Flood Control Analysis

by Philip B. Williams, Ph.D., P.E.

Background

For
centuries the Chinese people have been building earthen dykes to
prevent the Yangtze River from overtopping its banks during the flood
season and inundating the adjacent floodplain. Over the past four
decades, the Chinese government has mobilized the people of the Yangtze
Valley to substantially upgrade the existing system of flood protection
dykes and diversion works. Flood storage reservoirs and control
structures have been constructed, overflow diversion areas have been
established, and roughly 30,000 kilometres of dykes now line the
Yangtze and its tributaries, and protect urban centres situated in
flood-prone areas. The main dykes protecting the valley have been
reinforced and raised to an average height of 12 metres and along the
critical 180-kilometre Jinjiang reach, the dykes stand 16 metres high
in places. Today the Yangtze Valley is capable of safely diverting and
storing roughly half of a flood equivalent to that which occurred in
1954.

Ironically, as the dykes have been raised to increase the river’s
channel capacity, the risk of a flood disaster has grown dangerously high.
If the Jinjiang Dyke – the main dyke in the middle reach which Yangtze
Valley Planning Office engineers view as the critical component of the
existing system – were to fail, a huge flood would spread across the densely
populated floodplain, killing at least 100,000 people, and inundating
major urban centres. Conceivably, the river might change course entirely
and rush headlong into the city of Wuhan with a population of six million
people.

The Existing Flood Management System

The
fertile floodplain of the middle Yangtze Valley has been formed over
hundreds of thousands of years by sediments deposited from the flood
waters of the Yangtze and its major tributaries. Over the last two
thousand years, as the population of the valley grew and farming became
more intensive, a sophisticated flood management system was constructed
to reduce the risk of flooding. This system has been substantially
improved in the last 40 years and now consists of a network of river
dykes, overflow diversion areas, control weirs, floodplain lakes and
ring dykes. The guiding philosophy in the management of this system has
been to attempt to absorb the huge volumes of floodwater by temporarily
storing them in emergency diversion areas on the floodplain and in
floodplain lakes rather than attempt to ‘control’ all floods.

With this system, three types of flooding can occur:

• Along the river itself, between the main river
  dykes, there are settlements and cultivated land. Referred to as ‘beach
  areas’ in the CYJV report, they are susceptible to flooding from
  frequent high river stages. Here people know they are likely to be
  flooded and therefore take reasonable precautions to limit flood
  damage. According to CYJV, flood damages can occur in beach areas if
  the flood exceeds the expected 5-year flood level.
• In the overflow diversion areas, covering
  about 800 square kilometres, about 4 million people live and farm. The
  use of these overflow areas to store flood waters is infrequent, and
  according to CYJV, occurs about once in 20 years. Although the
  residents of the overflow diversion areas know that they live in flood
  overflow areas, the level of protection varies. Many of the towns are
  protected by ring dykes and some areas have constructed refuges
  protected by dykes. Other areas have little protection.
• Former floodplain areas protected by major
  river dykes have virtually eliminated flooding. Referred to as ‘primary
  protected areas’ in the CYJV report, these floodplains have been
  intensively developed. Of particular importance is the Jingbei Plain
  protected by the Jianjiang Dyke against flood levels expected only once
  every 250 years (or a 1:250 flood probability). The Jingbei Plain and
  major cities, such as Zhijiang, Shashi, and Wuhan, depend on
  maintaining the integrity of the 12 to 16-metre-high Jianjiang Dyke
  during extreme floods. The integrity can be threatened by seepage,
  erosion, and overtopping.

This flood management system has
functioned well, significantly limiting flood damages in the worst
flood of the century, the 1954 flood, which CYJV estimates to be a once
in 200-year event. The flood management system was successful because
of its ability to temporarily store huge volumes of water during the
flood. The 1954 flood is estimated to have discharged 366 cubic
kilometres of water in 60 days. At least 100 cubic kilometres were
probably stored in floodplain lakes, overflow diversion areas, and in
river channels.

This puts into perspective the comparatively
minor role the Three Gorges Project would have on reducing flood
volumes. If designed safely, and operated correctly, the dam is
intended to store 31 cubic kilometres which is less then 10 percent of
the 387 cubic kilometre volume of the 1000-year flood.

Review of the CYJV Flood Control Analysis

Although
most of the economic benefits claimed for the Three Gorges Project are
for electricity generation, the dam’s advocates have maintained that
flood protection is the primary need for the project. In fact, it has
been frequently stated that the dam must be built to avert catastrophic
flooding affecting millions of people living along the middle reaches
of the Yangtze River. For a project of this magnitude – one of the
world’s largest civil engineering projects that could potentially
affect hundreds of millions of people – the CYJV feasibility study
should provide an objective and systematic analysis of the reduction in
flood risk which would result if the dam were built, and evidence that
the project would be the most cost-effective approach for providing
additional flood protection. Moreover, such an analysis should be
accurate, use the best available techniques and have a consistent
methodology.

Unfortunately, the study does not do this, for the following reasons:

Purpose of the Project Is Misrepresented

Throughout
the feasibility study there appears to be confusion over the main flood
control goal. The study summary, which is written for potential
financiers and key policy makers, states that the project’s main flood
control function is to protect 20,000 square kilometres of downstream
floodplain inhabited by 10 million people. Design Volume 4, on the
other hand, states that the project ”can provide flood protection to
approximately 1 million square kilometres.”¹ And then Volume
7 on flood control states that the ”primary flood control objective of
the project and its operation must be to guarantee the integrity of the
major dykes protecting the Jingbei Plain and the Jingnan Region” – an
area of 6700 square kilo-metres.²

The
computed flood benefits* described in the detailed technical analysis
do not substantiate even the least grandiose of these statements.
According to CYJV, in the event of a 1000-year flood, the Three Gorges
Dam would protect 4000 square kilometres from floods, which is only 30
percent of the total area that would be inundated without the project.
But in the benefit analysis, even these figures are misleading. About
three-quarters of the benefits claimed for the project are attributable
to the reduction in the frequency, and not the elimination, of flooding
of just three areas – two flood overflow diversion areas and the beach
areas on the river side of the dykes. Only about 15 percent of the
total economic benefits are attributable to protecting the highly
populated Jingbei Plain and Jingnan Region, but the flood control
storage and operation at Three Gorges is designed specifically for this
15 percent.

Only a detailed reading of the study reveals that
the goals and operational design of the project are to upgrade
protection for the Jingbei and Jingnan regions from the current
250-year flood level to 1000-year flood level. This means that the
flood control operational design is not based on an objective
cost-benefit analysis, but on a subjective judgement that these areas
require levels of protection higher than the 250-year flood.

Unrealistic Operational Criteria

CYJV
has recommended an operational plan for the dam – CYJV’s Flood Control
Operation – to release or store floodwater in the reservoir depending
on water levels in the river downstream:

• During flood season, reservoir levels would be
  lowered from the normal pool level (NPL) of 160 metres to the flood
  control level (FCL) of 140 metres above sea level. Flood storage would
  be provided above 140 metres and moderate floods up to the 50-year
  flood level could be stored without the reservoir exceeding the NPL.
  For greater floods, the reservoir would rise higher depending on the
  magnitude of the inflows and the water level in the main channel
  downstream.
• In the event of a 100-year flood, water
  would be stored in the reservoir so that the water levels in the middle
  reach (at Shashi) would not exceed 44.5 metres. As such, diversion into
  the Jinjiang diversion area would be avoided – a practice which,
  without alternative flood protection measures, is becoming increasingly
  impractical due to rapid urban expansion and industrial development in
  the area.
• In the event of a 1000-year flood, water
  would have to be diverted into the Jinjiang diversion area, but the
  water level at Shashi would not exceed 45 metres, thereby avoiding a
  breach in the Jinjiang Dyke.
• To achieve the flood benefits claimed for
  the project means that this operational schedule would have to be
  rigorously followed – particularly during large floods. Actual
  operating experience of large multipurpose reservoirs during large
  floods indicates that there can be substantial deviations from
  prescribed operating procedures which result in greatly reduced flood
  benefits.

There is no reason to believe that the
Three Gorges Project operational design would not share these problems,
specifically, because of the following factors:

• Conflict with people living in the reservoir area

CYJV
fails to recognize the most significant operational problem which would
negate many of the flood benefits claimed. Roughly half a million
people are currently living in what would be the designated flood
storage area between 162 metres and 182 metres above sea level. These
people would be inundated when floodwater was stored in the reservoir
area above 162 metres.* In the event of a flood requiring storage above
162 metres, the dam operators would have to choose between flooding out
large numbers of people living in the reservoir area or large numbers
of people living downstream adjacent to the river and in overflow
diversion areas. For such a flood, the CYJV flood analysis fails to
demonstrate that more people downstream would be protected by the
proposed flood control operation at the dam, than would be flooded in
the reservoir area.

• Conflict with power generation

Generally
speaking, the higher the level of water in the reservoir, the more
power can be generated. Because power generation provides cash revenue
there would be strong institutional pressure to operate the dam for
maximum power generation. This could be done either by delaying
emptying of the reservoir, which has to be done prior to the flood
season, or by keeping the reservoir higher than required for flood
control. In fact, CYJV acknowledges that if its 160-metre recommended
scheme is built, the reservoir may, in future, be operated at higher
levels than presently stated, thereby sacrificing flood control in
order to increase power generation. Such potential changes in operation
are not reflected in the analysis of benefits.

• Conflict with downstream development

Most
of the expected flood benefits are based on the assumption that
population and development would increase along the beach areas and
overflow diversion areas. In general, when the area downstream of a dam
becomes increasingly populated and developed, dam operators frequently
disregard operating rules and fail to make required large flow releases
for flood control during major floods.

At the Three Gorges Dam, operators would be
faced with the difficult choice of opening the spillway gates and
flooding downstream property, as required by the design, versus
allowing water levels in the reservoir to rise and waiting until the
last minute to open the gates while hoping that the flood would abate.
If the flood did not abate and the dam operators had delayed opening
the spillway gates, they would then be forced to make large emergency
releases from the reservoir which could cause a catastrophic flood
downstream – negating expected flood benefits. This was not considered
in the probability analysis.

• Unrealistic assumption of perfect spillway operation

Actual
operating experience with large flood control dams show that when large
floods occur there is a substantial risk that designated overflow
spillway capacity is not achieved due to operator errors or
malfunctions of the spillway (e.g., the experience with Tarbela,
Pakistan, or Glen Canyon, U.S.).

• Inadequate analysis of flood levels

In
order to design the dam for flood control operation and to calculate
flood benefits associated with the Three Gorges Project, it is
necessary first to estimate flood risk along the middle reach by
pinpointing when and where peak flows would first overtop the dykes.
For large rivers like the Yangtze, with its complex flow network,
diversion areas, floodplain lakes and large tributary flows, it is a
complicated task to determine how water levels during a flood would
vary with space and time.

The most accurate tool for this kind of analysis
is by computer simulation using a hydrodynamic model, which is capable
of simulating downstream flows and water levels for various flood
scenarios throughout the river system at different times. CYJV
recognizes that analysis using the hydrodynamic model would provide the
most accurate results but rejects its use in favour of a more
analytically crude model because the ”accuracy of the input data is
much beyond the scope of the present study.”³ (Presumably,
the input data referred to is a detailed mapping of downstream channel
dimensions.) This flood routing model predicts water levels at only 12
locations along roughly 740 kilometres of river in order to identify
the times and locations where the flood flow would first overtop the
dykes. Use of this simplistic model in a developed country would not be
acceptable for such an important flood benefit analysis.

Generally, to validate the results of such an
analysis, flows computed using the flood routing model are compared
with actual observed flows along the river. But CYJV’s validation is
not convincing because there is a discrepancy between observed and
computed flows during the peak flow period of a magnitude about the
same as the total flood storage capacity of the Three Gorges reservoir.
CYJV acknowledges that the main reasons for this discrepancy are
hydrodynamic effects, such as backwater effects and channel conditions,
which it has declined to model, but nevertheless concludes that ”the
model represents reasonably well” the river flows.⁴

CYJV
emphasizes the importance of accurate water level predictions by noting
that a 20 centimetre change in predicted water level against a dyke
could mean a 20 percent change in flood benefits. But the use of a
flood routing model instead of a hydrodynamic model means that there
are likely to be substantial errors in predictions as well as in flood
control operational design. In addition, because key hydrodynamic
factors affecting flood risk have been ignored, expected flood benefits
achieved by upstream storage would not be as large if all these other
factors had been considered. It is also possible that because the model
does not predict where the peak flows are likely to first overtop the
dykes, the operational design could increase flood risk in some areas.

Flood Benefits Exaggerated

The
inadequate flood analysis and unrealistic operating assumptions
together tend to overstate the flood benefits of the project. In
addition, CYJV makes two other major assumptions that greatly
exaggerate the value of the flood benefits:

• Economic growth rates that increase property value between 10 and 30 times present values

Some
flood protection agencies in developed countries consider the use of
any predictions of future growth in flood benefit analysis as
conjectural and invalid. CYJV’s assumption that such a drastic increase
in investment would occur over the next 60 years for land that would
remain flood prone is highly questionable.

• Use of the same economic growth rates with and without the dam

One
of the basic tenets of flood management planning is that the perception
of flood damage potential and the type and value of land use are
closely related. Obviously people prefer not to build in flood-prone
areas and, even if they do, they would want to flood-proof their
property. These actions would greatly reduce the flood damage potential
in the ”no dam” scenario and hence the net benefits claimed for the
project. If, for example, the growth rate without the project was 40%
of that with the project, total flood benefits would be zero.

Both these assumptions illustrate a
fundamental planning error that permeates CYJV’s flood control
analysis. It focuses on flood control storage as a goal in itself,
whereas it is merely one tool for managing floods. Rather, CYJV should
have recognized the primary goal as flood damage reduction, which is
achieved through an integrated strategy that includes dykes, diversion,
zoning, and flood proofing.

Because of this, the project designers have built
a methodological flaw into their analysis – they fail to analyze flood
management of the Yangtze River as a complete system, which would
require incorporating not only ”plumbing” but also hydrologic,
geomorphic, social and economic factors.

Project Costs Ignored

• Relocating people out of the reservoir area

If
any realistic flood control operation were to be carried out, the
half-million people living between 162 and 182 metres elevation in the
reservoir region would have to be relocated, which could increase the
total project budget by at least 20%.

• Backwater flood damages upstream

CYJV
predicts that the 100-year flood level would rise at least one to two
metres within 30 years due to sedimentation in the backwater near the
city of Chongqing. Although the increased level of flooding would
affect hundreds of thousands of people, CYJV failed to consider this in
its analysis of flood damages. Rather, it defines a ”critical” flood at
a level of 198 metres or higher, which corresponds to the elevation of
the Chongqing-Chengdu railway. This ”critical” level is inconsistent
with CYJV’s own figures, which show damaging flows to occur at much
lower flood levels.*

CYJV circumvents
the problem of sediment deposition by claiming that ”future upstream
reservoirs and conservation measures” would reduce the high sediment
load carried by the river during floods.⁵ Neither the cost
of reducing sediment input to the reservoir nor the cost of increased
flood damages in Chongqing are included in project costs.

• Repairing downstream dykes

CYJV
acknowledges that clearwater flows* would erode the downstream dykes in
the middle reaches and claims this problem would be managed by river
training works and dredging. Neither of these measures would prevent
undercutting of the dykes downstream and it is worth noting that
elsewhere in the report, when discussing alternative flood protection
improvements, CYJV lists dredging as ”not very promising because it
interferes with the natural equilibrium of the river, and would require
frequent and costly maintenance.”⁶ Such costs, which CYJV recognizes can be exorbitant, are not included in the cost-benefit analysis.**

• Flood damages due to coastal erosion

Adjacent
to the Yangtze River mouth, extensive offshore mud flats are formed by
the sediment discharged from the river in a cycle of sediment
deposition and erosion. Several hundred kilometres of low-lying
shoreline depend on the mud flats for protection against coastal
flooding. CYJV failed to analyze the shoreline erosion and coastal
flooding that will result from both the reduction in sediment delivery
due to the effect of reduced flood peaks and the capture of sediment in
the reservoir – a serious omission considering that the Yangtze River
ranks fifth in the world in terms of its sediment discharge to the
ocean. The costs of either increased coastal flooding or additional
shoreline protection are not included in the cost-benefit analysis.

Failure to Consider Cost-Effective Alternatives

Because
the primary purpose of the project has been defined as flood control
rather than flood damage reduction, the importance of other components
of the flood management system have been downplayed. For example, the
existing system of dykes and diversion areas provides substantially
greater flood benefits than flood control storage upstream on Yangtze
tributaries. Combined with flood retention in the floodplain lakes, the
total flood storage capacity downstream of the Three Gorges appears to
be about 200 cubic kilometres, compared to the Three Gorges reservoir’s
maximum flood storage capacity of 31 cubic kilometres.

CYJV states that a long-term program of flood
control improvements is already under way but does not present this as
an alternative nor discuss how the benefits of small improvements to
the dykes would affect the cost-benefit ratio. On the one hand, CYJV
states without substantiation that such improvements ”would be
uneconomic and impracticable,”⁷ and, on the other, its
analysis shows that a 20-centimetre increase in the height of dykes
could increase flood benefits by 20%.⁸

One
important alternative component of an improved flood management system
(which was relegated to an appendix of the study and not even mentioned
in the study’s summary of alternatives) is the provision of ring dykes
and refuge centres to protect populated areas within the diversion and
beach areas. Existing ring dykes average eight metres high and
encompass some 21 towns, protecting 93,000 people in the Jinjiang
Diversion Area. Also, there are a number of refuge centres protected by
ring dykes in this area that would serve 150,000 people in the event of
flooding. CYJV concludes that ”protective dykes could be justified if
the annual probability of flooding is more than 1.5%,” which is
equivalent to a 75-year flood.⁹ Since this is the case for
the three diversion and beach areas which already provide most of the
project’s claimed flood benefits, then this would be a more
cost-effective alternative to building the Three Gorges Project.

Flood reduction strategies could achieve flood
benefits equivalent to or higher than those expected with the Three
Gorges Project by a combination of upgrading critical dykes, modifying
overflow areas, providing refuge centres and ring dykes,
flood-proofing, flood-warning systems, and development zoning.
Non-structural measures such as development restrictions in flood
hazard areas can greatly reduce flood damages. CYJV considers
development restrictions along the river proposed by the Chinese
government as a strategy to minimize flood damages caused by the Three
Gorges Project in the reservoir region:

This restriction would prohibit future construction
of new buildings, whether public, commercial, or residential, and new
industries around the reservoir perimeter below El 182 [182 metre
elevation mark]. This restriction would also effectively limit future
population growth below El 182.¹⁰

But
CYJV did not consider development restrictions as a potential strategy
downstream. Using CYJV’s own figures, a restriction on development in
the beach and diversion areas to less than 40% of the growth predicted
in the major protected areas (such as the city of Wuhan) would equal
all flood benefits expected with the project.

Failure to Consider Costs of Potential
Catastrophic Failure

CYJV
implicitly claims that the Three Gorges Project would prevent major
loss of life in an extreme flood such as the one-in-1000-years
category. There is repeated emphasis that failure of the Jinjiang Dyke
would have catastrophic consequences. So, the flood control operation
of the reservoir is directed towards managing water levels to prevent
failure in the downstream channel, even though the probability of flood
damage is much larger in other areas – for example, in the beach areas
and diversion areas.

A systematic impact analysis of the project would
probably indicate that the benefits of preventing loss of life are
likely to be negated because the dam itself would increase the
potential for loss of life as follows:

• Increased risk of failure of downstream dykes

CYJV
acknowledges that there would probably be an increased incidence of
dyke failure due to a downcutting of the river channel which would
undermine the river’s banks. This vitally important impact has been
dismissed without substantiation by CYJV, with their statement that
”channel morphology should not change significantly because the dykes
that presently border the river are in many cases protected by rip-rap.”¹¹*
As well, undercutting and erosion would be significantly aggravated by
the wide, daily fluctuations in flow because of power generation
demands. Equally important as the physical impact of the project would
be the institutional impact, which could increase the risk of dyke
failure. Construction of the dam would draw on funds allocated for
upgrading and maintaining dykes. As experience with other large flood
control dams has shown, the mistaken perception of improved flood
protection due to the dam could lead to reduced maintenance and
deterioration of downstream dykes and other flood management
infrastructure.

• Potential failure of the dam

As
is discussed in the later chapter on dam safety, the risk of
catastrophic failure at the Three Gorges Project is probably of the
same order of magnitude as the probability of a 1000-year flood. CYJV
failed to prepare a map of the area which would be flooded if the dam
were to fail, even though the potential loss of life would number in
the millions. Property damage would be so extensive that even if the
probability of failure were 1 in 10,000 years, any flood benefits
claimed for the dam would be negated, according to CYJV’s own figures.

• Large increases in population induced to settle in flood-prone areas

Although
CYJV recognizes that the presence of dykes can create a false sense of
security leading to more fatalities when they fail, it has not applied
this same relationship to the presence of the dam itself. CYJV’s
analysis of flood benefits is clearly based on the assumption that an
additional population would settle in flood-prone areas. What CYJV does
not discuss is that the flood risk in these areas would increase as a
result of any of the operational problems described earlier.
Furthermore, CYJV does not discuss how risks would increase over time
as the reservoir becomes clogged with sediment and its capacity to
store flood waters is reduced.

It is reasonable to assume that, as
sedimentation increases, the dam would be operated to maximize
protection against smaller floods. This would lead to a scenario where
cities and towns have been built up in what were thought to be
protected areas and ring dykes and refuge centres have long been
neglected. In the event of a large flood, the dam would no longer be
able to control flood waters as originally intended, and, inevitably,
the loss of life in such circumstances would be greater than if the dam
had never been built.

Sources and Further Commentary

*Flood benefits are calculated as the expected
reduction in economic losses due to flood damage, if the Three Gorges
Project were built.

*This figure does not include the additional
hundreds of thousands of people who would be affected by backwater in
the upstream vicinity of Chongqing.

*In 1981, Chongqing was hit by a flood that cost
$5.4 million in flood damages and affected approximately 137,000
people. The flood stage reached 194 metres elevation above sea level
which is four metres below CYJV’s ”critical” flood stage of 198 metres.

*Clearwater flows: when the river’s sediment is trapped by the reservoir, the river carries less sediment downstream of the dam.

**As an example, the estimated cost (as of 1980) for an ongoing program of dyke improvements is $1.8 billion.

*Rip-rap is the term used for large rock which is
commonly used to protect the surface of earthen dykes, embankments, and
river banks.

Continue to Chapter 9

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Chapter 7