Wednesday, April 22, 2009

Hydrometry Introduction

I found this book at the NC State Library:

Hydrometry, 3rd edition: A comprehensive introduction to the measurement of flow in open channels
UNESCO-IHE Lecture Note Series

Wubbo Boiten, Wageningen University, Delft, The Netherlands

Here's the publisher's description.

Based on this book and other sources I may find along the way, I hope to post a few times about the basic measurements taken along streams and rivers.

Initially, water resource measurements can be broken into two categories; quantity and quality. On the quantity side, we want answers to questions such as: do we have enough to drink, is there a risk of flooding in a certain location, are impermeable surfaces such as parking lots creating too much increase in stream flows, etc. The quality side seems obvious because we all want clean drinking water, but there are also less obvious concerns such as sediment transport, nutrient levels, wildlife impacts, and so on.

First let's look at quantity measurement.
When news about a flood hits the airwaves, we're always given stats like flood stage and water height. So water height is necessarily of interest. How do we measure it?
In talking to Dr. Birgand and in reading this book, I find five basic measuring techiques for water height.
1.) Staff Gauge
2.) Float/Counterweight Systems
3.) Pressure Transducers
4.) Bubble Gauges
5.) Ultrasonic sensors

The image on the left is a USGS Stream Gauge Station near Pullen Park in Raleigh, NC. In the background, you can see a white "stick" in the water. That is a staff gauge, which looks a lot like a measuring stick.

In addition to height, we also like to know how much water is flowing in a stream or contained in a reservoir. In streams this is measured as volumetric discharge, in units such as cubic meters per second or gallons per minute. Here are some classic methods for calulating discharge:
1.) Velocity Area method
2.) Slope Area method
3.) Stage Discharge method
4.) Acoustic Methods
5.) Electromagnetic Methods

I've used the Velocity Area method for calculations in class. The remaining four methods I hope to learn more about.

There's plenty to read about quantity measurements, so I think I'll leave the discussion of quality for another time.

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