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

Muller Engineering Company, Inc.
Happy Canyon Creek Major Drainageway Plan - DRAFT
SECTION 5 – ALTERNATIVE ANALYSIS
5.1 Alternative Development Process
Three classifications of alternatives were developed for Happy Canyon Creek: detention
improvements, channel improvements, and crossing improvements. Alternatives were evaluated on
a reach by reach basis, with consideration for the pairing of channel and detention improvements
based on the impact of additional detention on the degree of channel improvements necessary.
5.1.1 Detention Alternatives
Detention alternatives were developed with an aim toward neutralizing the impacts of development
within the watershed. While all jurisdictions within the watershed require detention and water
quality treatment for any new development, it is often provided on an on-site basis rather than a
regional basis. On-site facilities can be effective in reducing storm peaks and improving water
quality, however their size limits their impact on a watershed-wide level. Additionally, because
operation and maintenance is the responsibility of individual property owners for on-site facilities,
it is difficult to ensure that their effectiveness does not diminish with time. For these reasons,
UDFCD policy precludes consideration of the impact of on-site facilities in the master planning
process. Regional or sub-regional facilities provide greater impact due to their increased size and
are more reliable due to regularly scheduled maintenance. Regional facilities, therefore, are a key
solution to improving water quality and flood control from a master planning perspective. Private
landowners benefit from regional facilities as well, as they can eliminate the need for on-site
detention and water quality capture volume (WQCV).
Reduction of flows from the many small runoff events to storms slightly larger than the 2-year event
is considered of primary significance, as flows near the 2-year return period are considered
important channel-forming discharges and occur frequently enough to cause substantial impacts to
streams. Therefore, detention alternatives aim to reduce peak flow rates for all storm events to
historic levels. Currently developed areas were examined for adequacy of existing regional
detention; retrofits and new facilities were explored to address inadequacies. Planned regional
detention in areas of future development was reviewed, and additions or modifications were
considered to improve the effectiveness of future detention.
5.1.2 Channel Alternatives
Channel improvement alternatives were developed to address areas of concern within the channel,
including degraded or unstable reaches, threats to existing infrastructure, limited channel
conveyance, and floodplain concerns. Improvements targeted varying levels of channel stabilization,
from a traditional fully stabilized approach to a softer, more strategic approach. Hydraulic
evaluation of channel capacity was performed as necessary, and was based on the peak flow rates
developed during the hydrologic analysis.
5.1.3 Crossing Improvements
Improvements to roadway crossings with less than 100-year capacity are included in the alternative
analysis. New crossings were assumed to be the same type as the existing crossing (e.g. culvert or
bridge), with the exception of Dogwood Avenue. Douglas County provided guidance on the two
desired alternatives for Dogwood: replacement with a steel truss bridge that will span the 100-year
floodplain, and permanent closure of Dogwood to allow removal of the crossing.
5.2 Criteria and Constraints
5.2.1 Detention Alternatives
Detention improvements aim to provide Full Spectrum Detention within the watershed where
possible. Full Spectrum Detention combines 100-year detention with the capture of Excess Urban
Runoff Volume (EURV), or the additional runoff over historic levels that results from urban
development. By capturing this excess runoff and releasing it slowly, peak flows from a range of
events from small, frequently occurring storms through large, extreme flood events are maintained
near pre-development levels. As with WQCV facilities, EURV facilities capture and treat the “first
flush,” or the initial runoff from any storm event which will carry a substantial pollutant load;
however, EURV facilities have approximately double the treatment capacity of WQCV facilities.
Douglas County, the City of Lone Tree, and Arapahoe County have adopted Full Spectrum Detention
as the minimum detention requirement for all new developments and the Town of Parker is
currently updating its criteria to require Full Spectrum Detention.
5.2.2 Channel Alternatives
Channel improvements consider the criteria presented in the Urban Storm Drainage Criteria Manual,
the Douglas County Storm Drainage Design and Technical Criteria Manual, the Town of Parker Storm
Drainage and Environmental Criteria Manual, and the Arapahoe County Stormwater Management
Manual. These criteria include recommendations on channel slope and drop structure height, as well
as the overall approach to channel restoration. All jurisdictions favor natural channel preservation
and restoration to engineered channels. This approach seeks to maintain a wide, shallow floodplain
and make use of healthy riparian vegetation to help stabilize the channel, reduce stream velocities,
and improve water quality.
5.2.3 Crossing Alternatives
Overtopping of any roadway at a major drainageway crossing in the 100-year event is prohibited
under the criteria presented in both Douglas County’s Storm Drainage Design and Technical Criteria
Manual and Arapahoe County’s Stormwater Management Manual. Proposed crossing improvements
provide 100-year capacity according to criteria, including freeboard at bridge crossings.
August 2013 Page 5-1 Alternative Analysis