Morris & Fan. Reservoir Sedimentation Handbook

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CHAPTER ONE

INTRODUCTION

1.1 Need For Sediment Management

Most natural river reach are approximately balanced with respect to sediment inflow and

outflow. Dam construction dramatically alters this balance, creating an impounded river

reach characterized by extremely low flow velocities and efficient sediment trapping. The

impounded reach will accumulate sediment and lose storage capacity until a balance is

again achieved, which would normally occur after the impoundment has become “filled

up” with sediment and can no longer provide water storage and other benefits. Declining

storage reduces and eventually eliminates the capacity for flow regulation and with it all

water supply and flood control benefits, plus those hydropower, navigation, recreation,

and environmental benefits that depend on releases from storage. The Camaré reservoir

in Venezuela (Fig. 1.1) offers an example of the consequences of sedimentation; less than

15 years were required for loss of all storage. This site differs from most other reservoirs

in only one aspect, the speed at which storage capacity was lost. Sediment management

was not practiced at this site.

Storage loss is but one of many sedimentation problems that can affect reservoirs.

Operation of storage reservoirs is severely impacted by the time half the volume has been

sedimented, but severe sediment-related problems can appear when only a small

percentage of the storage capacity has been lost. As reservoirs age and sediments

continue to accumulate, sediment-related problems will increase in severity and more

sites will be affected. At any dam or reservoir where sustainable long-term use is to be

achieved, it will be necessary to manage sediments as well as water. This is not a trivial

challenge.

Many type of sediment-related problems can occur both upstream and downstream of

dams, and sediment entrainment can also interfere with the beneficial use of diverted

water. Sediment can enter and obstruct intakes and greatly accelerate abrasion of

hydraulic machinery, thereby decreasing its efficiency and increasing maintenance costs.

Turbid density currents can carry sediments tens of kilometers along the bottom of the

impoundment, eventually entering deep intakes and accumulating in front of low level

outlets. Localized sediment deposits in the delta region and streambed aggradations

upstream of the reservoir can produce flooding, cause soil water logging and salinization,

impair navigation, alter ecological conditions, inundate powerhouses discharging into

delta areas, and bury intakes (Fig. 1.2). In arid regions the growth of phreatophytic

vegetation on delta deposits can significantly accelerate water loss. The combination of

sediment trapping and flow regulation also has dramatic impacts on the ecology, water

transparency, sediment balance, nutrient budgets, and river morphology downstream of

the reservoir; dam construction is the largest single factor influencing sediment delivery

to the downstream reach. The cutoff of sediment transport by the dam can cause stream-

INTRODUCTION 1.2

(a)

(b)

FIGURE 1.1 Fully sedimented Camaré irrigation reservoir in Venezuela: (a) overview, (b) looking

upstream from the spillway showing incipient formation of floodplain deposits above the spillway

elevation on either side of the channel. The water surface in the photo is at the spillway cres

elevation (G. Morris).

INTRODUCTION 1.3

FIGURE 1.2 Sedimented water supply intake in the delta region upstream of Khasm El Girba

Dam on the Atbara River, Sudan (R. Hotchkiss).

bed

degradation, accelerate rates of bank failure, and increase scour at structures such as

bridges. The streambed will coarsen and become armored, degrading or eliminating

spawning beds. Even coastal processes can be affected; accelerated coastal erosion

affecting the Mississippi and Nile deltas is attributed to sediment trapping behind dams

more than 1000 km upstream.

Based on the inventory published by the International Commission on Large Dams

(ICOLD, 1988) and the current rate of dam construction, as of 1996 there were about

42,000 large (over 15 m tall) dams worldwide. There are several times as many lesser

structures. An overwhelming majority of these structures are designed and operated to

continuously trap sediment, without specific provisions for sustained long-term use.

Neither current nor projected levels of population and economic activity can be sustained

if today's inventory of storage reservoirs is lost to sedimentation, and, as population and

economic activity grow, reliance on the services provided by dams is increasing.

Reservoir-dependent societies range from technically advanced urban and agricultural

systems in the western United States to village irrigators on the Indian peninsula. Sudden

loss of the world's reservoir capacity would be a catastrophe of unprecedented magnitude,

yet their gradual loss due to sedimentation receives little attention or corrective action.

Reservoirs have traditionally been planned, designed. and operated on the assumption

that they have a finite "life," frequently as short as 100 years, which will eventually be

terminated by sediment accumulation. Little thought has been given to reservoir

replacement when today's impoundments are lost to sedimentation, or to procedures to

maintain reservoir services despite continued sediment inflow. There has been the tacit

assumption that somebody else, members of a future generation, will find a solution

when today's reservoirs become seriously affected by sediment. However, sedimentation

problems are growing as today's inventory of reservoirs ages, and severe sediment

INTRODUCTION 1.4

problems are starting to be experienced at sites worldwide, including major projects of

national importance. Sediment management in reservoirs is no longer a problem to be put

off until the future; it has become a contemporary problem.

Traditional approaches to sediment management have not considered the need for

sustained use. Large initial storage volumes and erosion control have traditionally been

recommended to reduce sediment inflow and delay the eventual "death" of reservoirs, but

erosion control alone cannot achieve the sediment balance required to stabilize reservoir

storage capacity and achieve sustainable use. Furthermore, many erosion control

programs are poorly conceived and implemented, and fail to achieve the desired

reductions in sediment yield. As a result, reservoirs worldwide are losing storage capacity

rapidly, possibly as fast as 1 percent per year (Mahmood, 1987). Reservoir construction

requires sites having unique hydrologic, geologic, topographic, and geographic

characteristics, and existing reservoirs generally occupy the best available sites. Because

of the high cost and multiple problems associated with sediment removal and disposal on

a massive scale, the sedimentation of large reservoirs is to a large extent an irreversible

process. If future generations are to benefit from essentia1 services provided by

reservoirs it will be largely through the preservation and continued utilization of existing

reservoir sites, not the continued exploitation of a shrinking inventory of potential new

sites. The water supplies and other benefits derived from reservoirs do not constitute

renewable resources unless sedimentation is controlled.

1.2 ELEMENTS OF SEDIMENT MANAGEMENT

Conversion of sedimenting reservoirs into sustainable resources which generate long-

term benefits requires fundamental changes in the way they are designed and operated. It

requires that the concept of a reservoir life limited by sedimentation be replaced by a

concept of managing both water and sediment to sustain reservoir function. Sustainable

use is achieved by applying the following basic sediment control strategies:

1. Reduce sediment inflow. Sediment delivery to the reservoir can be reduced by

techniques such as erosion control and upstream sediment trapping.

2. Route sediments. Some or al1 of the inflowing sediment load may be hydraulically

routed beyond the storage pool by techniques such as drawdown during sediment

laden floods, off-stream reservoirs, sediment bypass, and venting of turbid density

currents.

3. Sediment removal. Deposited sediments may be periodically removed by hydraulic

flushing, hydraulic dredging, or dry excavation.

4. Provide large storage volume. Reservoir benefits may be considered sustainable if

a storage volume is provided that exceeds the volume of the sediment supply in the

tributary watershed. The required sediment storage volume may be included within

the reservoir pool or in one or more upstream impoundments.

5. Sediment placement. Focus sediment deposition in areas where it’s subsequent

removal is facilitated, or where it minimizes interference with reservoir operation.

Configure intakes and other facilities to minimize interference from transported or

deposited sediments.

The cost and applicability of each strategy will vary from one site to another and also as a

function of sediment accumulation. However, even the largest reservoirs will eventually

INTRODUCTION 1.5

be reduced to small reservoirs by sedimentation and, sooner or later, will require

sediment management.

1.3 HANDBOOK APPROACH

This handbook seeks to generate an awareness of sedimentation problems, outlining

practical strategies for their identification, analysis and management. Basic concepts and

tools are presented which, when applied in an integrated manner, can achieve what we

will term sustainable sediment management in reservoirs. Sedimentation is the single

process that all reservoirs worldwide share in common, to differing degrees, and the

management strategies and techniques presented herein are applicable to reservoirs of all

ages, types, and sizes. An understanding of these principles will also aid in the effective

design and management of sediment-trapping structures such as debris basins and

detention ponds. Although complex mathematical and physical modeling studies may

often be required to finalize the design and operational procedures for sediment

management, this handbook makes no attempt to provide an in-depth treatment of these

analytical methods or sediment transport theory. Extensive treatments of these topics are

already available by others (Yang, 1996; Simons and Senturk, 1992; Julien, 1995; Chang,

1988; Vanoni, 1975; Graf, 1971). Rather, this handbook presents a broader view of basic

principles and methods essential for the conceptualization and assessment of

sedimentation issues associated with reservoirs, and for identifying management

strategies to be explored in subsequent detailed and site-specific studies.

Sedimentation problems and management techniques vary widely from one site to

another, and by studying specific sites one can appreciate the complexity of sediment

problems and the manner in which they can be addressed. Seven chapters have been

devoted to detailed case studies. These cases have been selected to demonstrate a variety

of management techniques and cover a wide range of geographic and hydrologic

conditions. Case study reservoirs range in size from 2 to 1700 Mm

(2 to 1700 × l0

Two sites are located in the United States, two in China, and one each in Costa Rica,

Switzerland, and Iran.

China has 82,000 reservoirs which are losing storage capacity at an average annual

rate of 2.3 percent, the highest rate of loss of any country in the world (Zhou, 1993).

China's Yellow River has the highest sediment concentrations of al1 the world's major

river systems, and China also has half of the world's large dams. Not surprisingly, China

has considerable experience in the management of reservoir sedimentation. This

handbook has made a particular effort to distill useful lessons from the Chinese

experience.

Finally, this handbook introduces the overall concept of sustainable sediment

management with the goal of converting today's sedimenting reservoirs into resources

that will benefit future generations as well as our own. Whereas the twentieth century

focused on the construction of new dams, the twenty-first century will necessarily focus

on combating sedimentation to extend the life of existing infrastructure. This task will be

greatly facilitated if we start today.

CHAPTER 2

RESERVOIRS AND SUSTAINABLE

DEVELOPMENT

2.1 WATER SUPPLY AND WATER SCARCITY

2.1.1 Global Water Resources

Natural lakes and rivers worldwide contain about 93,000 km

of fresh water, about

0.27 percent of the earth's total fresh water resource (Table 2.1). While the absolute

volume of groundwater storage is large, it is much less accessible than surface waters

because most is located too deep or in formations of insufficient permeability for

economic exploitation, the rate of replenishment is much slower than surface water, and

extraction from coastal areas is limited by saline intrusion. Thus, groundwater is much

less important than surface supplies. For example, total groundwater availability in 21

Arab countries is estimated at only 14.8 percent of total streamflow (Shahin, 1989).

Groundwater recharge in India has been variously estimated at 15 and 25 percent of total

runoff, and in the United States and Russia respectively groundwater use accounts for 20

and 9.3 percent of total withdrawals (Gleick, 1993). In the long run, withdrawals from

either surface or groundwater sources cannot exceed runoff, defined as rainfall minus

evapotranspiration, except to the extent that water is reused. However, reuse potential is

not large because the most voluminous water use, irrigation, is consumptive.

Water storage in natural lakes (91,000 km

) plus reservoirs (7000 km

) is slightly

less than the annual global precipitation of 119,000 km

over land areas, and about twice

the annual runoff into the ocean of 47,000 m

, a figure which includes surface water,

groundwater, and glacial discharges. The remaining 41 percent of global precipitation is

returned to the atmosphere as evapotranspiration. Runoff is unevenly distributed both

TABLE 2.1 World Water Resources

Relative amount of

Volume, 10

Total water, % Fresh water, %

Saline water: 1,350,939.4 97.47

Oceans 1,338,000.0 96.54

Lakes 85.4 0.01

Groundwater 12,854.0 0.93

Fresh water: 35,028.6 2.53 100.00

Antarctic ice 21,600.0 1.56 61.66

Other ice and permafrost 2,764.0 0.20 7.89

Groundwater 10,546.0 0.76 30.11

Lakes 91.0 0.01 0.26

Swamps 11.5 0.00 0.03

Atmosphere 12.9 0.00 0.04

Rivers 2.1 0.00 0.01

Saline water: 1,350,939.4 97.47

Oceans 1,338,000.0 96.54

RESERVOIRS AND SUSTAINABLE DEVELOPMENT 2.2

geographically and in time. While the Amazon basin accounts for 15 percent of global

runoff, Australia, with a slightly larger land area, accounts for only 0.7 percent

(Shiklomanov, 1993). Seasonal variation is particularly evident in areas such as India,

projected to be the world's most populated country by year 2050, where the monsoon

season lasts 3 months or less and the remainder of the year is essentially dry.

2.1.2 Water Scarcity

The amount of fresh water circulating through the hydrologic cycle is fixed, but the

demand for water use has grown rapidly during the twentieth century and fresh water is

becoming an increasingly scarce resource. Modern society is, above all else, a hydraulic

society. Population and economic activity are increasingly tied to the diversion of fresh

water to human use, particularly as agricultural expansion becomes increasingly

dependent on irrigation. Water withdrawals in the United States grew more rapidly than

population through the twentieth century, but withdrawals have slowed since the 1970s,

reflecting the increasing efficiency of water use because of scarcity (Fig. 2.1). Recog-

FIGURE 2.1 Water withdrawals in the United States over time, as compared

to population (data from USGS Water Summary and Census of Population).

RESERVOIRS AND SUSTAINABLE DEVELOPMENT 2.3

nition that there is inadequate water to pollute freely has led to increasing levels of

wastewater treatment, greater efficiency in use, and recycling in the industrial sector.

Scarcity has also led to increasing irrigation efficiency in the agricultural sector, and new

plumbing codes are gradually taking the United States from 20-L (5-gal) to 6-L (1.6-gal)

flush toilets. Municipal water rationing has been imposed not only in dry areas such as

southern California but also in humid areas ranging from Florida to New York.

Increasing efficiency of use, combined with low rates of population growth, are also

poultry, 125 kg of

ploys

ds. Reservoirs are necessary to divert

ABLE 2.2 Projected Global Po

Population, millions by year

ge erally characteristic of highly industrialized nations in Europe.

The patterns of increasing water scarcity evident in the United States are also

occurring on a global scale, but in many regions the scarcity is much more acute. Most of

the world's population growth is occurring in less developed countries, especially in areas

of low rainfall with high and increasing dependence on irrigation, which greatly increases

per capita water requirements since food and fiber must be produced from runoff diverted

to agricultural use rather than natural precipitation on rain-fed cropland. Thus, in Sweden

it is possible to maintain one of the world's highest standards of living with water

withdrawals of only 620 m

/capita/yr, whereas in irrigation-dependent Arab countries

withdrawals of 1205 m

/capita/yr are needed to produce an individual's annual

requirement of 375 kg of fruits and vegetables, 35 kg of meats and

cereals, plus 55 m

for domestic use (Falkenmark and Lindh, 1993).

Total runoff can be used to give a crude, and admittedly optimistic, estimate of the

total volume of water that could be made available if water supplies are fully developed

for human use, leaving little if any water to sustain natural aquatic or wetland

ecosystems. Engleman and LeRoy (1995) used a per capita runoff of 1700 m

/yr as a

benczmark for "water stress" and 1000 m

/yr as a benchmark for "water scarcity," and

combined national data on total runoff with the 1994 United Nations medium population

projection by country (summarized in Table 2.2 by continent) to compute runoff per

capita. This was compared to the benchmark values to examine trends in water

availability. By year 2050 some 58 nations containing nearly 4.4 billion people will be

experiencing either water stress or scarcity, as compared to a population of less than 0.4

billion so classified in 1990 (Fig. 2.2). Population and water availability for several

countries are compared in Table 2.3, where it can be seen that the United States, despite

large deserts, as a whole is a water-rich nation. Israel, with only 461 m

/capita, em

the world's most technologically advanced and efficient water management system.

The continuously sustainable or firm yield from unregulated stream diversions

generally ranges from zero to 20 percent of the mean annual discharge, with the lower

values applicable to more and and smaller watershe

T pulation Growth

Continent 1950 1990 2020 2050

annual %

Growth rate

990-2050,

Europe 549 722 723 678 - 0.1

North America 1 3 3

1 4, 5,

merica

orld Total 2,521 5,286 7,889 9,834 1.0

66 278 58 89 0.6

Ocean 13 27 40 46 0.9

Asia ,403 3,186 744 741 1.0

Latin A 166 440 676 839 1.1

Africa 224 633 1,348 2,141 2.1

RESERVOIRS AND SUSTAINABLE DEVELOPMENT 2.4

FIGURE 2.2 Current and projected world population, showing the population in nations having

t levels of runoff per capita (after Engleman and LeRoy, 1994)

differen

TABLE 2.3 R

unoff per Capita for Selected Cou

ntries

Population illions cap

Runoff per ita, m

Country

depth, mm

1990 2050

1990

Runoff

2050

Israel 4 4.7 8.9 461

241

Algeria

850.6 1639.9

can Republic

1606.0 2,424

ingdom

y 561

former

U.S.S.R.)

B .5 264.3

46,809

26,291

24.9 55.7

690

309

Egypt 59

56.3

117.4 1,046 502

Ethiopia 98

47.4

194.2 2,320 566

Moroc 573

4.3 47.9

1,151 585

Haiti 396 6.5

18.6 1,696

593

Nigeria 333 96.2 338.5 3,203 910

Pakista 582 121.9 381.5 3,838 1,227

India 634 2,451 1,271

Domini 410 7.1

13.2 2,813

1,519

China 293

55.3

1,743

Turkey 260 56.1 106.3 3,619 1,910

United K 492 57.4

61.6 2,090

1,947

Mexico 182 84.5 161.5 4,224 2,211

France 338 56.7

60.5 3,262

3,059

Germ 79.4

64.2 2,520

3,113

Iran 72 58.9 163.1 4,428 4,972

Japan 448 123.5 110.0 4,428 4,972

United States 253 249.9 349.0 9,915 7,101

Russia ( 248 280.4 318.9 19,493 17,142

razil 816 148

Source: Adapted from Engleman and Leroy (1994).

RESERVOIRS AND SUSTAINABLE DEVELOPMENT 2.5

a greater percentage of the surface runoff to human use on a continuous basis by

equalizing irregular stream flow; water is stored during periods of high flow and

delivered during periods of low flow. As water demand increases relative to runoff the

reservoir storage volume available for flow equalization must increase, and as demand

centers grow it also becomes necessary to develop increasingly distant water sources.

Because the most easily developed resources are used first, each additional cubic meter of

sustained water yield typically requires more effort to extract than the previous one

because of the law of diminishing returns. The cost of each new project; the marginal cost

of water supply, tends to increase over time. Biswas (1991) states that the unit cost of

water from the next generation of domestic water supply projects is often 2 or 3 times

higher than the present generation; nearly all the easily available sources of water have

already been or are now under development. With many of the best irrigable areas

already in use, the real cost of new irrigation schemes in Asia has also doubled or tripled

between the mid-1950s and mid-1980s (Table 2.4).

In summary, there is a clear trend toward increasing water scarcity relative to its tra-

ditional uses. Reservoir sedimentation will exacerbate this scarcity by reducing the yield

from already-developed supplies.

Morris &amp; Fan. Reservoir Sedimentation Handbook

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Morris & Fan. Reservoir Sedimentation Handbook