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,p ,.. Plan of Study for City of North Olmsted Phase I by Floyd Browne Group January 4, 2006 STORM WATER SYSTEM EVALUATION Storm Collection System Study The storm drainage basins for the LeBern and Dover pump stations were examined for adequacy of capacity in order to better understand storm water collection and transport as a factor in the area's sanitary sewer overflow challenge. The two storm basins containing both the LeBern and Dover pump stations were appraised as these basins contained the majority of flooded residents. The suspect basins were screened for adequate transport by delineating the drainage basin into sub-basins and examining the capacity of the arterial culverts transporting the storm water out of the basin Methodology Both the LeBern and Dover drainage basins were previously established from the City's storm map and transferred to the County topographical maps shown in Exhibits 1 and 5. Sub-basins were determined and common storm sewer collection points are indicated in the respective Exhibits. The Rational Method was used to determine the intensity/duration frequency storm flows for each sub-basin using 2-month, 6-month, 1-year, 2-year, 10-year, 20-year, 50-year and 100-year frequency storms. The results for each sub-basin may be found in Tables 2 and 3. The details of how the Rational Method was employed in this screening may be found in Appendix C. Culvert/Conduit sizes were examined using ODOT's CDSS program to determine the flow capacity of each conduit and the type of control the conduit is under as the flow through the culvert/conduit increased. The ODOT analysis may be found in Appendix B. Culvert/conduit capacity was then compared to the storm flow intensity/duration frequency curves in each sub basin to determine the capacity of the culvert/conduit relative to the storm frequency for the sub-basin. The screening revealed areas that do not have the capacity for reasonable storm flow using storm frequency as the deciding factor in areas considered for further study. LeBern Pump Station Drainage As may be observed from Table 2, the LeBern drainage area culvert capacities range between 2-year and 100-year storm capacities. For the most part, the LeBern storm system is designed adequately with the smaller subdivisions being designed for 10-year storms and increasing in frequency as the collection branches combine to arterials. Three (3) deviations were found to normal engineering design practices used for storm drain design in the LeBern drainage basin. Hall Ditch (sub-basin A2-6) has the capacity of a 2-year storm. Upon further examination it was found that this was intentional to include some detention time in the system. There were flooding complaints on Ambour Drive, which drains into Hall ditch, during the August 20, 2005 storm but it is believed that these flooding complaints stemmed from poor water transport to Hall ditch through the residents' back yards. This should be further investigated as a neighborhood problem but local observations should be collected to determine how the Hall ditch performed during the August 20th storm. Another area of low frequency storm flow was through the box culvert under Hawkins Road (sub-basin A3-2). The ODOT analysis indicated that the culvert had the capacity of a two (2) year storm. This should be further investigated although, when the review of flooding 20