One method to accurately measure flow Q for small and medium-size streams is through the use of a slotted Weir.
Methods for Measuring Your Stream Flow…
Stream Flow x Head Pressure = Power
Stream levels will change through the seasons, so it is important to measure FLOW at various times of the year. We will need these varied flow measures to create an FDC or flow Duration curve, more on the FDC in a later post. If this seasonal variable flow measure is not possible, attempt to determine various annual flows by discussing the stream with a neighbor, or finding US geological survey flow data for your stream or a nearby larger stream. Also keep in mind that fish, birds, plants and other living things rely on your stream for survival. Especially during low water seasons, avoid using all the water for your hydro system. FLOW is typically expressed as volume per second or minute. This is also called a “FLOW rate” since it is a dynamic volume per time interval. Common examples of volume units are gallons or liters per second (or minute), and cubic feet or cubic meters per second (or minute):
A rectangular slotted Weir consists of a temporary dam structure with a rectangular slot are opening gate.
This slotted Weir gate has the following characteristics;
- All stream flow to be measured, Q. is constrained to go through the slotted gate.
- The bottom of the rectangular slotted Weir gate is leveled horizontally.
- A reference stake or pole is driven into the stream bed below the water line. So that it is exactly level with bottom of the Weir gate.
- The stake must be placed upstream at least four times the distance of the maximum Weir gate water depth.
- Water must be allowed to exit the Weir gate freely, such that there is an air gap beneath it as it flows over the Weir. A “sharp” 90 degree edge lip helps here.
- Water upstream of the Weir must move freely and not have major disturbances.
Water will contract or shrink in width x depth, as it increases speed, when it approaches and flows through the opening.
Given both the width and depth of the water flowing over the Weir; it is a simple procedure to look up the value for the water flow using a Weir table.
The following table is based on a reference Weir gate 1 inch wide.
An example of use is as follows:
Assume your Weir gate is 1 foot wide or 12 inches, you measure the water passing over it at 6 1/4 inches.
Using the table, you look up 6+1/4 and read 6.2 5 CFM per inch of width.
Multiply 6.25 CFM/in x 12 in = 75 CFM. That’s a pretty decent flow, if you have enough head you may be in business.
FYI – Metric Formula for a rectangular notched Weir is: Q = 2/3 x Cd x , 2g^1/2 x (L – 0.2h) x h^3/2, Where Cd is the coefficient of discharge.
Take Cd = 0.6 (normal case) then Q = 1.8 x (L – 0.2h) x h^3/2 in liters/sec