Final report on laboratory measurements Samphire Hoe

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Samphire Hoe Physical Model Studies

Report TR 147

Rev 0.0

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Document Information

Project CLASH Workpackage 4

Report title Samphire Hoe Physical Model Study

Client European Community

Client Representative Prof. J. de Rouck, University of Ghent

Project No. CAS 0314

Report No. TR 147

Doc. ref. TR 147 Rev 0.0 V02.16.doc

Project Manager Dr Tim Pullen

Project Sponsor Dr Phil Besley

Document History

Date Revisio

n

Prepared Approved Authorised Notes 22/10/2004 0.0 TAP NWHA PB

Prepared Approved Authorised

This report is a contribution to research generally and it would be imprudent for third parties to rely on it in specific applications without first checking its suitability. Various sections of this report rely on data supplied by or drawn from third party sources. HR Wallingford accepts no liability for loss or damage suffered by the client or third parties as a result of errors or inaccuracies in such third party data. HR Wallingford will only accept responsibility for the use of its material in specific projects where it has been engaged to advise upon a specific

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Summary

CLASH Workpackage 4

Samphire Hoe Physical Model Study Report TR 147

October 2004

Under CLASH (“Crest level assessment of coastal structures by full scale monitoring, neural network prediction and hazard analysis on permissible wave overtopping”), HR Wallingford were committed to a programme of field measurements of overtopping at Samphire Hoe, Kent, England. Following the successful completion of those field measurements those storms were simulated in physical model studies so that the field and laboratory results could be compared. This report describes the work carried out during the physical model studies, which include a 2d flume study at the University of Edinburgh and a 3d basin study at HR Wallingford.

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1. Introduction

The processes of wave overtopping of seawalls are not yet understood fully; particularly those that may cause risks to people close behind seawalls. There remain important gaps in knowledge, despite significant improvements in recent years. To help reduce uncertainties in the prediction of coastal flooding, HR Wallingford (HRW) have been commissioned to develop improved prediction methods for use by coastal engineers. This research is supported by the EC under the CLASH project led by University of Gent (contract EVK3-2001-0058), and by defra / EA under project No FD2412. CLASH is an extensive study by twelve partners at universities and research institutes across Europe, under the EC 5th Framework programme.

Under CLASH (“Crest level assessment of coastal structures by full scale monitoring, neural network prediction and hazard analysis on permissible wave overtopping”), HR Wallingford were committed to a programme of full scale measurements of overtopping at Samphire Hoe, Kent, England (See, Pullen & Allsop, 2003). This report describes 2d & 3d laboratory simulations of those and reports the results.

1.1 BACKGROUND

CLASH is intended to benefit low lying and populated coastal regions, which depend critically on the performance of coastal structures for defence against storm surges, wave attack, flooding and, erosion. Continuing sea level rise and climate change emphasis the need for reliable and robust predictions of overtopping hazards as higher storm surges and more severe storms may lead to flooding. Population pressures on land use in some coastal regions have sometimes ignored coastal hazards. The CLASH project will produce generally applicable prediction methods on the required crest height of most coastal structure types, based on permissible wave overtopping and hazard analysis.

A particular motivation for this research was the suggestion by earlier research in another EC project, OPTICREST, that there might be unexpected scale effects in some hydraulic modelling in which small-scale tests might under-predict overtopping at full scale. While these suggestions were not subsequently supported by large scale tests by the VOWS team in the large flume at Barcelona, see Pearson et al (2002), it is clear that this uncertainty could have substantial impacts.

HRW in collaboration with the University of Edinburgh, as part of CLASH Workpackage 4, undertook to simulate in the laboratory the field measurements at Samphire Hoe described by Pullen & Allsop (2003) and Pullen et al. (2003). This report describes each of the test facilities, the equipment used and the results of these field measurement simulations. The HRW physical model was tested in a large wave basin in 3d at a scale of 1:20 and in a 2d wave flume at a scale of 1:40 at Edinburgh.

1.2 REPORT

OUTLINE

Following this introductory section, Chapter 2 discusses the Samphire Hoe field measurements and results, Chapter 3 the design and operation of the laboratory measurements and the results are described in Chapter 4. Finally in Chapter 5 the data are compared and the principal conclusions are given.

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2. Samphire Hoe Field Measurements

This chapter will give a brief overview of the field measurements carried out at Samphire Hoe, which have been described in detail by Pullen & Allsop (2003). It will describe the site, indicate how the measurements were obtained, and present the results in a format that will provide a basis for the analysis in the later chapters. Additionally, the bathymetry at Samphire hoe will be discussed to provide the relevant information required during the discussion on the model design.

2.1 SAMPHIRE

HOE

Samphire Hoe is located in the Southeast corner of England (Figure 1) and is an area of reclaimed land, comprising 4.9M m3 of chalk marl excavated from the channel tunnel. The area of approximately 300,000m2_{is enclosed by a vertical Seawall with a crest} level at +8.22(mODN) and a toe level at –2.42(mODN) (Figure 2). Samphire Hoe has been landscaped and is used by the public as a recreational area (Figure 3). The site is owned by Eurotunnel, and is run on their behalf by the White Cliffs Countryside Project (WCCP). Eurotunnel and WCCP agreed to allow HRW to conduct full scale measurements at Samphire Hoe. Samphire Hoe is exposed to waves from the southwest and southeast. The seawall is subject to overtopping on approximately 30 days per year as a result of waves breaking over the rubble toe berm and impacting on the seawall face, and so this site offered good opportunities for field measurements.

The area in the foreground of Figure 3 was the location for the field measurement equipment, which was placed approximately 40m from the corner. From the cross section though the seawall shown in Figure 2, it can be seen that there is a wide concrete promenade behind the parapet wall. The monitoring equipment was deployed across this promenade, and is shown in Figure 4. This equipment ostensibly consisted of a series of continuously draining tanks, which enabled instantaneous volumes for individual overtopping events to be determined. From these data it was possible to determine mean overtopping discharge rates along with the peak volumes and relate these to the incident wave and water level conditions. Additionally, the positioning of the tanks across the promenade enabled data on the spatial distribution of the overtopping to be collected. A more detailed discussion describing these field measurements has been given by Pullen & Allsop (2003) and by Pullen et al. (2003).

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Figure 2: Section of the Samphire Hoe Seawall

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Figure 4: The overtopping tanks in position at Samphire Hoe

2.2 FIELD

RESULTS

A total of three storms were monitored at Samphire Hoe, and these occurred on 10 March 2003, 1 May 2003 and 2 May 2003. During the storm of 10 March 2003, overtopping discharges were too small to be determined accurately with the field monitoring equipment and will not be discussed here; refer to Pullen & Allsop (2003) for further details. Successful measurements were achieved during the remaining two storms, and these are described below.

2.2.1 1 May 2003 Storm

The storm of 1 May 2003 lasted from approximately 10:00 until 15:00, with wind speeds in the range 15~20m/s (up to gale force 5). The maximum recorded overtopping discharge was approximately QBar = 1.0 l/s/m and the maximum predicted discharge was QBar = 1.4 l/s/m. During the storm the overtopping water was arching over the parapet wall as it was caught by the wind, and as a consequence of the high wind speeds, it was blown over a wide area. From observations made during the storm, and from subsequent video analysis, it has been estimated that approximately 2/3 of the overtopping discharges were not collected in the overtopping tanks. The discharges have, therefore, been multiplied by a factor of 3 to represent more accurately the true discharges. This factor is close to the value of 3.2 that de Waal et al. (1996) determined for their “spray transport factor.” This described the difference between the discharge that would travel over the crest of a vertical wall with and without wind. Whilst strictly not the same mechanism in this instance, the effect on the subsequent travel trajectory beyond the crest is likely to be of the same magnitude.

The results from the data processing for this storm, described by Pullen & Allsop (2003), are summarised in Table 1, and shown graphically in Figure 5. The table shows the principal results of interest presented at half hourly intervals (1800s) during

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was assumed that a period of 1800s was a suitable time interval that might represent a change in the general conditions for assessing overtopping. The spectral wave conditions at the toe have been derived from the extraction point data (see Figure 7), as described by Pullen & Allsop (2003), and have been adjusted according to the offshore / inshore ratios derived during the calibration of the laboratory wave conditions. The data in Figure 5 have been plotted against Besley’s (1999) empirical prediction method for composite vertical walls. Further discussion on these data is given in Section 5.1.

Table 1: Storm 1 May

Hm0 Toe (m) Tp Toe (s) Tm-1,0 Toe (s) Tm0 Toe (s) Tm0,2 Toe (s) h (m) d (m) Rc (m) QBar (m3_/s/m) 2.02 6.39 5.55 4.83 4.48 4.52 2.27 6.12 9.38E-04 2.08 6.39 5.55 4.84 4.49 4.72 2.47 5.92 2.70E-04 2.13 6.40 5.56 4.84 4.49 4.92 2.67 5.72 4.23E-04 2.14 6.56 5.70 4.96 4.60 4.86 2.61 5.78 2.89E-04 2.16 6.72 5.83 5.08 4.71 4.80 2.55 5.84 2.77E-04 2.06 6.56 5.70 4.97 4.61 4.52 2.27 6.13 1.07E-04 1.96 6.41 5.57 4.85 4.50 4.23 1.98 6.41 4.94E-05 1.81 6.56 5.70 4.96 4.60 3.76 1.51 6.89 7.65E-05 1.65 6.70 5.82 5.07 4.70 3.28 1.03 7.36 1.41E-04 1.49 6.70 5.82 5.07 4.70 2.72 0.47 7.93 1.65E-04 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0.0 0.2 0.4 0.6 0.8 1.0 Rd Qd Besley (1999) 1 May 2003 Field       = = = = − − − 2 * * 2 * 3 79 . 2 4 _, _, _, 2 10 x 63 . 4 m s s c d d d d _gT h H d d d H R R d gd Q Q R Q π

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2.2.2 2 May 2003 Storm

The storm of 2 May 2003 lasted from approximately 21:45 until 04:15 the following morning. Before overtopping was recorded at the monitoring site wind speeds had been at storm force, but by the time that overtopping started to be recorded wind speeds had become insignificant. The overtopping discharges were being directed vertically upwards and coming down in the area directly behind the parapet wall. Little or no overtopping discharge was being blown by the wind during the time that the storm was being observed visually. HRW staff left the site at approximately 23:00 when it became too dangerous to remain in the observation position. At this time the winds were still not affecting the overtopping discharges. In this case no multiplication factor was applied to the data.

The results of this storm are summarised in Table 2, and are shown graphically in Figure 6. The highest recorded mean overtopping discharge during the storm was QBar = 3.3 l/s/m and the prediction according to Besley (1999) was QBar = 3.1 l/s/m, which is excellent agreement.

Table 2: Storm 2 May

Hm0 Toe (m) Tp Toe (s) Tm-1,0 Toe (s) Tm0 Toe (s) Tm0,2 Toe (s) h (m) d (m) Rc (m) QBar (m3_/s/m) 2.00 7.11 6.18 5.38 4.99 3.63 1.38 7.01 8.63E-05 2.37 7.05 6.12 5.33 4.94 4.30 2.05 6.34 1.38E-03 2.53 7.05 6.13 5.34 4.95 4.70 2.45 5.94 3.30E-03 2.51 7.06 6.13 5.34 4.95 4.95 2.70 5.69 3.24E-03 2.47 7.06 6.14 5.34 4.96 5.04 2.79 5.60 1.81E-03 2.37 7.07 6.14 5.35 4.96 5.05 2.80 5.59 1.15E-03 2.22 7.07 6.14 5.35 4.96 4.97 2.72 5.67 6.07E-04 2.07 7.22 6.27 5.46 5.06 4.76 2.51 5.88 1.71E-03 1.92 7.51 6.53 5.68 5.27 4.42 2.17 6.22 4.94E-04 1.75 7.74 6.72 5.85 5.43 3.98 1.73 6.67 2.52E-04 1.56 7.89 6.85 5.97 5.53 3.43 1.18 7.22 5.98E-04 1.40 7.74 6.73 5.86 5.43 2.87 0.62 7.78 1.91E-04 1.26 7.30 6.34 5.52 5.12 2.30 0.05 8.35 6.16E-05

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1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0.0 0.2 0.4 0.6 0.8 1.0 Rd Qd Besley (1999) 2 May 2003 Field       = = = = − − − 2 * * 2 * 3 79 . 2 4 _, _, _, 2 10 x 63 . 4 m s s c d d d d _gT h H d d d H R R d gd Q Q R Q π

Figure 6: 2 May Field Measurements

2.3 APPROACH

BATHYMETRY

It was not possible to carry out a survey of the bathymetry of the approach to the field monitoring site as part of this study, but an accurate assessment was made as part of a detailed desk study. The available material for this study included: Admiralty Chart No 1892, English Channel (Figure 7); aerial photographs; photographs taken from the cliffs (Figure 8); measurements made at the site by the research team (Figure 2); historical data relating observed overtopping events to the wave direction (Figure 9); and the detailed description given by Maddrell (1996).

The approach from the extraction point outside Folkestone to the area adjacent to Samphire Hoe ranges from –15.6 to –28mCD (Figure 7), but generally the overall variation shows a –20mCD contour across the seafloor. The levels given in Figure 2 are in mODN and these differ from mCD by 3.67m, thereby –20mCD is equivalent to a depth of h = 23.67m with respect to 0.00mODN. Due to this general variation in the bathymetry, it was assumed that the wave conditions would not vary significantly between those determined at the extraction point and those arriving at the –10mCD contour. At this point the depth is h = 13.67m, with respect to 0.00mODN, and is approximately the depth at which overtopping begins.

From Figure 9 it is clear that overtopping generally occurs when waves arrive from 180ºN. The monitoring site was along the western splay wall of Samphire Hoe, and this is aligned approximately along West by South to East by North giving an obliquity of approximately 10º to the incident waves. The approach slope was therefore determined by assuming a constant slope in a line between the monitoring site and the extraction point from the –0mCD to the –10mCD contour, which gives a slope of approximately 1:30.

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In the area immediately in front of the seawall at the measurement site the evidence from photographs (see Figure 8), and the behaviour of the incoming waves, suggested that there is a flat plateau in front of the seawall. The exact extent of this is uncertain, but an allowance was made for this in the design of the physical models.

Figure 7: Approach bathymetry and wave extraction point (From Admiralty Chart 1892)

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0 45 90 135 180 225 270 315 360

Wave direction (deg N)

Hs

(m)

Historical Data

Overtopping Observed

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3. Laboratory

Tests

This section describes the methods and model set up for each of the two physical model studies conducted for Samphire Hoe. These tests include a 2d model at the University of Edinburgh flume and a 3d model in one of the deep water basins at HRW. All dimensions will be in prototype units unless specifically stated otherwise.

For each of the two studies described here, the principal objective was to simulate the field conditions as closely as practicable, and to test sufficient additional wave conditions to examine the general overtopping behaviour of the tested structures. For each wave condition and water level combination, mean and wave-by-wave overtopping discharges were recorded. Data on the spatial distribution on a selection of the Edinburgh tests were recorded, and on all of the HRW tests, which also included repeat tests with the addition of wind.

The methods and reasoning adopted for the wave and water level combinations will be discussed in the sections that follow, but to save space and to avoid repetition, these will be reported along with the results in Chapter 4.

3.1 MODEL

SCALING

The Froude scaling law is applied to the physical model studies described below, where gravity is the predominant factor in the fluid motion. Wave models, since wave motion is essentially a gravitational phenomenon, are therefore designed according to this law. Froude's law states that the Froude number, Fr, should be the same in model and prototype where Fr is defined

as:-gD u

Fr= (3.1)

where u is a characteristic velocity, g is acceleration due to gravity and D is a characteristic length.

Wave models are not distorted and have the same horizontal and vertical scale. The linear scale of the model, to which the offshore bathymetry and seawall were constructed, is known as the geometric scale. In the design of the physical models the principal concern is to ensure that the main aspects of the wave-structure interaction are reproduced faithfully at a scale that avoids significant scale effects. A scale of 1:40 was selected for the Edinburgh flume model, and a scale of 1:20 for the HRW basin model. Refer to Sections 3.2.2 & 3.3.1, respectively, for further discussion on the selected scales.

The Froudian scaling relationships for various different parameters, where λ is the ratio between model and prototype dimensions, are outlined in Table 3. It should be noted that acceleration could be scaled if measurements were required, however, since g is constant a scaling factor of unity is used here.

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Table 3: Froudian scaling ratios

Unit Dimensions Scaling

Length L λ Time T λ1/2 Velocity LT-1 _λ1/2 Volume L3 _λ3 Overtopping L3_T-1_L-1 _λ3/2 Acceleration (g) LT-2 ₁

3.2 2D EDINBURGH STUDY

3.2.1 Test Edinburgh Wave Flume

The 2-d experimental investigations at small-scale were all completed in the wave channel in the School of Engineering at University of Edinburgh, UK. The channel is 20 m long, 0.4 m wide and has an operating water depth of 0.7 m (Figure 10). The sidewalls and the bottom of the flume are made of glass. The facility is equipped with a moveable impermeable beach that allows a range of slopes from approximately 1:5 to 1:50. For this study, the approach beach was installed at a fixed slope of 1:50.

Figure 10: The Edinburgh Wave Flume

Waves are generated by a flap type wave paddle that is capable to produce regular and irregular waves with wave heights up to 0.11 m and wave periods up to 2.0 s for a fixed water depth of 0.70m at the paddle. The paddle is equipped with a non user controllable active absorption system that significantly reduces reflected waves returning from the structure.

The model tests on the Samphire Hoe structure required measurements of the incoming waves, the waves directly in front of the structure, and wave overtopping characteristics. (discharges, velocities, individual volumes, and distribution). Different measurement devices were deployed to determine the required information, and are described in more

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detail in the following sections.. The measurement devices deployed on the facility in Edinburgh were:

wave gauges: typical resistance gauges

overtopping measurement: measurement container hung from load cell overtopping distribution: segmental container

video analysis: camera for observing wave run-up, wave breaking and wave overtopping

Wave Gauges

To determine the wave characteristics, up to 8 resistance type wave gauges were used (see Figure 11). A quoted precision of ±2% can be achieved with these wave gauges. The gauges consist of a pair of resistance wire which are placed vertically in the water and fixed in position above the tank. The resistance from the gauge is converted to a voltage. By moving the gauge up and down known amounts in still water allows the voltage water elevation relationship to be determined. The gauge relationship between water level and voltage is linear.

The electronics allow the gain and offset of each gauge to be tuned / adjusted. For this study the electronics of the gauges were tuned such that the calibration on each gauge were set each morning to 1volt = 20 mm. This was done by zeroing the gauge at still water level, then moving the gauge a fixed distance (usually 50mm) and then adjusting the gain of the gauge until the required voltage was achieved (usually 2.5 volts). The gauge was then returned to its initial position and the voltage recorded. If the voltage was within 0.02 volts of zero (0.4mm), then the calibration was deemed to be acceptable. If not, then the voltage was re-zeroed and the calibration procedure repeated again until the required voltage acceptability was achieved. The recorded voltages of the gauges during measurements were converted to water surface elevations during later analysis.

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Overtopping discharges

Overtopping discharges were directed via a chute into a measuring container suspended from a load cell, as shown in Figure 12. Individual overtopping events were detected by two parallel strips of metal tape run along the structure crest that acted as a switch closed by the water. Wave-by-wave overtopping volumes were measured by determining the increment in the mass of water in the collection tank after each overtopping event following the general approach first used by Franco et al. (1994), and subsequently applied at other laboratories in the UK and Europe.

Figure 12: Overtopping collection container attached to load cell measurement device

Overtopping distribution

Selected conditions of varying period and wave heights were used to establish the mean spatial overtopping distribution. Mean overtopping volumes were determined in each compartment of the collection chamber, as shown schematically in Figure 13. For larger volumes the water was emptied and weighed at the end of the test, however, for the chambers farthest form the wall volumes were collected using tissue paper. By subtracting the initial weight of the paper from the wet weight the volume was found. By expressing the volumes in each of the individual chambers as a proportion of the total collected volume, a model of the spatial distribution can be established; further discussion will be given in Section 4.1.2.

Video Analysis

A number of tests were recorded on videotape for later analysis. T he camera was positioned on the side of the flume to record the wave breaking regime and the overtopping characteristics.

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Figure 13: An overtopping wave observed during the experiments, and a schematic of the collection chambers

3.2.2 Wave paddle characteristics and model scale

Figure 14 shows the paddle performance function for the wave maker in the flume, a test matrix has been selected to correspond with lines of constant steepness, ranging between 0.073 and 0.044, the suggested test matrix is shown in Table 4.

The scaling of the proposed model, was limited to either the characteristics of the paddle, or the physical dimensions of the actual model itself. The maximum significant wave height the paddle was able to generate was approximately 0.11m at a period of 1.1seconds. The 1:1 year prototype wave was 2.2m with a period of 5.8 seconds. Scaling of the wave height gives 0.11: 2.2, i.e. a scale of 1:20 was possible, scaling of the wave period gave 1.1 : 5.8, i.e. (5.8/1.1)2 = 27.8, thus a scale of 1:30 was possible. However, at a 1:30 scale the model wall would have been higher than the outer walls of the flume for certain tests, thus it was decided to adopt a 1:40 scale model.

Table 4: Offshore wave conditions for overtopping measurements

Wave period Ts (s)

Significant wave height Hs (m)

0.6 0.035, 0.041

0.8 0.044, 0.053, 0.063, 0.730

1.0 0.068, 0.083, 0.098, 0.113

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0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Period, T [s] Wav e hei ght, H [c m] Hmax (irregular) Proposed Hso Sop = 0.034 Sop = 0.044 Sop = 0.053 Sop = 0.063 Sop = 0.073

Figure 14: Wave paddle performance curve and lines of constant steepness

3.2.3 Replication of storm conditions

Prototype measurements from the 2003 winter storms were outlined briefly in Section 2. Overtopping measurements were collected on 1 & 2 May, and comparison measurements have been performed in the wave flume at the University of Edinburgh. These 1:40 scale investigations were completed in the small wave channel at Edinburgh University. A 1:50 beach and the Perspex model wall were installed as shown in Figure 15.

System accuracy

Prior to undertaking any tests, the accuracy of the overtopping measurement system was checked. A series of simulated overtopping events were performed in which known volumes of water were ‘thrown’ into the measurement container. The resulting data from the load cell were then passed through an algorithm to identify and quantify individual overtopping events. These results indicated that derived and actual total volumes differed by no more than 0.7%, suggesting that any errors in the measurement system were negligible. More detailed descriptions on quality control and variability were given by Pearson et al. (2001).

Measurement of inshore wave conditions

As the wave flume was equipped with active wave absorption systems to remove reflected waves from the structure and to reduce possible uncertainties in determining incident and reflected inshore wave conditions, all measurements of the inshore wave height at the structure (Hs) were made by repeating the test sequence without the structure in place, and placing a wave gauge at the same location of the structure.

Test length and sequence

Due to the change in water level at Samphire Hoe during the storm conditions, the overtopping discharges were grouped into 30 minute sections, which gave around 300 waves. For the replication of storms in the flume in Edinburgh, a test duration of 1024 seconds was selected which represented approximately 1000 waves.

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3.3 3D HR WALLINGFORD STUDY

3.3.1 Model design and facilities

The Samphire Hoe model was constructed in a deep water wave basin at a nominal scale of 1:20. A general plan of the model is shown in Figure 16. The offshore bathymetry was horizontal leading into a 1:20 slope that was used to reduce the depth, and then the final approach bathymetry at a slope of 1:30. In the area directly in front of the seawall, horizontal bathymetry was modelled to represent the plateau in the foreshore area, as discussed in Section 2.3. The battered seawall was constructed from marine ply at the rear of the foreshore area, and the profile of the seawall was reproduced accurately by including the Larsen piles and the set back parapet wall, as can be seen in Figure 17 & Figure 18. The toe berm rock used in the physical model was Carboniferous Limestone of density 2.71t/m3_{, and was scaled (refer to Appendix A) to} reproduce a porosity n = 35% and a Dn50 = 1.5m. The porosity and permeability of the berm was considered to be more important than the stability during selection of the model rock, nonetheless, it remained stable throughout the test programme.

A maximum Hs = 2.75m was assumed for the design, whereby the minimum offshore water depth needed to be approximately h = 4 x 2.75m = 11.0m, where 4Hs is generally assumed to be the minimum depth required in which to reproduce the wave accurately. This depth corresponds to a water level at 0.00mODN, and for convenience, a final depth of h = 11.08m was chosen which is equivalent to the –14.75mCD contour. The western return wall was modelled for approximately half its length (40m) and the western splay wall was modelled for approximately 80m. This allowed for the direction of the waves and any hydraulic affect that may be expected from waves diffracting around the corner of the walls. The promenade and the recurve wall at the back were not modelled, as this does not affect the overtopping. Taking account of the expected wave heights and periods and considering the outline seawall cross-section and plan, a suitable scale for the physical model was chosen as 1:20.

Waves were generated by a 4m long multi-element wave paddle at the –14.75mCD contour. It has active absorption on each of the paddle elements with each being driven by a linear motor. Two wave probes are mounted on the front face of each paddle element to measure the surface elevation continuously. This signal is then compared with that generated, and the feedback loop is adjusted to the paddle to ensure that only the required incident wave train is generated, and that reflections from the structure are absorbed at the paddle. The input signal to the wave generator is produced by a random wave synthesiser, which uses HRW’s HRWavemaker software. HRWavemaker is capable of producing random sea states to any required spectral shape, and for this study JONSWAP spectrum were used. Wave guides were used at either end of the generator to contain the waves within the modelled area. These guides were extended close enough to the modelled area to prevent excessive energy loss, but far enough back from the seawall to prevent spurious reflections from entering the study area.

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Figure 17: Front view of the Samphire Hoe physical model

3.3.2 Wave conditions and water levels

A selection of the wave conditions and water levels from the 1 & 2 May 2003 storms were calibrated so that simulations of these storms could be tested and compared with the field data. Additionally, parametric wave conditions and water level combinations were chosen to examine the general overtopping behaviour of the structure. Model test runs each consisted of 1000 (Tm) irregular waves generated from JONSWAP spectra for

γ = 3.3.

Each Test Part was run for a single water level, wave height (Hs, specified at the wave maker) and peak period Tp. The calibrated wave conditions at the toe of the berm range generally from Hm0 = 1.1~2.4m and Tp = 5.9.3s, and all arrive from 180ºN which is approximately 10º oblique to the western splay wall.

3.3.3 Wave calibrations

Wave calibrations were completed prior to construction of the seawall. A shingle spending beach positioned in the intended location of the seawall ensured that wave reflections were minimised during calibration. A twin wire resistance type wave gauge was used to monitor the offshore wave conditions (equivalent to the extraction point, refer to Figure 7) and at the toe of the rock berm. The nominal wave conditions were calibrated at the offshore gauge. Additional wave gauges were placed at other points along the centreline of the model, so that these data could be used for any subsequent numerical analysis, but which will not be reported here.

During wave calibration statistical measurements of the waves were made using a long, non repeating wave sequence, defined by a JONSWAP spectrum. Output from the wave gauges was monitored and a wave counting analysis was carried out. This type of analysis gives a more accurate representation of the wave conditions in shallow water, particularly where the waves are breaking. The conditions were measured over 1000 waves, based on the mean wave period Tm. Measurements of surface elevation were

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made relative to the mean value of the water level. A wave is defined as lasting between two successive down crossings of the mean water level. At the end of each ‘batch’ of waves, wave heights calculated from the sum of the maximum departure above and below the mean water level, are sorted in descending order from which statistical values of Hs and Hmax et cetera can be found. The total length of the calibration period is divided by the number of waves to give the mean wave period Tm. Using an iterative process of altering the input conditions to the wave generator and measuring the response, the required wave heights were achieved.

When the correct settings for each spectrum had been found, a spectral analysis of the wave sequence was carried out. This allowed the entire energy content of the spectrum to be measured giving values of wave height, Hmo (an estimation of the significant wave height Hs in deep water) and mean wave period, Tm via a Fast Fourier Transform technique (refer to Appendix B for further details). Additional, spectral analysis was carried out using WaveLab 2.6 (supplied to the CLASH partners by the University of Aalborg), which provided the values of Hm0, Tp, Tm-1,0, Tm0 & Tm0,2 presented in Chapter 4.

3.3.4 Overtopping measurements

The methods of measuring overtopping discharges were the same for all tests. The equipment was capable of determining the total volume and the individual wave-by-wave volumes. Discharges entered into the main overtopping tank via a chute located at the top of the parapet wall (8.22mODN). The main tank was suspended inside a larger tank, separating it from the water in the wave basin, via a load-cell positioned above the model; as can be seen in Figure 18. The load-cell for the main tank was capable of measuring up to 100 kg(model) and the overtopping tank had a capacity of 60 litres(model). However, to ensure greater accuracy for the results, and because lower volumes than the capacity of the tank were anticipated, the load-cell was calibrated over a range of 10kg. The load-cell operates between 0 & 10 volts (DC) and this signal passes through an analogue to digital converter to produce a signal from 0 to 4096 bits. With a calibration range of 10kg, the sensitivity of the load-cell was approximately 10,000(ml)/4096(bits) ≈ 2.5 ml/bit. Once the noise had been filtered from the signal, individual volumes could therefore be determined with a nominal accuracy of approximately 5ml (equivalent to 2 l/m prototype). Overtopping detectors were placed at the rear of the chute, and inside the entrance to the tank, as can be seen in Figure 19. A typical trace showing the cumulative overtopping discharges and the detected events is shown in Figure 20.

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Figure 18: General arrangement of the overtopping tank and chute

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0 1 2 3 4 5 6 7 8 0 200 400 600 800 1000 Time (s) V o lu m e (l t) & E v ent s

Cumulative Overtopping Volumes (lt/m)

Overtopping Event

Figure 20: Typical record of cumulative overtopping discharges and event detection

During the field measurements, as described by Pullen & Allsop (2003), the overtopping tanks were placed in such a way as to be able to record both the individual discharges and their spatial distribution. For the physical model tests it was not feasible to record the data using directly scaled equipment; this may be appreciated with reference to the size of the single tank shown in Figure 18. Instead, the individual wave-by-wave discharges were recorded in the main tank, and the spatial distribution was recorded in a series of 6 x 1 litre tanks positioned normal to the seawall adjacent to the main tank as shown in Figure 21. This allowed the spatial mean discharges to be recorded progressively farther back from the seawall. During the field measurements the discharge was specified at the centre of each tank compartment, and similarly the same technique was used for the model results, where the central positions are given in Table 5.

Each of the distribution tanks had a circular opening of 0.062m internal diameter and a triangular spout with internal dimensions of 0.012m(spout extent) by 0.018m(on the circumference), giving a total area in prototype of approximately 1.251m2_{. The tanks} were placed as closely together as possible, but this left an area around then where overtopping discharges could not be collected, which is shown schematically in Figure 22. To compensate for this a nominal width of 0.816m was used to determine the discharges, which was derived from the total area of all the tanks (7.505m2_{) divided by} the distance from the front to the back of the tanks (9.200m). During a limited number of tests the forward tanks would fill before the test was complete. For these cases the tanks were covered, and the time noted for determining the mean discharge.

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Table 5

: Location of the spatial distribution tanks

Tank

Landward Distance

_{(m) (prototype)}

1

1.580

2

3.120

3

4.660

4

6.205

5

7.840

6

9.480

Figure 21: General arrangement of the 6 overtopping distribution tanks

seawall

6 tanks

Missing area

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3.3.5 Wind measurements and overtopping

Wave overtopping is generally associated with wind, and its effects have been discussed by Ward et al. (1996) and de Waal et al. (1996). Wind may cause overtopping of part of the breaker spray that would otherwise have fallen back into the sea in a situation without wind. It may cause the breaker type to change by deforming the incident wave, or it may cause overtopping by spray generated by the wind on the sea. These are general effects that may not always be pertinent when discussing overtopping at vertical structures. In this case the significant point is whether the overtopping discharge passes over the crest of the structure or falls directly back into the sea. This particular effect is a well known phenomenon that has been discussed by de Waal et al. (1996) and has been observed during these model tests.

Scaling wind remains a difficult task, especially with regard to its effect on air / spray mixtures, where surface tension, viscosity and droplet size are the same for both prototype and model. Moreover, spray trajectories will be turbulent and should therefore be modelled using Reynold’s scaling, which is incompatible with Froude scaling. law is applied to physical model where gravity is the predominant factor in the fluid motion. Nonetheless, for these tests Froude scaling was used on the wind speed so that some measure of the effect of different speeds on the spatial distribution of the overtopping could be established.

To study the effects of wind, four large fans (approximately 4 x 0.75m x 0.75m) were placed directly in front of the seawall in the area immediately in front of the measuring equipment, as can be seen in Figure 23 & Figure 24. The fans did not produce a homogeneous wind speed across the front of the seawall, and a number of measurements were taken at different positions, and these were averaged to obtain basic wind speeds. The reason for placing them here was to ensure that they did not effect the incident waves, but rather they assisted in “pushing” the overtopping discharge over the parapet wall in a manner analogous to the paddle wheel used by de Waal et al. (1996). Once the overtopping had travelled over the crest, the wind would then effect the distribution. The results from these tests are described in Section 4.3.3.

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Figure 23

: The bank of four fans seen from the seawall

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4. Test

Results

This chapter describes the basic results obtained from each of the various physical model test programmes. Detailed discussion on these results will be given in Chapter 5, and only those points that are considered necessary to understand the presentation of the data will be given here. To that end, the method for determining the mean overtopping discharge at composite vertical walls will be used for presenting the data in this section and is described below. Additionally, a brief description of the model for presenting the spatial overtopping discharge is also given.

4.1 FORMATTING OF THE RESULTS

4.1.1 Overtopping at composite Vertical Seawalls

Overtopping performance of vertical walls is dependent upon the type of incident wave. In deep water, waves hit the structure and are generally reflected back seawards (reflected / non-breaking / pulsating waves). However as the waves become limited by water depth, they are prone to break over the sea wall (so-called impacting waves), this causes a change in the overtopping performance.

The characteristics of the incident wave conditions determine the overtopping performance of seawalls. For vertical walls exposed to breaking waves Allsop et al. (1995) have discussed how impacting wave conditions occur, and this leads to a greater number of waves overtopping the structure. They demonstrated that the overtopping characteristics were highly dependent upon the type of incident wave. This lead to the formulation of a wave breaking parameter, h*, a parameter which allows the parameterisation of the incoming wave type. h* may be determined from the following expression; 2 2 *

2

m s

gT

H

h

=

π

(4.1)

where h is the depth, Hs the significant wave height and Tm the mean wave period at the

toe of the structure. Reflecting waves predominate when h*>0.3, and impacting predominates when h*<=0.3.

For composite vertical structures including toe berms, such as that at Samphire Hoe, the

h* parameter can be modified, as discussed by Besley (1999), to take account of the

relative size of the berm, and is given by;

2 * 2 m sgT H dh d =

π

(4.2)

where d is the water depth over the berm. Besley (1999) suggested that the berm is classified as large when d*<=0.3, whereas when d*>0.3 the mound is classified as small and the wave at the structure behaves as with plain vertical walls. The formulation of

d* is essentially dependent upon the water depth and the wave steepness and reflects the

fact that the waves are more likely to break if the wave length or the wave height is large compared to the water depth. From Besley’s study, an empirical formulae was established to determine the mean overtopping discharge on a composite vertical

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79 . 2 4 10 x 63 . 4 − − = _d d R Q (4.3)

where Qd is the dimensionless discharge, and Rd, the dimensionless crest freeboard,

which is given by;

s c d H d R R ₌ * _(4.4)

and where Rd is valid for 0.05<Rd<1.0.

From Equations 4.2 to 4.4, the mean overtopping discharge QBar(m3/s/m) can be

obtained from the following expression;

( )

₃ 0.5 2 * gd d Q Q_Bar = _d (4.5)

4.1.2 Overtopping Distribution

To establish a conceptual model for the spatial distribution of the overtopping discharges, the parameter q* is defined which is non-dimensionalised with the offshore wavelength. q* is described as follows:

























=

o Bar o Bar

L

x

Q

L

x

Q

q

Total i

*

(4.6)

where the QBar i are the discharges at landward distances of x/Lo from the crest, and

where Lo is the offshore wavelength. By plotting the q* as a function of x/Lo it is

possible to establish a general relationship of the form,

( )

(

kx_L_o

)

e

q*= − (4.7)

where the constant k determines the spatial distribution. An example of Equation 4.7 is shown in Figure 27 for the 2d Edinburgh results.

By application of Equation 4.7, it is possible to plot the results as a function of the dimensionless landward distance to show the percentage of the discharge that has landed after a distance of x/Lo. Figure 28 shows this for Vi and VTotal, the ratio of QBar i

to QBar Total being the same, where the bold red line shows the limiting trend (k = 30).

The Vi are used in preference because they represent the recorded volumes in the

individual tanks and this simplifies the analysis. In each of the later figures showing the distribution, the limiting trend is also shown in bold red. Similarly, the individual distributions are not labelled, as the purpose is to demonstrate the general trend in the data rather than to discuss the behaviour of individual waves.

4.2 EDINBURGH

TESTS

4.2.1 Field Measurement Simulations

The results from the Edinburgh 2d flume test programmes for 1 May 2003 storm and the 2 May 2003 storm are summarised in Table 6 and Table 7, respectively. The results include the spectral wave conditions at the toe, the water depth h at the toe, the water

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depth d over the berm, the crest freeboard Rc and the mean overtopping discharge q.

These results are shown against Besley’s (1999) empirical prediction in Figure 25 and Figure 26, respectively.

Table 6: Edinburgh 1 May

Hm0 Toe (m) Tp Toe (s) Tm-1,0 Toe (s) Tm0 Toe (s) h (m) d (m) Rc (m) QBar (m3_/s/m) 2.01 5.40 5.59 5.14 5.27 2.27 6.12 7.97E-04 2.08 5.49 5.58 5.16 5.54 2.54 5.85 1.41E-03 2.20 5.49 5.84 5.33 5.67 2.67 5.72 2.01E-03 2.20 5.49 5.84 5.33 5.65 2.65 5.74 2.01E-03 2.20 5.49 5.84 5.33 5.55 2.55 5.84 2.01E-03 2.10 5.49 5.83 5.28 5.33 2.33 6.06 9.64E-04 2.09 5.49 5.70 5.24 4.98 1.98 6.41 5.44E-04 1.97 5.49 5.80 5.17 4.51 1.51 6.89 2.16E-04 1.78 5.49 6.18 5.34 4.03 1.03 7.36 1.90E-04 1.60 5.49 5.99 5.17 3.47 0.47 7.93 9.82E-06 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0.0 0.2 0.4 0.6 0.8 1.0 R_d Qd Besley (1999) 1 May 2003 2d Lab       = = = = − − − 2 * * 2 * 3 79 . 2 4 _, _, _, 2 10 x 63 . 4 m s s c d d d d _gT h H d d d H R R d gd Q Q R Q π

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Table 7: Edinburgh 2 May Hm0 Toe (m) Tp Toe (s) Tm-1,0 Toe (s) Tm0 Toe (s) h (m) d (m) Rc (m) QBar (m3_/s/m) 2.47 5.78 6.12 5.64 5.80 2.70 5.69 3.91E-03 2.49 5.78 6.09 5.60 5.92 2.79 5.60 3.63E-03 2.49 5.78 6.09 5.60 5.92 2.80 5.59 3.63E-03 2.47 5.78 6.12 5.64 5.80 2.72 5.67 3.91E-03 2.09 6.35 6.63 5.80 5.28 2.17 6.22 1.06E-03 1.65 7.20 7.29 5.84 4.32 1.18 7.22 1.34E-04 1.58 6.89 6.94 5.63 3.68 0.66 7.73 5.97E-05 1.44 6.89 7.14 5.63 3.40 0.19 8.00 1.46E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0.0 0.2 0.4 0.6 0.8 1.0 Rd Qd Besley (1999) 2 May 2003 2d Lab       = = = = − − − 2 * * 2 * 3 79 . 2 4 _, _, _, 2 10 x 63 . 4 m s s c d d d d _gT h H d d d H R R d gd Q Q R Q π

Figure 26: Edinburgh 2 May Simulation Results

4.2.2 Spatial Overtopping Distribution

Nine selected conditions of varying period and wave heights were selected to establish the mean overtopping distribution from the Edinburgh tests, and these are shown in Figure 27 and Figure 28. Figure 27 shows the underlying trends that are used to establish values of k for Equation 4.7, and Figure 28 shows the cumulative distribution of the discharge landward of the crest. In general this shows that approximately 90% of the discharge has landed at a distance of x/Lo = 0.05 and 100% by x/Lo = 0.15.

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q* = e-35.977x/Lo 1E-04 1E-03 1E-02 1E-01 1E+00 1E+01 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Landward distance x/L_o q* Figure 27: q* Edinburgh 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.00 0.05 0.10 0.15 0.20 0.25 Landward distance (x/L0) Σ Vi (x/L 0 ) / V Tota l

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4.3 HR

WALLINGFORD

RESULTS

4.3.1 Parametric Tests

The results from the HRW 3d parametric test programme are summarised in Table 8. The results include the spectral wave conditions at the toe, the water depth h at the toe, the water depth d over the berm, the crest freeboard Rc and the mean overtopping

discharge q. These results are shown against Besley’s (1999) empirical prediction in Figure 29. 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0.0 0.2 0.4 0.6 0.8 1.0 Rd Qd Besley (1999) Parametric 3d Lab       = = = = − − − 2 * * 2 * 3 79 . 2 4 _, _, _, 2 10 x 63 . 4 m s s c d d d d _gT h H d d d H R R d gd Q Q R Q π

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Table 8: Parametric Tests Hm0 Toe (m) Tp Toe (s) Tm-1,0 Toe (s) Tm0 Toe (s) Tm0,2 Toe (s) h (m) d (m) Rc (m) QBar (m3_/s/m) 1.33 6.50 5.78 4.81 4.39 2.42 0.17 8.22 1.59E-05 1.35 7.36 6.59 5.42 4.92 2.42 0.17 8.22 2.72E-05 1.28 6.80 5.91 5.35 5.02 3.42 1.17 7.22 2.33E-05 1.40 8.29 6.78 5.84 5.42 3.42 1.17 7.22 1.62E-04 1.63 6.17 5.28 4.67 4.36 3.42 1.17 7.22 1.18E-04 1.66 6.81 5.68 4.87 4.51 3.42 1.17 7.22 1.43E-04 1.24 8.10 6.28 5.52 5.19 3.42 1.17 7.22 1.00E-04 1.70 6.23 5.50 4.84 4.52 4.42 2.17 6.22 1.00E-03 1.08 6.21 5.80 5.13 4.77 4.42 2.17 6.22 4.91E-06 1.56 7.89 7.03 6.22 5.78 4.42 2.17 6.22 6.19E-04 1.63 6.17 5.27 4.73 4.44 4.42 2.17 6.22 5.49E-04 1.22 7.00 6.35 5.62 5.22 4.42 2.17 6.22 1.25E-04 1.73 6.86 5.68 5.07 4.77 5.42 3.17 5.22 8.34E-04 1.09 6.80 5.96 5.26 4.85 5.42 3.17 5.22 2.21E-05 1.72 7.89 7.18 6.54 6.16 5.42 3.17 5.22 4.62E-04 1.55 5.89 5.26 4.70 4.42 5.42 3.17 5.22 3.24E-04 1.82 6.81 5.98 5.32 4.97 5.42 3.17 5.22 1.97E-03 1.98 7.36 6.53 5.79 5.38 5.42 3.17 5.22 5.05E-03 2.34 7.81 7.16 6.23 5.75 5.42 3.17 5.22 1.18E-02 2.31 7.53 6.86 6.01 5.57 5.42 3.17 5.22 8.64E-03 1.74 6.50 5.92 5.34 5.01 5.05 2.80 5.59 1.16E-04 1.78 6.81 5.92 5.34 5.02 5.04 2.79 5.60 2.59E-04 1.63 6.50 5.92 5.37 5.05 4.97 2.72 5.67 3.24E-04 1.59 5.89 5.36 4.85 4.56 4.92 2.67 5.72 1.79E-04 1.62 6.18 5.60 5.06 4.75 4.80 2.55 5.84 2.34E-04 1.51 6.64 5.86 5.31 4.99 4.76 2.51 5.88 2.02E-04 1.59 6.16 5.33 4.83 4.55 4.72 2.47 5.92 7.96E-05 1.46 6.04 5.50 4.92 4.58 4.52 2.27 6.13 7.76E-05 1.40 6.91 6.53 5.96 5.61 4.42 2.17 6.22 9.57E-05 2.39 8.59 7.34 6.30 5.83 4.42 2.17 6.22 9.54E-03 2.26 7.89 6.93 6.01 5.57 4.42 2.17 6.22 5.37E-03 2.02 6.48 5.94 5.35 4.99 4.30 2.05 6.34 2.97E-04

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4.3.2 Field Measurement Simulations

The results from the HRW 3d basin test programmes for 1 May 2003 storm and the 2 May 2003 storm are summarised in Table 9 and Table 10, respectively. The results include the spectral wave conditions at the toe, the water depth h at the toe, the water depth d over the berm, the crest freeboard Rc and the mean overtopping discharge q.

These results are shown against Besley’s (1999) empirical prediction in Figure 30 and Figure 31, respectively.

Table 9: Storm Simulations 1 May

Hm0 Toe (m) Tp Toe (s) Tm-1,0 Toe (s) Tm0 Toe (s) Tm0,2 Toe (s) h (m) d (m) Rc (m) QBar (m3_/s/m) 2.14 6.56 5.70 4.96 4.60 4.86 2.61 5.78 1.22E-04 2.06 6.56 5.70 4.97 4.61 4.52 2.27 6.13 5.24E-04 1.96 6.41 5.57 4.85 4.50 4.23 1.98 6.41 1.60E-03 1.81 6.56 5.70 4.96 4.60 3.76 1.51 6.89 1.02E-04 1.65 6.70 5.82 5.07 4.70 3.28 1.03 7.36 1.25E-04 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0.0 0.2 0.4 0.6 0.8 1.0 Rd Qd Besley (1999) 1 May 2003 3d Lab       = = = = − − − 2 * * 2 * 3 79 . 2 4 _, _, _, 2 10 x 63 . 4 m s s c d d d d _gT h H d d d H R R d gd Q Q R Q π

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Table 10: Storm Simulations 2 May Hm0 Toe (m) Tp Toe (s) Tm-1,0 Toe (s) Tm0 Toe (s) Tm0,2 Toe (s) h (m) d (m) Rc (m) QBar (m3_/s/m) 2.00 7.11 6.18 5.38 4.99 3.63 1.38 7.01 2.64E-04 2.07 7.22 6.27 5.46 5.06 4.76 2.51 5.88 4.95E-04 1.92 7.51 6.53 5.68 5.27 4.42 2.17 6.22 2.55E-03 1.26 7.30 6.34 5.52 5.12 2.30 0.05 8.35 2.22E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0.0 0.2 0.4 0.6 0.8 1.0 Rd Qd Besley (1999) 2 May 2003 3d Lab       = = = = − − − 2 * * 2 * 3 79 . 2 4 _, _, _, 2 10 x 63 . 4 m s s c d d d d _gT h H d d d H R R d gd Q Q R Q π

Figure 31: HRW 2 May Simulation Results

4.3.3 Spatial Overtopping Distribution

Figure 32 and Figure 33 show the ensemble spatial distribution for 5400s periods, from the storms of 1 & 2 May 2003. Figure 34 shows the distribution for the waves from the parametric test series. Figure 35 to Figure 37 show the results from selected tests with wind speeds of 15m/s, 26m/s & 28m/s, respectively. To each of the data, a general trend (Bold Red) has been fitted. These are not the “best fit” nor average trends in the data, rather they are conservative approximations that keep 80% of the overtopping to the left of the generic trend at all times. That is, up to 0.8Σ Vi/VTotal will generally be

estimated conservatively at any given x/Lo. The distribution of the remaining

0.2Σ Vi/VTotal may, therefore, be slightly over or under predicted, but the hazards will

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Landward distance x/L0 Σ Vi (x/L 0 )/V Tot al

Figure 32: Spatial distribution 1 May 2003 field results (limiting k = 30)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Landward distance x/L0 Σ Vi (x/L 0 )/V Tot al

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.00 0.05 0.10 0.15 0.20 0.25 Landward distance x/L0 Σ Vi (x/L 0 )/V Tota l

Figure 34: Spatial distributions from the parametric tests (limiting k = 30)

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Figure 36: Distribution with a wind speed of 26m/s (limiting k = 17)

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5. Conclusions and Discussion

5.1 MEAN OVERTOPPING DISCHARGE

Figure 38 and Figure 39 show the results for the field and both the two laboratory test programmes plotted against Besley’s (1999) empirical prediction for the 1 & 2 May 2003 storms, respectively. Generally, the results are in excellent agreement with the prediction, and there are very few exceptions.

In Figure 38 there are a number of results where the values of Qd are below the

prediction line, and this is particularly the case for the field and the 3d simulation results. The wave conditions and water levels show that h* is very close to 0.3 for these waves, where h* = 0.3 is the transition from impacting to pulsating waves. Pulsating waves will have large run-up values but will not necessarily overtop the crest. Those that are more impacting will send large plumes directly into the air but are less likely to travel over the crest in the absence of any significant wind. During the tests it was observed that when potential overtopping discharges did occur they fell back into the sea as a result of the absence of wind. The results are therefore below the prediction line mainly as a result of modelling effects

An allowance for the loss of discharges, that not falling into the tanks, was considered during the analysis of these data, however, in this case it is clear that considerably more water than was originally estimated was lost due to wind blown effects. Moreover, it should be recalled that there were gaps between the overtopping tanks at Samphire Hoe, and that the discharges landing between the tanks have been estimated. It is therefore not possible to be certain what those discharges were, only that they are the best that can be determined. Any differences between the field and laboratory results for certain cases may be entirely due to the difference between the methods used for measuring them. The Edinburgh results show a generally better agreement with Besley because there was no obliquity (β = 0°) for these tests, whereas the field and the 3D tests had β = 10°.

The 2 May results, shown in Figure 39, are much less scattered than the 1 May results and this be attributed to two significant factors. There was no wind associated with this storm and the wave and water level conditions were generally more severe than for the previous storm. These results are in excellent agreement with Besley’s prediction, with many lying directly on the prediction curve.

Figure 40 shows all the results from the field and laboratory measurements and the parametric tests. Generally the parametric results are in excellent agreement with Besley’s prediction except when the vale of Rd increases. It has been described by Goda (2000), that the scatter is greatest when crest is high and the discharge small, and less as the freeboard reduces and overtopping increases. These discharges are small and in most cases potential discharges do not travel over the crest, falling back into the sea in the absence of any wind.

The data from the field and laboratory tests have been compared here and found to be in excellent agreement with Besley’s (1999) empirical prediction for composite vertical walls. However, the principal investigation here is how the field measurements and the direct laboratory simulations compare. Figure 41 shows these comparisons for the field

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clear that the results are in agreement, with two being directly on the one-to-one comparison line, and the others being mainly clustered around these. There is one significant outlying point, but this has already been explained above. There is a general indication from the cluster of data above the line that the laboratory measurements may record higher discharges than the field, but these are generally balanced by those below the line. To summarise, these comparisons show that there are no scale effects when the field and laboratory measurements are compared directly.

1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0.0 0.2 0.4 0.6 0.8 1.0 Rd Qd Besley (1999) 1 May 2003 Field 1 May 2003 2d Lab 1 May 2003 3d Lab       = = = = − − − 2 * * 2 * 3 79 . 2 4 _, _, _, 2 10 x 63 . 4 m s s c d d d d _gT h H d d d H R R d gd Q Q R Q π

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1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0.0 0.2 0.4 0.6 0.8 1.0 Rd Qd Besley (1999) 2 May 2003 Field 2 May 2003 2d Lab 2 May 2003 3d Lab       = = = = − − − 2 * * 2 * 3 79 . 2 4 _, _, _, 2 10 x 63 . 4 m s s c d d d d _gT h H d d d H R R d gd Q Q R Q π

Figure 39: Field and 2d & 3d results for 1 May 2003 storm

1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 0.0 0.2 0.4 0.6 0.8 1.0 Rd Qd Besley (1999) 1 May 2003 Field 1 May 2003 2d Lab 1 May 2003 3d Lab 2 May 2003 Field 2 May 2003 2d Lab 2 May 2003 3d Lab Parametric 3d Lab       = = = = − − − 2 * * 2 * 3 79 . 2 4 _, _, _, 2 10 x 63 . 4 m s s c d d d d _gT h H d d d H R R d gd Q Q R Q π

(50)

1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00

1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 Qd(Meas Field) Qd( M ea s La b) 1 May 2003 2d Lab 1 May 2003 3d Lab 2 May 2003 2d Lab 2 May 2003 3d Lab       = = − 2 * 2 * 3 2 , m s d gT h H d d d gd Q Q π

Final report on laboratory measurements Samphire Hoe

Samphire Hoe Physical Model Studies

Report TR 147

Rev 0.0

Document Information

Document History

Summary

Contents

1. Introduction

1.1 BACKGROUND

1.2 REPORT

OUTLINE

2. Samphire Hoe Field Measurements

2.1 SAMPHIRE

HOE

2.2 FIELD

RESULTS

2.2.1 1 May 2003 Storm

2.2.2 2 May 2003 Storm

2.3 APPROACH

BATHYMETRY

3. Laboratory

Tests

3.1 MODEL

SCALING

3.2

2D EDINBURGH STUDY

3.2.1 Test Edinburgh Wave Flume

Wave Gauges

Overtopping discharges

Overtopping distribution

Video Analysis

3.2.2 Wave paddle characteristics and model scale

3.2.3 Replication of storm conditions

System accuracy

Measurement of inshore wave conditions

Test length and sequence

3.3

3D HR WALLINGFORD STUDY

3.3.1 Model design and facilities

3.3.2 Wave conditions and water levels

3.3.3 Wave calibrations

3.3.4 Overtopping measurements

: Location of the spatial distribution tanks

Tank

Landward Distance

(m) (prototype)

1

2

3

4

5

6

6 tanks

Missing area

3.3.5 Wind measurements and overtopping

: The bank of four fans seen from the seawall

4. Test

Results

4.1

FORMATTING OF THE RESULTS

4.1.1 Overtopping at composite Vertical Seawalls

2

gT

H

h

h

=

π

π

( )

4.1.2 Overtopping Distribution

















_{(m) (prototype)}