PDF (53.8 MB) - Urban Drainage and Flood Control District

Muller Engineering Company, Inc.
Happy Canyon Creek Major Drainageway Plan - DRAFT
Reach 3, located adjacent to I-25, is another severely degraded reach. Though the floodplain width
through this reach has always been limited due to the steep topography, construction of I-25 further
constricted the Happy Canyon Creek floodplain and reduced the stream length by eliminating
meanders in the channel. The constricted floodplain combined with increased flow rates from
upstream development and from I-25 itself have led to channel instability in this reach, with the
potential to threaten the integrity of I-25. There are numerous instances of slope paving failure, and
a sheet pile failure where the Oak Ridge tributary crosses below the interstate. Ongoing bank scour
is evident through recent bank collapses. Happy Canyon makes a 90-degree turn to cross under the
I-25 bridge; the channel invert has scoured several feet along the north (outer) side of the bridge
crossing, potentially threatening the bridge abutment.
A second exposed utility is located just upstream of
Birch Avenue in Grandview Estates in an area of
localized channel degradation. The type and size of
the utility are unknown. Channel degradation in
this area is also apparent at the pier for the Birch
Avenue bridge; the paint line is 2-3 feet above the
channel invert. This degradation may pose a hazard
to the bridge structure.
Flood hazards and problem areas are identified in
Figures C-2 through C-4.
4.3 Previous Analyses
Paint line indicates possible degradation below
Birch Avenue bridge
Previous hydraulic analyses along Happy Canyon Creek include the 1977 FHAD, the 1993 OSP, and
the 2001 analysis and preliminary design of channel improvements within Grandview Estates,
completed by HDR.
Failed slope paving along Happy Canyon
Creek below I-25 barrier wall.
Channel scour below I-25 bridge; original
invert to right
Active cattle grazing along Reaches 4 and 6 in the City of Lone Tree poses a water quality hazard to
the watershed as a result of cattle manure in the stream bed and overbanks. During field
investigations, manure was noticed as far north as E-470, two miles beyond the grazing area. It is
likely that, during large storm events, manure is
washed down Happy Canyon Creek into Cherry
Creek, eventually reaching Cherry Creek Reservoir.
With a single cow producing 12 tons of manure per
year, this is potentially a significant nutrient
source for the Reservoir. In addition to the water
quality impacts, the cattle threaten the health of
the stream by trampling the banks, killing off the
wetland and riparian vegetation adjacent to the
channel and leaving the banks vulnerable to
Cattle near Happy Canyon Creek in Lone Tree
erosion.
The FHAD identified few flood hazards along Happy Canyon Creek, as most of the watershed was
undeveloped at the time. Crossings at Dogwood Avenue, Birch Avenue, and old West Parker Road
(now Lincoln Avenue) within Grandview Estates were undersized at the time; bridges have since
been installed at Birch and Lincoln. It appears the Dogwood crossing has also been replaced, though
not with sufficient capacity; the FHAD identifies a triple culvert crossing consisiting of one 36” CMP
and two 36”x60” CMP arches while the current crossing is a triple 48” CMP. Existing development at
the time was outside the floodplain limits.
The 1993 OSP identified inadequately sized road crossings at Clydesdale Road, Lincoln Avenue, and
Dogwood Avenue. As mentioned above, the Lincoln Avenue crossing has since been replaced with a
bridge. West Parker Road was found to be adequately sized based on a 100-year peak of 3800 cfs;
the increased peak of 6200 cfs in this study exceeds the structure capacity. Floodplain delineation
was not performed for the 1993 OSP; structure flooding was based on the floodplain limits
established in the 1977 FHAD, which was superimposed on the OSP mapping. According to the text,
structure flooding was limited to barns and other outbuildings within Grandview Estates, though
review of the OSP plan drawings shows three homes within the boundary.
The HDR study built upon the findings of the 1993 OSP with regards to flood hazards; additional
problem areas within Grandview Estates were noted including areas of bank instability and channel
degradation. Multiple phases of construction were identified and prioritized; several of these
August 2013 Page 4-5 Alternative Analysis