There are several reasons why reducing sediment losses from construction sites should be implimented. – Prevent sediments from entering storm drainage system
– Prevent sediments from entering watercourses
– Prevent loss of organic soil materials
– Protect human health and safety along with maintaining compliance
Assessing potential erosion and employing the proper ESC practices during the planning stage is crucial towards determining their success.
Processes
There are four processes associated with erosion, sedimentation, and transport of sediment.
– Prevent sediments from
entering storm drainage system – Prevent sediments from entering watercourses
– Prevent loss of organic soil materials
– Protect human health and safety along with maintaining compliance
The intensity and duration of each process determines to a great degree the severity of the erosion process on the site.
Detachment: This is the breaking the bonds that holds materials together. The tractive forces that come from soil erosion are resisted by cohesive forces between the particles of soil. The shape of the soil particles, their roughness, velocity, and discharge can be measured. This is how erosion starts when raindrops impact the soil.
The makeup of the soil will determine its resistance to erosive forces. While all soil will erode when faced with such forces, some resist longer than others. Such particles include gravel, clay, sand, and silt. Clay because of its small size tends to have a higher resistance to erosion compared to gravel which is quite large and loose. The two basic detachment methods are as follows.
– Impact: The force of the raindrops on the soil. The heavier and faster the drops, the greater the impact.
– Abrasion: Particles of soil moved by the water or the wind exert abrasive forces on other soil particles that are not in motion, such as those still on the ground. The abrasion results in additional detachment.
Entrainment: This is the picking up of particles which have been detached by wind or water. Entrainment follows detachment and can be caused by the following.
– Gravity: The greater the degree of sloping, the more gravity acts on the particles
– Fluid Forces: Wind and runoff exert drag on the particles.
It takes greater force to detach particles compared to when they are entrained. In addition, gravity affects particles whether in the air or in water. For fluid forces, the density of the liquid will determine the drag that is affecting the particles.
Transport: For materials that are not dissolved in the flowing water they tend to be transported by one of three methods.
– Suspension: Moving through the flow without touching the bottom
– Saltation: Particles dislodged by other particles in the flow
– Traction: Particles slowly moving along the surface in the flow
Particles that move by traction tend to be much slower compared to those that move by suspension or saltation.
Sedimentation and Deposition: This is the buildup of particles that can no longer be propelled by the runoff or airflow. Such particles tend to slow the flow velocity which in turn means a greater buildup or sedimentation occurs. Given that larger particles react to a greater degree in flow velocity to smaller particles, they tend to gather first.
Tag: Erosion, Control, runoff, sediement, fluid, processes, traction
