Context: The Central Water Commission under Ministry of Jal Shakti organized a one-day national workshop on ‘Integrated Management of Sediments in River Basins and Reservoirs for Sustainable Development” at India International Centre in New Delhi. It was largely agreed that short and long term action plans need to be prepared along with measures to control silt deposition in Reservoir which may be implemented in phases.
What are sediments?
- Sediment is a naturally occurring material, organic and inorganic, that is moved and deposited in a new location by water, wind or ice.
- It is broken down and transported by processes of weathering and erosion.
- They can consist of rocks, minerals, remains of plants and animals.
- It can be as small as a grain of sand or as large as a boulder.
- Sediment that is light enough to be carried by water without touching the stream bed is called suspended sediment.
- Sediment created and deposited by glaciers is called moraine.
What is siltation and sedimentation?
- Rivers flowing in the alluvial plains tend towards a stable flow condition. In other words, they try and maintain a balance between the silt load carried and the volume and velocities achieved by the flow. This is usually called a stable sediment regime for the river.
- When the core parameters of volume and velocities are disturbed, either due to a low gradient (while it enters the plains) or encroachment in flood plain, widening of the channel (braiding of river streams) suspended silt particles in the river water settle down. This is called siltation.
- This phenomenon is normally called sedimentation when it occurs in a reservoir.
Why sediments are important?
- Source of nutrients: Sediment is important because it usually enriches the soil with nutrients forming large fertile plains. Areas rich in sediments are often also rich in biodiversity. The rivers lacking in sediments (mostly downstream rivers of dams,) are known as ‘hungry waters’.
- Landform development: Sediment deposition create several landforms like natural levees, embankments, deltas and river banks, flood plains beaches, spits and sand bars etc.
- Ecological functions: Sediment processes are an extremely important part of many ecosystems as well as of primary importance to particular species. For example, various organisms in both marine and freshwater environments rely on replenishment of sediment for their reproductive habitat.
- Natural water filtration: When water passes through soil, it is cleaned via physical, chemical and biological processes. In addition to soil’s physical filtration capacity, soil contains important biota that helps transform and decompose certain chemicals and other contaminants from soil, thus helping filter them out of the water.
- Disaster management: Sediment is one of the main tools in coastal zone management. Huge amounts of sediment are being used for flood protection (e.g. beach nourishment), and habitat and wetland protection.
- Following chart can help us to analyse the role of sediments in various spheres;
What is the effect of sand mining on sedimentation in rivers?
Mining of sand, if done at an optimum level, removes excessive sediment deposit in rivers.
- Unscientific and excessive sand mining depletes the mineral at rates at which the river system cannot replenish it.
- Excessive mining creates high over banks with steep slopes, which are not stable due to the limited shear strength of the constituent soil materials.
- Continued collapse of such banks leads the river course to deviate towards developed lands and poses an ever-increasing threat to them.
What are the causes of reservoir sedimentation ?
Generally, soil erosion is the major cause of reservoir sedimentation and subsequent sedimentation of reservoirs is a complex process dependent upon a number of natural and anthropogenic factors.
The causes are classified into two with respect to the factors, namely;
- Natural Causes
- Geomorphology – configuration of the land surface including the location, size and shape of such physical features as hills, ridges, valleys, streams and lakes.
- Hydrology – To be successful, a dam and reservoir project must have an adequate and continuous supply of water. The annual rainfall, the ratio of watershed area to reservoir area, and the volume of stream of the year must be known.
- Hydrogeology – to determine whether groundwater would contribute to the reservoir or whether the reservoir would lose water to the groundwater system is also essential.
- Soil characteristics – The type of soil and its properties such as porosity and permeability can cause or lead to erosion within and around the reservoir.
- Anthropogenic Causes
- Tillage practices: Wrong tillage practices can cause loose soil thereby leading to washing away of top soil.
- Overgrazing: Too much grazing of vegetation by animals can lead to exposure of the soil in an area thereby causing erosion.
- Mining and logging: Mining activities can lead to erosion due to wearing off of the surface through surveys and excavation as well.
What are the problems with sedimentation in dams?
- Reduction of storage capacity: When a river is put to halt behind a dam, the sediment it contains settles at the bottom of the reservoir. Reservoirs have become settling tanks for the sediment in most cases. This means the dam has less and less space for water, which it was supposed to hold.
- Impact on stability: Higher siltation of reservoirs exerts pressure on the reservoir thereby compromising the dam stability.
- Reduced efficiency of power generation: The high rate of reservoir sedimentation pose a challenge to dam operators due to the abrasion of turbines and other dam components. The cracking of the tips of turbine blades by waterborne sand and silt considerably reduce their generating efficiency and needs costly repairs.
- Reduced discharge capabilities: Sediments will often block low-level outlets designed to allow for reservoir drawdown. Reduction of spillway capacity can occur as a result of the loss of approach depth when the sediment front reaches the dam.
- Environmental impacts: Plant and animal species are sensitive to alteration of both the sediment supply and flow regime. Increases in sediment concentration in upstream areas can create turbid waters with a smaller euphotic zone. This decreases plant productivity, negatively impacting fish and bird species and can also cause visual impairment for predatory fish, affecting their feeding habits. Finally, sediment is a primary carrier of suspended pollutants such as nitrogen, phosphorous and heavy metals. Sediments released as a result of sediment management or a dam breach may have environmental effects that can persist for decades.
In India, government statistics on 11 of the country’s reservoirs with capacities greater than one cubic kilometre show that all are filling with sediment faster than expected, with increases over assumed rates ranging from 130 percent (Bhakra) to 1,650 percent (Nizamsagar). A 1990 World Bank paper on watershed development concluded that in India, “erosion and reservoir sedimentation are not only severe and costly but accelerating. It is now obvious that the original project estimates of expected sedimentation rates were faulty, based on too few reliable data over too short a period”.
What steps can be taken in this regard?
- Reducing sediment inflow: Sediment delivery to reservoir can be reduced by techniques such as erosion control and upstream sediment trapping.
- Routing the sediments: Some or the entire inflowing sediment load may be hydraulically routed beyond the storage pool by techniques such as drawdown during sediment-laden floods, off stream reservoirs, sediment bypass etc
- Sediment removal: Deposited sediments may be periodically removed by hydraulic flushing, hydraulic dredging or dry excavation.
Adopting Erosion Control and Soil Conservation Measures:This includes all those general methods which are adopted to reduce erosion of soil and to make it more and more stable. They may include: plantation, control grazing, terracing benching, cover cropping like grassing and contour binding, etc.