Flumes – installing for decades of flow measurement, Part 1

What is a flume?  Most people think of long flumes that carry water across a canyon, or along the edge of a mountain, to get water through steep country.  These flumes are expensive and time-consuming to build so they have to make economic sense.  In early
California, flumes were used to get water from a stream to gold-bearing gravels where there wasn’t water.  Gold was certainly worth the expense!  It takes water to wash gravel over a washboard so gold can settle out in the ribs or slats.  Flumes were then used to transport cut logs from the mountains down to mills in the valleys.  Lumber also brought in enough revenue to make flumes worth it.

The kind of flume for measuring flows is a concrete, metal, fiberglass, or wood structure built to exact dimensions.  The newly-built flume shown below is formed concrete.  It took 4 days for a crew of 5 people to make this.  This flume is 3.0′ wide, and will be used to measure diversions of up to about 16 cfs.  This device could last for 40 years before it becomes too worn to be accurate, or develops cracks that let parts of it settle.

Flumes are much more expensive than a weir box with boards.  It costs 3 or 4 times as much to install.  On the plus side, there are no boards to change, it measures a wide range of flows with good accuracy (+/- 5% in the first 10-15 years of its life), and it will pass debris and gravel through without clogging.

The photo below is of a flume that has been installed for 30 years.  It shows what can become a common problem: the ditch below has not been kept as deep as it should be, so the flume is “flooded out”.  The flow computed by using the staff gage depth is about 40% more than actually goes through the flume…so the ranchers who use the water could be shorting themselves.

Rehabilitating a flume is not impossible, but it is not often done.  The whole floor could be raised by pouring a higher concrete floor, making sure it slopes exactly the way the old floor sloped.  Usually a new measurement device is installed nearby, and the old flume is not used anymore.

More on the details and how-to’s of flumes later.  For now, we sure appreciate the snow and rain!