INVESTIGATING MORPHODYNAMIC CHANGES OF TANG ESTUARY INLET, IRANIAN COASTLINE OF THE OMAN SEA Aref Farhangmehr1, S. Abbas Haghshenas2, Mohammad Dibajnia3 and Ali Naeimi4 1) Visiting Researcher, Institute of Geophysics, University of Tehran, Iran, Email:
[email protected] 2) Assistant Professor, Institute of Geophysics, University of Tehran, Iran, Email:
[email protected] 3) Collaborating Scientist, Institute of Geophysics, University of Tehran, Iran,
[email protected] 4) Assistant Professor, Islamic Azad University, Saveh Branch, Email:
[email protected]
1. Introduction Tang Inlet with its complex morphodynamic behavior is of high interest not only from a theoretical coastal engineering perspective, but also it deserves attention due to its potential for development on the Iranian Makran Coastline. The estuary is located approximately 75 km east of Chabahar Port. It is a tidal estuary formed landward of a crescentic bay on the Oman Sea. The estuary’s inlet is located at the end of this bay in an area sheltered by an approximately 1.7 km long island (Figure 1). This island acts as a natural offshore breakwater, which results in formation of a tombolo connecting the estuary’s inlet to the island. An important feature of the system is the migration of the inlet and its alternative opening to the west and east sides of the tombolo feature behind the island. This phenomenon has occurred at least three times during the past 50 years.
Tang Estuary experiences infrequent flood events during occasional intense rainy periods. The upstream watershed is not large; however, it supplies the narrow estuary with moderate sediment load once a year and heavy sediment load once every five years, on average. Tang estuary lies on riverine remains of a major flood channel of the region experiencing considerable floods during rare but intense rainy periods. In the current study, morphodynamic changes of Tang Estuary are investigated by analyzing 33 historical Landsat satellite images taken over the period of 1966 to 2016. All the images were analyzed in a GIS framework to investigate the evolution of Tang Estuary over the past three decades. Three inlet migration events were detected which occurred in 1987, 1998 and 2015. Figure 1 shows 2015 events on its corresponding satellite image.
Figure 1. Sample inlet migration event captured through historic shoreline change analysis for the Tang area [1].
2. Probable Effects of wave Characteristics and Rain Fall on Migration Inlet Considering the importance of this dynamic system and corresponding wave and current characteristics, physiography and watershed analysis of Tang Estuary is a matter of importance. The average annual precipitation of the study area is estimated to be about 100 mm/year, while the maximum daily precipitation during the past 30 years is recorded 116 mm in January 1998, the year that one of the inlet migrations had occurred. On the other hand, Figure 2 shows annual wave roses for the period of 1996 to 1999 adopted ISWM (2002) data [2]. The wave roses for 1998 and 1999 show remarkable difference in direction comparing with those for 1996 and 1997. The dominate orientation of waves changed from South and South-East in 1997 and before to South-East in 1998 and later (See Figure 2.). This may be considered as another reason for the captured migration in 1998.
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Figure 2. Yearly wave rose in front of estuary at depth of 30m. (a) 1996, (b) 1997, (c) 1998 and (d) 1999 [2]
3. Discussion Tang inlet is characterized by a migrating tidal channel which demonstrates two semi-stable westerly and easterly orientations. It seems that the inlet has been opening alternatively to the west and to the east every 10~15 years. The switching from the westerly to easterly channel orientation seems to be predominantly caused by floods and diffraction currents caused by the dominant southerly
and south-easterly waves diffracted around the rocky headland. It is while another possibility refers to the inlet response to decadal variations in alongshore sediment transport. Generally, inlet stability is a result of balance between tidal prism volume and longshore transport rate. If easterly transport is strong, then it closes off the inlet and it has to breach/open to the west. When there is strong westerly events, then easterly transport closes off the inlet and it opens up towards the east. These two possibilities are examined in this paper.
4. References [1] Haghshenas (2015): Morphodynamic Changes of Tang Estuary Inlet. Researh Report, Institute of Geophysics, University of Tehran. 63p. [2] Technical report, ISWM Iranian sea wave modeling. Phase 3, ports and maritime organization. See URL:, http://coastseng.pmo.ir/coastalengineeringoffice-proj17-proj19fa.html; 2008.
