Cover Image
close this bookHandbook for Agrohydrology (NRI)
close this folderChapter 2: Measurement of runoff
View the document(introduction...)
View the document2.1 Estimates of runoff
View the document2.2 Collecting runoff data
View the document2.3 Water level recording instruments
View the documentEquipment costs
View the documentAppendix A: Measurement of runoff

2.2 Collecting runoff data

2.2.1 Volumetric data

Simple Tanks

Complex Tank Systems

Multislot Dividers
Multipipe Dividers
Rotary Dividers

2.2.2 Continuous Systems

2.2.2.1 Natural Controls for Runoff Measurement

Rating Curves

Methods of Flow Measurement

Velocity Area Method
Float Gauging
Chemical Gauging

Stream Flow Networks

2.2.2.2. Artificial Controls for Runoff Measurement

Flumes

HS, HL and H Flumes
Parshall Flumes

Weirs

V-notch Weirs
Triangular Weirs

Culverts and Existing Structures

Methods to calculate the runoff that is likely from various rain storms on catchments of various sizes and with a range of conditions have been discussed. The following chapter describes the equipment that is needed to measure runoff using these systems. There are two main types:

Volumetric equipment

Continuous or Through-flow equipment

A theoretical estimation of runoff peaks and volumes will enable the choice between these two methods to be made, as choice is determined essentially by the size of runoff volumes. Other considerations such as the risk of sedimentation, debris in the flow, site and cost also play a part in the decision, but the amount of water to be measured is by far the most important criterion. In general, the two methods of data collection are used in the following circumstances and have the concomitant advantages and disadvantages that are outlined below. After considering these advantages and disadvantages and after estimating the size of flows that are likely, the most suitable method of measurement should be clear. If doubt remains between the suitability of using simple or complex volumetric measurement systems, further discussion is presented in section 2.2.1.

Volumetric Data Collection Methods

Advantages:

- Can be used easily to measure small volumes of runoff.

- The most basic equipment is a simple tank, although more complex systems will be described later, whereby a small, known proportion of the flow is collected and the total is found multiplying the collected flow according to this proportion.

- This equipment is relatively cheap.
- It can be manufactured locally and is relatively simple to use.
- Lends itself to the easy replication of experiments.

Disadvantages:

- The main limit on the equipment is the physical size, in particular the depth of the collection vessel, the top of which must be installed at a lower elevation than the runoff area.

- It also has the disadvantage of only collecting "lump sum" runoff volumes and gives no other hydrological information. This limits the usefulness of the data.
- No idea of the varying contributions within complex storms is available.
- No information on runoff duration can be found, nor how much rain fell before runoff started.

- The equipment must be well-serviced and be emptied (of sediment as well as water) after every runoff event. It is therefore not a good system for field station runoff measurements, because runoff may not be suspected and an arduous and often fruitless visiting schedule would be necessary to cover all eventualities on a routine basis.

- There is the risk of over-filling of the tanks which can result in the loss of accurate data, though a limited number of experiments with very small vessels can be used successfully, if a reliable field observer is engaged at the site.


Continuous Data Collection Methods

These can be undertaken in many different ways, depending on the physical properties of the flow and characteristics of the site, but the basis of measuring the runoff remains the same for all. Runoff is channeled to flow through a "control" section. This control section may be artificially constructed and as such, will have pre-determined hydraulic properties. Alternatively, a suitable section of a natural channel may be used, though the hydraulic properties of a natural channel must be determined by measurement. In either case, the volume of water passing at any time is found by measuring only the height of water in the channel (the "stage" of the flow). The measurement of stage is effected by the use a water level recorder (WLR) which records changing flow stages over a desired period. Integration of the various stage heights over the period of record gives the total flow.

Advantages:

- These methods also give information on flow durations, peak flows and on when runoff started in relation to rainfall.

- WLRs can hold the data relating to many runoff events.

- The equipment can be left unattended for months if necessary and is well-suited for use at remote field sites.

- The disruption of routine visiting schedules is not a serious problem.

- There is no limit to the flow volumes that can be measured, if the control section is large enough to pass the runoff.

Disadvantages:

- The greatest restriction on the use of this method is the cost of the water level recorders (similar to that of recording rain gauges) and, in common with any complex machinery, the possibility of malfunction. Artificial control sections can be designed to be built locally, for an outlay similar to that of volumetric collection tanks.

2.2.1 Collection of Volumetric Data

Simple Tanks

Simple tanks are used to collect runoff from the very smallest catchments. It is important to put into perspective the size of plot for which simple tanks are suitable:

Example

For every 1 mm of rainfall that is shed per 1 m² of catchment, the collection tank will receive 1 litre of runoff. A 1 in 10 year storm is an appropriate return period for which to design.

- A 100 mm storm (assumed to be a 1 in 10 year rainfall) over a 1 m² plot with a runoff efficiency of 50 % ( for example a sandy loam soil, with a slope of 2%, relatively bare of vegetation ) would give 50 litres of runoff.

- Over a 10 m² (1 m × 10 m) plot runoff would be 500 litres

- Over a 100 m² (10 m × 10m) plot runoff would be 5000 litres.

A tank built to contain 5000 litres would need to have dimensions greater than 2.5m × 2.0m × 1.0 m, (an adequate freeboard is always essential), about equal to the capacity of 25 large oil drums. This is too big for easy manufacture, installation and replication. Even to contain the runoff from the 10 m² plot, the tank would have to be greater than 1.0 m × 1.0 m × 0.5 m. Alternative complex tank systems can be designed to collect runoff amounts of this order of magnitude, and these systems are discussed later. It is reasonably obvious, therefore, that even though most runoff events will be much smaller than the example above, simple tanks should not be used on plots larger than a few square metres. The importance of measuring extreme event runoff volumes, to ensure the widest range of data for analysis, cannot be over emphasized.

Design

On the whole, the design of simple tanks is not a difficult task. The dimensions should be appropriate to the size of the estimated maximum runoff volume. Remember that an adequate freeboard is necessary. Generally it is best to have the tanks made specially, as the modification of containers used previously for other purposes may be as expensive and can lead to compromises in design.

An example simple tank and plot layout is illustrated below in Figure 2.9.

Galvanized steel plot boundaries, about 15 -20 cm above ground


Figure 2.9: Typical Design of Simple Runoff Tank and Plot Layout

Features to note are:

- Light gauge galvanized steel boundaries, about 15 cm high to avoid rain shadow. These can be easily bent into shape and be knocked into the ground or, if soils are very hard, dug in. Any seam or joint that is not sealed should have its outer edge pointing downslope to prevent the entry of outside runoff.

- "Funnel" neck to direct flow into tank. This should be large enough to allow unrestricted peak flow into the tank. It should not be liable to blockage by debris; a wire mesh may be fitted.

- Tank cover to prevent rainfall entering tank directly and prevent animals interfering with collected runoff.

- The tank should be made of inexpensive mild sheet steel, painted against rust.
- Welding destroys zinc protection at seams, if galvanised steel is used.
- Plastic containers usually have a short life, due to rotting by UV light.

One suitable alternative to the specially manufactured tank is the ubiquitous large oil drum (usually about 40 imperial gallons or a little more than 200 litres). These can be cut to size along a horizontal axis to give 2 × 100 litre containers with some freeboard. These tanks are generally too deep to allow easy installation as a whole unit (deep soils would be needed and emptying a deep container is awkward) and their horizontal installation is very inconvenient.

A section must be cut to allow a funnel arrangement to enter the tank and lids need to be manufactured as separate items. Oil drums are often prized objects in developing countries, put to many varied uses and their cost or scarcity may make them unsuitable. A typical installation is shown below in Figure 2.10.


Figure 2.10: Modified oil drum as 100 litre collection tank

Installation

The tank, of whatever type, must be sunk into the ground, therefore ensure that depth is the smallest dimension where possible and completely fill in the whole of the excavated area to avoid water collecting around the tank and disturbing it by flotation or subsidence. Settlement of the soil may take some time. Ensure that animals cannot remove the lid to drink from the water. Regard theft (and possibly vandalism) as a serious threat in field locations.

Cementing the tank into the ground should be avoided as it is likely that the tank will have to be removed on occasions for a thorough cleaning and possibly repair. Easy removal is especially important if the installation is in a farmer's field where pre- and post-ploughing runoff data may be required and removal will be necessary. The tank's presence could prevent correct ploughing. The plot boundaries should not cast a serious rain shadow and should be tapped into the ground or eased in with a spade. The galvanized sheeting recommended can be re-used over many seasons and is cheap and easy to work with. In all respects metal sheeting is superior to earth-dug bunds for small plots. During and after installation, ensure that the minimum disturbance is made to the catchment. In particular ensure that digging etc. does not impede or encourage runoff flow into the tank. Remember that surface flow on small catchments is very shallow, perhaps less than 1 cm deep and even very small surface features, inadvertently produced, can affect runoff. If installed at a field site, a permanent reader will probably be necessary.

Measurement

Carelessness while measuring the amount of runoff that has been collected can lead to as inaccurate results as careless installation of the equipment. A standard data sheet should be drawn up, such as the one below. A separate sheet per site is recommended for each visit. Each individual tank should be clearly numbered where several tanks are installed together. A record of the capacity of the tanks is essential to check against spurious measurement. The tanks can be emptied by scooping out the water using a small measuring vessel. This is preferable to having to remove the tank when measurement is made. The quickest and most reliable way to read the tanks is to have 2 × 20 litre (marked with 1 or 2 litre graduations) and 2 × 5 litre containers (marked in 0.5 litre graduations) available. The runoff is scooped out in a marked vessel (2- 3 litres capacity is the most manageable) and poured into one of the 20 litre containers. When this is full, it is noted on the sheet. This tank is emptied while the other is filled. When most of the runoff is emptied, the 5 litre vessels are used. Unless runoff amounts are very small, results to the nearest 0.2 litre are adequate. Care is needed during the process. Sediment is likely to be present.

Dip-stick measurement of runoff is difficult since accurate leveling of the tank is necessary during installation and the tank may later re-settle or be disturbed. Graduations marked on the tank are an unsatisfactory method of measurement, they become erased or covered with mud, etc. Emptying the tanks carefully is surprisingly time-consuming and an adequate period should allowed, especially where groups of many tanks are involved. It is best that at least two people should undertake the task together and three is to be preferred, one concerned only with the recording of data.


Figure 2.11: Collection tank data sheet example

Complex Tank Systems

The examples of possible runoff amounts, given in the previous section on simple tanks, indicate that small tanks can only be used on runoff plots of about 10 m or even less. For larger catchments, that for one reason or another are not fitted with flow-through measuring devices, a more complex system of collection tanks is needed.

Design

The basic arrangement of these systems is to install a first tank which collects initial runoff. When this tank is full, further runoff entering the tank causes overflow into a second tank. However, only a small, known proportion of this overflow runoff is passed into the second tank, most of the runoff is allowed to run to waste. Such systems can deal with runoff from catchment areas with an upper limit (depending on storm size and catchment conditions) of about 100 to 200 m², though this will be determined by the character of the individual site. There are two main types of complex tank systems:

Multi-slot dividers and Rotating slot dividers.

The major limitations are:

- Tank size

The manufacture and installation of tanks with capacities of several hundred litres can be difficult. Generally, they are made of heavy-gauge galvanized or mild steel sheet to retain some rigidity and are therefore quite expensive to manufacture and unwieldy to install. Because of their size and the fact that they lie one behind the other, downslope, considerable earth-moving may be necessary, especially where land slopes are low. Shed-like constructions may be needed to prevent rain from entering the tanks and flow channels. Dug earth channels behind the installation are necessary to remove the waste water during rainfall/runoff and to prevent the ponding of water which may otherwise enter the tanks and confuse measurement. These channels are also essential to remove the water as the tanks are emptied, which requires the use of a small pump. Pumps of about 0.1 horse power combine sufficient power with precision of control. The tanks should be protected from accidental runon from areas around the installation. Installations for rotary dividers are considerably smaller than those of multi-slot dividers.

The tanks cannot be considered portable in any sense of the word and a permanent location is required for their use. It is important therefore that very careful consideration is given to site suitability from the viewpoints of installation and experimental objectives. In particular, problems can occur in areas of low slope (< 2%) where back-up ponding and drainage difficulties can be severe.

- Number of tanks

If small tanks are preferred, a greater number must be used for each installation and this number is limited by accuracy, as well as the space that they occupy. The first tank in line measures all runoff, conventionally the second tank measures 1/10 th of the overflow and a third tank would usually do the same (though these proportions can be altered to suit individual needs). Thus by the third tank, only 1% of the real runoff amount is actually being collected. If the tanks are not well-manufactured and properly installed, every 1 litre inaccurately measured will affect the calculated volume by 100 litres. If yet another tank were added to increase the catchment area from which runoff was collected, then each litre measured would represent 1000 litres (1 m³) of runoff. Small inaccuracies of collection or measurement would lead to seriously flawed data. Although the difficulties of tank size can be overcome by making the tanks smaller, three tanks in line is probably the sensible limit. The problems of accuracy can be overcome by calibration (see below), but some inaccuracies in measurement and accidental spillages must be accepted as a fact of life.

Rotating slot dividers do not suffer from exactly the same limitations as multislot dividers, since the division of runoff into various proportions is only undertaken once. However, high quality design and manufacture is essential and any flaw in the mechanism may prevent operation. Peak flow estimates by calculation should be made to ensure that all inlets/outlets can cope with the flow.

a. Multislot Divider

Runoff draining from a collection gutter (which should be covered with a wire trash screen ) on the downslope side of the plot flows into a conveyance channel or pipe to the first tank. Heavy sediment will settle here. When this tank is full, 10% (for example) of the excess is passed through a vertical slot and drops down into the second tank. Various sizes and numbers of slots can be used, according to need, examples of approximate slot sizes and capacities are given below in Table 2.18.

The remaining portion of the runoff continues along the channel to be discharged as waste, or to further tanks where a similar proportion is retained for measurement. The tanks and slot plate should be made of suitable galvanized steel sheet with welded, water-tight seams. Slots should be made in the plate as accurately as possible and all angles should be 90°. However, sophisticated workshop facilities may not be available and equipment may have to be transported under difficult conditions to a field site. Thus, while it is important to construct the dividing system with care, it is necessary to calibrate the equipment after installation, to correct for any inaccuracies of manufacture.


Table 2.18: Approximate Number, Dimensions and Capacities of Divider Slots

Figure 2.12: shows a typical arrangement for a multislot dividing system. Construction details are given in Appendix


Figure 2.12: Multislot Divider System a) Plan b) Side Elevation


Figure 2.12 continued

Where the materials and workshop facilities are not available to manufacture a multislot system, a cheap alternative "multi-pipe" divider can easily be made, so long as basic welding equipment is obtainable. The system uses 200 litre oil drums, sheet steel and neoprene pipes. Calibration, as described later in this section, is essential for this system.

A metal box, with a handled lid, is welded from cut pieces of steel plate. On one side, a rectangular section, approximately 15 cm square, is cut out to accept an intake gutter. Ten 5 cm. stub pipes are welded over 5 cm. holes cut into the opposite side of the box to the intake, to form spouts. A 200 1 drum (usually 90 - 100 cm high), is cut into two portions, the first 60 cm and the second 30 cm high or thereabouts. One 5 cm hole is cut into the side of the larger piece of drum, leaving a freeboard of 5 - 10 cm. A short metal stub pipe is welded on to the hole to a form an intake. On the opposite side of the drum ten 2.5 cm spouts are fitted radially in the same way, at a slightly lower level than the intake. A 2.5 cm intake pipe is fitted on the smaller piece of drum. The box, large section of drum and small section of drum are fitted together with the neoprene pipes. All containers should have lids to prevent direct rainfall from entering them. The nine spouts that are not connected act as waste drains. The containers should be leveled when installed.

Figure 2.13 below shows how the containers are fitted together. All dimensions of containers and pipes are approximate and can be altered to suit locally available materials. The number of pipes can be selected according to need.

Installation

The installation of a system requires care more than expertise.

Large tank sets (Figures 2.13, 2.16 and 2.17) must have the site completely prepared by earth-moving if necessary, beforehand. All the tanks in the series should be carefully placed into their excavations and the conduits fitted. They should be leveled and completely assembled together before being set in concrete, where appropriate. All conduits should be leveled.

Where lowering of the tanks may be necessary later, for instance if the surface of the plot is expected to erode as part of the experimental intention, tanks should not be concreted in but placed on durable stands, the elevation of which can be reduced. Multi-pipe systems are best in such circumstances, because they are smaller. Great care should be taken not to distort the dividing system on installation. The system should be protected from rain and runon as soon as possible, where open tanks are used.

The free flow of water, with no ponding, from all tanks in the series should be ensured. Calibration should not be attempted until the concrete is set, or the tanks are secured to their stands. Procedures for the small tank system are essentially the same, but less labour will be required and alterations to the site can be made during installation.

The detailed procedures of calibration are given below.


Figure 2.13 Multipipe Divider System

Calibration

Calibration is straightforward, though the large tanks will need several oil drums of water.

- Ensure the tanks are firmly set in the ground and all the components are secure.

- Ensure the outlets are as level as possible

- Ensure the whole system is clear of debris.

- Ensure all seals are water tight

- Fill the first tank with water until it overflows to a small degree into the second tank.

- Ensure there is no ponding between the tanks.

- Remove any water from the second tank

- Using an accurately graduated vessel ( for example 10 litres), pour water carefully into the (primed) first tank.

- Record the amount poured.

- This should be repeated until an easily recoverable quantity of water can be removed from the second tank. The waste water from the first tank can be ignored, but ensure that it flows away from the site easily.

- Measure the water recovered from the second tank.

- Repeat the process but start by filling tank 2 and measuring from tank 3

The calibration factor of the tanks, or more correctly the dividing system is then the ratio: Water Poured / Water Recovered.

For example:

Tank 2
275.0 litres / 25.6 litres,
Calibration Factor (C.F.) = 10.74

Tank 3
109.0 litres / 11.2
Initial Calibration Factor = 9.73
Actual Calibration Factor- CF Tank 2 × CF Tank 3, therefore = 10.74 × 9.73 = 104.50

All quantities runoff measured from the second tank should be multiplied by the calibration factor to calculate the true runoff volume. A mean value should be obtained from a number of calibrations undertaken for each tank. For a series of three tanks, the same procedure is followed. The total runoff from rainfall represented by the portions actually collected in all the tanks would be (for the examples given above):

Runoff from tank 1 × Calibration Factor ( = 1.0) + Runoff from tank 2 × Calibration Factor (= 10.74) + Runoff from tank 3 × Calibration Factor (=104.5).

Routine data collection sheets for a multiple tank should be prepared in a manner similar to Figure 2.14:


Figure 2.14: Collection tank data sheet example

b. Rotating Slot (Coshocton) Divider
The rotating slot divider is a much smaller device than the equipment that has been described above, but involves a high degree of precise manufacture. Welding must be accurate and discrete, bumps and distortions of the metal wheel must be avoided. High quality bearings are needed. Detailed drawings for the construction of this runoff sampler are given in Appendix A3. Figure 2.15 shows a sketch of the mechanism fitted below an H flume.

Runoff is directed from a collection conduit and pours over a horizontal sampling wheel, the slot divider. The action of the water forces the wheel to rotate and the sample slot cut into the wheel continually passes under the water then away as the rotation continues. Runoff that goes through the slot enters a sump and then is conducted away to a collection tank. That which does not pass through the slot, runs to waste.

Installation

Plot design and collectors are as described above.
Installation design often depends on the land form of the runoff area.
Figure 2.17 shows an example.

The complex tank systems described above are frequently used for sampling sediment load and the combination of runoff and sediment measurement is obviously a cost-effective manner of organising activities, the value of which should be carefully considered at the planning stage of any project.


Figure 2.16


Figure 2.17

Figures 2.16 and 2.17 Alternative Installations of the Rotating Slot Divider Source: USDA Handbook 224

2.2.2 Collection of Data from Continuous or Flow-through Systems

Flow-through systems are used where catchments provide too much runoff to be collected in tanks. They are also necessary where knowledge of the start, the duration, peak and end of flow of runoff (the flow hydrograph), is needed. These systems are particularly useful at remote sites, where visits cannot be made after every rain storm. In humid climates, they can be used to make continuous readings of permanent streams. Measurements are made at a control section with known hydraulic properties, where the flow volume can be measured simply by recording the depth (stage) of water passing through the control section at any time.

Controls fall into two main categories:

Natural controls

exploit the physical features of the stream channel or other waterway, (be it ephemeral or permanent) to provide a location for the measurement of flow. This is not as simple as it may appear.

Artificial controls are made to pre-designed specifications, according to their use and are placed where required. There are many designs of artificial control.

In general, the most useful controls for work in agrohydrology and water harvesting are artificial. However, it is possible that natural controls may be used of necessity, especially where natural channel flow in large amounts is present. These volumes of water may be too large for the use of a pre-calibrated device or conditions may be unfavourable for its operation. The cost of building artificial controls on any but the smallest river channels will probably be prohibitive.

2.2.2.1 Natural Controls for Flow Measurement and Stream Gauging

The first problem to overcome is the identification of a suitable site. Bends in channels should be avoided. Eddying and spiral flow occur and cause changes in the river bed and the undercutting of banks, making such locations unsuitable. Straight sections of channel are desirable. Sites should be located where the bed is as uniform as possible, away from tributaries and other flow disturbances. Changes in vegetation, human activity etc. can all affect the control at a site.

The effects of controls (the influence of the channel having a particular form that restricts flow) can be present at low water, high water or may change as the depth of the river alters in flood. Usually low water controls become ineffective as stage increases. Contracted sections such as bridge openings may operate at highwater which may be a disadvantage, though bridges are frequently convenient access locations for measuring stream discharge.

At gauging sites, locations where frequent flow measurements are made, gauges that facilitate the recording of stream depth are placed. If possible, an automatic water level recorder and manually-read posts are used, though this is expensive. Where a continuous record of stage is not required, manual posts alone are sufficient. The hydraulic properties of a natural channel, which must be determined to allow the use of stage-only recordings to measure flow, are defined by rating curves, sometimes by rating tables.

Rating Curves

A rating curve is a calibration curve, a graph of the relation between stream depth (stage) and flow (discharge). Obviously, as stream depth increases, so does discharge. However, this relation is unique at each location on the channel and is rarely if ever a straightforward linear relation. When sufficient depth/discharge data are collected (see below) to define this relation, unknown discharge can be found by simply reading the known depth from the rating curve. Further discharge measurements are taken on a routine basis to up-date the rating curve. This will be necessary if the stream channel changes, for example after a severe flood. It is a serious problem for short-lived projects that it may take many years to compile a rating curve which, of necessity, should include a wide range of discharge, from low to high.

A typical rating curve will be similar to Figure 2.18. The relation can also be defined in a rating table, which allows more convenient use in computer programs. Note that the curve will plot as a number of straight lines when logarithmic axes are used, indicating a change of control at the inflection point(s) as stage/discharge relations change.


Figure 2.18 Example Rating Curve

Data points collected after the compilation of the rating curve, should lie within 10% of the curve. Values that do not, indicate a change in control of the river section or large measurement error. Values not within +/- 2% of rated discharge can indicate that re-drawing of the curve may be necessary in some circumstances, but it is assumed here that moderately accurate values of discharge are satisfactory, and that agricultural and water harvesting projects will not wish to invest the time nor resources to delve deeply into the theory and practice of hydrometry, nor undertake the rigorous field schedule of data collection that would be necessary to achieve wholesale rating curve revision. A simple stage/discharge plot should be adequate to define the relation.

From a graph such as 2.18, the flow hydrograph can be obtained from a continuous reading of stream height.

There is no easy way to extrapolate extreme discharges from a rating curve and although the general equation of any curve is assumed to be q =k (g - a)b, where a, b and k are site constants, this formula cannot account for changes of channel geometry at higher stages and an abrupt discontinuity of the relation will be seen when bank-full conditions are experienced; that is when the river floods and is no longer confined to the channel.

Stream gauging procedures measure the stage and discharge of a stream and provide the basic data for rating a channel location. Rough estimates of discharge for different stages can be made by taking the cross-sectional area of the channel and multiplying the channel area by stream velocity. Values of velocities can be estimated by using Manning's equation (see later in this chapter).

Methods of Flow Measurement

a. Velocity-Area method

This most accurate and usual method uses a current or flow meter and associated equipment. The meter consists of a set of horizontally mounted cups (vertical axis) that move a contact breaker as they rotate. This breaker, wired to a battery and either an automatic counter or head phones, registers each rotation of the cups. Each meter is individually calibrated and provided with a table (usually) fixed to the carrying case. This table is used to convert the number of rotations per time period into flow velocity in feet or metres per second. For very small streams or highly vegetated conditions a horizontal axis propeller-type meter is used, but the method of measurement is the same for both instruments. Figures 2.19 and 2.20 show typical cup-type and propeller meters.


Figure 2.19: Typical Cup-type Flow Meter


Figure 2.20: Propeller-type Flow Meter

A marked cable or tape is stretched across the stream channel at the gauging site, at right angles to the flow. The tape is used to divide the stream width into convenient sections. In Figure 2.21 below, 14 × 2.0 m sections are used, starting from the initial point (0) on the left bank. This leaves one section of 1.2 m at the right bank, to complete the full stream width of 29.2 m. To ensure a sufficient number of velocity readings across the stream width, no section should be greater than 10% of the total width, where possible 5% gives greater accuracy. The subdivision of the channel width allows the measurement of the different velocities and discharge within each section, due to friction and eddying. On the other hand, the time taken to complete the procedure should not be too lengthy, because minimum change in stage of the stream, during the time of gauging, is desirable. Any large change in stage can cause an inaccurate measurement of discharge, unless corrections are made. These are quite complicated and best avoided. Stage readings should be taken from the staff gauges located at the site, before starting and after completion of discharge measurements to check for excessive change of stage.


Figure 2.21: Velocity-Area Method

Figure 2.21, shows the arrangement of channel sections used in the velocity-area method. The flow meter is positioned at 0.2 and 0.8 times the vertical depth of the water, pointing against the flow. At each position the number of rotations in a given time (or given number of rotations in a measured time) is counted and the velocity calculated from the calibration table of the flow meter. The average velocity of the two readings is taken as the overall average velocity of the whole vertical section. The distribution of velocities from bed to surface in a stream is parabolic and the average of the two measurements gives an accurate measure of true mean velocity.


Figure 2.22: Sample Discharge Measurement Form

The depth of the stream is noted from the graduated bar that holds the current meter. The procedure is followed for all the sections of the stream. Where the stream is too shallow (< 0.50 m) to allow two velocity readings to be taken, the 0.6 of stream depth position, below the stream surface, alone is used. Calculations of depth, velocity and thereby, discharge are made as illustrated in Figure 2.22.

In some cases, usually when the river is at a high stage, wading the stream to effect measurement will be impractical. In such circumstances a bridge or other convenient structure must be used. Readings are taken with the meter suspended from a cable and held down by a large, streamlined weight. The current will carry the instrument downstream and it will not hang vertically, but a small correction can be made to overcome this. With the cable 12° from the vertical the inaccuracy of measured depth is about +2%, but corrections will depend on exactly how much cable is paid out and how much is in and out of the water. A table can be drawn up and used, according to Figure 2.23 below.


Figure 2.23: Geometry of a Cable-suspended Flow Meter

Operation and maintenance

On the whole, the equipment is easy to operate and maintain. Regular oiling of the cup bearings with light lubricant is essential (sewing machine oil is a good substitute if manufacturers' oil is unavailable, but do not foul the contact points) and spare bearings and a spare set of cups should be purchased. The cups in particular should be treated with care as any damage will alter the rating of the instrument. Equipment should be cleaned and dried after use. The electrical contacts should be kept clean as they tend to burn out at the tips with use. Propeller meters usually have bearings of synthetic plastic material and usually should not be lubricated.

It is desirable for current meters to be re-calibrated by the manufacturer or a hydraulics laboratory every few years, therefore careful use of the equipment is essential if this costly inconvenience is to be kept to a minimum.

Equipment suspended from a bridge or similar structure will necessitate the use of a winch. This is purchased with meter weights, cable and fittings and will include an integrated depth-counter. A simple board can be made to which the winch can be fixed for manual operation. Figure 2.24, below shows a vertical view of the winch board.

It is suitable for use by one person with all but the largest of weights used for very large rivers. Some help may be needed when the weight and meter is lowered over the side of the bridge. Purchased stands for winches tend to be expensive and large.

It assumed that suspended cable ways, which are sometimes used in operational hydrology, will be far beyond the resources of an agrohydrological or water harvesting project and that less accurate but cheaper methods of flow volume estimation, such as float-gauging would be more appropriate. Inflatable dinghies are not too expensive, but the need for an outboard motor adds to the cost.


Figure 2.24: Simple Hand-held Winch Board

b. Alternative methods of discharge estimation

Float gauging

This method probably provides the most suitable alternative way to measure stream velocity and discharge. Surface floats travel at about 1.2 times the mean stream velocity. See Figure 2. 25 below which shows the distribution of stream velocity with depth.


Figure 2.25: Depth / Velocity Relations of Stream flow (Velocity Profile)

Floats should be clearly visible and of uniform size and material. A straight stretch of channel should be used to avoid velocity changes and eddy currents. Where possible, changes in velocity should be accounted for by placing floats across the width of the stream. Several floats should be used and average reading taken.

Remember that a cross-sectional profile of the stream with depth measurements to calculate area will be essential to allow the computation of discharge. This will require a survey of the channel at a later date. Permanent staff gauges can be emplaced and a rating curve determined. This method may be useful when a river is at a very high stage and impossible to gauge by the velocity-area method. In general the cost of an automatic water level recorder to measure river stage would not be warranted if float gauging is used to the exclusion of more accurate methods.

Chemical gauging, whereby salts, dyes or radioactive materials are introduced into river flow is sometimes used. With this " dilution" or "tracer" method, a concentration ct of the tracer is injected into the flow at a rate qt. Downstream, samples are taken when equilibrium concentration cc has been achieved and the discharge qt is

qt = (ct/ce - 1) qt (2.18)

Methods of concentration determination however, involve expensive detection equipment that would not be appropriate to most projects.

Stream Flow Networks

It is likely that a project that measures stream flow will require a number of stations, but at the same time will need to keep down costs. A basic network should consider:

- Determination of the minimum catchment area to be monitored, perhaps as large as 250 km² per station in developing regions .

- A station should always be located at the catchment outlet.

- Gauging of major tributaries should be undertaken.

- Locations/streams of particular significance such as those in areas of future development should be targeted.

- Regard should be given to the kind of use the information will be put to: flood forecasting; irrigation development etc.

- A good sample of hydrological, topographical and geological types could be monitored.

- Probability forecasts are usually an important factor in collecting stream flows and the longer the station records, the better.

- However, if budgets are severely restricted, then stations can be moved after 5 or 10 years and synthetic data derived thereafter.

- Where possible sites should be located near bridges etc. for ease of measurement and instrumentation and should have good, all year access.

- Gauge height readings from manually-read, graduated posts will require a site reader.

2.2.2. Artificial Controls for Runoff Measurement

Natural controls are limited by the occurrence of natural channels, whereas artificial controls can be placed wherever there is need for them. This can be in natural channels if desired, but bunds and channels can also be installed to bring dispersed surface flow to a point suitable for measurement. Furthermore, artificial controls are pre-calibrated with known rating curves which do not have to be compiled using flow-discharge information. These advantages make artificial control structures the most suitable for agrohydrological applications. There are many designs of artificial controls, each developed to be suitable in different circumstances and it is important to select the correct design of structure for the job in hand. Figure 2.26 presents a diagram to aid selection.

The basic assumption is made here that agrohydrology and water harvesting projects will measure runoff from relatively small catchments, fields and experimental plots, though equipment suitable to measure runoff from areas in the order of square kilometres is considered. Peak flows will be relatively small, probably no more than 1- 2 cubic metres per second, in many cases peaks will be only a few litres per second. Therefore, from the wide range of artificial controls available, those that are most appropriate to small peak flow measurement have been selected and are described below in detail, with examples of installation, problems of operation, etc.


Figure 2.26: Selection of Artificial Control Structures

Situations may be encountered where small structures are inadequate, and examples of large artificial controls are given, but in less detail. These are minor works of civil engineering and their construction is usually undertaken only by River Authorities and similar organizations. They are costly and permanent, but in some situations may be essential if stream flow data are to be collected where natural controls are unsuitable. Any project proposing to enter into the construction of such controls is urged to approach the relevant Authority and seek advice as to those which have proved most suitable for local conditions, their likely cost and problems of installation.

2.2.2.1 Flumes

Flumes are essentially long, box-like structures that allow the flow of water to retain or increase its kinetic energy as it passes through them. They have the advantages of being able to measure small flows accurately while allowing debris and sediment to pass. They can be made light and portable and can be located in most situations. They may be fitted to small experimental plots which do not have natural channels, or be placed in steam beds. They are probably the most suitable of all artificial controls for agrohydrological applications.

a. H, HS and HL flumes

The H flume is the basic instrument of this group. HS flumes are designed to measure very small flows accurately (flows < 28 1 s-1), while HL flumes are capable of measuring much greater flows (up to 3.3 m³ s-1). The materials from which they are made, their installation and operations are similar to the H flume, though their dimensions are somewhat different. H flumes fill a wider niche of runoff measurement than HS and HL flumes, therefore this section will discuss them in detail. The design criteria of HS and HL flumes are given below in Figures 2.27 and 2 28. Rating tables for the conversion of recorded stage to discharge are given in Appendix A1. The main point to note is that the dimensions and quality of manufacture strictly determine the rated capacities of the flume, therefore they should be made as accurately as possible to retain the correct rating characteristics.

HS and HL Flumes


Figure 2.27: Design Specifications of HS Flume


Figure 2.28: Design Specifications of HL Flume


Table 2.19: Capacities of HS and HL Flumes

Rating tables for these HS and HL designs are given in Appendix A1.

H flumes

For the range of flow measurement met in agrohydrology, especially runoff plot and farmer's field studies, H flumes are very useful measurement devices, but it is important that they are manufactured and installed with precision. Figure 2.29 gives the design dimensions.

Construction

The flume is the component of the whole structure to which the water level recorder is attached to measure stage.

- To manufacture the flume, prepare detailed drawings of the design with the maximum capacity needed.

- Make a paper and then a thin sheet metal template of the flume that can be used many times.

- Use either heavy gauge galvanised or mild steel sheet that can be rust-resist painted to make the flume.

- The thickness of metal should be appropriate to the overall size of the flume.

- Support all edges with angle iron or structural steel to prevent warping.

- Welded joints should be water tight, strong and ground smooth.

- Vertical sides should be exactly vertical and made from one piece, the bottom plate should contain no more than one joint and it should not be closer than 30 cm to the outlet.

- Avoid all distortions, dents and warps when cutting and fixing plates.

- Before installation, the flume should be checked for adherence to the proportional dimensions in Figure 2.29.

Approach section and stilling well

The flume head or measuring section acts as an artificial control to allow stage measurement, in addition all flumes need an approach section, attached to and upslope of the flume. They also need a stilling well upon which to site the water level recorder (WLR). Remember that the top of this well must be of a suitable design to take the specific manufacture of WLR. It is usually essential therefore, to obtain the WLR before the well is designed and fitted. Stilling wells are best made of the same metal as the flume and welded to the head measuring section. Openings allow the passage of water between the flume and stilling well.

H Flume Specifications

For small flumes (D = 20 to 60 cm), it is good procedure is to construct the approach section out of flume metal according to the specified dimensions and weld it to the flume in the workshop, with the required slope of 2%. In the field, the flume measuring section is installed with its floor horizontal (use a spirit level) and the approach section will then be set at the correct slope without further action. Such an installation has the advantage of being portable and can easily be removed for ploughing or relocation. Large flumes may require under-floor support of the approach section. Handles on the walls allow ease of portability (see Figure 2.30 below).


Figure 2.29: Design of H flume

Note: for flumes with D < than 30 cm, length of flume is made greater than 1.35 D to allow for float and stilling well.

Source: USDA Handbook 224

For large flumes (depth 1.2 m+), construction of the approach section can be completed in the field using cement block walls to the appropriate dimensions in Figure 2.32. All block work and cement floors should be rendered smooth and the join between flume and approach section should be well sealed. Alternatively, treated wood (tongue and groove with water tight joints) can be used for large or small flumes. This can be sheet metal-covered if preferred and makes a good, cheap temporary structure, but consider the problems of termite damage and rot.


Figure 2.30: H Flume, Approach Section and Stilling Well

A concrete approach floor with a 2% slope can be used, with the (metal) flume discretely bolted to it and the join sealed. However, concrete floors do not allow the same flexibility of removal and may be problematic where installation is dependent on seasonal ploughing. In all cases, angle iron should form the sloping edges of the flume, to prevent any distortion.

Installation
Before installation, it is well worth considering the following points:

- Has the flume been checked for correct manufacture?
- Has a test fit of WLR, float and counter weight been made?
- Has permission to install been given where required?
- Will the installation be permanent / be there for many seasons ? or
- Will it have to be removed for ploughing and then replaced?
- Is the design too big for easy transport and installation?
- Is it located in the correct position?
- Would many cheaper but shorter-lived flumes serve the purpose better?

Installation should take place with the approach section just below ground level and at the lowest elevation of the plot or catchment. This is convenient if in a natural stream channel, but for agrohydrological measurements, this may entail the construction of bunds (typically earth or galvanised steel) to concentrate the flow. If gutters are used instead, they should be covered or the runoff from rain falling directly into them must be taken into account in runoff calculations. To avoid scouring and undermining of the approach section, a small clay or cement apron can be positioned where water runs into it from the plot. A hard surface (tiles, cement) is placed below the outlet point to prevent erosion.

Remember that the measurement of runoff from ploughed fields in particular involves sedimentation as a problem. If so, then the flume should have a 1 in 8 sloping floor fined as shown in Figure 2.31. A sloping floor makes no significant difference to runoff measurement.


Figure 2.31: Front Elevation of H Type Flumes Showing Sloping Floor

Setting the water level recorder
Flumes require water level recorders to be fined at the measuring section to record water height. The simplest way to fit the WLR is as follows.

- Test that the WLR float and counterweight move freely up and down within the stilling well, with the WLR sitting on top of the well, but not fixed.

- The stilling well should be designed to have its base at a lower level than the flume floor by about 10 cm, thus forming a sump.

- The sump is filled with water until it flows out into the flume.

- The WLR is fined with tape, float and counterweight.

- When the float is lowered, water is displaced until the float rests at the zero position.

- The WLR pen or electronic level is set and the WLR stand can be bolted into fixed position.

- The WLR is then set in relation to the flume floor and any accidental subsidence will not affect readings.

- As the sump water evaporates the WLR will register negative readings, but in the event of runoff, the sump will fill and the float will rise rapidly.

- The amount of runoff needed to fill the sump is negligible.

- Alternatively, a wire frame in the sump set to a level whereby the float rests on it at zero level with the flume floor can be used to prevent negative values. - Check the sump for sediment and clear as necessary.

Rating Tables and Equations

Strictly speaking, current metering checks should be made on the operation of flumes, to ensure that design specifications have been followed precisely and rating is accurate. However, in practice the facilities and time to do this will rarely be available for project staff working in the field, especially in developing countries where suitable facilities may not exist. It is essential therefore to construct the flumes accurately and avoid accidental damage to them. Damage is most likely during transit to the installation location, or during seasonal removal.

The equations that govern the rating (stage/discharge) of H flumes are complex and it is advised that the rating tables provided in Appendix A1 are used.

The rating equations for H, HS and HL flume stages are given below for discharge in cubic feet:

Low Flow: Transition:

Q = A0 (280 + B1H) H( H - 0.01)A1
Q = (K0B0 + K1B1H) (2g)0.5 H1.5 (2 19), (2 20)

Medium and High Flows:

Q = {(E0 + K1D) B0 + (F0 + F1D) B1 (H +v2/2g )}(2g)0.5 (H + v/2g)1.5 (2.21)

with D = 1 and v = average velocity at the head measuring section


Table 2.20: Coefficients of Rating Equations 2.19, 2.20 and 2.21

Siting and Plot Construction

For small catchment and plot runoff measurement, the simplest location for the flume is in the lowest-lying corner. Use of a simple levelling instrument from one base will identify this point, without the need to undertake a comprehensive plot survey. Few plots, even those that have been land-levelled are absolutely square to the land contours and it is likely that the lowest point will lie in a corner of the plot, which is conveniently the focus of the defining bunds or walls. However, the use of a level is necessary as an assessment of elevation by eye can be misleading. Galvanised sheeting cut to 30 cm wide strips and dug into the ground will form a durable perimeter for small plots. The edges must overlap well, with upslope ends of the metal sheets on the inside of the overlap. Where they meet the flume approach section, they can be bolted to it and provided with a water-proof seal. Earth banked against the inside of the sheets at this point can help prevent scouring.

Rarely, scouring around the flume approach mouth can be a problem. The metal approach in Figure 2.30 has a step dug under the ground to help prevent this. Compacted soil, clay or cement aprons can also be used, but a solution will depend on the particular circumstances of the site and workers should be prepared to use their imagination in overcoming small problems such as these. Earth bunds will be cheaper for larger catchments. Experience shows that for a 0.4 ha plot (4,000 m², 100m × 40m) a perimeter bund built to 50 cm takes about 4-8 days to complete by 4 people on a hard, compact sandy loam soil. Picks, shovels and mattocks will be needed. Settling of the soil reduces the height of the bund to approximately 35 cm after a few weeks, with no further reduction. Weed growth soon aids stability. As a guide to perimeter bund construction, pegs with string at the desired height are adequate. Soil is dug and thrown in, with the trench on the outside of the plot. Obviously 35 cm bunds enclosing a plot with a flume of greater depth would not be adequate, or if they were, they would indicate that the flume had been over-designed. For any but the smallest plot, it is not necessary to cover the flume or account for rain falling directly onto it. In the example plot above, the H flume and approach section represent only about 0.01% of the runoff area.


Figure 2.32: Alternative Installations for H Flumes

Submergence

Wherever possible, flumes should not be located where submergence, that is the ponding of discharged water around the outlet, will occur. Drainage channels (where necessary) should be adequate to deal with the removal of discharge. This is not a problem at locations with any reasonable slope, but in low-slope areas (1% or less) it can cause difficulties. H flumes are well designed to cope with the submergence problem, 30 and 50 % submergence cause less than 1 and 3% inaccuracies in the measured flow, respectively.


Figure 2.33: Head and Submergence, H flume

Figure 2.33 gives the relation between the increase of flume water head due to submergence and depth of submergence. It is defined by the equation:

H = d1/ 1 + 0.00175 (ed2/d1)5.44 where (2.22)

H = free flow head;
d1 = actual head with submergence;
d2 = tail water depth above flume zero head
e = base of natural logarithms (2.71828)

However, before seeing the head / submergence relation as a way out of this problem it must be remembered that a second WLR, or some other method, is needed to measure the depth of submergence. Given the high cost of WLRs, (as well as the extra time needed to analyze the data) it is best to choose a less problematic site for installation, if at all possible.

b. Parshall flumes

Parshall flumes are a particular type of Venturi flume, their chief advantage being that they cause only a low loss of head during operation. Their design is based on a long constricting section or throat, the floor of which is flat. They are more difficult to construct than H-type flumes, having a more complex shape, but in general they have no significant advantages for measuring runoff in most circumstances, except that they can be constructed on site to measure very much larger flows. Field calibrations, with velocity recordings and large flows of water, which are difficult to arrange, could be necessary. Small Parshall flumes can be bought relatively cheaply, but they are too small to be fitted with WLRs and therefore are only suitable for regular, predictable flow, such as that in irrigation channels. In these circumstances, stage can be measured manually on a regular basis. Small flumes can be manufactured from welded sheet metal, following the careful practice outlined above in this section, though the design is complex and the tolerances of dimensions are very small. They are installed with the flume floor level and care must be taken that they are stable and undermining by erosion cannot take place in front of the converging section. The stilling wells are located in the adjacent banking (see Figure 2.34) and hydraulic connection to the water level recorder is provided by a connecting pipe at flume floor level. WLR installation procedure is the same as for the H flume.


Figure 2.34: Parshall flume

Source: USDA Handbook 221

Rating Equations
The general rating equation for small Parshall flumes is:

Q=4Wha1.522 W0.026 where (2.23)

Q = discharge in cubic feet
W= throat width or length of crest in feet (the size of the flume)
Ha = gauged head, 2/ 3 {(W/2)+4)}feet back from the crest in feet

In metric form, with dimensions in metres, the rating equation is:

Q= 4 ( 0.3048)2-1.57(W)0.026 WHa1.57(W)0.026 where (2.24)

Q = discharge in m s-1
W = throat width in m
Ha = upper gauge head in m at a point 2/3 (W/2 + 1.219) metres back from the crest.

The general formula for large flumes (> 3 m) is given by Parshall as:

Q= (2.29265 W + 0.47376) Ha1.6, with all values in metres (2.25)

Large flumes are constructed of reinforced concrete in the field and their manufacture is a difficult task and accordingly expensive. They are generally used where flows are large (they can measure flows much greater than H-type flumes) and where backing up of water and submergence can be a problem. They do need more than one water level recorder where this latter condition is met, however. Where flows are regulated and orderly (for example during irrigation procedures) they can be used with manual gauges, which should be read at short, regular intervals. This saves greatly on the cost of WLRs, as is also the case for H flumes. Capacities and dimensions for Parshall flumes are given in Appendix A 5.

Submergence

Submerged conditions occur when water in the diverging section impedes flow in the converging section and they demand a more complex formula than for H flumes. It is accurate for values of Hb / Ha up to 0.96:

Q = C1 (H2 - Hb)n1 / {-(log Hb/Ha + M2)}n2 (2.26)

Values for submerged flow coefficients and exponents, C1, Ha, Hb, n1, C2 and n2, are given below in Table 2.21. St is transition submergence, where free flow changes to submerged flow.


Table 2.21: Submerged low Coefficients and exponents for Parshall Flumes (m)

2.2.2.2. Weirs

A weir is a low dam or wall built across an open channel and has a specific shape and size. Water flows over in a free-falling sheet (nappe), but if the nappe is partially under the water downstream of the dam, it is said to be submerged. In this condition the accuracy of measurement is reduced. There are many types of weir, but none are suitable for locations other than those with light concentrations of sediment. Some common designs are described below.

a. V-notch Weirs

These are often used to measure low flows, as they do so accurately. They are therefore relatively useful in agrohydrological situations. The common V-notch is a 90° opening (usually cut from a metal plate) with the sides at 45° to the vertical. The approach velocity of flow can be ignored if the distance from weir to bank is twice the head and the height from channel bottom to the crest is twice the head. To fulfil these criteria, modification to the approach section is not usually difficult. V-notch weirs are also useful in the agrohydrological context, because not only can they be used to measure flow from plots and small catchments, they are relatively easy to make and install. Their rating equation for various flows is simple. Their biggest disadvantage is that they are unsuitable for locations with any other than low concentrations of sediments. The V-notch should be kept clean and sharp at all times.

Manufacture and Installation

The 90° V-notch is cut from rigid 10 mm sheet mild steel, which is galvanised or carefully painted to resist corrosion. This is bolted to the cement block approach section (with a rubber gasket sealed joint), which also acts as a sediment sump.

Figure 2.35 below shows a typical installation of a small V-notch weir.


Figure 2.35: Installation of V-notch Weir

The V-notch is bevelled to a sharp edge and must be maintained in this condition. The stilling well is located away from the weir, at a convenient point and hydraulic connection is made to the sump (at the level of the V apex) by a 5 cm diameter pipe. The stilling well can be any convenient dimensions. A small oil drum fixed into the ground, with a suitable outlet for the connecting pipe, makes a good form for the stilling well. It should be treated to resist corrosion. The top of the drum has welded or bolted onto it, fixings appropriate to the type of water level recorder to be used. It should also be fitted with a lid with small holes adequate for the passage of the float / counterweight tape or wire. The connecting pipe should be fixed to the drum prior to installation, laid horizontal by levelling and sealed at the sump end. Bunds are raised to direct flow to the weir and should be solid enough to resist erosion. The simplest way to set the level of the WLR and to check that the levels of the V-notch and pipe are the same, is to fill the sump and stilling well with water (though this may take several hundred litres, depending on the size of stilling well and sump). The water level should be allowed to settle until it is just at the apex of the V-notch and at the bottom of the pipe in the sump and stilling well. The float can then be lowered, a small amount of displaced water will drain and the pen or electronic counter on the WLR set to zero. The WLR will register negative values due to evaporation from the stilling well during long periods without runoff. Some account must be made of runoff collected in the sump and refilling of the stilling well after rainfall, especially if the runoff event is small. The capacity of the sump may be several hundred litres. This procedure will depend upon the exact circumstances of the installation, the size of runoff event and the degree of sedimentation of the sump, etc.

Rating Equation

The rating equation for a 90° V-notch weir is relatively simple and from it a rating curve or table can be derived. It is:

Q = 2.49 H2.48 where (2.27)

Q = discharge in cubic feet ( 35.3 ft3 = 1 m³ = 1,000 litres)
H = head above lowest part of the V-notch in feet ( 1 foot = 0.305 m)

For V-notch weirs with angles not equal to 90°, the rating equations are complex and discussed below for large structures. For small weirs, however, there is little or no advantage in diverging from this orthodox design.

b. Large Weirs

Examples of large artificial controls are given below, but in less detail than those used for the measurement of small runoff flows. These structures are minor works of civil engineering usually undertaken only by River Authorities and similar organisations with the necessary equipment and skills. They are costly and permanent, though in some situations they may be essential if stream flow data are to be collected and natural controls are unsuitable. Any project proposing to enter into the construction of such controls is urged to approach the relevant Authority and seek advice as to those which have proved most suitable for local conditions and their likely cost.

Broad Crested V-notch Weirs (Triangular Weirs)

Large versions of the 90° V-notch weir can be used to measure large volume of runoff, but as runoff amounts increase so does (usually) the presence of debris which may block the outlet. Triangular weirs pass floating debris easily.


Figure 2.36: Broad Crested Weir

They are large, permanent concrete structures, capable of measuring flows greater than 30 m³ s-1 and involve a considerable input of finance and labour. Backwater ponding is not permitted. However, they are relatively simple in design and construction' compared to alternative weirs and so are discussed here in some detail. Figure 2.36 shows the dimensions of such a weir with a 3:1 sloping section.

A straight section of channel is needed for 20 m upstream and a concrete apron 4m long is needed downstream. A large end cutoff wall is necessary to prevent the structure being undermined. The calibration of these weirs is affected by the approach velocity, the cross-sectional area of the approach 3 m upstream from the weir being a measure of this. Rather than providing a series of rating equations, which are very complex for these weirs, rating tables are given in Appendix A 4.

c. Culverts and Similar Existing Structures

In some instances it is possible to use existing structures such as road culverts to measure runoff. The advantages of such structures are that they may be fairly common and will be already in place. Sometimes they may have to be built out of necessity for other project activities and so impose no extra cost on the hydrology budget. It is important to remember however, that in most cases existing structures will not have been made with runoff measurement in mind and modification may be necessary.

This can be costly and time-consuming. They may not be conveniently located and serious errors of estimation can occur when such structures are used without knowledge of suitability. The basic aspects of flow in culverts are discussed here because culverts and their runoff capacities can be an important aspect of water harvesting schemes, farm layouts and irrigation projects. In particular, square concrete and circular corrugated metal culverts are frequently encountered.

Flow in culverts

Culvert capacity can be controlled by the inlet section or the conduit. In either case the head water elevation may be above or below the top of the inlet and the solution to calculating culvert flows depends on the head and tail water conditions. Square or circular sectioned culverts may be used, but neither are accurate meters of low Bow when compared to pre-calibrated artificial control sections. The three main types of flow in culverts are:

1. Where the slope of the conduit is less than the neutral slope. The conduit is full and therefore controls the flow. Inlet submerged, outlet submerged or not.

Use Pipe Flow Equation (2.29).

2. Where conduit slope is greater than neutral slope. Inlet submerged, outlet is not submerged. Entrance controls exist (inlet submerged) .

Use Orifice Flow Equation (2.30) or Figure 2.38

3. Inlet not submerged, outlet not submerged, culvert slope less than neutral slope.
Conduit controls exist and Entrance controls do not.

Use Manning's Open Channel Formula (2.31)

Neutral slope is defined for small angles of the conduit to the horizontal by:

Neutral slope=tan x = sin x = Hf/L = Kc (v2/2g) where (2.28)

x is the slope of the conduit,
Hf is friction loss in conduit of length L (m)
Kc is the friction loss coefficient v is the velocity of flow in m s-1 g is the gravitational constant in m s-2

Figures 2.37 (a) to 2. 37 (c) illustrate these conditions, respectively.


Figure 2.37 (a)


Figure 2.37 (b)


Figure 2.37 (c)

1. Pipe flow (that is when the conduit controls the capacity of flow) usually occurs when the slope of the conduit is less than the neutral slope. The pipe flow equation is:


where (2.29)
Q = flow capacity in units of L3 T-1
a = cross-sectional area of conduit in units L2
H = head causing flow in units of L
Ke = entrance loss coefficient
Kb = loss coefficient for bends in the culvert and can therefore often be ignored.
Kc =head loss coefficient (which = (1,244,522 n2)/ d1.33 where d =diameter in SI units and n = Manning's n)

Values of a, L and H are measured.

To see if conduit slope ( x ) is less than the neutral slope, the latter is found by equation 2.28:

Sn = tan x = sin x = Hf / L = Kc (v2 / 2g) where

x = slope angle of the conduit
Hf = friction loss in conduit length L in m
L = conduit length
Kc = friction coefficient
v = velocity of flow in m s-1
g = gravitational constant in m s-2

Worked example

What is the capacity of a 600 mm diameter culvert, 15.0 m long with a square edged entrance? Survey shows the inlet elevation to be 456.35 m, the outlet elevation = 456.20 m, the head water elevation is 457.95 m and tail water elevation is 455.25 m ?

The first step is to assume that pipe flow prevails and use equation 2.29

Ke = 0.5 (square inlet)
Kc= 0.319 (with Manning's n estimated at 0.036)
Kb is only used for conduits with bends and therefore is not included
H=0.94 m
a =3.14 m²

Therefore:

m² s-1

To determine if the pipe flow assumption is correct, the neutral slope is calculated from equation 2.28 and substituting discharge / area for velocity in SI units,

Sn = 0.319 × (0.283)2/ 2 × 9.8 × (0.28)2 = 0.0166,or 1.7%

As the actual slope of the culvert S = 456.35 - 456.20) / 15.0 = 0.01, or 1.0 %, then as culvert slope < than neutral slope, pipe flow conditions prevail. To check whether orifice conditions (as opposed to pipe flow) prevail, the orifice equation 2.30 below, is used. The values of h and C in this equation are 0.85 and 0.6 respectively. The discharge is:

m³ s-1

This discharge is greater than the full pipe capacity and therefore pipe flow must prevail. Using the procedures above and in cases where the culvert slope is found to be greater than the neutral slope, pipe flow cannot prevail, the orifice equation should be used. Outlet not submerged.

2. Orifice flow is the second type of flow and is found when the conduit slope is greater than the neutral slope, the inlet is submerged but the outlet is not (i.e. inlet controls exist). The equation for orifice flow is:

where (2.30)

a = cross sectional area
h = head to the centre of the orifice
C = 0.6 for sharp-edged orifices
g = the gravitational constant in m s-2

Alternatively, Figure 2.38 can be used to determine flow from an orifice with inlet (entrance) controls where H= head and D= diameter of a circular sectioned culvert in m.


Figure 2.38: Stage Discharge Relation for Control by Square Inlet to Circular Pipe

3. Channel Flow (Manning's Formula).
In some cases the headwater elevation is lower than the top of the inlet and control is by the channel or conduit itself. This occurs where the conduit slope is too shallow to allow the maximum possible flow that could be provided. Manning's formula is used to calculate flow. In other cases, despite the low head, the inlet section still provides the control and restricts the flow. This occurs when the slope of the conduit is greater than that required to move the possible flow through the inlet. Figure 2.38 can be used in these cases. To determine whether the results as given by either channel flow or inlet restriction calculations are correct, follow the steps below.

Assume restriction at the entrance does not exist.

Calculate channel flow using Manning's formula:

v = R0.667 S0.5/ n where (2.31)

v = average velocity of flow in m s-1
n = roughness coefficient of the channel
R = the cross-sectional area divided by the wetted perimeter (a/p) in m
S = hydraulic gradient (channel slope)

The use of this formula depends on making the dimensions of the channel such that v = Q/a and where Q the flow rate of the channel.

Then use Figure 2.38 to check the flow through the inlet. If the Manning's solution gives a flow greater than that from Figure 2.38 then obviously the latter is correct since this represents the upslope control of all flow through the conduit. Where the situation is reversed (channel flow < inlet flow), channel flow will prevail.

Manning's Formula Worked Example

Determine the capacity of a pipe of 1.20 m diameter, 20 m long with a square-edged entrance. Elevation of the inlet is 224.0 m, outlet is 223.95 m . Head water elevation is 225.0 m and tail water elevation is 220.0 m. In this case, the conduit has a very shallow slope and channel conditions may prevail. In the first instance assume a flow depth in the conduit of 0.6 m. Then

a = 0.57 m²
n = 0.015
R = 0.185 m
S = 0.0025

Substituting in Manning's formula (2.31), v= 1.08m s-1 and Q=0.62m³ s-1

If it is assumed that the approach velocity is negligible, then the loss of static head due to acceleration is = v2/2g, = 1.082 / 19.6 = 0.06 m. The depth of water at the entrance (headwater elevation minus inlet elevation) is 1.00 m and a loss of 0.06 m would give 0.94 m, which does not correspond with the assumed depth of 0.6 m. The process of iteration can be continued. If the flow depth is now assumed to be 0.90 m, then:

a = 0.94 m²
n = 0.015
R = 0.269
S = 0.0025

Substituting into Manning's formula: v = 1.39 m s-1 and the loss of head, v2/ 2g, now = 0.10 m, which when subtracted from 1.00 m = 0.90 m, the assumed depth. Thus flow is limited by the conduit and the discharge of the flow is: Q =1.39 × 0.94 = 1.31 m 3 s-1

Very many different designs of culverts are constructed and it is recommended that specialist manuals be consulted if work in the area of culvert structures and their hydraulic properties in to be studied in great detail.