Robert J. Danehy
James M. Hassett
The analysis of aquatic habitats in streams has been the subject of much research
during the past 40 years. The traditional visual classification of pool and riffle
categories has been modified to include numerous descriptive classes. However, visual
classification schemes are qualitative and subjective and do not allow for objective
quantification and statistical comparisons across a range of stream habitat types.
Use of the Froude number provides an objective way to classify and analyze habitat
units.
The Froude number (Fr) is used by hydraulic engineers to describe types of flow.
Fr, derived from a force balance on an element of incompressible fluid, can be expressed
as
(1)
where:
Fr can be thought of as the ratio of kinetic energy (proportional to V2) to
potential energy (proportional to gy). Fr values greater than 1 describe supercritical
or shooting flow; Fr values less than 1 describe subcritical or tranquil flow.
In field practice, Fr can be calculated as
(2)
where
Equation (2) is exactly equivalent to equation (1). Consequently, the Froude number
can be computed by measuring channel width, cross sectional area, and discharge at
specific stream cross sections.
Biologists have recently begun to use this tool in the analysis of habitat. Heede
and Rinne (1990) suggested that a hydraulic approach to aquatic habitat analysis
could improve our understanding of the relationship between fish and flow. Statzner
and Higler (1986) used hydraulic character to describe patterns of aquatic insect
distribution. Jowett (1993) analyzed 1,112 stream sites in New Zealand and showed
Fr values for pools to be less than 0.18 and Fr values for riffles to be greater
than 0.41.
Danehy (1994) examined aquatic habitat in a small (116 km2) central New York watershed.
Stream flows and 31 cross-sections one bankfull width apart were obtained at 13 sites
during summer low flow conditions. Fr (for each cross section) and Fr variance (for
each site) were calculated using equation 2.
Figure 1. Froude number longitudinal and frequency distributions
at 31 cross sections at two sites on Onondaga Creek, Tully, New York.
Figure 1 is a comparison of two sites by longitudinal and frequency distribution
of Fr. The lower Fr values at the Cows site indicate the predominant pool habitat
while the higher values at Tully Farms Road describe a more riffle dominate environment.
The distribution and size of brook trout (Salvelinus fontinalis) and brown trout
(Salmo trutta) reflect those differences with much larger brown trout found at the
Cows site predominated by pools while brook trout were found only at the Tully Farms
Road site containing more riffles.
A comparison of the means of the 13 sites (Table 1) separates the sites into distinct
groups. Mean Fr ranged from 0.13 to 0.47. Using Jowett's criteria, a single site
(Webster Rd) had a mean in the pool range and three sites (Rattlesnake 1, Woodmancy,
and Emerson) had means in the riffle range. The rest of the sites fell between these
extremes.
Fr alone does not tell the entire story with respect to fluvial fish habitat. Fr
variance for a reach provides additional important information. High variance indicates
heterogeneous habitat which should be reflected in more diverse communities. In Danehy
(1994) Fr variance differences between pool-riffle channels from higher gradient
step-pool channels were evident. Pool-riffle channels had a Fr variance of less than
0.50 whereas the more complex step-pool channels had a Fr variance > 0.50.
While mean Fr is a measure of habitat type, Fr variance is a measure of habitat
complexity.
The Froude number can be an important addition to the aquatic ecologist's tool box.
The ability to describe habitat quantitatively is a clear improvement over the many
qualitative approaches that have been proposed. From a more practical standpoint,
stream restoration projects are frequently managed by an engineer. The ecologist
who can express habitat concerns in terms meaningful to the engineer is more likely
to be heard.
References
Danehy, R.J., 1994. Geomorphic, hydrologic, and hydraulic determinants of fish and
macroinvertebrates in a small watershed. Ph. D. Dissertation, SUNY College of Environmental
Science and Forestry, Syracuse, NY, 158 pp.
Heede, B. H. and J. N. Rinne, 1990. Hydrodynamic and fluvial morphological processes:
Implications for fisheries management and research. North American Journal of Fisheries
Management, 10:249-268.
Jowett, I.G. 1993. A method for objectively identifying pool, run, and riffle habitats
from physical measurements. New Zealand Journal of Marine and Freshwater Research,
27:241-248.
Statzner, B. and B. Higler. 1986. Stream hydraulics as a major determinant of benthic
invertebrate zonation patterns. Freshwater Biology, 16:127-139.
About the Authors
Robert J. Danehy is a Fisheries Biologists with Weyerhauser Company's Oregon Watershed
Team based in Springfield, Oregon.
James M. Hassett is an Associate Professor in the Faculty of Forest Engineering at
the State University of New York College of Environmental Science and Forestry (SUNY-ESF),
Syracuse, NY. He specializes in engineering hydrology, specifically the development
of rainfall-runoff models.
