Boneyard Creek Flow Drop Study
The Boneyard Creek is a small urban waterway that runs through Champaign and Urbana, Illinois. The creek’s existing channel geometry cannot contain the high discharges that are a result of watershed urbanization, and thus water flows out of the stream’s banks on a regular basis. The University of Illinois is in the process of implementing a plan to protect buildings on its campus from floods of up to a 100-year return period.
A one-to-twelve scale physical model was constructed on the floor of the Ven Te Chow Hydrosystems Laboratory in order to investigate the hydraulic behavior of the Lincoln Avenue bridge and surrounding floodplain. The model was tested with the existing channel geometry in place for a range of discharges that included floods with return periods greater than 100 years. The influence of several variables was investigated, including effects of the 54-inch storm sewer that joins the creek under Lincoln Avenue, seasonal variability of the upstream channel roughness, and different flow stages at the downstream boundary.
The model was then changed to reflect the geometry of the proposed channel improvements. Tests were again run for flows up to and beyond the 100-year flood, and the hydraulic characteristics of both configurations were compared.
The study determined that the major controlling feature for upstream water surfaces is the drop structure that currently exists underneath the Lincoln Avenue bridge. At lower flows, the water passing over the drop achieves supercritical flow conditions, thus controlling upstream water surfaces elevations. The drop structure geometry has great influence at higher discharges as well, when it restricts the flow so that water backs up behind the bridge. This effect is detrimental to the surrounding area, however, because water that would normally pass into the culvert without incident becomes overland flow that runs east to rejoin the channel downstream of the culvert.
The proposed lowering of the channel bottom with the consequent removal of the drop structure were found to decrease stages within the channel upstream of Lincoln Avenue. The proposed improvements also protected the Hendrick House parking lot from the 100-year flood, and reduced flood stages on Lincoln Avenue and downstream in Urbana by approximately .87 and .74 feet, respectively. The modifications also performed well for discharges up to 1800 cfs, which could occur if upstream stormwater storage facilities do perform as planned and thus do not reduce the discharge rate downstream.