Regional-Scale Study on Sediment Processes of Khuran Strait at Persian Gulf with Implications for Engineering Design

In this article, the sediment transport processes in the Khuran Strait between the mainland Iran and Qeshm Island at North Central Persian Gulf are studied in regional scale in a comprehensive manner. The objectives of this study include defining the type and origin of the sediment, the influencing factors, and the dominant mode of sediment transport. Four possible scenarios of sediment processes in terms of sediment sources and influential environmental forces are examined. The results of the conceptual and 2D numerical model of MIKE21 applied for this purposes indicate that the sediment sources in the transport processes are mainly provided by the sediments suspended from the central and eastern zones of the strait bed. Other sources including input from rivers do not have direct influence on the processes. The results are applied to the study of morphological changes for engineering applications including the pattern and amount of deposit in the Rajaee port approach channel and harbor basin. The pattern and amount of annual sediment deposits in the approach channel predicted by the model is satisfactory, compatible with annual dredging records.


Introduction
Impacts caused by human interferences on coastal system result in a change in the coastline and its surroundings' behavior and consequently influence the functionality of the coastal facilities. Prediction of these mutual influences is therefore vital for the long term operation of the coastal infrastructures such as ports and navigation channels. Assessment of the environmental impacts also requires true understanding of the involved processes (Reible, 2008). Most active studies in the field of coastal sediment problems in recent years focus on morphodynamics, i.e. the modeling of the processes determining the coupled evolution of sea bed topography and the wave-current field (Battjes, 2006). As a general practice, the local scale of a given area is mainly employed in project designs for engineering applications, while, larger scale area and longer time including regional processes within coastal cells boundaries have been well applied at managerial level and decision makings (Shanehsazzadeh and Parsa, 2013;Luo et al., 2013;Li, 2008;Larson et al., 2002).
Tidal networks which consist of a complex system of bifurcating channels, are one of the most remarkable natural patterns observed in environments. Morphodynamical pro-cesses in tidal networks depend on a balance between sedimentary processes and hydrodynamics. In fact, coasts partly provide local sources and sinks of sediment, which may interact with shelf-sea evolution (van der Molen et al., 2004;Bird, 2008). In modeling the geomorphology evolutions for engineering purposes such as siltation in navigation channels and harbor basins, true designation of sediment source and pathway contribute to correct simulation of the processes and accurate predictions.
In this article, the sediment transport processes in the Khuran Strait (KS hereafter) between mainland Iran and Qeshm Island at North Central Persian Gulf (Fig. 1) are studied in a comprehensive manner. The study area is mostly a shallow channel (maximum depth of 42 m) which is located in the Hormozgan province coastline (Fig. 1). The detailed feature of the coastline was presented by Shanehsazzadeh et al. (2014). KS consists of a tidal network with a broad range of ecosystem functions. Mangrove forests and deltas of several permanent and seasonal rivers are among the main features of this strait (Fig. 2). Understanding the underlying physics that govern the formation and evolution of these landscapes, their functionality and evolutionary processes are of the highest environmental priority (Danehkar, 1996).
In addition, Shahid Rajaee (Rajaee hereafter) commercial port which is one of the strategic ports in this area is located in the Strait. A bridge with a 2.2 km span linking the Qeshm Island and the mainland is also under construction. The objectives of this study include defining the type and source of the sediment, the influencing factors and the dominant mode of sediment transport pathway in the area. The pattern and volume of deposit transported in the port approach channel and harbour basin and sedimentation/erosion around the entrance and berthing structures are of the engineering concerns as well.
The first extensive study on this area was conducted in 1972 during the hydrodynamics and sedimentation studies of the Rajaee Port, where the local mathematical and physical models were applied (Adibi-Harris, 1973). Based on the records of periodical dredging during operation years, average gross annual volume of dredging of the basin is about 370000 m 3 . Assuming that the net dredging volume is about 60% of its gross volume, the mentioned volume equals averagely 15 cm of silt (ibid).
A series of field measurements were made for the con-struction of the Bahonar Port, another important port in the area (Fugro-Sesco, 1972). These measurements in addition to the more recent measurements (Khak Baft Consulting Engineers, 2007) indicate that the sediments of the project area mostly consist of sand with some cohesive fine particles. The studies conclude that: although coastal sediments mostly consist of sand, the deep water seabed contains finegrained sediments of clay and silt, which can cause major problems in terms of sedimentation after construction of ports in both internal and external areas of the basin. The only regional study conducted in this area was by the national project of Integrated Coastal Zone Management (ICZM) with a focus on the erosion/accretion of coastal area and the evolution of coastlines; hence the processes of deeper water are not considered. Consequently, a comprehensive regional study on the source and pattern of sedimentation and erosion including the coupled modeling of sediment, wave and tidal current is of major necessity in order to better realize the sediment transport processes and refine the predictions of morphodynamic changes applied in managerial decision makings; therefore the purpose of this study is to introduce crucial results regarding this issue. In this article, the hydro-physical characteristics of the area are described first. Next the methodology of the study on the possible scenarios of sediment sources is introduced. The results of the 2D numerical model for prediction of sediment transport are then summarized followed by discussion and conclusion on the predictions and the engineering applications derived from the study. Study on all the possible sources of sediments and their contribution to the processes in this region through a comprehensive area model is the distinguished advantage of this study.

Study area hydrodynamics and sediment characteristics
The general wave rose (left) and tidal current rose (right) of the KS measured in 12 m depth in front of the Rajaee Port area are presented in Fig. 3. As observed in this figure, the area has a weak wave field with 99 percent of the year wave height being below 0.5 m, i.e. calm condition. There are two wave systems coming from different directions, seas from Persian Gulf and swells from Oman Sea. The dominant wave direction here is from south-westward, although it is turned to westward at the west of the port (not shown). The waves are of low energy with very low sediment transport potential; consequently, coastlines of the entire area particularly at both sides of the Rajaee Port have very slow progress. Generally, waves are more active in coastal area with minor effects on processes in deeper parts, which is the subject of the present study. Moreover, the Strait of Hormuz (between Oman Sea and Persian Gulf, Fig. 1) including the KS is tide-dominated (Azizpour et al., 2016). Therefore, the present article is focusing on sediment transport by tidal current; thus wave generated transport, which dominates nearshore sediment transport and shoreline  A. Shanehsazzadeh, H. Ardalan China Ocean Eng., 2019, Vol. 33, No. 3, P. 356-364 change is not included.
The current rose indicates coast-parallel tidal currents in the study area, with the speed higher than 1.0 m/s. Recent simulations and measurements indicate that the current velocity reaches up to 2 m/s in some areas of the strait (Mahmoudov et al., 2011;Zaker et al., 2011;Razavi et al., 2014); therefore, the tidal current is considered as the main acting force for sediment transport in this area.
In terms of sediment sources, river Kal, about 40 km west of the Rajaee Port, is the main outsource of sediment in this area. The total estimated input of the river into the strait is about 37 million m 3 (Fara Darya Arsheh consultant Engineers and Sogreah, Sediment Study of Hotspots: Sediment Study of the Rivers Ending to the Hormozgan Province Coasts, Submitted to PMO, 2010). There are some seasonal rivers which flush considerable sediment during their annual flooding on which no reliable data are available. One of the most important seasonal rivers is located at the far east of the strait (close to Bahonar Port, Fig. 1), for which the average discharge of 0.16 m 3 /s with approximately 10% sediment content is estimated.
Moreover, the seabed deposits at far west of the strait can be transferred into the area. At the narrow section of the strait the tidal current is high enough to convey sediments to the eastern parts where the current decreases, providing condition for deposition.
In order to identify bed sediment grading, at coastal zone and deep water, numerous sampling cases have been conducted in the area (Fara Darya Arsheh consultant Engineers and Sogreah, Sediment Study of Hotspots: Shahid Rajaee Port, Submitted to PMO, 2012). In general, the sediment in the area is classified as fine sand, while, the particles in coastal areas are coarser than those in deep water; averaging 0.1 mm and 0.05 mm, respectively. However, based on the samples taken during field measurements carried out under the present project in and around the internal area of the Rajaee Port, d 50 of sediment particles is smaller than 0.02 mm, consisting of about 5% of clay, 80%-90% of silt and the remaining sediments are larger. A general sediment distribution curve in coastal area is presented in Fig. 4. Small amount of clay in the sediments indicates a non-co-hesive behavior. Moreover, at the five locations along the strait simultaneous measurements of current and suspended sediment are made in this study in various depths and tidal conditions of spring and neap (Fig. 5). The results are applied in calibration and validation of the numerical models.

Conceptual and numerical modeling
Although rivers put in significant amounts of sediment to the coast, it is generally expected that only a small portion of the transported materials are deposited in the nearshore; the finer fractions, the dominant ones, move offshore into deeper waters (Larson et al., 2002). In order to   examine this general expectation closely and study the pattern and amount of sedimentation/erosion in KS, based on previous studies and historical evidences, the following four possible scenarios of sediment processes in terms of sediment sources and influential environmental forces are introduced which have the capability of influencing the process in the area, separately or jointly: (1) Direct transfer of sediments of river Kal and main seasonal rivers into the area during flooding occurrences; (2) Mobilization of sediments on the bed of Khuran canal at the west of KS (only) and their transfer into the east; (3) Sediment sources of the seasonal rivers at the eastern sections; (4) Mobilization of sediments from the entire KS bed. The sources of sediment considered for the four scenarios are marked in Fig. 6. Each one of these scenarios is numerically simulated and their effects on sedimentation and erosion are investigated.

Numerical model establishment
A two-dimensional (in surface) coupled hydrodynamic and sediment transport numerical model is applied to the area covering approximately 900 km 2 , including the Rajaee Port area, in order to investigate the pattern and volume of sedimentation/erosion at KS. In the modeling set up, the unstructured mesh sizes are appropriately considered as variable and are reduced to 45 m at the port basin and channel, as presented in Fig. 7. Bathymetry is based on maps provided by National Geographic Centre of Iran in 1999. In consideration of the dominant force and type of sediment in the strait, the selected model is the Mud Transport (MT) module, from MIKE21 software package. This module is linked to the 2D hydrodynamic model of the current (HD) and uses hydrodynamic data for calculation of sedimentation/erosion processes, simultaneously (DHI, 2007).

Numerical model calibration
Before application for sediment transport modeling, the hydrodynamic model of the current (HD) is firstly calibrated and validated through comparison with field data. A sample curve of comparison of the numerical model predic-tions and measurements is presented in Fig. 8. In the figure, arrows represent the tidal current direction.
The critical aspect in application of the sediment transport numerical model is always to consider model parameters in a proper manner in order to fit the situation of the study area. Most of such parameters indicate the effect of combinations of some mechanisms, provided simply as one parameter and thus, it is of high importance to determine such parameters empirically or through measurements. In this study, a number of model parameters are measured directly or derived indirectly from the measurements. These parameters include sediment size, fall velocity and erosion coefficient. As for other parameters including bed roughness and critical shear stress, sensitivity analysis is made and the appropriate factor range is obtained with regard to the field data. Twenty-five hours simultaneous current and suspended sediment profiles at seven stations are applied for this purpose. The obtained factors are compared with the standard values introduced by profound references (Julien, 2002;DHI, 2007). One of the most sensitive parameters is erosion coefficient (M), which is considered as 0.000025 kg/m 2 /s after calibration with field data.
The models for four scenarios are then calibrated and validated through various field data including history of changes at shoreline, instantaneous measurement of current and sediment concentration and review of the dredging reports (FDA consultant Engineers and Sogreah, Sediment Study of Hotspots: Shahid Rajaee Port, Submitted to PMO, 2012). The time series of tidal fluctuation and prediction of simultaneous sediment concentration for the location close to the Rajaee Port basin is presented in Fig. 9. As observed, sediment concentration increases in spring tidal conditions up to 0.14 kg/m 3 and decreases to 0.01 kg/m 3 in neap conditions.
The suspended load concentration predicted by this  A. Shanehsazzadeh, H. Ardalan China Ocean Eng., 2019, Vol. 33, No. 3, P. 356-364 model is compared with the measurements at 7 points in spring and neap tides; results of the selected ones are tabulated in Table 1, where, Points 2 and 3 are located at eastern, close to the port approach channel, Points 4 and 5 at central and Point 6 at western sections of the strait, close to the strait mouth. The agreement between the measurements and modeling results in different locations of KS verifies the appropriate reliability of the model in estimating sediment concentration in different tidal conditions. As observed in this table, the concentrations increase from east to westwards, both in model predictions and the measurements.

Results and discussion
The calibrated model is applied to predicting sediment concentration at the entire strait and annual morphodynamical evolution in and around the port. One-year numerical simulation is considered for scenarios, which is appropriate for the regional-scale processes. The results of the numerical model for the above mentioned four scenarios are briefly described. The modeling area consists of the entire KS including the Rajaee Port basin and approach channel (see Fig. 7).

Scenario 1: Direct transfer of sediments of river Kal and the main seasonal rivers
Location of the river Kal estuary and the two main branches entering into KS is shown in Fig. 10. To implement the effect of the river Kal in sediment transfer modeling, two sediment sources corresponding to the two main branches are taken into account for river water discharge into the KS.
To investigate the influence of river Kal on sedimentation processes of its surroundings, the maximum instantaneous discharge of the river is considered in this study. Sediment capacity is 11%, estimated based on assuming annual sedimentation of about 36859717 million ton/a and flood discharge of about 320 m 3 /s (FDA and Sogreah, Sediment Study of the Rivers Ending to the Hormozgan Province   Coasts, Submitted to PMO, 2010). The distribution of suspended load sediment concentration in worse condition i.e. maximum tidal states (full spring ebb) for Scenario 1 is shown in Fig. 11. As observed, the sediment yield of the river Kal during flooding is quite local and has limited effect on the whole strait. The measurements of suspended sediments in several locations including the eastern section of the strait indicate the existence of sediment concentration all over the strait in the mentioned tidal state. This phenomenon prevents the sediments from the river to have direct effects on sedimentation in the whole area. Nevertheless, it is obvious that the sediments from river may provide sediment source for the sea bed, which will in long term cover the entire strait through consecutive tidal processes.

Scenario 2: Mobilization of sediments on the bed of
Khuran canal at the west of KS One of the alternatives of the sediment source of sedimentation at the east of KS and in particular in and around the Rajaee Port basin and its approach channel is the direct suspended sediment transfer from the western bed of the Khuran. As observed in Fig. 2, there are numerous narrow waterways at the west of the strait where the tidal current is potentially high, more than enough for supplying suspending bed materials. However, the waterways are covered with mangrove forests, which hinder the current velocity; hence a complicated current pattern is in this area.
The probability of the effect of sediments suspended from the above-mentioned area is assessed in the most critical case, i.e. maximum suspended sediment. Here, only the western section of the bed in KS is considered to get activated while the dispersion and movement of the mentioned plume is accounted for in different tidal conditions. The distribution of suspended load sediment concentration for Scenario 2 in maximum tidal states (full spring ebb) is shown in Fig. 12. Mangrove forests at the western part of the KS cause extra shear stress and might increase the potential of sediment transport. Modeling results indicate that an increase in shear stress in the model indeed causes an increment of the concentration while the extent of the plume does not face a substantial change. The results here indicate that the suspended sediments produced from erosion of the bed at the western section of the strait are forced eastward by the ebb tide but before reaching the eastern zone, direction changes westward by the successive spring tide. This is a constant cycle and finally has no direct effect of suspended sediments of the western sections of Khuran on the eastern sections.

Scenario 3: Sediment sources of seasonal rivers at the eastern parts
Seasonal rivers at the eastern sections of the strait (close to Bahonar Port, Fig. 1) could be the source of sedimentation in the strait. The sediments' samples taken from the Rajaee Port basin and approach canal prove fine-grained nature of the materials. Thus, fine-grained sediments of the seasonal rives can remain suspended and probably get deposited in the central sections of the strait. Consequently, based on the available information, a sediment source of the seasonal rivers in this area with 160 lit/s flood outflow and 11% sediment are assumed in the numerical model. The river discharge is estimated based on the outcome of comparison between the area of the river basin and similar adjacent basins, for which the data are available.   A. Shanehsazzadeh, H. Ardalan China Ocean Eng., 2019, Vol. 33, No. 3, P. 356-364 361 The sediment distribution pattern introduced by the mentioned sediment source under most critical tidal condition of the spring flood is shown in Fig. 13. The concentration pattern in this figure similarly indicates that the sediment source has only local effect and thus does not have the potential to reach the entire strait.

Scenario 4: Mobilization of sediments from the entire KS bed
Mobilization of sediments from the entire strait bed is considered as one of the important alternatives among these scenarios. These sediments may get suspended under tidal conditions from different sections of Khuran canal and even from sections near the Rajaee Port and deposited under tranquillity conditions or due to the depth change. Major source of the bed sediments of Khuran is the geological reef materials as well as the inflow from river deltas including Kal, Jalabi etc. These sediments make a permanent potential which get activated and begin moving gradually in spring and neap cycles. These suspended sediments will begin redeposition in appropriate locations and conditions. The Rajaee Port basin and its approach channel are among the locations appropriate for sedimentation, due to the considerable artificial changes made in the depth of the sea bed and flow pattern.
The distribution of suspended load density in maximum tidal states (full spring ebb) for Scenario 4 is shown in Fig. 14. As observed, density at the west of KS is more than that of other quadrants. This is due to higher current velocity at west which increases bed erosion; hence, an increase in regional density of suspended sediment. However, in some areas of eastern sections, close to the Rajaee Port basin and the shipyard (Fig. 1), higher concentration is predicted.
The study on all possible scenarios reveal that the main sedimentation process in the strait including the Rajaee Port and its approach channel is subject to mobilization of bed sediment at KS; other sources have indirect effects on the process. Intuitively, with the varying cross section in the strait (Fig. 2), the flow first accelerates at the inflow on the western side, then decelerates in the area where the mangroves are present, accelerates again through the narrowest point, and finally decelerates at the eastern end. Since tidal flow controls the sediment transport, the sediment rate is more or less proportional to the velocity to some power and would indicate areas where the source of sediment is available at the sea bed.

Engineering applications
The coupled hydrodynamics and sediment transport models are vastly applied to prediction of morphological changes caused by human interventions. The advanced 2D and 3D models once appropriately calibrated and validated, are considered as the reliable and more economical tools compared with physical models for predicting coastal process and beach face evolution in response to coastal infrastructures (van Rijn, 1993;Kamphuis, 2000). Many successful practices regarding the application of the approved models at regional scale for various circumstances, are reported in the related literature (Hibma et al., 2003;Margvelashvili et al., 2008;Kuang et al., 2012;Luo et al., 2013;Bever and MacWilliams, 2013). In this study, the results of one-year regional modeling in KS are applied to the prediction of the sedimentation and erosion pattern at important locations in KS. The most important application of the regional modeling of the processes in KS is the identification of the sedimentation/erosion pattern in and around the area of Rajaee Port basin and its approach channel which are located at the eastern section of the strait (Fig. 1). Rajaee Commercial Port Complex is the largest and most strategic port of Iran.

Sediment deposition in the Rajaee Port approach channel
Predicting critical areas of sedimentation and defining an optimum dredging policy for the approach channel are of great importance for the port authorities. The representative corresponding result of the model in terms of annual bed level change at and around the port basin and approach channel are shown in Fig. 15. In this figure, light colours (positive) represent annual deposition, and dark colour (negative) represent annual erosion, as predicted by the model.  As observed, sedimentation is concentrated in the central part of the channel. Compared with available dredging reports of the channel and the surveyed hydrographical patterns, it is deduced that the location, amount and pattern of sedimentation obtained from the model are well compatible with the location required for dredging and dredged volumes in the recent years (168000 m 3 annual). Thus, the model results are promising in determining the quantity and quality of sediment behaviour. According to the model results and experiences, sedimentation/erosion in other sections of the approach channel is not considerable. In brief, it can be concluded that sedimentation problem of Rajaee port approach channel is at its mid one-third, as shown in Fig. 15.
In order to investigate the reason behind this phenomenon, as the feature of the sea bed bathymetry at the westward of the approach channel, presented in Fig. 16, due to a geological fold at the sea bed (indicated in the figure) the tidal flow diverted accelerates in a narrow channel up-drift of the approach channel toward the middle of the channel and then decelerates due to extended depth of the channel and therefore the deposition rate is higher in this area.

General pattern of erosion/sedimentation in the Rajaee
Port basin The results obtained from modeling in the port basin area are expressed in Fig. 17. Once more, in this figure light colours (positive) represent annual deposition, and dark colours (negative) represent annual erosion, as predicted by the model. According to this figure, sedimentation is concentrated in the central section of the basin and in front of the jetty close to the entrance, reaching 27 cm and 70 cm in height, respectively. At the outskirts of the basin at the western and eastern arms' lees (left and right, respectively in Fig. 17) of breakwater, bed erosion is observed; as noticed, at the eastern arm the intensity is lower. The maximum annual erosion depth of this part has been predicted by the model of 1 m and 0.5 m, respectively. Such increase in the depth at the lee of breakwater head is reported by the port authorities as well, which is well predicted by the model. Erosion of the bed at the heads caused by the local intense tidal current at the entrance should be of major concern, since it could undermine the stability of the breakwaters in the long term. In general, the breakwater arms cause deviation and convergence of powerful tidal currents of the area and the secondary currents caused by entrance intensify erosion opposite to the breakwater.

Conclusions
In this article the hydrodynamics and sediment processes in the Khuran strait (KS) between mainland Iran and Qeshm Island at Northern Persian Gulf are studied in a comprehensive manner. Several processes with various possible scenarios of sediment sources are considered for the conceptual modeling. A regional large scale 2D numerical model is developed to investigate the effective sediment sources, pattern of sediment concentration and morphodynamical changes in the strait. The outputs of the models in terms of hydrodynamics and sediment intensity are calibrated and validated through the collected field data. From modeling the conceivable sources of sediment it is revealed that the sediment activity in the KS is mainly affected by the sediments suspended in the central and eastern zones of the strait (Scenario 4). Other factors including the input from rivers, even in their maximum discharge, do not have direct influence on the sediment processes in the entire area, though they provide the source of fine-grained sediments of the sea bed in the long term. Concentration of suspended sediment in the west of KS is more than that in the eastern sections. This is due to the increase of the tidal current velo-   A. Shanehsazzadeh, H. Ardalan China Ocean Eng., 2019, Vol. 33, No. 3, P. 356-364 363 city from the east to the west. The results of the study are applied to the prediction of morphodynamical changes including sedimentation/erosion in the Rajaee Port basin and approach channel. The model predicts the pattern and amount of the annual sediment deposits in the approach channel quite well. The erosion at the bed close to the breakwater heads, which was predicted up to 1 m in height by the model, could undermine the stability of the breakwaters. More precise details of such local morphological changes clearly require a three dimensional modelling study.