Claypool Drainage District Analysis

The purpose of this analyses was to evaluate the hydraulic efficiency of selected ditches and culverts within the Claypool Drainage District and to determine their bank-full capacity at critical points in the system.  This analysis focused on the main drain of the Claypool Drainage Ditch and the three ditches tributary to the main drain.  The basis of this study included ditch and Culvert topographic information previously provided by the District to Lindley & Sons, Inc., calculations previously by Lindley & Sons on the previously mentioned information, previously mentioned plans and data prepared by others, calculations previously prepared by Lindley & Sons regarding, "Eikey Ditch" and preperation of additional ditch and culvert capacity calculations using available topographic information.

A Hydrologic analysis of the existing watersheds within the Claypool Drainage District was also prepared.  The analysis in this report was used to evaluate the conveyance of storm water runoff through the existing District drainage system.

Hydrologic Analysis

The U.S. Army Corps of Engineers Hydrologic Engineering Circular No. 1 (HEC-1) hydrologic modeling computer software routine was utilized in the determination of the existing storm water runoff rates within the drainage district. Watershed specific information, including drainage area, runoff curve number, time of concentration and rainfall data from the Illinois State Water Survey Bulletin 70 publication, was input into the model.  The model calculated the existing peak runoff rate for each watershed and the combined watersheds.

The rainfall data was received from the Illinois State Water Survey Bulletin 70 publication. Huff distributions were applied to more accurately depict the time-depth distribution of the rainfall for the selected rainfall events.  The rainfall frequencies evaluated for this analysis were the 2-year, 10-year, 50-year, and 100-year storm frequencies.  The rainfall duration events evaluated for each storm were the 1-hour, 3-hour, 6-hour, 12-hour, 24-hour, and 48-hour rainfall durations.  A table identifying the expected runoff rate at each section analyzed in this report can be found in Appendix B.

The watershed specific information was obtained from interpretation of United States Geological Survey mapping, Grundy and Will Counties Soils Survey and Aerial Photographs.  The watershed areas were delineated based on USGS contour mapping.  The runoff curve numbers were calculated based on ground coverage information from the aerial photos in combination with soils information from the Soils Surveys using the methodology from the Soil Conservation Technical Release 55 (TR-55).  The times of concentration were estimated utilizing the Clark Method, detailed in "A Technique for Estimating Time of Concentration and Storage Coefficient Values for Illinois Streams" prepared by U.S. Geological Survey as Water-Resources Investigations 82-22.

Hydraulic Analysis

There were a total of eight ditch sections and four culvert crossings analyzed in this report.  The ditch sections were analyzed to determine their bank-full capacity.  This bank-full capacity was compared to the calculated runoff determined by the hydrologic model in order to evaluate the capacity of the exisiting ditch as it relates to the conveyance of runoff through the drainage district.  Each channel was evaluated assuming an "un-maintained" channel roughness, Manning's n-value of 0.070 and a "maintained" channel roughness, Manning's n-value of 0.035.  The "maintained' channel roughness value is only applicable if the District adheres to a strict channel maintenance program including removal of accumulated debris, removal of woody growth within the channel banks, and mowing of the grassed channel slopes on a regular basis.The culvert sections were analyzed in a similar manner to evaluate whether the culvert crossings are acting as restrictions to flow of storm water in the ditches.

The first ditch section analyzed, labeled number 1 in the Ditch/Culvert Identification Exhibit Found in Appendix A, is located immediately downstream of the confluence of the Claypool Drainage Ditch from the east with the unnamed tributary from the south. Assuming a "un-maintained" condition this ditch is capable of conveying approximately 1160 cfs (0.05 cfs per tributary acre).  This is approximately equivelant to the expected runoff from a 5-year rainfall event.  If the channel was part of a regular maintenance program, the ditch capacity may increase to approximately 2330 cfs (0.10 cfs per tributary acre), or the equivelant of a 25-year rainfall event.

The second ditch section analyzed, labeled number 2, is located on the Claypool Drainage Ditch just up stream of the confluence with a unnamed tributary from the south.  Assuming a "un-maintained" condition, this ditch is capable of conveying approximately 600 cfs (0.03 cfs per tributary acrea).  This is less than expected from a 2-year rainfall event.  If the channel were a part of a regular maintenance program, the ditch capacity may increase to approximately 1200 cfs (0.07 cfs per tributary acrea), or the equivelant of just less than a 5-year rainfall event.

The third ditch section analyzed, labeled number 3, is located on a un-named tributary to the Claypool Drainage Ditch just upstream of the confluence with the Claypool Drainage Ditch.  Assuming a "un-maintained" condition, this ditch is capable of conveying approximately 210 cfs (0.06 cfs per tributary acrea).  This is slightly greater than the expected runoff from a 5-year rainfall event.  If the channel were a part of a regular maintenance program, the ditch capacity may increase to approximately 430 cfs (0.11 cfs per tributary acrea), or the equivelant of a 25-year rainfall event.  It is assumed that this section is typical of the entire un-named tributary as there was no additional information available to evaluate this un-named tributary any further.

The fourth ditch section analyzed, labeled number 4, is located on the Claypool Drainage Ditch just downstream of the ditch's of Interstate 55.  Assuming a "un-maintained" condition, this ditch is capable of conveying approximately equivelant to the expected runoff from a 10-year rainfall event.  If the channel were a part of a regular maintenance program, the ditch capacity may increase to approximately 1460 cfs (0.28 cfs per tributary acrea), or the equivelant of a 50-year rainfall event.  It should be noted that required peak release rate for new development within the district is 0.15 cfs/acre for a 100-year, 24-hour storm event.  If the area upstream of this ditch section were to be entirely developed and adhering to the peak discharge requirements, the ditch may have the capacity to safely pass the 100-year storm event.

The fifth ditch section analyzed, number 5, is a culvert crossing on what is commonly referred to as the "Eikey Ditch".  This culvert crossing is a combination of 8'x6' concrete box culvert and a 50"x32" corrugated metal pipe arch culvert.  It is located at the ditch crossing under East Valerio Rd.  Assuming full-flow of the culverts with no surcharge of "head" on the upstream side, the combination of culverts can convey approximately 403 cfs (0.13 cfs per tributary acrea).  This is approximately equivelant to the expected runoff from the 25-year rainfall event.

The sixth section analyzed, number 6, is a culvert crossing of an un-named tributary to the "Eikey Ditch".  This culvert crossing is a 48-inch diameter reinforced concrete pipe culvert.  It is located at the ditch crossing under Illinois Route 113.  Assuming full-flow of the culverts with no surcharge of "head" on the upstream side and an assumed water depth of two feet, the culvert can convey approximately 78 cfs (0.06 cfs per tributary acrea).  This is approximately equivelant of the expected runoff from just less than a 10-year rainfall event.

The seventh ditch section analyzed, number 7, is a culvert crossing of an un-named tributary to the "Eikey Ditch" just upstream of  the culvert under Illinois Route 113.  Assuming a "un-maintained" condition, this ditch is capable of conveying approximately 50 cfs (0.05 cfs per tributary acrea).  This is approximately equivelant to the expected runoff from the 5-year rainfall event.  If this event were apart of a regular maintenance program, the ditch capacity may increase to approximately 100 cfs (0.10 cfs per tributary acre), or the equivelant of just less than a 25-year rainfall event.

The eighth ditch section analyzed is labeled number 8 in the Ditch/Culvert Identification Exhibit Found in Appendix A.  It is located on what is commonly referred to as the "Eikey Ditch" at approximately the end of East 2nd Street.  This section of the Eikey Ditch has been improved per plans prepared by Lindley & Sons, Inc. last revised 5-27-99.  Assuming the district is following the maintained plans for this ditch, this ditch is capable of conveying approximately 167 cfs (0.10 cfs per tributary acrea).  This is approximately equivelant of the expected runoff from just more than a 10-year rainfall event.

The nineth section analyzed, labeled number 9, is a culvert crossing on the "Eikey Ditch".  This culvert crossing is a 60-inch diameter reinforced concrete pipe culvert with head wall.  It is located at the ditch crossing under Illinois Route 113.  Assuming full-flow of the culverts with no surcharge of "head" on the upstream side and an assumed water depth of two feet, the culvert can convey approximately 120 cfs (0.19 cfs per tributary acrea).  This is approximately equivelant of the expected runoff from a 20-year rainfall event.

The tenth section analyzed, labeled number 10, is located on the "Eikey Ditch" approximately 1700 feet upstream of Illinois Route 113.  Assuming a "un-maintained" condition, this ditch is capable of conveying approximately 330 cfs (0.27 cfs per tributary acrea).  This is approximately equivelant to the expected runoff from the 100-year rainfall event.  If the channel were apart of a regular maintenance program, the ditch capacity may increase to approximately 656 cfs (0.54 cfs per tributary acre), in excess of the 100-year rainfall event.

The eleventh section analyzed, number 11, is located on the "Eikey Ditch" approximately 5000 feet upstream of Illinois Route 113.  Assuming a "un-maintained" condition, this ditch is capable of conveying approximately 113 cfs (0.14 cfs per tributary acrea).  This is approximately equivelant to the expected runoff from the 50-year rainfall event.  If the channel were apart of a regular maintenance program, the ditch capacity may increase to approximately 226 cfs (0.29 cfs per tributary acre), in excess of the 100-year rainfall event.

The twelfth section analyzed, labeled number 12, is a culvert crossing on the "Eikey Ditch".  This culvert crossing is a 78-inch Corrugated Metal Pipe culvert, projecting from fill.  It is located at the ditch crossing under Spring Road.  Assuming full-flow of the culverts with no surcharge of "head" on the upstream side and an assumed tail water depth of five feet to mimic the downstream channel depth, the culvert can convey approximately 220 cfs (0.34 cfs per tributary acrea).  This is in excess of the expected runoff from a 100-year rainfall event.