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Improvement study for the Parachique tidal inlet
Coastal Engineering, 1 (1977) 323--348 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
323
IMPROVEMENT STUDY FOR THE PARACHIQUE TIDAL INLET
RONALD MOOR Fishery Harbour Development Project, Dutch Bilateral Technical Cooperation, Lima (Per~) (Received December 13, 1976; accepted July 6, 1977)
ABSTRACT Moor, R., 1977. Improvement study for the Parachique tidal inlet. Coastal Eng., 1: 323--348. A fishing village became established at Parachique due to the nearness of good fishing areas and natural protection against wave action, offered by the tidal inlet. Not until after Peru's Ministry of Fisheries improved the infrastructure (quaywall, fish reception terminal, iceplants, cold stores, travel lift, etc.), were the problems often found at unstable tidal inlet's observed, i.e.: location instability of the entrance and a shallow access channel. Under a Dutch Technical Cooperation programme, the Engineering Services Office of the Ministry of Fisheries carried out an extensive field survey to determine the natural process of the tidal inlet and a study for possible improvements. The survey included detailed bathymetrics outside and inside the tidal inlet, float measurements, current measurements to calculate the flow rate and simultaneous tide registration at several locations, to enable reproduction of the tidal wave propagation in a mathematical model, in order to study the effect of several possible changes. This paper summarizes the most important aspects of the study, in which two alternative solutions emerged as technically feasible, both solutions involving jetties and a sand bypassing arrangement.
INTRODUCTION M o r e t h a n 2 0 0 vessels w o r k i n g in t h e smaU-scale f i s h e r y (less t h a n 10 t o n s d e a d - w e i g h t ) , utilize P a r a c h i q u e ( F i g s . l , 2 a n d 3) as a h o m e basis f o r f i s h e r y activities, l a n d i n g a b o u t 1 5 , 0 0 0 t o n s o f fish a n n u a l l y . P a r a c h i q u e is l o c a t e d 1 k m inside a t i d a l inlet (Fig.2), fish landings b e i n g f a c i l i t a t e d b y a fish r e c e p t i o n t e r m i n a l having a 2 0 0 m l o n g q u a y w a l h Also, a travel lift h a s b e e n c o n s t r u c t e d f o r vessel m a i n t e n a n c e a n d repair. H o w e v e r , t h e inlet p r e s e n t s p r o b l e m s , r e l a t e d t o t h e i n s t a b i l i t y o f t h e e n t r a n c e a n d t h e s h a l l o w n e s s o f t h e access c h a n n e l d u r i n g l o w - w a t e r tides, b o t h t h e s e p r o b l e m s b e i n g e m p h a s i z e d a f t e r a b r e a k t h r o u g h , c r e a t i n g t w o e n t r a n c e s (see Fig.12). INLET STABILITY F o r a s t a r t t h e inlet h a d t o b e classified a c c o r d i n g t o t h e p r e d o m i n a n t m o d e
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INLET
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SCALE: Z, STE
INDICATED
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327 o f littoral d r i f t by-passing, e i t h e r b y " b a r " , " t i d a l f l o w " o r a c o m b i n a t i o n o f b o t h . T h e classification o f t h e tidal inlet can be a c c o m p l i s h e d o n t h e basis o f t h e r e l a t i o n b e t w e e n n e t littoral d r i f t (S) and tidal prism ( ~ ) or a l t e r n a t i v e l y b e t w e e n S and t h e m a x i m u m tidal f l o w (Qm) ( B r n u n et al., 1 9 5 5 ; B r u u n , 1966). T o calculate t h e q u a n t i t i e s involved, a h y d r o g r a p h i c survey was carried o u t , including extensive b a t h y m e t r y covering all t h e o u t l y i n g zones and d e e p i n t o t h e inlet (Fig.2). T h e ebb and f l o o d c u r r e n t s w e r e m e a s u r e d with floats (Fig.4), and d u r i n g spring t i d e v e l o c i t y profiles were m e a s u r e d at a section o f t h e inlet's e n t r a n c e (Figs.5A, B) w i t h c u r r e n t meters. T h e m a x i m u m flow rate was calc u l a t e d f r o m t h e s e d a t a as Qm = 500 m 3/s (Fig.6).
NOTATION List of symbols C co
Ch~zy friction coefficient deepwater wave speed
D
grain size
H0 Hb Hs 11, I~ K K~ m Q Qm S Sb Ss Ts U0
deepwater wave height breaker wave height significant wave height parameters ripple height refraction coefficient beach slope tidal flow rate maximum flow rate total sand transport transport of bottom material transport of material in suspension significant wave period orbital velocity at the bottom
V
velocity
A
relative density coefficient breaking wave angle of incidence ripple factor tidal prism
~b
u
T o d e t e r m i n e t h e n e t littoral drift, existing swell a n d sea d a t a in d e e p w a t e r were a n a l y z e d ( A n o n y m o u s , 1 9 6 0 ) . T a b l e I s u m m a r i z e s wave o c c u r r e n c e f o r swell over a year. It s h o u l d b e o b s e r v e d t h a t t h e d i r e c t i o n s n o t i n c l u d e d in t h e t a b l e d o n o t o c c u r , even over t h e s h o r t e r t h r e e - m o n t h periods given in A n o n y m o u s (1960). Swell waves arriving at t h e coast o f Peru are o r i g i n a t e d
328
TABLE I Deep w a t e r wave d i s t r i b u t i o n in percentages, f o r swell H (m)
SE
0.30--1.80 1.80--3.60 >3.60 Total
S
SW
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39.00 19.50 1.25
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E
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1.00
Calm 8%; u n d e f i n e d 0 . 5 0 %
between 35 ° and 40 ° of southern latitude. The sea data (not included here) observe a similar pattern. Furthermore, a wind rose for Bayovar (Fig.18) indicates almost continuous winds from between the south and southeast. Swell from the west occurs only during 1% of the time, and refraction diagrams show a slight tendency to induce northward sand transport. Thus, it can be concluded that for littoral drift computations it is sufficiently accurate to base the calculations on swell from the south and southwest. To calculate the net littoral drift, the deep-water wave distribution was determined (Fig.7), obtaining Hss ° = 1.50 m as the significant wave height exceeded 50% of the time. Refraction diagrams were drawn for the predominant south and southwest directions (Figs.8, 9, 10) finding a refraction coefficient of Kr = 0.20 and an average wave incidence angle at the breaker depth
of ~b = 8°.
For a first impression of the littoral drift, Galvin's (U.S. Army Coastal Engineering Research Center (CERC), 1973) formula was utilized: S = 2 . 1 0 s (Hb 2 )
(Hb in feet; S in cub. yards per year)
For Hb = 0.25 m (from visual observations during the survey) a north-bound littoral drift rate of 105,000 m 3/year was calculated. Later on, with more data available the CERC (1973) formula was utilized: S ~- 0.014 H~coK2r sin ~ b COS ~ b with H0 = 1.50 m, Co = 21.8 m/s (for Ts = 14 sec), ~ b = 8 ° , and a 74% occurrence, littoral drift was calculated as: S = 88,000 m 3 ]year. Also the Bijker (1972) formula was utilized, together with the Longuet-Higgins (CERC, 1973) formula to calculate the longshore current: V = 20.7 m ~ For ~b = 8°, Hb =
sin 2~b 0.30 m and a 1 : 40 beach slope this gives V = 0.24 m/s.
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PARACHIQUE TIDAL INLET REFRACTION DIAGRAM
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SOUTH AND SOUTH WEST Tzl4 809. DATE~
gEP~. 11976
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SGALE:
I 0 ORAWN : P.A,L. REV:
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337 Bijker (1972) gives the bed load as:
--
exp
=0.27 2\
-I V / "~
and suspension transport as: Ss = 1.83Sb [I, ln ( ~ h ) + I2] For: U0 = 0.70 m/s; g = 0 . 5 ; D = 0.18.10 -3 m, C = 50 m ~ n / s and A = 1.65, Sb can be calculated as: Sb = 6,200 m 3/year. With a ripple height of K = 0.04 m and a water depth of h = 0.60 m, the relation S J S b = 14 can be found from Bakker (1971). This gives a total littoral drift of S = 92,000 m 3/year. In addition the (south) spit accretion was measured b e t w e e n t w o soundings, taken with a 4 months interval ( F i g . l l ) , as 25,000 m 3 . Under the assumption that this represents an average accretion, since the wave regime over the year shows no exceptional changes -- apart from the somewhat higher waves (swell) during the winter months (June, July, August, September) than during the summer months (January, February, March) -- the annual accretion would be in t h e order of 75,000 m 3 , and obviously the littoral drift will be greater by the a m o u n t by-passed. As a conclusion, a north-bound littoral drift in the order of 100,000 m 3 ]year seems a reasonable quantity for further calculation purposes. The b a y considered as a whole, has a crenulate shape and is possibly tending toward an equilibrium form. The source of sand to satisfy the available transport capacity of the waves is thought to be available in the existing beaches south of Parachique and sand is also supplied by the continuous winds blowing from the south to southeast over the desert which constitutes a major part of the whole surrounding land area. A relation S / Q m = 200 has thus been established, so that the tidal inlet's stability can be classified as transitional according to Fig.13, which has been prepared based on information from Bruun (1966) and Battj'es (1967), littoral drift being by-passed b y wave action over the half-moon-shaped offshore " b a r " formation in front of the inlet ( F i g s . l l , 12) and by tidal currents. In addition the quantity accreted at the south spit is by-passed after a breakthrough, whereafter the process is repeated. POSSIBLE I M P R O V E M E N T S
To improve the existing situation, the relation S / Q m should be appreciably reduced. This can be accomplished in principle either b y increasing Qm or decreasing S. It was thought that the removal of the causeway located a b o u t 9 k m inland from the entrance (Fig.2), obstructing the tidal wave advance, would increase the tidal prism and Q respectively.
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TIDAL
TIDAL INLET CLASSIFICATION DATE : SEP. It97111
SCALE: ,
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343 To study the effect of the causeway removal, 15
After discarding a great number of alternatives,two possible solutions emerged as technically feasible: (a) a curved south jetty including a submerged weir and sand trap behind it (Fig.16) in tandem with a north jetty; and (b) two curved jettiesaligned according to the flood and ebb currents (Fig.17). The main factors considered for jetty alignment are the following: jetty
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alignment parallel to tidal currents (ebb and flood); jetties perpendicular to depth contours to restrict capital outlay; protection of access channel from wave action; and avoid erosion and/or accretion in the access channel. Obviously all factors cannot be reconciled in one design so that a compromise solution has to be accepted. The t w o final alternatives considered are shown on Figs.16 and 17. Albeit alternative (a) would permit dredging the sand trap, it being somewhat protected b y the south jetty, the risk of n o t trapping all the littoral drift over the weir is quite great, increasing the possibility of shoal formation within the approach channel. Also, if future demand would require a greater depth in the access channel, the extension of the south j e t t y 1 in alternative (a) would impose some problems. It should also be mentioned that the way of establishing the weir length and height is quite arbitrary. Finally, alternative (a) obviously demands a higher initial capital outlay. For the above reasons alternative (b) was chosen as the solution involving the lesser risk of failure; an important fact in a country where knowledge of coastal engineering is just developing, and not being taught at universities, and where all breakwater and j e t t y construction is regarded with a degree of suspicion, specially if maintenance dredging is involved. The required width b e t w e e n the jetties was calculated for current velocities n o t exceeding 0.90 to 1.00 m/s at springtide, allowing some extra width for eddy formation at the inside of the south jetty, during flood currents, for alternative (b).
By-passing plant Continuous by-passing of littoral drift can be accomplished b y a fixed or a movable by-passing plant on a j e t t y or trestle, or it can be arranged b y means of a sand trap dredged continuously or at regular intervals using a hydraulic stationary suction dredge, since wave conditions are generally not bad. A fixed by-passing plant, would not be able to remove any shoals formed within the navigation channel by tidal currents or littoral drift by-passing the south jetty. Moreover the risk of the p u m p becoming choked after a major b r e a k d o w n or severe weather conditions is higher whereas this is non-existent for a movable pump. Therefore, a flexible solution was adopted in the form of a stationary suction dredge. COST ESTIMATE
The cost of the t w o jetties involved in solution (b) was calculated at 1975 prices at US $ 2 million, the investment required for dredge and by-passing equipment was calculated at US $ 1 . 2 million. Required annual maintenance dredging was estimated for a m a x i m u m of 150,000 m s of accretion due to littoral drift and 50,000 m 3 for improvement of the access channel. The first t w o years, spoil should be used to raise the spit adjacent to the south jetty, improving resistance to short durations of heavy
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347
CALM 0.9%
-I $
PARACHIQUE TIDAL INLET FIGURE :
WlNDROSE FOR BAYOVAR NOTE: Averoge Over
18
1963/1969 DATE : JUNE 1 t 9 7 7
SCALE:
DRAWN: W.M.R REV: R, MOOR
348
swell conditions. This would also allow a longer construction time for the bypassing installations. Spoil from the navigation channel dredging can be used for channel alignment improvements and for land reclamation behind the terminal; to be used for further fishery industry expansion.
REFERENCES Anonymous, 1960. Sailing directions for South America, Vol. III, 6th ed. Oceanographic Office Publications, 25, USA. Bakker, W.T., 1971. De Dynamica van Kusten. Post doctorate course, Delft University of Technology, Delft. Battjes, J.A., 1967. Quantitative Research on Littoral Drift and Tidal Inlets. Coastal Engineering Laboratory, University of Florida, Gainesville. Bruun, P., 1966. Tidal Inlets and Littoral Drift, Vol. 2. Skipnes Offsettrykkeri, Trondheim, 200 pp. Bruun, P., 1976. Port Engineering, 2nd ed. Gulf Publishing Co., Houston, Texas, 600 pp. Bruun, P. and Gerritsen, F., 1955. Stability of Coastal Inlets, Vol. 1. North-Holland, Amsterdam, 125 pp. Bruun, P., Gerritsen, F. and Bhakta, N., 1974. Evaluation of overall entrance stability of tidal entrances. Proc. Int. Conf. Coastal Eng., 14th, Copenhagen. Bijker, E.W., 1972. Lecture Notes for Topics in Coastal Engineering. Delft University of Technology, Delft. Direcci~n General de Extracci6n, 1974. Estudio Portuario Parachique. Ministry of Fisheries, Lima. Groot, R., 1971. Rehabilitatie van stranden. Ports and Dredging, 71. IHC Holland publication, Rotterdam. Herron, W.J. and Harris, R.L., 1966. Littoral bypassing and beach restoration in the vicinity of port Hueneme, California. Proc. Int. Conf. Coastal Eng., 10th. Magnuson, N.C., 1967. Planning and design of a low-weir section jetty. Proc. ASCE, 93 (WW 2). Moor, R., 1976. Guia para el Disefio de Puertos Pesqueros. Direcci6n General de Extracci6n, Ministry of Fisheries, Lima. US Army Coastal Engineering Research Center, 1973. Shore Protection Manual. Government Printing Office, Washington, D.C.
323
IMPROVEMENT STUDY FOR THE PARACHIQUE TIDAL INLET
RONALD MOOR Fishery Harbour Development Project, Dutch Bilateral Technical Cooperation, Lima (Per~) (Received December 13, 1976; accepted July 6, 1977)
ABSTRACT Moor, R., 1977. Improvement study for the Parachique tidal inlet. Coastal Eng., 1: 323--348. A fishing village became established at Parachique due to the nearness of good fishing areas and natural protection against wave action, offered by the tidal inlet. Not until after Peru's Ministry of Fisheries improved the infrastructure (quaywall, fish reception terminal, iceplants, cold stores, travel lift, etc.), were the problems often found at unstable tidal inlet's observed, i.e.: location instability of the entrance and a shallow access channel. Under a Dutch Technical Cooperation programme, the Engineering Services Office of the Ministry of Fisheries carried out an extensive field survey to determine the natural process of the tidal inlet and a study for possible improvements. The survey included detailed bathymetrics outside and inside the tidal inlet, float measurements, current measurements to calculate the flow rate and simultaneous tide registration at several locations, to enable reproduction of the tidal wave propagation in a mathematical model, in order to study the effect of several possible changes. This paper summarizes the most important aspects of the study, in which two alternative solutions emerged as technically feasible, both solutions involving jetties and a sand bypassing arrangement.
INTRODUCTION M o r e t h a n 2 0 0 vessels w o r k i n g in t h e smaU-scale f i s h e r y (less t h a n 10 t o n s d e a d - w e i g h t ) , utilize P a r a c h i q u e ( F i g s . l , 2 a n d 3) as a h o m e basis f o r f i s h e r y activities, l a n d i n g a b o u t 1 5 , 0 0 0 t o n s o f fish a n n u a l l y . P a r a c h i q u e is l o c a t e d 1 k m inside a t i d a l inlet (Fig.2), fish landings b e i n g f a c i l i t a t e d b y a fish r e c e p t i o n t e r m i n a l having a 2 0 0 m l o n g q u a y w a l h Also, a travel lift h a s b e e n c o n s t r u c t e d f o r vessel m a i n t e n a n c e a n d repair. H o w e v e r , t h e inlet p r e s e n t s p r o b l e m s , r e l a t e d t o t h e i n s t a b i l i t y o f t h e e n t r a n c e a n d t h e s h a l l o w n e s s o f t h e access c h a n n e l d u r i n g l o w - w a t e r tides, b o t h t h e s e p r o b l e m s b e i n g e m p h a s i z e d a f t e r a b r e a k t h r o u g h , c r e a t i n g t w o e n t r a n c e s (see Fig.12). INLET STABILITY F o r a s t a r t t h e inlet h a d t o b e classified a c c o r d i n g t o t h e p r e d o m i n a n t m o d e
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325
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PARACHIQUE TIDAL INLET rF;;UME: OVERALL PLAN 2 DATE: SEF'
SCALE: Z, STE
INDICATED
DRAWN: W.M.R. REV, R. MOOR
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327 o f littoral d r i f t by-passing, e i t h e r b y " b a r " , " t i d a l f l o w " o r a c o m b i n a t i o n o f b o t h . T h e classification o f t h e tidal inlet can be a c c o m p l i s h e d o n t h e basis o f t h e r e l a t i o n b e t w e e n n e t littoral d r i f t (S) and tidal prism ( ~ ) or a l t e r n a t i v e l y b e t w e e n S and t h e m a x i m u m tidal f l o w (Qm) ( B r n u n et al., 1 9 5 5 ; B r u u n , 1966). T o calculate t h e q u a n t i t i e s involved, a h y d r o g r a p h i c survey was carried o u t , including extensive b a t h y m e t r y covering all t h e o u t l y i n g zones and d e e p i n t o t h e inlet (Fig.2). T h e ebb and f l o o d c u r r e n t s w e r e m e a s u r e d with floats (Fig.4), and d u r i n g spring t i d e v e l o c i t y profiles were m e a s u r e d at a section o f t h e inlet's e n t r a n c e (Figs.5A, B) w i t h c u r r e n t meters. T h e m a x i m u m flow rate was calc u l a t e d f r o m t h e s e d a t a as Qm = 500 m 3/s (Fig.6).
NOTATION List of symbols C co
Ch~zy friction coefficient deepwater wave speed
D
grain size
H0 Hb Hs 11, I~ K K~ m Q Qm S Sb Ss Ts U0
deepwater wave height breaker wave height significant wave height parameters ripple height refraction coefficient beach slope tidal flow rate maximum flow rate total sand transport transport of bottom material transport of material in suspension significant wave period orbital velocity at the bottom
V
velocity
A
relative density coefficient breaking wave angle of incidence ripple factor tidal prism
~b
u
T o d e t e r m i n e t h e n e t littoral drift, existing swell a n d sea d a t a in d e e p w a t e r were a n a l y z e d ( A n o n y m o u s , 1 9 6 0 ) . T a b l e I s u m m a r i z e s wave o c c u r r e n c e f o r swell over a year. It s h o u l d b e o b s e r v e d t h a t t h e d i r e c t i o n s n o t i n c l u d e d in t h e t a b l e d o n o t o c c u r , even over t h e s h o r t e r t h r e e - m o n t h periods given in A n o n y m o u s (1960). Swell waves arriving at t h e coast o f Peru are o r i g i n a t e d
328
TABLE I Deep w a t e r wave d i s t r i b u t i o n in percentages, f o r swell H (m)
SE
0.30--1.80 1.80--3.60 >3.60 Total
S
SW
9.45 5.50 0.25
39.00 19.50 1.25
9.75 4.25 0.25
15.50
59.75
14.25
W
E
0.50 0.75 0.25 0.25 0.25 - 1.00
1.00
Calm 8%; u n d e f i n e d 0 . 5 0 %
between 35 ° and 40 ° of southern latitude. The sea data (not included here) observe a similar pattern. Furthermore, a wind rose for Bayovar (Fig.18) indicates almost continuous winds from between the south and southeast. Swell from the west occurs only during 1% of the time, and refraction diagrams show a slight tendency to induce northward sand transport. Thus, it can be concluded that for littoral drift computations it is sufficiently accurate to base the calculations on swell from the south and southwest. To calculate the net littoral drift, the deep-water wave distribution was determined (Fig.7), obtaining Hss ° = 1.50 m as the significant wave height exceeded 50% of the time. Refraction diagrams were drawn for the predominant south and southwest directions (Figs.8, 9, 10) finding a refraction coefficient of Kr = 0.20 and an average wave incidence angle at the breaker depth
of ~b = 8°.
For a first impression of the littoral drift, Galvin's (U.S. Army Coastal Engineering Research Center (CERC), 1973) formula was utilized: S = 2 . 1 0 s (Hb 2 )
(Hb in feet; S in cub. yards per year)
For Hb = 0.25 m (from visual observations during the survey) a north-bound littoral drift rate of 105,000 m 3/year was calculated. Later on, with more data available the CERC (1973) formula was utilized: S ~- 0.014 H~coK2r sin ~ b COS ~ b with H0 = 1.50 m, Co = 21.8 m/s (for Ts = 14 sec), ~ b = 8 ° , and a 74% occurrence, littoral drift was calculated as: S = 88,000 m 3 ]year. Also the Bijker (1972) formula was utilized, together with the Longuet-Higgins (CERC, 1973) formula to calculate the longshore current: V = 20.7 m ~ For ~b = 8°, Hb =
sin 2~b 0.30 m and a 1 : 40 beach slope this gives V = 0.24 m/s.
329
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SER 1,976
DATE:
INDICATED
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F L O W R A T E AT ENTRANCE
PARACHIQUE T I D A L
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6
FIGUREI
INLET
CO CO b~
333
334
CREST
FqJNTA
CAUSE WAY
I~tTA
SECHURA•
DEPTH IN
HOMD
20 I ~ .
PARACHIQUE WAVE REFRACTION FROM Fk~,U~: THE SOUTH T ' I 4 Sec. 8 DATE~ 3EP- ll978
SCALE :
D~RAWN~P.A.L*
INDICATED REV: R.MOOR.
335
~UNTA NE(~RA
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PARACHIQUE TIDAL INLET REFRACTION DIAGRAM
! FIs~is :
SOUTH AND SOUTH WEST Tzl4 809. DATE~
gEP~. 11976
l
SGALE:
I 0 ORAWN : P.A,L. REV:
R ,MOOR
337 Bijker (1972) gives the bed load as:
--
exp
=0.27 2\
-I V / "~
and suspension transport as: Ss = 1.83Sb [I, ln ( ~ h ) + I2] For: U0 = 0.70 m/s; g = 0 . 5 ; D = 0.18.10 -3 m, C = 50 m ~ n / s and A = 1.65, Sb can be calculated as: Sb = 6,200 m 3/year. With a ripple height of K = 0.04 m and a water depth of h = 0.60 m, the relation S J S b = 14 can be found from Bakker (1971). This gives a total littoral drift of S = 92,000 m 3/year. In addition the (south) spit accretion was measured b e t w e e n t w o soundings, taken with a 4 months interval ( F i g . l l ) , as 25,000 m 3 . Under the assumption that this represents an average accretion, since the wave regime over the year shows no exceptional changes -- apart from the somewhat higher waves (swell) during the winter months (June, July, August, September) than during the summer months (January, February, March) -- the annual accretion would be in t h e order of 75,000 m 3 , and obviously the littoral drift will be greater by the a m o u n t by-passed. As a conclusion, a north-bound littoral drift in the order of 100,000 m 3 ]year seems a reasonable quantity for further calculation purposes. The b a y considered as a whole, has a crenulate shape and is possibly tending toward an equilibrium form. The source of sand to satisfy the available transport capacity of the waves is thought to be available in the existing beaches south of Parachique and sand is also supplied by the continuous winds blowing from the south to southeast over the desert which constitutes a major part of the whole surrounding land area. A relation S / Q m = 200 has thus been established, so that the tidal inlet's stability can be classified as transitional according to Fig.13, which has been prepared based on information from Bruun (1966) and Battj'es (1967), littoral drift being by-passed b y wave action over the half-moon-shaped offshore " b a r " formation in front of the inlet ( F i g s . l l , 12) and by tidal currents. In addition the quantity accreted at the south spit is by-passed after a breakthrough, whereafter the process is repeated. POSSIBLE I M P R O V E M E N T S
To improve the existing situation, the relation S / Q m should be appreciably reduced. This can be accomplished in principle either b y increasing Qm or decreasing S. It was thought that the removal of the causeway located a b o u t 9 k m inland from the entrance (Fig.2), obstructing the tidal wave advance, would increase the tidal prism and Q respectively.
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PARACHIQUE
TIDAL
TIDAL INLET CLASSIFICATION DATE : SEP. It97111
SCALE: ,
INLET
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8GALE:
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343 To study the effect of the causeway removal, 15
After discarding a great number of alternatives,two possible solutions emerged as technically feasible: (a) a curved south jetty including a submerged weir and sand trap behind it (Fig.16) in tandem with a north jetty; and (b) two curved jettiesaligned according to the flood and ebb currents (Fig.17). The main factors considered for jetty alignment are the following: jetty
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345
alignment parallel to tidal currents (ebb and flood); jetties perpendicular to depth contours to restrict capital outlay; protection of access channel from wave action; and avoid erosion and/or accretion in the access channel. Obviously all factors cannot be reconciled in one design so that a compromise solution has to be accepted. The t w o final alternatives considered are shown on Figs.16 and 17. Albeit alternative (a) would permit dredging the sand trap, it being somewhat protected b y the south jetty, the risk of n o t trapping all the littoral drift over the weir is quite great, increasing the possibility of shoal formation within the approach channel. Also, if future demand would require a greater depth in the access channel, the extension of the south j e t t y 1 in alternative (a) would impose some problems. It should also be mentioned that the way of establishing the weir length and height is quite arbitrary. Finally, alternative (a) obviously demands a higher initial capital outlay. For the above reasons alternative (b) was chosen as the solution involving the lesser risk of failure; an important fact in a country where knowledge of coastal engineering is just developing, and not being taught at universities, and where all breakwater and j e t t y construction is regarded with a degree of suspicion, specially if maintenance dredging is involved. The required width b e t w e e n the jetties was calculated for current velocities n o t exceeding 0.90 to 1.00 m/s at springtide, allowing some extra width for eddy formation at the inside of the south jetty, during flood currents, for alternative (b).
By-passing plant Continuous by-passing of littoral drift can be accomplished b y a fixed or a movable by-passing plant on a j e t t y or trestle, or it can be arranged b y means of a sand trap dredged continuously or at regular intervals using a hydraulic stationary suction dredge, since wave conditions are generally not bad. A fixed by-passing plant, would not be able to remove any shoals formed within the navigation channel by tidal currents or littoral drift by-passing the south jetty. Moreover the risk of the p u m p becoming choked after a major b r e a k d o w n or severe weather conditions is higher whereas this is non-existent for a movable pump. Therefore, a flexible solution was adopted in the form of a stationary suction dredge. COST ESTIMATE
The cost of the t w o jetties involved in solution (b) was calculated at 1975 prices at US $ 2 million, the investment required for dredge and by-passing equipment was calculated at US $ 1 . 2 million. Required annual maintenance dredging was estimated for a m a x i m u m of 150,000 m s of accretion due to littoral drift and 50,000 m 3 for improvement of the access channel. The first t w o years, spoil should be used to raise the spit adjacent to the south jetty, improving resistance to short durations of heavy
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348
swell conditions. This would also allow a longer construction time for the bypassing installations. Spoil from the navigation channel dredging can be used for channel alignment improvements and for land reclamation behind the terminal; to be used for further fishery industry expansion.
REFERENCES Anonymous, 1960. Sailing directions for South America, Vol. III, 6th ed. Oceanographic Office Publications, 25, USA. Bakker, W.T., 1971. De Dynamica van Kusten. Post doctorate course, Delft University of Technology, Delft. Battjes, J.A., 1967. Quantitative Research on Littoral Drift and Tidal Inlets. Coastal Engineering Laboratory, University of Florida, Gainesville. Bruun, P., 1966. Tidal Inlets and Littoral Drift, Vol. 2. Skipnes Offsettrykkeri, Trondheim, 200 pp. Bruun, P., 1976. Port Engineering, 2nd ed. Gulf Publishing Co., Houston, Texas, 600 pp. Bruun, P. and Gerritsen, F., 1955. Stability of Coastal Inlets, Vol. 1. North-Holland, Amsterdam, 125 pp. Bruun, P., Gerritsen, F. and Bhakta, N., 1974. Evaluation of overall entrance stability of tidal entrances. Proc. Int. Conf. Coastal Eng., 14th, Copenhagen. Bijker, E.W., 1972. Lecture Notes for Topics in Coastal Engineering. Delft University of Technology, Delft. Direcci~n General de Extracci6n, 1974. Estudio Portuario Parachique. Ministry of Fisheries, Lima. Groot, R., 1971. Rehabilitatie van stranden. Ports and Dredging, 71. IHC Holland publication, Rotterdam. Herron, W.J. and Harris, R.L., 1966. Littoral bypassing and beach restoration in the vicinity of port Hueneme, California. Proc. Int. Conf. Coastal Eng., 10th. Magnuson, N.C., 1967. Planning and design of a low-weir section jetty. Proc. ASCE, 93 (WW 2). Moor, R., 1976. Guia para el Disefio de Puertos Pesqueros. Direcci6n General de Extracci6n, Ministry of Fisheries, Lima. US Army Coastal Engineering Research Center, 1973. Shore Protection Manual. Government Printing Office, Washington, D.C.