Summary and Recommendations

The Claypool Drainage District and three unnamed tributaries have been evaluated in this report.  A hydrologic analysis has been performed in an effort to approximate the expected storm water run-off rates that would occur in the drainage district during the 2-year, 10-year, 50-year, and 100-year storm events.  These expected flows were then compared to the capacities of various sections throughout the drainage district.  It appears that the majority of the drainage ditches in the district have sufficient bank-full capacity to convey the runoff from a storm event with a reccurence interval of 50-years.  this is includes the unnamed tributary to the Claypool Drainage ditch along the far western boundary of the district, the unnamed tributary that more or less parallels the Atchison, Topeka, and Santa Fe railroad and the Illinois Central railroad through Coal City and the main channel of the Claypool Drainage ditch from the Atchison, Topeka, and Santa Fe railroad crossing to its eastern limits.

This analysis revealed three area that may be of concern to the district.  The first, and possibly the most significant, is the main channel of the Claypool Drainage Ditch from the railroad crossing to its western limits.  This portion of the ditch only has bank-full capacity for the 10-year event.  Additional capacity above the bank-full evaluation may be available along the Claypool Ditch as most of the land adjacent to the ditch is undeveloped and the potential for flood damage is reduced.  However, flooding impacts to developed areas near the channel should be considered.  Additional topographic cross-sections along this portion of the drainage ditch will provide additional data on the impacts and capacity of floods overtopping the channel banks.  The district should evaluate the cost of obtaining this additional topographic information and the subsequent analysis against the risk for potential flood damage in this area.

A second area of concern to the district is the culvert crossing identified as Section 6 in this report.  This culvert is a 48-inch diameter concrete pipe culvert under Illinois Route 113.  The calculated pipe full capacity of the culvert is approximately 78 cfs.  This is equivelant to the expected runoff from just greater than a 10-year rainfall event.  Capacity calculations of the ditches upstream and downstream of this culvert suggest the ditches have the bank-full capacity to convey the 50-year storm event.  This culvert may be acting as a restriction to the conveyance of this ditch system.  If frequent flooding is occuring upstream of the culvert, it may be a result of the flow restrictions of the culvert, causing a backwater condition and not a problem with the cross-sections of the upstream ditch.

The third area of concern to the district is that portion of the "Eikey Ditch" between East Valerio Road and Illinois Route 113.  This portion of the ditch has been reconstructed based on plans prepared by Lindley & Sons, Inc. in an effort to reduce the potential for the "Sloughing" of the ditch side slopes.  The analysis in this report indicates that the reconstructed ditch along this section has a bank full capacity of 167 cfs, which is just more than the expected runoff from a 10-year rainfall event.  Capacity calculations of the ditch section upstream and downstream of this section of the ditch suggest that those ditch sections have the bank-full capacity to convey the 50-year storm event, possibly even the 100-year event upstream of Illinois Route 113.  If the district has not received compaints of flood damage along this reach of the "Eikey Ditch", there may be sufficient over-bank capacity to pass the expected storm water runoff flows.

There are two tributaries to the Claypool Drainage Ditch with no available topographic information.  These two tributaries are large enough to be shown as intermittent streams on the USGS maps.  The first flows north from the west side of Carbon Hill and intersects the Claypool Ditch just west of North Carbon Hill Road.  The second flows north from Braidwood and intersects the Claypool ditch just east of Interstate 55.  Sufficient topographic information was not available to evaluate these two tributaries.  These tributaries may cause significant flooding impacts to developed portions of Carbon Hill and Braidwood.  We recommend collecting topographic data and further hydraulic study of these tributaries.