Sediment Generation and Transport Mechanism in Watersheds
Sediment is primarily generated because of fluvial (water) and Aeolian (wind) processes. The forces which are involved in this are: a) attacking forces, which remove and transport the soil particles, b) resisting forces, which retard the erosion.
Fig. 17.1. Effect of Weathering and Gravity on Sediment Generation.
Sediment generation in a landscape is a factor of multiple natural and anthropogenic influences. Selby (1993) identifies climate and geology as the predominant factors of erosion with a close interdependency of soil type and vegetation. Out of all the sediment generating agents, the role of water in detachment, transportation and deposition is indeed very significant (Fig. 17.1). Since recent past, human activities favoring more to it because of increased land degradation.
Sources of Sediment Generation are:
Sediment transport is the movement of solid particles (sediment), typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport due to fluid motion occurs in rivers, the oceans, lakes, seas, and other bodies of water, due to currents and tides, in glaciers as they flow, and on terrestrial surfaces under the influence of wind. Sediment transport due to gravity can occur on sloping surfaces in general, including hill slopes, cliffs, and the continental shelf-continental slope boundary. Flow and sediment transport are important in relation to several engineering topics, e.g. erosion around structures, back ﬁlling of dredged channels and near shore morphological change.
Estimation of the sediment flow rates by a stream is necessary to understand the extent of erosion in the stream’s catchment and for estimating the reservoir life if one is planned to be constructed on the stream.
There are three main processes through which sediment is transported in the streams. These processes are:
Suspension: Suspended sediment is that which remains in suspension in the flowing water for a considerable period of time without contact with the stream bed.
Saltation: Movement of sediment along the stream bed because of bounce and skip on the bed.
Surface Creep: It is the movement of sediment almost in continuous contact with the stream bed.
Fig. 17.2. Velocity and Sediment Distribution in a Straight Natural Stream.
The Figure 17.2 (a, b and C) shows how the velocity, sediment concentration and sediment discharge varies with the depth of flow. In first case i.e a) which is showing variation of velocity of flow with the depth, from the profile it is analyzed that maximum velocity is not at the top surface but lies somewhere below that (0.2 depth). From b) it is analyzed that as the depth of flow increases sediment concentration increases in the flow and transportation of sediment increases. In case c), the sediment discharge profile is showing that as depth increases it is increases up to some depth and later on starts decreasing.
17.2 Types of Sediments Transported along with Streams
A channel flowing in watershed transports the runoff along with the sediment. The total sediment load that is transported out of the watershed by a stream is classified into components depending upon their origin as:
Bed Material Load comprises of bed and sediment load.
17.2.1 Wash Load is the sediment originating from the land surface of the watershed and is transported to the stream channel by means of splash, sheet, rill and gully erosion. Wash load is generally composed of fine-grained soils of very small velocity.
17.2.2 Suspended Load is relatively finer bed material that is kept in suspension in the flow through turbulence eddies and transported in suspension mode by the flowing water. These particles move considerably long distances before settling on the bed and sides. The suspended load is further divided into the wash load which is generally considered to be the silt+clay sized material (< 62 μm particle diameter) and is often referred to as “fine-grained sediment”. The wash load is mainly controlled by the supply of this material (usually by means of erosion) to the river. The amount of sand (> 62 μm particle size) in the suspended load is directly proportional to the turbulence and mainly originates from erosion of the bed and banks of the river. In many rivers, suspended sediment (i.e. the mineral fraction) forms most of the transported load. Suspended load can also be classified into three grades as:
Coarse sediment: Particles diameter > 0.20 mm
Medium sediment: Particles diameter between 0.20 to 0.075mm
Fine sediment: Particle diameter < 0.075mm
17.2.3 Bed Material Load is sediment load composed of grain sizes originating in the channel bed and sides of the stream channel.
Bed Load is relatively coarse bed material load that is moved at the bed surface through sliding, rolling and saltation. Bed load is stony material, such as gravel and cobbles that moves by rolling along the bed of a river because it is too heavy to be lifted into suspension by the current of the river. Bed load is especially important during periods of extremely high discharge and in landscapes of large topographical relief. It is rarely important in low-lying areas.
17.3 Methods of Stream Sediment Measurement/Sampling
In order to prevent environmental and associated engineering issues, it is very necessary to measure total sediment load. Many methods are designed and available to measure the sediment load. A few of these methods are discussed as below.
17.3.1 Location of Measurement/Sampling
To reduce the errors in sediment measurements, site selection is required with careful observation and precautions. Sampled sediment data can be influenced by several sources of variability, including spatial which is defined by the sample location relative to the investigated activity (MacDonald, 1992) i.e, near the stream bed sediment concentration is more, in the middle relatively less while at the top surface it is too less. Thus it is difficult to select the sampling point. To avoid or possibly reduce these errors, following points should be taken in to care for the site selection.
If the sampling is to be done only at one point, then sample should be collected at a depth of 0.6 times depth of stream (d), measured from top.
In case of double point sampling, one sample should be collected at a depth of 0.2d while other at 0.8d and then the resultant concentration should be averaged.
For three points sampling one sample should be taken near the top of water surface, second from mid depth of stream and third near the stream bed and weighted equally.
In case of multiple sampling, several samples from several points of vertical section of the stream are taken, which helps to get more accurate result.
17.3.2 Frequency of Sampling
Frequency of sampling depends on the sediment concentration in the stream flow. It is well known that sediment concentration increases rapidly on rising phase of hydrograph. This happens due to the action of rain drops on the soil which displaces the soil from their position resulting in sediment formation. Therefore, sediment samples should be collected more frequently at the beginning of runoff and it should be continued up to peak. The sample should be taken at every 15 minutes interval.
17.3.3 Number of Sediment Monitoring Stations
In case of watershed, from where total runoff is disposed off through the outlet, the collection of sample is carried out as it is the most ideal point in the whole watershed for sample collection. While in case of drainage system, gauging station should be provided at every outlet and samples are collected.
17.3.4 Observation and Collection of Sediment Samples
220.127.116.11 Location of Observation Post
Silt observation posts should be located on the basis of the following points:
Wherever possible, the site of stream gauging should be utilized for installation of silt observation post because it represents the yield of sediment rate from the entire watershed.
The observation sites should be free from any disturbing points such as change in site configuration and back water effect etc.
A silt observation site should be located at that point of stream, from where the constant soil erosion is taking place due to turbulence.
The site should be easily accessible (facility of transportation).
Wherever available, the sediment monitoring station with overhead platform like arrangement should be used. It helps in collection of sediment sample from the entire width of the stream.
In the hilly regions, where torrential flow exists, cable bridge may be used for taking the observations.
The site should have straight stream length of about 150m, both towards upstream and downstream face.
18.104.22.168 Collection of Sediment Samples
(a) Suspended Load Sampling
The concentration of suspended load varies in the stream cross-section both horizontally and vertically. For the sampling of this suspended sediment for calculation of the overall quantity of sediment carried by the water, various samplers are available. Different types of suspended load samplers are:
Instantaneous Vertical Sampler
Instantaneous Horizontal Sampler
Bottle Type Sampler
Vertical Pipe: A vertical pipe sampler as its name indicates consists of a vertical pipe which is lowered to desired depth. The water sediment mixture flows upwards and when filled, valves at either end of the pipe are closed.
Instantaneous Vertical Sampler: The instantaneous vertical sampler obtains specimen from a smaller part of the vertical depth after the sampler is lowered to the sampling point.
Instantaneous Horizontal Sampler: It consists of horizontal cylinder equipped with end valves which can be closed suddenly to trap sample at any desired depth. This sampler can operate close to the stream bed. It is designed to minimize the disturbance losses.
Bottle Type Sampler: Bottle type sampler resembles similar to the milk bottle with the necessary provision for lowering the bottle to the sampling point (depth) and opening the bottle at the desired depth. Air within the bottle is displaced by the incoming sample, which escapes through the intake opening causing disturbances at the intake end.
Integrating Sampler: Modified form of bottle type sampler is integrating sampler, which takes representative sample from the entire depth. No disturbance to the flow is there as entrance and exit are different. An example of this sampler is Delft bottle type sampler, practically used in sediment sampling of rivers and channels.
Delft bottle type sampler is used to measure suspended sediment transport in rivers and other water courses from the surface down to 0.1 m above the river bottom. The sediment containing water flows through a bottle shaped sampler. The shape of this sampling body induces a low pressure at the rear face in such a way that the water enters the nozzle of the sampler with almost the same velocity as the undisturbed flow. The sharp decrease of the velocity in the wide sampling chambers causes the sediment material to settle there.
Fig. 17.3. Suspended Load Sampler Delft Bottle Type.
(b) Bed Load Sampling
The rate of bed load movement is determined by placing the sampler on the stream bed and measuring the amount of material collected in a given time. The bed load sampler is usually held in position by a rod if the stream is shallow, or by a cable boat or by trolley etc.
Bed load samplers are of different types according to their construction and principle involved. Three main types of bed load sampler are in maximum use:
Tray or Pan Type
Pressure Difference Type
Basket Type: Basket type sampler consists of a box or basket, generally made of meshed material. The sampler is lowered to rest on the stream bed with the open end on the upstream to catch a sample of moving material/sediment. The introduction of the sampler into the stream causes an inward resistance to flow and a resultant lowering of the stream velocity. Hence the entrance velocity is decreased from that of the undisturbed stream, causing some of the material to drop out before entering the sampler. Thus, the efficiency, i.e., the percentage of the material moving towards the sampler, which is actually caught by it is less than 100%.
Tray Type Sampler: It consists of flat pan or tray-shaped device with baffles or slots to check the moving material. It has the same disadvantage as that of basket type sampler.
Pressure Difference Type: It is designed to overcome the objection of decreased velocity at the entrance to the sampler. A pressure drop at the exit just sufficient to overcome the energy losses is formed, thus giving the same entrance velocity as in the undisturbed stream.
17.3.5 Estimation of different Loads from Samples
The collected samples from different locations and depths of streams are brought back to laboratory, where the sediment particles are filtered out from the solution. The filtered particles are dried up for 24 hours at 104 °C in the lab and then weighted up. This weight represents the sediment concentration in the sampled water. By estimating the total water flowing off from that point in stream per unit time (second, hour or day) the total sediment flow in the same unit of time can be calculated. In practical, the “parts per million” (ppm) equivalent to mg/l is used as unit of sediment concentration in water, where the concentration sediment in mg is shown as available in per liter of flowing water in channel or river. For an example, let in a one liter bottle sampler, the sediment concentration has been measures as 315 gm after filtering the drying up the sample. In this way the sediment concentration measures in the river is 315 mg/l (or 0.315 g/l).
Keywords: Sediment Yield, Stream Sediment, Sediment Transport, Sediment Measurement