#net head

Introduction

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.     

Dumpy levels and theodolite

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.  

Sighting meters

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 %.     

Water-filled tube and pressure gauge

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.     

Water filled tube and rod

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.     

Spirit level and plank

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%.     

Maps

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.     

Altimeters