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Home , Arabian Sea, Gulf of Khambhat, Khambhat

J. Syst. Sci. (2021) 130:112 Ó Indian Academy of Sciences

https://doi.org/10.1007/s12040-021-01601-6 (0123456789().,-volV)(0123456789().,-volV)

Tidal hydrodynamics and Cushing characteristics of Gulf of , west coast of – A numerical approach

1,2 1, ADITI MITRA and V SANIL KUMAR * 1CSIR-National Institute of Oceanography, Engineering, Dona Paula, 403 004, India. 2Bharathidasan University, Tiruchirappalli, 620 023, India. *Corresponding author. e-mail: [email protected] MS received 7 August 2020; revised 9 February 2021; accepted 9 February 2021

Tidal hydrodynamics of the (GoK) were studied with the observed and simulated datasets. The Cushing time of the Gulf was estimated using the 2D-model calibrated and validated with the tide and current speed observations. Model simulations were carried out with the inCuence of tide, , and tide and wind combinedly. Wind-driven Cushing was found to be the least with the Cushing time magnitude of more than 25 days. The Cushing time was found to be much lower for the surrounding irrespective of the seasonal and tidal variations. The model-derived tidal circulation revealed the seasonal variability and the dominance of Cood tide over ebb in GoK. Maximum current speed was during spring-Cood conditions in the southwest , whereas minimum current speed was during the neap- ebb condition of pre-monsoon. High-frequency radar data, from the southern GoK ascertain the increase in current speed from south to north. The Simpson–Hunter parameter, calculated in several locations of the Gulf established the enhancing mixing capacity of GoK from south to north. During low current speed conditions, the Gulf experiences the formation of a barrier at the southern which interrupts Cushing between the Gulf and the north-eastern during northeast monsoon and pre-monsoon season. Keywords. Tide; current; hydrodynamics; Gulf of Khambhat; Cushing time; large water body.

1. Introduction successfully simulate the tidal ampliBcation inside GoK and surroundings by a barotropic numerical Gulf of Khambhat (GoK) is a converging channel model. Later it was concluded that the shape, and a highly energetic macro-tidal regime of the varying bottom friction coefBcients (Nayak and northeastern Arabian Sea. It provides navigational Shetye 2003), and the large width of the conti- connectivity to industrially important states of nental shelf oA the central west coast of India and (Bgure 1). The Gulf (Joseph et al. 2009) were the main agents behind boundary in the present study is taken as the 153 the tidal ampliBcation of the Gulf. In the north km stretch between Pipavav in the state of Gujarat , the maximum tidal range of more and Daman in the of Daman and than 10 m is found in GoK (Indian Tide Diu. Unnikrishnan et al.(1999) were the Brst to Table 2017), which is the second maximum over

This article is part of the topical collection: Advances in Coastal Research. 112 Page 2 of 14 J. Earth Syst. Sci. (2021) 130:112

Figure 1. Bathymetry of the study region where the deployment locations are clearly indicated: the red line indicates gulf boundary; negative, positive and zero in bathymetry indicates land, water and inter-tidal , respectively. the globe after , Canada. The tides obtain the tidal circulation, propagation and to are of mixed semi-diurnal type with diurnal dis- understand the physical processes. parity and variable amplitudes, which increase Coastal regions are very dynamic systems that from the south to north along GoK. The maximum steadily transform over different temporal and current speed of 3.3 m/s was recorded in GoK spatial scales as a result of geomorphic and and current Cows towards north–northwest and oceanographic processes (Cowell 2003). In such south–southeast during Cood and ebb respectively areas, tidal hydrodynamics play a major role to (Kumar and Kumar 2010). The tidal currents of understand the complexity and characterizing GoK have several small-scale distinct features due features for construction as well as protection to the complex topography and varying tidal measures. Since the tidal strength is the most amplitude. The principal factors, which regulate important agent in driving and modulating the the seasonal variability of GoK are the Arabian Sea Gulf dynamics, it is the fundamental interest of the at the Gulf’s entrance and the Indian monsoon. present study to have a meticulous knowledge of The major rivers which drain into the Gulf are the tide-derived dynamics of GoK. The divergent Narmada, Tapi, Mahi, and Sabarmati. The mon- response of the Gulf towards tidal and seasonal soonal freshwater discharges of these rivers are reversal is the principal goal of this work. immense and the maximum contribution is from In the case of tide-dominated, semi-enclosed River Narmada, ranging between 10,000 and basins with negligible freshwater interference, 60,000 m3/s during monsoonal Coods. High resi- three transport time scales are used to quantify the dence time is recorded from almost every part of mixing and water exchange capability, i.e., age, the Gulf, which makes the Gulf more turbid (Mitra exposure time, and Cushing time. An understand- and Kumar 2021). Understanding the hydrody- ing of the Cushing rate of estuaries and semi-en- namic characteristics of the GoK is extremely closed basins into the sea is important for important as hydrodynamics have a direct impact monitoring and management of the aquatic system on the design of any engineering infrastructure, since it helps to obtain the trophic state health built in this area. The GoK has been the site for (Lucas 2010; Brodie et al. 2012). Since water poli- many research studies for the past two decades or cies over the globe exhibit, the requirement of more. These studies including the present one managing the aquatic systems and their health, a deployed various instruments in the attempt to hydrodynamic as well as water quality descriptor J. Earth Syst. Sci. (2021) 130:112 Page 3 of 14 112 such as the Cushing time may be utilized as a good concentration within the systems (Monsen et al. proxy of health and mixing criteria. Flushing time 2002; Valle-Levinson 2010). In general, it has been (FT) is deBned as the rate at which the entire presumed that the water of a shallow water system volume of water is Cushed out of an estuary or predominantly gets replaced by water from the shallow water sea and therefore could be used to connected outer seawater. This phenomenon of estimate the rate of removal of a pollutant carried water egress/ingress could remove/replace mate- by the water (Thomann and Muller 1987). Long rials associated with terrestrial discharge/oAshore Cushing time indicates a longer period Cush pollu- water. This eventually impacts the ecosystem tant out of the Gulf/estuary. The freshwater frac- health and status of the shallow water sea and its tion method is a popular approach for practical associates. Thus, an attempt has been made to estimations of estuarine Cushing time for unsteady determine the tidal and seasonal inCuence on Cow and variable tidal conditions. On the other the Cushing characteristics of the GoK and the hand, the tidal prism method is implemented in the surrounding . shallow water basins, where freshwater interven- tion is negligible. Numerical tracer experiments have also been carried out as another perspective 2. Materials and methods to understand the long-term mixing time scales. It was observed that the water exchanges and tracer 2.1 In-situ data Cushing time scales became substantially longer Measured tide and current data from locations during neap tide and shorter at spring tides in the such as Pipavav (1 Jan–7 Feb 2014) and Dholera experiment at (Prandle 1984;LuA and (inside GoK) (31 Aug–3 Oct 2008) and Diu (out- Pohlmann 1995). The complexity of tracer distri- side GoK) (1 Jan–7 Feb 2014) by using Seaguard bution is largely a consequence of complex tidal Tide Gauge and Current Meter (RCM), Acoustic current patterns. In general, Cushing time pos- Doppler Current ProBler (ADCP) data oA Bhav- sesses an inverse relationship with the freshwater nagar (25–28 Jan, 24–27 Mar and 9–12 April 2013) input. Hence, it could be used to determine fresh- and high-frequency radar (HFR) data (01 Jan water exchange/withdrawal scenarios in view of 2014–31 Dec 2014) retrieved from three other planning and management of water resources. locations of southern Gulf separated at a distance Fifty percent of the global livelihood depends of 12 km from each other are used in the study. upon the coastal zone. Thus, there is an inCuence of anthropogenic activity upon aquatic environments. Modeling a semi-enclosed coastal embayment as a 2.2 Hydrodynamic model discrete system within or connected with a larger sea is often beneBcial. Since water quality processes Continuity and momentum balance equations take place at times scales much larger than the given by Stoker (1957) have been used to obtain average tidal period, hydrodynamic and water the tide-elevations, tidal currents, circulations in quality models could be coupled together by com- the GoK. These equations are two-dimensional and bining the rate of tidal exchanges with the Cushing vertically integrated, applicable for an unsteady time scales. In this case, the FT is considered as the water column. In this study, the 2D model applied measurement of the entire system and a single is the same model used to determine the Lagran- magnitude would depict the status of the entire gian circulation of the Gulf by Mitra et al.(2020a). water body. The advantage of such an approach is Same model was used to estimate suspended sedi- the quantiBcation of the relationship between ment (SS) transport and residence time (RT) of the alteration in water quality and the feedback of the GoK (Mitra and Kumar 2021). Other than the ecosystem in a compliant manner (Lee and Wong Gulf, the model has been applied in several loca- 1997). On the other hand, the residence time is a tions (Naidu and Sarma 2001; Naidu et al. 2016). similar concept but is usually considered to have The northern boundary of the model domain was variability within a region as well as differs for closed and boundary tides were applied at the various material inputs (Zimmerman 1976; southern, western, and eastern boundary. Data of Takeoka 1984). However, FT is an integrative zonal (u) and meridional (v) components of wind at parameter used to delineate the water mass 10 m height having 1-hr interval with a resolution exchange between two water bodies, without of 0.25° 9 0.25° for the entire study area used as identifying spatial variability in the mass model inputs is from the ERA5 reanalysis produced 112 Page 4 of 14 J. Earth Syst. Sci. (2021) 130:112 by European Centre for Medium-Range Weather 2.4 Mixing

Forecasts (ECMWF) (Hersbach et al. 2020). The 3 river discharge data of major rivers (Narmada, Simpson–Hunter parameter (h/U ) (Simpson and Tapi, Mahi, and Sabarmati) was obtained from the Hunter 1974) was calculated to determine the Central Water Commission, New Delhi. Standard mixing ability of the Gulf, where h is the water seawater density has been used in the entire model depth of a particular location and U is the average domain and GoK was assumed to be a one-layer current velocity. system. The model simulations were continued for three typical months of three seasons, viz., summer monsoon (July), winter monsoon (January) and 3. Results pre-monsoon (March) in the year 2016. The monthly averaged simulation results are presented 3.1 In-situ data in the study. Tide data collected at Diu (outside GoK) and Calibration and validation of the model based on Pipavav (inside GoK) are shown in Bgure 3. The measured tide and current data at were data depicts the tidal ampliBcation inside the Gulf presented in Mitra et al.(2020a). Correlation where within a stretch of 68 km, i.e., between Diu coefBcients of 0.87 and 0.75 were obtained for tide and Pipavav, tide-elevation has a drastic increase and current respectively which indicate a well- of about 3 m with a rate of 0.04 m/km. Current match between observed and simulated datasets. speed and direction data recorded with the help of Positive bias values (0.04 m for tide and 0.09 m/s ADCP at central GoK during January, March, and for current) of these parameters indicate slight April indicate that maximum current speed was over-estimation by the model. recorded during April followed by March and Since in the earlier studies, validation of the January (Bgure 4). The resultant direction during model was not done for the northern region, pre- January was towards SSW which tended to shift dicted tide elevation and current speed were esti- westward during March and April. mated and compared with the datasets collected at The seasonal averaged current speeds recorded Dholera in the northern GoK for the year 2008. Even by HFR are presented in Bgure 5. Even though the though the station locations are situated inside a locations are very close to each other, the increase channel, the model could predict the tidal amplitude in current speed from south to north is quite accurately, although a slight discrepancy was noticeable, where minimum current speed was encountered in the case of the tidal phase (Bgure 2a). observed in the southern-most location and vice- On the other hand, the model-derived current speed versa. The northward ampliBcation of the u-com- matched well with the observation (Bgure 2b). ponent of current was prominent (Bgure 5a–c). This was not found in the case of v-component, 2.3 Estimation of Cushing time (FT) where a negligible increase was recorded (Bgure 5d–f) which is well-portrayed in the FT was estimated by applying the tidal prism Bgures due to the uniformity of scales for both the model (Alber and Sheldon 1999). components.

Tf ¼ Vh=Vtp; ð1Þ 3.2 Circulation of GoK where Tf = FT, Vh = high water volume of the estuary volume (Luketina 1998), Vtp = the tidal The model simulations were carried out for differ- prism (volume that entered during low water and ent physical forcings, viz., tide (Bgure 6), wind high water). (Bgure 7) and for the combined inCuence of tide Freshwater inCuence is taken using the equation and wind (Bgure 8) during three typical months of by Luketina (1998). the summer or southwest monsoon (July), winter or northeast monsoon (January) and pre-monsoon V ¼ Q þ Q ; ð2Þ tp FW FL season (March). The maximum and minimum where QFW is the input of freshwater during tidal magnitude of current speeds were recorded during period (high or low) and QFL is the volume of oA- the summer monsoon and pre-monsoon seasons, shore water entering into the estuary during the respectively. Simulations revealed that the circu- Cood tide period. lations inside the Gulf are more complex than that J. Earth Syst. Sci. (2021) 130:112 Page 5 of 14 112

Figure 2. Model validation with data collected at Dholera in the northern gulf of Khambhat.

Figure 3. Comparison of tidal amplitude of Diu (outside GoK) and Pipavav (inside GoK). of the surrounding sea irrespective of the seasonal tide and wind (Bgure 8a), tide-driven circulation conditions. Tide-driven current speed magnitudes (Bgure 6a), and wind-induced circulation (Bgure 7a), were found to be much higher inside the Gulf than respectively. that of the sea. On the contrary, wind-induced current speed was found to have more dominance 3.3 Overall hydrodynamics and mixing in the Arabian Sea. After a month of model simu- properties of GoK lation, a barrier was noticed in the southern region of GoK mostly during the pre-monsoon season Cross-sectional maps of the distribution of mean (Bgure 6b, c and 7c). The barrier gets indistinct tide-elevation and current speed over the domain during the winter monsoon period and eventually of interest were plotted by using in-situ as well as disappears during the summer monsoon season. model-derived data over several locations of the The maximum current speeds of *3, 2.5, and 1.2 Gulf and surroundings (Bgure 9). The distributions m/s were recorded for the combined circulation of revealed the occurrence of minimum current speed 112 Page 6 of 14 J. Earth Syst. Sci. (2021) 130:112

Figure 4. Current speed and direction data recorded by acoustic doppler current proBler (ADCP). (a)25–28 Jan 2013, (b)24–27 Mar 2013 and (c)9–12 April 2013. at southern GoK which had an unsteady increase The Simpson–Hunter parameter was calculated towards the north. The undulating topography and at several locations of the Gulf which is presented the varying bottom friction coefBcients are the in table 1. The data indicates maximum and main factors behind this. Observations revealed a minimum magnitude at locations HF3 and maximum current speed of * 2 m/s near Bhav- , respectively. The dataset portrays nagar in the central GoK which had a decrease enhancement of the mixing capability of GoK further northward. The cross-sectional proBle of from south to north. tide-elevation also depicted the same, i.e., increasing from the south towards north till Bhavnagar and again having a further northward 3.4 Flushing time decrease. These datasets reveal that maximum current speed was observed in the regions between The model domain has been segregated into two and Bhavnagar which are the places of sub-domains, i.e., GoK and surrounding sea (SS) maximum tide height. In general, tidal ranges are for separate estimation of Cushing time. Three found to increase with a decrease in water depth separate simulations have been carried out to from mouth to head, up to 20 m depth contour, determine the Cushing characteristics of GoK, SS, thereafter, it decreases with decreasing water and GoK+SS, considering the seasonal and tidal depth. variability. Further, simulations were continued J. Earth Syst. Sci. (2021) 130:112 Page 7 of 14 112

Figure 5. u and v component of high frequency radar data collected in three locations of southern GoK: (a) u summer monsoon, (b) u winter monsoon, (c) u pre-monsoon, (d) v summer monsoon, (e) v winter monsoon and (f) v pre-monsoon.

Figure 6. Monthly-averaged tide-driven circulation during (a) southwest-monsoon, (b) northeast-monsoon, and (c) pre- monsoon. considering the tidal forcing, wind forcing, and the monsoon (Bgure 10). Spring FTs are quite higher combined forcing of tide and wind. than the neap ones. The maximum magnitude of Flushing time of GoK was found to be high FT (66 tidal cycles) was obtained considering wind during the winter monsoon and pre-monsoon sea- as the single Cushing forcing. Overall considering sons which gets decreased during the summer the seasonality, the Cushing time varied between 112 Page 8 of 14 J. Earth Syst. Sci. (2021) 130:112

Figure 7. Monthly-averaged wind-driven circulation during (a) southwest-monsoon, (b) northeast-monsoon, and (c) pre- monsoon.

Figure 8. Monthly-averaged tide+wind-driven circulation during (a) southwest-monsoon, (b) northeast-monsoon, and (c) pre- monsoon.

15 and 55 tidal cycles in realistic conditions and pre-monsoon. Flushing is thereafter antici- (tide+wind). pated between 7 and 26 days during realistic The Cushing time of the SS was found to be spring neap cycles. invariable irrespective of the tidal conditions The depth-averaged Cushing times were esti- (Bgure 10). However, seasonal variation was quite mated in GoK for different seasonal and tidal evident with maximum Cushing recorded during conditions (table 2). Maximum Cushing was the summer monsoon (7 days) followed by winter obtained with the combined simulation of tide and monsoon and pre-monsoon. During the summer wind during the summer monsoon season. Wind- monsoon season, the FT was found to be quite driven Cushing was found to be minimum for each similar for the three aquatic systems (GoK, SS, and seasonal and tidal conditions. A marginal decrease GoK+SS). in FT was obtained from spring to neap irrespec- The FT of the entire model domain was found tive of the seasons. As a whole, minimum and to be quite similar to that of GoK (Bgure 10). maximum Cushing times of around 7 and 31 days Even in this case, maximum Cushing was obtained were obtained for neap and spring tides of summer during summer monsoon followed by winter monsoon and pre-monsoon, respectively. Seasonal monsoon and pre-monsoonal months. The region variability is more prominent in case of the open requires 7 days to Cush considering repetitive ocean region, on the other hand, tidal conditions neap tide conditions and 26 days for spring tide also intervened in the case of GoK along with the simulations, respectively, during summer monsoon seasonality. J. Earth Syst. Sci. (2021) 130:112 Page 9 of 14 112

Figure 9. Cross-sectional proBle of tidal current speed and direction.

Table 1. Simpson–Hunter parameter calculated in GoK.

Average current Magnitude Location Depth h (m) speed U (m/s) of h/U3 HF1 30 0.88 44 HF2 22 0.93 27 HF3 17 0.96 19 Daman 4.65 1.5 1.38 Surat 10 2 1.25 Dahej 15 2.5 0.76 Bhavnagar 20 3.3 0.55

4. Discussion speed was obtained from south to north. A study in areas of mid-Atlantic Bight identiBed high seasonal In the present study, tidal hydrodynamics were variability of the tidal properties; it has shown that examined from Eulerian and Lagrangian points of HFR is a better resource for annual mean surface view and both of them are in pace with each other. tidal properties and has the advantage of providing Propagation of tide and current speed in GoK and information on seasonal alterations (Brunner and surroundings manifested that the current speed is Lwiza 2020). In the present study, HFR data pro- mainly modulated by the tidal action of the Gulf. vided information on spatio-temporal variation of Thus, a northward increase was obtained in the current components as well. The component of current speed as well as tide-elevation, reaching HFR current speed (u) was found to have a maxima in Bhavnagar and decreasing further prominent increasing trend both spatially and northward. In the Mandovi and Zuari estuaries of temporally, whereas the component (v) did not the west coast of India, the tidal height remained reCect such and was found to be more or less unchanged over a large distance from the mouth invariable. Maximum u and v-component speeds of due to the balance between geometric ampliBcation 0.69 and 0.7 m/s were obtained from HF3 (south- and the bottom friction (Unnikrishnan et al. 1997). ern-most location) during the southwest monsoon. The present study has contrasting characteristics The HFR data provided adequate temporal cov- due to the combined inCuence of geometry, friction, erage of the surface ocean current even though the and bottom topography. Similar results were spatial coverage was low, but it allowed the present obtained by the in-situ data (HFR) from southern study to characterize the mean seasonal current of GoK where a small, but steady increase in current that region. The data as a whole reinforced the 112 Page 10 of 14 J. Earth Syst. Sci. (2021) 130:112

Figure 10. Flushing rates of (a) Gulf of Khambhat, (b) surrounding sea and (c) entire model domain.

Table 2. Assessment of Cushing time of the model domain.

FT tide (days) FT tide+wind (days) FT wind (days) Neap Spring Neap Spring Neap Spring Southwest monsoon (SWM) 13.4 14 7.3 7.8 16 17.3 Northeast monsoon (NEM) 22.5 23.3 18.5 20.7 26.2 28.4 Pre-monsoon (PM) 25.5 26.8 23.7 25.5 29.5 31.3

general circulation of GoK. The ADCP data indi- speed during northeast-monsoon than pre-mon- cated the seasonal variability of the current speed soon, both these datasets manifested the impor- where a higher value was found during tance of seasonal cycles in the Gulf dynamics. The March–April (pre-monsoon) than January (north- general circulation data reCected considerable east-monsoon). Even though the seasonal vari- variation in the seasonal as well as the tidal cycle of ability of the ADCP data showed slight the Gulf circulation. It also showed variability discrepancies with that of the HFR counterpart considering various physical forcings. The circula- which obtained a higher magnitude of the current tions exhibited the presence of a barrier between J. Earth Syst. Sci. (2021) 130:112 Page 11 of 14 112 the Gulf and the Arabian Sea on a seasonal scale. due to the ampliBcation of tide and tidal current, The tide-driven barrier is prominent during but due to the presence of the barrier such char- northeast-monsoon and pre-monsoon when the acteristics are not reCected in the surrounding sea. current speed was comparatively less. The south- The exact tidal contribution towards the overall ward shift of the barrier during northeast-monsoon dynamics of the Gulf was not quantiBed in the revealed high current speed inside the Gulf which present work but it discerns that the maximum helped the barrier for a southward proceed; contribution comes from the tidal part even though whereas the barrier forms further northward due to the Gulf is exposed to seasonally reversing wind comparatively low current speed during pre-mon- regime. soon season. The monthly averaged simulations The physical forcings impact the overall revealed the presence of the barrier considering the dynamics of GoK irrespective of the seasons. Since tide-driven and wind-driven circulations; whereas the southern region of the Gulf is in the acquain- it was found to be vague in the case of the com- tance of the Arabian Sea, it experiences seasonally bined circulation of tide and wind. A study by reversing wind patterns more than the northern Mitra et al.(2020a) concluded that the typical region. The Cushing characteristics of the model bathymetry of the Gulf along with the tidal vari- domain reveals that the entire system is cleaned up ability played a key role behind such type of for- comparatively quickly when both the physical mation. A similar type of barrier was noticed in the forcings are considered and the Cushing was found southern region of Cochin estuary during the pre- to be least when the wind was the single physical monsoon season which led to the formation of a factor. That is why the seasonal reversibility was standing wave (Balachandran et al. 2008). The quite evident in this case where minimum FT was previous studies (Mitra et al. 2020a, b), as well as observed during the summer monsoon. This the present one, infer that the Gulf dynamics is resembles the Cushing characteristics of Azhikode inCuenced more by Cood tide than the ebb. These estuary and Cochin estuary where the minimum could have a great implication on eAluent and and maximum Cushing were obtained during dry marine debris transport as well as sediment trans- and wet seasons respectively (Revichandran and port and the erosion-accretion mechanism of the Pylee 1998; Vinita et al. 2015). They also con- Gulf. The Simpson–Hunter parameter calculated cluded that the FT is directly correlated with the in the Gulf showed a decreasing trend from south plankton biomass and the amount of contaminant. to north reaching a minimum value at Bhavnagar. In general, spring Cushing is found to be more than The low value in northern stations indicates that neap due to higher current speed during spring the mixing capacity of the Gulf increases with (Sridevi et al. 2015). On a contrary, spring Cushing increasing tidal action. The higher mixing capacity time of the Gulf was found to be more than neap of the Gulf portrays the Gulf as a single layer due to higher tidal prism during spring. Even system where the use of a 2D model could be well- though the tidal prism was found to be much justiBed. The high value of the parameter at the higher in the case of the surrounding sea, it was southern GoK stipulates chances of stratiBcation found to have much better Cushing than that of the at those locations. The Lagrangian particle track- Gulf. This could be explained by the general cir- ing, done by Mitra et al.(2020a) briefed higher culation of the model domain wherein the southern advection of particles inside GoK which got region of the Gulf experienced a formation of a reduced outside. Lagrangian circulation in the barrier which hinders the easy Cow of water European continental shelf has established the through it and prevents the Gulf to Cush out to the association between the larger magnitude of open sea. A study in the also had excursion of particles in the tide-dominated regions inferred that the FT was found to be comparatively (Ricker and Stanev 2020). They have also found smaller in the southern region of it than in the that the tidal forcing considerably inCuences the northern or northwestern region (Sadrinasab and accumulation of particles in the shelf which Kampf 2004). The complex topography leads to resembles the study by Mitra et al.(2020a) where more complex circulation and tidal characteristics they have established that there is a tendency of of GoK which eventually impacts the FT. Due to the Gulf to accumulate particles at the northern quite simpler bathymetry of the surrounding sea, region where tidal intervention is maximum. The the FT was found to be smaller. Considering the present study manifests the fact that both the entire model domain, the FT was estimated to be advection and mixing get intensiBed inside the Gulf quite larger than the surrounding sea, but was 112 Page 12 of 14 J. Earth Syst. Sci. (2021) 130:112 comparable with that of GoK. This indicates that Arabian Sea during the pre-monsoon and northeast the Cushing characteristics of the model domain monsoon seasons due to the combined eAect of tide depend more on the FT of GoK than the SS. In a and bottom topography of the Gulf. It prevents the long run, this could be harmful to the Gulf, as the easy Cushing of the Gulf water to the open sea. On water exchange between the shallow sea and the the other hand, the surrounding sea had lower surrounding region is the foremost factor to mod- Cushing time due to its bathymetry leading to easy ulate the water quality and ecosystem (Winter water movement. The Gulf was found to have the et al. 2020). Since several industries and ports are least Cushing during pre-monsoonal months, and there along the banks of GoK, the longer FT could the Cushing time was found to be minimal during be of concern for the Gulf ecology. The untreated the southwest monsoon. The surrounding sea had eAluent, if released from the industries could be comparatively lower Cushing time. The high present in the Gulf water for a prolonged period Cushing time of the Gulf could be of potential risk (even more than 25 days) and could lead to for its health and status in long run. eutrophication as well as harmful in that area since the tidal time scales as well as the biophysical interactions provide an important Acknowledgements mechanism for algal bloom development and evo- lution (Lucas et al. 1999). This not only will aAect The ADCP and HFR data used in the study are the lower trophic level organisms such as planktons provided by the National Institute of Ocean or small Bshes, but also large Bshes and eventually Technology, Chennai. We thank the Director, human beings. Studies have revealed that the National Institute of Ocean Technology, Chennai phytoplankton growth rates vary from less than 1 and Dr B K Jena, Scientist G, National Institute of day to around 3 days (Laws 2013). The large FT of Technology (Ministry of Earth Sciences), Chennai, the Gulf could lead to the entrapment of hazardous for helping with the ADCP and HFR data. The material which in the end might be uptaken by the authors thank Dr V S Naidu for the support pro- planktons. Since the southern region of the Gulf is vided to carry out the model simulations. Director, directly connected with the Arabian Sea, better CSIR-National Institute of Oceanography, Goa Cushing is expected in this region. provided facilities to carry out the study. We thank all colleagues for the help provided during the Beld data collection. We thank the reviewers for the 5. Conclusion comments and suggestions, which improved the scientiBc content of the paper. This is NIO con- The present study clearly shows that the tide- tribution 6681 and forms a part of the PhD thesis induced hydrodynamics most significantly modu- of the Brst author. The Brst author wishes to lates the Gulf characteristics. The observed as well acknowledge the Council of ScientiBc & Industrial as simulated datasets are well in agreement with Research, New Delhi for the award of a Senior each other. The HFR data indicated clear spatio- Research Fellowship. temporal variability of the current components, especially the u-component where maximum mag- nitude was recorded in the northernmost location Author statement during the southwest monsoon. Although distinct seasonal alteration is noticed in the Gulf circula- Aditi Mitra: Conceptualization, formal analysis tion as a whole, the tidal impact played a key role and original draft. Sanil Kumar: Supervision, in the overall dynamics. The present study writing-review and editing. resembles the previous ones where a northward increasing trend in current speed with increasing tide elevation was observed. This eventually leads References to the ampliBcation in the mixing capacity of the Gulf towards the north which is quantiBed with the Alber M and Sheldon J E 1999 Use of a date-speciBc method to help of the Simpson–Hunter parameter (lower examine variability in the Cushing times of Georgia estuaries; Estuar. Coast. Shelf Sci. 49 469–482. magnitude in the northern region). A barrier could Anonymous 2017 Indian tide table: Indian and selected foreign be noticed in southern GoK which causes distinct ports; Survey of India, Government of India, Dehradun, characteristics in the Gulf compared to the India. J. Earth Syst. Sci. (2021) 130:112 Page 13 of 14 112

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