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

Muller Engineering Company, Inc.
Happy Canyon Creek Major Drainageway Plan - DRAFT
SECTION 6 – RECOMMENDED PLAN
6.1 Plan Description
The recommended plan, shown in Figures 6-1 through 6-3, consists of the following elements:
1. Detention Alternative C. The maximum detention approach is recommended because it
most effectively reduces peak runoff during frequent and large, infrequent storms to levels
similar to historic conditions. This in turn reduces stresses imposed on Happy Canyon Creek
during runoff events and allows a more modest program of stabilization improvements to be
implemented. The maximum detention alternative also provides the most water quality
treatment via extended detention, and significantly reduces 100-year peaks through the
flood-prone Grandview Estates. EURV and FSD detention facilities are recommended to be
located offline from mainstem Happy Canyon Creek to attenuate increased peak flows prior
to entry into the creek and to avoid dramatic imbalances in sediment movement and stream
equilibrium. Online peak-shaving ponds within RidgeGate are recommended to maximize
100-year detention, minimize impact to developable area, and avoid “stacking” the
development’s peaks with the main channel peaks.
2. Reclamation of degraded reaches. It is recommended that the portions of Happy Canyon
Creek that have experienced severe degradation be rehabilitated as soon as possible. This
includes the eroded length of channel downstream of CPNMD Pond #12 in Reaches 1 and 2
and the I-25 Corridor (Reach 3). Risks to an exposed high pressure gas line in Reach 1 and to
I-25 in Reach 3 make these repairs an immediate priority; in addition, if the degradation
continues, the damage will grow worse and the reclamation improvements will need to be
more extensive and costly. To help diminish a cycle of rapid downstream degradation, it is
recommended that the channel be filled slightly steeper than the predicted equilibrium slope
and the sides of the active channel be left relatively soft to maintain a moderate sediment
supply in these reaches. It is recommended that this approach be refined based on further
evaluations, including quantitative sediment equilibrium analyses over the anticipated range
of flow conditions. With the implementation of Detention Alternative C and the resultant
reduction in peak flows during frequent events, Channel Alternative D2 can be applied to
these reaches.
3. Timed, incremental approach to keep reaches stable. One of the most important benefits
of this major drainageway plan will be to coordinate a strategy to protect the 8 miles of
Happy Canyon Creek that are currently in a relatively stable condition (several reaches are
already severely degraded and require more immediate, substantial stabilization measures).
If not managed well, these currently stable reaches could deteriorate quickly. The
recommended approach has a number of characteristics. It is:
• Incremental. Reclamation improvements are tailored to meet the current and nearterm
needs of the stream rather than totally re-constructing channels for assumed
long-term conditions. Improvements seek to be small and surgical, reinforcing weak
points but preserving stable areas of the floodplain.
• Timed. Improvements are to be implemented at “just the right time” – proactively
addressing “hot spots” soon after early signs of instability are observed rather than in
a delayed manner to repair major degradation after it has already occurred.
• Distributed. Since Happy Canyon Creek is a system that responds to actions in
individual sub-reaches, improvements are intended to be implemented in a
coordinated approach and distributed throughout the study reach based on the
condition and needs of the creek in any given year. Otherwise, too many dollars may
be spent on completely stabilizing one reach to a level that may not be needed for
years into the future, while others are left unprotected and vulnerable to impact with
the potential to create a downstream ripple effect of impacts.
• Progressive. Since Happy Canyon Creek will respond to changes in the watershed
over time, there may be a need for follow-up phases of work within the same reaches,
adding grade control as the stream adjusts to a flatter equilibrium slope or addressing
new “hot spots” of bed or bank erosion.
• Managed. Annual stream inventories are recommended to observe signs of instability
that need to be addressed and to assess how initial reinforcement improvements are
functioning, how revegetation establishment is progressing, and whether any signs of
additional instability are noticed in reaches addressed in initial improvements. The
results of the monitoring are to be used by UDFCD and the local jurisdictions to plan
for subsequent improvement measures.
The four alternatives identified for the relatively stable reaches of Happy Canyon Creek
represent a progression of channel improvements that may be necessary both over time and
with increasing stress from developed runoff conditions. Potential improvements that may
be necessary could start with the more modest stabilization measures of Alternative S4 (or
even less) and move through Alternatives S3, S2, and perhaps ultimately to the measures of
Alternative S1 if time and stresses call for more and more stabilization.
If the stresses imposed on the creek are managed well – with the maximum Full-Spectrum
Detention Alternative C and other recommended measures – it is possible that lesser
stabilization measures will suffice to allow these reaches to maintain a relatively steep
equilibrium slope even over the long term. In this case, Alternative S4 may represent all the
channel stabilization work that is ever needed. If additional measures become necessary,
August 2013 Page 6-1 Alternative Analysis