Erosion and Sediment Process in Highway Construction Sites, Schemes and Mind Maps of Environmental Science

Download Erosion and Sediment Process in Highway Construction Sites and more Schemes and Mind Maps Environmental Science in PDF only on Docsity! SECTION 4 - EROSION AND SEDIMENT PROCESS June 2011 4-1 4.0 EROSION AND SEDIMENT PROCESS 4.1 Mechanics of Erosion Erosion is the wearing away of material by naturally occurring agents through the detachment and transport of soil materials from one location to another, usually at a lower elevation. Natural agents are mostly responsible for this phenomenon but the extent to which erosion occurs can be considerably accelerated through human activities. Water is the predominant agent of erosion on highway construction sites. Wind erosion is not considered a major contributing factor to erosion on highway construction projects because of the localized nature of the exposed areas and the relatively short construction time periods. Thus, methods of controlling water erosion will be the principal focus of this manual. However, many of the methods effective in reducing erosion caused by water are also effective in reducing erosion caused by wind. 4.2 Sedimentation Sedimentation is the deposition of soil particles previously held in suspension by flowing water. The phenomenon of sedimentation takes place at those locations experiencing a reduction in the velocity of flow. Initially the larger particles settle out. As the flow velocity reduces further, the smaller particles settle, eventually, leaving only the clay sized particles, being the smallest, as the last to be deposited. Sedimentation can also occur in slower-moving, quiescent waterbodies, or in treatment facilities such as stormwater ponds. For the purpose of this document, the process of sediment control is equivalent to the control of the sedimentation process. Suspended material, particularly fine organic material such as organic silt, can have low total suspended solids (TSS) test values but high turbidity measurements. TSS is the mass of suspended solids per volume of water whereas turbidity is an indication of the ability of light to pass through the water. Both TSS and turbidity can have detrimental effects on an aquatic environment. Clay particles will only settle out after extended periods of time due to their fine particle size and, the potentially, elevated pH of the water. As a result, settling by gravity alone is often ineffective for clay size particles. 4.3 Types of Water Erosion There are generally four types of erosion that result from water which are illustrated in Figure 4.1. 1. Raindrop (Splash) Erosion: Movement of soil particles caused by the direct impact of raindrops on unprotected exposed soil surfaces. 2. Sheet Erosion: Movement of soil particles by runoff flowing over the ground surface as an unconcentrated thin sheet layer. Erosion is caused by shear stresses associated with water flow. 3. Rill and Gully Erosion: Movement of soil particles due to concentration of runoff in the depressions (rills) in the ground surface. Erosion potential is greater than with sheet flow due to the greater velocity and depth of flow. Further increases in the SECTION 4 - EROSION AND SEDIMENT PROCESS June 2011 4-2 velocity and depth of flow will increase the erosion potential which may gradually enlarge the rills into gullies. Rills are 75 mm or less in depth. Once the depth exceeds 75 mm then formation of gullies occurs (Fifield, 2001). 4. Stream and Channel Erosion: Movement of soil particles on the bed and banks of streams and channels due to concentration of runoff. Scouring, another facet of channel erosion, occurs along channels where eddies form as a result of sudden expansion, contraction or change in flow direction. Scouring may lead to rapid soil loss from the channel bed or sideslopes. Figure 4.1: Types of Water Erosion Erosion potential is reduced by minimizing rainfall impact and by reducing the velocity and depth of surface water flow. The erosion potential increases with increasing flow velocity and depth. 4.4 Factors Affecting Erosion 4.4.1 General Erosion occurs as a result of a number of interacting factors and processes. Four broad factors that affect erosion are as follows:  Climate;  Soil characteristics;  Vegetative cover; and  Topography. Each of these factors is described in the following sections. 4.4.2 Climate The regional climate varies across the province of Alberta. As such, design rainfall event duration and intensity may vary for a given return period based on the location of SECTION 4 - EROSION AND SEDIMENT PROCESS June 2011 4-5 Figure 4.3: Unified Soil Classification System (modified by PFRA) SECTION 4 - EROSION AND SEDIMENT PROCESS June 2011 4-6 Figure 4.4: Grain Size Description According to Various Engineering Soil Classification Systems Source: Holtz and Kovacs, 1981 SECTION 4 - EROSION AND SEDIMENT PROCESS June 2011 4-7 Figure 4.5: Estimated Runoff from Precipitation for Different Soil Types Source: Fifield, 2001 4.4.4 Soil (Vegetative) Cover In nature, the extent of vegetative cover determines to a large extent the erosion that takes place. Vegetative cover is a very durable and highly effective erosion control measure. It achieves its objective by:  Shielding the ground from direct rainfall impact;  Improving the soil permeability;  Reducing velocity of runoff; and  Holding soil particles in place with a root structure from living and dead vegetation (topsoil). Because of the effectiveness of vegetative cover in controlling soil erosion, it is usually the primary choice for long-term erosion control unless there are reasons for doing otherwise.