where Cf is the skin friction coefficient which depends of
the flow type and is calculated using L and height of the pits
and crevices; W and L are width and length of the surface
pieces (m) and r is the fluid density (kg/m3). shows
the drag force variation among roughness level and
between flumes. The experiment consisted of four different
roughness groups under two different water velocities, hence,
a total number of eight treatment groups with different drag
forces. The coverage of algae was recorded 10 days after the
installation of all surfaces. This period is considered suitable
for algal settlement. The surface area
coverage (SAC-%) of the biofilm in the pictures was calculated
in a standardized centered area (1 cm2), to avoid border
effects, using Adobe Photoshop. This
technique was adapted from and recent studies have used percent coverage as an
algal metric. Differences in algae growth in relation to drag force
were analyzed with ANOVA tests. The influence of velocity
in algae SAC was analyzed with logistic regression tests,
using water flow as a dichotomist variable (i.e. slow vs.
fast) and covered area as a continuous variable to assess
the probability of algae to settle in slow or fast streams.
All statistical analyses were made using the software
Statistica 10.