BOARD01425

• Board Meeting Date: 3/29/1999
• Description: ISF Section - Methodology Report
• Board Meetings - Doc Type: Memo

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Wetted Perimeter Method
The Wetted Perimeter Method, like R2CROSS, is an example of a habitat retention, standard
setting model. This method is routinely utilized in six states (Figure I).

Wetted perimeter is the linear distance along the stream channel bottom that is covered with
water (Figure 3). It requires quantification of the relationship between wetted perimeter and
streamflow (Figure 4). This relationship may be derived empirically from multiple channel
geometry-discharge field measurements or modeled hydraulically from a single, channel geometry-
discharge dataset. A Wetted Perimeter instream flow recommendation is determined by locating the
inflection point of the wetted perimeter-discharge relationship (Nelson 1980, Stalnaker et al. 1995).

The methodology uses wetted perimeter as an index of habitat quality to estimate a desired
instream flow value. Channel geometry-discharge field data is collected in a critical habitat area,
most often a riffle. The critical habitat type serves as an index of habitat for the rest of the stream.
The method relies on the assumption that preserving the wetted perimeter in a critical habitat area

Figure 3. Wetted Perimeter defined.

Vertical Distance (ft)
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(10)
o

(2)

0,>"
Stt88mbsnk

(4)

Water Surface Elevation

(6)

Wetted Perimeter ~
(= Total D;~tanc8 of Sold Line)

(8)

t

4

8 12 16 20 24 28
Horizontal Distance (ft)

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