Adequacy Check of Existing Crest Level of Sea Facing Coastal Polders by the E...
project presentation
1. Bolivar Roads Surge Barrier:
How to estimate the optimal barrier
height after a Risk Assessment
1
Katerina Rippi
2. Conception of Ike Project
Existing Levee system
Proposed floodgate in
Bolivar Roads pass
Ike Dike
2
3. Galveston Bay region
Why protecting Galveston bay area is
urgent?
6 million population and growing
Nationally-important population
and economic center
Supports the largest
petrochemical complex in the US
(second largest in the world)
Port of Houston alone generates
over $178 billion in economic
activity annually
Galveston Bay provides valuable
ecosystem services
3
4. Galveston Bay region
Year Hurricane Category Damage [$]
1900
Galveston
Hurricane
4 20 million
1957 Audrey 4 147 million
1961 Carla 5 325 million
1971 Edith 5 25 million
1983 Alicia 3 2.6 billion
2005 Rita 5 12 billion
2008 Ike 4 30 billion
Rita Ike
AliciaSome of the most catastrophic
hurricanes in Galveston Bay region
4
6. Design considerations
Location of the barrier
Type of barrier (sector, barge gate, caissons etc)
Afloat or not?
Open or (partially) closed structure?
6
7. Bolivar Roads Surge Barrier
Why a storm surge barrier?
Avoid large bottom protection
• During hurricane conditions, this section
will have to deal with large flows. This
may require a large bottom protection as
costly as a storm surge barrier.
Give people the feeling of safety
• The perception of safety is also taken into
account in order to decide for a closed or
an open barrier
7
8. Bolivar Roads Surge Barrier
Objective of this study:
What is the optimal height of the storm surge barrier in terms
of cost efficiency and damage prevention inside the basin?
Sub-questions:
1) What is the influence of local wind setup and inflow on
increasing the water level inside the basin under hurricane
conditions for different barrier heights?
2) What are the water levels caused by different barrier heights
inside the basin?
3) What is the extend of the damage under different inundation
levels?
4) What is the risk reduction of a certain water level for
different barrier heights?
5) How can the damages and the initial investments for the
construction be quantified?
8
9. General Methodology
9
Fundamentals
Hurricane principles
Preliminary research and
boundary conditions
Area characteristics
Safety level
Hazard
Surge development inside
Galveston basin:
Semi-enclosed basin
Main components of the basin
water elevation:
Overflow through
the coastal spine
Local wind setup
Local wave setup
Inflow from rivers
Balance equation for
calculating the water level
inside the basin
Simulation Model:
Variables definition
Assumptions
Storm surge in the open coast
Monte Carlo
Consequences
Inundation maps
Existing protection
level
Exposure:
Residential
Industrial
Vulnerability:
Depth-Damage curves
Flood Risk
Assessment
Investment costs:
Discount and growth rate
Construction and
maintenance costs:
Bolivar Roads storm
surge barrier
Ike Dike
Benefits: Damage
prevented
Total Costs
Final conclusions and
recommendations
10. Fundamentals
Hurricane Principles
A hurricane is a type of tropical cyclone, which is a generic term for a low
pressure system that generally forms in the tropics. The cyclone is accompanied
by thunderstorms and, in the Northern Hemisphere, a counterclockwise
circulation of winds near the earth's surface
Maximum sustained winds of 74 mph (33 m/s) or higher
Forerunner component
10
11. Fundamentals
Landfall location
11
The hurricane's right side (relative to the
direction it is travelling) is the most
dangerous part of the storm because of
the additive effect of the hurricane wind
speed and speed of the larger
atmospheric flow
12. Fundamentals
Return period of the storm surge in the Gulf side (Stoeten, 2013)
12
2.5
3
3.5
4
4.5
5
5.5
6
0.00010.00100.01000.1000
hsurge[m]
Return period [yr-1]
13. Fundamentals
•Report Galveston Bay
(TU Delft, Iv-infra,
Royal Haskoning):
Design, Flood risk
•DeVries, 2014: Design
of the environmental
section
•Stoeten, 2013: Risk
Reduction for Galveston
Bay
•Karimi, 2013: Design of
the navigational section
•Master Project, 2014:
Ike Dike in Bolivar
Peninsula
13
Preliminary research
15. Fundamentals
Meteorological data
Dominant wind directions: south, southeast
Average rainfall over the Galveston Bay Area over 48 hours during Ike
amounts about 200 mm, resulting in just a slight increase in water level
in the Galveston Bay that does not coincide with the peak storm
intensity.
15
17. Fundamentals
Existing Protection
Galveston island seawall
Tide barrier in city of Kemah (100-yr event)
Diversion channel and dam in the City of La Marque (500-yr event)
Levee system in Texas City (500-yr event)
17
Existing Levee system
18. Hazard
Surge development inside Galveston Bay
Approximations
Inlet: narrow channel with a zero storage capacity
Basin: semi-enclosed, quasi steady response, constant depth
Main components of the basin water elevation during hurricane event
Overflow only over the storm surge barrier. Overflow over barrier islands neglected due
to Ike Dike full retaining height
Local wave setup neglected : fetch and depth limited
River inflow: river discharging negligible, tide dominance (Ruijs, 2011)
18
20. Hazard
Assumptions
Prevailing wind direction south/southeast, constant
Landfall location at the west side of Galveston bay
The mean bay level increases solely due to inflow from the storm surge
barrier
Western shore-normal winds effect on the bay
Linear behavior of wind velocity and storm surge level in the gulf side
20
21. Assumptions
Cumulative distribution function of the storm surge (Generalized Pareto
distribution )
Relationship between maximum storm surge and maximum wind velocity
(NOAA, Introduction to Storm Surge)
y = -0.365ln(x) + 2.15
R² = 0.9882
2.5
3
3.5
4
4.5
5
5.5
6
0.00010.00100.01000.1000
hsurge[m]
Return period [yr-1]
y = 0.138x - 4.1459
R² = 0.9915
0
1
2
3
4
5
6
20 30 40 50 60 70 80
Surgelevel[m,+MSL]
Maximum Wind Speed [m/s]
21
Hazard
22. Hazard
Monte Carlo
A broad class of computational algorithms that rely on repeated random
sampling to obtain numerical results of a variable.
Consequently, a probability distribution of a stochastic variable can be
estimated from the simulated values.
22
Probability distribution
of the water level inside
the basin for each
barrier height
23. Halfway conclusions
23
1. Water elevation seems to converge after barrier height of 3.5 m which suggests
that after this height the influence of the barrier inside the basin tends to be
almost the same (see right figure).
2. Almost linear relationship between water level inside the basin and the
maximum occurred storm surge
24. Halfway conclusions
24
3. Local wind setup dominates inflow
4. Inflow tends to decrease almost exponentially from lower to
higher barrier heights
5. Wind setup increases from lower to higher heights
25. Halfway conclusions
6. Higher barrier leads to a lower water level inside the basin
7. CDF tends to be the same for barrier heights above 3 m
8. Probability of exceedance (or hazard) is almost the same for barrier heights
above 3 m
9. For events with a large return period, reduction of the probability of
exceedance is larger for higher barriers but with a slight difference in
comparison with lower barriers
25
26. Halfway conclusions
Probability of exceedance reduction
26
0
0.002
0.004
0.006
0.008
0.01
0.012
Hb=1 m Hb=2 m Hb=3 m Hb=6 m
Probabilityofexceedance
reduction
1/100 yr-1 surge level
0.0009
0.00092
0.00094
0.00096
0.00098
0.001
0.00102
Hb=1 m Hb=2 m Hb=3 m Hb=6 m
Probabilityofexceedanace
reduction
1/1,000 yr-1 surge level
0.000195
0.000196
0.000197
0.000198
0.000199
0.0002
0.000201
Hb=1 m Hb=2 m Hb=3 m Hb=6 m
Probabilityofexceedanace
reduction
1/5,000 yr-1 surge level
9.5E-05
9.6E-05
9.7E-05
9.8E-05
9.9E-05
0.0001
0.000101
Hb=1 m Hb=2 m Hb=3 m Hb=6 m
Probabilityofexceedanace
reduction
1/10,000 yr-1 surge level
27. Halfway recommendations
1. 3D simulation model
2. FEMA cumulative probability function for the storm surge in the
open coast
3. Flow under the barrier
4. Sediment allocation inside the basin (basin environment)
27
30. Consequences
Exposure & Vulnerability
Residential (Single Family Residential 2-story structures)
30
0
20,000
40,000
60,000
80,000
100,000
120,000
1/100
[1/yr]
1/1,000
[1/yr]
1/5,000
[1/yr]
1/10,000
[1/yr]
Number of residential properties within the floodplain
Other areas
Texas City
Bolivar Peninsula
Galveston island
31. Consequences
Exposure & Vulnerability
Residential
31
0
1000
2000
3000
4000
5000
6000
7000
8000
1/100
[1/yr]
1/1,000
[1/yr]
1/5,000
[1/yr]
1/10,000
[1/yr]
Direct tangible damage in million $US
Other areas
Texas City
Bolivar Peninsula
Galveston island
32. Consequences
Exposure & Vulnerability
Industrial
32
0.0
5.0
10.0
15.0
20.0
25.0
30.0
1/100
[1/yr]
1/1,000
[1/yr]
1/5,000
[1/yr]
1/10,000
[1/yr]
Direct tangible flood damage to industry in billion
$US
Port of Houston
Texas City
33. Consequences
Exposure & Vulnerability
Indirect damage in million $US
33
Return period [1/yr] 1/10 1/100 1/1,000 1/10,000
Industrial
Port of Houston 0 26,000 60,000 120,000
Texas City 0 4,500 16,000 20,000
Public
Critical infrastructure 1,900 2,850 3,610 3,800
Transportation & Evacuation 537 805 1,019 1,073
Navigation & waterways 3,200 4,800 6,080 6,400
Agriculture and fisheries 600 900 1,140 1,200
Total 6,237 39,855 87,849 15,2473
34. Consequences
Total estimated damage
34
0
20
40
60
80
100
120
140
160
180
200
0.00010.00100.0100
Totaldamageinbillions$
Probability of exceedance per event
Probability-Damage curve without the coastal spine
36. Flood Risk Assessment
Benefits • Damages avoided
Costs
• Investments
(storm surge
barrier+Ike Dike)
• Maintenance
36
To assess the performance of the risk reduction of different barrier heights, a Cost-
Benefit Analysis (CBA) methodology is carried out. A cost-benefit analysis
estimates the present day monetary value of cost and benefits of an intervention
and provides a measure of how well a project performs over its life time.
CBA
38. Flood Risk Assessment
Discount and Growth rate
Interest rate ≈ 7%
Growth rate ≈ 5%
38
Per Capita Personal Income in Harris County, TX (source: Economic Research, 2014)
1996 2006
Land use change
Light green
areas in the
right picture
indicate the
development
gain until
2006
39. Flood Risk Assessment
Construction and Maintenance costs
Strom Surge Barrier in Bolivar Roads
39
Maintenance 1% of the initial investments
per year (maintenance every 10 yr)
40. Flood Risk Assessment
Construction and Maintenance costs
Ike Dike (Master project, 2014)
40
Return period [1/yr] 1/10 1/100 1/1,000 1/10,000
Bolivar Peninsula
[million $]
311 418 582 696
Galveston island
[million $]
217 292 406 486
Total [million $] 528 710 988 1182
Maintenance costs 1.5% of the initial costs every 5
years
41. Flood Risk Assessment
Obtaining a sense of maintenance costs
For a 100 yr lifetime and a maintenance procedure to take place every 10
years for both the storm surge and the land barrier
41
Discounted maintenance costs [million
$]/ Safety level [yr-1]
1/100 1/1,000 1/10,000
Hb=3 m 132 172 201
Hb=6 m 158 205 240
42. Flood Risk Assessment
Benefits
Conceptualization of Benefits and Risk
42
Integrated Risk:
Barriers protect against a
wide range of events up
to a predefined
probability of occurrence.
Likewise, risk is the
integrated risk up to the
desired safety level
starting from the most
frequent event.0.1 0.0001
44. Flood Risk Assessment
Determination of the optimum safety level
Total Costs (=Cumulative Risk + Investment)
44
1/1,000-1/10,000
45. Flood Risk Assessment
Determination of the optimum safety level
Net Benefits & Benefit-Cost ratio
45
Discounted over 50 yr Discounted over 100 yr
46. Flood Risk Assessment
Determination of the optimum safety level
Net Benefits & Benefit-Cost ratio
46
Discounted over 200 yr The larger the
lifetime the larger the
NB and the BC ratio.
Positive NB are
noticed from 100 yr
lifetime and on, for a
barrier height of above
2 m.
1/10,000 yr-1 safety
level gives the largest
NB.
200 yr lifetime also
leads to positive NB
but with larger total
costs.
48. Conclusions
Lifetime: 100 yr?
Safety level: 1/10,000 yr-1
Optimal barrier height: 2. 6m → 4 m+MSL inside the basin for a 1/10,000 yr-1
storm surge level in the gulf side → 1/3 inflow and 2/3 wind setup
CBA led to height close to 3 m after which water levels inside the basin tend to
be almost the same (probabilistic analysis)
The higher the barrier the lower the water level inside the basin
48
Water level inside the basin [m]
Return period [yr-1
]
Water level in
the gulf side
[m]
Without
coastal spine
Hb=1 m Hb=2 m Hb=3 m Hb=6 m
1/100 3.9 3.6 3.5 3.2 3.1 3.05
1/1,000 4.9 4.7 4.1 3.6 3.5 3.4
1/5,000 5.2 5.3 4.4 3.9 3.8 3.7
1/10,000 5.4 5.7 4.6 4.2 4 4
51. Recommendations
Detailed investigation of direct and indirect damage
More precise estimation of the benefits
Split Galveston bay region into zones according to their vulnerability and their
existing protection level
Risk assessment based also on the individual risk (life loss)
More detailed calculation of the investment and maintenance costs (to be
estimated after design)
Interest and growth rate estimation (constant?)
Combinations of different heights of the storm surge with the land barrier
heights
Reliability issues→ probability of failure of the barrier?→independent
hurricane events?
51
(additional to the halfway recommendations)
The storm surge barrier will consist of two sections; a navigational section and a section for refreshing the Galveston Bay in normal conditions.
Firstly the shortest path from the Galveston Island to the Bolivar Peninsula can be an appropriate location for the storm surge barrier. But considering the subsoil and depth along the alignment it may be better to position the barrier deeper inside the bay. A drawback of locating the barrier more inside the bay is that an modifications to the entrance channels to Galveston could be required. Other factors play a role as well, such as the impact on environment and shipping but also the amount of exposure of waves from the Gulf of Mexico.
Deal with negative hydraulic heads (barge gate is preferable)
3) Afloat: cope with high stream velocities, less vulnerable for negative hydraulic heads (the gate simply swings open again), if gate is set down additional measures are required such as (larger) valves, skirts and fenders, this discharge decreases the positive effect of the storm surge barrier on the water levels behind the barrier. But: the dynamic forces in the gate are higher and the wear and tear on the bearings are higher if the gate stays afloat.
4) If a partially closed environmental section of the barrier provides sufficient flood protection the navigational section could remain open (without barrier). This section will have to deal with large flows during hurricane conditions and will therefore require a large bottom protection. It is noted that keeping a part of the barrier completely open can harm the image of the storm surge barrier in the perception of the general public.
Therefore, the hurricane track that is going to be considered in this study in order to estimate, the water level inside the basin, will be west of the basin. Besides, the regions of the Galveston bay that have the highest potential of damage and are the most social and physical vulnerable
Report Galveston Bay: proposed the location of the barrier, the division into 2 sections (navigational+environmental), characteristics of the barrier (dimensions, open or closed navigational section etc)
De Vries: an equal height for both navigational and environmental part is preferable (more cost efficient)
Stoeten: return period of storm surges, constructing a coastal spine is more risk averse that upgrading Texas levees or ship channel’s levees
Karimi: design navigational part, costs, proposed a research for a concrete navigational section rather than steel
Rainfall can be neglected. 200 mm in comparison with the wind setup and inflow effect which can cause much more amplification of the water level inside the basin in just an hour or so.
Discharge Q was divided into discharge over the barrier Qw (equation from weirs) and discharge through an entire open navigational section, Qc (Chezy equation)
2) West sided landfall location as it is the worst case in terms of the induced damage
3) No inflow from the barrier islands-Ike Dike is considered to fully block the surge
4) According to Sebastian et al. (2014) conclusion that during Ike hurricane the rising water levels in the west part of the bay were driven by the combined effect of counterclockwise, western shore-normal winds in the bay and shore-parallel winds along the Louisiana-Texas (LATEX) shelf. An increase of 30% in water levels has been implemented.
5) The reason the maximum wind speed does not coincide with the peak surge is because according to Harris (n.d.) and Pore (1964), peak surge was observed to coincide with alongshore wind components rather than onshore components.
In each Monte Carlo iteration, a random value was given in the extreme storm surge elevation (hocmax) according to their cumulative probability distributions for a certain probability of occurrence while the maximum wind speed (U10) was computed by a linear relationship between wind speed and storm surge level. Then, for different barrier’s heights from 0 to 6 meters, the water level in the west part of the basin is derived after an iteration loop of time steps equal to the time of closure of the barrier.
Left figure: Water elevation inside the basin as a function of storm surge for 3 different barrier heights (1 m, 3 m and 6 m) for 4000 realizations. Right figure: Water elevation as it has been generated from 4000 realizations for barrier heights between 0.5 m and 6.5 m (on each box, the central mark is the median, the edges of the box are the 25th and 75th percentiles and the whiskers extend to the extreme data points). Both graphs were made for the 32th hour of closure where the peak surge meant to be appeared
Certainly, the fact that a linear increase and decrease of storm surge were assumed over time may affect their relationship. However, it is expected that as long as the maximum storm surge becomes bigger so does the water level inside the basin
Inflow tends to decrease almost exponentially from lower to higher barrier heights while the wind setup increases from lower to higher heights. This can be explained from the fact that a larger barrier height prevents a certain amount of inflow which incurs lower mean water levels than a lower barrier height. On the other hand, wind setup is inversely proportional to water depth and subsequently lower water levels lead to higher wind setup. This lead for barrier heights between 3 and 6 m to generate almost the same water level inside the basin.
The effect of the western shore normal winds was taken into account. An allowable inflow could increase this effect as it increases the water depth. However, this effect and its magnitude can only be determined after a detailed hurricane simulation with several landfall locations and intense. Probably for another landfall location other than west sided, this effect might be negligible.
CDF = Generalized Pareto, the probability of exceedance for heights between 4 and 6 m tends to be almost the same. This leads also to similar risk reduction as we will see later on.
Hazard reduction = reduction in the probability of exceedance of a specific event
9) This may be attributed to the number of iterations that have been carried out (4000 iterations). Probably, with more realizations the difference in the hazard reduction of different barrier heights would be obvious also for extreme events with return periods up to 1/10,000 yr-1
Account for the more complex form of a hurricane (circular motion, change in wind speed over space and time) and basin’s bathymetry and coastal attributes (for example bathymetry)
What is more, the cumulative probability function of the storm surge in the gulf side that was used as an input to the calculations, was this one estimated by Stoeten (2013) synthetic model simulation. However, FEMA has proposed different return periods that should certainly be investigated and included in the any similar research.
MEOW= Max Envelope of Water (Stoeten, 2013)
From Jonkman et al. (2013). The estimated safety level was estimated from Kasper Stoeten (2014)
Data on land use was obtained through TNRIS (2013) while the US Census (2010) provides estimates on median home value within Census block groups.
Estimated damage for a 1/100 yr-1 event on the Galveston West End and Bolivar Peninsula are assumed equal to zero because of existing FEMA BFE requirements. The estimated direct tangible flood damage for a 1/100 yr-1 event is lower than observed after Hurricane Ike because it is assumed that lost or severely damaged property was rebuild to contemporary building standards.
The Port of Houston is the second largest port in the United States with an estimated direct economic impact of 178.5 billion dollars a year (Martin Associates, 2011). Its largest asset is the chemical and petrochemical industry with a total crude oil processing capacity of 1,120,000 bpd, almost 10% of the
nationwide capacity. The Port of Houston suffered minor damage during Hurricane Ike as surge levels remained approximately 0.6 meter below the docks (Bedient, 2012).
Texas City is located about 40 kilometers South of Houston and home to three oil refineries with a combined processing capacity of 707,000 bpd (EIA, 2013). Texas City did not suffer flood damage during hurricane Ike, however the Texas City Dike was overtopped.
(Perry, et al., 2008) (Stoeten, 2013) (Master Project, 2014) The U.S. Coast Guard estimates that a one month closure of the Port of Houston will cost the national economy $60 billion dollar (USCG, 2013). Studies indicate that a five week disruption at a large oil refinery adds about $US 5,000/bpd to nationwide refined product expenses (CPRA, 2007). Additional losses relating to loss of sales and earnings amount to $US 12,000/bpd per five weeks of
downtime (CPRA, 2007).
Rebuilding oil refineries, docks or other industrial areas may take months. It is assumed that a 1/10,000 yr-1 event results in a 2-month disruption attributable to flooding. Short and thereby relatively inexpensive when compared to rebuilding efforts after Hurricane Katrina, which lasted up to one year (Admiraal, 2011). Indirect tangible damage figures of the 1/1,000 yr-1 and 1/5,000 yr-1 relate to the 1/10,000 yr-1 event by expressing them as a percentage of the direct damage.
Public damage related to the damage after hurricane Ike
!!!!!!!!!!!Hazard is different for each barrier height
r’ discount rate and g growth rate
We consider that total investment costs occur directly after the construction of the structure. Benefits are defined as the damage avoided in comparison with the “do nothing” alternative.
Μssb// maintenance costs of the storm surge barrier
Mid// maintenance costs of the Ike Dike
Growth rate for Galveston bay is about 4.6% as well (3 counties are present in Galveston bay: Harris, Galveston and Chambers. The last one is excluded as its contribution to the growth is negligible and it is located in the east part of the bay while our consideration is limited to the west part)
C// cost of the barrier
B//width of the barrier
H//retaining height of the barrier
Δh//maximum head across the barrier (for simplicity, it was considered to be equal to the water level outside the barrier. More precisely, the head is the difference between the water level outside and inside. It determines more or less the costs for the width and the foundation of the barrier. However, the water level inside the basin changes every hour under hurricane conditions and under away that needs investigation in order to come up with a reasonable result).This may underestimate costs. A negative head is also possible!
Cunit// unit cost [$/m3], Karimi: 18,900 $/m3 for navigational, De Vries: 15,000 $/m3 for the environmental part, Van de Toorn: 30,000-40,000$/m3. The unit cost that was taken in this research was a combination of those estimations. More specifically 30,000 $/m3 for the navigational and 20,000 $/m3 for the environmental. This unit value takes also into account maintenance costs discounted (as I saw from the references that I took them from). They are thought to be considered reasonable values after an overview of similar projects and after a discuss with Iv-infra structural department.
However, this indeed depend on the maintenance costs and their more precise elaboration. A well constructed barrier needs maintenance every 20 years while for the dike a reassessment of the safety level takes place every 6 years. Of course, there are also unpredictable maintenance costs such as after a severe hurricane strike which may have a dramatic impact on the structure.
From a brief overview of maintenance costs, they may rise the barrier costs up to approximately 1 billion $ for 1/10,000 yr-1 event and 0.5 billion for a 1/100 yr-1 event. Which again shows that a safety level of 1/10,000 yr-1 maybe relatively beneficial. But, according to the aforementioned method, if we want to add the maintenance costs we should decrease the unit cost respectively.
Maintenance is considered to take place every 5-6 years
Varies for different barrier heights and safety level
Most frequent event: 1/8 yr-1 event (Keim&Muller)
I found the risk from the integration of these graphs for each barrier height. The risk without the coastal spine was calculated from the integration of the damage-probability curve in slide 31.
From the graph we can determine the area of the safety levels within which we can move. Obviously, below 1/1,000 yr-1 safety level, the risk without the barriers is less than the total costs of the structure. Thus, it is not cost efficient to choose a safety level below this one. Of course, here we assumed a land barrier that fully blocks the surge. If overflow of the land barrier is considered then costs might decrease, but this requires further investigation.
This graph is from a discounting over 100 years
Safety level refers to the probability of exceedance of the water level in the gulf side
Safety level refers to the probability of exceedance of the water level in the gulf side
1)However, for a larger structure lifetime, initial investment costs would probably be larger. On the other hand, maintenance costs are reduced.
2)The proposed safety level (1/10,000 yr-1) also coincides with Kasper Stoetens (2013) results about the optimum safety level.
3)100 yr lifetime is the first to give positive NB with the lowest costs.
For 100 yr lifetime, the lower the safety level is the lower the total costs are. However, NB determines the optimum safety level and thus for this level, the optimum barrier height is chosen.
Numbers refer to the conclusions in sequence:
1)As structure’s lifetime increases, risk as well as total costs also increases. In terms of total costs (risk+investment), for 100 yr and 200 yr lifetime the optimum safety level is 1/100 yr-1, for 50 yr is 1/5,000 yr-1. However, the value of the net benefits determines the optimum safety level. We can compare the total costs of each alternative to the “status quo” (doing nothing alternative) in order to decide if it is worthwhile to undertake a specific alternative (total costs of alternative< risk of “doing nothing” alternative). The chosen lifetime depends also on the B/C that US uses for such investments.
5) However, the difference is not significant and that is why in the end a height of 6 m is not beneficial as the risk reduction does not overweigh its investment costs
Numbers refer to the conclusions in sequence:
6) Calculated for a barrier of 3 m, safety level 1/10,000 yr-1 and 100 yr lifetime
7) Lower premiums due to risk reduction: more clients for being insured, development in the new protected areas, win-win situation (better protection, lower insurance for homeowners)
Low BC ratio? Then this picture might help reconsider the until now benefit cost ratio and probably NFIP will increase the available budget for Texas in terms of flood protection measures.
2) Benefits: i.e. more development in the surrounding areas due to safer standards, multiple functions of the coastal spine (road, recreational area), lower insurance
3) In order to make a better estimation of the potential damage. Separate areas with different flood protection level from the main regions (i.e. Texas city, port of Houston, Kemah etc)
4) Rapidly increasing population
5) Issues like: bed protection, unforeseen costs, costs due to an increased lifetime of the structure (new materials (concrete, steel etc), innovative design), maintenance
6) Also different heights of the dike should be investigated in combination with the barrier’s height
8) In this research, hurricane events were considered independent concerning their effect on the barrier. For each hurricane, barrier maintains its initial strength. However, after repeated storm events it will gradually loose its strength (not only from storm events but also from natural phenomena like corrosion etc.). In such a case, either you accept the decrease in the strength and thus the risk increases or a maintenance procedure takes place which consequently increases the investment costs.
