Erosion and Sedimentation FAQ’s


Q: What is erosion and sedimentation?

A: By definition: “Erosion” – The natural process by which the surface of the land is worn away by water, wind, or chemical action.

“Sedimentation” – The action or process of forming or depositing sediment into waters of the commonwealth.

Q: How does earthmoving and land development affect erosion and sedimentation?

A: Land development and earthmoving activities often expose soils to the erosive forces of water through precipitation and stormwater runoff.  The shaping of land for construction or development purposes removes the soil’s protective cover and changes the characteristics of the soil itself in many ways that are often detrimental to runoff patterns, stream flow characteristics and infiltration.  Protective vegetation is reduced or eliminated, topsoil is removed, and cuts and fills are made, changing topography and runoff characteristics for the site.  This can increase the rate at which erosion takes place to 10 or many more times the natural rate, depending on site conditions. 

Q: Why should I be concerned about Erosion and Sedimentation?

A: What is the harm if a little mud washes into the stream? Erosion takes place all the time naturally, so what’s the big deal? Do you know how much these BMP’s are costing me?  - These are all legitimate questions often asked by earthmovers that do deserve an answer.

Accelerated erosion and sedimentation due to earthmoving activities has negative impacts not only on the environment but also economically on all of us residents and tax payers.

Environmental Impacts:  Stormwater discharges generated during construction have a potential for serious water quality impacts.  The biological, chemical, and physical properties of the waters may be severely affected.  The erosion and transportation of sediment into aquatic ecosystems is the primary pathway for delivering nutrients (especially phosphorus), metals, and organic compounds.  According to the Pennsylvania Fish and Boat Commission, “Sediment pollution in lakes, reservoirs and bays can introduce excess nutrients resulting in algal blooms; block the amount of available sunlight for aquatic plants; reduce water depth resulting in warmer water temperatures; and speed up the water bodies natural aging process (eutrophication).”  Excess sediments are associated with increased turbidity and reduced light penetration in the water column, as well as more long-term effects associated with habitat destruction and increased difficulty in filtering drinking water.  In addition to reducing light penetration, fine sediment (fine sand and smaller( impedes sight-feeding, smothers benthic organisms, abrades gills and other sensitive structures, reduces habitat by clogging interstitial spaces within a stream bed, and reduces the intergravel dissolved oxygen by reducing the permeability of the bed material.  Introduction of large volumes of sediment also has the potential of filling lakes and reservoirs as well as clogging stream channels. 

Economic Impacts:  It has been estimated that over four billion tons of sediment reach the ponds, rivers, and lakes of the United States each year, and approximately one billion tons of this sediment eventually reaches the ocean.  Sediment entering small streams in upland areas can be carried downstream into larger, navigable rivers and reservoirs necessitating costly dredging.  Every year in the United States about 497 million cubic yards of material are dredged by the U.S. Army Corp of Engineers and private operators to create and maintain navigable waterways and harbors.  Without such efforts, many waterways and port facilities would soon become impassable by most large commercial and defense vessels.  The cost of keeping these waters passable is approximately $500 million dollars annually.  In addition, the disposal of dredged material has become increasingly difficult with the dwindling supply of suitable sites available.


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