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Napa County Resource Conservation District |
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Napa County Resource Conservation District |
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Wire weight stream gage measurements will be looked at in conjunction with rainfall data and velocity data. This information will be entered into the MIKE 11 Computer model at the Napa County Resource Conservation District (NCRCD) office. The model can be used to ask "what if" types of questions to help us better understand the dynamics of the watershed and subwatersheds during storm events and how the interaction of the flow from tributaries impacts flooding. The gages give us the opportunity to collect data specific to a particular watershed or creek. Consistent tracking of stream flow by citizens in their own subwatershed can significantly contribute to the knowledge and understanding of the relationships between local rainfall, stream ecology, and watershed health.
Through the Stream Gage Monitoring Program, volunteers throughout the Napa River watershed use wire weight stream gages to measure the height of several tributaries to the Napa River. These measurements are taken during storm events, and data is entered and stored at the NCRCD.
Flow from tributaries into the Napa River varies dramatically depending on rainfall in a given year, topography and micro-climates of the subwatershed. Rain and runoff captured in a stream can significantly impact the amount of water delivered to downstream tributaries and the Napa River. The interactions of rainfall, runoff and stream flow in tributaries influence the occurrence of downstream flooding.
There are two types of wire weight stream gages, permanent and hand-held. The permanent wire weight stream gage is attached to the streambank and the hand-held stream gage is used from a bridge. Each stream gage is calibrated for a specific stream site. The volunteer monitor reads the gage during storm events. During storm peaks, the gage is read more often. The volunteer lowers the weight to the surface of the stream and measures the water level of the stream. The measurement is recorded on a data sheet. After taking the measurement, the weight is pulled up and secured to the gage until the next measurement is taken.
A rainfall gauge is attached to a post in an area free from obstructions and easily accessible. The monitor records the amount of precipitation (in inches) on the data sheet, at least once a day when it rains. (Note the different spellings for the word "gage/gauge." "Gage" is used when associated with hydrology, and "gauge" when used with rainfall.)
Velocity measurements are taken using orange peels and a stopwatch. The measurement is the time it takes for an orange peel to travel thirty feet while floating downstream.
The stream gage, velocity and rainfall Monitoring Program always needs additional volunteers who are willing to spend time collecting data. Contact the RCD to sign up for training. For more information connect to the links below.
Site Selection
Site selection is coordinated between a member of the Napa County Resource Conservation District (NCRCD) staff and the volunteer monitor. The monitor may select a tributary or a reach of the Napa River in which he or she has interest, or may take on a site that is recommended by the NCRCD. The stream gage will be positioned in an area on or over the stream bank that is convenient to reach and that will allow a clear view of the wire weight at different levels of stream flow.
Stream Flow Monitoring:
Wire Weight Stream Gage
Reading a wire weight stream gage requires little equipment and a small amount of time. The greatest need is dedication to the activity during the rainy season and especially during storm events.
A stream gage will be provided and/or installed by the Napa County Resource Conservation District (NCRCD). The level of the stream flow, or water in the stream, is referred to as stream stage. Stage is what will be measured with the stream gage. The cross section of the stream channel where the gage is placed will be surveyed to determine the channel area.
Measuring stream stage goes hand-in-hand with measuring stream velocity and rainfall. In addition to the wire weight gage, you will be provided with a rainfall gauge, and a stopwatch to perform velocity measurements. Data sheets for recording rainfall, stage and velocity will also be provided by the NCRCD.
Training and a detailed protocol will be provided to monitors by the NCRCD.
Stream Flow Monitoring:
Velocity
After recording a stage reading, a velocity measurement is taken. The two combined give a relationship between the stage and the velocity of a given stream. This relationship is referred to as "flow."
We recommend using orange peels for taking the velocity measurements. They work well because they are easily available, highly visible and float. A thirty foot length of stream is measured and marked by stakes near the stream gage site. An orange peel is tossed upstream of the upstream marker and the stopwatch is used to record the seconds that pass while the peel floats downstream to the downstream marker.
Repeat velocity measures at least three times. Ideally, one peel should represent velocity near the right bank, one in the middle of the channel, and the third near the left bank. Averaged together they represent the average speed of the surface of the stream.
Stream Flow Monitoring:
Rainfall
Twenty-four hour readings
This reading gives the accumulated rainfall for the past 24-hour period.
- At the same time each day, preferably early morning, read the amount of captured precipitation. The water in the gauge is likely to appear rounded at the surface when observed at eye level. This is caused by water surface tension and is called a "miniscus." The gauge is read at the center point of the miniscus.
- Record the reading in inches and tenths of an inch. Remove the rain gauge and shake out all collected water before replacing.
- Use the "comments" column to record observations (such as "steady rain all night," etc.)
Mid-day readings
It can be informative to know when the bulk of rain falls during storms. "Mid-day" readings can give a more precise picture of the pattern of rainfall. An inch of rain falling in an hour, for example, may affect the environment quite differently than an inch of rain falling throughout the day.
- At periodic intervals thorough out the day/storm make a note of the accumulated water in the gauge, but do not shake out the gage. As you continue to take readings throughout the day, each reading will be higher than the previous one. You will not empty the gauge until the following morning when you take the 24-hour reading.
- Make notes about when the rain stopped, started, and any other comments in the space provided.
Glossary of Terms
cfs: Cubic feet per second.
Flow: A measure of the volume of water which passes a point in one second; expressed in units of cubic feet per second (cfs). Flow is the product of velocity and channel-cross sectional area; expressed mathematically in the formula Q = VA; where, Q = flow, V = velocity, and A = channel cross-section area.
Thalweg: The deepest point in a channel cross-section.
Velocity: Surface velocity of the water as it flows between two points. Velocity is an expression of length per unit of time (feet per second). To calculate velocity divide distance by time.
Watershed (or Drainage Basin): A geographic area from which water, sediment, and dissolved materials are drained by a river and its tributaries to a common outlet. This outlet can be a larger river, a lake, an estuary or an ocean. Also called a drainage basin. A watershed is separated from adjacent watersheds by a ridge or drainage divide. Within watersheds there are subwatersheds. Watersheds and subwatersheds usually take their name from the river or creek which drains them, such as the Napa River watershed and Dry Creek subwatershed.
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