Current Regime in the Gulf of Kutch

Indian Journal of Marine Sciences Vol. 6,June 1977, pp. 39-48

P. S. SRIVASTAVA& V. C. JOHN Engineers India Ltd, New Delhi 110001

Received 10 June 1976; revised received 17 January 1977

Long term in situ current data have been collected for a number of locations in the Gulf of Kutch and analysed. Tides in the area are of mixed type and are predominantly semidiurnal with a large diurnal inequality. The mean spring tidal range from mouth to the closed end of the Gulf ranges between 2·1 and 6·2 m. A correlation between tidal ranges and tidal current speeds has been established for Gurur and Bobby Shoals and off Narara Bet. The data reveal that the major steady currents in the area are of tidal origin. However, during south-west monsoon period, strong westerly winds would generate wind driven currents, with surface speeds reaching about 0·5 mjsec. The width of the Gulf narrows down between long. 69°E and 69°20'E causing an increase in the tidal current speeds, with speeds reaching up to 2·5 m/sec. The density currents in the Gulf are negligible.

URRENT regime in an area is one of the im- The currents prevailing in the Gulf are controlled portant factors affecting navigation, fisheries, by the various parameters viz. wind, tide, bathy- C offshore structures, harbour construction and metry, coastal configuration and density distribu- coastal climate. Systematic study of current regime tion. A brief description of the above is presented in the Gulf of Kutch (Fig. 1) has not been done in this paper. previously, though general knowledge of the surface curren ts for a few localized places can be had by General Description of the Area referring to the west coast of India Pilot or Naval There is no major river discharging into the Hydrographic office charts. Gulf and the average annual rainfall in the area is

JAKHAU

. . l 13

I r: \.j '-'-, \. ./_.rI \ ,--._/

• _JAMNAGAII LEGEND - 3·66m CONTOURS to·97m CONTOURS _. --- 18·26n'1 CONTOURS - .. - ..- 36'56m CONTOURS

Fig. 1 - Station locations in Gulf of Kutch

39 INDIAN J. MAR. SCI., VOL. 6, JUNE 1977 less than 52·5 cm ; hence 2 layer flow as encountered inequality. Various tidal elevations viz. mean low in a normal estuary is not expected in the area. water springs, mean low water neaps, mean sea Mean spring tidal range- from the mouth to the level, mean lligh water neaps and mean high water closed end of the Gulf ranges between 2·1 and 6·2 rn, springs for 4 locations are presented in Table l. hence high speed tidal currents are expected. From a study of the tidal variations at various The time that a tidal wave takes to proceed from locations in the Gulf the following conclusions re- open sea to the closed end is about 2t hr. Bathy- garding the movement of tidal wave could be made: metry, coastal configuration, wind pattern and Tidal wave approaches the Gulf from the west. On tidal range in the Gulf are such that they can entering the Gulf the speed of the wave is reduced create very complicated current regime; therefore, on the shallower waters near the coast while in a detailed current study has been planned. the deeper waters the speed is maintained until liVinds - Wind pattern in the area is predomi- long. 69°22'E. The time taken by the wave to nantly seasonal with rare cyclonic disturbances. travel from long. 69°E to 69°22'E is about 1 hr 20 In order to understand monthly wind distributions min. From long. 69°22'E onwards the speed is in the area. nearly 4000 ship observations covering considerably increased and the wave reaches the the area 21° to 25°N lat. and 67°E long. to coastline closed end in about 1 hr, the direction of the tidal for the period 1961 to 1966 have been analysed. wave being from west-southwest following the orien- Predominant wind directions in the area are tation of the Gulf. west-southwesterly and north-northeasterly during Tidal range increases faster from long. 69°E to June to September and December to March respec- 69°30'E; the increase is of the order of about 80%. tively. Higher wind speeds are likely to occur From long. 69°30'E to the closed end, the rate of during June to September with winds up to 74 kmjhr increase in tidal range is slower, being of the order from west or west-southwest. July is the windiest month in the area with wind speed exceeding 20 km/hr for 81·5% of time with predominant direction TABLE 1 - TIDAL ELEVATIONS AT VARIOUS ST,\TIONS from west-southwest. The calmest month is February with winds exceeding 20 kmjhr occurring for 13·3% Station Mean Mean Mean Mean Mean low low sea high high of time only, the predominant wind direction being water water level water water north -p orth westerly. springs neaps neaps springs Tides - Tides in any area can be grouped under (m) (m) 3 broad classes: semi-diurnal, diurnal and mixed. Okha +0·41 +1'20 +2·04 +2·96 +3-47 Tidal variations at Okha, Sikh, Kandla and Sikka +0'71 +1·73 +3·04 +4'35 +5'38 Navalakhi are compared for April 1975 and presented Kandla +0·78 +1-81 +3-88 +5'71 +6·66 in Fig. 2. Tides in the area are of mixed type and Navalakhi +0·78 +2·14 +4·15 +6·16 +7·21 are predominantly semi-diurnal with a large diurnal

10 11 12 13 14 15 16 J7 18 19 DATE AND TIME 20 APR.':IS

Fig. 2 - Tidal variations at different stations in April 1975

40 SRIVASTAVA & JOHN: CURRENT REGIME IN THE GULF OF KUTCH of 30%. The total increase in tidal range from The locations where the current data have been the mouth to the closed end of the Gulf is about collected are presented in Fig. 1. Details of the 150%. data collected for various stations are given in Bathymetry and coastal configuration - The Gulf Table 2. of Kutch is an elongated gulf in the east-west Tide data for Okha, Sikka, Kandla and Navalakhi direction with an average depth of about 30 m.At have been obtained from the Survey of India. the mouth of the Gulf, Lushington shoal with Wind data for Okha, Jamnagar and Kandla have depths nearly 5 m below chart datum is present. been obtained from the India Meteorological Depart- On the south-east side of Lushington, a channel mer.t. with the depth of water varying between 30 and Detailed analysis for Gurur and Bobby shoals and 50 m is present. Besides Lushington, there are off N arara bet -- Long term current records- are other shoals in the area, e.g. Gurur, Bobby, Ranwara, available only for locations Gurur shoal (station 2), etc. The presence of Ch anka reef and Ranwara Bobby shoal (station 3) and off Narara bet (station 5). shoal narrows down the Gulf at about long. 69°20'E. Current records have been analysed to determine The coastal configuration of the Gulf is very irregular half hourly current speeds and directions. The with a number of islands, creeks and bays. Density distribution - Data on the density distribution in the Gulf is not available except at the mouth of Gulf and at station 5 (Fig. 1). Since the $TILL WAHl'. LEVEL tidal range and the tidal currents in the area are quite significant, large scale mixing takes place in the Gulf. There is no significan t fresh water influx in the area and the average annual rainfall J211t"'

TABLE 2 - DETAILS OF CURRENT DATA COLLECTED

Station Location Type of Depth of Horizontal Period of Remarks No. current meter measurement control recording used below C.D. (m) used

Lushington Float Surface 3.4 to 4.4.68 Collected by shoal observation chief hydro- grapher 2 Gurur shoal H ydroproducts About 3 Theodolites 8.3 to 2.4.75 Collected by self recording Engineers model 505-A India Ltd 3 Bobby shoal do About 3 do do do 4 Ranwara shoal Roberts At every meter 6.4 to 9.4.74 do direct from surface reading to bottom 5 Proposed SPM Bendex Q-15 7, 27 Theodolites 13.7 to 15.8.73 do self recording 23.8 to 25.8.73 6 do One's direct 1, 5, 10, 20, 30, 42 do 26.3 to 27.3.74 Collected by reading chief hydro- grapher 7 do Roberts Surface, Mid do 21.4 to 23.4.74 Collected by direct depth and near Engineers reading bottom India Ltd 8 Off Narara bet do do do 26.4.74 do 9 do do do do 18.4.74 and 24.4.74 do 10 do do do do 25.4.74 do 11 do do Near bottom do 22.4.74 and 23.4.74 do 12 do do do do 21.4.74 and 22.4.74 do 13 do do do do 22.4.74 do

41 INDIAN J. MAR. SCI., VOL. 6, JUNE 1977 speeds an d directions so obtained have been sorted Tidal currents in an area normally increase with out to determine the percentage of occurrence of increase in the tidal range. A comparison of flood different speed classes in different directions, The and ebb current speeds with different tidal ranges percen tage frequencies so obtained for stations 2, recorded at the nearest port has been made. These 3 and 5 are further analysed to determine predomi- data are presented in Fig. 8. nant speed, maximum speed and predominant General anaiysis for whole Gulf - Simultaneous directions (Table 3). tidal data in the Gulf are available only for Okha, In order to understand the variation of current Sikka, Karidla and Navalakhi. For understanding at spring, neap an d transition period the cumulative the variation of tides from the mouth to the closed frequency tables of curren t speeds for various periods end of the Gulf, tide data for April for all the 4 and for different stations have been prepared. Based stations have been plotted (Fig. 2). on these tables current speed exceedence graphs Maximum surface currents recorded at each station have been drawn and are presented in Fig. 4. during the present study together with the data In order to get an idea of the distribution presented in the Indian Hydrographic charts have of currents in different directions the available data been presented in Fig. 9. on current speed and directions for each station have been plotted on a polar diagram. The scatter Results and Discussion diagrams of curren t speeds and directions so Gurur shoal- Predominant and maximum surface obtained are presented in Figs. 5-7. curren ts recorded are 0·95 and 1·59 m/sec respectively. The recorded directions show considerable variations (Fig. 5). The probable reason for this

TABLE 3 - PREDOMINANT AND MAXIMUM CURRENT could be that the direction recording mechanism SPEED AND PREDOMINANTDIRECTION of the current meter did not function properly. Current speed exceedence diagram (Fig. 4) shows Station Location No. of Pre- Ma.x. Predominant that the spring currents are 60 to 65% stronger No. observa- dorni- speed direction tions nant m/sec than the neap currents. Current speed exceeds speed 1 m/sec for 18, 50 and 60% of time in the case of m/sec neap, transition and spring tides respectively. In order to understand the variation in current speed 2 Gurur shoal 376 0·95 1·59 Sou therlv and So~th- with the variation in tidal ranges, tidal data for Sou theasterl y the nearest available port, i.e. Okha, were plotted 3 Bobby shoal 719 0·95 1-46 Westerly and against the recorded current speeds (Fig. 8). From Easterly this figure it can be seen that for the same tidal range 5 Off Narara Bet* 792 0·20 0·76 Easterly and (proposed SP:'.I Southwesterly ebb currents are stronger than the flood current. site) The probable reason for this can be that since the recording is at an elevated place compared to the *\Vave current filtered out. surrounding area the down ward run of the water will add to the ebb flow, A study of the wind

100 """'~"'=------~G:-:-U::R-:-:U':'R-S::-:H::O:-:A-:-L--t~':;;:"S:=------;;-B:::;OB~B:V:;-;:S~H:-;:;O~A-;-L --I\~~-;N::-A;:;R:;-;A;-;:R;-;A~B:-;;E:-;T-'"

' «,\" ST. No. 2 .~" ST. No.3 ST.No.6 ~ '." ~-' ".\" '\ \.~, " " ' " § \ \ \" ~ 7S ", \ "\ -, ~ \ \ \ : ", \ . \ § \ \ \. \

~ , \ \ \

W05Q \ \ \ ~ ..• \ \ \ ; \ \ \ ~ \. \ \

i25 \\ \\ IL __ SPRING \ \ \ ~ - - - TRANSIllUJ \ \ \ ~ - ... -NEAP \ \ \ \ ~ \ \ \ \ ~ OL-~~~-L-L~~~~~~~''-~'~~''~'~:-~~~~~~~~~~~_~~\~~~~''~~~~~~~~~~~~ 1.5 o I o.s 1.0 i.s CURRE~i5 SPEED Im/sl 1,0 0 0,>

Fig. 4 - Current speed exceedence curves for different stations

42 SRIVASTAVA & JOHN: CURRENT REGIME IN THE GULF OF KUTCH

GURUR SHOAL (STATION NO.-2) 1e-3-75 TO 3-4-75

Fig. 5 ~ Scatter diagram of currenl speeds and directions at station 2 (Gurur shoal) from 18.3.75 to 3.4.75 [Currents were recorded using Hydroproducts model 505-A current meter.Depth of measurement 5 m above sea bed.Current speed measured in m/sec and direction in degrees from North. Current speed values averaged over 5 min recording period.Spring (X), neap (e) and transition (A)]

43 INDIAN J. MAR. SCI.,VOL. 6, JUNE 1977

BOBBY SHOAL (STATION NO.- 3) 16 - 3 -75 TO 3 - 4 . "IS Fig. 6 -Scatter diagram of current speeds and directions at station 3 (Bobby shoal) from 18.3.75 to 3.4.75 [Details same as in Fig. 5. Depth measurement 6 m above sea bed)

44 SRIVASTAVA & JOHN:CURRENT REGIME IN THE GULF OF KUTCH

N O·

180" s NARARA BETcSTATION NO.-S) 13-7-73 TOI5-8-73 Fig. 7 - Scatter diagram of current speeds and directions at station 5 (Narara bet) from'13.7.73 to 15.8.73 [Details same as in Fig. 5. Scatter plotting are not shown separately for spring, neap and transition periods]

45 INDIAN J.MAR. SCI., VOL. 6, JUNE 1977 1·5 .. GURUR SHOAL BOBBY SHOAL OFF NARARA BET "/ ST. No.2 ST. No.3 ST. No.5 ):.1r :.. / .. .. I. "..../....~IJI, . a / ~ 'j' . ,/. .' >!. .. . • ...... s I- I a .. ;. /.' . 0"25 " ". :&. .: lJJ , I . 'j / lJJ • 1 ...... ". . . c, " . . oJ> : / . / I- /: a .. Z / .. lJJ ./ a: a: (. :l /" ./ u 'I: .I. / s /. . '. ~ "0 J ~'/ /. •... ~ ~ / x FLOOD • EBB - ---FITTED RELATION FOR FLOOD --FITTED RELATION FOR EBB

2'5 2·5 5'0 7·5 TIDAL RANGE AT OKHA (m)

Fig. 8 - Comparison of flood and ebb current speeds with different tidal ranges recorded at different stations [Total ranges for Okha and Kandla are taken from tide tables. Current speed values are averaged over 5 min recording period. Recording period for Gurur and Bobby shoals is 18.3.75 to 2.4.75 and Narara bet is 13.7.73 to 15.8.73] pattern at Okha during March reveals that the surface currents recorded are 0·20 and 0·76 m/sec predominan t and the next predominant directions respectively. The 2 predominant directions ot're- of wind are north-westerly and westerly respec- cording are 78° ± 10° during flood and 2l3° ± 10° tively. These wind pattern can create north- during ebb period. These 2 predominant directions easterly a-id easterly surface currents in the area are separated by l35° rather than UlO°. This may and hence cannot add to the ebb flow in the be due to the concave nature (Fig. 1) of the southern area. coast of the Gulf of Kutch which causes an additional Bobby shoal- Predominant an d maximum surface southerly component thus changing current direc- currents recorded are 0·95 and 1·46 m/sec respec- tion. tively. The 2 predominant directions of recordings Curren ts are dorninan tly tidal with a semi-diurnal are 80° + 10° and 260° + 10°, which is also the periodicity. For 5% time, speed remains less than orientation of the Gulf, ~howing thereby that the 0·05 m/sec which represents slack period between tidal currents are predominant in the area (Fig. 6). flood and ebb tides. Current speeds during spring Curren t directions are bimodal with flood current and exceeded 0·5 m/sec for 37% of time with the peak ebb current flowing for 47 and 41 % of the time speed of 0·76 nt/see towards 75°. Current speed respectively. During the remaining 12% of the time, exceeden ce diagrams (Fig. 4) show that surface current direction is variable between 90° to 250° but currents exceed 0·5 m/sec for 4'5, 27 and 37% of speed remains generally below 0·4 m/sec. Current time in the case of neap, transition, and spring tides speed exceedence diagrams (Fig. 4) show that surface respectively, which are lower than those recorded speed exceeding 1 m/sec occurs 5, 45 a-id 56% of at Bobby and Gurur shoal. Spring tidal currents the time in the case of neap, transition and spring are about 82% stronger than neap currents. tides respectively. These percentages are lower From Fig. 8 it is seen that in general flood than those recorded at Gurur shoal. The spring currents are stronger than the ebb currents unlike tidal currents are nearly 45 % stronger than neap in the case of currents at Bobby and Gurur shoals. currents. During the months of June to September (south- A comparative study of the tidal range and west monsoon) winds exceeding 20 kmjhr occur for maximum recorded current speed (Fig. 8) show 58% of time (Fig. 2) with predominant directions that ebb current is in general higher than the flood ranging between west to west-southwest. The wind- current. The probable reason for this is the same driven currents generated during this period range as discussed for the Gurur shoal. from easterly to east-northeasterly and hence these Off N arara bet - Surface current: - The current curren ts will superimpose over th e flood tidal directions off Narara bets are bimodal, with pre- currents. This may be the probable reason for dominant directions and peak speeds nearly parallel flood currents being stronger than ebb currents off to the bottom contour. Predominant and maximum Narara bet.

46 SRIVASTAVA & JOHN: CURRENT REGIME IN THE GULF OF KUTCH

) r, -... \)

-JAMNAGAR LEGEND:- 3·66 m CONTOURS 10 97 m CONTOURS -·-·-18·28m CONTOURS - .. - .. - 3656 m CONTOURS

"KHAMBHALIA

5 tAL E •...1:665,1 SO

(b)

Fig. 9 - (a) Maximum surface currents recorded at each station during the present study and (b) surface currents recorded at area marked [A] in the top figure

47 INDIAN J. MAR. SCI., VOL. 6, JUNE 1977

TABLE 4 - NORMAL WIND-DRIVEN SURFACE CURRENTS

Season Max. wind Max. current Predominant Predominant wind current Speed Dir. Speed Dir. (kmjhr) (rnjsec) Speed Dir. Speed Dir. (kmjhr) (mjsec)

December-March 49 3300 0·34 191 16 300 0·09 225 April-May 49 2400 0·34 101 16 3000 0·09 165 June-September 74 2700 0·49 116 24 2400 0·15 105 October-November 61 600 0·42 275 9 3300 0·08 195

Bottom currents _. An analysis of the data on bottom currents reveals that the curren ts are strongly TABLE 5 - MAXIMUMWIND RECORDED DURING THE periodic with bimodal current directions. The pre- CYCLONE ON 22 O(T 1975 dominant directions are 1l00± 10° during flood Wind Time Maximum wind and 169° ± 10° during ebb. The predominant recording hrs recorded directions differed from the surface current directions station with the flood current turned more onshore. The Speed Direction* ebb curren ts instead of following the directions (krnjhr] (degrees) 2900± 10° as expected, flow towards 169°+ 10°. Veraval 0800 90 180 The bottom currents are normally 70% of the surface Dwarka 1100 67 360 Okha t 100 110 currents. 90 Surface currents in the Gulf of Kutch - The width Jamnagar 1500 167 of th e gulf narrows down between long. 69°10'E *Variation in direction of wind is due to the location of and 69°26'E due to the presence of Chanka Reef station lying in different quadrants of the cyclone. and Ranwara shoal. This causes an increase in +Exact time not known. the tidal current speeds with speeds reaching up to 2·5 m/sec. The current speeds at the mouth of Jamnagar. Hence the maximum wind current Gulf and east of 69°26'E are comparatively lower, which could have occurred during the pre ient with speeds reaching up to 1·5 m/sec. The general cyclone may be 0·57 m/sec towards 275°. direction of curren ts in the area are parallel to the As explained in the section on general description bottom con tour suggesting thereby the dominance of the area, the density currents in the area are of tidal currents. expected to be negligible. The wind-driven current will play an important part in modifying the tidal circulation in the area Acknowledgement when the wind speed exceeds 20 krnrhr. The The authors wish to express their thanks to Dr normal wind currents for different seasons in the A. K. Malhotra, for his keen interest in the work. The area have been computed using the Ekmans authors are grateful to their colleagues Dr A. K. relationsv" and the wind data. The analysed data Lyal.l, Mcsse~s P. Vijay Kumar, V. Gopalan and J. S. is presented in Table 4. Maximum wind-driven Nagi for then valuable field work. The basic data currents in the Gulf are expected to occur during usc~ in ~hi.s paper ~avc b.een collected by Engineers June-September. In order to know the maximum In~Ia Limited for ItS clients, Indian Oil Corpo- surface current the wind-driven currents should be ration and Department of Lighthouses and Light- combined vectorially to the tidal currents presented ships. in Table 4. Very high wind speeds are expected during cyclonic period. A very severe cyclone hit References the Gulf of Kutch during October 1975. The 1. Survey of India, Indian tide tables, 1975. maximum wind speed recorded during the above 2. Engineers India Ltd, Draft report on the marine surveys in cyclone at various shore stations are given in G.ulf of Kutc.h for the construction of lighthouses, 1975. 3. Engmeers India Ltd, Draft project report - offshore Table 5. terminal in Gulf of Kutch. 1974. The maximum wind speeds (167 km/hr) were 4. EKMAN, V. W., Astronomi Fysik, 2 (1905), 11. recorded at J amnagar. The over water wind speeds 5. NEUMA~N, G. & PIERSON,W. J. Ull. Principles of are expected to be 10% higher and hence wind speed phys1Cal oceanography (Prentice Hall Inc., Engle Wood of the order of 184 km/hr can be expected off Cliffs, New Jersey), 1966.