SSI FreshWater Catalogue

VOLUNTEERS - FIELD NOTES

Flow Method 3. Float & Profile

LAST UPDATE 2021/02/27

Flow 3. Technique - Deploying a Float and Profile Measurements for Flow Determination

Deploying a Float (AKA "A Duck") for Velocity Measurements and Flow

For the float (flow) method 3. (and method 2.) the first step is the measurement of a creek profile (details below). The stream profile gives an area, the float gives a velocity - 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 options for flow measurements and to float (duck) method described below are available here. For useful reading on effectively measuring Float-Flow have a look at these online guidelines for this simple technique.

Note - the creek profile measured/used in the float (flow) 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).

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 the velocity meter flow measurement (method [2]).

The Float and Velocity Measurements

The float is a device that will remain bouyant in the creek and will give a surface water measure of creek flow velocity (in metres per second). Various items can be used and natural is good, though it is important that a similar float is used for each measurement (for consistency).

Over a representative (straight, low turbulance flow) stretch of the creek, measure a defined distance (2m, 4m) along the bank and record the time (seconds) it takes for the float to travel over this distance. Three values of time should be recorded for the distance defined.

Note - this measured float-distance section should be within the profile measured.

Keeping Stream Cross-section Measurements Practical & Manageable

For any steam profile data being acquired for use with float and flow, or a velocity meter, up to 15 “profile points” can be logged in the FWC App. What we are after for a stream profile is a representative (“good first pass”) cross-section, to allow us to use the depths and widths captured to determine the area of “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 alot smaller) a practical limit of 10 representative float-flow profile points per stream is recommended.

Creek Profile Sampling Intervals for Flow Measurements - A Rule of Thumb

A creek profile flow calculation sensitivity analysis, and the guidelines on profile 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" 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 cross-section with more than 10% of flow…..

Stream profiles...

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 sampling in small(ish) creeks - an example.

Flow calculated using all cross-section points 47.0 l/sec

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

Flow using odd 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"!

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