Site Assessment: Measuring Flow & Head

Matt Deady, Professor of Physics, Director of the Physics Program, Bard College

The two main factors that determine the amount of power and energy you can attain from your hydropower site are the flow rate, or the amount and the head. Learn about the relationship between flow rate, area, and speed here: Khan Academy: Flow Rate

Measuring Flow

Flow rate, Q, is the volume of fluid which passes through a cross section per unit of time, measured in cubic meters per second (m³/s).  Because water is highly incompressible, for a given passageway of water, the volume of the water must always remain the same.

As Figure 1 shows for water moving through a pipe, the volume flow rate remains the same, but as the cross-sectional area of the pipe reduces, the speed of the water increases.  This relationship between flow rate, water speed, and cross-sectional area is characterized in the equation, Q = A⋅ V, where A is the cross-sectional area and V is the water velocity.  That is, in the example of Figure 1, Q = A1⋅ V1 = A2⋅ V2 .   This relationship also pertains as the width of a stream or weir changes.

In order to obtain the most accurate flow rate data for your site over longer periods of time, it is important to look at historical flow rate data.  We have provided a guide to using the USGS website on flow data in Using the National Water Information System.  If recent data for your site is unavailable it might be helpful to look at data from nearby sites, usually within a 50 mile radius.  This distance is ideal because only sites with similar weather and geological patterns will allow for accurate comparisons between sites.

Flow rates right at the site can be measured on site using a flowmeter, or the other techniques described here, but just a few measurements will not give a complete picture of the flow you can expect throughout the year or over many years, which you will need to predict the reliability of the site as a source of hydropower.  Throughout the year, bodies of flowing water will experience natural variability as the seasons change.  These bodies may also experience droughts or flooding depending on the season and knowing the timing of these events are very important when it comes to the feasibility of your site.  A common method of visualizing the reliability of your site is by producing a flow duration curve.  You do so by taking flow rate data from your site and organizing this data on a graph in order to see how often your site will produce a certain amount of flow.

Measuring Head

Head is the height difference between the surface of the water at the location of the intake and the turbine wheel.  There are several methods which can be used in order to determine whether a site is feasible for hydropower.  The first method in estimating the head of a site is utilizing geographic information system (GIS) data, available through the United States Geological Survey (USGS).  The most consistent data to support this kind of analysis over statewide areas is an elevation model with 10 m resolution.  This data is too inaccurate to be useful for small-scale hydro sites, as half a meter (1.5 feet) of additional head can already decide whether a project is economically feasible or not.  Other sources have some data of higher resolution but are inconsistent across the state.  There are locations within the U.S. where a higher resolution is available, and quality and detail of these maps may also improve for other locations in the near future.

Remote sensing is another method than can be used to determine whether you have a feasible hydropower site.  This method may consist of multiple approaches including imagery captured from aerial robotics (Small Unmanned Arial Systems or sUAS), ground based close range photogrammetry (CRP), and ground based laser scanning or a combination of some or all of these.  All three approaches need specific equipment and a skilled professional to be successful.  Shrubs, trees and the site specific geologic topography can lead to complications when considering remote sensing technology. The effort necessary to perfectly analyze the present head differences with remote sensing technology may easily exceed the appropriate effort to assess the potential head at a small-scale hydro site.

Direct measurement relies on traditional survey measurement techniques to directly measure elevation differences between two points. This approach can be used without any special equipment, even though laser distance meters with a built in level can improve the efficiency and accuracy of this approach.  Direct measurement requires a two-person team.  One method is to work downhill by using a sight level while your assistant uses a tall pole with graduated measurements along its side.  At the surface of the intake shine the sight level horizontally where the height of the level laser above the ground can be measured by your assistant with the tall pole.  Once this height difference has been measured, relocate the sight level to the base of the pole and repeat the first step.  Continue to repeat these steps until you have reached the proposed turbine location.  Once you reach this location, add all of your measured heights to calculate the total head of your site.  To measure the head of a site working uphill, shine the sight level horizontally at a fixed distance above the proposed turbine location.  Have your assistant stand where the light hits the ground uphill in front of you.  From this location, repeat the first step continually while elevating towards the height of the intake.  Once you have reached the intake, add all of the measurements to calculate the total head of your site.

What is presented here is rewording of the method described at Micro-hydro-power.com and similar locations. Other methods are also described at Energy.gov.