Module 3 - 65
To measure the W
of a watercourse:
ɷ Use a fibre survey chain or nylon tape measure for all length
ɷ Include all unvegetated gravel bars in the measurement. These
generally show signs of recent scouring or deposition.
ɷ Where multiple channels are separated by one or more
vegetated islands, the width is the sum of all the separate
channel widths. The islands are excluded from the width
ɷ Widths should be measured at right angles to the direction of
flow at a minimum of six sites, taken at equally spaced
intervals. The interval between measurement stations should
be approximately equal to the channel width at the first
ɷ Generally, watercourse widths should not be taken near
watercourse crossings, unusually wide or narrow areas [e.g.,
impoundments or disturbances; see the
(FPC, 1995)]. However, width
estimates for these areas should be outlined in the comments
section of the Site Card.
Measure the wetted width of the watercourse to the nearest 0.1m (Fig.
3.16). Wetted width is the width of the wetted portion of the channel,
measured at right angles to the direction of flow. If multiple channels
occur, then the separate widths should be added together, [refer to the
Riparian Management Area Guidebook
(FPC, 1995) and the
(FPC, 1995)]. Water beneath undercut banks,
protruding rocks, logs, stumps, and bars surrounded by water are
included in the wetted width measurement.
Active Floodplain Width
The presence of flood signs indicates the extent of historic flooding
events, where the watercourse’s flow has exceeded its bankfull channel
capacity. Typical flood signs are debris on the banks outside the bankfull
width, recent scarring of trees or other vegetation, and newly deposited
fluvial sediments on the forest floor, tree trunks, or vegetation. Using a
metre stick or other measuring rod, measure the height (in metres) of the
flood signs above the bankfull height. Using a fibre tape or tight chain,
measure the horizontal extent that the flood signs extend beyond the W
on each side of the channel. Sum these measurements with the W
determine the active floodplain width.
Document under “comments” the type of flooding evidence observed.
Record the bankfull depth of the watercourse to the nearest 0.1m (Fig.
3.16). Bankfull depth is the depth that the watercourse would have if its
channel were flooded to the top of the banks.