Soil & Water Conservation Engg.

Lesson 19 Sedimentation of Water Resources

 19.1 Sedimentation

Sediment is fragmented material, which is transported or deposited by water, air or ice as natural agents. Eventually sediment settles out and accumulates after transport; this process is known as deposition. Among all, water is the most widespread agent of sediments transport. Therefore, sediment yield from soil surface and its transportation by water is commonly considered for study purposes. When land disturbance activities occur, soil particles are transported by surface water movement. Soil particles transported by water are often deposited in streams, lakes, and wetlands. This soil material is called sediment. Sediment is the largest single nonpoint source pollutant and the primary factor in the deterioration of surface water quality.  Land disturbing activities such as road construction and maintenance, timber harvesting, mining, agriculture, residential and commercial development, all contribute to this problem. There are three basic types of sediments: rock fragments, or clastic sediments; mineral deposits, or chemical sediments; and rock fragments and organic matter, or organic sediments. Dissolved minerals form by weathering of rocks exposed at the earth's surface. Organic matter is derived from the decaying remains of plants and animals.

Sedimentation: It is the processes of letting suspended material settle by gravity. Suspended material may be particles, such as clay or silts, originally present in the source water. More commonly, suspended material is created from material in the water and the chemical used in coagulation or in other treatment processes, such as lime softening. Sedimentation is accomplished by decreasing the velocity of the water being treated to a point below which the particles will no longer remain in suspension. When the velocity no longer supports the transport of the particles, gravitational force will remove them from the flow. Sedimentation is a general term for the processes of erosion, transport and deposition.

Sedimentology: It is the study of sediments and sedimentation.

19.2 Sources of Sedimentation

Sediment is delivered from two broad erosion sources. The first being sheet erosion and second being channel type erosion. Sheet erosion is primarily an upland source of sediment while channel type erosion; resulting from the concentrated flow of water; is comprised mainly of gully erosion, valley trenching, streambed and stream bank erosion.

The sources of sediment can be listed as below:

1. Erosion from agricultural, forest and waste lands,

2. Movement of soil mass due to landslides, slumps and soil creeps,

3. From gully by concentrated runoff, 

4. Stream bank erosion including cutting of banks and scouring from bed, 

5. Erosion caused by occurrence of flood in the watershed,  

6. Incident to the roads, railroads, cleaning of houses, industries etc. and

7. Mining and dumps left as waste materials over the ground surface.

In sediment analysis, the estimation of total sediment load carried away through any stream has primary importance because based on the total sediment load, several preventive measures can be adopted. The relative contribution of different sediment sources varies from catchment to catchment. Therefore, consideration must be given to those sources whose contribution is more effective and steps should be taken for controlling them.

19.3 Factors Affecting Sedimentation of Water Resources

Several factors affect the separation of settleable solids from water. Some of the common types of factors are:

1.     Land Use and Soil Type: Sediment yield is closely related to the soil type and land use. Vegetation provides cover on the soil surface in the form of blanket to protect it from the impact force of the rainfall. The energy of rain drop is dissipated resulting in reduction of splashing effect over the ground surface. At the same time vegetation also creates a hindrance in the flow of runoff; resu1ting in the reduction of flow velocity and ultimately causing minimal scouring of soil particles from the soil surface. Furthermore, the infiltration rate gets enhanced, which reduces the runoff and thereby sediment yield, too.

Soil type is an important variable to affect the sediment yield. For example if there are two types of soil, one is sandy and the other is clay soil; the sandy soil has greater problem of particles detachment due to its coarser characteristics, while the clay soil can not be detached easily due to finer nature. In sandy soil, the soil loss (sediment yield) is more compared to the later one. However, once detached, the clay particles can be transported more easily.

2.     Catchment Size: There is an association between the rate of sediment production and size of catchment area, because of the fact that the total runoff yield is dependent upon the aerial extent of watershed. The peak flow per unit area decreases as the area increases while the period of surface runoff increases with area. The reason behind this is that; a catchment of lager area has greater time of concentration. As a result, more time is available to the water for infiltrating into the soil. Ultimately there would be higher runoff and soil loss or sediment yield. In a small size catchment there is a reverse trend. The relationship between sediment yield per unit area and catchment area is shown in Fig. 19.1.

 

Fig. 19.1. Relationship between Sediment Yield and Area of Watershed. (Source: Suresh, 2009)

3.   Climate and Rainfall: The relationship between sediment production and mean annual rainfall of the area has been investigated by several scientists. The general relationship between them is shown in Fig. 19.2 from which it can be seen that under dry conditions there is no surface runoff and no sediment movement, while under high rainfall conditions, there is a peak flow of surface runoff resulting in greater sediment yield.

 

Fig. 19.2 Relationship between sediment yield and mean annual Rainfall. (Source: Suresh, 2009)

4.   Particle Shape: The shape of the particle affects its settling characteristics. A round particle, for example, will settle much more readily than a particle that has rugged or irregular edges. All particles tend to have a slight electrical charge. Particles with the same charge tend to repel each other. This repelling action keeps the particles from congregating into flocs and settling.

5.    Water Temperature: Another factor responsible for sedimentation process is the temperature of the water. When the temperature decreases, the rate of settling becomes slower. With the highest temperature, the settling process becomes much faster as density of water decreases at higher temperature.

19.4 Losses due to Sedimentation of Water Resources

Worldwide reservoir sedimentation is a serious problem and considered as a silent enemy. The gradual reduction of capacity reduces the effective life of dams and diminishes benefits for irrigation, hydropower generation, flood control, water supply, navigation and recreation. On the other hand sediment deposition propagates upstream and up tributaries, raises local groundwater table, reduces channel flood carrying capacity and bridge navigation clearance and affects water division and withdrawals. On the other hand, the reduction of sediment load downstream can result in channel and tributary degradation, bank erosion and changes of the aquatic habits suited to a clear water discharge.  

1. Loss of Reservoir Storage Capacity: The key impact of reservoir sedimentation is the reduction in the useful life of the reservoir. Sediment deposition is a key factor reducing the life of dams around the world. Reservoirs are expensive to build and their construction usually entails high social and environmental costs. Entire communities may be forced to relocate and ecosystems are destroyed due to their construction. However, it is recognized that dams also bring many benefits such as water storage, power generation and flood mitigation. Extending the life of dams through careful management of sediment, therefore, should be a key priority.

2. Effects of Sediment on Hydropower Operations and Reservoir Operations: The build-up of sediment in front of power intakes may result in significant costs for hydropower operations. Dredging is often required to remove excess sediment and allow a full flow of water through the intakes. If the sediment accumulation is high, the reservoir outlet works (intake and bottom outlet structures) may also become clogged. Abrasion of hydraulic machinery may also occur, decreasing its efficiency and increasing maintenance costs.

3. Impacts on Infrastructure: Too little sediment, especially downstream of dams, may encourage accelerated erosion around structures on/in riverbeds or riverbanks due to a lack of sediment recharge. In extreme cases, excessive erosion has led to incidents such as bridge collapses. Cracks in infrastructure such as bridges and other structures are typical results of such erosion. Aside from dams, sediment can have impacts on other man-made infrastructure. Too much sediment can disrupt the normal functioning of irrigation pump house intakes and can also disrupt irrigation when excess sediment is deposited in canal systems. Deposition in canal systems can lead to high costs for those reliant on these systems as a water supply. Dredging may be required to remove surplus sediment. Sediment deposition may also result in blockages or inefficiencies in irrigation infrastructure (including pumps and distribution networks) and may even impact upon the produce. Sediment also has negative impacts on domestic water supplies, causing problems in both water treatment plants and distribution networks. Failure of water treatment plants, especially in poor regions, can mean that the water is unsuitable for human consumption. As a result populations may suffer from health problems.

4. Flooding: Flooding occurs when a watercourse is unable to convey the quantity of runoff flowing downstream. The frequency with which this occurs is described by a return period. Flooding is a natural process, which maintains ecosystem composition and processes, but it can also be altered by land use changes including river engineering. Increased sediment accumulation in river systems can raise the level of the riverbed, subsequently increasing water levels. This deposition can have significant implications for flooding, and may cause floods to pose a risk to human settlements which would otherwise be contained by banks and levees.

5. Navigational Issues: The sedimentation of water courses can also make them unsuitable for navigation without regular dredging work. This dredging is often costly to operate.

6. Impacts on Wetlands and In-stream Ecosystems: Where dams do not exist to trap sediment, excessive sediment inputs may have negative impacts on wetland areas. This is especially the case where wetlands occur close to agricultural areas and where land use change has resulted in increased rates of soil loss, increased downstream sediment loads and increased rates of sedimentation in wetland areas. The impact of excessive sediment deposition in wetlands may create ecological disruption. Sedimentation results in alteration of aquatic food webs, nutrient cycling and biogenic processes that transform and sequester pollutants. Eventually sediment deposition may entirely smother wetlands resulting in limited biological diversity.

7. Impacts on Water Quality (Turbidity and Sediment-associated Pollutants): Sediment is a pollutant in its own right. Even where sediment is uncontaminated by agricultural fertilizers and pesticides and industrial or human waste, they cause turbidity in the water which limits light penetration and prohibits healthy plant growth on the river bed. The accumulation of sediments on the river bed can smother or disrupt aquatic ecosystems by reducing food sources, and degrading spawning grounds (such as gravel and rocky environments) and the habitats of desirable fish species. Turbidity may also result in eutrophication where nutrient rich sediments are present (particularly sediments from agricultural land with high fertilizer contents). Eutrophication creates a situation where the dissolved oxygen present in the water system is reduced and the fish species may be unable to survive in the water column. Eutrophication, where it results from toxic algal blooms, can also be a serious risk to human health. Sediments in areas with high human activity often contain chemical pollutants which may pose a risk to human health and the health of surrounding ecosystems. Potable water supplies can be compromised by the presence of excess sediment (whether contaminated by toxins or not) as purification facilities may not be able to cope with the sediment in the water leading to temporary breakdowns and subsequent risks to the safety of the drinking water. Contaminated surface waters also cause a risk by altering the metabolic processes of the aquatic species that they host. These alterations can lead to fish kills or alter the balance of populations present. Other specific impacts are on animal reproduction, spawning, egg and larvae viability, juvenile survival and plant productivity.

Keywords: Sedimentation, Sedimentology, Eutrophication, Turbidity.

References

Suresh, R. (2009). Soil and Water Conservation Engineering, Standard Publishers Distributors, 951 p.

Simons, D. B., & Şentürk, F. (1992). Sediment transport technology: water and sediment dynamics. Water Resources Publication.

Suggested Readings

http://www.bae.ncsu.edu/programs/extension/wqg/sri/sediment5.pdf

http://iahs.info/redbooks/a059/059023.pdf

http://www.nbcbn.com/Project_Documents/Progress_Reports_2010/River_Morphology_Research_Cluster/River_Morphology_Research_G1.pdf

http://www.irtces.org/isi/isi_document/2011/ISI_Synthesis_Report2011.pdf