Coastal currents play an important role in the hydrodynamic transport processes near the coastlines. Since early seventies the increase in oil and gas production, operational vessel activities, coastal and industrial development, desalination, and wastewater treatment are the main sources of damage to the marine environment in the Sea of Oman [1]. The pollution caused by accidental and intentional spills, ballast water discharge, brine disposal, toxic waste discharge, dredging, and land reclamations has led to severe ecological damage to the coastal environment and the marine ecosystem. Preventive and control strategies are put in place to fight both land and sea-based pollution and thus to safeguard the marine ecology of the region. Therefore, continuous monitoring of hydrodynamic and seawater quality parameters along the Omani coast is imperative to improve the understanding of sediment transport, erosion, and deposition mechanisms in this region [2].

The Sea of Oman (previously known as Gulf of Oman) is situated in subtropical zone with depth varying from 3,000 m in oceanic part and gets shallower towards the Strait of Hormuz with the varying depth of 70-110 m separating it from the Persian Gulf and has, a total area of 94,000 km². The Strait of Hormuz is 56 km wide and roughly 90 m deep but for a trough near and around the Musandam Peninsula. While the Arabian Sea stretches 1200 km along the Omani coast from Ras Al-Hadd in the north to the Oman-Yemen border in the south. The open Arabian Sea has a total area of 3,862,000 km², with a depth up to 4652 m.

Atmospheric conditions over Persian Gulf and Sea of Oman are north-westerly winds, with seasonal variations over the Sea of Oman, high pressures in winter (1015 hPa) with relatively low in summer (995 hPa) and air temperatures varying between 32–34 oC in summer and 18^oC–20^oC in winter. Hydrologically, Sea of Oman during summer experiences the Southwest Monsoon (SWM) oscillated from the Persian Gulf transmitting toward the Arabian Sea through it. In winter, Northwest Monsoon (NWM) from the oceanic regions of the Arabian Sea moves toward the Gulf passing through Sea of Oman along the Iranian coast in the North. Circulation in Sea of Oman is dominated by a clockwise gyre in the west and counterclockwise gyre in the east, creating upwelling circulation in both summer and winter, whose strength depends on prevailing winds [3].

The tide from the Persian Gulf oscillates to the Sea of Oman through narrow Strait of Hormuz which further co-oscillates towards the Arabian Sea. Those tides are complex standing waves and vary from primarily semi-diurnal to diurnal pattern [3]. Akbari et al. [4] observed the tides in the Persian Gulf which is standing wave, and progressive waves passing by the sea surface in the Sea of Oman and the Arabian Sea. Wind and tides predominantly result in high water level fluctuation in the Sea of Oman and the Persian Gulf with tide height varying from 1.5m to 4.5m [5]. Although the coast of Sea of Oman has a strategic importance by virtue of the oil pollution risks due to heavy tanker traffic (approximately 35% of global ship-borne oil passes through Strait of Hurmuz according to a report [6]), huge coastal developments along the coast of Oman and possible climate change effects on the seawater quality and marine life, the hydrodynamic measurements are scarce. Probably the first detailed measurements were carried out after the second Gulf War in 1992 [3]. Kwarteng et al. [7] carried out few hydrodynamic measurements to study coastal erosion along Al-Batinah coast of Oman. Sana and Baawain [8] carried out seawater quality and hydrodynamic measurements at 44 points along the northern coast of Oman. Although these measurements were carried out in shallow depths (about 10m), very low dissolved oxygen values (2 mg/L) were observed in the bottom layers. In other words, stressful conditions for marine life existed even in the shallow waters. Higher levels of the concentrations of Lead and Vanadium were found in the samples collected from the locations near Mina Al-Fahal and Qalhat. In October-November 1999 GOGP99 project was carried out to carry out hydrodynamic and seawater quality measurements and detailed results were reported about the Strait of Hormuz and Gulf of Oman (presently Sea of Oman) [9,10]. Recently Gerd et al. [11] reported coastal currents along the northern coast of Oman. A huge number of short-term studies have been conducted in the past 50 years by the consultants who were involved in the design and analysis of several coastal structures along the coast of Oman. These structures include ports, harbors, intakes and outfalls of desalination and power plants and other industrial plants located along the coast. Unfortunately, most of these studies are unpublished and the collected data is either lost or considered classified by the public and private organizations who were the owners of the relevant projects. Sultanate of Oman is working on Oman Vision 2040 (https://www.oman2040.om/vision-en.html) according to which sustainability is one of the pillars of the future development. To ensure sustainable coastal development procurement and maintenance of historical data of coastal hydrodynamics, seawater quality and marine ecology is imperative. To maintain coastal data relevant public authorities can play vital role in the establishment, operation and management of permanent coastal monitoring stations and data collections, quality control and dissemination to the interested stakeholders. The government of Oman has established National Centre for Statistics and Information (https://data.gov.om/) for dissemination of important data and information. However, the website is in its infancy and a limited range of data is available through it.

In the present study, waves and current measurements were carried out at eleven locations along the coast of Oman. This study will help the public authorities in establishing a permanent coastal hydrodynamic monitoring system to mitigate coastal problems that include pollution and coastal erosion along the coast of Oman. Moreover, wave estimates for various return periods are presented that will be useful in the design of coastal structures along the coast of Oman.

The waves and current were sampled using Nortek Acoustic Doppler Current Profiler (ADCP) AWAC AST 600 kHz deployed at the sea bottom with external battery canister that supplies power in stand-alone mode and raw data was stored in internal memory (Figure 1). Therefore, the instrument was recovered timely to charge and download and process the raw data using personal computer. The AWAC software was used for instrument configuration before deployment and to convert raw data to ASCII code. Storm software was used for the graphical interpretation and processing of the raw data.. This equipment can measure ocean current profiles to a range of 50 m and waves to that of 60 m. In the present study the ADCP settings had the vertical bin resolution of 1 m and blanking distance of 0.5 m along the water column. The current and wave data were obtained at every 10 min and 20 min profile interval, respectively. The wave measurements were processed offline using the software by Nortek. The measured data of waves and current profiles requires strict quality control by virtue of the accuracy of deployment, location, and maintenance. For the deployments of longer durations, it was ensured to clean the ADCP head because barnacles start depositing on the sensors. The effect of such deposition does not affect the efficiency of measurements much. Nonetheless regular in situ cleaning of the equipment was carried out by the divers.

Along the coast of Oman field measurements of waves and currents were carried out from year 2017 to 2019 at eleven coastal locations (Figure 2). At each location, the hydrodynamic data were collected for at least two months duration in each season (summer and winter). The coordinates and duration of measurements for all these locations are shown in Table 1.

The measurement locations were selected due to their strategic importance for the country and the region. For example, Oman’s oil refineries are located at Mina Al-Fahal and Sohar. The largest one is expected to start operating by the end of year 2023 at Duqm. Moreover, major shipping ports are located at Sohar and Duqm. The most important desalination plants are located at Al-Ghubrah, Barka and Taqah. Shuwaimiyah is a potential location for future industrial activities. Khaburah, Quriyat and Shinas are important due to fisheries-related activities. Large scale aquaculture projects are located in the offshore area of Quriyat. Masirah island is one of the most promising tourist destinations in Oman for which a comprehensive development plan is to be started soon. All these locations either currently have ongoing or will undergo coastal development in the country’s plan namely Oman Vision 2040.

Since early seventies the coasts of the Arabian Peninsula are observing huge coastal developments resulting in more job opportunities and enhanced economic activities. At the same time the environment along the coasts is rapidly deteriorating due to lack of monitoring and improper management of the coasts. In the present study the hydrodynamic measurements were done using available state of the art equipment. The results of the present study may be used as the baseline measurements and permanent monitoring stations could be established by the public authorities by selecting some of the sites from this study. Current and wave behavior in different months from year 2017 to 2019 at the eleven coastal location of the Oman was investigated based on the in situ current and wave data obtained from the ADCP.

The wave measurements showed that the most frequently occurring waves had the significant wave height ranging from 0.15m to 0.65m and peak wave period from 2 s to 6 s. The occurrence of swell waves with longer period (> 8 s) was more at the stations located on the southern coast (> 40% of total waves) as compared to those in the vicinity of Muscat (<10% of the total waves) and northern (Al Batinah) coast (<20% of the total waves). The effect from the dispersion of the storms or weather conditions at the Arabian Sea rather than the effect from the local wind conditions supports the occurrence of higher swell waves at the Southern coastal stations. Generally, the maximum wave height is estimated from Raleigh probability distribution according to which maximum wave height is approximately twice the significant wave height. Whereas the measurements in the present study suggest that Hmax ≈ 1.5 Hs. The statistical analysis based on Gumbel distribution shows that the 100-year return period significant wave height varies from 1.67m to 3.31 m for the eleven monitored location.

The averaged current magnitude at the upper surface was found to be 0.3 m/s with the dominant direction towards South at measurement stations in the vicinity of Muscat; 0.15 m/s with the dominant direction towards Southwest at northern (Al Batinah) coast; and, 0.2 m/s with the dominant direction towards Southeast at the Southern coastal stations. The mixed semidiurnal tidal cycle with a tidal range of about 2 m was observed in the study area. Higher current magnitudes were observed during the spring tide than those during neap tide.

The measurement and assessment of the current, wind and waves parameters helped understanding the sea state and hydrodynamics along the coastal location of Oman. The analysis of these environmental forces plays an important role in research related to planning and development of coastal and offshore engineering, ocean wave generation, wave energy converter and other applications. The real-time continuous measurement of these forces would further help in the calibration and validation of the coastal and ocean models in evaluating the pollution transportation, sedimentation, plume dispersion and flushing at the coastal area of Oman.

The continuous hydrodynamic measurements have numerous advantages due to the rapid pace of development along the coast. Because of a lack of historical data of hydrodynamic parameters, the design of coastal structures in the region is generally based on the statistical inferences from short-term measurements. Due to which extremely expensive coastal structures are at risk of partial or total failure caused by utilization of improper data. A continuous monitoring program could improve this situation by providing historical data to the stakeholders. As observed from the hydrodynamic measurements in the present study the wave, tide and current parameters along the Sea of Oman are different from those along the Arabian Sea. Therefore, permanent monitoring stations are needed along both the water bodies. To investigate the relationships between the deepwater waves and current profiles and those near the coastline, deepwater wave buoys equipped with downward looking ADCPs would be useful. In addition, the wind speed sensors should also be installed on the measurement stations because the wind data from nearby weather stations or ECWMF may not correctly represent the local wind conditions.

This study is extremely important considering the forthcoming environmental problems in case of an improper design and management of coastal structures, marine discharges, and the anticipated effects of climate change in the region.