Holcomb Ditch Watershed Study
Below are the Conclusions and Recommendations from the Franklin County Engineer’s Office’s final report regarding the Holcomb Ditch Watershed Study. This report, in its entirety, is on file at the Norwich Township Administration offices, located at 5181 Northwest Parkway, Hilliard Ohio 43026. For more information on this study, please contact Kate Cavanaugh, Township Administrator, at (614) 876-7694 or email at Kate_Cavanaugh@NorwichTownship.org.
Franklin County Engineer's Office - Holcomb Ditch Watershed Study - Final Report
Conclusions and Recommendations
A detailed hydrologic model was created as part of the study of the Holcomb Ditch Watershed. Analysis of this hydrologic model has indicated that there are not any significant opportunities within the watershed to modify existing detention basins to reduce peak flow rates to Holcomb Ditch west of Dublin Road. There are limited opportunities within the watershed to create a regional detention basin as the basin would have to be located in-line with Holcomb Ditch to have any impact on reducing peak flow rates. An in-line basin would also require a great deal of permitting and a detailed analysis of the upstream impacts would have to be conducted to assess the properties that would be impacted by the basin. It appears that the most valuable and obtainable solutions to mitigate the flooding and erosion of Holcomb Ditch directly west of Dublin Road would be a reduction in stormwater runoff volume and increased conveyance capabilities of the stream. While a majority of the recent commercial developments are designed to meet their calculated allowable release rates which equate to be less than the pre-developed peak flow rates from the same area, these developments do contribute a larger amount of stormwater runoff. The prolonged release of this excess runoff volume could be a factor in the erosion seen within Holcomb Ditch.
The most viable option appears to be an aggressive downspout disconnection program. The Holcomb Ditch Watershed has approximately 160 acres of residential area that is directly connected to the storm sewer system. Implementing a downspout disconnection program for these residential areas could reduce runoff volume and peak flow rates by up to 40% which would have a significant impact on the smaller and more frequent rainfall events. Downspout disconnection within the Scioto Run Subdivision would be a priority area in conjunction with stream channel improvements and could eliminate some of the more frequent occurrences of flooding and reduce erosion.
It is recommended that the City of Hilliard further investigate the stormwater controls, if any, provided by the commercial developments west of I-270. Section 2.1 of this report notes several subareas within the watershed that have commercial developments that are likely to utilize parking lot ponding or underground detention as their means of detention. If plans are available from these developments, the detention features can be updated within the hydrologic model. It will also be important to note that it detention systems were once utilized on these sites, and discontinued (modifications made to outlet controls), excess stormwater runoff could be discharging at a rate higher than permitted. If the commercial sites are providing a detention feature on-site, these features should be investigated to ensure that the existing outlet structure is compliant with the stormwater regulations to which it was designed.
A temporary storm sewer pipe that is connected to the wet basins within the Hickory Chase development (northwest corner of Britton Parkway and Anson Drive) directly to Holcomb Ditch should be removed or abandoned. While this pipe is unlikely to be contributing substantial flows to Holcomb Ditch, it should be addressed as it was never meant to be a permanent feature. A temporary work easement will have to be granted from the property owner to have access to the storm sewer pipe. To avoid excess cost, it is recommended that the pipe be bulkheaded at both ends and filled with a CDF material (Control Density Fill, “Flowable Concrete”) to keep runoff from the Hickory Chase wet basins from discharging to Holcomb Ditch.
Based on the results of this watershed analysis, we feel that the next steps in this process would be to assess the feasibility of the recommended improvements. An investigation into the possible grants and funding opportunities for the downspout disconnection program and stream improvements should be conducted as well as gauging the impacted resident’s willingness to participate in either option. Funding for the downspout disconnection program could possibly be obtained through a 319 grant. Grant applications for FY2012 will be due in May of 2011 and awarded at the beginning of 2012. It is recommended that a public meeting be held for the residents within the Scioto Run Subdivision to assess how many residents will participate in the program with a follow-up mailed survey. The documentation provided within Appendix C includes several case studies that discuss how the downspout disconnection program was developed as well as some of the incentives offered to residents that produced greater participation in the program. The receptiveness of the residents within the Scioto Run Subdivision to the downspout disconnect program will help determine whether or not a downspout disconnection program should be implemented within the other recommended subdivisions. Possible stream improvements should also be addressed within these public meetings. The acquisition of easements to perform these improvements will need to occur in conjunction with applying for grants. The 319 grant would also seem most appropriate for this endeavor.