Measuring River Flows

For example:

Say that water flow in the Waimakariri River measured by the recorder at the Old Highway Bridge site is 49 cubic metres per second (49 m3s-1).

What does this mean?

If you imagine a cube with one metre sides and then try to imagine 49 of these cubes, that's how much water is flowing past the recorder every second.

Now imagine flushing your toilet 5,444 times (or 8,166 economy flushes).  That's how much water we're talking about.  In one second!

How are river flows measured?

Environment Canterbury measures river flows by:

1. Measuring water levels;
2. Measuring river flows; and
3. Combining the information to produce a flow record.

Automatic water level recorders measure the level of water.  By recording this information every 15 minutes we can capture rapid changes in the river flow.

The most common recorders are made of hollow towers.  At the top of the tower is a machine which records the data, known as a logger.  At the bottom of the tower is an "intake pipe" which allows the tower to fill up until it reaches the same level as the water in the river.

To measure river flow, we first measure the river's width, depth and velocity.  This is called "gauging," and there are different ways to do this, depending on the river.

When we do a gauging we measure the velocity of the water from at least 20 points across the river cross-section.  These points are called "verticals."  Current meters are used to measure velocity and they have buckets or propellers that rotate at a speed in proportion to the water velocity.

At each of the verticals we also measure the depth of the water and the distance from the river bank, so we know the area between each vertical.  From these measurements we can calculate the "discharge flow" between each vertical and add these up to calculate the discharge of the entire cross-section.

At water level recorder sites we carry out gaugings at least once a month and, at some sites, every fortnight.