What's Causing This Erosion?

Understanding erosion causes is critical to choosing an appropriate response:

• Severe storm erosion usually deserves a ‘wait and see’ response, as nature often provides rapid, efficient and cost-effective (!) recovery.

• Setbacks or development buffers are practical responses to unstable coastal landforms, which can create cycles or episodes for decades or longer.

• If erosion is due to short-term (e.g. seasonal) or local fluctuations in sediment transport, coastal protection systems such as groyne fields can be used to spatially redistribute erosion pressure, balancing accretion and erosion.

• Where erosion is due to insufficient sediment supply, then it may be offset by renourishment, or erosion pressure can be redirected to a sacrificial area, such as an artificial dune.

• With sea level rise, changes to coastal processes can cause landform migration (e.g. onshore or offshore sediment transfer). The progressive nature of this change prompts use of adaptive management, potentially using different forms of coastal management as interim responses to recession.  

Despite the elegance of this ‘cause and response’ logic, it is rarely applicable. Firstly, because most erosion mechanisms interact with one another, many are active simultaneously, challenging identification of critical processes. Secondly, available information commonly gives only a narrow window to coastal change, potentially obscuring contributing processes, or focusing our perspective on a singular process. The latter is particularly important when using numerical modelling to assess coastal erosion hazards, as they can introduce biases in the representation of different processes.

Identification of what is causing coastal erosion may require an almost forensic assessment. Some of the steps involved may include:

1. Identifying the natural sensitivity of the coastal landforms, generally based on morphology.

2. Characterising physical changes that have occurred.

3. Evaluating ‘external’ changes, such as new structures, or coastal management changes.

4. Assessing changes to drivers of coastal processes, including storms or variation of prevailing conditions.

5. Cross-checking different physical and temporal scales of coastal dynamics. This generally involves using different levels of information.

An example is provided for Damb Harbour, Pakistan, located within Chowdry Channel, which links Miani Hor tidal lagoon to the Arabian Sea. Porali River flows into Miani Hor. This configuration provides opportunity for both marine and fluvial processes to be active. Previous studies of the system had described Miani Hor and Chowdry Channel as 'dynamic'.

Construction of Damb Harbour, including training walls, was undertaken in 2009, with anecdotal reports of severe foreshore erosion occurring from 2014. Unsurprisingly, this timing prompted an interpretation that erosion was due to the new structures. However...

• Starting with Google Earth imagery, which effectively covers 2004 onwards, it was apparent that erosion had started earlier (!) and extended along most of Chowdry Channel. This implies a 'bigger problem'.

• Also from Google Earth imagery, the ocean entrance showed increasing sediment influx - was the channel migrating due to increased tidal flood shoal deposition? More frequent imagery from Sentinel-2 confirmed this behaviour was active over teh last decade.

Miani Hor Entrance 2016-2025.ppsx

Download PPSX • 3.19MB

• A move to nautical charts, only available from 1984 and 2021, indicated something different... Significant scour of the whole channel, typical of river flooding. Further evidence of this mechanism was provided by a century of rainfall records, with drought conditions from the 1920s until early 2000s.

Was this the 'answer'...? Landsat imagery allowed checking further back in time, to 1987... This demonstrated that erosion of the northern bank of Chowdry Channel has been active for the last 50 years, with consistent loss of up to 10m/year.

In the absence of regular river flooding cycles, the most progressive driver for change has been sea level rise, which has been recorded with 2mm/yr rise at Karachi since 1920. Now that might not sound like a lot... but with the very shallow margins of Miani Hor, it has been sufficient to expand the inundated area significantly, increasing the tidal prism and supporting channel growth and migration.

Looked at with this context, recent change shows a mix of dynamics, with foreshore shear, infilling of tidal wetlands, and expansion of tidal systems into new landward basins. It's a mix of erosion and inundation.

All that started to give an 'answer' for the harbour site, until the site survey came in!!

Between the 2021 nautical chart and the 2025 survey, an exceptional flood occurred on the Porali River. This caused continued channel expansion, equivalent to what had occurred over the preceding 40 years!

Change included significant steepening of the bank adjacent to Damb Harbour, as well as a massive scour hole, 38m total depth, and 15m deeper than the adjacent channel. Development of the hole occurred upstream of the harbour, adjacent to previous efforts to stabilise the bank.

This investigative sequence, while unfortunately identifying significant challenges for ongoing sustainability of the harbour, demonstrated how each piece of information could provide a well-supported 'cause' of erosion. The reality, apparently, is more complex, with a background process of sea level rise, overlain by a change in river flood conditions, and interacting with more recent human construction and stabilisation efforts.