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

4 Stream-Profile Analyses Using a Step-Backwater Model for Selected Reaches in the Chippewa Creek Basin in Ohio
The Chippewa Subdistrict is concerned about the
impact that existing and future development and channel
conditions will have on the hydraulic and hydrologic characteristics
within the watershed. To aid in the assessment of
these impacts and to assist communities making land-use and
development decisions, the Chippewa Subdistrict requested
that the U.S. Geological Survey (USGS) develop a step-backwater
model for selected reaches of Chippewa Creek, Little
Chippewa Creek, and River Styx.
Description of Study Area
The Chippewa Creek basin (fig. 1) is located in northeast
Ohio, largely in Medina and Wayne Counties but also in small
portions of Summit and Stark Counties. The Inlet, which is
the headwaters of Chippewa Creek, is a roughly 2-mi reach
of stream located west of the city of Medina, and it flows into
Chippewa Lake. Chippewa Creek flows out of Chippewa
Lake, the largest natural lake in Ohio. Chippewa Creek generally
flows south through south-central Medina County, through
the village of Seville, and into Wayne County. After flowing
through Sterling, Chippewa Creek flows east, passing through
the southern edge of the city of Rittman, and eventually flows
into the Tuscarawas River in southwestern Summit County
(fig. 4). Near the outlet of Chippewa Lake, some of the flow
in Chippewa Creek escapes into the headwaters of Westfield
Ditch, a tributary of Chippewa Creek, and so Westfield Ditch
was modeled as well. Little Chippewa Creek starts in Wayne
County near the city of Orrville and generally flows north to
join Chippewa Creek at the south side of the city of Rittman
(fig. 4). River Styx begins in Medina County near Wadsworth
and generally flows south into Wayne county, along the eastern
edge of the city of Rittman, and joins Chippewa Creek at
the southeastern side of Rittman (fig. 4). The entire Chippewa
Creek basin covers 188 mi 2 , with Little Chippewa Creek covering
32.2 mi 2 and River Styx covering 29.6 mi 2 .
The Chippewa Creek basin is largely agricultural, and a
majority of the basin is used for cultivated crops, pastures, and
hay. In cities and villages, a majority of the development is
of low intensity or is considered open space (U.S. Geological
Survey, 2001). The 2009 population estimates, according to
the U.S. Census Bureau, are 174,035 in Medina County and
114,222 in Wayne County.
The USGS currently operates one streamflow-gaging
station in the Chippewa Creek basin (fig. 1). That station,
Chippewa Creek at Miller Road at Sterling, Ohio (USGS
03116077; fig. 4), was installed in 2002, and data collected
by it was used to calibrate the model for Chippewa Creek.
Two other streamflow-gaging stations were operated in the
Chippewa Creek basin but have been discontinued: Chippewa
Creek at Easton, Ohio (03116200), operated from 1959
to 1981, and Little Chippewa Creek near Smithville, Ohio
(03116100), operated from 1957 to 1972 (fig.1). Data from
these stations were not used to calibrate the model because the
stage-discharge relationships have changed over the years.
Flood-Control Modifications
The SCS built structures and modified channels to mitigate
the effects of flood flows, as listed in the Chippewa Subdistrict’s
Official Plan (Muskingum Watershed Conservancy
District, 1961). Sections of Chippewa Creek, Westfield Ditch,
Little Chippewa Creek, and River Styx were modified to
increase conveyance of flood flows by straightening and modifying
the channels. The modifications called for a trapezoidal
cross section, and the dredged materials were removed from
the channel and distributed along the tops of the banks. The
embankments formed from the dredged materials increased
the main-channel cross-sectional area to convey greater flood
flows and provided small impoundments behind the embankments
for temporary storage. Any water storage behind the
embankments drained back into the channel through small culverts
or infiltration through the soil. All of the embankments
and structures are within the MWCD jurisdiction.
The plan also called for the construction of eight floodretention
structures. All eight of the SCS flood-retention
structures are located on tributaries to Chippewa Creek (fig. 1;
table 1). Chippewa Lake is not considered one of the floodcontrol
structures, although it provides some natural storage
and flood attenuation. Structures II–A, III–A, IV–A, and V–D
have conservation pools above the dam, but VIII–C, VIII–D,
V–C, and VII–C are dry dams. The combined flood-storage
capacity of all eight flood-control structures, not including the
conservation pool storage up to the emergency crest, is 4,987
acre-feet (table 1).
Purpose and Scope
This report presents the methods used for and results of a
study to analyze the hydrology and hydraulics of flow within
channels of the Chippewa Creek basin to aid in an assessment
of various flood-mitigation alternatives. The USGS used
the Hydrologic Engineers Center’s River Analysis System
(HEC-RAS; U.S. Army Corps of Engineers, 2008 a–c) stepbackwater
model to develop water-surface profiles within
selected streams in the basin (table 2) and the reach containing
Chippewa Lake (fig. 4). Streamflows determined by use
of the USGS Ohio StreamStats program (U.S. Geological
Survey, 2008) were used as estimates of bankfull flows in the
models; cross-sectional elevation data, hydraulic structure
geometries, roughness coefficients (Manning’s n), and peakdischarge
estimates also were used as model inputs. Potential
modifications to the channel were evaluated to determine their
effects on water-surface elevations. The roughness coefficients
(Manning’s n) of the main channel were then reduced
by 5, 10, 15, and 20 percent to simulate potential effects of
channel clearing and/or modifications on the water-surface
profiles.