Diurnal variation in salinity and currents in Vasishta Godavary estuary, east coast of India

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1 Indian Jurnal f Marine Sciences Vl. 18, March 1989, pp Diurnal variatin in salinity and currents in Vasishta Gdavary estuary, east cast f India Y V Ramana, V Ranga Ra & B S R Reddy Department f Meterlgy and Oceangraphy, Andhra University, WaItair , India Received 30 March 1988; revised 28 Nvember 1988 Distributin f salinities and currents in Vasishta branch f Gdavari estuarine system, in relatin t the fresh water discharge and tides has been studied in 3 representative mnths. In July, when the river discharge is very high, the estuary exhibits sme characteristics f a salt wedge type. In September, when the river discharge is usually smaller cmpared t that during July, the estuary tends t becme a partly mixed type while during February (river discharge being minimum) the estuary becmes a well mixed type. These features are well reflected in the hurly (fr 12 h) bservatins f salinities and currents at the 3 selected statins and in the stratificatin - circulatin diagram. Lngitudinal exchange cefficients shw a decreasing trend with decrease in the fresh water discharge and with the increase in the distance frm the muth upstream. In an earlier study, variatins in salinity, current and temperature in the Gautami branch f the Gdavari estuarine system, east cast f India have been investigated I. The present paper pertains t diurnal variatins (12 h) f salinities and currents in the Vasishta branch f Gdavari estuarine system. Current and salinity distributin within the estuary in relatin t tide and fresh water discharge is discussed. Materials and Methds The study area (Fig. 1) is the lwer mst 25 km f the Vasishta Gdavari branch, which pens int the Bay f Bengal at Antarvedi. The tidal influence, in general, is bserved up t 25 km upstream frm the muth f the estuary with the tidal range f m at the muth f the estuary. The estuary has an average width f 500 m and depth f 10 m. The fresh water discharge is maximum during June t Octber because f suthwest mnsn and is minimum during January t May. Hurly bservatins (0600 t 1800 hrs, 1ST) f salinity and current ver a full tidal cycle were made at each f the 3 statins (Fig. 1). The statins were fixed alng the axis f the channel, where greater depths were encuntered. July and September 1986 and February 1987 were selected fr bservatins crrespnding t different fresh water discharge cnditins. Salinities and currents were measured 1 at 2 m depth intervals frm surface t 1 m abve the bttm t btain vertical prfiles. The time f ccurrence f high water and lw water at each statin and estuarine depths was studiedl. Results and Discussin Hurly bservatins f salinities and currents- July: During this mnth, the river discharge was very high (abut 2,35,000 cusecs) int the estuary. Fr st 1 (Fig. 2a), salinity curves at the surface and intermediate depths cincided "'35 'E 8 A Y OF 81 45' E 8 ENGA L Fig. 1 - Statin lcatins in Gdavari estuary 54

2 RAMANA et az.: DIURNAL VARIATION IN SAUNITY & CURRENTS : 10 :~""l~ lj).~ 4 20 FLOOD Time(hrs,IST) 'l" 6 7 ~LOOD i /" EB B,.// /~ '-" '. ~/,... " 40 EB B FIG.2, 7 8 (0 7 e " I SU RF ACE \ \\\ ---_ INTERMEDIATE DEPTH _ 1m ABOVE BOTTON \ \\\ \,, 40 ~ 20 CII~ 0-_ 0 III Iv C 3: 20 ~ E 40 ~ u <3 ~ :c 100 Cll_ ~E 75 CII~ 50-0 i= 25 (Q) 100 7S <;0 25 (e) FIG.3 Figs 2 and 3 - Salinity and velcity variatins ver a tidal cycle n (2) July 1986 and (3) September 1986 at st I(a), st 2 (b) and st 3 (c) 55

3 INDIAN J MAR SCI, VOL. 18, MARCH 1989 with the zer axis since the estuary at this statin was filled with fresh water bth at surface and intermediate levels. Even at the bttm layers, the estuary was filled with fresh water at the time f lw tide. Salinity at the bttm gradually increased during the fld perid reaching a maximum f 14.8 x 10-3 arund the time f high water and decreased t zer during the succeeding ebb perid. Ebb current dminated thrughut the tidal cycle at the surface and at the intermediate levels (Fig. 2a). Weak fld currents were hwever, nticed at the bttm during fld perid. At the surface as well as at the intermediate levels, the ebb currents were maximum at the mid ebb perid (abut 3 h after high water) while the currents were minimum arund the mid fld perid (abut 3 h after lw water). Maximum currents (128 cm sec - I) were bserved at the surface during this mnth. The fld regime was nticed nly at the bttm layers; the maximum fld current velcities being 10 cm sec - I. Near the bttm, the change in the directin f the current frm ebb t fld ccurred arund the time f lw water and the change frm fld t ebb tk place 30 min after the ccurrence f high water. These cnditins clearly shwed that the seawater penetrated int the estuary in the frm f a thin saline wedge acrss the muth. At st 2 (Fig. 2b) the estuary was cmpletely filled with fresh water frm surface t bttm during the entire tidal cycle. Hence, nly the distributin f currents is shwn in Fig. 2b, alng with the tide phase diagram. The fld regime was ttally absent at all depths during the entire tidal cycle. This clearly shwed the dminance f ebb flw at this statin as a cnsequence f very high discharge int the river. At the surface, maximum current velcity (135 cm sec-i) was bserved during mid ebb perid while minimum (68 cm sec - I) was at the mid fld perid. Mre r less similar variatins in currents were bserved at the intermediate depth during the tidal cycle. Even at the bttm, the currents were flwing twards the sea (ebb) during the entire tidal cycle, but the velcities were very small (10 t 30 cm sec-i ). At st 3 (Fig. 2c), salinity was zer thrughut the tidal cycle at all levels since the river was cmpletely filled with fresh water frm surface t bttm. Hence, the current variatins nly are shwn in the figure alng with the tide levels. The currents were fairly strng at all levels and the tidal influence was almst absent at this statin. On the whle, salinity and current variatins in July reflect the fllwing features. Excessive fresh 56 water discharge int the river caused the predminance f ebb current at all statins. Current velcities were maximum at surface and decreased twards the bttm. The effect f tide was nticed nly at st 1, where seawaters entered the estuary, making the waters sme what saline at the bttm layers. At sts 2 and 3 since they are away frm the muth, seawater was nt encuntered at any level. September: The river discharge during this mnth was slightly less (abut 37,500 cusecs) int the estuary. At st 1 saline waters mixed with the river water t varying degree at all levels frm surface t bttm (Fig. 3a). An increase f salinity frm surface t bttm was generally nticed. At the surface, salinity gradually increased during fld perid reaching a maximum f 13 x 10-3 arund high water time and decreased t zer arund the mid ebb perid. At intermediate level, maximum salinity f 20 x 10-3 was bserved 30 min after the ccurrence f high water, which decreased t 12 x 10-3 during ebb perid. At the bttm layers variatins in salinity were generally less (maximum 22 x 10-3 at high water time and 14 x 10-3 arund lw water time). Salinities at all depths were greater than thse bserved in July. Current distributin at st 1 shwed that the fld regime was als well develped during this mnth but was still nt as dminant as the ebb regime. The fld regime persisted fr a lnger time at the bttm, cmpared t the ebb regime. Cmpared t July, ebb current velcities during this mnth were generally lw at the 3 levels but the fld currents at the bttm were slightly high. These bservatins indicated partially mixed cn~ ditins in the estuary at this statin during September. At st 2 (Fig. 3b), the estuary was filled with fresh water near the surface during all phases f tide. Seawater reached this statin thrugh the subsurface layers. At intermediate level, salinity increased during fld tide frm zer and attained a maximum f 14.8 x 10-3 at the time f high water. Thereafter, salinity decreased rather shatply and attained zer value at abut 3 h after the ccurrence f high water. At the bttm, salinity started increasing at abut 1 h befre the ccurrence f lw water, reached a maximum value f 19.8 x 10-3 at 30 min after the ccurrence f high water and again decreased gradually during ebb perid. The flw at the surface was always directed twards sea ver the entire tidal cycle. The ebb current velcities were greater cmpared t the fld velcities. At the bttm layers, there was a clear dminatin f the fld regime ver

4 RAMANA et al.: DIURNAL VARIATION IN SALINITY & CURRENTS the ebb regime; the fld current persisted fr abut 7 h, while the ebb current lasted fr the rest f the perid in the tidal cycle. At the bttm maximum current bserved during bth ebb and fld was 26 cm sec - 1. At st 3 (Fig. 3c), the estuary was cmpletely ccupied with fresh water thrughut the tidal cycle. The flw at all levels was directed twards the sea. Salinity and current variatins during September reveal the fllwing features: At st 1, seawater entered the estuary during the fld perid and mixed with the river waters bringing abut hurly variatins in salinity with the tide at all levels. At st 2, saline waters were encuntered nly at subsurface layers while at surface it was filled with fresh water. This is due t the predminant effect f ebb regime at the surface due t fresh water discharge. At st 3, fresh water discharge was clearly seen and the tidal influence was negligible. The ebb regime dminated frm tp t bttm at this statin. Similar features were reprted earlier in MandvF and Zuari3 estuaries. February: The river discharge was negligible during this mnth and n fresh water was discharged int the estuary. Salinity and current variatins with tide were clearly reflected during this mnth (Fig. 4). Salinity variatins at intermediate and bttm levels were mre r less similar at st 1 (Fig. 4a). Salinity difference between surface and bttm was very small cmpared t that during September. The surface t bttm salinity difference was minimum at the mid fld perid when the fld. currents were fairly strng. But this difference was smewhat high at bth high water time and lw water time. The same feature was als bserved in the narrws f the Mersey estuary4. Dumg this mnth the salinities at all levels were very high cmpared t thse bserved in July and September. The currents at all levels at st 1 appeared t be cntrlled ttally by the tide (Fig. 4a) and the current variatins were almst in phase with the tide. Maximum ebb currents (46 cm sec - 1) were bserved at the intermediate level. The fld currents bth at the surface and at the bttm were relatively weak. Similar bservatins were made earlier in Gautami Gdavari1,s estuary. Salinity and velcity variatins at sts 2 and 3 (Fig.4b and c) exhibit mre r less the same features as thse bserved at st 1. But the salinity values shwed a decreasing trend as ne mves frm st 1 t 5t 3. Change in the directin f currents tk place at all levels, a feature that was nt existed fr st 2 during July and September. At st 3, maximum salinities were t.8 x t-3 and 11.4 x 1O~3, respectively at surface and bttm. Maximum ebb current velcity f 46 cm sec - 1 and a maximum fld current f 36 em sec - 1 were bserved at the surface. At the bttm, currents were generally weak due t the effect f bttm frictin. In February, the effect f tide was nticed even at st 3 situated abut 25 km, upstream frm the muth f.the estuary. Well mixed cnditins were encuntered at all statins and the surface t bttm salinity difference was quite small. Strng fld currents were encuntered at intermediate ~..,..-..l _ 7.2 : '024 ~ 20 C -... ~ Ii'l ~22 / '~ l4 10 "~ : 125 0> iii~l00 ~E QI u 75,,~ i= (0) (b) Fig. 4 - Salinityand velcity variatins ver a tidal cycle n Feb at st 1 (a),8t 2 (b) and st 3 (c) 57

5 INDIAN J MAR SCI, VOL. 18, MARCH 1989 level while strng ebb currents were bserved in surface layers. The effect f bttm frictin in reducing the current velcities at the bttm is clearly seen. Classificatin f estuary using $(ratificatin-circulatin diagram - Based n measurements f salinity and current velcity, the Vasishta Gdavari estuary has been classified int different types using the methd develped by Hansen and Rattray6. Tw dimensinless parameters t characterise estuaries - (i) a stratificatin parameter (() s/s), rati f surface t bttm difference in salinity ()s) divided by mean crss-sectinal salinity (s) and (ii) a circulatin parameter (V/Vr), rati f net surface current (V.) t mean crss-sectinal velcity (Vr) - have been used. The values f the abve parameters have been evaluated fr different mnths at the 3 statins and lcated in the stratificatin-circulatin diagram (Fig. 5). In July, st 1 fell in type 4, representing salt wege type with intense stratificatin. Sts 2 and 3 were cmpletely filled with fresh water at all levels and hence did nt appear in the stratifica.:. tin-circulatin diagram. In September, st 1 fell in type 2b (partly mixed type with cnsiderable stratificatin). St 2 fell in between type 4 and type 2b indicating that the estuary at this statin was tending twards partly mixed type frm the salt wedge type. St 3 was cmpletely ccupied with fresh water and hence did nt appear n the diagram. In February, all 3 statins fell in type la, representing well mixed cnditins. Flushing rates and lngitudinal exchang~ cefficients f the estuary were als calculated fr the 3 representative mnths based n all the data cllected. The flushing rate, which represents the rate at which the ttal vlume f the water in the estuary is exchanged, was calculated using a simple relatin?. R Flushing rate = ~ f where R = river run ff flw and f = average fractin f fresh water in the estuary. Fresh water fractin (f) at a given lcatin is given by: f = (S- S)/S where S= nrmal cean salinity f the castal waters int which the estuary empties and s = salinity at that particular lcatin. The flushing rates, estimated frm the bserved data were 5611, 1234 and 87 m3 sec-i respectively fr July, September and February. The effect 10 Fig. 5 - Stratificatin-circulatin diagram (A= st 1, N = st 2 and R = st 3. Suffixes: J =July, S = September and F= February) Table 1 - Lngitudinal July 1986 September 1986 Febru:.:'Y 1987 exchange cefficients [V.aluesrepresent: ( x 1(6) cm2 see I] St 1 St 2 St f river discharge is thus clearly reflected in the flushing rate values fr different mnths. Lngitudinal exchange cefficients which are als very imprtant in understanding the characteristics f an estuary were caiculated8. The exchange cefficient was very large at st 1 during July when the fresh water discharge is very high (Table 1). The cefficient decreased with the decrease in river discharge and was minimum in February. The exchange cefficients shwed a decreasing trend frm the muth upstream. Similar features have been reprted earlier frm Yaquina9, TaylOand Ptmac II estuaries. Acknwledgement The authrs (Y V R & V R R) express their sincere gratitude t the CSIR, New Delhi fr the award f research fellwships during the perid f this study. References 1 Ranga R.a V, R.amana Y V & Reddy B S R, Indian J MarSci, 17(1988) Varma K K, Gangadhara R.a L V & Cherian T, Indian J Mar Sci, 4 (1975) Cherian T, Gangadharl). R.a L V & Varma K K, Indian J MarSci, 4 (1975)

6 RAMANA et al.: DIURNAL VARIATION IN SAUNITY & CURRENTS 4 Hughes P, GephyJ RyAst Sc, 1 (1958) Rama Sarma D V, Hydrgraphy f the Gdavary estuary, Ph.D. thesis, Andhra University, Hansen D V & Rattray M Jr, Limnl Oceangr, 11 (1966) Officer C B, Physical Oceangraphy f estuaries, (Jhn Wiley & Sns, USA) 1976, Stmmel H, J Sewage and Industrial Wastes, 25 (1953) Burt W V & MlpTiage L D, J Sewage and Industrial Wastes, 29 (1957) West J R & Williams D J A, ASCE, Prc 13th Cn! n Castal Engg, (1972) Hetling L J & O'Cnnell R L, Water Resurces Research, 2(1966)