Upload Now
ADVERTISEMENTS:
Everything you need to know about measuring soil loss!
1. Multi-Slot Divisor:
The multi-slot divisors are the device used for measuring the runoff and soil loss from runoff plots or small areas. It is an integral component of runoff plot. Its constructional feature consists of several slots of same dimension. The slots are fitted at the end of divisor box. The water flow from runoff plot leads towards the divisor. The divisor divides the entire water flow into different parts.
The uniformity in flow division depends on uniformity of flow velocity. As for as possible the flow velocity should be uniform in divisor box. Variations in flow velocity also cause unequal distribution of sediment concentration in divided water flow, as result there is introduction of error in measurement.
ADVERTISEMENTS:
In operation, the water flow is allowed to pass through the device; and flow from one of the slots is directed into the collecting tank; and water from the remaining slots is allowed to drain out. The water samples are taken from the collected water in the collecting tank for analysis purposes.
A multi-slot divisor requires following information’s for its design:
i. Maximum runoff volume expected in 24 h
ii. Peak runoff rate (generated from plot) of the design frequency
ADVERTISEMENTS:
iii. Maximum soil loss likely to be from heaviest storm
Multi slot divisor assembles following components:
1. Boundary wall
2. Runoff collector to catch and concentrate the flow from the plot
ADVERTISEMENTS:
3. Stilling tank
4. Divisor box; and
5. Collecting tank.
ADVERTISEMENTS:
Various advantages of multi slot divisors are mentioned as under:
i. Useful for measurement of low discharge rates.
ii. It is simple in design, construction and operation.
iii. There is no risk of mechanical failure.
ADVERTISEMENTS:
iv. Data processing is relatively simple.
v. It can measure the runoff and soil loss, both.
Size of Divisor:
The use of suitable size MSD for prediction of accurate sediment load from the runoff plot is very important. The divisor’s size is based on the expected rate of overland flow generated from the runoff plot and the amount of water to be stored in the collecting tank.
The choice on divisor is mainly based on the following parameters:
i. Capacity
ii. Number of slots
iii. Width of slot; and
iv. Length of slots.
The number of slots and slot’s dimension together, indicate the divisor size. The slot dimension determines the divisor’s capacity. The divisor ratio is the Aliquot size. A 5-slot divisor refers to the divisor ratio of 1:5.
Number and Size of Slots:
To handle an expected maximum flow rate the required number of slots (N) in MSD can be determined by using the following formula:
N = 10 APF/C … (9.1)
Where,
F = expected maximum runoff percentage (fraction)
A = plot area (ha)
C = capacity of storage tank (m3)
P = precipitation (mm).
The research studies reveal that in multi slot divisors the maximum number of slots should be 15. However, if it is essential to use the MSD of greater number of slots than 15, then at the place of single MSD, two MSD should be installed in series to have a required divisor ratio. The size of slots is decided on the basis of expected flow rate from the field plot. The accuracy in measurement gets reduce, especially when a large size MSD is used for low flow depth.
Due to this reason it is always suggested to select the divisor with the capacity equal to the expected flow rate from the field plot. The divisor ratio and expected flow rate from plot also determine the size of collecting tank. After determining the dimension of different components of MSD, it is fabricated using suitable materials; and is installed in the field for collecting the sediment samples.
Calibration:
The calibration of MSD is very essential to verify its accuracy or correction required in measured data. It is done after installation of MSD system.
For calibration, the following steps can be followed:
1. Fill the stilling tank with water up to the crest of precision plate. And stop adding of water when it is about to over flow from the crest.
2. Now, add a fixed amount of water at uniform rate into the stilling tank; and collect the aliquot.
3. Determine the total amount of water by multiplying the aliquot and the number of slots.
4. Calculate the percentage error by determining the difference of estimated (step-3) and the actual amount of water added.
5. Repeat the procedure for several observations. And compute the values of percentage error.
Precautions and Maintenance:
The followings are the few important points to follow for successful operation of multi-slot divisor:
i. The calibration should be done each year before using for measurement.
ii. If the MSD has been fabricated using such metallic materials, which is likely to get affected by corrosion and rust formation, then the entire body of MSD should be painted with good quality paint.
iii. The slots should be cleaned, after each runoff event during monsoon season.
iv. The deposited trash in the tank should be removed.
v. During operation, an attention should be given about its proper functioning and any leakage in the system or extraneous entry of water in the collecting tank. Also, the outlet of collecting tank should be checked for its leakage. If there is any leakage that must be removed, immediately.
vi. After taking the measurements, all the lids should be tightly closed.
2. Depth Integrating Sediment Sampler (DIS):
In experimental studies on runoff-soil loss evaluation using runoff plot, the sediment samplers are widely used for predicting the sediments yield/soil loss. There have been developed a variety of sediment samplers but they have their own limitations about contributing watershed area etc.
Few samplers cannot be used for the watersheds having the area less than 1 ha, because of their limited capacity. The depth-integrating sampler, which is suitable for the watersheds up to 400 ha, is very common for measuring the soil loss.
However, it also has few limitations, are outlined as under:
1. For the flow containing very high percentage of medium and coarse sands, this sampler is not efficient.
2. For the storms producing multi peaks (more than two), it does not measure sediment load, so accurately. Normally, measures lesser value.
3. For determining the sediment load or soil loss from large watersheds, i.e., more than 400 ha, this sampler is not suitable. It is only suitable for small watersheds, i.e., less than 400 ha area.
Design Criteria:
The design criteria of depth-integrating samplers are given as under:
1. As compared to horizontal and vertical variations in sediment concentration, the variation in sediment load with respect to time is relatively more important.
2. The sampler must be capable to quantify the sediment load efficiently at or near the hygrograph’s peak stage, because major portion of soil loss/sediment yield takes place during this stage.
The events of momentary or instantaneous fluctuations in sediment concentration across the flow section should not be considered for design. It can be done by selecting the sampling site at such a point where sediment variation across the flow section is minimum.
To account for the time variation in sediment loads, the rapidly fluctuating runoff flows from small watersheds can be considered. This can be obtained by taking representative samples during different stages of flow, and from different depths of stream flow.
In order to collect the sediment samples from different depths using DIS, the small-diameter pipes are placed at specified heights from the streambed; and all are connected to an individual plastic made pipe leading to their separate container.
Amongst several pipes, the lower most pipe collects the sediment sample throughout the runoff period, and upper most pipe collects the sample for shorter duration depending on relative position of flow stage.
The collected sediment samples are analyzed for determining the sediment concentration of each container, individually. Simultaneously, the hydrograph data are also recorded at each sampling point.
The actual sediment concentration and total soil loss for each container is computed by using the following formula:
The fabrication of depth-integrating sediment sampler is very simple. It can be done by using the readily available materials.
The construction guidelines are outlined as under:
i. The materials and size of rod to be used should be such that there is very low vibration in the rod, when kept for sample collection in the flow. For accurate sediment sampling the minimum vibration is very important.
ii. The intake rate should be uniform for all sampling pipes. If different pipes have different intakes then there is variations in sampling rate.
iii. In comparison to sampling pipe, the diameter of plastic pipes should be slightly bigger (diameter) to eliminate the resistance likely to get develop due to sample flow.
iv. The number of sampling pipes and their spacing should be on the basis of desired accuracy and sediment flow conditions. Normally, towards lower side of flow depth (near stream bed) a wider spacing, while towards upper side of flow depth a closer spacing between the sampling pipes is recommended.
v. Since, the volume of sample collection in each container is different, therefore, the containers to be used, they should also be of different sizes.
vi. The metal rod which holds the sampling pipes, should be firmly fixed on firm foundation at the channel bed.
vii. The location of sampling point should be such that, it is free from high turbulence of stream flow; otherwise, sampling will not be authentic. The location should be based on the limited degree of flow turbulence.
3. H-Types Flumes:
In natural field plots the measurement of overland flow/runoff and soil loss or sediment yield is carried out by using pre-calibrated devices such as flumes and weirs, because of their high degree of accuracy in measurement. These devices are installed at the gauging site adopting proper guidelines.
The H-type flumes and weirs used for runoff measurement, are described as under:
These flumes are suitable for free-flow condition. The submerged-flow condition is not suitable. The free flow at d/s of measuring device docs not affect the flow condition within and at u/s section of the flume. The free flow condition takes place when there is sufficient fall near the outlet of flume.
On the other hand, the submerged flow occurs when d/s flow gets affected within and u/s section of flume. These flumes are also not being suitable for measuring the flow having heavy coarse bed-loads.
H-flumes offer various advantages; few of them are outlined as under:
i. It provides accurate measurement of low and high water flows, because of having the provision of increasing the throat opening as per rise in flow stage.
ii. There is no need of cleaning the device. Its converging section causes automatic cleaning by increasing the flow velocity.
iii. This flume is suitable for measuring the runoff containing the sediments in suspension form and low bed-loads.
iv. Very simple in construction.
v. Stable in operation.
vi. Requires minimum maintenance.
vii. Installation is very simple; and not affected by the steep gradient of channel bed.
Types of H-Flumes:
Various types of H-flumes are mentioned below:
i. HS-FIume:
These flumes are found suitable to measure the small flow rates ranging from 1.4 x 10–3 to 2.27 x 10–2 m3/s.
ii. H-Flume:
This type of flume is used for measuring the medium runoff discharge, i.e., from 9.0 x 10–3 to 0.85 m3/s.
4. Weirs:
These are very suitable and simple device for measuring the discharge rates in varying situations. Especially, where a fall of about 18cm or 0.6 ft or more in the waterway grade is there, this device can be suitably used. Also, where submergence at u/s section is not desirable, it can be used.
Although, the weirs are of various types; but the most commonly used is described as under:
These are also called sharp-crest triangular weir; are used for measuring the runoff from small plots. The 90° and 120° V-notches, are very common. Its construction consists of sharp edge metal blades.
These blades are constructed with the angle iron 89′ 89′ 13 mm, or non-eorrodible metal plate of 6 mm or 0.25inch thickness. V-notches are suitably used for measuring the high and low discharge rates with high accuracy. The ratings of V-notch are presented in Table 9.2.
For better performance of sharp-crested V-notch, the following conditions are essential, as suggested by USDA (1979) and Pathak et al. (1981):
i. The blade thickness should not be more than 6 mm.
ii. The u/s corners of notch should be sharp.
iii. The corners should also be filed perpendicular to the u/s face without scratches; and should not be smoothed with abrasive cloth or paper.
iv. The knife like edges should be avoided, because their maintenance is difficult.
v. The d/s edges should be relieved by chamfering, particularly when plate is thicker than the recommended crest width.
vi. The chamfer should be at the angle of 45° or more to the crest surface.
vii. The distance between lowest crest point and bottom of the approach channel should preferably not be less than the twice the depth of water flow above the lowest crest point. And in no case, it should be less than 30 cm.
viii. The distance between sides of the weir and the sides of approach channel should preferably not be less than twice the depth of water flow above the lowest crest point. However, it should never be less than 30 cm.
ix. The nappy should touch the upstream edge of the crest, only.
x. The head which is the difference in elevation between the lowest crest point and the water surface at a point upstream of the weir, should be at the distance of 4 times the maximum head on the crest.
xi. The cross-sectional area of approach channel should be at least 8 times of the overflow at the crest, for the distance of 15 times the depth of flow. And if it is less, then it should be corrected.
5. Stage-Level Recorder:
The water level recorders are used for measuring the water flow rate. It records the flow measurements in the form of graph between flow stage and corresponding time. The measurements recorded by this instrument are being accurate and very reliable.
There have been developed several types of stage-level recorders; they are given as under:
1. Mechanical type stage-level recorder; and
2. Digital automatic stage-level recorder.
The drum type stage level recorder is very common type mechanical stage-level recorder. It records the runoff observations, continuously. Its construction consists of gauging unit which includes a float and counterweight-graduated float pulley.
Float is kept over the water surface during measurement. Corresponding to the variation in flow depth (stage) the movement of float is transmitted to a cam with the help of set of gears. During this action the pen moves on the chart in vertical direction; which traces a curve in the form of graph on the chart wrapped over the drum.
The drum is equipped with a clock. This recorder modifies the vertical movement of counter-weighted float into a curvilinear trend, mechanically. Recording of observation is in the form of graph between the flow stage relative to a datum plane and corresponding time. This recorder has the problem about movement of chart, and drying/clogging/blotting of pen.
The Thalimedes is the digital type automatic stage-level recorder. It is a float operated shaft encoder consisting digital data logger. This recorder can measure the runoff continuously from the watershed/field. Its operation is easy and cost-effective. It eliminates all the problems related to the mechanical type recorders. It ensures an uninterrupted measurement of runoff stage under fluctuating water level conditions over long period.
The measurement of runoff by automatic stage level recorder is based on the principle, that the change in water level is transferred to the float pulley on the shaft encoder unit, via float cable and counterweight system. By this action the pulley gets motion (rotation), which is converted into an electronic signal. The signal is transferred to the data logger by the transducer cable, which is saved as the measured value.
The salient features of digital automatic stage-level recorder (Thalimedes) are outlined as under:
i. It is a microprocessor-based electronic data logger, considered as an ideal stage-level recorder.
ii. It can be suitably operated with a single 1.5 Volt DC type battery.
iii. Its EEProm can store up to 30000 measured values.
iv. The sampling and logging intervals can be maintained between 1 minute to 24 hours.
v. For data transfer, no need of wires and cables.
vi. About 11000 measured data can be processed in 4 seconds.
vii. It has no moving parts, as result there is no problem related to gear/clock and chart mechanism.
viii. There is no switches/connectors, as result this device docs not has any contact problems.
ix. It is compact, rugged, and light in weight (0.32 kg); its shaft encoder weight is 0.14 kg.
Installation of Stage-Level Recorder:
The stilling well, intakes and recorder shelter are the essential components of gauging station equipped with stage-level recorder.
These components along with their installation, are described as under:
Stilling Well:
The recorder is installed over it. Inside stilling well a float with counterweight is equipped to indicate the rise/fall of water level without affected by surge or waves. Stilling well is located at one side of stream, with the care that it does not get interfere with the flow over spillway; and near measuring point at flow section of pre-calibrated structure. The size of well is decided on the basis of required stability, depth, type of material used and space required for the float and counterweight for proper functioning.
These wells are normally constructed by using brick masonry materials. However, the galvanized iron and concrete pipes can also be used for this purpose. They should be constructed at firm foundation with waterproof feature.
In order to have better result, the construction of stilling well should be done based on the following considerations:
i. The lower most intake of well should be about 30 cm above the bottom of well.
ii. The portion of well below the lowest intake should be watertight.
iii. The inside diameter of well should not be less than the sum of diameter of recorder pulley, half the diameter of float, half the diameter of counterweight, and 7.0 cm extra.
iv. Inside surface of well should be smooth. It can be done by plastering or by lining with the help of thin metal sheet.
v. The depth of well should be about 20 cm more than the double of the maximum expected head, to avoid the possibility of submergence of float counter-weight.
vi. The well size (diameter) should not be very large, because there is possibility of lag between the rise/fall of water level in the well.
Intake:
The function of intake pipes is to connect the stilling well with the pre-calibrated structure. The intakes may be made of one or more galvanized pipes with 2.5 cm diameter holes. The exact number of intake pipes can be determined based on the thumb rule, that their total cross-sectional area should be at least 1% of the cross-sectional area of stilling well.
More than one intake should be provided at different elevations in the well as safeguard against clogging due to deposition of sediments or other materials, and also have a better connection with the water level during rise or fall.
Maintenance of Flumes and Stilling Well:
The maintenance of flumes and stilling well is carried out in following aspects:
i. The structure and upstream pond area should be free from weeds and trashes.
ii. The accumulated sediments should be removed.
iii. The crest level should be checked at least yearly to ensure zero positioning of gauge.
iv. The crest should also be checked for nicks/dents, because they affect the measurement accuracy.
v. The well and intake pipes should be free from silt deposition.
vi. At the time of well cleaning or debris removal from the intake pipes, the recorder pens should be raised from the chart because its surge in the well can release excess ink on the chart, as result the pen can tear the paper.
vii. After every major flow events, the intake pipes should be checked; and if necessary the deposited sediments should also be removed.
Comments are closed.