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

Stream-Profile Analyses Using a Step-Backwater Model
for Selected Reaches in the Chippewa Creek Basin
in Medina, Wayne, and Summit Counties, Ohio
By David E. Straub and Andrew D. Ebner
Abstract
The U.S. Geological Survey, in cooperation with the
Chippewa Subdistrict of the Muskingum Watershed Conservancy
District, performed hydrologic and hydraulic analyses
of selected reaches of three streams in the Chippewa Creek
basin in Medina, Wayne, and Summit Counties, Ohio to aid in
the assessment of alternatives for channel improvements for
mitigating flood hazards. Peak flows with a 50-percent annual
exceedance probability (the 2-year flood) were estimated from
regional regression equations as an approximation of bankfull
discharges. Drainage areas of the three stream reaches studied
ranged from 29.6 to 188 square miles.
The Hydrologic Engineers Center’s River Analysis System
step-backwater model was used to compute water-surfaceelevation
profiles for the 2-year flood along selected reaches
of the streams. The base model was modified to determine
the effects of reducing main-channel roughness coefficients
(Manning’s n) by 5, 10, 15, and 20 percent on the computed
water-surface profiles. Reach-averaged reductions in watersurface
elevations ranged from 0.11 to 1.29 feet over the four
roughness reduction scenarios.
Introduction
The Chippewa Creek valley was a series of swamps until
channel modifications drained it and turned it into a productive
agricultural region by 1900. Without a systematic maintenance
program, the channels reverted back to their natural conditions
and flooding became a problem. In June 1956, the Soil
Conservation Service (SCS) developed a watershed-protection
and flood-control plan (U.S. Department of Agriculture,
1956) that called for 33.2 miles of modified channels and 8
floodwater-retention structures (fig.1). The channel modifications
included straightening the channel of selected stream
reaches of Chippewa Creek, Westfield Ditch, Little Chippewa
Creek, and River Styx. A trapezoidal channel geometry was
constructed to increase the efficiency of the channel to convey
the floodwater out of the basin. The floodwater-retention
structures were designed to attenuate flood-peak streamflows.
The effects of these flood-control features were estimated
to reduce the annual average flooded area by approximately
4,210 acres. (Muskingum Watershed Conservancy
District, 1961).
After three amendments to the original plan, in which
the primary changes were the locations of conservation
pools upstream from the floodwater-retention structures,
the final version of the Chippewa Subdistrict’s Official Plan
(Muskingum Watershed Conservancy District, 1961) was
adopted in March 1977. All channel modifications and floodwater-retention
structures of this plan were completed by 1980.
The floodwater-retention structures have been maintained
by the Chippewa Subdistrict of Muskingum Watershed Conservancy
District (MWCD). Over time, however, the streams
have developed significant tree and brush growth along the
channel, thus decreasing the ability of the stream to convey
floodwaters out of the basin. In 2007, a section of Chippewa
Creek between Shorle Road and Miller Road was cleared of
debris, trees, and brush (figure 2). The downstream channel
reach of Chippewa Creek still (as of 2010, figure 3) has appreciable
tree and brush growth along the channel banks.
The Chippewa Creek basin is undergoing significant
development pressure in several areas of the watershed.
Traditionally, communities could make use of Flood Insurance
Studies (FIS) as an important tool for determining areas
that should not be developed. A majority of FIS’s conducted
by the Federal Emergency Management Agency (FEMA)
in the Medina County portion of the Chippewa Creek basin
were completed in the mid-to-late 1980’s. A FIS for Medina
County and incorporated areas (Federal Emergency Management
Agency, 2008) was released on August 4, 2008,
but no new detailed studies were completed as part of this
new study. The flood-hazard-inundation areas of the older
studies were redelineated over new digital base map information
provided by Medina County. A FIS for Wayne County
and incorporated areas (Federal Emergency Management
Agency, 2009) was adopted on August 18, 2009, and it
included new detailed flood profiles that were developed for
Chippewa Creek and River Styx for recurrence intervals of
10, 50, 100, and 500 years.