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
aUniversity of Zagreb, Faculty of Civil Engineering, Kačićeva 26, 10000 Zagreb, Croatia,
bUniversity of Zagreb, Faculty of Civil Engineering, Kačićeva 26, 10000 Zagreb, Croatia,
cUniversity of Zagreb, Faculty of Civil Engineering, Kačićeva 26, 10000 Zagreb, Croatia,
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 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 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.
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.
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 intensityRainfall of low intensity
(< 6,5 mm/h) / Rainfall of moderate intensity
(cca 6,5 mm/h) / Rainfall of high intensity
(> 25,4 mm/h)
Suspended solids, mg/l
Particle size, μm / 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 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.