Open Space Advisory Board Agenda and Packet 2011 08 11

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Preliminary PUD and Plat July 7, 2011 th 96 and Dillon stopped at the railroad tracks, despite the existing historic patterns indicating otherwise, existing culverts carried water under the tracks, and the track beds are somewhat porous as well. Currently, there is a 12” pipe under the railroad tracks at this low point that is collected in the CTC system and conveyed with their drainage to the east. Also, the railroad track embankment is not impervious and drainage would migrate through the embankment and into the CTC property. There is also a private irrigation ditch lateral group that has indicated the surface runoff that is tributary to the railroad tracks continues underground and emerges in existing laterals east of the CTC. The CTC essentially ignored the offsite basin in its entirety at any level or capacity. The City of Louisville enacted an Outfall System Plan in 1982. This plan acknowledged the runoff from this area and that the CTC did not include a conveyance through its property and in the historical route. That OSP anticipated development on the lands west of these parcels with a reduced runoff rate based on standard mitigation practices. That property is now open space, so the historic runoff will be the prevailing design parameter and the historic runoff is typically at a greater peak and lesser volume than the mitigated runoff. The OSP indicated acceptance of the offsite runoff and conveyance in a pipe through these parcels and to the intersection of Dillon Road and the BNSF, which is the southeast corner of the Church ownership. th In the mid 2000’s, the City reconstructed 96 Street and Dillon Road along the frontage of this th site. As part of that project, the City designed and installed drainage pipes under 96 Street that have in part dictated the design of these parcels drainage outfall and collection systems. There are now two needed drainages that impact this site as opposed to the one in the OSP. The second th outfall occurs at the intersection of 96 and Dillon Road where there are now buried pipes for future connection or extension. The development will allow for the acceptance and conveyance th of the runoff from these two locations through the parcels and to the intersection of 96 and Dillon Road. The main central outfall will route under the main access Road B in a large pipe and will be routed through the site drainage mitigation pond. The other outfall along Dillon Road will be an open channel, except where it is crossed by the Church site access from Dillon Road. Since this channel only conveys offsite runoff and is not necessary or required for the development, we are proposing that this area, which will be in an Outlot, be dedicated to the City and counted as land dedication. The downstream drainage system now starts at the recent phase of the CTC as an open channel and small pipe. The drainage system does not extend any further to the west from that point at this time. In order to meet the City requirements, a runoff mitigation pond will be introduced into the project in the area of open fields on the Church’s parcel and north of the southeast corner of the th project. This pond will accommodate the site development, the runoff from the site side of 96 Street and the northern offsite basin. Until the outfall system becomes available, the drainage from this pond will be mitigated, minor storm events conveyed to the drainage outfall on the south side of Dillon Road, and major storms and bypass flows released back into the railroad right of way as it historically has. Page 7 of 10