Safety Analysis of ComCoast Concept at
Neßmersiel
PRINCIPLE: CUR PROJECTNUMBER: 05i105 VERSION: Final version 04-04-2006
Projectdata
• Titel: Safety Analysis of ComCoast Concept at Neßmersiel Version: Final versioninal version
Principle: CUR
Projectnumber: 05i105 Partners:
Project description: This report provides a safety analysis of the ComCoast concept at Neßmersiel in Germany. The ComCoast concept is to use a wide coastal area for water defence purposes. The wide coastal area can be referred to as a coastal defence zone. The primary dike will be provided with a overtopping resistant revetment allowing more seawater to overtop the dike and the area behind the primary dike will be used to store overtopping seawater.
Table of contents
1 Introduction 1
2 Description of pilot area 2
3 Hydraulic boundary conditions 4
3.1 Water level and wind statistics 4
3.2 Wave generation and wave reduction 6
4 Results of safety calculations 9
1
Introduction
The average sea level is expected to rise as a result of climate change. This will increase hydraulic loads on coastal water defences. Until now, traditional dike improvements (step-by-step heightening) compensate for increasing hydraulic loads or reduction in flood risk.
However, with ongoing rise in sea level traditional dike improvement might not be the optimum way to strengthen coastal water defences. From both economical and technical points of view, alternatives are desired.
ComCoast is a European project, which develops and demonstrates innovative solutions for flood protection in coastal areas. The project organisation consists of ten partners from five North Sea countries. The ComCoast concept is to use a wide coastal area for water defence purposes. The wide coastal area can be referred to as a coastal defence zone.
The aim of this report is to test the applicability of the ComCoast safety concept for the German situation by conducting a safety assessment according to the ComCoast concept for the Neßmersiel pilot area in Germany. The safety analysis is based on the method described in the report ‘Safety analysis of the ComCoast concept’ [CUR – 2005]. The description of this method will not be discussed in detail in this report.
Chapter 2 gives a description of the site area. Chapter 3 presents the hydraulic boundary conditions and the methods used to determine these conditions. The results of the safety analysis calculations are given in chapter 4. Finally, in chapter 5 some remarks and conclusions concerning the results of the calculations are given.
2
Description of pilot area
The pilot area is situated in the North-western part of Germany in the coastal region of the federal state of Niedersachsen (Lower Saxony) and stretches over approximately 9.4 km along the mainland coast.
The pilot area consists of six polders. Three adjoining polders bounded by a primary dike in the north and a secondary dike in the south. Three summer polders in front of the primary dike are bounded by a summer dike in the north and a primary dike in the south. Figure 1 gives an overview of the site area.
A
B
C
Figure 1: Overview of pilot area Neßmersiel
Three different cross sections are taken into consideration. All cross sections used in this analysis are derived from a LIDAR-campaign. The height information represents a
generalisation and may be influenced by errors due to the method of generalisation, LIDAR-measurement or post-processing. The design height of the primary dike is lower than the herein used LIDAR-height. The difference results, among other things, from the safety margin of ground settlement. Cross section A is at the western part of the pilot area and cuts through the Mande polder. Figure 2 gives a schematisation of cross section A.
Seaside Landside
Primary dike Former dike
+8.3m NN +5.0m NN 660m 250m +1.3m NN +1.1m NN 1 : 6
Cross section B is in the middle part of the pilot area and cuts through the Lütetsburger Sommerpolder and the Lütetsburger polder. Figure 3 gives a schematisation of cross section B.
Figure 3: Cross section B
Cross section C is situated in the eastern part of the pilot area and cuts through the Wester Neßmersieler Sommerpolder and the Wester Neßmersielerpolder. Figure 4 gives a
schematisation of cross section C.
Figure 4: Cross section C
Seaside Landside
Summer dike Primary dike Former dike
+3.0m NN +8.0m NN 350m 440m +5.0m NN +2.0m NN + 2.2m NN +1.3m NN 120m 1 : 6
Seaside Landside
Summer dike Primary dike Former dike
+3.5m NN +7.9m NN 410m 300m +5.0m NN +1.7m NN + 2.1m NN +1.5m NN 500m 1 : 6.5
3
Hydraulic boundary conditions
3.1
Water level and wind statistics
This section describes the water level and wind statistics used for the safety analysis. Water level statistics
Water level statistics are based on data from the Norderney gauge station. Figure 5 gives the relation between a certain water level and corresponding annual exceedance probability. This relation can be described by a lognormal function. The mathematical description of the lognormal function and corresponding parameters are given in appendix A. The water level is a combination of storm set-up and tide. The maximum tide at the Norderney gauge is 1.21m. The water level with a return period of 1/4,000 years is 5.4m NN.
Figure 5: Annual exceedance probability of water level
Wind statistics
Wind statistics are based on statistics from Terschelling in the Netherlands as wind statistics as Neßmersiel wind statistics are not available at the moment. Terschelling is located approximately 125km west of Neßmersiel and has a comparable geographical setting in the Wadden Sea area. Figure 6 gives the exceedance probability for four wind directions (from westerly to northerly wind). The exceedance probability of the wind speed is based on the maximum hourly averaged wind speed per tide. The mathematical description of the probability function and corresponding parameters are given in appendix A.
0 1 2 3 4 5 6 7
1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00
Exceedance probability [-] Water level [ m NN] Storm set-up Tide
0 10 20 30 40 50 60
1.0E-11 1.0E-09 1.0E-07 1.0E-05 1.0E-03 1.0E-01
Exceedance probability [-] W inds pe e d [ m /s ] 270 degrees 300 degrees 330 degrees 360 degrees
Figure 6: Wind statistics from Terschelling for four different directions
In section 3.2 the prevailing wind direction is determined for cross sections A, B and C. Correlation between water level and wind speed
The wind speed and water level are assumed to be correlated. This means that high water levels and high wind speeds will coincide often. More information about the correlation between the water level and the wind speed can be found in [CUR – 2005] and [HKV – 2003].
3.2
Wave generation and wave reduction
This section discusses the wave generation and wave reduction models used. First information is given about the wave generation model. Subsequently, based on this model and on water level and wind statistics the prevailing wind direction for cross section A, B and C is
determined. Wave generation
The generation of waves can be described by the Brettschneider method. The method gives an expression for wave height Hs and wave period Ts as a function of the windspeed V, water depth d and fetch F. The expressions for Hs and Ts are given in appendix B.
A
B
C
Figure 7: Fetches for cross sections A, B and C
The input parameters wind speed and water level are derived from the statistics described in section 3.1. Fetches for different wind directions and cross sections are derived from Figure 7 and are given in Table 1. The fetch for cross section A with a direction of 270 degrees has a length of 11,2km. In this approach waves entering the Wadden Sea through the gap between Juist and Norderney, might be underestimated. When more detailed calculations will be made in future this aspect should be taken into account.
Wind direction [degrees] Cross section A Fetch [m] Cross section B Fetch [m] Cross section C Fetch [m] 270 (WEST) 15,000 10,000 11,200 300 11,200 6,100 5,000 330 5,000 3,700 2,900 360 (NORTH) 4,500 3,000 2,400
Determining prevailing wind direction
Based on the water level, wind speed and fetch the wave height in front of cross section A, B and C is determined. Subsequently the overtopping discharge is calculated for every wind direction taking into consideration a reduction factor for oblique incoming waves. The prevailing wind direction is set to be the direction with the highest overtopping discharge. The overtopping discharge calculation is based on a 1/4,000-years water level and a 1/4,000 wind speed. Table 2 gives the overtopping discharges for cross section A for four wind directions. The prevailing wind direction is 300 degrees (north-westerly wind).
Wind direction [degrees]
Wind speed [m/s]
Fetch [m] Hs [m] Wave angle [degrees] Overtopping discharge [l/s/m] 270 23.5 15,000 1.93 70 0.020 300 26.5 11,200 1.80 40 0.026 330 30.0 5,000 1.37 10 0.002 360 30.5 4,500 1.10 20 < 10-4
Table 2: Determining prevailing wind direction - cross section A
Table 3 gives the overtopping discharges for cross section B for four wind directions. The prevailing wind direction is 270 degrees (westerly wind).
Wind direction [degrees]
Wind speed [m/s]
Fetch [m] Hs [m] Wave angle [degrees] Overtopping discharge [l/s/m] 270 23.5 10,000 1.78 70 0.019 300 26.5 6,100 1.55 40 0.009 330 30.0 3,700 1.18 10 0.0007 360 30.5 3,000 0.97 20 < 10-4
Table 3: Determining prevailing wind direction - cross section B
Table 4 gives the overtopping discharges for cross section C for four wind directions. The prevailing wind direction is 270 degrees (westerly wind).
Wind direction [degrees]
Wind speed [m/s]
Fetch [m] Hs [m] Wave angle [degrees] Overtopping discharge [l/s/m] 270 23.5 11,200 1.82 70 0.012 300 26.5 5,000 1.46 40 0.002 330 30.0 2,900 1.09 10 < 10-4 360 30.5 2,400 0.89 20 < 10-4
Table 4: Determining prevailing wind direction – cross section C
It should be noted that the overtopping discharges are extremely small. An overtopping discharge of 0.1 l/s/m is seen as very small and negligible.
Wave reduction
Wave reduction can be caused by the existence of a shallow foreland or a summer dike. In case of shallow foreland the wave height will be limited to about half of the water depth. In case of a summer dike the wave height can be reduced by wave transmission. The reduction is expressed by a transmission coefficient Kt. Figure 8 gives the relation between the relative freeboard (quotient of the freeboard of the summer dike and the wave height) for a summer dike [INFRAM - 2005]. 0 0.2 0.4 0.6 0.8 1 -3 -2 -1 0 1 2 3 hk / Hs Kt [-]
Figure 8: Relation between relative freeboard and transmission coefficient
In cross section A and cross section B a summer dike is present. Based on the 1/4,000 year water level of 5.4m NN and the 1/4,000 year wave height of 1.8m the relative freeboard of the summer dike in cross section B and C is respectively -1.3 and -1.0. These relative freeboards lead to a transmission coefficient Kt of 0.8 in cross section B and 0.7 in cross section C. The transmitted wave height can be determined by multiplying the incoming wave height with the transmission coefficient.
4
Results of safety calculations
This chapter discusses the results of the safety calculations for cross sections A, B and C. First the failure probabilities of the current primary dike and the failure probability when using a coastal defence zone are determined. Secondly, return periods for the inundation depth between the primary dike and the former dike are determined. Finally, some future failure probabilities are determined, based on a sea level rise of 60cm per century. With regard to the calculations the following should be mentioned:
1. The calculations are based on the assumption that the primary and secondary dikes are infinitely strong. In other words, the calculated failure probability represents only failure due to a shortage of water storage capacity. This means that failure due to instability, erosion and piping is neglected.
2. The failure probability calculations are carried out on a two dimensional schematisation of the case study situations.
3. The failure probability calculations are carried out for one wind direction (the prevailing wind direction).
The calculations therefore give an order of magnitude of the failure probability of the primary dike or a coastal defence zone. Figure 1 schematises the failure of the current primary dike and failure of the coastal defence zone. It is assumed that the current primary dike fails when the amount of overtopping discharge exceeds 0.1 l/s/m. It must be noted that this is a stringent rule. It is assumed that the coastal defence zone fails when 2% run up overtops the secondary dike.
Figure 9: Schematisation of failure of current primary dike and failure of coastal defence zone.
Failure probabilities
Table 5 gives the probabilities of the current primary dike and the coastal defence zone as they are explained above.
Primary dike Secondary dike
Failure of current primary dike = overtopping discharge primary dike exceeds 0.1 l/s/m
Failure of coastal defence zone = overtopping secondary dike exceeds 2% run up criterion
Ru2%
Cross section A Cross section B Cross section C
Failure probability of the primary dike
8.6E-5 4.1E-5 2.5E-5
Failure probability of the coastal defence zone
4.2E-7 6.0E-7 7.2E-7
Table 5: Failure probabilities for the current primary and a coastal defence zone (based on a simplified model)
Table 5 shows that the failure probability of the current primary dike is in the order of magnitude of 10-5 – 10-4. In other words dike failure due to overtopping will take place on average less than every 10,000 years. In case of a coastal defence zone the failure probability due to a shortage of water storage is in the order of magnitude of 10-8 – 10-7. In other words failure due to a shortage of water storage can be neglected.
Furthermore Table 5 shows that cross section A has the failure probability concerning the current primary dike whereas cross section C has the highest failure probability when the coastal defence zone is considered. This is caused by difference in effective width of both cross sections. The storage capacity of cross section A is twice as large as cross section C.
Return periods of inundation depths
This section gives the return periods of corresponding inundation depths between the primary dike and the former dike. Inundation is caused by water overtopping the primary dike. The overtopped water will be stored in the area between the primary dike and the former dike. Figure 10 gives the return periods of the inundation depths for cross sections A, B and C.
1.00E+03 1.00E+04 1.00E+05 1.00E+06 0 0.05 0.1 0.15 0.2 Inundation depth [m ] R et u rn p er io d [ year s] Section A Section B Section C
Figure 10: Return periods of inundation depth for cross sections A, B and C (based on a simplified model).
Figure 10 shows that even small inundation depths of 0.01m rarely occur. For example, the return period of an inundation depth of 0.01m for cross section A is 36,000 years. For other cross sections this return period is a little higher.
Future development
First the future development of failure probability for a traditional dike like the current primary dike and for a coastal defence zone is given. The future development of the failure probability is based on a sea level rise of 60cm per century. Figure 11 gives the future development of the failure probability for a traditional dike.
Failure probability of current primary dike
1.0E-05 1.0E-04 1.0E-03 2000 2050 2100 Year F a il ur e pr ob a bi li ty Section A Section B Section C
Figure 11: Future development of the failure probability of a traditional dike (based on a simplified model)
Figure 11 shows that the failure probability of the traditional dike will rise. However the failure probability remains quite small. For example, the failure probability of cross section A in 2100 will be in the order of magnitude of 3.0E-4. In other words dike failure due to overtopping will occur on average every 3,000 years.
Figure 12 gives the future development of the failure probability of a coastal defence zone.
Failure probability of coastal defence zone
1.0E-07 1.0E-06 1.0E-05 2000 2050 2100 Year F a il u re p ro b a b il it y Section A Section B Section C
Figure 12: Future development of the failure probability of a coastal defence zone (based on a simplified model)
Figure 12 shows that the failure probability of a coastal defence zone will rise. The failure probability of cross section C in 2100 will be in the order of magnitude of 4.0E-6. In other words failure due to shortage of storage capacity will occur on average every 250,000 years. Future development of certain inundation depths in the area between the primary and secondary dike is discussed now. Figure 13 gives the return period of certain inundation depths for section A for the current situation (2000), year 2050 and year 2100.
1.00E+03 1.00E+04 1.00E+05 1.00E+06 0 0.05 0.1 0.15 0.2 Inundation depth [m] R et u rn p e ri o d [ y ea rs ] Section A - 2000 Section A - 2050 Section A - 2100
Figure 13: Future development of certain inundation depths for cross section A (based on a simplified model)
The return period of 0.01m inundation depth for section A in the current situation is about 36,000 years. The return period of this inundation depth will be 17,400 years in year 2050 and 8,500 years in year 2100. Figure 14 gives the return period of certain inundation depths for section B for the current situation (2000), year 2050 and year 2100.
1.00E+03 1.00E+04 1.00E+05 1.00E+06 0 0.05 0.1 0.15 0.2 Inundation depth [m] R e tur n pe ri od [ y e a rs ] Section B - 2000 Section B - 2050 Section B - 2100
Figure 14: Future development of certain inundation depths for cross section B (based on a simplified model)
The return period of 0.01m inundation depth for section B in the current situation is about 52,000 years. The return period of this inundation depth will be 24,000 years in year 2050 and 11,000 years in year 2100.
Figure 15 gives the return period of certain inundation depths for section C for the current situation (2000), year 2050 and year 2100.
1.00E+03 1.00E+04 1.00E+05 1.00E+06 0 0.05 0.1 0.15 0.2 Inundation depth [m ] R e tu rn p e ri o d [ y e a rs ] Section C - 2000 Section C - 2050 Section C - 2100
Figure 15: Future development of certain inundation depths for cross section C (based on a simplified model)
The return period of 0.01m inundation depth for section C in the current situation is about 66,000 years. The return period of this inundation depth will be 30,000 years in year 2050 and 14,000 years in year 2100.
The figures above show that the return period of a certain water depth will roughly halve every 50 years.
5
Conclusions and interpretation of results
The main conclusion is that the current primary dike is already high and will hardly experience overtopping. Therefore, the calculations give very small failure probabilities. At present the ComCoast concept is not necessarily needed to provide the safety of the polder area between primary and secondary dike line in this context.
The results of the calculations in chapter 4 show that failure probability of the primary dike due to overtopping is small (< 10-4 / year). Failure due to overtopping takes place when more then 0.1 l/s/m water overtops the primary dike. When a coastal defence zone is considered, the failure probability due to shortage of water storage capacity is very small (<10-4 / year). When looking at future development of failure probabilities and return periods it can be said that generally:
- the failure probability doubles every 50 years based on a sea level rise of 60cm per century.
- the return period of a certain water depth halves every 50 years based on a sea level rise of 60cm per century.
The small failure probabilities of both the primary dike and the coastal defence zone can be explained by a number of reasons.
First dikes along the Wadden Sea coast face a moderate wave climate. Due to the limited depth of the Wadden Sea and the limited fetch due to the existence of Wadden Islands wave generation is limited. Typical 1/4,000 year wave conditions in the Wadden Sea are a wave height of 1.7-1.8m and a wave period of 4-5s.
In a certain way the Wadden Sea already created its own coastal defence zone since the existence of islands and the limited water depth reduce hydraulic loads on the water defences. Secondly, the outer slope of the primary dikes in regarded cross sections are approximately 1 : 6. This relative gentle slope leads to relative small amounts of overtopping discharge.
Finally, the effective width of the polder and therefore the storage capacity of the polders are quite large. This results in large return periods for small inundation depths. In general, the applicability of the ComCoast safety analysis concept on German areas is limited due to limited available data. For different reasons a number of important parameter and input data must be assumed. For instance, most of the statistic relation parameter used in the ComCoast concept had to be taken from locations in the Dutch wadden sea area, since these parameters are not collected or investigated at the German wadden sea area.
References
[HKV – 2003] Diermanse, F., Thonus, B., Lammers, I., De Veiligheid van Nederland
in Kaart – Inventarisatie hydraulische randvoorwaarden (in Dutch) –
fase 2, DWW, Delft, 2003
[CUR – 2005] CUR, Safety Analysis of the ComCoast concept, CUR, Gouda, 2005 [INFRAM – 2005] Meer, J.W. van der, et. al., Wave transmission at low-crested
structures, including oblique wave attack, Infram, Marknesse, 2005
[TAW – 2002] Technische Adviescommissie voor de Waterkeringen (TAW),
Technisch Rapport Golfoploop en Golfoverslag bij dijken (in Dutch),
Appendix A: Water level and wind statistics
Water level statisticsThe exceedance frequency of the water level is given by the lognormal distribution. The probability density function is given by:
[ ] 2 2 2 ) ln( 2
2
1
)
(
σ μπσ
− −=
h he
h
h
f
where h Water level [m NN]μ mean of the lognormal density function σ standard deviation of the lognormal function
The mean for the Norderney data is set at 1.76m NN and the standard deviation is 0.47m. The lognormal function describes the water level rise due to storm set-up. The maximum tide (1.21m) is added to gain realistic water level statistics during storm situations.
Wind statistics
The exceedance frequency of the wind speed is given for every wind direction by the conditional Weibull distribution:
V V V V V exp a ;a a) (V V V ω σ α ≥ ⎪⎭ ⎪ ⎬ ⎫ ⎪⎩ ⎪ ⎨ ⎧ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − ⋅ = > α σ ω ρ μ (A-2) where:
μ Exceedance frequency [1/tide] V Wind speed [m/s]
ρV Exceedance frequency of threshold value ωV [1/tide] aV Threshold value of wind speed V [m/s]
σV Scale parameter αV Form parameter
ωV Threshold value of statistical information
Table A.6 gives the parameter for the Weibull distribution of the wind speed for four different wind directions.
Table A.6: Parameters for the Weibull distribution of the wind speed for four different wind directions
Parameter 270 (WEST) 300 330 360 (NORTH)
ρV 0.011 0.015 0.019 0.016
σV 12.19 11.74 10.40 8.86
αV 2.16 2.12 2.18 2.10
Appendix B: Wave generation model
The expression for the wave height Hs according to Brettschneider is given by:
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎣
⎡
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛ ⋅
⎟
⎠
⎞
⎜
⎝
⎛ ⋅
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛ ⋅
⋅
=
⋅
75 . 0 2 42 . 0 2 75 . 0 2 2578
.
0
tanh
0125
.
0
tanh
578
.
0
tanh
283
.
0
V
h
g
V
F
g
V
h
g
V
H
g
sThe expression for the wave period Ts according to Brettschneider is given by:
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎣
⎡
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛ ⋅
⎟
⎠
⎞
⎜
⎝
⎛ ⋅
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛ ⋅
⋅
=
⋅
⋅
375 . 0 2 25 . 0 2 375 . 0 2833
.
0
tanh
077
.
0
tanh
833
.
0
tanh
20
.
1
2
V
h
g
V
F
g
V
h
g
V
T
g
sπ
Where: V = wind speed [m/s] h = water depth [m] F = Fetch [m] g = acceleration of gravity [m/s2]Appendix C: Overtopping discharge model
The overtopping discharge is based on a model developed by Van der Meer [TAW – 2002]. The model gives the overtopping discharge for breaking and non-breaking waves.
Breaking waves
In the case of breaking waves the overtopping discharge qo can be determined by:
op 3 S b o Q g H tanS q = ⋅ α (A-4) where: qo Overtopping discharge [m2/s]
Qb Dimensionless overtopping discharge for breaking waves g Acceleration of gravity [m/s2]
HS Wave height [m]
tan α Angle of the outer slope of the dike [-] Sop Wave steepness
The wave steepness is given by:
2 P S op T g H 2 S ⋅ ⋅ = π (A-5) where:
TP Wave peak period [s]
The wave peak period Tp can be derived fron the wave period Ts by the following expression:
Tp = 1.1 · Ts (A-6)
The dimensionless overtopping discharge Qb for breaking waves can be determined by:
⎪⎭ ⎪ ⎬ ⎫ ⎪⎩ ⎪ ⎨ ⎧ ⋅ ⋅ − ⋅ − = γ 1 tan S H h h f 0.06exp Q op S I b b α (A-6) where:
fb Factor for breaking waves hI Crest level of the primary dike h Sea water level
γ Combined reduction factor
The combined reduction factor γ is composed of reduction factors for the angle of wave attack, the existence of a berm and a reduction factor for the roughness of the outer slope. The reduction factors for a berm and the roughness of the outer slope are both 1.0 since there is no berm present and the revetment of the outer slope is a grass revetment which has a reduction factor of 1.0.
The reduction factor for wave angle is given by the following expression: γβ = 1.0 – 0.0033· β
where:
β = wave angle with respect to the dike normal in degrees. The maximum value of β is 80°.
Non-breaking waves
In the case of non-breaking waves the overtopping discharge qo can be determined by: 3 S n o Q g H q = ⋅ (A-7) where:
Qn Dimensionless overtopping discharge for non-breaking waves
The dimensionless overtopping discharge Qb for breaking waves can be determined by:
⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ⋅ − ⋅ − = γ 1 H h h f 0.2exp Q S I n n (A-8) where:
fn Factor for non-breaking waves
factor Mean Standard deviation
Breaking waves 5.2 0.55
Non breaking waves 2.6 0.35
