Watershed Erosion and Sediment Yield During
Construction Activities
Assistant Dražen Vouk, B.Sc.CE a
Professor Davor Malus, Ph.D.CE b
Assistant Damir Bekić, M.Sc.CE c
University of Zagreb, Faculty of Civil Engineering, Kačićeva 26, 10000 Zagreb, Croatia,
dvouk@grad.hr
b
University of Zagreb, Faculty of Civil Engineering, Kačićeva 26, 10000 Zagreb, Croatia,
malus@grad.hr
c
University of Zagreb, Faculty of Civil Engineering, Kačićeva 26, 10000 Zagreb, Croatia,
dbekic@grad.hr
a
Abstract
Soil erosion problems on construction sites and resulted transport of sediment downstream
the watershed, as well as, its final settling are comprised within this paper. Negative aspects
of such processes are primary manifested through sediment buildup in downstream area
and its influence on disturbance of natural biological balance in nearby water bodies and on
surface area.
Strong correlation between the volume of earth works and generating quantities of
suspended sediment puts the highway construction sites under special concern. Extensive
highway network development in Croatia during the last few years has witnessed that little
or no attention's been given to this problem. Since new highway directions are planned and
overall process of large surface area disturbance hasn't been finished, implementation of
sophisticated and well-known best management practices (BMPs) in erosion control should
be considered.
The importance of implementing such practices was shown on the case of highway “Istarski
Ipsilon” located in the middle-southern Istra region (Croatia). Due to the world-wide
acceptance, Revised Universal Soil Loss Equation (RUSLE) was used to estimate the
degree of soil loss. Investigation results emphasize that good after construction
maintenance program is also one of the essential elements in reducing the erosion rate of
the soil and avoiding downstream problems with sediment deposition. The problems
concerning additional expenses for erosion control measures and their descriptive
comparison with damages resulted from plugging the drainage lines, constant loosing of
hydraulic capacity in pertinent separators (grit and grease chambers) and lagoons as well as
the reduction in their treatment efficiency were also implied.
This paper proposes systematic approach concerning soil loss prediction together with
different measures to control surface erosion and transport of sediment.
Key words: erosion, sedimentation, highway construction, BMP's
1
Introduction
Problems concerning surface soil erosion together with resulted transport of
sediment have been influencing the natural water ecosystems since the earth was formed.
Surface erosion is considered to be among the most destructive phenomenon on earth. It is
natural geological process which occurs primarily under the force of raindrops, flowing water
and wind by detaching the soil particles from the surface, allowing them to be washed or
blown off and eventually deposited downstream the watershed (in nearby water bodies). All
types of soil are more or less vulnerable to the erosion processes even though the erosion
potential depends on large number of different factors. Four principal factors and their
interrelation have the highest influence on erosion potential of an area: the climate (rainfall –
intensity and duration), soil characteristics (texture and structure), topography and
1
vegetative cover. Although wind erosion might take high influence on the total quantity of
detached material, it would be left out of any further consideration within the given analysis.
The main purpose of this work was to investigate the influence of water forces as the main
erosion actuator on the watershed, which involves the action of raindrops and surface
runoff.
In comparison with natural soil erosion with relatively small and uniform rates and
quantities of lost soil almost identical to amount that is produced, accelerated or
anthropogenic erosion could be distinguished. Accelerated erosion is the result of human
activities on earth. It is caused by land disturbance and removal of vegetative cover or other
natural protection of the soil. Loosing the topsoil protective layer, soil becomes vulnerable to
erosion processes. Next to agriculture, construction activities are principle causes of
accelerated erosion. Natural soil compactness is temporarily disturbed on construction sites,
especially those with large amount of earth works. Stormwater runoff from construction sites
could strongly influence the quality of nearby water bodies. For that reasons construction
zones require detailed analysis and related control measures to protect natural environment
habitats, especially water ecosystem. In respect to overall scope of earth works, special
erosion protection care must be taken during highway construction. Furthermore, highway
construction has linear character, which is likely to intersect the surface waterways and
increase the potential of accumulating higher quantities of sediment. These negative
aspects also affect related management practices in controlling the highway runoff quality –
drainage channels, grit and grease chambers, lagoons, grassed swales, constructed
wetlands, etc. Washing out topsoil layer in highway corridor including cut and fill slopes,
specific amount of yielded sediment reaches the drainage system. Sediment accumulation
rate augmentation in drainage channels may result with culverts plugging and further
sediment transport with its final deposition in the runoff treatment structures. Due to constant
deposition of detached particulate matter, the settling layer at the structure bottom increases
and affects its efficiency by decreasing effective volume. Within the runoff treatment
structures of "wet" character (with permanent submersion) like wet lagoons and constructed
wetlands, diverse water ecosystem is emphasized which is very sensitive on sediment
augmentation. Hence, erosion and sedimentation control on highway construction sites and
subsequent highway maintenance requires additional efforts and careful planning.
This investigation comprises erosion rate prediction on particular segment of
highway "Istarski Ipsilon" (Vodnjan-Pula) during its construction and maintenance, as well
as, the influence of yielded sediment on related runoff treatment structures – separator (grit
and grease chamber) and lagoon. Special interest on selected area is given from the aspect
of hilly terrain configuration and flysch geological characteristics, which altogether result with
increased erosion potential. Among dozens of available erosion prediction models, widely
accepted RUSLE (Revised Universal Soil Loss Equation) method was used for this purpose.
In contribution to better understanding of problems analyzed within the given investigation,
basic erosion, as well as, sediment transport and deposition principles will be described.
2
Surface erosion processes and deposition of sediments
2.1
General
Soil erosion is the process of loosing the topsoil layer by the action of different forces
and the water action as the most dominant one. The process involves detachment of soil
particles within the surface layer and further transport of sediments by flowing water with its
final deposition downstream the watershed. Rainfall is described as one of the basic
initiators of water erosion processes. Raindrops strike the soil surface and detach the soil
particles (Splash erosion - Figure 1). Destructive character of raindrops primarily depends
on the soil properties especially on the cover type of land surface. Therefore, areas that are
covered with some type of vegetation are less susceptible to erosion impacts in comparison
with bare and sparsely vegetated soils. Soil particles detached from compact soil mass are
2
picked up and carried away by stormwater surface runoff. Shallow sheets of water flowing
are formed at the beginning of surface runoff causing "sheet erosion" (Figure 1). That type of
erosion has uniform character without distinct turbulence and it rarely acts in destructive
manner on soil surface. Its basic activity includes transport of sediment detached by
raindrop impact. Sheet erosion is related to relatively narrow area (less than a few meters)
before shallow surface flow begins to concentrate. The sheet flow concentration results with
velocity and turbulence increase, which in turn causes the detachment and transport of
more soil particles. That negative aspect of the concentrated flow is called rill erosion as its
action forms tiny channels of a few centimeters deep, called rills (Rill erosion – Figure 1). As
the rills flow into each other larger and larger channels are formed resulting with higher
flowing energy and more destructive action in the term of "gully" erosion. Finally, the mixture
of runoff and sediment reach the stream or drainage channel and is transported further on.
When the stream velocity slow down to a certain degree (retention, detention, runoff
treatment structures), suspended particles begin to settle out.
SPLASH EROSION
SHEET EROSION
RILL EROSION
GULLY EROSION
STREAM OR CHANNEL
Figure 1. Types of soil erosion (NCSCC, 1988)
2.2
Factors influencing erosion
Different factors influence the inherent erosion potential of an area, but its rate is
primarily determined by four principal factors: soil characteristics, climate, topography and
vegetative cover. In spite of their different character, all of these factors are interrelated.
Regarding the natural erosion genesis caused by action of rainfall and surface runoff,
the most significant soil characteristics are those pertinent to infiltration capacity of the soil
and topsoil stability in the meaning of resistance of the soil to detachment and sediment
transport by surface runoff. Thereby, factors such as soil texture and structure, organic
matter content and soil permeability must be emphasized.
All soils consist of certain percentage of clay, sand and silt particles, which defines
its texture characteristics. Soils that contain higher percentage of silt and fine sand are more
susceptible to erosion impact and described as the most erodible. Increase of clay and
organic matter content results with lower erosion rates. Soils with higher fractions of course
particles are least susceptible to erosion. Besides its moderate to rapid permeability, good
resistance to destructive action of rainfall characterizes those soils.
The source of organic matter in soil might be human or animal and found in different
stages of decomposition. Increased content of organic matter in soil improves the structure
stability, as well as, permeability which results with higher erodibilty resistance.
Texture and structure rank among basic parameters that determine soil permeability.
Higher permeability rate results with decrease quantities of surface runoff and additional
increase in erodibilty resistance.
3
Rainfall intensity, duration and frequency are fundamental factors in determining the
surface runoff characteristics. Increased runoff volume and velocity affect the topsoil
stability, increase the capability of soil particles detachment and downstream transport of
sediments. Frequent, persistent and high intensity rains contribute to additional increase in
soil erodibility. As an example, runoff quality analyses at 70 construction sites in the
Birmingham area, USA, will be shown (Nelson, 1996). Table 1 shows the measured values
of suspended solids concentrations and particle sizes in relation to rainfall intensity.
Seasonal climate characteristics define the higher erosion risk periods of the year.
Table 1. Runoff quality at construction sites (Nelson, 1996)
Rainfall intensity
Suspended solids, mg/l
Particle size, μm
Rainfall of low
intensity
(< 6,5 mm/h)
Rainfall of
moderate intensity
(cca 6,5 mm/h)
Rainfall of high
intensity
(> 25,4 mm/h)
400
3,5
2000
5
25000
8,5
Among the factors that describe the site topography, the biggest influence on erosion
potential have size, shape and slope characteristics. Volume and velocity of runoff are
primarily dependent on slope length and gradient with proportional relation between them.
Slope orientation is also considered to be the important element in determining the erosion
potential.
Vegetative cover is one of the most influencing factors in generating erosion process
on surface area and its control. Besides the soil protection from destructive character of
raindrops, vegetation also improves soil stability, reduces the runoff velocity and decreases
the runoff volume. Bare and disturbed soils without protective cover are more susceptible to
erosion impacts.
3
Problem analysis
With respect to the existing data (projects) availability, the segment Vodnjan-Pula of
highway "Istarski Ipsilon" has been analyzed within this investigation. The segment is
located in south-western part of Istrian peninsula near Pula town (Figure 2). Observed area
is characterized by hilly terrain with lightly U-shaped valleys. Altitudes range between 100,0
and 150,0 m.a.s.l. The area receives about 800 mm of annual precipitation and most of it
falls as rain. Snow is a very rare condition and lies for insignificant periods with annual mean
of 5 days. From geological aspect the flysch is dominant top layer earth formation, which
describes the specified area with increased erosion potential.
The main purpose of the study was to mathematically express the soil loss on given
area due to generation of erosion processes and further transport of sediments. Altogether,
two investigations were carried out. The first one presumes no protection while the second
one includes the use of erosion and sedimentation control measures on disturbed and
surrounding area. Thereby, intension was to indicate the magnitude of problems concerning
erosion and sedimentation in construction and maintenance of roads, especially those with
wider corridor such as highways. Erosion rate was estimated first and the influence of
sedimentation in runoff treatment structures was investigated afterwards. Two such
structures are implemented within the analyzed segment of highway – separator (grit and
grease chamber) and treatment lagoon. Separator is located along the highway on the
extended plateau with outlet connected on lagoon via concrete pipe Φ600mm in total length
of 42,0m. Dimensions of the separator were determined based on hydraulic calculations, so
length/width of 8,0/3,0m were chosen. Lagoon is designed with average depth of 1,0 m and
effective volume of 660m3. In accordance with applied design, lagoon is shaped as
extended detention pond, which represents the main treatment structure and most of
4
sediments settle therein. Separator manages to detain only 30% of washed material since
the transported sediment contain large portion of small-sized particles, which easily reach
the lagoon. Such design with separator placed in front of lagoon describes common practice
in Croatia, although in general it's not a prescribed standard. Lagoons could be designed
with all necessary functions undertaken, without the need for additional separator as it is the
case accepted world-wide. For that reason, additional scenario will be analyzed that
includes the design of treatment lagoon without separator. The outlet structure of the lagoon
is connected to the drainage well as the final disposal practice with percolation into the
ground. Perforated concrete pipe (Φ600mm) in total length of 25,0 m is installed for that
purpose. So, collected sediment could not further spread downstream the watershed, but
instead remain deposited in the lagoon by decreasing its effective volume. There is also a
thought that sediment might reach the drainage well and plug its pores, which increases the
magnitude of analyzed adverse effects.
Observed segment is characterized by combination of cut and filled zones in laying
out the main road. Regarding the generation of erosion process within the highway corridor
and further transport of sediments there are certain distinctions between cut and fill zones.
Soil particles from cut slopes transported with surface runoff reach the internal and closed
drainage system of the highway and are easily transported to the treatment structures
without possibility of settling along-the-way. Surface runoff on filled slopes tends to reach
the external drainage system with open channel flow and final disposal in nearby
depressions or water streams with constant linear infiltration into the soil. Thus, significant
amount of transported sediments is captured at the same watershed area without reaching
the treatment structures but it doesn't diminish the negative aspects of erosion actions on
filled slopes to the full extend because the character of soil loss is permanent. Concerning
the main purpose of the study, which is mentioned earlier, the segment area with cut slopes
only has been analyzed. It includes the segment area from chainage 9+701,50 to
12+915,00.
Figure 2. Analyzed area
Regarding the erosion processes generation on observed area and its negative
aspects including the soil loss and downstream transport of sediments, two different time
periods could be analyzed separately. The first one includes construction period during
which the most intensive erosion impacts are expected, considering the greatest portion of
bare and disturbed surface area. According to prescribed maintenance program all residual
material on constructed highway must be removed, inner drainage system must be washed
and runoff treatment structures must be cleared with all settled material removed from it,
before putting the traffic in operation. Finalizing the highway construction, cut and fill slopes
only remain potentially erodible zones. Their erosion potential is particularly emphasized
until the permanent vegetative cover restores. Due to mentioned factors, the study analysis
comprises the negative aspects of soil loss and sediment settling in the adjacent treatment
structures (separator and lagoon) for the operational period of time only, after highway
construction.
5
4
Methods
First soil loss prediction models, based on water actions (precipitation and runoff),
were developed during the mid 20th century in USA. Perennial data collection and its
processing from thousands of different locations served the research scientists to define the
most of relevant factors in erosion potential prediction. The given empirical soil loss equation
was found under the term Universal Soil Loss Equation (USLE) and was described by
Wischmeier and Smith (1978). Even though it became the most widely accepted tool, at first
it was used in estimation the soil loss on agriculture surface area only. With addition
research and experiments, scientists tried to adopt the model for the use on different areas
such as construction sites, which led to the further upgrading of USLE and development of
new and modified models. Since the main object of this study describes the erosion
problems within the highway corridor during its use and maintenance, Revised Universal
Soil Loss Equation (RUSLE) was chosen as competent model adopted to construction sites
and other disturbed areas. RUSLE has the same formula as USLE, but has several
improvements in determining factors. These include some new and revised isoerodent
maps; a time-varying approach for soil erodibility factor; a subfactor approach for evaluating
the cover-management factor; a new equation to reflect slope length and steepness; and
new conservation-practice values (Renard, et al., 1997). RUSLE is used to predict the soil
loss in the area until the concentrated flow is formed (splash, sheet, rill and gully erosion
zones). Concentrated flow erosion impact is excluded because the highway drainage
system is piped-closed and doesn't agitate any further soil loss. RUSLE is an erosion model
for computing the soil loss per unit area on yearly basis:
A  R  K  LS  C  P
(t / ha  y )
(1)
where is:
R ........... erosivity index – rainfall and runoff factor (N/h)
K ........... soil erodibility – soil factor with respect to soil loss (t∙h/ha∙N)
LS ......... slope length and steepness factor
C ........... cover and management factor
P ........... support practice factor
RUSLE enables identifying the critical areas, which are particularly susceptible to
erosion impact and whereon the most intense soil loss is expected. Thereby, it makes the
selection of optimal management (protection) practice easier, as well as, increases their
efficiency and reduces additional expenses. Hereafter, each of these factors will be
discussed separately.
Erosivity index, R, represents the energy that is referred to splashing effect of the
falling raindrops and its further surface flow (runoff). Destructive effect of rainfall kinetic
energy is manifested by the impact of raindrops, which causes the topsoil particles to
detach. When surface flow is formed, detached particles are easily transported downstream
the watershed. R factor must not be misinterpreted as rainfall factor only because runoff
also participates in generating the erosion processes. Peak rate of runoff, as relevant value,
is determined by the maximum 30-minute-rainfall intensity (I30). Erosivity index value could
be defined as the average annual value of the sum derived by the products of rainfall kinetic
energy (E) and I30 -value. Therefore, rainfall intensity data (daily based) must be available to
define the R factor. Fournier (1960) defined a simplified method for determining the R factor
in the form of empirical equation:
p
R
P
2
(N / h )
(2)
where is:
p ........... average monthly precipitation value for selected time period (mm)
P ........... total annual precipitation (mm)
6
Having the relevant rainfall data at multiple raingauges, Fournier method helps to
generate an isoerodent map for the observed region. Isoerodent map is defined as irregular
lined mesh of equal average annual values of the R factor. Figure 3 shows the isoerodent
map for Republic of Croatia, based on Fournier method and data collected in period 19651984 (Bašić et al., 1992).
Erosivity index (R) values based on Fournier method
Figure 3. Isoerodent map for Republic of Croatia (based on period 1965.-1984.)
Soil erodibility factor, K, is described by the rate of soil vulnerability to erosion
processes. K factor value is primarily dependent on soil properties – its texture, structure,
organic matter content and soil permeability. The basic textural characteristics are described
by the content of silt+very fine sand (0,002 – 0,1mm) and the content of certain fractions of
sand (0,1 – 2,0mm). Increased content of silt and very fine sand reduces the soil resistance
to erosion resulting with higher K factor values. Organic matter content is also an important
parameter in determining the soil erodibility and is proportional with soil resistance to its
particles detachment. Removal of the topsoil humus layer at construction sites causes
reduction in organic matter content resulting with further increase in soil erodibility. Having
these soil properties known, K factor can be easily calculated by using the different
nomographs based on results from large number of pilot projects. One of the most common
nomograph used in calculation the K factor is given in Figure 4 (Wischmeier et al., 1971).
Both length and steepness of the slope considerably affect soil erosion caused by
action of water. Slope length and steepness factor (LS) is dimensionless parameter and
describes the influence of these two parameters on the surface erosion rate. Longer slopes
with higher gradients are more susceptible to erosion impact. In fact, LS factor represents
the expected ratio of soil loss per unit area of observed slope (with particular slope and
gradient) to that from a standard unit plot. The length of 22,1m and uniform slope at 9%
define standard unit plot with pertinent LS value of 1. Specially defined empirical equations
7
for both slope length (L) and steepness (S) are used in determining the LS value at
construction sites. Based on these empirical expressions, different tables, graphs and
nomographs are generated with purpose of simplifying the LS factor determination. The
Table 2 shows LS factors for highly disturbed soil condition with little or no cover, such as
construction sites (Renard et al., 1997).
Figure 4. Nomograph for estimating the soil erodibility factor (K)
Table 2. LS values at construction sites and other highly disturbed areas
Slope
gradient
(%)
Slope length (m)
2
3
4
5
0.2
<1
0,05
0,05
0,05
0,05
0,05
10
0,05
15
0,05
20
0,05
30
0,05
50
0,05
0.5
0,07
0,07
0,07
0,07
0,07
0,07
0,08
0,08
0,09
0,09
1.0
0,09
0,09
0,09
0,09
0,09
0,1
0,13
0,14
0,15
0,17
2.0
0,13
0,13
0,13
0,13
0,13
0,16
0,21
0,25
0,28
0,33
3.0
0,17
0,17
0,17
0,17
0,17
0,21
0,3
0,36
0,41
4.0
0,2
0,2
0,2
0,2
0,2
0,26
0,38
0,47
0,55
5.0
0,23
0,23
0,23
0,23
0,23
0,31
0,46
0,58
0,68
6.0
0,26
0,26
0,26
0,26
0,26
0,36
0,54
0,69
0,82
8.0
0,32
0,32
0,32
0,32
0,32
0,45
0,7
0,91
10.0
0,35
0,37
0,38
0,39
0,4
0,57
0,91
12.0
0,36
0,41
0,45
0,47
0,49
0,71
1,15
14.0
0,38
0,45
0,51
0,55
0,58
0,85
1,4
16.0
0,39
0,49
0,56
0,62
0,67
0,98
20.0
0,41
0,56
0,67
0,76
0,84
1,24
25.0
0,45
0,64
0,8
0,93
1,04
30.0
0,48
0,72
0,91
1,08
1,24
40.0
0,53
0,85
1,13
1,37
50.0
0,58
0,97
1,31
60.0
0,63
1,07
1,47
60
0,06
75
100
125
200
250
300
0,06
0,06
0,06
0,06
0,06
0,06
0,1
0,1
0,1
0,11
0,12
0,12
0,13
0,18
0,19
0,2
0,22
0,24
0,26
0,27
0,37
0,4
0,43
0,48
0,56
0,63
0,69
0,5
0,57
0,64
0,69
0,8
0,96
1,1
1,23
0,68
0,79
0,89
0,98
1,14
1,42
1,65
1,86
0,86
1,02
1,16
1,28
1,51
1,91
2,25
2,55
1,05
1,25
1,43
1,6
1,9
2,43
2,89
3,3
1,1
1,43
1,72
1,99
2,24
2,7
3,52
4,24
4,91
1,2
1,46
1,92
2,34
2,72
3,09
3,75
4,95
6,03
7,02
1,54
1,88
2,51
3,07
3,6
4,09
5,01
6,67
8,17
9,57
1,87
2,31
3,09
3,81
4,48
5,11
6,3
8,45
10,4
12,23
1,64
2,21
2,73
3,68
4,56
5,37
6,15
7,6
10,26
12,69
14,96
2,1
2,86
3,57
4,85
6,04
7,16
8,23
10,24
13,94
17,35
20,57
1,56
2,67
3,67
4,59
6,3
7,88
9,38
10,81
13,53
18,57
23,24
27,66
1,86
3,22
4,44
5,58
7,7
9,67
11,55
13,35
16,77
23,14
29,07
34,71
1,59
2,41
4,24
5,89
7,44
10,35
13,07
15,67
18,17
22,95
31,89
40,29
48,29
1,62
1,91
2,91
5,16
7,2
9,13
12,75
16,16
19,42
22,57
28,6
39,95
50,63
60,84
1,84
2,19
3,36
5,97
8,37
10,63
14,89
18,92
22,78
26,51
33,67
47,18
59,93
72,15
The cover and management factor (cropping factor), C, is dimensionless and defined
as the ratio of the soil quantities that were actually lost from an area with specified cover and
management to the corresponding loss from an completely bared and tilled area of equal
size. The value of C factor in such aforementioned surface conditions of complete bareness
8
equals 1. Any protection against the impact of falling raindrops and surface runoff will
reduce soil loss resulting with lower values of C factor. Thus, C factor value depends on the
type of vegetative cover and its characteristics as well on the efficiency of additional
management practices. Protective character of vegetative cover is manifested not only
through reduced destructive effect of falling raindrops, but the leakage loss
(evapotranspiration) and reduced runoff velocity as well. Removing the natural protective
topsoil layer increases the soil susceptibility to erosion impact, which finally results with
higher quantities of material that was washed out. Growing stage of vegetative cover during
the rainfall periods is also one of the essential elements in generating the erosion processes
on observed area. Protective role of natural vegetative cover could be substituted with
different forms of management practices such as hydroseeding, chemical stabilization, straw
mulching, covering the area with geosynthetic materials, etc. Moreover, determination of C
factor is one of the most delicate stages in erosion rate prediction, since it is a function of
different interactive relationships among the mentioned parameters.
Support practice factor, P, is dimensionless and evaluates the efficiency of specific
support practices (concerning the soil cultivation) in soil loss prevention. Herein, the
preparation of soil for seeding and the way of further cultivation are the most important
factors. In the most adverse case, described by cropping up and down as the standard
tillage procedure, the value of P factor equals 1. Changing the direction of cropping by
converging it to perpendicular to the slope reduces the P factor and overall erosion impact.
Applying the P factor within the RUSLE method is primarily associated with agricultural
areas. At construction sites the P factor is generally irrelevant and taken into consideration
with value of P=1.
5
Calculations and Results
Entire calculation procedure is based on determination of soil particles that were
washed out at cut slopes and transported throughout the highway drainage system with final
settlement in separator and lagoon. Altogether two different cases (scenarios) were
analyzed. The first one presumes a separator integrated next to the lagoon as a first
(preliminary) stage in overall treatment process. Second scenario considers the runoff
treatment without installation of separator. Implementation of different management
practices were analyzed within the both scenarios as well as the case without any erosion
control measures. Management practices (erosion BMP's) were selected in a manner to
best suite the local conditions.
The use of RUSLE method requires determination of all necessary input parameters
(defined in previous chapter), which coincide with the local conditions. The first three factors
from equation (1) describe general parameters that are only assigned to characteristics of
observed locality. For that reason, these three factors have equal values within each
scenario analysis.
As described earlier, determination of rainfall erosivity index (R) is based on Fournier
method and adjacent isoerodent map given in Figure 3. For selected location the R factor
could be easily determined with the value of 80,0 N/h.
Soil erodibility factor (K) is primarily defined by the soil properties. Having the soil
texture, structure, organic matter content and soil permeability known, it is easy to determine
the value of K factor by using the nomograph given in Figure 4. Finally, the K factor for
selected location is determined with the value of 0,55.
Slope length and steepness factor (LS) is determined by using the simplified method
given in Table 2. Considering irregular characteristics of cut slopes along the selected
segment of highway, mean slope length and gradient values (L=8,0m; S=100%) were taken
to simplify the overall analysis. Extrapolation of these values in Table 2 gives the competent
LS factor value of 4,6.
C factor value is primarily dependent on the implementation of management
practices regarding the protective character of different types of soil cover (vegetative,
geosynthetics, mulching). Therefore, different C factor values corresponding to selected
9
erosion BMP's will result with different quantities of soil loss and will be given in separate
interrelated comparison as the final result of the given analysis. Two erosion BMP's were
selected as the appropriate ones at selected area – MUBV (geonet made of natural
biodegradable materials such as straw and/or coir fibres) and MIZMO (protective polymers
net filled with soil material and additional mixture of fertilizers and seed for permanent
greening). Since no field investigations were carried out on selected location, C factor
values were estimated based on literature data (Wischmeier et al., 1978). With respect to
the MUBV application the C factor was estimated with value of 0,05. In the second case of
implementing the MISMO two different stages were observed regarding the time period
needed for the full vegetative cover establishment and afterwards. The corresponding value
of C factor within the first stage equals 0,45 and the second one 0,1. Time period until the
permanent vegetative cover is established depends on different parameters (vegetation
type, climate, etc.) and was estimated for the purpose of given analysis with 3 months. For
the case that implicates no erosion control measures the C factor value equals 1,0.
Results of the first stage in overall calculation procedure are presented in Figure 5
with respect to implementation of selected erosion BMP's as well as the case without any
protection measures. Results were given in the form of calculated quantities of soil material
that was washed out on observed cut slopes, in the first year after the highway was put in
use.
809,6
Quantities of soil loss (t/y)
900
800
700
600
500
400
300
172,04
200
40,48
100
0
Without protection
MUBV
MIZMO
Figure 5. Quantities of soil loss on adjacent cut slopes
Based on given results, the volume of settled material in adjacent runoff treatment
structures (separator and lagoon) could be easily determined. As it was mentioned earlier,
separator manages to detain about 30% of soil particles that were washed out. The
comparison regarding the generation of different volumes of settled material in separator
and lagoon is given in Figure 6. The resulted volumes correspond to the previously derived
results.
Sediment settling in separator and lagoon reduces their effective capacity and
causes further reduction in runoff detention time, which finally results with lower runoff
treatment efficiency. Equation 3 could be used to calculate the treatment efficiency of the
lagoon that was shaped as extended detention pond (Young et al., 1996).
Lagoon treatment efficiency was calculated for each scenario and comparison of the
results is given in Table 3. Coefficients 'a' and 'b' are empirical values based on field
investigations and taken from literature (Young et al., 1996).
R  a  t bd
(3)
where is:
R
= treatment efficiency (%)
td
= detention time (h)
a, b
= coefficients
10
Lagoon
Volume of settled material (m3/y)
500
Separator
450
400
350
347
300
250
200
150
74
149
100
17
50
32
7
0
Without protection
MUBV
MIZMO
Figure 6. Volume of material that was settled out in separator and lagoon
Table 3. Treatment efficiency depending on application of erosion control measures
Treatment efficiency (%)
Parameter
Lagoon with separator
Lagoon without separator
Coefficients
Without
protection
MUBV
MIZMO
Without
protection
MUBV
MIZMO
a
b
Total suspended solids
77
90
88
66
89
87
41,5
0,2
Lead
77
90
88
66
89
87
41,5
0,2
Zinc/copper
45
50
49
42
50
49
31,4
0,12
Phosphorus
45
50
49
42
50
49
31,4
0,12
Total COD
45
50
49
42
50
49
31,4
0,12
Nitrogen
33
40
39
27
40
38
15,2
0,25
The results show moderate reduction in lagoon treatment efficiency disregarding the
implementation of erosion control measures at disturbed cut slope areas. Designing the
runoff treatment system without separator it is certain that all soil particles that were washed
out reach the lagoon and settle down in it. Therein, the lagoon losses more of its effective
capacity, which results with additional reductions in its overall efficiency.
Transport of sediment and its settling throughout the highway drainage system
address the need for additional efforts in maintenance program, especially pertained to
treatment structures such as separators and lagoons in which most of the sediment is
deposited and requires frequent discharging. Additional maintenance caused by plugging
the drainage channels or culverts might also be of special concern especially during
construction and initial period of highway use in which the most of the sediment
accumulation is likely to happen.
In some cases, increased costs caused by intensive maintenance might overtake the
costs needed for erosion control measures implementation, which would prevent such
adverse actions to happen. Even in such circumstances of lower maintenance costs
compared to erosion BMP's implementation, additional qualitative aspects (which are hard
to express on money basis) must be encountered within the analysis. That includes primarily
the aspects of sustainability, which comprise valorization of permanent soil loss on selected
area, sound of esthetics, etc.
6
Conclusion
The main objective of the study was to indicate the magnitude of erosion adverse
effects on disturbed and bare soils as well as the further transportation of sediments and
their deposition in nearby water bodies including the runoff treatment structures.
The results emphasize the importance of erosion control measures implementation
not only during construction phase but the highway use as well. Good after construction
11
maintenance program is one of the essential elements in reducing the negative aspects of
processes such as soil loss and sedimentation of soil particles downstream the watershed.
The implementation of protective measures that will reduce the erosion rate to acceptable
levels is often in opposite direction to the investor endeavor in reducing the total
construction, operation and maintenance costs to the maximum extend. Accepting the
further economy development under the basic principles of sustainable development, it
would be preferable to take all available protection measures at sites with higher erosion
potential, no matter on eventually required additional expenses.
Taking the climate characteristics of the selected area into consideration, which are
described with average R factor values (analyzing the Croatian territory only), it could be
realized the importance of the erosion BMP's implementation at sites with bigger portion of
high intensity rainfall.
A special concern of analyzed problem is coming out if the natural water bodies are
taken as the runoff recipients. In that case there is disturbance potential of natural biological
balance as the result of large quantities of sediment buildup in pertinent water ecosystems.
The results also suggest a need to divert the collected runoff besides the treatment
structures just for the initial period of highway use until complete vegetation establishment
on disturbed areas, even in the case of erosion BMP's implementation.
7
References
Bašić, F., Vidaček, Ž., Petraš, J., Racz, Z. (1992): Distribution and Regional Peculiarities of
Soil Erosion in Croatia, 203-518 pgs., Polj. znan. smotra, Zagreb, Hrvatska.
Fournier, F. (1960): Climat et erosion; la relation entre I erosion, du sol par I ean et les
precipitations atmosferiques, Presses, Univertsitaces de France, Paris, France.
Kisić, I., Bašić, F., Butorac, A., Mesić, M., Nestroy, O., Sabolić, M. (2005): Erozija tla vodom
pri različitim načinima obrade, 95 pgs., Agronomski fakultet Sveučilišta u Zagrebu, Zagreb,
Hrvatska.
Nelson, J. (1996): Characterizing Erosion Processes and Sediment Yields on Construction
Sites, MSCE thesis, 94 pgs., Dept. of Civil and Environmental Engineering, University of
Alabama at Birmingham, USA.
North Carolina Sedimentation Control Commission - NCSCC (1988): Erosion and Sediment
Control Planning and Design Manual, Raleigh, NC, USA.
Renard, K.G., Foster, G.R., Weesies, G.A., Cool, D.K., Yoder, D.C. (1997): Predicting Soil
Erosion by Water: A Guide to Conservation Planning with the Revised Universal Soil Loss
Equation (RUSLE), USDA-ARS, Agriculture Handbook Number 703, USA.
Young, G.K., Stein, S., Cole, P., Kammer, T., Graziano F., Bank, F. (1996): Evaluation and
Management of Highway Runoff Water Quality, Office of Environment and Planning, HEP40, Federal Highway Administration, Washington, DC, USA.
Wicshmeier, W.H., Johnson, C.B., Cross, B.V. (1971): A soil erodibility nomograph for
farmland and construction sites, 189-194 pgs., Journal of Soil and Water Conservation,
26/5, USA.
Wischmeier,W.H.;Smith,D.D. (1978): Predicting Rainfall Erosion Losses — A guide to
conservation planning, USDA Agric. Handbook No. 537, US Gov. Print. Off., Washington,
D.C., USA.
12