Coastal Processes

Sediment transport along the Hastings frontage is primarily driven by the wave climate. As a result the drift is bi-directional dependant on the direction of the waves, which in turn is dictated by the predominant wind direction at any given time. Both modelled data and actual records demonstrate the predominance of waves in from a south-westerly direction. These drive the annual net sediment transport in an easterly direction. Controlling structures in the form of groynes and more significantly the harbour arm restrict sediment transport and help to maintain a suitable beach.

Figure1 illustrates the sediment movement in and around the harbour beach, with the width of the arrows indicative of the amount of shingle transport. The net easterly drift has resulted in material building up on the western side of the harbour arm. This in turn feeds the harbour beach when it overtops the structure, and to a lesser extent circumnavigates the seaward extent. Due to the large height difference, either side of the harbour arm, shingle that bypasses the harbour arm is unable to move back in the event of a reversal in drift direction. In a similar fashion material moving through the open sluice gates (Figure 2) and round the terminal groyne is lost to the system.

Figure 1: Sediment Movement Within and Around the Harbour Beach

 

Figure 2: Open Sluice Gate inside the Harbour Beach

At the eastern end of the harbour beach shingle moves in both directions, in response to wave direction, as is observed along the rest of Hastings beach. At the western end the transport patterns behave differently as a result of the sheltering effect of the harbour arm. Material moves in a westerly direction at similar rates to the rest of the coastline when subjected to waves from this direction, however very little material is able to move back in an easterly direction. The material that does move also tends to be finer given the reduced intensity of the hydrodynamic conditions. This has three effects:

  1. Net sediment drift is in a westerly direction, resulting in accretion of material on the eastern side of the harbour arm.
  2. Orientation of the beach berm is in a SW – NE direction as opposed to W – E that is observed along the rest of the frontage, producing a crenular shaped bay.
  3. A natural sorting mechanism exists whereby large material is driven to the western side of the beach with aggressive easterly waves. This is then effectively trapped as the sheltering effect of the harbour arm results in only smaller material moving back eastwards.

Beach face gradients are a function of sediment size/composition and hydrodynamic climate. As a general rule coarser sediment is able to produce a beach with a steeper stable gradient, this phenomenon has resulted in the undesirable 1:5 gradient in front of the fleet at the western end of the harbour.

No sediment grading analysis records exist, so it is unknown if the quantity of coarse material is increasing and exacerbating the problem. Despite no factual record, it is probable that as sediment moves into, and through, the harbour beach more coarse material is being naturally sorted and trapped.

Sediment Budget

Evolution of the harbour beach over the last thirty years has resulted in a relatively stable beach with similar mounts of material entering and leaving the system. Figure 3 demonstrates the evolution of the western side of the beach, a 70m increase in beach width and the development of the crenular bay. Since then the beach has maintained the same approximate plan shape.

Figure 3: Hastings Beach through the years (click to enlarge)

More detailed records of beach changes are provided by the Strategic Coastal Monitoring Project. It is difficult to reach any conclusions with a high degree of confidence given the relatively short-term trends produced over a seven-year monitoring period. The data collected to date does however provide an insight into the accuracy of conclusions drawn in previous studies.

Loss rates for shingle along the Hastings frontage are quoted as 5,000m³/a in the Cooden-Cliff End Strategy Plan (2002). The monitoring data collected from 2003-2006 indicated a loss of 7,200m³/a for the whole frontage. However, since then, there has been a 43,242m³ gain in sediment, which equates to an accretion rate of 4,252m³/a. This shows that sediment change rates cannot be definitively calculated on the basis of short data sets.

Net sediment transport rates are quoted as 4,500-5,000m³/a easterly in the South Foreland to Beachy Head SMP (1996, 2006), whereas the Cooden to Cliff End Strategy Plan (2002) estimates rates at 10,000m³/a with a variability of +/- 5,000m³. In reality transport rates vary throughout the frontage dependant on type/condition of controlling structures, amount of beach material, orientation of coastline and exposure to hydrodynamic conditions. Previous BMP reports have highlighted that actual transport rates appeared higher than predicted, the likely result of the replenishment works at Bulverhythe. Notwithstanding this anthropogenic distortion of the sediment transport process, the Strategic Regional Coastal Monitoring Project results do correlate with the predictions of the aforementioned studies.

 

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