Beach Sediment Tracer Study 2010 - 2012 - ESCP & Portsmouth... · Beach Sediment Tracer Study 2010 - 2012| 3 The hand-held wand has a limited read range, typically less than 0.2m,

Beach Sediment Tracer Study 2010 - 2012

Beach Tracer Report for Portsmouth & Hayling Island Frontage

2 | Beach Sediment Tracer Study 2010 - 2012

Introduction The SCOPAC Sediment Transport Study (2004) identified several limitations in the current understanding of coastal sediment transport in the East Solent. To help overcome the knowledge limitations there is a need to:

1) Confirm the presence of a drift reversal at Gunner Point, and identify the location of the resulting drift divide;

2) Ascertain both the net direction and rate of littoral drift at Southsea;

3) Estimate attrition losses to inform revised estimates of the longevity of recharged beaches;

4) Ascertain the feed of material to both Eastoke and Eastney from tidal deltas.

The knowledge limitations can all be addressed, to varying degrees, using tracer pebbles to track the movement of material along the beach. Historically various tracer techniques have been utilised, ranging from painting native pebbles to aluminium tracers and copper cored resin pebbles. Each of these techniques has limitations when deployed over longer periods of time, e.g. months to years, as required to study attrition rates, and establish long-term sediment pathways.

A new tracer technique has been developed and deployed in the US, utilising radio-frequency identification (RFID) technology to track objects in rivers and beaches. The system has three main components; PIT tags, an antenna & reader, and PDA or datalogger. PIT (Passive Induced Transponder) tags are small glass encapsulated capsules that broadcast a unique identification number when detected. The tags do not contain an internal power source, and can be embedded within pebbles taken from beaches in the study area. When embedded within suitable pebbles the tags can be used for long-term deployments with no risk of identification markings wearing off the exterior of the pebble. Once detected the position of the pebble can be recorded using GPS technology, and if required the pebble can be recovered from the beach to establish a precise location and burial depth.

RFID Antennas & Readers

Three RFID detection units have been purchased or constructed to enable the efficient location of tagged pebbles once deployed; a short-range wand, a hand-held antenna & a towed antenna array. The detection range varies between antennas, and is also affected by the size and orientation of PIT tag being detected.

Beach Sediment Tracer Study 2010 - 2012| 3

The hand-held wand has a limited read range, typically less than 0.2m, and is used during tagging of the pebbles and to home in on individual tags for recovery from the beach. The hand-held antenna has a greater read range, upto one metre, and is used to sweep the beach on foot. The towed array consists of four antennas, covering a 5 metre width of beach. The towed array also has a read range of upto one metre, and is suitable for use on wide open flat areas of the beach where a quad bike can be used to tow the array across the surface.

Tracer Tagging & Measurement

A total of 2,800 23mm and 32mm PIT tags were purchased and embedded in native pebbles collected from the 2 sites. In order to insert the PIT tags, a 6mm diameter hole was professionally drilled. Initial trials indicated that drilling holes in flint using a conventional drill resulted in prohibitively high shatter rates. A new technique was developed utilising a water jet containing fine grit to drill a hole through the centre of each pebble, with virtually no shattering of the encased pebbles. Once drilled, the PIT tags are sealed in place using epoxy resin mixed with mineral filler. This could then be filed smooth to match the pebble surface.

The PIT tag detection range is greatest when orientated towards the antenna, thus to increase the likelihood of the tag being orientated in this manner when lying on the beach, the tags were inserted along the short-axis of the pebble.

Short range wand Hand-held antenna Towed antenna array


Once the tags were inserted each pebble was individually weighed, and the long-, medium- and short-axis measured using callipers. These catalogued measurements may be used as a baseline against which to calculate attrition rates for any pebbles recovered.

Tracer Deployment & Monitoring

An initial trial deployment of 300 tracers was carried out on the Eastoke frontage between 13th and 26th September 2010. The tracers remained within three adjacent groyne bays, and the average detection rate was >70%. The Eastoke site was chosen to reduce the risk of pebble losses either offshore, or into tidal inlets, during the first deployment. These initial tracers were left in the field to allow attrition estimates to be gained over a longer period of deployment.

A further 1000 pebbles were deployed at Eastoke during March 2011 and 250 at 3 sites near to the entrance to Langstone Harbour during January 2011. This made a total of 14 deployments along the Hayling Island frontage (see Figure 1). Pebbles were deployed either side of the Harbour entrance to ascertain the direction of littoral drift and location of any drift divides. There were a total of 12 deployment sites at Portsmouth, with the release of 1800 pebbles at various sites along the whole open coast frontage (Figure 2). Pebbles were released along the beach at the approximate location of Mean Sea Level (0.17mOD)

During May 2012, 100 pebbles were released at MSL (0.01mOD) for a trial at Lee-On-Solent as part of the sediment transport study for the River Hamble to Portchester Strategy (RHPS). Following this trial, a further deployment of 1500 pebbles occurred at 3 sites along this frontage during March 2013. These deployments used a 12mm RFID tag as the sediment grading on these beaches was smaller than that of Portsmouth or Hayling.

Monitoring surveys were conducted after the first week of deployment and then approximately every 1-2 months for the first year in the field. After the first year the frequency of survey was once per annum, with the tracers now within the field environment for over 2 years.

After 300 days in the field, recovery rates for the tracer pebbles can be expected to be less than 10% of initial deployment numbers.


Results

Hayling Island - Eastoke

Deployments at Eastoke (Figures 3 –12) would suggest the presence of a drift divide along this section of frontage, approximately at the junction of Southwood Road and Creek Road (Figures 11-12). Pebbles deployed east of this point generally move in an eastwards direction, around into the harbour entrance, at an estimated rate of up to 60m/per month. The largest movement can be found near the harbour entrance where the tidal currents are strongest. Pebbles deployed west of the drift divide generally move in a westerly direction, at an estimated rate of up to 130m/per month although these pebbles may have been artificially influenced by beach recycling activities in this area. However, a further deployment on Hayling beach frontage (Figure 13) indicates pebble movement to the west of 4800m over a 2 year period (or approximately 200m/per month).

Hayling Island – Gunner Point

There have been 3 deployments at Gunner Point, all showing strong westerly transport towards the entrance to Langstone Harbour (Figures 14-16). Transport rates are approximately 60m/per month westwards, although within the harbour entrance there is evidence to suggest a transport direction south towards the sand banks, probably due to the strong tidal currents. However pebbles here are difficult to locate and quickly lost in deep water channels, buried or swept away.

Portsmouth – Eastney

There have been 4 deployments at Eastney, 2 around the harbour entrance and 2 along the open frontage. The deployments within the harbour entrance both show movement of pebbles northwards into Langstone Harbour, with transport rates of approximately 50m/per month (Figure 17-18). There is evidence that the long groyne at the harbour entrance and the pier to the north, both allow sediment transport around and beyond the structures.

The deployments along the open frontage both show movement west and east, although the predominate direction appears to be westerly transport (Figures 19-20).

Portsmouth – Southsea

To the west of South Parade Pier there have been 2 deployments which have resulted in a predominately easterly sediment transport direction of approximately 50-60m/per month (Figures 21-22). For 2 deployments east of the pier the pebbles have moved in both directions, although there is a greater number found moving west (Figures 23-24).


Portsmouth – Clarence Parade & Old Portsmouth

There have been 4 deployments along this complex stretch of heavily engineered coastline (Figure 25-28). The general trend is for pebble movement north west towards the harbour entrance. There are numerous structures, narrow beaches and a deep water channel close to the foreshore, thus pebbles here are easily lost from the system. Recovery rates are especially low for these deployments.

Findings & Limitations

Recovery Rates

The collected data suggests that the recovery rate of the tracer pebbles decreases rapidly after deployment, with very few results returned after 1 year in the field. The poor recovery rate may be partly influenced by the nature of the bespoke detection kit, which is limited in its capabilities for long term tracer studies. Recent deployments of pebbles for the River Hamble to Portchester Strategy have resulted in a detection rate of less than 20% returned after 1 month in the field, with several locations as low as 5% detection rate. These deployments have used a smaller pebble with a smaller PIT tag which may account for these low readings.

In the majority of deployments, it is generally rare to find the same pebble more than once or twice, making it difficult to draw any definitive sediment transport rates from the study.

Tracer Burial Depth

Following a tracer burial depth experiment conducted for SCOPAC, it has been ascertained that the detection range of the pebbles is influenced by a number of different factors. The results from the trial indicate that the detection range is greatest for the larger RFID tags and that type of sediment can have an influence on the detection range. The actual detection range is up to a maximum of 0.8m in gravel for the 23mm RFID but this is reduced to around 0.35m for the 12mm RFIDs used for the RHPS. Further details of the tracer burial depth can be found in the report presented to SCOPAC in February 2013.

Attrition

Initial proposals for the project were to estimate attrition rates for the beaches along this study frontage in order to estimate the longevity of recharged beaches.

Attrition in the coastal environment is a form of erosion occurring to beach material, due to the regular impact between the bedload causing them to be broken up into smaller fragments. This process also produces pebbles which are rounder, smoother and over time smaller.


In theory the tracer pebbles released into the environment can be measured over time and compared to the initial cataloguing measurements recorded prior to deployment. However with very low recovery rates after a year and potentially ever decreasing, the practice of recovering pebbles within the field to re-measure and weigh them would be extremely costly.

Localised Conditions

A tracer pebble study relies on long term trends in order to produce useful and meaningful results, as localised conditions can alter the pattern of sediment transport daily. Although the greatest detection rate is within the first surveys, it should be noted that results obtained after only several weeks or months may not necessarily be representative of the longer term trends within that stretch of coastline, particularly if prevailing weather and sea conditions have been adverse.

More reliable results should be built up over the longer term of the study, although the retrieval rates will be proportionately lower as time goes on.

SCOPAC sediment transport model


The SCOPAC sediment transport study (2004) provides a broad scale overview of sediment transport along the Portsmouth and Hayling Island frontage. It identifies several drift divides along the coastline, transport northwards in to the harbour entrances, as well as a general east to west transport rate along the open coast.

Part of this tracer study was to investigate the sediment transport pathways around the coastline and in particular to

1) Confirm the presence of a drift reversal at Gunner Point, and identify the location of the resulting drift divide.

The evidence to date would suggest that the sediment transport direction at Gunner Point is from east to west and then northwards in to the harbour entrance. There is little evidence to suggest that there is a drift divide at this point in the coastline although there may be movement of material further offshore that is not recorded by the tracer study.

2) Ascertain both the net direction and rate of littoral drift at Southsea

The direction of littoral drift at Southsea appears to be complex, with material moving both east and west along this section of coastline. West of the pier, material generally moves east towards the pier and beyond but east of the pier material appears to move west. However further towards Eastney the net sediment transport appears to be easterly moving in to the harbour entrance. This is in agreement with the drift divide shown at this location in the current SCOPAC sediment transport model.

3) Estimate attrition losses to inform revised estimates of the longevity of recharged beaches

At this stage it is unlikely this study will proceed due to the time and cost of recording the pebbles combined with the very low recovery rates.

4) Ascertain the feed of material to both Eastoke and Eastney from tidal deltas.

Whilst the feed of material for both Eastney and Eastoke is from the beaches to the west, it is difficult to comment at this point in the study regarding material from the tidal deltas. They have not been surveyed due to the expanse of the sand banks and the limitations associated with the detector kit such as burial depth detection.

Recommendations & Successes Whilst the tracer study has so far produced some useful results, the limitations of the study are such that the results are generally qualitative providing approximate sediment transport directions along the coastline. Whilst further detailed analysis could be performed on the results to date, it is unlikely to fundamentally alter the main findings.


However, as a communication tool to non technical audiences the study and the presentation of results have proved extremely successful and generated notable worldwide interest. In particular the detection of pebbles using the survey kit has been widely received as a useful engagement method with people unfamiliar with the study who would like hands on experience of coastal processes and the methods used to currently study them.

The tracer results and any further analysis could provide a useful additional source of information to feed in to the latest SCOPAC sediment transport study which is currently undergoing review and updates. Much of the current findings within this report are broadly similar to the 2004 SCOPAC sediment transport study, however the level of detail could prove a useful addition.

The main limitations currently associated with the study lie in the detection range of the pebbles and the uncertain nature of the survey kit itself. The results become more valuable as the length of time the pebbles are in the field increases, however this also increases the associated time and costs of detecting the remaining tracers along with ever diminishing returns.

At present this technique would be useful for short term localised projects to examine the movement of material around a structure or short section of coastline. However, longer term studies of complex beach frontages are not realistically possible with the present setup of survey equipment used or the PIT tags employed in the pebbles.

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Solent Sediment Tracer Study - Portsmouth

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±Figure 23. Pebble Movement: Deployment 10

_̂ Deployment LocationSeptember 2011November 2011March 2011July 2011January 2012December 2012

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Beach Sediment Tracer Study 2010 - 2012 - ESCP & Portsmouth... · Beach Sediment Tracer Study 2010 - 2012| 3 The hand-held wand has a limited read range, typically less than 0.2m,