l a t e d c h a n g e s o f Red r iv e r bed to 2 0 3 2 . T h e r e r s u lt s are shovved in F ig u r e 3.6. T h e se r e s u l t s s h o w th at:Deep scour p h en om en on s resulting from regulating eíTect o f Hoa Binh reservoir happen alm ost only in Da river and transm it only to T ru n g H a station.
T h e deep sco u r sp e ed in Da river red uces grad u ally (S e e ta b le 3.1). Da river bed w ill b e c o m e g r a d u a lly stability (to reestab lish th e n e w b a la n ce State).
Figure 3.6.
P r e d ite d r e s u lt o f Rccỉ river bed c h a n g e s to 2032 (reach from Hoa B in h to Ha Noi)T a b l e 3.1 A v e r a g e d eep sco u r sp eed at s o m e cross section in d e í ử e n t period (m/year)
Cross sectio n 1992 - 2 0 0 2 2002 - 2012 2012 - 2 0 2 2 2022 - 2032
S ectio n 5 0 .3 8 0 0.1 6 8 0 .062 0 .0 0 0
S ectio n 18 0 .0 6 1 0.003 0.001 0 .0 0 0
S ectio n 48 0.021 0.017 0.014 0.013
S i m u i a t i o n o f c h a n g c s o f R e d rivcr systern. 81
T à i l i ệ u t h a m k h a o
1. C h ih T e d Y a n g , S e c l im e n t T r a n s p o r t - T h e o r y a n d P r a c t i c c, M c G ra v v - H i l l C o m p a n ie s , Inc. 1996.
2. N g u y ề n T h ị N g a , T r ầ n T h ụ c , D ộ n g lự c h ọ c d ò n g s ô n g, G iá o t r ì n h T rư ờ n g Đ ạ i h ọ c Khoa học Tự n h iê n , Đ H Q G H N . H à Nôi, 2 0 0 1 .
3. T r a n T u ấ t và n n k , Đ ặ c t r ư n g h ì n h t h á i
lưu
v ự c s ô n g V iệ t N a m , V iệ n K h í tư ớ n g T h ủ y v ã n , H à N ộ i, 1985.4. u . s A r m y C o rp s o f E n g in e e rs , 1 I E C - 6
version
4 .1 . U s e r 's M a n u a ỉ , H y d r o lo g ic E n g i n e e r i n g C e n te r , 1993.ĨA P CHÍ KHOA HỌC ĐHQGHN. KHTN & CN, T XIX, Nọ1, 2003________
M Ỏ P H Ỏ N G D IỄ N B IẾ N L Ò N G D A N H Ệ T H Ố N G S Ô N G H ồ N G B Ằ N G M Ỏ H ÌN H H E C -6
(Đ o ạ n s ô n g Đ à từ H o à B ìn h đ ế n n g â ba T h a o Đ à v à đ o ạ n s ô n g
H ổ n gtừ n g ả ba T h a o -Đ à đ ế n H à N ô i )
Nguyển T h i Nga
K h o a K h í
tượng
T h ủ y v ă n& H ải
d ư ơ n g h ọ c Đ ạ i h ọ c K h o a h ọ c T ự n h iê n , Đ H Q G H à N ộ iv ỏ i hai n h iệm v ụ q u a n tr ọ n g h à n g đầu là p h á t đ iệ n v à p h ò n g lù c h o h ạ du, từ khi v ậ n h à n h , hồ H o à Bình đà v à đ a n g đ em lại n h ử n g lợi ích h ế t sứ c to lớn n h ư n g c ủ n g đă và đ a n g gãy ra n h ữ n g t h a y đổi kh á b ấ t lợi n h ư bồi la n g lò n g hồ, xói cục bộ hạ lưu đập v à xói la n tr u y ề n v ê hạ du... Đ á n h giá, m ô p h ỏ n g v à dự báo các t h a y đỏi sa u khi xây dự n g c á c cô n g tr ìn h trên s ô n g đê c!ê x u ấ t các b iệ n p h á p h ạ n c h ê các t h i ệ t hại luôn là đề tà i c ấ p t h i ế t và có ý n g h ĩa th ự c t i ễ n lốn. C ô n g cụ hữu h iệ u và kin h tê (lể g iả i q u y ết c á c dể t à i n à y là mô h ìn h to á n .
H E C -6 là mô h ìn h sô th ủ y động lực m ột c h i ề u t r o n g lò n g d ẫ n hở có b iên di độn g được T ru n g tâ m Kỹ t h u ậ t Thủy v ă n Hoa Kỳ t h i ế t k ê đ ể mô p h ỏ n g v à dự báo các th a y đôi tro n g trắ c d iệ n dọc s ô n g do xói hoặc bồi tr o n g các thời đ oạn vừa.
Bài báo này n g h i ê n cứu ứ n g d ụ n g mô hìn h H E C -6 đ ể m ô p h ỏ n g d iễ n b iế n lòng sô n g Dà p h ía hạ lưu đ á p H oà B ìn h (đoạn từ H oà B ình đ ê n n g ã ba T h a o -Đ à ) và lòng s ô n g H ồ n g (đoạn từ n g ã ba T h a o -Đ à đ ế n H à Nội). K ết q u ả h iệ u c h ỉn h v à k iểm đ ịn h m ỏ h ìn h cho th ấ y mô h ìn h H E C -6 vói bộ th ô n g s ố tôi ưu đà lự a c h ọ n có t h ể sử d ụ n g
dể mô phòng diẻn b iên lòng sông Hổng tr on g tương lai VỚI mức độ chín h xác có the
chấp n h ậ n được. K ết q u ả m ô p h ỏ n g (liền biến đáy s ô n g H ồ n g b ằ n g mô h ìn h H E C -6 vối bộ th ô n g sô đả lự a c h ọ n vỏi đ iểu k iệ n ban đ ầ u là lò n g s ô n g H ồ n g đo đạc n ă m 1992, điêu k iệ n biên tr ê n là đườn g quá trìn h lưu lư ợ n g đ iể n h ìn h (d ạ n g tr u n g bình n h iề u n ă m th ờ i k ỹ 1 9 8 8 -1 9 9 8 ) c ủ a các t r ạ m H ò a B ìn h , Y ê n B á i, V ụ Q u a n g , T h ư ợ n g Cát và điếu k iện b iên dưới là đườn g quá trìn h m ực nước đ iể n h ìn h (ciạng t r u n g bình n h iê u n ă m th ờ i k ỳ 1 9 8 8 -1 9 9 8 ) c ủ a t r ạ m H à N ộ i c h o t h à y :• H iện tư ợ n g xói s â u do ả n h hư ỏn g đ iê u t i ế t c ủ a hồ H oà B ìn h c h ủ y ê u c h ỉ x ả y ra trên sô n g Đà và c h ỉ la n t r u y ề n đên trạ m th ủ y v ă n T r u n g H à.
■ Tốc độ xói có xu h ư ớ n g g iả m dần. Lòng s ô n g Đ à d ầ n d ầ n trở n ê n ổn đ ịn h (lập lại trạng
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c â n b ằ n g mói).