IV Erosion Control Practices
IV Erosion Control Practices
IV Erosion Control Practices
A. Vegetative Method
Vegetation absorbs the energy of falling water and reduces the runoff volume
and erosive velocity. The effectiveness of vegetation in controlling erosion varies
according to density, composition, and structure.
1. Cover cropping
Cover cropping is the growing of crops to cover the soil and protect it from
the impact of raindrops which causes soil detachment and dispersion. It increases
the permeability and infiltration rate through biological loosening effect of the
root system. In critical slopes of the field, grasses and legumes should be used.
2. Crop Rotation
3. Contouring
4. Strip Cropping
Strip cropping is the growing alternate strips of different crops in the same
field. For controlling water erosion, the strips are always on the contour. The three
general types of strip cropping:
b) Field Strip Cropping – the strips of uniform width are placed across the
general slope. This practice may also be used for wind erosion control.
5. Mulching
Mulching is the covering of the soil with crop residues such as straw, corn
stalks, and standing stubbles to protect both crop and soil from damage by water
erosion. The mulch absorbs the energy impact of raindrops and prevents runoff
from gaining speed. To be effective, mulch should cover 70-75% of the soil
surface.
6. Multiple Cropping
7. Hedgerows
8. Wattling
Wattling
Contour Hedgerows
9. Reforestation/Afforestation
B. Engineering Method
1. Terrace
2. Grassed/vegetated Waterways
3. Diversion Ditch
Drop structures are small dams used to stabilize steep waterways. They are
placed at interval along the channel to stabilize it by changing its profile from
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continuous steep gradient to series or more gently sloping reaches which will slow
down the velocity of flowing water.
5. Checkdams
6. Farm Pond
Farm ponds are designed to store runoff water and minimize the potential
runoff that may cause soil erosion. The farm pond must have adequate capacity to
handle runoff from the drainage area. If necessary, it must be provided with
vegetated flood spillway to handle excess runoff.
7. Riprap Structure
Stones/rocks are filed along the contour of the sloping areas to form a wall.
This is used in areas with abundant rocks and where terracing is not appropriate.
A one meter wide area along the contour is leveled to provide a good base for the
wall. The height of the wall is from 30-50 cm, depending upon the slope gradient.
9. Jetties
Contour Rockwalls
C. Vengineering Methods
D. Terracing
Terrace Classification
1. Broadbase Terrace
2. Bench Terrace
Terrace consists of series of benches across the sloping land with slopes of
up to 30% with the main function is to provide more efficient distribution of
irrigation water.
The conservation bench terrace is designed for semi arid regions where
maximum moisture conservation is needed. It consists of an earthen embankment and
a very broad flat channel that resembles a level bench.
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Terrace Design
The design of a terrace system involves the proper spacing and location of
terraces, the design of a channel with adequate capacity, and development of a
farmable cross section.
Terrace Spacing
V.I. = Xs + Y
Where:V.I. = vertical interval between terraces, from the top of the slope
to the bottom of the first terrace, m
X = constant for geographical location
Y = constant for soil erodibility and cover conditions during critical
erosion periods.
s = average land slope above the terrace in percent
When soil loss data are available, spacings should be based on slope
lengths using contouring and the appropriate cropping-management factor that will
result in soil loss within the permissible loss.
Example 1:
If the soil loss was 16.8 Mg/ha (7.5 t/a), for K=0.1 t/a, L=122 m (400 ft.), S=8%,
C=0.2, and P=0.6 (contouring) in the USLE, what is the maximum slope length and
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corresponding terrace spacing to reduce the soil loss to the terrace channel to 6.7Mg/ha (3
t/a)?
Solution:
L = (l/22)x
where: x = a constant, 0.5 for slopes>4%, 0.4 for 4%, and 0.3 for <3%
l = slope length, m
s = field slope, %
LS = 2 x 6.7/16.8
= 0.80
From Figure 3.3, read from 8% curve a slope length of 21.3 m (70ft). By
similar triangles,
V.I. = 8 x 21.3/100
= 1.7 m (5.6 ft)
Terrace Grade
Terrace Length
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Size and shape of the field, outlet possibilities, rate of runoff as affected by
rainfall and soil infiltration, and channel capacity are factors that influence terrace
length.
The length should be such that erosive velocities and large cross sections
are not required. The maximum length for graded terraces generally ranges from
about 300 to 500 m, depending on local conditions. There is no maximum length
for level terraces.
Terrace cross-section
After smoothing, the ridge and bottom widths will be about 1 m, which
will give a x-section that approximates the shape of a terrace after 10 years of
farming.
The depth of flow determined from the runoff rate for a 10-year return
period storm or for the required runoff volume for storage-type terraces.
For graded terraces, the Manning equation is suitable for design using a
roughness coefficient of 0.04. The max design velocity should not exceed 0.6 m/s.
The depth should permit a freeboard of 20% of the total depth.
For graded terraces the design peak runoff rate should be based on a return
period of 10 years. The runoff volume for level, and conservation bench terraces
should be based on a 10-year, 24-hour duration storm.
E. Vegetated Waterways
1. Determination of Runoff
Flat triangular or parabolic channels with side slopes of 4:1 or flatter may be
easily maintained by mowing. Side slopes of 4:1 or flatter are also desirable to
facilitate crossing of farm equipment.
Parabolic cross section should usually be selected for natural waterways.
Below are the geometric characteristics of the shapes of cross sections (Figure 13)
4. Design Velocity
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5. Roughness Coefficient
For design purposes, the product velocity multiplied hydraulic radius (vR)
is a satisfactory index of channel retardance. Vegetation has been grouped into five
retardance categories (Table ). The figure below shows the n-vR curves for the
five retardance categories.
T T Berm
Berm
Freeboard
Freeboard t
D D
v d
d
h
b
D = total depth
D = total depth d = design depth
Rectangular Cross- Triangular Cross-section
d = design depth v/h = side slope
section Channel Canal
b = bottom width T = top width
T = top width
T Berm T Berm
Freeboard Freeboard
t
t
D
D
v d
d
h
b
D = total depth
d = design depth D = total depth
b = bottom width Parabolic Cross-section
Trapezoidal Cross-section d = design depth
v/h = side slope Canal
Canal
T = top width T = top width
Table 4-2. Channel x-section, wetted perimeter, hydraulic radius, and top width formulas.
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bd
Rectangular A bd R T=t
P b 2d b 2d
t 2dz
zd
Triangular A zd 2 P 2d z 2 1 R
D
2 z2 1 T t
d
bd zd 2 t b 2dz
Trapezoidal A bd zd 2 P b 2d z 2
1 R
b 2d z 2 1 T b 2Dz
a
t 2d t
R 0.67d
2 8d 2 1.5t 2 4d 2
Parabolic A 3
td P t
3t R
2d
approx. 1
2
3 D
T t
d
6. Channel Capacity
V = R2/3 S1/2 / n
Where: v = average velocity of flow, m/s
n = roughness coefficient of the channel
R = a/p, the cross sectional area divided by the wetted
perimeter, m
S = hydraulic gradient (channel slope)
V = q/a
Figures 15, 16, and 17 give solutions to the Manning equation for retardance
classes B, C, and D, respectively. For trapezoidal channels with 4:1 side slopes the
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depth of flow can be determined for the required cross sectional area, bottom width,
and hydraulic radius.
For small parabolic cross sections, d = 1.5 and for triangular cross sections d =
2R. A freeboard of 0.1 to 0.15 should be added to the design depth.
Retardance Average
Cover Condition
Class Height (cm)
Alfalfa Good Stand, uncut 11
Bermuda grass Good Stand, tall 12
Blue grama Good Stand, uncut 13
Brome, fall fescue Long
B Kudzu Dense or very dense, uncut
Lespedeza sericea Good stand, not woody, tall 19
Reed canary Long
Weeping lovegrass Good stand, tall or mowed 13-24
Wheat Mature, good vR>=1
Bermuda grass Good stand, unmowed 6
Brome, tall fescue Mowed
Centipede grass Very dense cover 6
Common lespedeza Good stand, uncut 11
Crabgrass Fair stand, uncut 10-48
C Grass mixture (orchard grass, red Good stand, uncut 6-8
top, Italian ryegrass, common
lespedeza)
Kentucky bluegrass Good stand, headed 6-12
Reed canary Mowed
Wheat Mature, poor vR>=0.5
Bermuda grass Good stand, cut 2.5
Common lespedeza Excellent stand, uncut 4.5
D Buffalo grass Good stand, uncut 3-6
Grass mixture (as above) Good stand, uncut 4-5
Lespedeza sericea Good stand, cut 2
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Example:
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Design a trapezoidal grassed waterway to carry 5.66 m3/s (200 cfs) on a 3 percent
slope on erosion-resistant soil. The vegetation is to be Bermuda grass, and the channel
should have 4:1 side slopes.
Solution:
From Table 4.2, permissible velocity is 2.4 m/s (8fps). Table 4.3 shows Bermuda
grass in retardance class D, when mowed and in class B when long. To design for stability
against erosion, the mowed condition is the more critical.
From figure 17, for class D retardance with v =2.4 m/s and a slope of 3%, R=0.31.
From figure 7.4, by trial and error solution, the cross sectional area must be
(5.66/2.4)=2.36 m2 and R = 0.31m.
From figure 7.8, for R=0.31m, a=2.36m2, and b=4.0m the depth of flow d=0.41m.
These dimensions will provide a stable channel with v = 2.4 m/s.
The design depth must now be increased when the grass is long with retardance
class B because the velocity is reduced. The previous bottom width of 4.0 must be
retained. By trial and error select a depth of 0.53 m that will have a=3.24 m 2 and R =
0.39.
From fig 7.5 with R=0.39 and a slope of 3%, gives v =1.75 m/s. At the 0.53-m
depth, q=3.24 x 1.75 = 5.67 m3/s, which checks or within 10% is adequate.
The example shows that the bottom width is determined by the need not to exceed
the permissible velocity under mowed condition of minimum retardance, and that the
depth is determined by the need to provide capacity under conditions of maximum
retardance.
References
Del Castillo, R.A., Dalmacio, R.V., Lasco, R.D., and N.R. Lawas. 1994. Soil and Water
Conservation Management. A Training Manual. UPLB Agroforestry Program.
Foster, A.B. 1964. Approved Practices in Soil Conservation. The Interstate Printers and
Publishers, Inc. Daville, Illinois.
PCARRD. 1977. The Philipines Recommends for Soil Conservation. Los Baños, Laguna.
PCARRD. 1991. The Philippines Recommends for Watershed Management. Los Baños,
Laguna.
Schwab, G.O., Fangmeier, D.D., Elliot, W.J., and R.K. Frevert. 1993. Soil and Water
Conservation Engineering. 4th Edition. John Wiley and Sons, Inc.
Troch, F.R., Donahue, R.L., and A. Hobbs. 1991. Soil and Water Conservation. 2nd
Edition. Prentice Hall, Englewood Cliffs, New Jersey.