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3.4 Setting up of measuring stations
The measuring stations will consist of a runoff control where the measuring and sampling devices previously described will be installed. The performance of the measuring station is primarily limited by the behaviour of this control, that is the more permanent device and the more difficult to be modified or substituted.
A runoff control is some man-made or natural constriction of the stream channel that provides a unique relationship between stream discharge and water level, flume or weir type, depending on the range of discharges to be measured, the amount of bedload transport, and other technical constraints. A sediment self-cleaning control is highly advisable.
The size of the control needs to be primarily adapted to measure events up to ten years of recurrence period, medium-sized events being more important than low flows for sediment transport. Controls adequate for catastrophic events are only acceptable if at least two events per year can be correctly measured.
Within the Project, a natural control is only reasonable for pre-existing stream gauging stations in the wider basins, and if calibration measurements are available (stage-discharge relationship). The acquisition of data for a range of discharges is usually a long-term task that could be longer than the duration of the Project itself, and is a difficult task for small basins because of their ephemeral response.
Whenever possible, the flow control structure should be one with a known calibration formula or table (calibration formula or table provided with the design). If field conditions make this impossible, a calibration exercise is to be made, through gauging water discharges with the current meter or chemical methods for a range of discharges as wide as possible.
The control has to be protected against malfunctioning caused by the transport of sediments, especially bedload. This protection can be afforded by a self-cleaning design or by a sediment trap located before the control, sufficiently sized to catch the bedload transported by a medium-sized event. This trap should be emptied as frequently as possible, especially after major runoff events. Self-cleaning designs are highly recommended because an adequate performance of the control during moderate or major events is more important than the higher precision during small events, for sediment evaluation purposes.
The water level is to be measured in a stilling well or pipe, provided with a drainage tap for sediment cleaning. In permanent streams, a pressure transducer can also be placed on the bottom of the runoff control, adequately protected against bedload impacts. The location of the water level sensing device must be selected according to the requirements of the runoff control design.
A graduated shaft should be installed close to the intake of the stilling well or the pressure sensor, in order to check the readings of the data logger with the actual water levels observed by the operator.
The sampling instruments have to be adequately located in order to keep the minimal modification of the hydraulics of the control. Locations with high turbulence of flow are recommended in order to obtain the best mixing of waters, provided they will not be destroyed during high events.
The data logger should be located in a shelter protected against weather and vandalism. Some data loggers are provided with weatherproof housings that behave well if vandalism is not a risk. If the station is provided with an automatic sampler, the best solution is to shelter both the data logger and the sampler in a steel or masonry housing, and to place the solar panel on its roof; this housing can also be very useful in remote locations to store other tools.
Summarizing, the recommended measuring stations should consist of:
• a runoff control:
• artificial (flume or weir), provided with calibration formula or table;
• medium -sized (in respect to the size of the catchment);
• self-cleaning (or provided with an adequate sediment trap);
• with a graduated shaft for visual readings of water level;• a stilling well or bottom housing for the water level sensor; and
• a steel or masonry housing to shelter the recording and sampling instruments.
This schedule has been prepared especially for the following instrumentation:
a) a rain recorder and limnigraph based on a data logger;
b) a stage sampler (siphon fed bottles);
c) an automatic sampler (ISCO 2700 or similar); and
d) a suspended sediment integrating sampler
3.5.1 Every week or more frequent/v in rainy weather: maintenance of samplers:
The equipment needed is:
• a field book, with a waterproof pen;
• a permanent marker (filter pen);
• a collection of bottles for stage sampler;
• a collection of bottles for transporting water samples from automatic sampler;
• a clock, a thermometer for water temperatures;
• a cleaning flask with distilled water;
• a shovel, a basket;
• a 5 kg hand scale; and
• keys.
The recommended procedure is as follows:
a) Take a new page of the field book and note:
• date and solar time; name of the station being visited;
• name of the operator, assistants and visitors;
• height of the water level in the control point (shaft); and
• temperature of the water.b) Open the stage sampler cabinet and search for filled bottles.
If these are present, take them orderly, plug them, and carefully write permanently on the bottle:
• Station name;
• S + number of the bottle within the stage (S1 for the lowest one); and
• date.Write on the field book the numbers of bottles recovered (write S0 if none)
Check siphons and pipes, rinse if necessary, but keep the empty bottles dry and clean. Check the protection against insects.
Replace the filled bottles with empty and clean ones.
c) Open the lid of the automatic sampler, and look at the screen to check if it is working and if it has taken some samples.
Remove the control unit and search for filled bottles (one unit less than the flashing number on the screen). If these are present, carefully shake the content of every bottle and transfer it to a transport bottle, plug it and write permanently:
• Station name; and
• A + number of the bottle in the sampler date.Rinse the bottles and replace them in the sampler.
Write on the field book the numbers of bottles recovered (write A0 if none).
Replace the control unit.
Reset the sampling programme. Replace the lid of the sampler.
Check cables and pipes.
Check the voltage of the solar panel.
d) Clean and weigh the bedload sediments from the trap or the control, note on the field book the weight and the kind of sediments (cobbles, gravel, sand or mud).
Clear the control and water level devices from any obstruction which could disturb water flowing along the control or the readings of the instruments.
Write in the field book any incidents.
e) Check the funnel of the rain gauge, clean it if necessary, but without water.
f) Close and secure all the doors and locks.
3.5.2 Every month: Data recovering
The equipment needed is: a field book, with a waterproof pen;
• a permanent marker (filter pen);
• a clock;
• keys;
• a portable computer (if you do not have one, you have to transport the data logger to the desktop computer, execute the data retrieval instructions, and replace the data logger in the operating station as quickly as you can, to allow minimum data loss); and
• diskettes for security copies.
The recommended procedure is as follows:
a) Take a new page of the field book and note:
• name of the station being visited;
• date and solar time;
• name of the operator, assistants and visitors;
• height of the water level in the control point (shaft); and
• temperature of the water.b) Open the weatherproof housing of the data logger (remove and transport it if necessary):
• turn on the computer;
• check the validity of the time and date of the internal clock (solar time!);
• connect the data logger to the computer via the appropriate cable;
• start the transference program;
• select the appropriate name of the scheme (station);
• check the file with the data recovered;
• reset the data logger;
• check the correct functioning of the connected sensors;
• quit the transference program;
• make a backup copy of the recovered file;
• disconnect the data logger from the computer and turn off the latter;
• write on the field book all the incidents.
3.5.3 During discharge events: Direct sampling
This task is not a routine procedure; it needs to be much more adapted to field and event conditions and, therefore, requires a more experienced personnel. As it is to be performed during discharge events, it is especially important to prepare all the necessary steps of the operation before starting the task, and not to take personal risks.
If stage and automatic samplers are in operation at the station, this task is first necessary to know the relationship between the sediment concentrations of samples from these devices and the mean actual discharge of the whole flow. If no automatic sampler is active, this is the main procedure to obtain suspended sediment data.
In both cases, the first strategy is to obtain measurements of suspended sediment concentrations characteristic for a wide range of flow conditions (discharge, rising or decreasing flow), then the sampling interval is to depend more on discharge variations rather than on time. A further strategy is to obtain a large number of sediment concentrations throughout the events.
The equipment needed is:
• a field book, with a waterproof pen;
• a permanent marker (filter pen);
• a collection of bottles to transport samples;
• a clock, a thermometer for water temperatures;
• a 10-12 litre bucket;
• a depth integrating suspended sediment sampler with accessories (rod or cable, reel and crane); and
• a cleaning flask with distilled water.
The recommended procedure is:
Small (less than 2 m wide) and turbulent reaches, or transparent waters:
Under these conditions, use the bucket to collect one or two samples that you consider representative of the whole suspended sediment discharge, shake well and take a sample in a bottle for laboratory analysis, seal it, and carefully write permanently on the bottle:
• B (for bucket sample);
• station name;
• date; time; and
• height of the water at the control point.
Take the field book and note in it for every sample:
• bucket sample;
• station name;
• date;
• time;
• height of the water at the control point;
• height of the water at the stage sampler (if different);
• rising or falling water discharge;
• water temperature; and
• your name.
Wider (more than 2 m wide) or low turbulence reaches:
Under these conditions suspended sediment concentrations can significantly vary in depth and position within the stream section. It is necessary to perform a rather intricate operation to obtain a representative sample.
There are several sampling strategies which can be used depending on the needs and the experience of the team; the easiest one, which produces results adequate for our purposes, is the so called "equal transit rate method" (see relevant Turkish case study in (PAP/RAC-UNEP, 1997)). This method is based on the fact that the intake velocity of the sampler is proportional to the flow velocity, then, if the sampler is moved along several verticals equally spaced in the cross section, at the same transit rate, a single sample can be obtained, representative of the mean suspended sediment concentration (discharge weighted) of the flow.
a) Select a section of the stream near the gauging control where the cross section is somewhat constricted and fairly uniform in depth. Determine a number of equally-spaced verticals along a cross-section of the stream, depending on its width (6 to 12 typically).
b) Place the suspended sediment sampler in the central vertical, and move it at continuous rate from the surface of the water to the bottom of the channel and reverse. Open the sampler and check the sample taken in the bottle: if it is filled one half to three-thirds of its capacity, the rate of movement is adequate; take a faster rate for a more filled bottle, or a slower one for a less filled bottle.
c) Make a transit on every one of the verticals at the selected movement rate, and pour every sample into the bucket (you can also change the sampling bottle).
d) Shake the content of the bucket well and fill a transport bottle with this averaged sample. Write permanently on the bottle:
• D (for depth-integrated sample);
• station name;
• date;
• time; and
• height of the water at the control point.Take the field book and note in it:
• Depth integrated sample;
• station name;
• date;
• time;
• height of the water at the control point;
• height of the water at the stage sampler (if different);
• rising or falling water discharge;
• wafer temperature;
• number of verticals; and
• your name.e) If you want to calibrate the automatic sampler, open its lid, note the number of the bottle to be filled in the field book, press the "HALT PROGRAM" key, then the "MANUAL SAMPLE" key, wait for the sampling operation to finish, and press the "RESUME PROGRAM" key. Replace the lid of the sampler.
It is necessary to perform at least the following determinations with the samples obtained during the former procedures (Hadley & Walling, 1984).
3.6.1 Total suspended sediments concentration
Weigh the sediment retained on a 0.45 µm filter. The filters have to be weighed before filtering, and, after filtering, dried at moderate temperature (40° C max) and weighed.
Transparent water requires the filtering of the whole sample, but only a small part of it (mix well!) is enough for more sediment-charged samples, to avoid saturation of the filter.
Express the result in milligrams per litre (net weight of the filtered sediment divided by the volume filtered). Label the used filters and store them in a dry cool place for eventual subsequent analysis.
3.6.2 Total dissolved sediments concentration
The electrical conductivity of the water is measured with a customary EC meter. At the same time, it is necessary to measure the temperature of the water and calculate the correction to be applied to the electrical conductivity in order to make it equivalent to the one prevailing at the standardized temperature of 25°C
To convert EC readings into dissolved sediment concentrations it is necessary first to construct the relationship, by obtaining the dissolved sediment concentrations by weighing the dry residue at moderate temperature (40° C max) of the water obtained by the previous filtering procedure.
Express the result in milligrams per litre (net dry residue divided by dried water volume).
3.6.3 Other determinations
If the laboratory incorporates the appropriate facilities, it is advisable also to perform analysis on:
• grainsize analysis of suspended sediment (laser scattering, pipette or densimeter analysis).
• mineralogical and chemical analysis of filtered sediments (by X-ray Diffraction and X-ray Fluorescence).
• pH, alkalinity and major dissolved ion concentrations.
4. Data processing and presentation
4.1 Benefits from the measurement programme
4.2 General aspects of relevant data management
4.3 Data processing, formats for data recording
4.4 Interpretation and presentation of results
4.1 Benefits from the measurement programme
It has been claimed that a lack of reliable information on erosion and sediment yield rates for large parts of the world exists, and that this lack is a major inconvenience to assessing the environmental consequences of erosion, as well as to designing adequate erosion control strategies (Lal, 1988).
This lack of reliable information is due to the fact that measuring erosion rates is an exercise subject to several and important sources of uncertainty that increase under semiarid conditions where runoff is ephemeral or highly varying. This uncertainty reoccurs throughout the measuring programme; it begins with sampling stream water and ends with the calculation of annual sediment yields.
The quality of the data obtained through this programme depends primarily on the adequacy and accomplishment of the sampling programme, but also on the data processing routines used. It has been shown that the use of different calculation procedures on the same data sets can give final results of sediment yield that differ by one order of magnitude (Walling, 1988). The purpose of this chapter is to provide a set of recommendations to process the field and laboratory information for assessing sediment yield volumes for individual events and for the whole measuring period; some instructions are also given for preparing the technical and management-oriented reports.
It is important to highlight that the present measuring programme seeks to obtain reliable information on erosion rates and sediment yields from small catchments, sufficient for understanding the erosion processes active in these areas and to orientate land management strategies and erosion control practices. Its development will provide information on the characteristics of events as well as an assessment on the sediment yield rates, together with some knowledge on the degree of uncertainty of this estimate. Very probably it will not provide "official" erosion rates that would appear authoritative, but it should provide reliable and, therefore, useful information.