Alafia River Minimum Flows and Levels - Southwest Florida Water ...

PHABSIM analysis required acquisition of field data concerning channel habitat
composition and hydraulics. At each PHABSIM site, tag lines were used to
establish three cross-sections across the channel to the top of bank on either
side of the river. The three cross-sections were sited to include a riffle, pool and
run sequence. Water velocity was measured with a Marsh-McBirney Model 2000
flow meter at two or four-foot intervals along each cross-section. Stream depth,
substrate type (sand, bedrock, mud, snags, exposed roots, snags, aquatic and
terrestrial vegetation) and habitat cover (present or absent) were recorded along
the cross-sections. Other measured hydraulic descriptors included channel
geometry (ground elevations), water surface elevations across the channel and
water surface slope determined from points upstream and downstream of the
cross-sections. Data were collected under a range of flow conditions (low,
medium and high flows) to provide the necessary information needed to run the
PHABSIM model for each stream reach.
4.2.3 Instream Habitat Cross Sections
Cross-sections for assessing instream habitats were examined at eight sites in
the Alafia River corridor. Triplicate instream cross sections, from the top of bank
on one side of the channel through the river and up to the top of bank on the
opposite channel, were established at each site perpendicular to flow in the
channel. One of the three cross-sections at each site was situated along the
floodplain vegetation transect line. Replicates were located 50 ft upstream and
downstream. A total of 24 instream cross sections were sampled (8 crosssections
x 3 replicates at each site).
For each instream habitat cross-section, the range in elevation and linear extent
(along the cross-section) of the following habitats were determined:
• bottom substrates (which was comprised by a combination of either sand,
mud, or bedrock);
• exposed roots;
• snags or deadwood;
• wetland plants; and
• wetland trees.
4.2.4 Floodplain Vegetation Cross Sections
Floodplain cross-sections (transects) based on the location of vegetation
communities identified from USGS Gap Analysis Program (GAP) maps were
established to characterize wetlands and soils within the Alafia River corridor. For
cross-section site selection, the river corridor was stratified into upstream and
downstream reaches based on differences in dominant vegetation types. Eight
representative floodplain vegetation cross-sections; four each in the downstream
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