1. Post groyne construction monitoring, Poole
Use of CCO survey data
12 December 2012
Jon Kemp

2. Introduction This presentation consists of two parts:
Numerical modelling of Sandbanks to Branksome coastal defences (2002 – 2009)
Study site and challenges
Study approach
Numerical modelling
Post construction monitoring (2009 – present day)
Assess performance of groynes
Compare beach widths either side of rock and timber groynes
Assess beach widths to the west of the new groynes
Compare modelled to measured beach widths
Review beach survey programme
12 December 2012
CCO Annual Partners Meeting

3. Study site and challenges
Narrow beaches
Overtopping
Scour and undermining of seawalls
12 December 2012
CCO Annual Partners Meeting

4. Study approach Literature review
Gather guidance on beach control schemes – both UK and abroad
Included sub-contracted reviews (from New Zealand and Italy)
Review of current standard of defence and beach conditions along study frontage and coastal processes
Initial numerical modelling of beaches/ groynes
Initial short-list of suitable options drawn up
Multi-criteria analysis to assess direct and indirect benefits of 10 short-listed options
Numerical modelling of 4 options from the MCA and recommend a preferred option
Refinement of preferred option – numerical modelling
12 December 2012
CCO Annual Partners Meeting

5. Beach modelling – initial scheme
12 groynes:
75m long at 225m spacing
Aims of numerical modelling
Comparison of four Phase 1 options
Maintaining adequate beach widths
Efficiency at retaining beach recharge
Impact on downdrift beaches
Relative comparisons
Recommend a “final” preferred scheme
Refinement of scheme
12 December 2012
CCO Annual Partners Meeting

6. Beach modelling – refined scheme
Only 5 groynes:
75m long (or less) at 150m spacing
No groynes to the west
2:1 groynes worked best
Another important conclusion was that there was no need for any structure along the western section – with groynes installed led to localised narrowing
More cost effective to undertake direct improvements to the seawall, expect some narrowing over time + further recharge
Big cost saving and better amenity and aesthtic value
12 December 2012
CCO Annual Partners Meeting

7. Completed scheme – May 2009 Photos courtesy of PBC 12 December 2012
CCO Annual Partners Meeting

8. Post construction monitoring
5 year post construction monitoring study
Post construction monitoring (2010/11 – 2014/15)
Assess performance of groynes
Compare beach widths either side of rock and timber groynes
Assess beach widths to the west of the new groynes
Compare modelled to measured beach widths
Review beach survey programme
Work to date
Year 1 (FY2010/11): Analysis of beach survey data along the Poole frontage up to June 2010 (HR Wallingford, 2010).
Year 2 (FY2011/12): Analysis of beach survey data along the Poole frontage up to June 2011 (HR Wallingford, 2011).
12 December 2012
CCO Annual Partners Meeting

9. Available survey data
12 December 2012
CCO Annual Partners Meeting

10. Available survey data
12 December 2012
CCO Annual Partners Meeting

11. Management and analysis of survey data
Data Management
Beach Data Analysis system (BDAS)
Generation of digital elevation models (DEMs) using ArcGIS
Data Analysis
Shoreline position – MHW (0.5 m OD) contour extracted from DEMs
Beach width – Statistical analysis of MHW contour positions at all CCO profiles
Cross-sectional area – Good indicator of the “health” of a beach. Extracted from CCO annual reports
12 December 2012
CCO Annual Partners Meeting

12. Management and analysis of survey data
12 December 2012
CCO Annual Partners Meeting

13. Shoreline positions 15 m increase Short-term variations of ±5m
Seasonal changes in beach levels
Drift reversals
Groynes reached capacity
Effective in slowing longshore drift
15 m increase
Groynes 4 5 and 6 have reached capacity For as long as there is sufficient sand supply from the west it is likely that any sand being transported eastwards (dominant direction) will no bypass the groynes
June 2010 shows westward drift accretion against eastern side of groynes
Overall the groynes have been effective in slowing down the drift – as the supply of sand diminishes the effectiveness of the groynes in slowing drift and retaining sand will become more apparent
12 December 2012
CCO Annual Partners Meeting

14. Change in beach levels
in this particular image shows a steepening of the beach- lowering of levels offshore of MHW and increase in levels landward of this
Result of natural re-profiling - This analysis also shows seasonal fluctuations –
Also Show an increase at back of beach by up to 0.5m (wind blown sand now a minor problem)
However from this type not possible to judge whether the volume has changed or if it is just a redistribution vertically
12 December 2012
CCO Annual Partners Meeting

15. Change in beach width
General increase in beach widths but quite a lot of short-term variation
Over time the comparison of the variability in beach widths between the two types of groyne will become more meaningful.
Will also be interesting to see whether the short-term variations in response to different wave directions, storms and variations in slopes will be any different for the different groyne types
12 December 2012
CCO Annual Partners Meeting

16. Change in cross-sectional area
Considering only beach widths alone can sometimes be misleading – short-term changes in the slope often occur in response to a storm as well as seasonally with the result that the width of the upper beach will alter but the overall volume (per metre run of frontage) might be the same.
Volume of beach is a more reliable indicator of the health of the beach – on a beach that has been recharged and will likely be need to be recharged in the future – studying the changes in volume is particularly important.
To investigate change in volume we used the CCO annual reports Instead of repeating the analysis of beach profile data
Western end
Reduction in volume at western end is inevitable due to net eastwards longshore drift
Expected that this pattern of erosion will continue into the future – indicating that the sand arriving is smaller than volumes being carried eastwards
Could become a concern at Flag head chine first because of seaward bulge in the plan shape of the seawall
Eastern end
Losses have been less indicating the groynes have been working
Previous problems of low beach levels, particularly in front of the seawall at Branksome Dene change have been solved at least for the immediate future
The table also suggests no localised beach erosion has occurred immediately to the east of the new rock groynes.
Western end - Gradual decrease in sand along western part of frontage
Eastern end – Less sand loss and haver stabilised in last three years
12 December 2012
CCO Annual Partners Meeting

17. Comparison with modelling results
Main differences are in the immediate lee of the groynes
Note that the picture shown is just a snapshot in time using hindcast wave data – the waves since are likely to be different than those modelled
We are now in the process of remodelling the beach evolution with new wave data measured by cefas and CCO
12 December 2012
CCO Annual Partners Meeting

18. Conclusions Monitoring data Groyne performance Western frontage
Useful resource for future studies of beach management and fundamental research into beach and groyne processes
Following recommendations from first report CCO changed the survey schedule to co-ordinate Poole and Bournemouth surveys
Groyne performance
Effective in slowing longshore drift and maintaining beach widths.
Not yet enough data to make robust conclusions on model vs. measured or timber vs. rock groynes
Western frontage
Beach width has decreased and rate of loss is in line with the modelling
No evidence to suggest the decision not to install groynes was unsound
12 December 2012
CCO Annual Partners Meeting