Measuring flow at a water site
“Flow” is the volume of water per unit of time that flows at your water site, usually measured in cubic meters per second or liters per second.
1 cubic meter per second = 1000 liters per second.
You need to measure the amount of flow wherever you plan on putting in a hydro system, because the flow rate also determines how much energy there is available in the water to extract.
Just like the amount of head, the flow rate is always changing. So you have to think of the flow rate not as a static number but in terms of the duration of time that each flow is present at your site during the course of a year. This is usually represented in terms of flow duration curve.
There are several methods of measuring water flow:
- Bucket Method
- Float Method
- Salt Pulp Method
- Weir Method
The Bucket Method can not realistically be used to measure flow for Archimedes Screw installations because if you can measure flow with a bucket (or a 200 gallon trough for that matter) you probably don’t have enough flow for an Archimedes Screw installation.
The Bucket Method however is a good way to measure flow for high head, low flow sites.
The Float Method is too limited for us to use for Archimedes Screw installations because of the requirement to determine the cross sectional area of the river and because of the requirement to use correction factors based on the type of river or stream.
The Salt Pulp Method still requires you to calculate the cross sectional area of the river or stream but instead of measuring water velocity using a float, you use salt dispensed in the water and a sensor to detect the salt. The flow rate is determined by measuring the speed and concentration of the cloud of salt as it passes downstream. The salt is a more accurate way of measuring water speed than the float which only rides on the top of the water.
Whenever possible we always use weir calculations to measure flow because usually we have some type of weir to work with and there are weir calculations available for various types of weirs and they are quite accurate if you pay attention to their specific requirements for use.
You’ll want to take many measurements over the course of a year and to aid in doing this you simply get “large rulers” for attachment to the weir somewhere appropriate to allow quick visual measurement of water height coming over the weir on many occasions.
As an alternative we use HOBO water level loggers above and below the weir to measure water height coming over the weir for flow calculation purposes and also to measure head as well over time.