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|You are here:||Comments and remarks to Wim Jonker Klunne|
Several methods exist for measurement of the available head. Some measurement methods are more suitable on low-head sites, but are too tedious and inaccurate on high-heads. If possible, it is wise to take several separate measurements of the head at each site. Always plan for enough time to allow on-site comparison of survey results. It is best not to leave the site before analyzing the results, as any possible mistakes will be easier to check on site.
A further very important
factor to be aware of is that the gross head is not strictly a constant
but varies with the river flow. As the river fills up, the tailwater level
often rises faster then the headwater level, thus reducing the total head
available. Although this head variation is much less than the variation
in flow, it can significantly affect the power available, specially in
low-head schemes where every half metre is essential. To assess the available
gross head accurately head water and tailwater levels need to be measured
for the full range of river flows.
The use of a dumpy level (or builder's level) is the conventional method for measuring head and should be used wherever time and funds allow. Such equipment should be used by experienced operators who are capable of checking the calibration of the device.
Dumpy levels are used with staffs to measure head in a series of stages. A dumpy level is a device which allows the operator to take sight on a staff held by a colleague, knowing that the line of sight is exactly horizontal. Stages are usually limited by the length of the staff to a height change of no more than 3 m. A clear unobstructed view is needed, so wooded sites can be frustrated with this method.
Dumpy levels only allow a horizontal sight
but theodolite can also measure vertical and horizontal angles, giving
greater versatility and allowing faster work.
Hand-held sighting meters measures angle
of inclination of a slope (they are often called inclinometers or Abney
levels). They can be accurate if used by an experienced person, but it
is easy to make mistakes and double checking is recommended. They are small
and compact, and sometimes include range finders which save the trouble
of measuring linear distance. The error will depend on the skill of the
user and will typically be between 2 and 10 %.
It is probably the best of the simple methods available, but it does have its pitfalls. The two sources or error which must be avoided are out of calibration gauges and air bubbles in the hose. To avoid the first error, you should recalibrates the gauge both before and after each major site survey. To avoid the second, you should use a clear plastic tube allowing you to see bubbles.
This method can be used on high-heads as
well as low ones, but the choice of pressure gauge depends on the head
to be measured.
This method is recommended for low-head
sites. It is cheap, reasonably accurate and not prone to errors. In this
case, if more bubbles are trapped in one rising section of the tubes than
in the other, then the difference in vertical height of the sets of bubbles
will cause an equal difference in the head being measured, though this
is usually insignificant. Two or three separate attempts must be made to
ensure that your final results are consistent and reliable. In addition
the results can be cross-checked against measurements made by another method,
for instance by water filled hose and pressure gauge.
This method is identical in principle to
the water filled tube and rod method. The difference is that a horizontal
sighting is established not by water levels but by a carpenter's spirit
level placed on a reliably straight plank of wood as dewscribed above.
On gentle slopes the method is very slow, but on steep slopes it is useful.
Mark one end of plank and turn it at each reading to cancel the errors.
The error is around 2%.
Large-scale maps are very useful for approximate
head values, but are not always available or totally reliable. For high-head
sites (>100 m) 1:50,000 maps become useful and are almost always available.
These can be useful for high-head pre-feasibility studies. Surveying altimeters in experienced hands will give errors of as little as 3% in 100 m. Atmospheric pressure variations need to be allowed for, however, and this method cannot be generally recommended except for approximate readings.