Stream-Profile Analyses Using a Step-Backwater Model for ... - USGS

10 Stream-Profile Analyses Using a Step-Backwater Model for Selected Reaches in the Chippewa Creek Basin in Ohio
Analyses of Hydraulic Structures
Four methods for computing losses through bridges are
available in HEC-RAS. The energy equation (standard-step
method) is applicable to the widest range of hydraulic problems
(U.S. Army Corps of Engineers, 2008b). The standardstep
method performs computations at bridges basically
as open-channel flow, except that adjustments are made to
account for the cross-sectional area and wetted perimeter 1
within the bridge opening. For most bridges modeled in this
study, the energy equation was used to account for energy
losses because very few bridges were submerged.
By definition, pressure flow occurs when the water
surface on the upstream side of a bridge equals or exceeds the
low-chord elevation, a condition that can cause the bridge to
function as a pressurized orifice. In these cases, pressure-flow
computations are warranted for use in HEC-RAS simulation.
The use of this type of solution was checked at all bridges
in the hydraulic models where the water-surface elevation
derived from the energy equation was found to be within
1.0 ft of the low-chord elevation of a bridge. Review of the
HEC-RAS model output indicated that pressure-flow computations
were not required.
When road overflow occurs at a culvert, HEC-RAS can
use a weir-flow computation to determine the amount of flow
passing over the road (U.S. Army Corps of Engineers, 2008c).
The validity of the weir-flow computation must be checked
by means of a submergence 2 test. The model default maximum
submergence for weir flow is set to 0.95 (95 percent).
For a weir-flow computation to be considered valid, the road
embankment must be high enough to cause flow over the road
to pass through critical depth 3 . If a weir flow computation
is not valid, computations are based upon contracted openchannel
flow. For situations where road grades are submerged,
Shearman and others (1986) recommend abandoning culvert
and weir hydraulics in favor of composite sections (the combination
of the road and culvert cross-section geometries) to
reflect pseudo-open-channel conditions.
Preliminary HEC-RAS simulation indicated that road
overflow would occur at two locations: Chippewa Creek Road
at river station 114,595 of the Chippewa Creek model and
a private driveway at river station 32,680 of the River Styx
model. A check for submergence was done at each location
where road overflow occurred to assess the validity of the
1 Wetted perimeter is defined as the length of the line of intersection of
the channel wetted surface with a cross-section plane normal to the
direction flow (Chow, 1959).
2 Submergence is defined as the ratio of the depth of water above the
minimum weir elevation on the downstream side of a structure divided by
the height of the energy grade line above the minimum weir elevation on
the upstream side of a structure (U.S. Army Corps of Engineers, 2008c).
3 Critical depth is the depth of flow at which the specific energy is a
minimum for a given discharge (Chow, 1959).
weir-flow calculation. The Chippewa Creek Road bridge was
modeled by use of the standard step method, and it was submerged
from backwater from Chippewa Lake; thus, weir flow
was not possible. The road grade of the submerged private
driveway was not elevated enough to provide sufficient fall
for the default weir-flow calculation. The culvert at the private
drive was removed and replaced with a composite section.
Reduction of Roughness Coefficient Analysis
To analyze the impact of reducing the roughness in the
main channel (for example, as might occur if channels were
cleared of vegetation and brush) on the water-surface-elevation
profiles, the roughness coefficients for the main channels
were reduced by 5, 10, 15, and 20 percent from their base values.
These reductions were made along the entire reaches of
Chippewa Creek, River Styx, and Little Chippewa Creek,
and were applied only to the main channel, not to overbank
areas. Table 6 shows the average decrease in the water-surface
elevation for each reduction percentage on Chippewa Creek,
River Styx, and Little Chippewa Creek. It should be noted that
the values in the tables are valid only when all downstream
roughness coefficients are similarly reduced. For example,
in table 6, a 20-percent reduction in the main channel roughness
coefficient along Chippewa Creek from Frick Road to
State Route 3 will only result in a 0.78-ft decrease in the
water-surface elevation if the main-channel roughness is also
reduced by 20 percent from the mouth to Frick Road. The
water-surface elevation changes shown in these tables are for
the discharges used in the models (see table 5).
Results of Hydraulic Analyses
Water-surface profiles corresponding to approximately
bankfull discharges and for selected reductions in roughness
coefficients are presented in tabular and graphical formats in
Appendix 3. These profiles show the computed water-surface
elevations as a function of distance from a reference location.
Reach-averaged reductions in water-surface elevations
ranged from 0.11 to 1.29 ft over the four roughness reduction
scenarios. Selected results of the final hydraulic analyses done
for this study before performance of the decreased roughnesscoefficient
scenarios are presented in Appendix 2.
The minimum channel elevations at each cross section
and the hydraulic structures are shown in figures 3–1, 3–2, and
3–3. All elevations presented in the profile plots are referenced
to the NAVD 88. The HEC-RAS model simulations indicated
that road overflow would occur at two locations: the Chippewa
Creek Road at river station 114,595 of the Chippewa Creek
model and a private driveway at river station 32, 680 of the
River Styx model (fig. 3–1 and 3–2, respectively). The Chippewa
Creek Road bridge was submerged from backwater from
Chippewa Lake.