Flow measurement methods

Much of the knowledge of hydrology in the fields of morphology and behavior of rivers is based on the results of direct water measurements. Many of the formulas used in the design of irrigation systems are derived from experimental data and field measurements. Completing or correcting these formulas also depends on such data. Among the measurements that are done under the name of hydrometry and hydrometry and their results are used in hydrological analyzes are: 1) water level measurement 2) water depth measurement 3) water velocity measurement 4) Measuring water flow (flow rate).

Discharge, in water sources, is the volume of water displaced from a specific section (river, canal, dam valve, etc.) over a specified period of time, denoted by the letter Q. Flow rate is expressed in metric system in cubic meters per second, cubic meters per hour or liters per second. The unit of cubic meter per second is used for large discharges such as rivers and large canals and the unit of liter per second is used for water flows from wells and water that enters leaks. It is possible to measure the average flow in a flow in different ways, which we will deal with:
Mathematical method

If we divide a section according to the following figure into several parts b1, b2,… and bn and the water depth is at the location of each of the divisions d2, d1,… and dn by measuring the average velocity at each of the division points v2, v1, … And vn can be calculated as the flow rate that each of the divided parts of the cross section passes. The same method as the ones mentioned, which some hydrologists prefer, is calculated as follows.

The method of lines is the same speed

By drawing lines of equal velocity, the average velocity of water in a section can be obtained. If Ai is the area between both lines at the same velocity and Vi is the average velocity on this area (average velocity at the top and bottom line), the flow rate will be:

Flow measurement
Flow measurement using chemicals

All chemical methods are based on diluting a substance that enters water at a point in a river. The chemical can enter the river continuously at once. If a volume (V) of a chemical with a concentration of C1 is introduced into the river water at once, assuming that the previous concentration of this substance in the river was equal to Co, at a point downstream the concentration of water in terms of this substance is initially Co, but then From the time the chemical reaches that point, the concentration gradually increases, and after reaching the peak point, it decreases again and reaches Co again. Usually Co is always zero because water lacks such chemicals from the beginning. If the concentration of the chemical in water is C2 and at measured intervals dt, the amount of river discharge will be:

Flow measurement from slope and transmission coefficient

This method is used only as an estimate and in places where direct water measurement is not possible. For this purpose, 2 or 3 points in the river route are considered and at each point by estimating the cross section (A), hydraulic radius (R), Manning roughness coefficient (n), the value of transfer coefficient (K) is calculated from the following formula and If the difference between the heights of the points equal to delta d and the flow rate for the cases in which we have measured two or three points can be calculated as follows:

In this method, sufficient care must be taken in selecting the points and the distance between them. So that the distance between the points is at least 20 meters for slopes of about 1%, 200 meters for slopes of 0.1% and 2000 meters for slopes of 0.01%.
Flow measurement from Ashl reading

If at one point of the river route (as in the indicator image) where a scale or water level measuring ruler is installed, in addition to reading the flow rate, the flow rate can be determined several times by measuring the average velocity and cross section. And Dubai won. The read points create a curve that is the result of direct measurements of flow and scale. Now, if only the scale is read, the flow can be estimated from the mentioned curve. In most rivers, it is enough to install a calibrated ruler (Gauge) to measure the water level. But if due to the turbulence of the water level, the ruler number cannot be read accurately, direct the water through a pipe into a cylinder that is built next to the river, and the water level in this cylinder, which will be equal to the water level in the river, It is measured by a recording device called a limnograph and recorded on paper or recorded by loggers.

Flow measurement in small flows

In small discharges it is not possible to measure in the ways mentioned above. In these situations, simple water measuring instruments are usually used, which include: overflow, orifice and partial flume. In the outlet pipes of wells, it is possible to measure the discharge in different ways. We generally know that overflows are metal plates that are cut into different shapes and placed vertically in front of the water stream so that water can pass over the cut part.

A) Triangular overflow: These overflows are made with the angle of the vertex alpha and if the height of the water on the vertex of the overflow is H, the flow rate is calculated from formula (a). The C coefficient for sharp-edged overflows is about 0.6.

B) Simple rectangular overflow: This overflow is a simple rectangular plate with width L (equivalent to the width of the canal or stream) that is placed in front of the water flow and water passes through it. If the height of water on the crown of the overflow is H, the flow rate is obtained from Equation (b).

C) Rectangular overflow with lateral compression: In these overflows, a rectangular cut is created in a part of a rectangular plate and when