WSP04979

• File Number: 8240.200.10.B
• Description: UCRBRIP - Riverine Fish Flow Investigations
• Basin: Colorado Mainstem
• Date: 6/1/1999
• Author: CDOW
• Title: Riverine Fish Flow Investigations 1999
• Water Supply Pro - Doc Type: Publication

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v2 1'2
h = LS +C!2.L_~
. f 2g 2g

(2)

where: L
Sf
C

= discharge weighted reach length
= representative friction slope between two cross sections
= expansion or contraction loss coeffiecient

The steady flow component is capable of modeling subcritical, supercritical, and mixed
flow regime water surface profiles (Brunner, 1998). HEC-RAS has a graphical user
interface (Gill) and requires station and elevation coordinates for each cross section.
Energy loss due to friction is accounted for with cross-section average values for
Manning n. Contraction and expansion of the channel is accounted for with the
inclusion of the distance between right, left and thalweg points at adjacent cross
sections. Simulation output can be expressed in tabular or graphical format and
generally consists of depth, average cross-sectional velocity, and permutations of depth
and velocity.

The HEC-RAS hydraulic model was used to determine wetted surface area and
depths as a function of flow. Water surface profiles were computed from one cross
section to the next by solving the energy equation with an iterative procedure called the
Standard Step Method. HEC-RAS determines water surface elevation and an average
velocity for each cross-section in an analysis. Thirty-one cross sections at
approximately 150 ft intervals were inserted into the digitized Duffy channel and 29
cross sections at approximately 130 ft intervals were inserted into the digitized Sevens
channel.

Water surface elevations were input into ARCView and endpoints of each cross
section and a triangulated irregular network (TIN) of water surface elevation was
created. Using a procedure called Cut and Fill, a TIN of the bed surface was subtracted
from the TIN of water surface creating a polygon representation of wetted area. In
order to determine the surface area for a given depth, the TIN's were converted to raster
data (GRID) and the grid of bed surface was subtracted from the grid of water surface
elevation. The resulting grid was turned into polygons and with integer values of
average depth for the interval. The average zero depth value included areas above the
water surface to 0.5 ft and "dry" area was removed. Wetted areas per depth categories
were calculated in ARCView with the "calcacre" avenue script.

HEC-RAS outputs a single average velocity for each cross section. Cross
sectional average velocities do not allow plotting the distribution and area of habitat
types based on combinations of both depth and velocity. Therefore depth was the only
habitat attribute available to compare differences in habitat between the two study areas
in this report.

RMA2 is a two-dimensional depth averaged finite element hydrodynamic model
created for the Corps of Engineers in 1913. RMA2 computes water surface elevations
and horizontal velocity components for subcritical, free-surface flow in two-dimensional
flow fields using a finite element solution of the Reynolds form of the Navier Stokes

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