Morris & Fan. Reservoir Sedimentation Handbook
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REDUCTION OF SEDIMENT YIELD 12.21
5. Are inflexible
6. Create political antagonism within the local community or the government
12.6 EROSION CONTROL ON MECHANIZED FARMS
Soil conservation was initiated in the United States in the 1930s, when dust bowl
conditions in the midwest were merely symptomatic of the lack of conservation that
characterized agriculture nationwide at the time. Soil conservation technology was in
its infancy, and farmers generally lacked knowledge of erosion control techniques. Land
abuses were widespread and readily apparent, not different from many less-developed
areas of the world today. Farms were literally being washed away by rain and blown
away by wind.
Present conditions are remarkably different in the United States, as in most other
mechanized agricultural areas in the world, where the most obvious soil erosion problems
have been controlled. On the basis of conditions in the mid-1980s, Colacicco et al. (1989)
estimated that two-thirds of U.S. cropland will suffer no yield loss over the next 100
years, since most land is eroding at less than the tolerable rate and suffers no productivity
loss. Despite erosion, productivity losses on the remaining lands can be largely offset by
increased fertilization.
Successful implementation of erosion control techniques across the diverse U.S.
landscapes is the product of many efforts: erosion control research in experiment stations,
sustained implementation efforts nationwide for some 60 years by the Soil
Conservation Service (now renamed Natural Resources Conservation Service),
increased public awareness and demand for enhanced water quality, an increasingly
stringent regulatory environment, and of great importance, the elimination of
cultivation on steeply sloping soils. Nevertheless, erosion, runoff quality, and reduced
infiltration rates are problems that still require more work in many areas. Two recently
formed watershed groups in Illinois were profiled by CTIC (1996), and are representative of
the approaches and concerns of watershed management in agricultural areas of the United
States. Both watershed groups are composed of area landowners, interested citizens, and
locally active personnel from a variety of government agencies, and were organized to
address problems of local significance.
In the watershed tributary to Lake Springfield, which had been dredged (Sec.
16.5.1), the two key issues were continued sedimentation and contamination of the
drinking water reservoir with a herbicide widely used by local farmers. Floods in 1994
deposited a volume of sediment equal to about 10 percent of the volume dredged, and
represented about $1 million in future dredging costs. Remediation includes best
management practices such as no-till, erosion control structures, filter strips, streambank
vegetation, and improved herbicide application practices.
Flooding during 1993 was the key problem that initiated watershed protection
activities in the Embarras River watershed. A nonprofit river management association
was formed to provide local landowners with the organizational structure needed to
implement a comprehensive resource plan developed for a 12-county watershed.
Although flooding catalyzed action, the list of concerns grew to include: log jams, water
quality, erosion, drainage, wildlife, better communication and accountability, loss of the
natural character of the river, private property rights, instream sedimentation, recreation,
water supply, land use change, wetlands, small bridges and culverts, and funding sources.
Implementation of conservation tillage on 75 percent of the 450,000 ha of farmland in the
watershed is expected to reduce peak flood discharge by 10 percent, and normally dry
REDUCTION OF SEDIMENT YIELD 12.22
flood control dams will provide an additional 15 percent reduction in peak discharge.
Together these measures are expected to reduce soil loss by about 1.5 million tons a
year.
Farmers are reluctant to implement soil conservation measures unless there is an
economic incentive to do so in terms of reduced production costs, increased yield, or
subsidies. Conservation tillage is often implemented as a cost-saving measure rather
than for erosion prevention. Sloping soils which cannot be tilled with tolerable rates of
erosion are recommended for permanent pasture or woodland, in accordance with the land
capability classification system, but farmers may be unwilling to take land out of crop
production. Because the on-farm cost of erosion is relatively low in the United States,
and off-farm costs may be more than double the on-farm costs, erosion prevention has
relied heavily on subsidies paid to farmers (Napier, 1990). Under the Conservation
Reserve Program initiated in 1985, farmers were paid an annual rent plus half the cost
of establishing a conserving land cover, in exchange for the retirement of highly
erodible or other environmentally sensitive land from crop production under 10-year
contracts. By 1993 some 36.5 million acres, or about 8 percent of U.S. farmland, was
included in the program at an average cost of $122/ha/yr (Table 12.2). The program was
TABLE 12.2 Conservation Reserve Program in the United States
Parameter Value
Contracts 377,000
Enrollment, ha (acres) 14.8 × 10
6
(36.5 × 10
6
)
Total erosion reduction t/yr (tons/yr) 635 × 10
6
(700 × 10
6
)
Total rental cost, $/yr 1.8 × 10
9
Average annual rental cost, $/ha ($/acre) 122 (50)
Area planted in trees, %
6
Source: Osborn (1993).
estimated to have reduced erosion by 635 million tons/yr, an average of 43 tons/ha,
representing a 22 percent reduction in the total erosion from cropland nationwide. A
limitation of this and similar subsidy-based programs is that when the subsidies expire,
the land may be returned to crop production (Osborn, 1993). This cost of avoided
erosion is about $2.83/ton, which is close to the cost of dredging sediment from reservoirs.
This makes it a very costly approach for curing reservoir sedimentation, especially if the
sediment delivery ratio is low.
Given the cost of subsidies for conservation set-asides, coupled with budgetary
limitations, Ervin (1993) postulated that erosion control in the agricultural sector may be
expected to gradually abandon subsidies and move increasingly into the regulatory
arena, as the demand for environmental quality increases and political power shifts
away from agricultural groups.
12.7 EROSION CONTROL ON SUBSISTENCE FARMS
Issues related to the implementation of soil and water conservation on subsistence farms
are discussed in Moldenhauer and Hudson (1988), Hudson and Cheatle (1993), and
Napier et al. (1994).
REDUCTION OF SEDIMENT YIELD 12.23
5.1.1. 12.7.1 Erosion Control Strategy
Hudson (1988) has pointed out that one of the keys to erosion control in the United
States is the system of land capability classification, which identifies slopes steeper than
12 percent as unsuitable for open cultivation. Adherence to this criterion is possible in the
United States, with its abundance of fertile lands on low slopes and dispersed pattern of
land ownership. However, in many tropical areas this system is entirely irrelevant
because the area of flat soils is small, the best lands are concentrated into large
landholdings, and the expanding population of the poor is forced onto soils of ever-
increasing slopes for survival.
When an erosion problem is identified in a watershed, there is a tendency to develop
and implement a "soil conservation" program. However, this top-down approach has
largely been ineffective against erosion, as evidenced by the failure of projects of this
type worldwide. Farmers control the use of the land they till, and are rarely willing to
implement costly soil conservation measures, or change their production practices,
unless there are tangible benefits to themselves and their families. The application of
structural erosion-control measures on subsistence farmers has often been unsuccessful;
the measures have not been maintained, and in some cases they were even dismantled
by the people they were supposed to benefit. Shaxson (1988) cites an example of well-
meaning conservation programs in some African countries requiring the construction of
contour banks on cultivated land. This provoked severe resentment of government
officials, especially when farmers who did not perform the required work were fined or
jailed. People were especially embittered when they saw that even where the banks were
installed, erosion did not end. In Nicaragua, Obando and Montalvan (1994) reported that
machinery was used to construct structural conservation measures on farms in the
watershed south of Lake Xolotlan to provide flood protection for the city of Managua.
These measures, which were constructed at no cost to the farmers, generally interfered
with common cultivation practices in the area. Many of the structures were either
abandoned or undermined by the farmers, who returned to their traditional practices.
This and other similar experiences noted in the literature underscore the importance of the
socioeconomic aspect of erosion control and involvement of the local people. An
important challenge is to develop conservation practices that not only reduce erosion but
also increase land productivity.
Successful erosion control projects will focus on practices that improve the farmer's
condition while simultaneously conserving soil and water. This concept has been
termed conservation farming or land husbandry by Hudson (1988). Conservation
measures are most likely to be profitable, and thus implemented, when they are cheap and
simple, or when they allow improved agronomic practices to be implemented. Even
when the principal impact of soil loss is offsite (e.g., reservoirs), the farm-level
approach is still appropriate because conservation measures must be implemented and
sustained by farmers themselves.
12.7.2 Terracing
The retardance of runoff and trapping of sediment is a key management practice on
sloping soils. Terraces (or earthen bunds) have been widely used and represent the basis
of sustained agricultural production in many areas of the world. Terraces may be
constructed of stone, or of earth with permanent vegetative cover to protect and stabilize the
terrace slope. Terraces are costly (labor-intensive) to construct and maintain, and will
not be sustained by the farmer unless there are tangible benefits. However, once the
benefits are realized, farmers will construct and maintain terrace systems. In
Ecuador (Nimlos and Savage, 1991), poor farmers constructed terraces because it was
REDUCTION OF SEDIMENT YIELD 12.24
part of an agronomic package that generated dramatic yield increases (more than double
for some crops), and the level soil was also easier to farm. In some areas of Kenya
terraces are required to trap enough moisture to prevent crop failure in dry years
(Wenner, 1988).
The benefits of terracing are highly site-specific. In drier climates terraces may be
required to trap enough water to undertake crop production, but on heavy clay soils,
terraces can create waterlogging and decrease crop production. Contour-grassed hedges
of vetiver may function much more satisfactorily than traditional structural terraces in
many situations.
Research and experiment stations have tended to favor terracing and similar soil
conservation methods which have high levels of technical efficiency, rather than methods
having high levels of cost-effectiveness to the farmer. Although terracing is a highly
effective method for reducing erosion, it is also costly to implement and maintain, and
because terracing may take some land out of production it may actually reduce the short-
term yield (Lutz et al., 1994). Terracing may also disturb the soil in a manner that brings
less productive soil to the surface. Depending on the setting, these costs can make
terrace construction economically unattractive to farmers. Structural measures have been
successfully employed in a number of regions, but are not the answer to all sloping soil
erosion problems. Methods such as construction of contour ditches and use of grass
strips and organic fertilizers are conservation measures that are less costly to implement
than terraces and may be considered more beneficial by farmers.
12.7.3 Agronomic Strategies to Reduce Erosion
Good agronomic practices produce a soil having a high infiltration capacity, achieved by
maintaining high levels of organic matter and soil cover in the form of either vegetation
or mulch. Sloping soils are modified to create terraces which retard runoff, thereby
enhancing infiltration and providing more water to sustain plant growth, and also
trapping soils and nutrients which would otherwise be lost with the runoff. Level soils
are also easier to cultivate than steep slopes. Retarding runoff and providing proper
drainage channels also helps preserve farm roads. All of these practices both benefit the
farmer and reduce erosion.
Agronomic conservation strategies used on subsistence farms focus on techniques
such as tillage reduction and intercropping to maintain a continuous cover of vegetation
or mulch, return organic material to the soil, and maximize water infiltration. Nitrogen-
fixing legumes are planted to improve soil fertility. Compost prepared from crop residue
and animal manure is spread back on the field as fertilizer. Crops are planted on the
contour, and strip cropping is used. Grass buffer zones which may be used for cut-and-carry
forage are set on the contour to trap sediment from fields, and stiff grass hedges may be
used to build natural terraces. Multiple cropping and multilayer cropping systems tend
to produce a more ecologically balanced environment, reducing both soil erosion and pest
problems. These systems can also reduce risk to the farmer, as when two crops with
somewhat differing tolerance to extreme wet or dry conditions, such as maize and
sorghum, are planted together in areas subject to drought to ensure against total crop
failure. The two following examples illustrate the application of these principles.
12.7.4 The World Neighbors Program in Honduras
World Neighbors (Vecinos Mundiales) is a nongovernmental organization (NGO) that
works in poor rural communities and extends knowledge about soil conservation and
REDUCTION OF SEDIMENT YIELD 12.25
other subjects. The organization's experience with soil conservation projects in Honduras
has been summarized by López-Vargas and Pío-Carney (1994).
In the initial stage of work with communities, farmers often do not believe in soil
conservation. Many claimed that breaking up the soil was foolishness, that constructing
ditches on the contour was for burying people, that collecting dung for composting was
a dirty job, and that because nothing grows on eroded lands it is better to move
elsewhere when the land becomes exhausted. The farmers may also not be familiar with
new or alternative crops.
Technical training must initially make individuals aware that they are the only
persons responsible for resolving their own problems and, above all, that they have the
capacity and intelligence to do so with a little effort. For long-term effectiveness farmers
must come to understand contributions of air, sunlight, water, nutrients, and soil
organisms to the formation of the crop, as well as the role that the forest plays in the
entire system and in human life. They must realize the responsibility they have to be
good administrators of nature and the potential they have to make nature useful and to
provide a better life, not only in the present but also in the future and for future
generations.
Practical and simple techniques are recommended for conserving soil structure,
moisture-retention capacity, and fertility. Diversion ditches are designed to reduce the
speed of flowing water, and in dry areas level ditches are used to capture runoff and
encourage infiltration. Vegetative barriers filter soil from water flowing downslope, and
the gradual accumulation of soil helps form natural bench terraces over years.
Vegetative barriers can also produce forage for livestock and vegetative material for
organic fertilizer. On rocky soils, rock barriers are constructed on the contour. Where
possible, minimum tillage techniques are used since they are less labor-intensive than
traditional tillage and also reduce erosion by minimizing the area of soil disturbance.
Organic fertilizers are prepared by composting vegetable matter and fresh dung from
livestock, and after composting the fertilizer is applied at the rate of 2 to 4 kg/m
2
. Dung
is not applied directly to crops without composting because it can attract insect pests.
Legumes such as velvet bean are planted as green fertilizers, since they not only fix
nitrogen but also produce seeds that can be processed to produce flour and tortillas, and
vegetable matter for incorporation into organic fertilizer. Simple conservation
structures such as diversion ditches, maximum use of organic material, narrow furrows
with minimal soil disturbance, and the minimal use of chemical fertilizer are the key
elements to the program.
The Vecinos Mundiales project workers start in each community on a small scale by
recommending a few simple techniques and applying them on small plots using native
varieties and tools. They build on existing local knowledge and techniques, and seek to
achieve gradual change. The objective is to slowly but surely alter production
techniques, incorporating the best traditional techniques with new ones as appropriate.
More people will become interested and the program will grow as the initial results are
proven in the field. Farmers are encouraged to share with others, and local extensionists
are recruited from those farmers who effectively practice what they preach. Total
acceptance is not expected, but participation levels of 60 to 80 percent are customary.
12.7.5 Implementation in Ecuador
Nimlos and Savage (1991) described the USAID-funded Sustainable Land Use
Management Project in Ecuador, which seeks to introduce sustainable soil conservation
practices and a conservation ethic to indigenous Quechua farmers cultivating steeply
sloping lands. The fertile level lands at lower elevations are owned by wealthy
REDUCTION OF SEDIMENT YIELD 12.26
landowners and are used mostly for grazing; poor subsistence farmers have access only
to marginal lands. The project area is located at elevations between 2300 and 4500 m,
slopes of 50 to 70 percent are common, and 80 percent of the parcels are less than 1 ha in
size. The men often travel to the city to seek work, leaving women to work the fields.
The program used 60 extensionists with 2 to 6 years of technical education, who
trained about 3500 farmers. About 9000 ha received conservation treatment, and the area
under improved cropping (strip-cropping, intercropping, crop rotation, contour cultivation,
and green manuring) approximately doubled every year. Construction of conservation
structures including hillside ditches, bench terraces, and earthen reservoirs increased by
about 70 percent per year. The program also provided assistance in the safe management of
pesticides, use of fertilizer, agroforestry, and native forest management. Crop production
increased dramatically: garlic by 92 percent, peas 421 percent, barley 216 percent,
onions 80 percent, beans 47 percent, potatoes 260 percent, melloco (a native fruit) 475
percent. The implementation rate for conservation treatments was highest in areas where
the increase in crop production was the most dramatic. The program was a success and
soil-conserving practices were expanded because the needs of poor farmers were
addressed and conservation practices increased yields.
Terracing was used to trap soil, and the combination of terracing and green manure
enabled farmers to grow crops in areas that had previously been unusable because of
severe erosion. Terraced lands are much easier to irrigate and cultivate, trap more
moisture, and produce much higher yields. In the project area, unterraced land was valued
at $160/ha, but its value increased to $2200/ha when terraced.
12.8 EROSION CONTROL AND FORESTRY
PRACTICES
12.8.1 Definitions
A silvicultural system refers to the methods in which trees are harvested and the stand is
regenerated. They include clear-cutting and variations thereof which produce even-aged
stands of timber, and single-tree or group selective-cutting techniques which maintains an
uneven-age stand structure. A logging system refers to the method used to fell trees, cut
them into logs, and transport the logs from the stump to a temporary storage area (the log
landing), from whence they are transported to the mill. The yarding system refers to the
method of transporting logs from the stump to the log landing and is the most critical
aspect of the logging system from the standpoint of erosion. Logs can be transported to the
landing by dragging or skidding with a tractor, a ground-level cable and winch, or
animals such as oxen and elephants. There are several types of skyline yarding systems
which either partially or wholly suspend logs in the air during transport, greatly
reducing soil disturbance. If a helicopter is used to remove logs, the method is termed
aerial yarding.
In some areas of the United States it is necessary to assess the cumulative impact of
timber harvest and other activities on water quality. The cumulative effect is the total
environmental impact resulting from the incremental impact of a proposed action, when
added to all other past, present, and reasonably foreseeable future actions, regardless of
who undertakes those actions. The idea behind a cumulative impact assessment is that
sensitive downstream uses, such as fishery habitat, can tolerate only a limited degradation
in stream quality. Therefore, all contributors to water quality degradation should be
considered when determining whether or not additional degradation is acceptable
(Cobourn, 1989; Sidle and Hornbeck, 1991).
REDUCTION OF SEDIMENT YIELD 12.27
Forest harvest activities, including erosion control measures, should be planned in
advance by preparing a timber harvest plan. A timber-harvest plan includes maps,
sketches, or photographs of the area to be logged. It specifies the logging methods to be
used, landing locations, the stream areas to be protected with buffer strips, and the width
of the buffers. It gives specifications for the construction, use, and maintenance of a
well-designed transport system, and it will also specify the methods to be used to repair
and stabilize logged areas, log landings, and roads upon completion of the harvest
operation. As part of timber harvest planning, areas of high erosion potential should be
identified and road construction avoided in these areas. Some flexibility should be
retained to allow adjustment to conditions encountered in the field.
12.8.2 General Strategies for Erosion Control
Natural forest represents the best type of vegetative cover available: forest soil is highly
permeable, and a layer of litter protects the soil from direct raindrop impact. Because of
the high infiltration rate, only rare and extreme rainfalls are capable of producing surface
flows. On steep slopes, a large percentage or even the majority of the long-term erosion
from forests can originate from slope failures during extreme rainfall events.
Commercial timber harvest disturbs a limited area of the forest floor for a very short
period of time, after which there is a cycle of regeneration and growth which may last
from 20 to 100 years before the next harvest. With timber harvest rotations of 20 years or
longer, and provided that vegetation recovers rapidly, the period of high erosion rates due
to sheet erosion is limited. Most sediment from timber harvest is derived from haul roads
and skid trails. Ill-designed and poorly constructed forest roads concentrate flows, create
gullying, and produce sediment for a period of many years. Consequently, sediment
control methods are oriented to: (1) reducing the amount of land that is disturbed by
roads and skid trails, (2) preservation of riparian stream corridors, (3) proper road design
and construction, and (4) rapid regeneration to protect the soil. General principles of
erosion control have been summarized from Gilmour (1977), Hyson et al. (1982),
Megahan (1983), and Wisconsin Department of Natural Resources (1990).
12.8.3 Yarding Methods
Large differences in impacts can be associated with different yarding systems, because of
the large difference in the amount of soil surface disturbed by the various systems (Fig.
12.10). In general, the yarding system that minimizes the area of disturbed soil will
produce the least erosion, other factors being equal. Specific recommendations for
yarding are given below.
Yard logs uphill rather than downhill, since this produces a pattern of skid trails that
tends to disperse runoff rather than to concentrate it (Fig 12.11).
Avoid stream channels insofar as possible in all phases of the yarding operation.
Never locate skid trails along stream corridors, and drain muddy water in skid trails
away from streams. Use temporary log or metal culverts at stream crossings, and,
when skyline yarding, lift logs across streams rather than dragging them.
Suspend logging on rainy days and periods immediately following rains when soils
are saturated. Operate logging equipment only when soil moisture conditions are such
that excessive soil damage will not occur.
Avoid tractor yarding on soils exceeding 30 percent slope, or 25 percent on soils of
high erosion hazard.
REDUCTION OF SEDIMENT YIELD 12.28
FIGURE 12.10 Average soil disturbance attributed to roads and yarding for differen
t
logging systems, based on 16 studies in the United States and Canada (
M
egahan,
1981).
FIGURE 12.11 Yarding in the uphill direction creates a pattern of skid trails tha
t
tends to disperse rather than concentrate runoff (redrawn from Gilmour, 1977).
REDUCTION OF SEDIMENT YIELD 12.29
Minimize logging roads and skid trails on very steep slopes or fragile soils by
using a skyline or aerial yarding system.
Obliterate and stabilize all skid trails by mulching and reseeding immediately upon
completion of logging. If necessary, also construct cross drains on abandoned skid
trails to divert runoff onto the forest floor and prevent gullying.
12.8.4 Log Landings
Log landings are relatively flat areas where logs are temporarily stored before being
transported to the mill.
Locate landings so that skid trails leading to landing areas will minimize impacts on
natural drainage channels.
Landings should be located on areas of gentle slope such as ridge tops or widened
road areas. Locate landing areas on firm ground away from stream channels, and
drain landings into well-vegetated buffer areas.
Landings and other intensive work areas should be kept free of chemicals and fuel,
and should be located away from streams, springs, and lakes, and outside of riparian
areas.
Upon abandonment, ditch and mulch the landing area to prevent erosion. If the
landing will be used repeatedly for thinning operations, plant with herbaceous
material. If it will not be used again for many decades, as in a clear-cut area, plant
trees.
12.8.5 Riparian Buffer Strips
Undisturbed strips of native vegetation which are preserved between streams and areas
disturbed by road construction or logging will reduce the quantity of sediment and
logging slash that enters the stream. Maintaining the environmental integrity of a riparian
buffer zone (also called a streamside management area, SMA) is highly beneficial to
water quality and aquatic habitat, and to terrestrial species as well. The riparian zone
should be maintained in native vegetation to provide bank stability, and to provide shade
that maintains low stream temperatures needed for the survival of some fish species.
The buffer should be of sufficient width to trap sediment and nutrients in the runoff from
adjacent disturbed areas. When trees in the buffer strip are to be harvested, fell trees
away from the stream and remove logs carefully to minimize disturbance. Large and
older trees should be maintained in this zone as a source of woody debris that is an
important component of the stream ecosystem. Slash and other logging residue should
never be deposited in streams or in the riparian buffer zone.
One of the purposes of a riparian buffer strip is to trap sediment that originates in an
upslope area, before it enters a stream. The recommended width of buffer strips will
vary depending on site conditions, but a minimum width of 25 m on each side of the
stream may be used as a guideline (Megahan, 1977). Strips might be narrower in areas
of low slope with careful control in adjacent logging areas, but much wider in heavily
disturbed areas with unstable soils. Buffer widths recommended by various states within
the United States generally vary from about 10 to 60 m on each side of the stream, with
larger widths required on soils having steeper slopes and more erodible soils. The
widths recommended by Maine (Table 12.3) are representative.
REDUCTION OF SEDIMENT YIELD 12.30
TABLE 12.3 Recommended Filter Strip Widths in the State of Maine
Source: AWWA (1995).
Width of strip along slope
12.8.6 Logging Roads
Logging produces a high density of roads, on the order of 2 km of road per square
kilometer of logged area. Roads concentrate runoff and can create long-term problems of
gully erosion if runoff is not handled properly. Important sediment sources related to
roads typically include stream crossings, fill slopes, poorly designed road drainage
structures, and roads close to streams. Roads run across the slope and intercept any
downslope runoff, and contribute additional runoff from the relatively impervious road
surface. This flow can become quickly concentrated along unlined ditches and can cause
gullying along the road; it can also cause gullying when it is diverted downslope.
Practices to reduce erosion from logging roads have been summarized by Megahan
(1977):
Design the transportation system and locate landings to minimize total mileage,
considering both skid trails and haul roads.
Design roads and skid trails to follow natural contours. Avoid the use of
switchbacks in steep terrain by using an alternative road layout. Avoid stacking
roads by using longer-span cable harvest techniques.
Space cross drains to provide adequate drainage and prevent the concentration of
roadside flows into erosive torrents.
To improve slope stability, design cut-and-fill slopes for stable angles less than the
normal angle of repose. If the terrain is too steep for this to be practicable, use
engineered slope stabilization techniques.
Use existing roads wherever practical when they meet adequate erosion control
standards, or can be upgraded to meet those standards, thereby minimizing the amount
of new disturbance and construction.
Orient road crossings at right angles to streams. Cross streams along straight reaches
having stable bank material. Size permanent culverts or bridges to pass large storms
(e.g., 25-year). At temporary crossings, remove metal or log culverts and
reconfigure, mulch, and replant the streambank as soon as operations are
completed.
Characteristics of poor and good road design and maintenance are schematically
compared in Fig. 12.12.
Land slope, % m ft
0 7.6 25
10 13.7 45
20 19.8 65
30 25.9 85
40 32.0 105
50 38.1 125
60 44.2 145
70 50.3 165