Public Works Engineering—Flood Control
Realtime Streamflow and Precipitation
How is it Done?
There are nine streams in SL County that have measurement equipment that radios-in or uses "telemetry" to report the water flow.
There are far more streams that the County monitors on an on-going basis but these nine have the radio equipment necessary to report the water flow on a real time basis. These are the stream gages we use to create these real time streamflow pages.
Stream Flow Measurements
The automated streamflow gages do not actually report the water velocity or water flow rate. The measurement taken is of the stream depth or "stage". The term stage refers to the height of the surface of the stream with respect to an arbitrary zero depth or stream bed.
Routinely, the stream gage technicians measure the actual stream flow and create a correlation table between this actual stream flow and the "stage". This correlation is called the stage-discharge relationship.
The measurements of the actual streamflow are normally taken by wading the stream and measuring the water flow rate at a carefully determined set of locations and at various depths.
These measurements are used to create a rating curve that allows us to measure the stream depth or stage and calculate the flowrate in cfs or cubic feet per second of water flow rate. On the charts you can see the stage and the flow rate both listed.
Stream Gaging Equipment
The gaging equipment on the actual stream measures the stream height using a simple float. A vertical tube or building, called a stilling well, provides a calm water surface at the same height as the stream.
A float rests on this calm water surface and is connected by a cable to a device called a rotary encoder.
This encoder measures the gage height and saves the record to a strip chart recorder for paper hard copy and to a data logger for an electronic copy.
In addition to the rotary encoder's output is sent to the telemetry radio where it is transmitted to the County's base station at the Government Center. Data is only transmitted when there is a change in stage in order to save battery life and to simplify matters.
Web Page Creation
Once the data is available at the Government Center on the base station computer it is a relatively simple matter to query this data to show you the data in real time using standard web development tools and web application servers.
Keep in mind that the table will only show data when there is a change of .01 feet in stage. When the streamflow is constant no data points are reported and displayed. The same thing happens when the stream is frozen or dry.
Historical Max Daily Flow
This statistic on each stream gage page refers to the maximum recorded daily flow for that stream gage. This is the highest recorded streamflow in any one day at this site.
Estimated Flood Flow
This statistic on each stream gage page refers to the ESTIMATED flow rate at which the stream floods or overflows its' banks. This is an estimate only since conditions change over time.