.png){kind=small}
Hydro Resource Evaluation Tool
physical characteristics of a hydro site
Tier 1. Rapid measurement of a low flow
Introduction
If you would like an idea of the magnitude of a low flow within your catchment, and hence an idea of the residual flow you can measure the flow in a river using a variety of methods. Please note that these methods will only give you a very approximate low flow. For instantaneous measurements of flow, you should use the velocity-area method where you estimate the cross-sectional area of the river and the velocity through that cross-section. The flow/discharge is the velocity multiplied by the area. To measure a low flow you should wait for a period of at least a week without significant rainfall to obtain an idea of the low flows.
If there is a structure such as a weir in place then you could estimate your flow using the methodology described in section 3.2 of the British Hydropower Association ‘Guide to UK mini-hydro developments’ (2006).
Estimation of cross-sectional area
The simplest method to estimate the cross-sectional area (m2) of your river is to measure the width (m) and multiply by the average depth (m). Section 3.3 of the ESHA (2004) ‘Guide on How to Develop a Small Hydropower Plant’ describes this in further detail.
Estimation of velocity
The velocity of the river can be estimated using a variety of different methods. Instantaneous measurements can be made using the following methods:
- By float
- By current meter (mechanical, electromagnetic or acoustic doppler)
- By dilution gauging
The simplest method to measure the flow is to use the float methodology. The current meter or dilution gauging methodologies should only be used if you fully understand the methodology; you will normally need to employ a hydrometrist experienced in using these techniques.
The float methodology is a very approximate estimate of velocity. For this rough preliminary survey of flow, the surface velocity can be measured with a float, such as a block of wood or an orange. You will need two people and:
- a measuring tape to measure the distance downstream and width;
- a measuring rod to measure water depth;
- a stop clock; and
- a number of floats of the same size and shape.
Drop the float into the centre of the river and over a known distance, measure the time it takes to float from the start to the finish. Calculate the velocity in meters per second. Repeat this process several times over the same distance and calculate your average velocity.
These measurements only give the surface velocity and a correction factor must be applied to give the average velocity over the depth. British Standard BS 3680 recommends a factor of 0.7 for rivers with 1m depth and 0.8 for rivers with over 6m depth.
Estimation of flow
Once you have your average velocity over the depth (m/s) and cross-sectional area (m2), you can multiply the two together to provide your flow (m3/s). Please note that there is quite a high hydrometric uncertainty associated with this method.
