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Capital Hill PUD, Tentative (14-020) March 3, 2017 As can be seen from the table, the areas discharging to sites 1 and 3 are quite comparable. Therefore, minimal detention to these discharge sites will be required. Through an iterative process, it was determined that multiple underground storage tanks would provide the most effective tool in reducing the peak discharge rates from the developed private road to Discharge Site 2. These tables, the HydroCAD analysis and the proposed stormwater design will be discussed in more detail in the following sections. 3.3 WATER QUALITY TREATMENT As noted previously, this report is intended to address the stormwater created by the improvements to the public roadway on Capital Drive, and the new private road circulating through the site. The primary stormwater management tools will be underground storage and mechanical treatment for the private roadway runoff due in part for the lack of available space for filtration facilities. The pervious pavement for the public improvements on Capital Drive is not analyzed in HydroCAD. According to the City of Eugene Stormwater Management Manual permeable pavements are impervious area reduction techniques and it is therefore assumed that this stormwater mitigation technique will match pre-existing conditions and provide treatment and match the exiting flow & discharge patterns. The private roadway improvements will have several mechanical treatment devices. The water quality runoff rates to discharge location 1 & 3 are 0.04 cfs and 0.02 cfs respectively. The runoff is hardly detained during the water quality storm so the peak treatment flow is relatively small and may be addressed with some kind of filter in the catch basin. The specific mechanical treatment will be determined when building permits are applied for and will have to approved by the City of Eugene. The basin area draining to Discharge location 2 likely would have a much larger peak flow rate if not for the layout of the multiple underground storage tanks located along the roadway. These detention facilities act as a pre-treatment, by collecting some solids and oils before releasing downstream. The peak discharge rate during the water quality storm, located just upstream of the final underground storage tank, is 0.08 cfs. It is at that location, just upstream of the Sth detention tank, that the mechanical treatment structure will be located. A treatment manhole or some other proprietary treatment technology may be selected and approved by the City of Eugene when applying for the building permit for the roadway and associated utility installation. 3.4 FLOW CONTROL AND DESTINATION Flow control devices or orifices were applied to the runoff directed to the Discharge locations 1 & 2 in the HydroCAD analysis. The basin area and peak runoff rate to the southwest discharge location is much less so only one underground storage tank is proposed. For design purposes the vault selected for this smaller basin is the Oldcastle Vault- 776. The pre-developed peak discharge rate for the 10-year storm to this location is 0.19 cfs. Under the post- developed analysis, through an iterative process, with the Vault-776, and an orifice size of 1.9 inches the peak discharge rate to this same location is 0.19 cfs. The peak under the post developed condition really isn't all that much higher without storage (only 0.23 cfs), but the reduced rate is certainly preferable. For the discharge to Location 2, it was determined through an iterative process that having various underground storage tanks collecting runoff every few hundred feet was more efficient than collecting all the runoff at one low point and then directing them to stepped/tiered underground storage facilities between lots 14 and 1 S. The second option required far more storage and would be more complicated to construct and install. The detention tank selected and used'for the multiple storage tank system design is the Oldcastle Vault-816. Branch Engineering, Inc. 4