VOLUNTEERS - FIELD NOTES

Flow Method 2. Velocity Meter (or Velocity Board) & Profile

LAST UPDATE 2024/02/12

Flow 2. Technique - Velocity Board, Swoffer (or Other) Velocity Meter and Profile Measurements for Flow Determination

For Flow method 2. (and Flow method 3.) the first step is the measurement of a creek profile (details below).  The stream profile gives an area, a velocity meter (or stream velocity board) provides a velocity (l/s) - together these can be used to determine flow.  Actual flow numbers are calculated in our SSIFWC webmap charting, in the dropdown menu "Charts" (so no calculations are required!

Some background to the velocity meters and/or steam velocity board being used are available under Flow Measurements.  For useful reading on effectively deploying our WPS Velocity Meters, or velocity board also have a look at these online, the: Swoffer field notes, Marsh-McBirney and Stream Velocity Board documentation.

Note - the creek profile measured/used in the velocity meter, or board method is dependant upon the type of stream being measured (and your available time!).  The ideal flow methodology is driven by the shape of the streambed (or its cross-sectional profile).  Simple stream beds are saucer-like in cross-sectional shape, more complex streams may be pan-like in cross-sectional shape, or even a pan/saucer shape with significant dents in the base! 

For a very simple creek profile a "V" shape can be used, as an an approximation only. For more on creek profiles cf. our V3 Creek Profiles notes.

A creek cross-section (profile)

The key elements of a creek's cross-sectional profile, recorded as part of float-flow measurement (method [3]), include "wetted width", subsection (or panel) width and depths and creek flow velocity (derived from float times, or a velocity meter).  

This example creek is a little saucer-like with tapering creek bank edges and a gently curved creek floor.

Note - the same creek cross-section procedure is used for both float (method [3]) and velocity meter (method [2]) flow measurements.

Keeping Stream Cross-Section Measurements Practical & Manageable

For any steam cross-section data being acquired for use with float and flow, a velocity meter,  or velocity board, up to 15 “profile points” can be logged in the FWC App. What we are after for a stream is a representative (“good first pass”) cross-section, to allow us to use the depths and widths captured to determine the area of stream “shallows” and “deeps” accordingly.  Given that our island’s streams are not in the same league as many of those in the rest of BC (i.e. are mainly a lot smaller) a practical limit of 10 representative cross-section points per stream is recommended. 

Creek Cross-Section Sampling Intervals for Flow Measurements - A Rule of Thumb

A creek cross-section (profile) flow calculation sensitivity analysis, and the guidelines on stream sampling from the province, help us to optimise the number of measurements we need to take in the field, and record and enter into our SSIFWC Cloud database.   

A field rule-of-thumb for creek cross-section measurements, for "velocity meter", "velocity board" and "float" measurement methods: 

- target using a smaller (but representative) number of panels for measuring/recording a stream cross sectional profile (eg 7-10 points depending on flow conditions and creek shape).  

- review the number of points being considered for measurement in conjunction with the guidelines from the province on stream sampling density, to try and ensure measured panels in a profile adequately sample all sections in the profile with more than 10% of flow…..  

Using the wading rod & Swoffer or Marsh-McBirney flow meter (Stream Velocity Board)

For the Velocity metres the fixed "wading rod" ("A") is used to determine actual water depths (recorded as part of a creek profile set), and to determine a proportional water depth for the sensor placement.  The sensor should be placed at 0.4 (or 40%) of the bottom of the water column (ie just below the middle of the water column).  When using the Stream Velocity Board the wading rod simply gives total water-depth, so is not necessary (the Velocity Board has fixed rulers for measuring water depth).

The main, fixed rod is marked to give the actual water depth.  The sliding foot rod, and the vernier scale on the fixed rod handle ("B"), are used to position the sensor at 40% from the bottom of the water column.  To adjust the sliding portion (sliding foot) of the wading rod, depress the trigger on the back side of the fixed rod handle.

To set the sensor at 40% from the bottom of the water column line-up the wading rod’s numbered sliding foot rod mark at the appropriate depth, aligning this with the appropriate vernier division on the the wading rod handle (on the top of the fixed actual depth rod). 
For example, if total water depth is 20 cm, then line up the “2” line on the rod’s sliding foot scale with “0” on the Vernier on the back of the handle (the sensor is then at ca. 7cm).  If the depth is 25cm, then line up the “2” sliding foot scale line with the “0.5" on the Vernier on the back of the handle (the sensor is then at ca. 10cm).

Once the sensor is located at the proper depth.  Hold the rod vertical at the appropriate measuring rule with the sensor pointing upstream (into the flow and perpendicular to the ruler).  Ensure that your position in the creek does not influence velocity measurement!

Measure the velocity readout (m/sec) after setting a suitable averaging ("2", "6", "20") for the flow conditions encountered.

Creek profiles and velocity measurements should be spaced to record and define:

See also notes on appropriate  "data density" below.

Notes - the Impact of Sampling - Data Density?

Sensitivity tests on a field wetted width-depth and flowmeter dataset recorded for the Weston Creek Outfall on Monday 4th May, 2020 suggests caution on over-zealous measurement intervals in small(ish) creeks - an example.

Flow calculated using all profile (panel) cross-section points                  47.0 l/sec

Flow using even profile (panel) cross-section points (i.e. 100, 120 etc)  45.8 l/sec

Flow using odd profile (panel) cross-section points (i.e 110, 130 etc)     47.9 l/sec

Basically the “error" between the different data subsets is  +/- 2 % (ie a very small number).  Note - do not oversample a creek cross-section in our "small island creeks"!