SUPPLEMENTAL MATERIALS (6-19-17)

Capital Hill PUD, Tentative (14-020) June 19, 2017 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 5th 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 15. 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. By having all the storage facilities fall within the footprint of the roadway, or just outside the roadway, the stormwater improvements are far less likely to encroach on the existing vegetation. The existing condition draining to discharge location 2 was assumed to be a 38-foot-wide section of sheet flow draining from the top of the hill, located about 380 feet away. The basin area of roughly 14,900 square feet created a peak discharge rate during the 10-year design storm of 0.16 cubic feet per second (cfs). With the current layout proposed for the private roadway, the basin area directing runoff to the 38' level spreader increases dramatically (3x) up to 45,520 square feet. It is for that reason that several tanks were selected, and multiple level spreaders are proposed. By matching the peak discharge rate of 0.16 cfs / 38 linear feet of level spreader, the post developed improvements match the pre-developed discharge rates and runoff conditions. The discharge to location 3 under the post developed condition shows a peak discharge rate of 0.11 cfs, where the existing condition was 0.10 cfs. This difference in peak discharge rate is minimal, and the discharge will be into a bubler that will runoff onto pervious pavement. The Branch Engineering, Inc. 5