Basic Guide to Develop Flow Rating for Practical River Measurement.

RIVER HYDROMETRY:

River Hydrometry is scheme of monitoring and record river flows. On a worldwide scale, almost 36,000 km3 of water moves through streams every year. Rivers & Streams were one of the key elements that help and groomed human settlement and civilizations in past. Their importance remains unchanged throughout these years, only difference is currently, rivers are supporting more populations. Managing water through optimum engineering solution are only strategy we have for fulfilling demands. River Hydrometry is primary activity that helps define these solutions and eventually executing it. Its applications include predicting and monitoring flows for flood control, water supply, agriculture, climate analysis and energy production.

How to Get Daily Flows?

1. Pre-requisite:

Knowing how to measure these flows is a prerequisite for water related scheme. Gauges are placed for depicting water level of rivers, they play a critical part in river hydrometry, Direct flow measurement is a complex and laborious operation that can be performed occasionally. It is the water level that is measured continuously (daily/hourly), However Weekly, direct measurement via current meter or any other flow measure device is considered as good practice. Below figures are show the basic data and setup of for hydrometry.

Daily Water Level Record from river site

Current Flow measurements with Record.

2. Developing Rating Curve:

Compiled water level from gouge and stream flow data from direct measurement can be graphical plotted against each. Points can be connected by drawing feasible trend line also called regression analysis, It is also instrumental to evaluate how well trend line represents the data. Normally, it can be achieved by deriving the correlation coefficient (R2). Range of correlation coefficient (R2) varies from (0-1), but value greater than 0.75 is considered acceptable depends upon conditions. Overall, this graphical representation is called "Rating Curve". 

Demonstration in MS Excel:

Step 1: Select data and go to insert > Plots > choose Scatter plot, as shown in Figure below.

 

Step 2: Right click on points and Choose "Add Trend Line".

Step 3: Right click on trend line and Select "Format Trend line".

From Format box displayed at right side you can select the type of line or curve. and also enable display of line/ curve equation (display option can be access by scrolling down to the end of box and tick mark the "Display Equation on Chart" and "Display R-squared value on Chart". In our case power equations and R-squared values are displayed in above figure on chart.

3. Daily flow from Rating:

For deriving daily flow, one must derive regression equation of trend line, parameters of equation will show flow and water level in their respected xy-plane. Daily measurements can be obtained by incorporating, daily water level into the equation.

Demonstration in MS Excel:

4. Calibration of Rating Curve:

Continuation of direct flow measurement can reveal finer characteristic of stream such as maximum & minimum flows and change in stream flow. Adding more record strengthen reliability of data which helps in calibration of daily flows.

By:

Zain Syed

Water Resources Engineer | Freelancer | Consultant

Email: zain.xane@gmail.com

LinkedIn Profile: linkedin.com/in/zain-syed-b663b2122