Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 117

Six stream classes are available in the NATHAT, but no national stream classiication tool is
available. The typical procedure would be to review the national classiication system (Poff 1996),
based on six stream classes and ten primary indices and then assign the study stream to a stream
class using the NATHAT (Poff 1996; Olden and Poff 2003). The NATHAT would then be used for
the analyses. Hersh and Maidment (2006) compared the IHA and NATHAT for use in developing
environmental lows for Texas streams and rivers.
4.6.2.2 Hydraulic Models
4.6.2.2.1 Wetted Perimeter Method
The wetted perimeter method assumes that there is a direct relation between the wetted perimeter
in a rifle and ish habitats in streams (Annear and Conder 1984; Lohr 1993). The wetted perimeter
is the width of the streambed and streambanks in contact with water for an individual cross section.
The concept may be illustrated considering a rectangular cross section of width B and depth Y. The
wetted area of the cross section would be B*Y, while the width of the wetted perimeter would be
B + 2Y. Another common hydraulic characteristic is the hydraulic radius, which is the wetted area
divided by the wetted perimeter.
The wetted perimeter method is used as a measure of the availability of aquatic habitats over a range
of discharges, and the relationship can be computed using ield data or hydraulic models (Annear and
Conder 1984; Nelson 1984). A plot of the wetted perimeter and discharge is used to determine the
streamlow required for habitat protection, often identiied by the point of maximum curvature.
4.6.2.2.2 R2-Cross Method
The R2-Cross method is also known as the “Colorado method” or the “critical area method”
(Nehring 1979; Espegren 1996). It has long been used by the Colorado Water Conservation Board to
establish instream low requirements to “preserve the natural environment to a reasonable degree.”
The method is based on one or more cross sections and the hydraulic modeling and selection of
established width, depth, and velocity criteria intended to provide for the “reasonable” protection
of aquatic resources.
The R2-Cross method requires the selection of a critical rifle along a stream and assumes that a
discharge chosen to maintain a habitat in the rifle is suficient to maintain habitats for ish in nearby
pools and runs for most life stages of ish and aquatic invertebrates (Nehring 1979). The streamlow
requirements for habitat protection in rifles are determined from lows that meet the criteria for
three hydraulic parameters: mean depth, percent of bankfull wetted perimeter, and average water
velocity. The R2-Cross method has been found to produce low recommendations that are similar
to those determined by more data-intensive techniques such as the instream-low incremental meth-
odology (Nehring 1979; Colorado Water Conservation Board 2001).
4.6.2.3 Habitat Models
4.6.2.3.1 IFIM
The IFIM is a commonly applied software system that is used to integrate microhabitat suitability
and macrohabitat suitability into habitat units that are then related to low over time (Schroeter et al.
2005). The habitat time series output displays the availability of a suitable habitat for the period of
interest. For example, the IFIM uses ive main steps in assessing the impacts of dams or abstractions
(Acreman and King 2003):
1. “The problems are identiied and broad issues and objectives are related to legal entitle-
ment identiication.”
2. “The technical part of the project is planned in terms of characterizing the broad-scale
catchment processes, species present and their life history strategies, identifying likely
limiting factors, collecting baseline hydrological, physical and biological data.”