Morris & Fan. Reservoir Sedimentation Handbook
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CHAPTER 12
REDUCTION OF SEDIMENT YIELD
12.1 INTRODUCTION
There are only two strategies to reduce the sediment yield entering a reservoir: either
prevent erosion or trap eroded sediment before it reaches the impoundment. This chapter
outlines basic principles of erosion control and sediment trapping.
The rehabilitation of degraded watersheds can dramatically reduce the rate at which
sediment, nutrients, and other contaminants are delivered to a reservoir. There is an
extensive literature on watershed management and erosion control. This chapter focuses
on erosion control, whereas the term watershed management encompasses a wide variety
of measures with objectives such as enhancing water quality, reducing flooding, and
improving wetland habitat. Watershed management programs may include both point and
nonpoint sources, such as New York City's $750 million program announced in 1993 to
improve water quality in the city's drinking water reservoirs, with the objective of
avoiding the much higher cost of constructing water filtration plants under the EPA's
Surface Water Treatment Rule. New York's watershed management program will include
the implementation of stricter watershed regulations, land purchases, upgrading of
sewage treatment plants, and agricultural pollution management (AWWA, 1995).
12.1.1 Applicability
Programs to prevent and correct erosion problems are invariably recommended to limit
reservoir sedimentation and enhance water quality, and at most reservoirs this is the only
measure recommended. However, many erosion control programs have been spectacularly
unsuccessful, despite large expenditures, because they were improperly planned and
executed, and lacked the long-term support and commitment of the land users. Many reservoirs
designed under the assumption that sediment yield will be controlled by upstream erosion
control practices have not only seen that promise unfulfilled, but have seen sediment yield
accelerate as population density and land degradation increased in the watershed.
In many developing areas, the progressive deforestation and cultivation of steep
slopes is the only survival option for the rural poor, and about 10 percent of the world's
population cultivates steeply sloping soils. These slopes may be worked by hand or by
animals such as oxen which can plow extremely steep hillsides. Where there are marked
wet and dry seasons, soil plowing may be done prior to the arrival of rains, thereby
maximizing the potential for erosion (Fig. 12.1). Even when conservation measures are
employed on steep soils, there can still be a lot of erosion.
REDUCTION OF SEDIMENT YIELD 12.2
FIGURE 12.1 Recently ploughed sloping soils in the Dominican Republic, prior to the
onset of the wet season (G. Morris).
It is both costly and difficult to implement improved land use practices over
watersheds of any size, and particularly in watersheds covering thousands of square
kilometers with multiple political jurisdictions and with tens of thousands of land users.
Nevertheless, land use improvement to reduce erosion and sediment delivery is often the
best method, and perhaps the only method, that is feasible for combating sedimentation at
many reservoirs, especially at hydrologically large reservoirs which are continuously
impounding.
Damage to reservoirs by sedimentation represents only a small part of the cost of
erosion. Estimates by Clark et al. (1985) suggest that loss of storage capacity in reservoirs
accounts for only 11 percent of the total cost of erosion in the United States (Table
12.1). This table underestimates the cost of reservoir sedimentation because it assumes
that existing reservoir capacity can be replaced at the same cost per unit volume used
for new reservoir construction, which is obviously not the case, especially for high-cost
operations such as dredging. However, it does point out that many users can be severely
affected by erosion, and the control of reservoir sedimentation is often not the principal
reason for implementing soil conservation measures. Measures to reduce erosion and
sediment yield typically benefit many parties in addition to dam operators, and a dam
owner attempting to combat erosion in the watershed may have many potentially helpful
alliances.
12.1.2 Limitations
Even when successful, erosion control does not necessarily represent a complete solution
to sedimentation problems. Some environments have naturally high rates of sediment
yield. Erosion control requires a long-term commitment, and produces long-term rather
REDUCTION OF SEDIMENT YIELD 12.3
TABLE 12.1 Summary of Damages Due to Soil Erosion in the United States (Millions of
1980 Dollars)
Type of Impact Single-value estimate Cropland's contribution
Instream effects:
Biological impacts
Not estimated
Recreational 2000 830
Water storage facilities 690 220
Navigation 560 180
Other instream uses 900 320
Subtotal 4200 1600
Offstream effects:
Increased flood damages 770 250
Water conveyance facilities 200 100
Water treatment facilities 100 30
Other off-stream uses 800 280
Subtotal 1900 660
Total 6100 2200
Source: Clark et al (1985).
than short-term results. Land use changes which reduce erosion may not produce a
short-term reduction in downstream sediment yield. For instance, Faulkner and McIntyre
(1996) did not detect any reduction in sediment yield from the 1150 km
2
Buffalo River in
Wisconsin, 20 years after a transition to less-erosive land use within the basin. Finally,
because erosion can never be reduced to zero, erosion control alone cannot achieve a
sediment balance across a reservoir. Additional measures will eventually be required to
remove sediment if the reservoir is to remain in operation, although erosion control can
greatly delay this requirement.
12.1.3 Additional Sources of Information
Moldenhauer and Hudson (1988) and Morgan (1995) provide good overviews of erosion
control issues and strategies in less developed areas. Schwab et al. (1993) is a basic
agricultural engineering text on erosion control techniques which focuses on structural
measures used in the United States. Sediment control in the urban environment is covered
by Goldman et al. (1986), Urbonas and Stahre (1993), and Haan et al. (1994).
Publications and extension support are available from the Natural Resources
Conservation Service within the U.S. Department of Agriculture, which has local offices
nationwide (Internet: www
.nrcs.usda.gov). Internet access to publications from the U.S.
Environmental Protection Agency can be obtained at www
.epa.gov.
A CD-ROM containing an extensive description of Best Management Practices
(BMPs) and a database on water utilities which have implemented watershed protection
programs is available through the American Water Works Association (6666 W. Quincy
Ave., Denver, CO 80235, Internet: www
.awwa.org). An electronic listing of 800
watershed management programs and contacts throughout the United States, plus
information on BMPs, is available from the Conservation Technology Information Center
(address below). Several organizations with publications, periodicals, and software
related to soil conservation are:
REDUCTION OF SEDIMENT YIELD 12.4
The Soil and Water Conservation Association
7515 NE Ankeny Rd., Ankeny, IA 50021-9764 Internet: www
.swcs.org.
Focus: Agricultural soil conservation, United States and worldwide
North American Lake Management Society
One Progress Blvd., Box 27, Alachua, FL 32615 Internet: www
.nalms.org
Focus: Water quality issues in lakes and reservoirs
The International Erosion Control Association
P.O. Box 774904, Streamboat Springs, CO 80477-4904 Internet: www
.ieca.org
Focus: Urban areas and construction industry
Conservation Technology Information Center
1220 Potter Dr., Room 170, West Lafayette, IN 47906-1383 Internet:
www
.ctic.purdue.edu
Focus: Mechanized agriculture in the United States
Metropolitan Washington Council of Governments
777 North Capitol St. NE, Suite 300, Washington, DC 20002-4201 Internet:
www
.mwcog.org
Focus: Urban best management practices
12.2 BASIC TECHNICAL PRINCIPLES
12.2.1 Technical Strategies for Erosion Control
Literally hundreds of specific structural and nonstructural management practices can be
used to reduce sediment yield. In the United States, recommended control practices are
termed Best Management Practices (BMPs). Many additional practices have been
developed and applied successfully in other countries, including indigenous terrace
systems in some parts of Asia and in the Andes which have been in continuous use for
over 1000 years. The wide variety of techniques employed worldwide to reduce soil
erosion and sediment yield may be synthesized into 10 basic strategies:
1. Fit the activity to the soil, climate, and terrain. Human activities ill-adapted to
the local soils and climate can produce erosion on a massive scale. In agriculture, avoid
the most erodible areas, and apply agronomic systems and land husbandry techniques
that are suited to the local soils and climate. In logging, avoid road construction on
highly erodible slopes, and select logging methods suitable for the slopes involved. In
urban construction, adjust development to the terrain by minimizing cut and fill. Erosion
control measures must also be matched to local conditions; some erosion control
techniques that work well in one region may increase erosion under different soil and
climatic conditions.
2. Minimize the area and duration of soil disturbance. The greater the soil area
that is disturbed, the greater the rate of erosion. In the agricultural sector, conservation
tillage can be used to minimize soil disturbance. In logging, different yarding methods
can produce a ten-fold difference in the soil area disturbed. On construction sites the
areal extent and length of time that disturbed soils are exposed to erosive rains can be
limited by proper scheduling of the work.
3. Protect denuded soils. Denuded soils are highly susceptible to erosion by
direct raindrop impact and rilling by prechannel flows. Protection may be provided to
agricultural soils by mulching with crop residue and related conservation tillage
REDUCTION OF SEDIMENT YIELD 12.5
techniques. Skid trails in forests can be covered with slash. Temporary protection at
construction sites can be provided by either natural or synthetic mulches, and permanent
erosion protection is provided by vegetation or impervious cover such as concrete and
asphalt.
4. Maximize vegetative cover. A good vegetative cover provides the best, least-
cost, long-term protection against erosion by insulating the soil against direct raindrop
impact, enhancing infiltration, and other methods. Even an incomplete cover of
vegetation or mulch can produce a dramatic reduction in the erosion rate (Fig. 12.2).
FIGURE 12.2 Reduction in erosion rate compared to bare soil, as a function of vegetative cover,
for: (a) ground-level vegetation, (b) vegetative canopy 1 m above ground level, and (c) oat straw
mulch. Note that a large reduction in erosion can be achieved prior to full coverage (redrawn from
Styczen and Morgan, 1995).
For
all types of land use, the rapid reestablishment of vegetative cover following
disturbance is essential to reducing erosion. The principal erosion control measure on
rangeland is to manage stocking levels and grazing patterns to maintain adequate
vegetative cover, especially in riparian areas.
5. Maximize infiltration. Rainfall which infiltrates the soil does not appear as
erosive surface runoff, and is also available to support plant growth. Measures which
retard runoff by adjusting slopes (contouring, terraces) and by improving soil structure
and permeability (maintenance of organic matter, maintenance of vegetative cover, use
of mulches) all improve infiltration capacity. Maintenance of infiltration capacity is a
critical measure on agricultural soils.
REDUCTION OF SEDIMENT YIELD 12.6
6. Manage slopes to prevent flow concentration. Channel erosion processes are
initiated and sustained by concentrated flows on sloping soils. Erosion control techniques
such as flow diversion, terracing, and cross drains on roads may be used to divert runoff
away from erosive slopes and to counteract the natural tendency for runoff to become
concentrated and cause channel erosion (Fig. 12.3). Erosion potential is also reduced by
limiting slope length and steepness.
7. Prepare drainageways to handle concentrated flows. Concentrated flows are
highly erosive, and adequate protection is required where runoff becomes concentrated
into channels. Use a low channel slope to reduce erosion hazard, and provide channel
protection in accordance with the channel slope, geometry, and discharge. Bed slope may
be controlled by orienting diversion channels across the slope, and by using check dams
or drop structures for downslope channels. Commonly used channel protection materials
include grass, geosynthetic mattings, stone, and concrete.
8. Trap sediment before it leaves the site. Use every opportunity to trap sediment
in runoff from disturbed areas. Broad filter strips of grass or woodland areas at the limits
of plowed fields, logging areas, and construction sites can infiltrate runoff, retard flow
velocity, and trap sediment from prechannel flows. Vegetated stiff grass hedges and silt
fences retard flow so that sediment will settle. Sediment transported by channel flow can
be trapped using sediment detention ponds.
9. Protect and preserve vegetation in natural riparian buffers. Sediment
delivery to stream channels by sheet flow can be reduced by maintaining a broad strip of
riparian vegetation, either grass or forest, between areas of disturbed soil and stream
FIGURE 12.3 Concentrated runoff released downslope produces rill and channel erosion. Avoid
this type of erosion by diverting flows away from unprotected slopes and providing protection
where the flow is released downslope (G. Morris).
REDUCTION OF SEDIMENT YIELD 12.7
channels. Avoid activities that will disturb the stability of natural stream channel banks
and the adjacent riparian zone.
10. Plan, monitor, and maintain control measures. Develop soil and water
conservation plans to facilitate implementation within the requirements of each land use
and site layout. Monitoring is necessary to schedule maintenance, and to identify which
strategies work and which do not. Periodic inspections are required to assure that control
measures are properly installed and maintained.
In the preparation of any conservation plan it is useful to review these 10 issues and ask
how satisfactorily each has been addressed.
12.2.2 Classification of Erosion Control Techniques
Techniques to reduce erosion and sediment yield may be broadly classified into three
categories.
Structural or mechanical measures control the movement of water over the earth's
surface to reduce the flow velocity, increase the storage of surface water, and safely
dispose of runoff (Morgan, 1995). Included in this category are: (1) all types of structural
terraces; (2) diversion channels, grassed waterways, and other flow conveyance
structures; (3) channel protection and stabilization measures such as riprap, gabions,
and check dams; and (4) sediment traps including debris basins, detention basins, and
reservoirs. Structural measures are characterized by high construction cost and the
requirement for maintenance over an indefinite period. Structural linings are required to
protect channels when the flow velocity (bed shear stress) exceeds critical thresholds.
Vegetative or agronomic measures rely on the natural regenerative properties of
vegetation or the management of crop and crop residue (mulch) to protect the soil. Contour
vegetated hedges can also form natural terraces. Vegetation is inexpensive and self-
renewing, and its use can minimize or virtually eliminate long-term maintenance
concerns. However, a significant effort may be required for the initial establishment of
vegetation, particularly on severely degraded sites and in semiarid areas. Vegetation
alone may be incapable of controlling erosion by concentrated flows.
Operational measures are management and scheduling measures employed to
minimize erosion potential. This includes strategies such as scheduling construction to
minimize the area of exposed soil and scheduling timber harvest activities to avoid
periods of excessive soil moisture. The focus of operational measures is inherently
preventive, seeking to avoid erosion and minimize the requirements for either vegetative
or structural remediation. Vegetative and operational measures are both classified as
nonstructural controls.
12.2.3 Types of Sediment Trapping Structures
Hydraulic structures which trap sediment may be broadly classified in the following
categories:
A check dam is a small grade control structure designed to trap bed load only and
used to prevent bed degradation and arrest gully erosion. Check dams normally do
not exceed about 2 m in height. They reduce stream slope to reduce the flow velocity
and channel erosion. Coarse sediment is trapped above each dam as the bed adjusts to
the new grade established by multiple check dams installed along the watercourse.
REDUCTION OF SEDIMENT YIELD 12.8
A debris basin is used to trap coarse sediment before it enters a downstream channel,
the performance of which would be adversely affected by sedimentation (Vanoni,
1975). They may also be used to trap hyperconcentrated debris flows (Mizuyama,
1993). Unlike a sediment detention basin, which is designed to maximize sediment
retention, a debris basin is designed to trap only the larger-diameter sediment that
will not pass through the downstream system as wash load, or the massive amounts
of sediment associated with debris flows. They are often sized to trap the sediment
transported by a single large design event. Provision is made for periodic sediment
removal to maintain sediment trapping volume.
A sediment detention basin is designed to trap suspended sediment for water quality
control, protecting downstream aquatic environments and structures from both
suspended and bed material load, and the associated pollutants. They are occasionally
used in rural areas, and are widely used for controlling sediment in runoff from urban
areas and construction sites. Sediment detention basins can also provide additional
benefits such as reduction in peak discharge and the creation of wetland habitat.
Most reservoirs are presently operated as efficient sediment traps, although sediment
trapping is not the intended purpose for their construction, and in most cases is
considered objectionable. Reservoirs are normally much larger than detention basins.
In some cases reservoir operation may be modified in the future to enhance the
downstream release of sediment.
Systems to remove coarse sediment from a water diversion, such as for hydropower
production, are discussed under sediment excluders (Chap. 13).
12.2.4 Sediment Trapping in Upstream Reservoirs
Although constructed for other purposes, upstream reservoirs of all sizes act as de facto
sediment traps, and downstream sites will experience greatly reduced sediment loading.
There are, for instance, 39 major structures along the Paraná River and its tributaries in Brazil,
above the Yacyretá dam below Posadas, Argentina. These upstream dams, including Itaipú,
currently the world's largest hydropower site, are responsible for dramatically decreased
sediment yields at the Yacyretá Dam site. What is less commonly recognized is that smaller
structures can also have a significant impact on sediment yield when constructed in large
numbers across the watershed. The National Inventory of Dams for the United States lists
over 74,000 reservoirs having over 60,000 m
3
of storage. However, the U.S. Natural Resource
Conservation Service estimates that there are 2 million farm ponds nationwide (see Table
2.5). Large numbers of smaller reservoirs and ponds have been constructed in many other
countries as well.
Sediment trapping by upstream dams is the most important factor controlling
sedimentation in many reservoirs. There are, however, two potential disadvantages to this
strategy as a long-term protection measure. First, the sediment-retention capacity of
upstream reservoirs may be limited. Second, owners of upstream sites may alter their
operation to pass sediment downstream.
12.2.5 Sediment Trapping versus Erosion Control
Under favorable conditions, sediment trapping can be a highly effective method for
reducing sediment yield. An engineered structure can provide reliable and predictable
sediment trapping as soon as it begins operation, eliminating the uncertainties
associated with implementation of an erosion control program. However, there are a
number of disadvantages to sediment trapping.
REDUCTION OF SEDIMENT YIELD 12.9
High cost. All structural measures are very costly to construct. The lower cost of
small check dams is offset by the large number of structures that is typically required.
Larger sediment detention structures must be properly engineered from the
standpoint of dam safety, spillway capacity, and long-term operation. Structures
of all sizes will eventually fail without maintenance. Because the cost per unit of
storage volume tends to decline as a function of dam height, to trap a given volume
of sediment, a few larger structures will be more cost-effective than many smaller
structures. The least costly measure might be to enlarge the reservoir itself.
Siting. With the exception of check dams, which are located within the gully
floor, a sediment detention pool will occupy a significant amount of land area. Sites
appropriate for a sediment detention structure may be difficult to locate and costly
to acquire.
Sustainability. For detention to represent a sustainable long-term sediment control
strategy, one of two criteria must be satisfied: (1) sediment must be removed from the
detention basin or (2) the sediment retention structure must continue to hold the trapped
sediment for an indefinite period. If sediment is to be removed periodically, scale
economies could make it less costly to remove sediment from the reservoir itself
rather than to construct and operate upstream detention basins. The long-term integrity
of sediment detention structures is a critical issue, because when a structure fails the
trapped sediment will be exposed to erosive forces and may be released. For
example, Gellis et al. (1995) examined detention structures and check dams constructed
in the 1930s on the semiarid Zuni Reservation in New Mexico. Of the 23 rock and brush
check dams investigated along 800 m of a single arroyo, 22 had failed. Of the 47 rock
or earthen detention structures located within a 172-km
2
quadrant, 60 percent had failed
by embankment breaching or erosion around the end of the embankment. This
situation is not unusual; the western United States is littered with failed detention
structures and check dams.
Limited benefits. Accelerated erosion is caused by unsustainable land use
practices, and reservoirs are only one of the many uses affected by erosion. Soil and
water conservation measures within a watershed can have many benefits beyond the
protection of reservoir storage, and many different interests and institutional
resources may be enlisted to combat watershed degradation. Conversely, sediment
detention structures will benefit only certain downstream users, but not the
upstream lands where the erosion problem originates. Economic resources
assigned to sediment detention structures will not be available to help cure the
underlying problems in the watershed, which may generate a wider range of long-
term benefits to the society.
Sediment detention structures are commonly used at construction sites, and for water quality
enhancement and flood detention in urban areas. However, they are infrequently used
specifically for the control of reservoir sedimentation because of the disadvantages
mentioned above.
12.3 FORMULATING AN EROSION CONTROL STRATEGY
12.3.1 What Causes Erosion?
Accelerated erosion is the earth's natural and predictable response to inappropriate land
use and poor management practices. Nobody sets out with the intent of accelerating erosion,
yet its occurrence is widespread. While many highly effective erosion control techniques can
be readily demonstrated on test plots or in small, closely controlled watersheds, it is quite
another matter to convince a community of land users to implement these practices on a
sustained basis.
REDUCTION OF SEDIMENT YIELD 12.10
Implementation can be highly problematic in less developed areas, where a tributary
watershed containing several thousand square kilometers of sloping and highly erodible
soils may be cultivated and grazed by tens of thousands of subsistence farmers. Most
subsistence farmers use wood for fuel, which also places woodlands under continuous
harvest pressure (see Fig. 7.10). About 90 percent of the population in Africa uses
fuelwood for cooking (Anderson, 1987). Factors that make erosion so difficult to control
include the large number of people and activities contributing to the problem and, in many
areas, the lack of consistent or effective government policy oriented toward soil and water
conservation. Napier (1990) pointed out that the implementation of soil conservation
measures over a large area is very expensive, and it is unrealistic to believe that effective soil
conservation programs can be implemented in any society without a permanent, adequately
funded service infrastructure staffed by trained professionals committed to soil conservation
goals. A good overview of the socioeconomic barriers to the adaption of soil conservation
measures by farmers of all income levels and on every continent is provided by Napier et
al. (1994).
12.3.2 Identifying and Prioritizing Sediment Sources
As pointed out in Chap. 7, sediment yield varies widely across watersheds of all sizes,
and most sediment will typically be contributed by a relatively small portion of the total
tributary area. To focus erosion control efforts, it is necessary to identify the geographic
areas and specific land use practices that are the major sediment contributors. In
developed areas, considerable information may already have been collected for the
project watershed, or for similar watersheds. In contrast, in less developed areas there
may be virtually no formal information on conditions in the watershed, no useful water
quality records, and incomplete mapping and photographic coverage.
Sediment sources can be characterized by modeling and sampling techniques
discussed in Chaps. 6 and 7. The importance of sampling is emphasized because of the
difficulty in converting erosion estimates to actual sediment yield, given the difficulty in
determining sediment delivery ratios. Even when formal analysis is not performed, a
screening analysis can differentiate between areas that produce little sediment and those
that produce large amounts of sediment. Lands of low or moderate slope with good
vegetative cover and an absence of gullying characteristically have low sediment yield.
Disturbed soils, especially those on high slopes or which deliver sediment directly to
stream channels, can be expected to have high sediment yield. Many types of sediment
sources should be obvious even to untrained observers. However, even low rates of
sheet erosion over a wide area can generate large amounts of sediment. An erosion depth
of only 1 mm across 1 ha represents 10 m
3
of annual soil loss, or about 16 tons of sediment.
Use particular caution in the interpretation of channel erosion. Incising channels with
expanding cross-sectional area can produce large volumes of sediment. However,
channel erosion by rivers having a stable cross-sectional area while meandering across a
floodplain will produce little net export of sediment downstream, and the river-
floodplain system is usually a zone of net sediment deposition.
Identify specific land use practices that are important contributors to erosion. On farms,
certain practices or selected crops may be much more important as sediment sources than
others. On construction sites, problems such as the lack of adequate enforcement may be
the principal problem (Fig. 12.4). In forests, high erosion rates may be associated with
specific types of yarding methods or specific aspects of logging road design.
In assessing erosional impacts on a reservoir, examine trends over periods of many
years or decades, and focus erosion control efforts in areas where the greatest benefits
can be anticipated within this time frame. For instance, in an area which is already