| Abstract | Risk-based design is a design approach developed for quantifying the probability of
failure and the expected annual damage cost of a structural system which is subject to the
variation of actual load and available resistance. The approach considers the variation of both
load and resistance and the interaction between load and resistance of the structure in the
estimation of probability of failure. Moreover, the expected annual damage cost can be
estimated by analyzing the variation of load and resistance along with the consequence function
of load and resistance relating to damage cost of failure. The solution will be the optimal
capacity of the structural system such as the flood control structure with maximum net benefit
for a design return period of flood protection failure. The concept of optimal risk-based design
is applicable not only to a single structure but also the combination of multiple structures such
as a flood control system.
In this study, an optimal risk-based design procedure of a flood control system is
developed. An interesting contribution is that the design approach is a computational
framework using four relevant techniques, namely flood flow simulation analysis, coincident
flood frequency analysis, load-resistance analysis and optimization of risk-based design. In this
design approach, the four techniques are linked together as a computational sequence for
determining the optimal design of a flood control system.
Large floodings in Bangkok and its suburban areas are mostly due to failure of existing
river dikes to prevent intrusion of flood water from the Chao Phraya river such as the river
flood overtopping the river dike into the protected area. Other floodings due to dike leakage
and dike gate closure failure are also possible. It is considered that in addition to the variation
of the river flood level due to various factors, the resistance of the river dikes is also variable
due to combined natural and man made effects. The river flood level is excessively high during
the simultaneous occurrence of the flood inflow from the north and high tide at the river
mouth. As it is impossible to properly solve such floodings by a single flood control
component, it becomes necessary to apply a flood control system to tackle the overall causes
of flooding. Therefore, this is interesting to apply an optimal risk-based design to a flood
control system to solve flood problems for Bangkok.
In this study, an optimal risk-based design of a flood control system is applied to
determine the optimal capacity of a flood control system based on maximum net benefit and
benefit/cost ratio with a design return period of flood protection failure for Bangkok. In this
study, a proposed Chao Phraya river flood control system for Bangkok recommended by the
Asian Institute of Technology (AIT) and the Thai-Austrian Consortium (TAC) (1986) is
considered. It is consisted of a diversion dam at Pak Kret (70 Km) upstream of Bangkok, a 52
km long diversion channel from the diversion dam to the sea with a diversion sea gate at the
end of the diversion channel, a sea barrier with a pumping station at the river mouth. The
diversion scheme will divert excessive portion of flood flow through the diversion channel and release the remaining flow through the diversion dam to Bangkok. The sea barrier will prevent
sea water intrusion into the river. The pumping station is a supplementary component of the
sea barrier to pump flood water to the sea when the sea barrier is closed. This proposed flood
control system will jointly utilize the existing river storage between the diversion dam and the
sea barrier to absorb flood volume during closure of the sea barrier. By this flow regulation,
the river water level at Bangkok can be controlled effectively during flood events.
The design approach is consisted of four steps of analysis. Firstly, the flood flow
simulation analysis is carried out to determine the flooding relationships of river flood levels
at Bangkok and at two flood boundary stations at Bangsai and Fort Chula for various capacities
of the flood control system. It considers the flood inflow from Bangsai which is mainly
diverted along the diversion channel from Pak Kret to the diversion sea gate and partly released
through the diversion dam at Pak Kret through the Chao Phraya river and the sea barrier to the
sea. The overdike flow into flood plains in Bangkok is considered. Secondly, the coincident
flood frequency analysis is undertaken to determine the probability density functions of the
natural and regulated flood levels at Bangkok for a mean dike crest level of 1.5 m.MSL.
Thirdly, the load-resistance analysis is done to determine the risk of flooding due to failure of
existing river dike and expected annual damage cost. Both load and resistance are considered
as variables. The floodings due to failure of existing river dike due to flood overtopping the
river dike, dike leakage and dike gate closure failure are considered. The respective resistance
are the dike crest level, the dike strength and the dike gate closure success while the common
load is the river flood level. Fourthly, the optimization of risk-based design is done to
determine the optimal size of diversion channel and capacity of pumping station at the sea
barrier based on maximum net benefit and benefit/cost ratio for a design return period of flood
protection failure (flooding). The flood control system with diversion channel basewidth
ranging from 0 to 80 m and pumping capacity ranging from 0 to 2,000 m3
/s are considered.
As the result of load-resistance analysis, it is found that the proposed flood control
system can increase the return period of flood protection failure at Bangkok and reduce the
expected annual damage cost considerably. When there is no flood control system, the return
period of flood protection failure is 2 years and the expected annual damage cost is 5,000
Million Baht. With the optimal risk-based design, it is found that the proposed flood control
system is remarkably profitable. The project period is considered for 50 years (1996-2045),
with initial 10 years for construction and subsequent 40 years for flood protection. The project
cost and benefit are estimated at present value of the year 1995. It is found that the optimal
flood control system is consisted of a diversion dam, a 40 m basewidth diversion channel with
a diversion sea gate and a sea barrier without pumping station. This scheme can protect
flooding in Bangkok and its suburban area for a return period of 270 years with maximum net
benefit and benefit/cost ratio. The project cost is 74 billion Baht while the net benefit is as high
as 200 billion Baht with the benefit/cost ratio of 3.7 and the economic rate of return of 10.2%.
If excluding the cost of improvement of city drainage system amounting to 43 Billion Baht,
the cost will decrease to about 31 Billion Baht. By including other indirect benefits arising
from this flood protection scheme in addition to the considered benefit from reduction of potential flood damage in the protected area, the profitability will be much larger. This flood
control system shows both high flood protection and large profitability, therefore justifying
viability for project investment. |