In a recent exercise, looking at dynamics on the Balochistan coast, Pakistan, two harbours have had significant persistent challenges, overwhelming successive structural interventions. However, applying standard coastal engineering approaches, 9/10 coastal engineers would likely have suggested similar actions, as they are the obvious response when considering littoral transport.
Case 1: Gaddani Harbour
At Gaddani Harbour, the southwest monsoon caused prevailing eastward alongshore drift and mild local downdrift erosion. Excessive waves at the harbour entrance were a more significant issue, restricting harbour use. This was addressed by a 60m extension of the harbour breakwater... leading to 16 years of increasing harbour restriction and eventually closure of the harbour basin.
Overall, the end effect has been counter-productive, with coastal accretion of ~60m completely blocking access to the original harbour basin.
Onshore migrating nearshore bars appear the likely culprit for this pattern of coastal change, as suggested by coastal change over 2015-2016. Initially a 30m wide bar (~12,500 m2 area) develops between September and November, after the end of the southwest monsoon. The bar progressively migrates landward and extends eastward, and is no longer visible by August 2016, in the next southwest monsoon.
Although limited capture of aerial imagery restricts clear resolution of nearshore bar formation and evolution, development is variable between years, with apparently larger bars in 2010, 2015, 2018 and 2021.
What went wrong?
In this case, nature has broken the 'rules' of littoral transport, which coastal engineers use widely for interpretation of coastal dynamics, largely focusing on shoreline position.
Determination of actions appears to have been based upon littoral transport, considering alongshore movement of sediment due to wave stresses. Installation of groynes on the east side of the harbour entrance suggests that behaviour was interpreted as a littoral transport reversal. This interpretation was likely supported by the timing of accretion, which occurred following relaxation of the southwest monsoon.
However, in this case sediment transport is not limited to alongshore littoral processes. Sediment supply from nearshore bars is onshore and eastward. Consequently, the groynes will have acted to compartmentalise the shore, increasing sediment capture at the harbour entrance.
Breakwater extension has provided an area of storage in its lee, so that the beach structure can form a position parallel to its original configuration, with more volume, with almost no change to wave patterns (i.e. it is landform change, rather than change to drivers).
How might a better solution have been identified?
Interpretation of coastal change at Gaddani Harbour is consistent with understanding that might be drawn from a sparse shoreline dataset, using the concept of littoral transport as a framework for interpretation.
However, the observed pattern of change is downdrift accretion. This is an unusual phenomenon. In such cases, it is important to evaluate the reliability of the frameworks being used for assessment. With the wisdom of hindsight - and more information than was available at the time - the presence and dynamics of nearshore bars is a critical component of the Gaddani coastal system.
In this case, a geomorphic interpretation of bathymetry may have shown bedforms indicating incipient bar formation - although this is likely to be ephemeral and might not have been detected.
Repeated bathymetric surveys provide the best means of capturing dynamic behaviour, including bar migration. However, it is important to acknowledge that collecting sufficient surveys requires a significant investment in time and expense, while the problem is ongoing. It can be challenging to sit and watch, while there is an imperative to fix the issue!
Case 2: Pasni Harbour
Pasni Harbour is a bit over 300km to the west of Gaddani, at the (eastern) tail end of a ~40km long segment of straight coast, and just south of the Dasht River mouth. The harbour was built on the eastern side of a headland, providing significant protection from the southwest monsoon conditions. The main harbour breakwater extended ~500m from the original shore and the deep section of the breakwater was built with a low core, to reduce the breakwater footprint and optimise cost. Due to the sheltered position of the harbour, and long-term stability of the updrift coast, low rates of alongshore transport might have been anticipated.
Following its construction in 1989, there was substantial rapid infilling to the south of the harbour basin. While the rate of accretion slowed from 2006-2011, sand started moving over the low core along the outer section of breakwater. Accretion subsequently accelerated, causing bypassing around the breakwater, progressively closing off the main harbour entrance, then severely restricting a secondary gap.
The nature of the sedimentation problem is partly explained by looking at the bay south of Pasni. A sand spit developed at the headland around 2006, which is likely to have been caused by increased eastward sediment transport along the long straight shore south of Pasni. In the lee of the headland, the spit lacked sufficient wave energy to be fully pushed onshore, and therefore it progressively extended northwest.
The spit bridged, although discontinuous, from the headland to Pasni by 2013, forming a 'superhighway' for sediment transport, rapidly increasing sediment supply towards the port. The subsequent formation of a tidal lagoon behind the spit barrier then contributed to gradual landward transfer of sediment towards the shore.
Note: Images are rotated, with north to the left.
Although the 2006 spit material is now largely distributed along the shore, with a narrow lagoon behind, substantial growth of a new spit provides a critical consideration for future management of Pasni Harbour.
What went wrong?
Like Gaddani Harbour, nature has broken some of the 'rules' of littoral transport used by coastal engineers to project coastal response to a new structure.
In this case, the site was likely selected due to its sheltered wave climate. This might have provided an expectation of low rates of sediment supply, as alongshore transport rates increase with wave energy.
Development of a spit breaks this 'rule', with localised form of the spit head (slope, orientation) to maximise the feature's ability to distribute sediment supply from the updrift source. This is almost the opposite behaviour to a beach, resulting in much higher rates of sediment transport than will be derived from littoral models.
How might a better solution have been identified?
Sedimentation at Pasni is principally controlled by the updrift supply, which is determined by conditions along the ~40km coast south of Pasni, rather than the locally sheltered coast. Regional morphology provided some indicators that alongshore sediment transport might be subject to variability, including small spits at the headland, and the partly truncated dune sequences southwest of Pasni.
Development of such an extended barrier spit may have been difficult to forecast without record of a historic precedent. This substantially reduced the effective along-coast storage of transported sediment and significantly increased the rate of delivery to Pasni. However, the spit took a number of years to extend across the bay, which might have provided the best opportunity for effective intervention. Growth of a new spit over recent years provides a further challenge to planning for rehabilitation of Pasni Harbour.
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