Environmental Impacts and Erosion

There are 2 main environmental impacts of changes in sedimentation rates on the marine environment:

(i) a lack of sediment input leading to coastal erosion; and,

(ii) an excess of sediment deposition leading to erosion upstream.

Erosion is the mechanism by which changes in sediment regimes are effected. Removal of vegetation cover is the most direct and obvious cause of sedimentation. Once deforestation has occurred, the topsoil is left exposed to water and wind erosion, leading to a reduction in the survival and germination of the remaining plants. The result of this positive feedback cycle is exposure of rock and lack of infiltration and percolation of water into the soil. Compounding this problem is the slow rate of new soil formation especially in tropical areas: hundreds of years can be required to develop 1 mm of topsoil.

Coastal ecosystems are particularly vulnerable to soil erosion when the following conditions prevail:

1. Thin soil cover on steep, tilled hillsides (especially when using poor agricultural practices) and where no floodplain is present;

2. Agriculture practice predominantly in a coastal zone;

3. Poor land management practices (such as cultivation at right angles to contours) in areas prone to high intensity rainfall;

4. Land abandonment causing slopes to be left unprotected without any soil or water conservation measures;

5. Development, urban input and discharges.

 

Soil erosion depends on 2 important factors among others:

1. Soil erodibility; the potential for soil to erode is dependent on the intrinsic characteristics of the soil such as particle size, permeability, cohesivity and porosity;

2. Rain erosivity; intense rainstorms, especially prevalent in the tropics, lead to high soil loss.

It has been estimated that only a very small proportion of sediments eroded from upland areas ever make it to estuarine areas and that the vast majority of sediments (around 90%) are the result of coastal processes. Agricultural sediments make up an accordingly small proportion of the other 10% or less. This is, of course, dependent on specific local conditions and may not be true in every case. This scenario is also more true in the developing world since in developed countries the rate of sedimentation due to agriculture is significantly higher.