Preventing the formation of a gully is much easier than controlling it once it has formed. If incipient gullies are not stabilized, they become longer, larger and deeper. Under certain climatic and geological conditions, vertical gully banks can easily become as high as 20-30 meters or more. This type of gully can engulf hillside farming areas, grass lands and even forest lands. In most cases, it is not possible to stabilize those gullies because of the huge landslides which occur on vertical (20-30m) gully banks after heavy rains and alternate freezing and thawing.
Prevention is also more economical because structural measures are considerably more expensive than preventive measures. Therefore, in erosion control or gully control, emphasis should be given to:
- Prevention of forest fires and illegal wood cutting in plantations and natural
forests,
- Prevention of grass fires,
- Control of grazing, and revegetation of open and grass lands,
- Maintenance of soil fertility and stability on land which is under agro-forestry
or agriculture,
- Control of road construction and mining,
- The immediate stabilization of moderate sheet and rill erosion, and incipient
gullies in forest, rangeland and cultivated areas.
In addition to proper land-management practices, specific slope-treatment measures, such as retention and infiltration ditches, terraces, wattles, fascines and staking, should be carried out above the gully area, and in the eroded area between the branch gullies, to reduce the rate and amount of surface run-off. These also decrease the cost of structural gully-control measures.
Diversions constructed above the gully area direct run-off away from gully
heads, and discharge it either into natural waterways or vegetated watercourses,
or onto rock outcrops and stable areas which are not susceptible to erosion.
Surface water must not be diverted over unprotected areas or it will cause new
gullies.
The basic aim of diversions is to reduce the surface water entering into the
gully through gully heads and along gully edges, and to protect critical planted
areas from being washed away. Small diversion ditches constructed either alone
or with other structures such as earth plugs and check dams, are commonly used
in gully control (Fig. 5).
To prevent scouring along the diversion channel, the gradient of small diversion ditches must be less than one percent - preferably 0.5 percent. However, if there is a permanent plant cover in the channel, the gradient may be as high as two to three percent. The protective vegetation must be maintained during the entire rainy season, or these steeper gradients will cause channel erosion.
Diversion ditches should be large enough to carry all the water that is discharged
from the gully catchment area during periods of maximum run-off (Fig. 6, Table
3). In regions subject to particularly heavy rains, in addition to diversions
established above the gullied area, a series of check dams must be constructed
along the gully channels.
Once gullies have begun to form, however, they must be treated as soon as possible, to minimize further damage and restore stability. The rest of this chapter is dedicated to basic gully treatment measures.
Gullies with very little water flow can be stabilized by filling and shaping,
that is, if the surface water is diverted, and livestock and fire are kept out.
Steep gully heads and gully banks should be shaped to a gentler slope (about
a one-to-one slope). Rills and incipient branch gullies may be filled in by
spade, shovel or plow (on cultivated lands).
In general, weeds will grow first, and then hardy plants capable of surviving
in a gullied area. After one or two years, the region's predominant vegetation
will cover the gully as part of this natural process.
Where natural growth does not occur, or if certain plant species of economic
value are desired, artificial vegetation may be considered. When making planting
decisions based on erosion control alone, it makes little difference whether
trees, shrubs or grasses are used. Any of these, when well established, will
provide good protection for the soil. Consequently, the kind of vegetation should
be chosen according to how the planted area will ultimately be utilized.
In regions with heavy rains, filling, shaping and diversions alone will not suffice to control gullies. Additional gully control and slope stabilization measures, such as check dams, stone terraces, wattles and revegetation, should be undertaken.
In gully control, temporary structural measures such as woven-wire, brushwood,
logs, loose stone and boulder check dams are used to facilitate the growth of
permanent vegetative cover. Check dams are constructed across the gully bed
to stop channel and lateral erosion. By reducing the original gradient of the
gully channel, check dams diminish the velocity of water flow and the erosive
power
of run-off. Run-off during peak flow is conveyed safely by check dams. Temporary
check dams, which have a life-span of three to eight years, collect and hold
soil and moisture in the bottom of the gully. Tree seedlings, as well as shrub
and grass cuttings planted in gullies can grow without being washed away by
flowing water. Thus, a permanent vegetative cover can be established in a short
time.
To obtain satisfactory results from structural measures, a series of check
dams should be constructed for each portion of the gully bed (see Table 2 for
selection of check dams). Because they are less likely to fall, low check dams
are more desirable than high ones. After the low dams silt up and rot away,
vegetation can control the low overfalls much more easily than on high dams.
Check dams are not necessary on those gully portions which are protected from
channel and lateral erosion by continuous rock outcrops along their gully beds.
Check dams may also be combined with retaining walls parallel to the gully
axis in order to prevent the scouring and undermining of the gully banks.
Stabilized watershed slopes are the best assurance for the continued functioning of gully control structures. Therefore, attention must always be given to keeping the gully catchment well vegetated. If this fails, the structural gully control measures will fail as well.
After surveying the longitudinal profile of the gully with simple field instruments, such as a clinometer (0-90 degrees) and 50 m measuring tape (see Appendix I), the number of check dams for each portion of the main gully channel can be calculated by using the following equation:
a: The total vertical distance is calculated according to the average gully
channel gradient and the horizontal distance between the first and last check
dam in that portion of the gully bed.
b: The total vertical distance is calculated according to the compensation
gradient and horizontal distance between the first and last check dam in that
portion of the gully bed (compensation gradient - see Spacing below).
H: The average effective height of the check dams, excluding foundation, to
be constructed in that portion of the gully bed (see Appendix I for example).
The spaces between check dams can be determined according to the compensation
gradient and the effective height for the check dams.
The gradient between the top of the lower check dam and the bottom of the upper
one is called "compensation gradient" which is the future
or final gradient of the gully channel. It is formed when material carried by
flowing water fills the check dams to spillway level. Field experience has demonstrated
that the compensation gradient of gullies is not more than 3 percent. Therefore
there is little practical value in some formulas (Steiger and Gravelius) used
to compute the compensation gradient.
The first check dam should be constructed on a stable point in the gully such
as a rock outcrop, the junction point of the gully to a road, the main stream
or river, lake or reservoir. If there is no such stable point, a counter-dam
must be constructed. The distance between the first dam and the counter-dam
must be at least two times the effective height of the first check dam.
The points where the ensuing check dams are to be built are determined according
to the compensation gradient and the effective height of the check dams. Fig.
8 shows how to use a clinometer, a clinometer stand and a target in order to
find the second check dam point in a gully bed. At the second point, the effective
height of the second check dam is marked at the edge of the gully by taking
into account the depth of the gully, the depth of the spillway and the maximum
height of the check dam.
Stand at this point marked at the edge of the gully, or if this is impossible,
at another position on the same level as the marked one. Since at this point
the compensation gradient is measured as three percent, the construction site
for the third check dam has been determined. In this way, all the other proposed
check dam points can be marked in the gully.
While spacing the check dams, give preference to the narrowest parts of the gully in order to reduce construction costs. In this case, to establish the compensation gradient between the proposed check dams, proportionately increase the foundation depth of the upper check dam when the space between the lower and upper check dam is extended. When the space is shortened, decrease the foundation depth. As the foundation depth is increased, the total height of the check dam (effective height plus foundation depth) should not exceed the permissible, maximum total height.
Estimate the maximum discharge (Qmax) of the gully catchment area by using the following run-off formulas.
Q max = CI A/3.6
C: Coefficient (varies from 0.20 to 0.50 depending on the type of land use
and topography).
I: Rainfall intensity, based on the concentration time of the flowing water
from the limit of the catchment to the site where the check dam is to be constructed.
Rainfall intensity is calculated according to the maximum (one hour) rainfall
intensity (I, mm/hour) which has a frequency of 5 to 10 years for that area.
A: The catchment area of the gully above the proposed check dam expressed in square kilometers.
3.6: Constant
Q max: Maximum discharge of the catchment at the check dam site expressed in
cubic meters/second.
Use of the rational formula is possible only when a rainfall intensity (I,
mm/hour) map of the country with the frequency of 5, 10, 25, 50 and 100 years
is available.
If there is no intensity map for the country, the following discharge formulas must be used instead: Kresnik, or the general run-off equation and Manning velocity formula.
1. Main Kresnik formula
2. Simple Kresnik formula
A: Catchment area of the gully above the proposed check dam, expressed in square kilometers.
Q max: Maximum discharge of the gully catchment at the proposed check dam site,
expressed in cubic meters/second.
a: Coefficient (0.6 - 2.0 depending on land use type).
1. General run-off equation
Q=AV
A: Cross-sectional area (wetted area) of main gully bed considering the highest flood water level at the proposed check dam site, expressed in square meters.
V: Velocity of the flowing water at that point, expressed in meters/second (to be determined by the Manning formula, below).
Q: Maximum discharge of the gully catchment at the proposed check dam point, expressed in cubic meters/second.
2. Manning formula
n: Roughness coefficient of the channels. For gully channels, n can be set at 0.025.
R: Hydraulic radius (wetted area divided by wetted perimeter), expressed in meters. R should be calculated for the point where the cross-sectional area of the
gully has been measured using the highest flood water level.
S: Gradient o£ the gully channel, expressed as a percent.
V: Velocity of flowing water at the proposed check dam site, expressed in meters/second.
The simple Kresnik formula gives more suitable results for gullies with catchment areas of less than 20 hectares. It can also be used in torrent control (catchment 300 ha maximum). The main Kresnik formula gives better results for torrents with catchment areas greater than 300 ha.
Q = CLD 3/2
C : Coefficient which is 3.0 for loose rock, boulder log and brushwood check dams; 1.8 for gabion and cement masonry check dams.
L : length of spillway in meters D : depth of spillway in meters
Q : maximum discharge of the gully catchment at the proposed check dam point, in cubic meters/second.
The catchment area of a gully (continuous gully) is 15 ha above the point where
a boulder check dam would be built. What are the dimensions of the check dams's
spillway?
By using the simple Kresnik formula, the maximum discharge can be estimated
as follows:
Q max = 25 A1/2
= 25 0.151/2
= 25 x 0.337
= 9.675 cubic metrs/second
Consequently, the spillway dimensions can be calculated by the spillway formula
as follows:
Q = CLD3/2
Q: 9.675 cubic meters/second as computed above by simple Kresnik formula
C: 3.0 coefficient for rock and brush structures
D: Depth of spillway varies from 0.5 to 1.5 m in general. (0.8 m is tried as shown below).
9.675 = 3L 0.83/2
= 3L 0.71
4.54 m = L
The length of the boulder check dam's spillway is 4.54 m (round 4.6 m), if
the depth of the spillway is accepted as 0.8 m.
Fig. 9 Common spillway forms used For check dams. Top, trapezoidal spillway for loose stone, boulder, gabion and cement masonry check dams. Centre, rectangular spillway for brushwood, log(pole), gabion and cement masonry check dams. Bottom, concave spillway for brushwood and loose stone check dams.
The spillway form of check dams may be concave, rectangular or trapezoidal (Fig. 9). For example, the length of the spillway which is the mean value of the parallel sides of the trapezium is computed as follows:
So, the length of the trapezoidal spillway is 4.6 m and its depth is 0.8 m. The sloping of unparalleled sides is 50 percent (1:2).
The length of the foundation must always be longer than the length of the spillway to prevent scouring and undermining by falling water.
The crest of rectangular and trapezoidal spillways should be level. Use the
leveling frame (Fig.10) for this purpose.
Loose-stone, boulder, gabion and cement-masonry check dams used for gully control
are also called "gravity dams" or "bulk dams". Their downstream
face slopes backward while their upstream face is generally vertical. Therefore,
the cross-section of a gravity dam is trapezoidal. The widths of a bulk dam's
crest and base are calculated according to their height, and empirical formulas.
A bulk dam's stability against overturning, collapsing, or sliding depends
on these empirical formulas. The weight of the dam and the water flowing through
its spillway section act as vertical forces
against it. The pressure on its upstream face, from material filled to spillway
level, acts as a horizontal force. These two forces create a pressure curve,
or resultant, which passes through the foundation of the check dam at least
two-thirds of its foundation width.
For stability, the weight of gravity dams on stony ground or new alluviums
must not exceed 50 tons/square m or 5 kg/square cm. If their pressure on the
ground is less than 5 kg/square cm, and if their
pressure curve crosses the foundation base at five-sixths of its width, bulk
or gravity check dams will not break.
Loose stone and boulder check dams are relatively small dams. Their maximum
height, including their foundation, is two meters. Therefore, their pressure
on the ground is always less than 5 kg/square cm.
Bulk dams constructed according to these formulas meet hydrostatic pressure
(soil pressure plus water pressure) because the space behind the check dam is
filled to the spillway with material excavated for constructing the foundation.
In other words, pressure from water or from the mixture of water, soil, sand
pebbles, etc. is first exerted on the filled material -- not on the dam's upstream
face.
In gully control, it is usually not necessary to design bulk or gravity dams
according to hydrostatic pressure, because the use of empirical formulas already
protects check dams from overturning, collapsing and sliding. If there is mud-flow
in the gully, however, the dimensions of the gravity or bulk dams must be determined
according to the hydrostatic calculations, because in this case, the hydrostatic
pressure varies from 1.8 up to 10 times that of water. To prevent mud-flow damage,
the space behind the check dam must be completely filled to the spillway.
Graphic methods can also be used to test the stability of gravity or bulk dams (Fig. 11).
The stability of other check dams such as woven-wire, log and brushwood will be determined by appropriate methods (described below) rather than empirical formulas.
The construction of check dams or earth plugs generally proceeds from the bottom
to the top of the gully. However, if rocks for gravity check dams have to be
transported from the bottom to the top of the gully through the gully channel,
then construction must start at the top.
Gully heads are usually stabilized by building suitable check dams in front
of them. The kind of check dam needed depends on the flowing water's falling
distance from the gully head, and the availability of construction material.
Field activities must be planned so that all structural work is completed before the rainy season. Otherwise, vegetative control measures cannot be undertaken, and the incomplete structures may be washed away.
In gully control, all structural measures generally protect soil against erosion
and conserve water. But earth plugs and check dams in particular, make more
water available to the soil's underground storage by retaining surface run-off.
As a result, well and spring water in the immediate vicinity is increased during
the first few years, and permanently regulated after a vegetative cover is established
in the gully. Cultivated lands on both sides, and at the base of the gully are
protected from flood and sediment damage, because structural measures, such
as diversion ditches, earth plugs, and check dams, prevent minor floods by diverting
and holding the surface run-off.
Gully control proposals can be appraised by using the cost-benefit ratio method, present net value or internal rate of return method. Even if the proposed treatment cost for the gully is high, it is usually economical to protect the soil from further damage.
Maintenance for structural measures must be continued for at least two years
after the treatment year. Treated areas must be inspected at least once a year.
The trees and grass established in gully catchment areas must be protected
against fire, illegal wood cutting, grazing and encroachment. If the revegetated
areas are properly managed for several years after the treatment, some fuelwood
can be produced from tree plantations and fodder can be obtained from grass
and fodder tree plantations.
