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

2.3 Water level recording instruments

Crest Gauges
Manual Gauges
Automatic Water Level Recorders
Electronic Logging Recorders
Chart Recorders
Bubble (Servo-manometer) Recorders

The measurement of flow volumes that use control sections, be they natural or artificial, necessitates the collection of water level data for the passing through the section, either automatically or by observer. The collection of these records is made by the use of one of three types of instrument. In order of providing least data first they are:

Crest gauges:

These gauges record only the highest level of


flow, but do so automatically.

Manual gauges:

These are simple gauges and provide records


whenever an observer is present to read them.

Water Level Recorders:

These are relatively sophisticated gauges that


provide a constant record of water levels.

The choice of gauge will depend upon the importance of the data: the first two types of instrument are cheap to manufacture from local materials, whereas the latter must be purchased at considerable expense. However, to help in correct choice, here is a list of the advantages and disadvantages of each, with a description of the circumstances for which they are suitable.

Crest gauges

Advantages:

- Cheap to manufacture out of local materials
- Easy to transport, place and maintain
- Very little instruction is needed for correct reading

Disadvantages:

- Provide very little data, only maximum peak flows
- Must be visited and read after a high flow

Suitable For:

- Situations where peak flow and maximum discharge only are required, for example estimating maximum flood levels, survey work, maximum flow probabilities. Some manner of converting flow stage to discharge must be available, if discharge values are needed.

Manual Gauges

Advantages:

- Cheap to manufacture from local materials
- Can be easy to install
- Need little maintenance
- Can provide good data from streams that flow regularly
- Essential backup and check on natural controls that have WLRs
- Data is a permanent, written record

Disadvantages:

- Do not give continuous records
- Need to be visited regularly or retain a gauge reader
- Data is as good as the reliability of the reader
- Can be washed away in flood
- Installation can be difficult
- Analysis demands manual input into computer storage

Suitable For:

- Commonly used on permanent streams
- Irrigation schemes
- Can be used on flumes etc. instead of WLRs where flow is regular and easily monitored
- Not suitable where a continuous record is needed
- Distant sites visited from base on a regular but not frequent basis can often be provided cheaply, but a local reader will be necessary.

Water Level Recorders

Advantages:

- Automatic, need infrequent visits

- Give complete runoff record: duration, peak flows, flow recession, volumes

- Data can be linked well to rainfall data (from intensity gauges)

- Data can often be downloaded directly into the computer for analysis with a great saving of time.

- Most suitable for remote sites, need no reader at site

Disadvantages:

- Expensive
- Need regular checks and maintenance
- May be difficult to repair.
- Solid state electronic instruments will have to be returned to manufacturers for repair.
- Need a higher level of training for correct usage.

Suitable For:

- Used where good quality data is essential
- Often used at remote sites where data collection would otherwise be impossible
- Especially useful at base stations where core research is being conducted

a. Crest Gauges

A typical crest gauge is illustrated below in Figure 2.39. Crest gauges provide useful information on peak flows when no observer is present. They can be fixed to bridges, stable stream banks or spillways. One of their most important advantages is the ease at which they can be constructed easily from cheap, locally-available materials.

Construction and Installation

Galvanised steel water pipe or plastic water pipe, with a 5 - 8 cm diameter is suitable. The former is less prone to damage from flood water, debris and rough handling, but plastic is easier to work with, lighter and cheaper.

Suitable caps for plastic pipe are sometimes more easily obtained and do not need to be screw-threaded to fit. Any suitable length from 1.0 - 1.5 m can be used conveniently. A series of 0.5 cm holes are drilled into the lower pipe cap and act as intakes to provide hydraulic continuity with the flow. A vent hole must be provided in the top cap or upper portion of the pipe. Inside the gauge a wooden measuring stick is placed, graduated with clear markings. One centimetre marks give adequate accuracy. Screwed to the measuring stick is a small, perforated container of plastic or non-corroding metal. For this purpose it is better if the stick has a square section. Within this container is placed powdered cork or fine polystyrene granules. When peak flow occurs, this material floats out of the perforated container and deposits itself on the measuring stick, from which the peak flow reading is taken.


Figure 2.39: Typical Crest Gauge

Note that the lower cap has a support for the measuring stick, which is securely screwed to the top cap. The gauge should be installed precisely in a vertical position, using rust-resistant, bolted brackets. The openings should face the direction of flow. The gauge should be levelled to a permanent bench-mark, so that in the event of removal, replacement can be effected from the same base level. Where other gauges are used (for example manually-read staff gauges), it should be levelled in sequence with these if possible, or at least to the same bench mark, so that a relative reference point is available. Care should be taken when replacing the measuring stick if the water level is higher than the bottom of the gauge, as a temporary displacement of water in the gauge could lead to a false reading.

As well as measuring peak flows, crest gauges can be used to measure volumes at unattended sites, or at times when the observation of maximum runon volumes is impractical. For example, some water harvesting systems direct runoff into small basins that provide supplementary water for fruit trees. Knowledge of the basin symmetry and water level can provide an estimate of total received runon, though infiltration losses need to be accounted for. Evaporation is unlikely to cause serious inaccuracy.

b. Manual Staff Gauges

Staff gauges are made of metal (often ceramic-covered) or plastic strips, about 12- 15 cm wide in 1 - 2 m long sections. They have numbered, graduated markings at 1 cm intervals. They are placed in a low-velocity location on structures (bridges, stilling wells, etc.) or on posts in the river bank. The water level is read from their graduated markings. They may be installed singly or in sequence, as illustrated below. In the case of ponds and reservoirs, the posts are set inclined for increased accuracy, as a small increase in water level can represent a large increase in volume. In this case the graduation and setting must be done very carefully.


Manual staff gauges installed in sequence

Staff gauges can be purchased and these are of high quality, but very expensive compared to those manufactured from local materials, especially when shipping costs are taken into account. On the whole, plastic gauges are best avoided as they eventually become brittle and breakage can result when they are placed in rivers that carry much debris. Resetting (which in flashy streams may be necessary each season) of these gauges can also lead to damage and they make popular targets for shooting practice.

Construction and Installation

The facility to manufacture locally is a great advantage. As well as providing large savings in cost, it is important to have replacement gauges available immediately. Perfectly serviceable gauge plates can be made in the following way, for less than one tenth of the commercial cost.

Cheap gauges can be made from flat wooden boards which have been treated to prevent rotting though these will last a few seasons. Aluminium sheet and galvanised steel sheet provide better alternative materials and can be cut into strips for use.

A suitable stencil with which to paint the graduations can be cut (0.5 m is a practical length) from acetate or thin, stiff card,. Thin metal makes a durable stencil, but tends to bend and work less well. The metal strip is painted black as a background. The stencil is placed on the sheet and the markings spray painted in white. Numerals 5 cm high are sprayed at 10 cm intervals in white on the black background Numerals 7.5 cm high are sprayed on at every metre interval. These can be sprayed on in the field according to particular need, another advantage over pre-marked, purchased plates. Two suitable designs are shown in Figure 2.40. The finished plates are then screwed (brass or stainless steel screws) to a treated back-board to maintain a suitable rigidity.

Staff gauges are emplaced at the gauging station during the dry season when permanent streams are at their lowest, or at any convenient time for ephemeral flows. Where possible, they should be fixed to bridges etc. to reduce the risk of loss in floods.


Figure 2.40: Example Graduated Markings for Gauge Plates

The type of fixing will depend on the structure available, but all plates should be vertical. Set the lowest one first and then in sequence. If they are to be placed on a stream bank, 5 cm galvanised water pipe sections make good posts. Alternatively rot-resistant local timbers can be used, but these are difficult to hammer into the stream bed or banks and may necessitate the use of a manual post hole digger. River Authorities and similar bodies have access to heavy installation equipment that is unlikely to be available to most projects. In all cases, the gauges must be levelled to ensure the sequence is accurately placed and the bottom of the gauge must be levelled to a permanent bench mark. If man-made structures are not available, nailed and painted marks on several large trees, well away from the river, will suffice. At many sites it will be too difficult to level into a national survey, but a site plan including all levelling details, should be made. Checks on the level of the gauges should be carried out at least once each year. Staff gauges can be used with artificial controls where flow is regular and reading can be arranged.

c. Automatic Water Level Recorders (WLRs)

There are many different manufacturers of WLRs. There are, however two main types:

Float and Counterweight Recorders
Pressure Sensing (Bubble gauge ) Recorders

Advantages and disadvantages:

In general, the former are the cheaper and more commonly encountered. They are the most suitable for agrohydrological applications because of their small size and ease of siting on artificial controls, especially H flumes which have integral stilling wells. They are easier to install. They do not need special housing, unlike bubble gauge recorders and are therefore more easily re-located. Either type will measure large differences in water level.

Float and Counterweight Recorders

There are two main types of these instruments, according to the manner in which data is recorded and stored: those with electronic data loggers and those which record with pens and paper charts set on a clockwork drum. The relative advantages and disadvantages of each type are listed below. The costs of both types are similar.

Electronic

Chart

Advantages


- Compact and robust

- Widely known

- Wide range of easily set recording times

- Sometimes possible to repair locally

- Good precision

- Do not need computer facilities

- Wide range of level differences


- Long periods between visits if necessary


- Download direct to computer


Disadvantages


- Batteries can fail

- Sensitive to rough handling

- Cannot be repaired locally

- Time/level adjustments limited by

- Need computer facilities

charts and clock which can be


difficult


- More limited recording time


- Manual data entry into computer

Operation

1. WLRs with Electronic Data Loggers

In keeping with the general trend towards solid state electronic instrumentation, this type of WLR is becoming increasingly common, but the float and counterweight, mechanical aspect of these recorders is still very much the same as orthodox chart recorders. The operation of these WLRs is discussed prior to installation procedures, as it is assumed that familiarity with the equipment will be desired before selection or installation in the field.

Changes in water level are detected by a float which sits on the water level in the stilling well, connected to a stainless steel tape or wire that ascends to and over a pulley connected to the recorder. At the other end of the tape or wire which descends from the pulley a counterweight is fitted to balance the mass of the float. As the float rises and/or falls, this movement is registered via the pulley axle. The rotational movement of the axle is converted into electrical signals by an electronic integrator. These signals are passed on to the main processing unit and then to the data logger, at pre-set time intervals specified by the operator. Figure 2.41 below shows a typical electronic WLR set to an H flume stilling well.


Figure 2.41: Electronic Water Level Recorder set in H flume Stilling Well

These recorders are compact and good designs are very robust. The main processing unit is powered by dry cell (preferably alkaline) batteries which should last for a year and which can easily be replaced. In many cases the loggers are powered by integral lithium batteries which last for up to ten years, but which can only be replaced by the manufacturer. These batteries enable the loggers to be removed from the recorder without the loss of data. The electronic components of the loggers are usually resin-sealed to prevent damage. Their operation is relatively simple. Once installed, facilities are available to label the recorder number, date and time. (Typically, these are recorded as a heading prior to the water level data and can be viewed when the data are down-loaded). This information is usually displayed on an LCD screen, which is located on the processing unit and is easily revised by using various switches.

The time interval that is desired for the data to be recorded is adjusted and displayed in a similar way and provides a very flexible facility. Time periods usually range from 1 minute to 24 hours, in 1 minute steps. Changes of date are normally recorded. Readings are precise to 1 mm, but the accuracy of this sensitivity depends on the correct installation and operation of the recorder and measuring section of the control. When a replacement logger is installed, the heading information is usually written on to it automatically. The recorders are capable of recording level differences (zero to maximum) of 100 m, but tapes and wires can be purchased or cut to any desired length. Ensure before purchase, that the correct type of power batteries are easily available.

Avoid the temptation to leave recorders untended for very long periods, just because the loggers allow this. It increases the probability of undetected faults, damage by flood, vandalism and theft. Data that are lost can never be replaced. In addition to the annoyance and loss of data, the misuse of such expensive equipment will greatly reduce its cost-benefit to the project. The more frequent the visits, even to automatically recording equipment, the better, though of course each project must decide upon the priority that this activity can take..

For small catchments and plots, which will provide short periods of runoff, it is important to make the time interval between the logging of water levels short, perhaps no longer than 5 to 10 minutes. A 32 kb logger should not need to be replaced more frequently than once each week or ten days with a 10 minute record interval. For much larger catchments with longer durations of flow, half or one hour periods may be adequate. For seasonal or perennial streams, records once, twice or four times each day may be suitable. Loggers in these circumstances can remain unchanged for many months. In all cases, the most suitable time interval is a balance between these factors:

- logger memory size;
- frequency of site visits;
- duration of runoff

Logger memories vary in size, but 100 kb+ or so is typical. An example data set is illustrated below, from a recorder on a 30 cm H flume: note that no-flow data are also recorded.

Level recorder Nr. 0045

Level in mm

Repeat Period 5 (min)

D T

"90/02/07

23:05 "

P O



P O



P 3



P 7



P 11



P 11



P 55



P 110



P 114



P 107



P 86

"90/02/08

00:00"

P 65



P 33



P 27



etc.



Spare batteries and the tools to replace them should always be carried on site visits, a note of the visit and logger change should be kept. It is difficult to check batteries with a voltmeter and experience is the best indication as to how long they will last. Recorders set on flumes etc. are unlikely to require replacement floats and counterweights if treated properly, though sometimes spares are useful.

Data are usually down-loaded into computer storage by a program provided by the manufacturer. When this is done, the data in the logger is marked for erasure by new data.

2. Chart Water Level Recorders

These recorders have a relatively complicated mechanical action, though this will vary to some extent according to manufacturer, whose instructions must be closely adhered to. The float and counterweight system is similar to that describe above. Typically, the action of the pulley, as the float rises, rotates a horizontal bar along which is a sunk spiral thread. Along this thread a pen and ink carriage is moved to the right by the rotation until it reaches the end of the bar, if the rotation of the bar continues the pen action reverses and it moves to the left along a counter-spiral. In this way the pen traces zig-zags along the chart. When the float falls, the action of the pen is reversed. This allows a wide range in levels to be recorded.

Particular care is needed on two points. First, the chart must be accurately placed on the drum according to its marked, correct position. Second, the pen must be accurately placed at the zero position after the chart is replaced. The adjustments for the speed of the drum can be altered to allow longer or shorter times between chart replacement. This is effected by changing part of the gearing mechanism (provided by the manufacture according to request) or engaging different cogs, often by a lever or sliding rod. Figure 2.42 below shows an example of this type of equipment.

It is important that the ink supply is adequate and that the pen functions properly, drying can be a problem in hot climates. The timing of the clock should be monitored and corrected if necessary. Before and after field installation, the pen should be checked that it turns at the correct place on the chart. Details of recorder number, date etc. can be written onto the chart. Analysis of the data is according to the level/time pen trace and can be undertaken (for flow volumes) by digitiser or by hand.

Charts should be clearly identified with station, recorder number date and time of removal, checks with manual gauges and checks on pen reversal. Change-overs from rising to falling flow, time corrections because of fast or slow clock running marked to the nearest minute and pen relocations should be recorded in pencil on the chart at the correct point. Make sure that the pen moves freely by rotating the pulley to raise the float tape. A free pen will make a perpendicular mark which should be noted as a check. The chart should be replaced on immediate arrival so that time can be spent to check that the equipment is working correctly. It is important to instil a regular routine for each inspection.

In the case of both electronic and chart recorders, the diameter of the float and the length of the counterweight should be appropriate to the size of the stilling well. No contact with the well sides should be allowed.

Installation of WLRs on Small Artificial Controls, Flumes and small V-notch weirs

The installation of water level recorders on to such equipment as H flumes and V-notch weirs was covered earlier. This procedure is straightforward and the same for both kinds of recorder. The main points of installation are the same for all control types: the tape and counterweight should move freely after the recorder has been set horizontally, as indicated by the spirit level provided on the recorder. The details of setting the recorder heading and time period or placing the chart and pen will depend on the manufacturer's specifications, but will be broadly similar to the above.

Installation on Natural Controls and Large Artificial Controls

The installation of the stilling wells for WLRs using large artificial and natural controls, whether electronic or chart, is a costly and difficult process. On the whole, agrohydrological and water harvesting projects will be concerned with small plot or catchment runoff, but the need to install WLRs to measure larger runoff amounts may be an important adjunct to these activities. The basic requirements of installation are described here.

The need in these circumstances, is to provide a large stilling well upon which the WLR can sit. The structure should be:

- Robust enough to withstand peak flows.

- Sited upstream of the control.

- It is of great advantage if the WLR can be secured to a solid structure such as a bridge, or attached to the control by supports.

- It should be placed in a relatively protected location.

- Access should be available at all stages of the river (for example by providing steps which ascend the structure or a walkway from the bank).

- Installation is best done in the driest season.

- Perforated steel water pipe or cemented pre-cast concrete sections can be used.

- To place the well in the lowest part of the channel, (perennial) streams must be diverted to allow access to bedrock. In sand rivers, air or water jetting can be used to sink the pipe, but the danger of it being washed away remains.

- If absolute minimum flows are not required, a diversion is not necessary.

- Heavy lifting gear will be necessary.

- Artificial controls may have to be dug clear of sediment during the dry season, especially in sand rivers.

- A series of staff gauges must be emplaced as a check on WLR operation.

Advice from local organisations familiar with the installation of such structures should be sought at the earliest planning stage. Construction may be beyond the time and resources of the project and less precise estimates of flows may release valuable resources for other work.

3. Bubble gauge (servo-manometer)

Generally, most advantages lie with the WLRs described above and so this description of bubble gauges, is brief, though this type of gauge is quite commonly used in the USA.

Where rivers are subject to violent peak flows with the likely loss of WLRs, these gauges have the distinct advantage of being sited away from flood water.

The manufacturer's manual should be consulted for detailed testing, installation and operation. Bubble gauges work on the principle of depth of water exerting an opposing pressure, registered by a mercury manometer, on that exerted by a regulated gas (nitrogen) supply from a cylinder. An orifice, with a vented cap to minimise sediment entry, is fixed to the river bed from which a plastic pipe leads to the equipment. The orifice should be located below minimum expected river stage. The changes of river level are recorded by a pen, on to a chart fixed to a clockwork drum.

Important points to note are:

- The equipment must be housed in a water and vandal-proof hut away from maximum floods.

- Sediment entry into the orifice vent must be prevented.

- The orifice installation can be jetted or driven into the channel bed, but changes in bed topography may arise.

- A spare nitrogen cylinder is necessary, though the rate of gas discharge may be regulated.

- One cylinder can last many months.

- Care should be taken to ensure the gas regulator operates correctly. This must be
tested before installation.

- The chart recorders are designed specifically for this kind of manometer instrumentation

- Especial care should be taken to avoid damage to the mercury manometer

Figure 2.43 shows how the components of a bubble gauge are assembled


Figure 2.43: Bubble Gauge with Servo-Manometer