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ANALYSIS AND DESIGN OF REFURBISHMENT FOR UNUSUAL DAMAGE TO A RUBBLE MOUND BREAKWATER

William Allsop1, Quintin Murfin

2, and Chris Sampson

2

A relatively simple rubble mound breakwater has been used to protect a large lagoon, itself then

progressively filled by dry waste materials, some (potentially) sensitive to possible erosion. The

breakwater is exposed to an extreme tidal range (~12m) and wave attack can exceed Hs ≈ 8m at

1:200 year return. Since construction, a part of the breakwater length has shown signs of unusual

damage. This paper describes analysis of that damage, the derivation of Joint Probability Analysis

conditions, and the development of refurbishment options taking account of current conditions, and

potential future changes to waves and water levels.

1. INTRODUCTION

1.1 Project background

The reclamation breakwater at La Collette, on the south coast of the island of

Jersey was constructed in 1994-95 to protect a large lagoon area to be filled by

dry waste, and (later) for light industrial and other uses related to the port and

nearby power stations, Figure 1. The site is exposed to an extremely large tidal

range (approx 12m) and wave attack along the exposed southern part of the

breakwater can reach Hs ≈ 8m at 1:200 year return. Wave conditions at the

1 Coastal Structures Group, HR Wallingford, Howbery Park, Wallingford, Oxfordshire OX10 8BA, UK.

w.allsop@hrwallingford.co.uk 2 Technical Services Department, States of Jersey, South Hill, St Helier, Jersey

Figure 1 La Collette reclamation breakwater, view towards North-West

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breakwater are reduced by refraction and depth-limiting, and by the conditional

probability of occurring with rare water levels.

The breakwater designed by Coode & Partners (now WSP), and tested by HR

Wallingford in 1993 for armour stability and wave overtopping, was of

relatively simple cross-section. The section design used core material up to

+11mCD with side slopes from 1:1.5, through 1:3, up to 1:4 on the upper part,

Figure 2. The steeper slopes at the lower elevations can be sustained as depth-

limiting reduces wave heights at lower tide levels. It was (probably) anticipated

that the coarse core material (50kg – 2t) would adequately act as an underlayer /

filter for the 6-10t armour. Photographs taken on site however suggest that the

core grading in practice may have been wider than specified for the model

testing, probably with more fine material than implied by the 50kg lower limit.

Filling of the large tidal lagoon formed by the breakwater started during 1995

and has continued to date, but probably at a slower rate than might have been

anticipated by the designers. By 2009, the major part of the initial lagoon had

been reclaimed, and some areas have already been converted for industrial /

commercial uses. One significant lagoon area remained, adjacent to the most

exposed section of the defence.

1.2 These studies

In 2009, States of Jersey Transport & Technical Services (TTS) required HR

Wallingford to assist them investigate reasons for damage to armour on the rear

face on the section over or adjoining the Dogs Nest rocks along the south to

SSW section of the breakwater (seen at the southern tip of the breakwater in

Figure 1). Damage to the armour had occurred along approximately 250m where

the armour crest width reduces from the 11.5m (used on the western facing

frontage) to around 3.5m for the southerly and eastern facing frontages. In 2007

interim remedial works had been started to reduce the probability of collapse of

the breakwater crest. These involved recovery of displaced armour from the rear

face and re-profiling of the crest to maximise the height of protection possible

Figure 2 Breakwater section tested in 1993

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using existing armour. During the remediation works, it was noticed that the

access roadway (see Figure 6) appeared to suffer disruption over periods of very

high tide, even in periods of relatively little wave overtopping. It was however

also noted that armour continued to damage during storms when presumably

wave overtopping (or transmission) was heavy.

1.3 Outline of the paper

The rest of this paper describes the analysis of damage, and presents calculations

of the wave exposure and of wave overtopping performance, from which

refurbishment options to reduce risks of damage have been developed. The

paper concludes with suggestions for enhancement options for conditions some

70 years ahead.

2. RECENT DAMAGE

The most exposed southerly section of the breakwater is difficult to access,

being backed by the tidal lagoon, accessed through a controlled tip area, and

having no roadway along its crest. Damage to crest armour on the section

adjoining the Dogs Nest rocks along the S / SSW section of the breakwater only

became apparent in aerial photos taken in 2003.

Figure 3 Damage to narrow crest showing disruption of crest armour (2003)

Analysis of those photographs, see Figures 3 and 4, suggested that the crest

armour had settled by 2-3m vertically along approximately 250m where the crest

width reduces from the 11.5m used on the western facing frontage to around

3.5m for the southerly facing frontage. It was initially assumed that damage to

the crest armour had been driven by overtopping, so initial effort concentrated

on calculating wave overtopping for a range of conditions. It was therefore

surprising that the crest level measurements in Figure 4 indicated that the

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landward part of the crest was more damaged, suggesting that the cause of

damage might be related to suffusion of fine material from the core driven by

steady and intermittent flows along the core / armour interface. This initial

conclusion was supported by evidence of such flows on that interface, see Figure

5, and by the absence of any obvious loss of armour from the seaward face.

Figure 4 Crest levels along seaward and landward edge of breakwater crest

Figure 5 Wave transmission flow along core / armour interface (January 2008)

It was therefore concluded that the main mechanism for the loss of crest level

was probably loss of fine material from the core, progressively removing

support for the armour, allowing settlement. Some armour rock was removed

under heavy overtopping, but only by being washed backwards into the lagoon

by heavy overtopping / transmission, made worse by the reduced crest level.

There was however little or no evidence of the same type or degree of damage

along sections of the breakwater with the wider crest, and none on those sections

where the lagoon behind had been filled, suggesting that the simplest remedy to

the suffusion problem would be to bring the fill against the rear face up to crest

level, removing the potential for substantial flows along the core / armour

interface. The challenge then remained as to how the crest details could be

configured to resist future extreme events.

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Figure 6 Remediation to crest during 2008 showing temporary access road

Figure 7 Interface between armour and core showing fine material and evidence of washout

3. WAVE CONDITIONS AND WATER LEVELS

3.1 Wave conditions

HR Wallingford had previously set up and calibrated a whole-island wave model

using 40 years of wind time series. In this study, nearshore bathymetry and

coastline were refined around the entrance to St. Helier Harbour and the La

Collette reclamation. For the joint probability analysis (JPA), new tide gauge

data were used to refine estimates of extreme sea levels, allowed analysis of the

joint probability of large waves with high water levels, for different positions

(Figure 8) and for different wave directions. Separate work in 2007 provided

guidance for Jersey on appropriate allowances for climate change to wind speed,

wave height, sea level etc., suggesting:

• sea levels 0.5m higher than present by 2080s;

• waves 10% increases in offshore wave height and period.

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Figure 8 Location of wave prediction points

The wave model was run to produce equivalent wave time series for each

nearshore point. Some of the extreme wave heights for Points B, C and D will

be reduced by depth-limited breaking. This reduction is not represented in the

main wave model, but was applied in subsequent calculations using methods by

Goda (1985) or Owen (1980). Those effects were however applied after

determining JPA conditions with appropriate water levels.

3.2 Joint Probability Analysis

Most overtopping at La Collette is expected to occur when both waves and sea

level are high. Simply taking high values of both waves and sea levels in any

overtopping assessment does not allow estimation of the overall return period.

Hence the need for JPA, involving not only high and extreme waves and sea

levels, but an assessment of their likelihood of occurring at the same time, i.e.

both variables reaching peak values within an hour or so of each other.

Measured sea levels from the St. Helier tide gauge were used to derive a Weibull

distribution. Extreme sea levels were then predicted from that distribution.

The dependence analysis was based on coincident wave and sea level data for

1993 to 2009. For each of the four nearshore points, the JOIN SEA program was

then applied to each of the simulations to determine the required extreme values.

The results are plotted in the form of joint exceedence extremes, i.e.

combinations of Hs and sea level expected to be exceeded (simultaneously by

both sea level and wave height) once, on average, in a given return period.

These results are multi-valued and are given for a range of joint return periods

by location, and by a number of direction sectors.

3.3 Local effects

Appropriate combinations of wave conditions and water levels were derived for

Positions B, C and D for overtopping calculations, but each condition needed to

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be checked for shoaling / depth-limiting. Waves at B around the western face of

the reclamation up to the start of the Dog’s Nest rocks were un-affected by a toe

level of -3mCD and bed slope of 1:50.

Test conditions, Position C, for 1:10, 1:200, and 1:200 climate

change

2

2.5

3

3.5

4

4.5

5

5.5

6

9 9.5 10 10.5 11 11.5 12 12.5 13

Water level (mCD)

Wa

ve

he

igh

t, H

s, (m

)

1:10 year

1:200 year

1:200 climate change

Figure 9 Initial JPA conditions for Position C

At C behind the Dog’s Nest, a local survey suggested that waves might track

across levels averaging +4mCD, but bed slopes are steep at 1:10. Local depth

limiting might therefore be expected to reduce wave heights for water levels

below 10.5-12mCD, but to a lower degree than for shallower slopes. Depth-

limited JPA conditions for C are shown in Figure 9. At D, waves are generally

shorter in period than at the western sites. For a bed level of +2mCD and bed

slope of 1:50, waves remained generally unaffected by depth-limiting.

B

C2

D2

C1 C3

D1

Figure 10 Positions B, C1, C2, C3, D1 and D3 for refined overtopping calculations

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3.4 Refined wave conditions

The wave and water level conditions at B, C and D derived in the first phase of

overtopping calculations were later refined to take more account of local

shoaling, depth-limiting and wave obliquity at intermediate positions around C

and D. Effects of local wave generation from easterly sectors were also

included. The adjustments were applied to give wave heights by direction sector,

for C1, C2, C3, D1 and D2, Figure 10.

The resulting wave conditions and water levels for C3 are shown for each wave

direction in Figure 11. These follow the general form seen previously, although

wave heights at C3 are reduced relative to C1 and C2, and at the shoreline

positions D1 and D2 are significantly smaller than at the nearshore location D.

Waves at C3, for 1:10, 1:200, and 1:200 climate change,

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13

Water level (mCD)

Wa

ve

he

igh

t, H

s, (m

)

1:10 year, 170degN

1:10 year, 190degN

1:10 year, 210degN

1:10 year, 230 degN

1:10 year, 250degN

1:200 year, 170degN

1:200 year, 190degN

1:200 year, 210degN

1:200 year, 230degN

1:200+CC, 170degN

1:200+CC, 190degN

1:200+CC, 210degN

1:200+CC, 230degN

1:200+CC, 250degN

Figure 11 Refined JPA conditions for Position C3

4. WAVE OVERTOPPING

As it was not possible to predict beforehand which direction, or which

combination of water level and wave height, would maximise the overtopping,

discharges were calculated for each combination of wave direction, JPA water

levels and wave conditions, for each of 1:10, 1:200 and 1:200 + climate change.

These calculations were repeated using different refurbishment scenarios. These

considered: Phase 1a - a near future conditions where land use continues as

present, tested against 1:10 year combinations of water level and waves; Phase

1b - a future condition where there would be no sensitive use close to the

defence, but it would be assessed against 1:200 year and 1:200 + climate change

conditions; and Phase 2 - possible future uses with industrial or similar

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developments close to the defences to be assessed against 1:200 year and 1:200

+ climate change conditions.

At C1, overtopping with the refined conditions was greater than previously, but

raising the crest from +14mCD to +19mCD does restore overtopping under the

1:200year + Climate Change case to below q ≤ 1 l/s/m, Figure 12.

C1 (wide crest at 19mCD) for 1:10, 1:200, & 1:200 +CC

0.001

0.01

0.1

1

8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13

Water level (mCD)

Ov

ert

op

pin

g d

isc

ha

rge

, q

, (l

/s/m

)

1:10 year, 170degN

1:10 year, 190degN

1:10 year, 210degN

1:10 year, 230 degN

1:10 year, 250degN

1:200 year, 170degN

1:200 year, 190degN

1:200 year, 210degN

1:200 year, 230degN

1:200+CC, 170degN

1:200+CC, 190degN

1:200+CC, 210degN

1:200+CC, 230degN

1:200+CC, 250degN

1:200 year, 250degN

Figure 12 Overtopping at Position C1, wide crest at +19mCD

In contrast, at C3, refining the wave conditions has substantially reduced the

overtopping, so much that reducing the (wide) crest back down to +14mCD can

give q ≤ 1 l/s/m under the 1:200year + Climate Change case, Figure 13.

Waves at C3 (wide crest +14mCD), for 1:10, 1:200, & 1:200 +CC

0.0001

0.001

0.01

0.1

1

8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13

Water level (mCD)

Ov

ert

op

pin

g d

isc

ha

rge

, q

, (l

/s/m

)

1:10 year, 170degN

1:10 year, 190degN

1:10 year, 210degN

1:10 year, 230 degN

1:10 year, 250degN

1:200 year, 170degN

1:200 year, 190degN

1:200 year, 210degN

1:200 year, 230degN

1:200+CC, 170degN

1:200+CC, 190degN

1:200+CC, 210degN

1:200+CC, 230degN

1:200+CC, 250degN

Figure 13 Overtopping at Position C3, wide crest at +14mCD

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5. REHABILITATION OPTIONS

For the section of the defence behind the Dog’s Nest rocks, re-examined as

Points C1, C2 and C3, the present narrow crest is not able to resist current

conditions, let alone credible future conditions. So for Phase 1, the armour crest

needs to be widened at +14mCD to a crest width of 11.5m.

Figure 14 Suggested defence at Point C1 and C2

For Phase 2 when developments behind the defence may need to be protected

against larger threats, the defence crest then needs to be extended upwards. For

the defence lengths covered by C1 and C2, the crest should be continued up to

+19mCD with an armoured slope (again at 1:3). This slope should end in a

horizontal crest, typically 3 armour stones wide, so (say) 3m width, backed by

the access road, see sections sketched in Figure 14. It might be possible to refine

these sections, perhaps using a recurved wave wall if space is at a premium, or if

an overall crest levels lower than +19mCD are desired.

Figure 15 Suggested defence at Point C3

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Along the defence length represented by point C3, the defence can however be

significantly reduced, with a simple (wide) crest at +14mCD being sufficient.

No modifications are required for the section at position D where waves are

smaller and/or relatively oblique to the revetment. Along those lengths, the

overtopping requirements are satisfied even by a narrower crest (~ 2m) under

the future 1:200 year conditions and the crest could (theoretically) be reduced to

+13mCD.

ACKNOWLEDGMENTS

The work reported was assisted by the Joint Probability Analysis by Peter

Hawkes (HRW). The authors are also particularly grateful for assistance

afforded this work by Nigel Johnstone, and colleagues in Jersey Met Office and

Jersey Harbours.

REFERENCES Allsop NWH, Briggs M.G., Denziloe T., & Skinner A.E. (1991) Alderney breakwater: the quest for

a 'final' solution Proc. Conf. Coastal Structures and Breakwaters, ICE, pp 303-320, ISBN 0-7277-

1672-7, publn Thomas Telford, London. Allsop N.W.H. & Williams A.F. (1991) Hydro-geotechnical performance of rubble mound

breakwaters, Report SR183, February 1991, HR Wallingford.

Goda Y. (1985) Random seas and design of maritime structures University of Tokyo Press, Tokyo. Goda Y. (2000) Random seas and maritime structures, 2nd edition, ISBN 981-02-3256-X, World

Scientific Publishing, Singapore.

HR Wallingford (1993) La Collette: random wave studies on the reclamation breakwater, Report EX2846, November 1993, HR Wallingford.

HR Wallingford (2001). Coastal processes and shoreline management study, St. Ouen’s Bay, Jersey.

Report EX 4020 (Rev A), HR Wallingford. HR Wallingford (2007). Climate Change, Jersey: Effects on coastal defences. Report EX 5516

(Release 2.0), HR Wallingford.

HR Wallingford (2011). La Collette Breakwater, St Helier: wave modelling, joint probability analysis and overtopping assessment. Report EX 6314 (Release 5.0), HR Wallingford.

Owen M W (1980) Design of sea walls allowing for wave overtopping Report EX 924, Hydraulics

Research, Wallingford. Pullen T., Allsop, N.W.H., Bruce T., Kortenhaus A., Schüttrumpf H. & van der Meer, J. W. (2007)

EurOtop: Wave Overtopping of Sea Defences and Related Structures: Assessment Manual , pdf

download available from : www.overtopping-manual.com Pullen T., Allsop, N.W.H., Bruce T., Kortenhaus A., Schüttrumpf H. & van der Meer J.W. (2008)

EurOtop: Overtopping and methods for assessing discharge Proc. FloodRisk, Oxford, UK.

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KEYWORDS – CSt2011

ANALYSIS AND DESIGN OF REFURBISHMENT FOR UNUSUAL

DAMAGE TO A RUBBLE MOUND BREAKWATER

1st Author: Allsop, William Allsop

2nd

Athor: Murfin, Quintin

3rd

Athor: Sampson, Chris

Coastal structures

Rubble mound

Suffusion

Wave overtopping

Joint Probability Analysis

Climate change effects