Morris & Fan. Reservoir Sedimentation Handbook
Подождите немного. Документ загружается.
REDUCTION OF SEDIMENT YIELD 12.11
severely degraded, future sediment yield may be limited by the supply of erodible soils.
Previously eroded material may already have collected in intermediate storage points in
the watershed, from which it will be delivered to the reservoir over the coming years or
decades, regardless of the improvements made in the upstream watershed. At the same
time, there may be another area of the watershed where destructive land uses are only
starting. It may make more sense to focus limited control efforts on stabilizing the area
of the watershed where soils have not yet suffered severe degradation.
FIGURE 12.4 Eroding soil at a construction site in the watershed tributary to Loíza Reservoir.
In addition to obvious sheet and rill erosion on bare slo
p
es, the stream passing through the site
has also been affected by earth movement. Although Puerto Rico has regulations requiring
erosion and sediment control at construction sites, enforcement is lacking (G. Morris).
Identify the land users who will need to change their actions. Does this group consist
of a dozen land developers in an urbanizing area, or a population of a million supported
by subsistence agriculture on steep slopes?
Erosion control is not primarily a "technical" problem, and factors limiting the
implementation of erosion control measures need to be identified. Cook (1988) identified
three types of factors that can contribute to erosion problems – informational, institutional
and technological – all of which must be tackled simultaneously for successful implement-
tation. Informational constraints may include inadequate extension services and non-
awareness of appropriate technology. Institutional problems may include lack of
enforcement; perverse legal, regulatory, and policy environments; and a wide range of
subsidy programs that promote resource exploitation and degradation. Technological
problems may include the lack of an erosion control package which is both technically
effective and economically attractive to land users.
12.3.3 Identification of Partners
Having characterized the problem of erosion and sediment yield from the standpoint of
reservoir operation, look for partners with whom to join forces to achieve improved
management within the watershed. A dam owner attempting to reduce sediment yield
from a watershed may join forces with governmental and nongovernmental institutions,
individuals, and grass roots organizations, who can in turn provide training, install
control measures, pass regulations, perform enforcement actions, and access a variety of
sources for cash and in-kind contributions to the erosion control effort. In the United
REDUCTION OF SEDIMENT YIELD 12.12
States, a logical first point of contact is the local office of the Natural Resources
Conservation Service within the Department of Agriculture. The electronic listing of
watershed organizations mentioned in the introduction to this chapter can provide
numerous contacts in the field, and watershed organizations can also be located on the
Internet. Erosion control is generally not a burden that the dam owner carries alone, and
well-directed contributions by the dam owner may catalyze sustained changes within
the watershed, making this an attractive long-term option. This approach is described in
the Feather River case study (Chap. 22).
12.3.4 Economic Costs, Benefits, and Erosion Control Strategies
Erosion damages may be broadly apportioned between on-site costs which affect the
eroding area itself, and off-site costs that accrue to downstream users or in distant areas.
The question of who bears the cost of erosion, and how large those costs are, is highly
relevant to the selection of an erosion control strategy.
The on-site costs of erosion on farms may include reduced crop yields, increased
cost of inputs such as fertilizer, damage to plants and fields by channelized water,
gullying of farm roads, and desiccation of local streams used for water supply. On
hillside farms with shallow soils and high erosion potential, the on-site cost of erosion is
typically very high. Agriculture cannot be sustained for many decades on shallow soils
that are rapidly eroding. If subsistence farmers on eroded soils migrate to urban slums,
erosion may impose a high cost to the urban sector as well.
Agronomic practices that trap moisture and nutrients to improve yields will also reduce
erosion. When these productivity-enhancing erosion control practices become an integral
part of the farmer's agronomic practices, they become self-sustaining. Shaxson (1988) has
referred to this as "erosion control by stealth," and described as an example of the
conservation approach taken on tea estates on sloping soils in Malawi. Managers were told
that production costs could be reduced significantly by reorganizing the road system to allow
uninterrupted access to the tea gardens during the rains, the main production and harvest
period. In areas of new plantings or replantings the dirt roads were realigned along
topographic crestlines whenever possible, where they would not be subject to washout by
cross-flows. Adequately drained lateral roads were extending along the contour or
controlled gradients, adjacent to conservation banks that controlled runoff. The tea rows
were then realigned parallel to the contoured roads, rather than in straight lines, thereby
impeding downslope runoff. Mulching was found to be beneficial to young tea plants for
agronomic reasons, but it simultaneously protected the soil against raindrop impact. While
the initial cost of developing this contoured system was higher than the traditional
practice, subsequent maintenance costs were much lower because of reduced runoff
which damaged roads, carried off soil and fertilizer, and clogged drainage channels and
conservation banks. Thus, while the changes in agricultural practices responded to economic
incentive, they simultaneously reduced land degradation.
A different economic balance between on-site and off-site costs occurs on
mechanized farms. The on-site cost of erosion on deep, gently sloping soils is typically
small, despite soil and nutrient losses which may be significant from the standpoint of
downstream users. The on-site cost of erosion is also perceived to be small in the forestry
and construction sectors: loggers and contractors are on the site for a very short period
of time, and erosion does not normally interfere significantly with their operations.
However, in developed areas, the off-site costs of erosion can be high because of the
larger inventory of downstream infrastructure such as reservoirs and navigation channels
and the high value assigned to stream quality for water supply, recreational, aesthetic,
and environmental reasons. In a competitive economic environment, few operators are
REDUCTION OF SEDIMENT YIELD 12.13
willing or able to incur significant costs for altruistic reasons, and when the on-site costs
of erosion are small it is necessary to resort to subsidies, informal or formal coercion, or
both, to implement and maintain effective erosion control measures for the benefit of
society as a whole.
Subsidies have been widely used to implement soil conservation measures in both
industrialized and less developed countries. However, subsidies should be used with
utmost caution, since they can create perverse incentives that may run counter to the
intended purpose. Lutz et al. (1994) cite an example of a reforestation incentive program
in Costa Rica that unintentionally encouraged farmers to deforest their land so they could
qualify for a reforestation subsidy. Farmers may also delay conservation practices in the
expectation of future subsidies for their implementation, and allow conservation measures
to fall into disrepair when subsidies end. In the United States, subsidies have been used to
temporarily retire erosive land from agricultural production. However, this is not only
very costly (see sec. 12.6), but the benefits are only temporary since the land is often
returned to production when the subsidies end (Napier, 1990).
In the construction and forestry sectors, erosion control has increasingly focused on
formal regulatory approaches. The U.S. Environmental Protection Agency now requires
sediment detention storage equal to 2.54 cm (1 in) of runoff depth from the disturbed
area of construction sites exceeding 4 ha, and a number of states now have erosion
control requirements at construction sites which exceed federal standards (Gilbert, 1996).
In the forestry sector some states have only voluntary guidelines for good timber harvest
practices, but enforceable standards are established in other areas. Timber harvest on
federal lands requires the preparation of a timber harvest plan which includes
specification of the appropriate erosion control measures which must be adhered to.
Nonregulatory techniques that can be helpful in combating erosion include:
1. Education and training of agency personnel, engineers, foresters, and operators
2. Promulgation of standards of practice by professional organizations
3. Adverse publicity, as might occur when a school biology class visits a stream below a
badly eroding site,and invites the local television news crew to film the event
4. Civil suits bought by parties injured by the erosion
5. Peer pressure
12.4 EROSION CONTROL MEASURES ON FARMS
This section provides a brief description of several basic erosion control strategies widely
used on farms. Many variations of these techniques have been developed to fit soil,
climate, crop, and socioeconomic conditions around the world, and local sources should
be consulted for information on specific techniques. Although not specifically discussed in
this handbook, wind erosion is also important in many agricultural areas, and some
measures which reduce erosion by water, such as strip cropping and reduced tillage, can
also protect against wind erosion.
REDUCTION OF SEDIMENT YIELD 12.14
12.4.1 Contouring
The practice of orienting field operations such as plowing and planting along the contour
is called contouring. It reduces surface runoff by trapping water in small depressions
and decreases the incidence of rill formation.
12.4.2 Strip Cropping
The practice of growing alternate strips of different crops in a field is called strip
cropping. For water erosion control the strips may be aligned along the contour, and the
crops follow a different rotational sequence so that the entire field is never bare. Buffer
strips may also be used to protect sensitive areas of the field from erosion, or to create
areas which will retard runoff and trap sediment.
12.4.3 Grass Filter Strips
A grass filter strip is a broad vegetated area, aligned on the contour insofar as possible,
which will filter sheet flow from cropped areas. Filter strips can trap both sediment and
the associated pollutants, including agricultural chemicals. The effectiveness of the
filter strip depends on factors including slope, filter strip width and soil infiltration rate,
sediment grain size, and amount of runoff. Filter strips are ineffective in treating
concentrated flows. Being a vegetative measure, filter strips tend to be self-
maintaining. However, they require a significant amount of space and can reduce the
amount of land available for crop production. They might be used for forage.
12.4.4 Conservation Tillage
One of the most important agricultural conservation measures being adopted in many
areas of the world in recent years is the reduction in tillage (turning of the soil). In the
United States the term conservation tillage encompasses a variety of specific techniques
such as no-till, mulch-till, ridge-till, and strip-till, all aimed at reducing the frequency
and surface area of the field subject to tillage. Approximately 40 million hectares (100
million acres) of U.S. cropland (about 20 percent of the total) had been converted to
conservation tillage by 1996.
Under conservation tillage, the crop stubble is left standing and residue is evenly
spread across the field as mulch instead of being plowed under. Weeds are controlled by
cutting and herbicide. Compared to conventional tillage, conservation tillage can
increase soil organic matter, reduce erosion by as much as 90 percent, enhance
infiltration, and reduce moisture loss. When implemented across a watershed, the
enhanced infiltration can reduce peak discharge and downstream flood damages.
Of greatest importance from the standpoint of sustained implementation are the
tangible short-term benefits to the farmer. On mechanized farms in the United States,
conservation tillage practices which greatly reduce erosion are "sold" to farmers, by
emphasizing their economic advantages in terms of reduced machinery costs and labor
(Fig. 12.5). On a 200-ha (500-acre) mechanized farm, Hill (1996) estimates that the
reduced number of passes through the field with equipment translates into an annual
savings of 225 hours of the farmer's labor and 1750 gallons of fuel, plus $2500 in reduced
mechanical wear on machinery. The equipment required for no-till agriculture is lighter
and lower-horsepower than machinery required for heavy plowing, which produces fur-
REDUCTION OF SEDIMENT YIELD 12.15
FIGURE 12.5 Advertisement promoting conservation tillage in the United States based on its
cost advantages to farmers. The key to the successful and sustained implementation of conservation
measures is making them productive to the farmer (Conservation Technology Information Center, by
p
ermission).
ther cost savings. Colacicco et al. (1989) indicated that farmers may be attracted to
conservation tillage more for its cost savings rather than for the purpose of soil
conservation.
12.4.5 Grassed Waterways
A grassed or vegetated waterway is a shallow drainageway in which vegetation protects
the channel against erosion, thereby increasing the permissible (nonerosive) velocity
compared to bare soil. Maximum permissible velocities depend on the type of grass, but
are generally limited to about 1.2 m/s and slopes not exceeding 5 percent. For streams
that have a base flow, a stone center is provided. Scour resistance may be enhanced by
geosynthetic reinforcement. Various grassed waterway configurations are illustrated in
Fig. 12.6.
REDUCTION OF SEDIMENT YIELD 12.16
FIGURE 12.6 Grassed or vegetated waterways. Either trapezoidal (as shown) or parabolic cross
sections may be used. An inner channel lined with stone is used when there is sustained base flow.
12.4.6 Terraces
The use of terraces is an ancient technique, and some terrace systems in Asia and the
Andes have been in service for over 1000 years. Beach and Dunning (1995) have
hypothesized that ancient Mayan agriculture in Central America, which sustained a
population much larger than today's population of subsistence farmers in the same area,
was based on the extensive use of terraces, of which only the faintest vestiges remain
today. Terraces work against gravity, interrupting the tendency of water to flow down-
slope. The layout and configuration of terrace systems or earthen bunds will depend on
the farming system, soils, crops, climate, etc. Several alternative terrace configurations
are illustrated in Fig. 12.7. Terraces are normally designed to discharge the collected
runoff to a lined channel or drain which carries excess water downslope. Because
structural terraces collect and concentrate water, the failure of a terrace can release a
concentrated flow of water which can cause gullying. While terraces can be highly
efficient at trapping both soil and water, and are required for sustained production on
steeply sloping soils, structurally formed terraces of earth or stone are costly to construct
and maintain.
12.4.7 Contour-Grassed Hedges
Because structural measures are costly to construct, and will fall into disrepair and fail
without continual maintenance, considerable attention has been given recently to the use
of self-sustaining stiff grass hedges. Natural terraces can be created by planting a dense
hedge of stiff grass on the contour, which retards runoff and causes water to pond and
deposit sediment on the uphill side of the hedge (Dabney et al., 1995). Because it is
living, the grass hedge will become stronger with time and can grow as sediment
collects and the terrace height increases (Fig. 12.8). Deposited sediment fills in low spots,
runoff tends to become more evenly dispersed and less erosive with time, and the hedge
tends to naturally build terraces that follow the contour (Dewald et al., 1996). Contour
grass hedges differ from grass buffers and filter strips because they are narrow, are
REDUCTION OF SEDIMENT YIELD 12.17
FIGURE 12.8 Slope profile illustrating the mechanism by which a stiff grass hedge will pond wate
r
and cause sedimentation, resulting in the creation of self-sustaining terraces (after Dabney et al.,
1995).
REDUCTION OF SEDIMENT YIELD 12.18
planted with stiff, erect grasses, and maximize the formation of berms or terraces due to
sediment deposition. Unlike structural terraces, which deliver a concentrated flow to a
grassed waterway or structural conduit, the grassed hedge allows water to seep through
(Fig. 12.9).
FIGURE 12.9 Runoff management on terraces. (a) Because structural terraces are impermeable,
they direct intercepted runoff to a grassed waterway, lined channel, or drain for transpor
t
downslope. (b) The contour stiff grass hedge is permeable, and disperses runoff across the slope
instead of concentrating it into a channel. (After Grimshaw and Helfer, 1995.)
The success in creating a vegetated hedge which forms natural self-sustaining
terraces owes much to the properties of the grass itself. Vetiver grass (Vetiveria
zizanioides) has been found to be a remarkably functional species in tropical and
subtropical areas (Grimshaw and Helfer, 1995). Vetiver grass, a wetland plant native to
northern India, grows under an exceptionally wide range of soils and climates. Neither
vetiver nor its roots are inherently spreading, and if planted in long lines it will create a
dense but compact hedge. It has been used successfully in areas receiving from 500 to
5000 mm of annual rainfall. It survives prolonged inundation, grows rapidly, has a
rooting depth exceeding 3 m, is resistant to fire, will become established on extremely
degraded soils, and will tolerate soil temperatures as low as -10°C. It is relatively
unpalatable and is not damaged by grazing by either cattle or goats, although young
shoots can be harvested for forage. It is easy to propagate and requires virtually no
maintenance. There are examples of vetiver hedges in southern India that are 200 years
old, and erosion control lines planted over 30 years ago in Fiji have developed a system
of stable noneroding terraces on steeply sloping soils, with no maintenance. Although
originally developed to control sheet erosion, the remarkable properties of vetiver are also
well-adapted for use in gully and channel stabilization, and for erosion control on
engineered slopes such as road embankments.
It is essential to use the proper "domesticated" south Indian vetiver cultivar, which
does not produce seeds. The wild variety from north India sets fertile seed and might
become a pest in some areas. Information on vetiver grass and cultivar sources is
available from Grimshaw and Helfer (1995), National Research Council (1993), and
the Vetiver Network (15 Wirt St. NW, Leesburg, VA 20176; Internet
www
.vetiver.org). Dewald et al. (1996) have described the use of switchgrass
(Panicum virgatum L.) and eastern gamagrass (Tripsacum dactyloides L.) for the
REDUCTION OF SEDIMENT YIELD 12.19
planting of stiff grass hedges in temperate climates. Both vetiver and switchgrass form a
dense hedge of stiff stems, effectively ponding water to depths of 0.3 m or more.
12.5 IMPLEMENTING EROSION CONTROL ON FARMS
12.5.1 Types of Farms
In discussing erosion problems on farms, it is necessary to distinguish between
mechanized and subsistence farms. Mechanized farms occupy generally flat or gently
sloping soils which often have a relatively low erosion hazard. These farmers typically
produce cash crops, use machinery, and buy their food in supermarkets. This group
includes most of the farmers in developed countries, and the wealthier farmers who
occupy the flat soils in less developed countries.
Subsistence farmers cultivate small plots, eat what they grow, and dedicate only a
limited part of their land to cash crops. They generally occupy lands too steep for
mechanization, or areas too remote to enable crops to be marketed. Much of the farm
work may be performed by women, especially when the men work off-farm to generate
cash income.
The two groups of farmers have many features in common, and the basic soil and
water conservation techniques have similar objectives regardless of the size and type of
farm. However, specific implementation measures and strategies are quite different for
the two systems, and will be discussed separately.
12.5.2 Criteria for Successful Erosion Control
Measures for the successful implementation of conservation practices on farms are
synthesized below from Moldenhauer and Hudson (1988), Hudson and Cheatle
(1993), Lutz et al. (1994), and Napier et al. (1994).
Successful conservation programs must be long-term. Successful soil and water
conservation requires that farmers in a region change their agronomic practices, a
process which occurs only gradually. Conservation can be achieved only if sponsors
and governments are committed to supporting long-term programs. Short-term
projects have proved to be ineffective. If conservation practices are to be effective
in reducing long-term sediment yield, they must be implemented permanently.
This means they must be either profitable for farmers, or enforceable. However,
profitability alone may not be sufficient. Problems such as land tenure, lack of
credit, poorly implemented extension programs, and perverse legal and
institutional arrangements may also need to be corrected.
Grass roots involvement and relevance is required. Work from the ground up to
develop a program that is relevant to local needs. Involve farmers, families and
communities in the planning and implementation of resource conservation activities.
It is particularly important to build on local social structures, cultural patterns,
knowledge, and practices. Conservation measures not relevant to the needs of the land
users may not be implemented, except with subsidies, and will not be sustained in the
long run.
REDUCTION OF SEDIMENT YIELD 12.20
Develop technically sound and economic conservation measures. Successful and
sustainable conservation measures will produce tangible short- and long-term
benefits to farmers and will fit within the farmer's economic and managerial
capabilities. To be sustained, conservation practices must be seen by farmers as ways
to minimize their risks, increase return, and reduce labor or other inputs. Focus on
agronomic practices which reduce the cost of inputs and labor and enhance crop
production, while also controlling erosion. Structural control measures can be
costly to construct and maintain, and land degradation is better controlled through
improved land husbandry than by engineering-based maintenance-intensive soil
conservation measures. Subsidies may be required to achieve large-scale
implementation of some measures, but should be used with extreme caution and
carefully targeted since they can also be counterproductive.
Training and communication. Appropriate forms of communication must be
used, such as adequately trained and motivated extensionists, appropriate technical
publications, and teaching by example. In less developed areas, the training of
extensionists from within each community can be an attractive alternative for
programs having a limited funding period from donor agencies, since the
extensionist' s knowledge and skills are more likely to stay within the community
after funding ends. Although mass media can provide awareness, they are not an
effective tool for promoting conservation.
Flexibility. The program must be flexible, with periodic reviews to discard
unsuccessful approaches and adopt and improve the strategies that work in each local
setting. Rigid plans with fixed timetables and targets are generally inappropriate.
Start small and grow systematically. A small success is always better than a large
failure. Successful projects start small, using the initial stage to test and perfect
techniques, and then expand gradually and systematically. The rate of expansion
will depend on the rate at which land users are prepared to change and the resources
available to the project.
Recognize and work within socioeconomic constraints. The adoption of conservation
practices appears to depend at least as much on socioeconomic factors as on the
technical effectiveness of the practices. The project must be fully aware of the
existing cultural, economic, and land tenure system, and tailor the program to fit that
system.
Administrative structures. Utilize existing organizations and administrative
structures, and strengthen them to the extent possible. In less developed areas, the
programs implemented by nongovernmental organizations (NGOs) have developed
many sound strategies for working with the rural poor and can serve as excellent
catalysts for general change, in contrast to the disappointing results often achieved by
government agencies. However, for widespread implementation the programs need
to become institutionalized within local government structure. Avoid the use of
special administrative units established to facilitate and administer the program,
because they tend to disappear when project funding ends.
In contrast to the strategies recommended above, several features appear repeatedly in
programs that have failed. Unsuccessful programs:
1. Have a top-down orientation and do not adequately consult the local communities
2. Do not consider the short-term needs of farmers
3. Do not provide for maintenance and follow-up to conservation works
4. Attempt to expand the program too rapidly