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High Line Canal Operational Hydrologic M

High Line Canal Operational Modeling Update 

Douglas County, CO

JR Engineering prepared an update to the overall High Line Canal Operational Hydrologic Model for use in the Urban Drainage and Flood Control District (UDFCD) High Line Canal Master Plan.  The High Line Canal is an irrigation ditch that is owned and operated by Denver Water and runs for 67 miles through multiple municipalities.  Much of the High Line Canal has adjacent trails and greenbelts, and several watersheds pass through the High Line Canal along its length.  Denver Water, in conjunction with UDFCD and its local partners, is permitting stormwater runoff to enter the High Line Canal on a case-by-case basis as they investigate plans to cease their use of the canal for irrigation deliveries.

The original Operational Model was prepared by UDFCD’s consultant as part of the Master Plan in 2017.  The Operational Model was intended to be amended and re-evaluated as developments propose discharging to the High Line Canal and as improvements to the High Line Canal are proposed.  The hydrologic modeling for the Operational Model was prepared in CUHP/SWMM formats.  The purpose of the Operational Model Update by JR Engineering was to examine the feasibility of routing developed conditions stormwater runoff from the proposed Solstice development into the High Line Canal, and to then formally update the Operational Model to include ultimate conditions development throughout the reach.

JR Engineering obtained the Operational Model from UDFCD and modified the sub-catchment delineation and hydrologic parameters to reflect the Solstice development as well as the Sterling Ranch development (by others).  The analysis involved a fully developed conditions analysis for existing, proposed, and future development within the watershed and coordination with other developers’ consultants to ensure an accurate representation of the ultimate build-out conditions hydrology.  Future conditions drainage patterns and planned conveyance infrastructure designed by others were integrated into the Operational Model Update.  Sizing and release rates from proposed and future water quality/detention ponds, both on- and off-site, were included in the modeling effort.  The Operational Model established baseline detention requirements for future sub-regional stormwater detention facilities.  At the upstream end of the reach, an inflow hydrograph was inserted, which represented the 100-year inflows from the adjacent drainageways and direct discharges from adjacent properties, and was provided by UDFCD’s consultant.

The Operational Model featured several locations where 100-year flows in the High Line Canal would potentially spill into the Solstice development and the adjacent Chatfield State Park.  The JR Engineering Operational Model Update analyzed the effects of creating one localized emergency spillway location to Tributary B, an existing drainageway tributary to Chatfield Reservoir.  The modeling prompted the design of an overflow inlet box located within the High Line Canal for high flows, and an armored spillway above the inlet box to pass flood-stage discharges to the existing drainageway to reduce the potential for flooding to existing and proposed residences below the Canal.  These facilities were fully designed and then incorporated into the Operational Model Update.

The High Line Canal reach adjacent to the Solstice development was designed with side slope modifications and landscape improvements to benefit the linear park aesthetic of the High Line Canal corridor.  The existing side slopes were designed to be laid back as compared to the steep banks seen in the existing Canal to promote safety and to turn the Canal into a water feature for the community.  These physical changes were integrated into the Operational Modeling Update, which had the effect of creating additional storage volume within the canal corridor and attenuating the routed peak discharge as the inflow hydrograph spread out and occupied the canal’s increased cross-sectional area.  The Operational Model Update approval process involved a detention analysis to determine the impacts on the timing of the developed peak discharge in Plum Creek when developed flows are routed in the High Line Canal to Plum Creek.
The Operational Model Update by
JR Engineering was approved by UDFCD, Denver Water, and Douglas County.