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Solstice Offsite Drainage Outfall

Solstice Offsite Drainage Outfall 

Douglas County, CO

The Solstice development is a single-family residential development in Douglas County, located just south of Chatfield Reservoir.  Storm drainage from the Solstice site was split between two existing un-named drainageways, both directly tributary to Chatfield Reservoir The main drainageway crossing the Solstice development was termed “Drainageway A” and consisted of over 180 acres of tributary area, which was carried through Chatfield State park where the drainageway was unimproved with the exception of a culvert crossing at the State Park Road with some existing channelization improvements.  Changing the existing drainage patterns through the Solstice site was not possible, so JR Engineering coordinated with Colorado Parks and Wildlife and the U.S. Army Corps of Engineers to create a drainage outfall through the State Park to Chatfield Reservoir.


JR Engineering met with the U.S. Army Corps of Engineers onsite and collaboratively decided on an outfall alignment as well as environmentally sensitive areas to avoid. To minimize the effects of development from the Solstice site, JR Engineering designed a Full-Spectrum detention pond at the downstream property line.  The pond was designed to over-detain stormwater releases down to a minimum practicable release, and in doing so the outfall pipe was able to be downsized to an 18” pipe from an original 42” required size based on historical peak flows.  The outfall pipe was roughly 1,200 linear feet in length, with the remaining 600 linear feet comprised of open channel and two culvert crossings.


The project required a drainage easement through Federal lands, which was accomplished via the Section 408 process. The 408 process also involved a U.S. Fish and Wildlife consultation to evaluate the effects of the project on the Preble’s Meadow Jumping Mouse habitat and nesting birds within the State Park.


JR Engineering prepared an overall basin hydrologic model using EPA SWMM, which took into account the over-detention release from the Solstice onsite detention pond.  JR modeled the proposed storm sewer outfall in StormCAD and the open channel segment in HEC-RAS using the calculated 100-year peak stormflows, and evaluated the replacement of the Drainageway A culvert under the State Park road, which was undersized for the tributary stormwater runoff.  JR Engineering designed the culvert replacement as well as a low-flow trail crossing for the Chatfield Reservoir perimeter trail.
JR Engineering  prepared final construction plans and a final design report that were reviewed and approved by Colorado Parks and Wildlife and the Omaha District of the U.S. Army Corps of Engineers.