HomeMy WebLinkAbout18 FloodplainsThe Lakes at Valley West Subdivision – Phases 1 and 2
Preliminary Plat Application
Floodplains
Baxter Creek:
During the initial stages of the Valley West Planned Unit Development, a detailed drainage plan
was completed that addressed/analyzed the major drainage courses running through the Valley
West property. One of those drainages, Baxter Creek, flows through a portion of the “Lakes”
subdivision currently under review as part of this Preliminary Plat submittal. This portion flows
through the far eastern edge of the proposed subdivision in an area that is planned to be
dedicated parkland. The 100-year high water limits were analyzed in the original PUD report
and were shown on the original Final Plat for Valley West Subdivision. Copies of the
applicable portions of the report are included within this Preliminary Plat submittal. The
100-year high water limits of Baxter Creek are also shown on the attached Exhibit A for your
reference.
The Durston Road culvert was also analyzed in the PUD report, providing an estimated capacity
of 150 cfs through the 54” CMP culvert existing at that time. That culvert has since been
replaced with a 54”x88” reinforced concrete arch pipe. The capacity of the new culvert, based
on surveyed inverts, is approximately 300 cfs before overtopping of the roadway. The culvert
itself can accommodate approximately 210 cfs without any headwater over the culvert (see
attached culvert calculations). The estimated 100-year flow at the Durston Road culvert is
114 cfs.
Based on the findings of the PUD report and existing trail elevations along the west bank of
Baxter Creek, the “Lakes” subdivision will not be impacted by the flooding of Baxter Creek
during the 100-year storm event. Please refer to the Baxter Creek Flood Analysis immediately
following Exhibit A.
Aajker Creek:
Aajker Creek was not part of the original Valley West PUD submittal, so a flood hazard
evaluation of this stream is attached herein, in support of “The Lakes at Valley West”
subdivision. The majority of runoff in Aajker Creek comes from areas south of Huffine Lane;
the 100-year flow was conservatively estimated to match the capacity of the two culverts under
Huffine, a total of 191 cfs.
USGS topography and project-specific (i.e., surveyed) topography between Huffine Lane and
Durston Road both indicate drainage generally flows to the north-northwest. This would direct
The Lakes at Valley West Subdivision – Phases 1 and 2
Preliminary Plat Application
any streambank overflow to the west side of the stream, and away from the project site.
Surveyed cross-sections and existing, surveyed culvert overflow points also indicate overflow
would occur to the west (offsite) rather than to the east, and would not re-enter Aajker Creek
downstream of the point(s) of overflow. The attached report and calculations estimate the
100-year peak flow within the Aajker Creek stream channel along the project’s west boundary to
be 54.4 cfs. The 100-year high water limits of Aajker Creek are shown on the attached Exhibit A
for your reference.
Based on the findings of this analysis, the “Lakes” subdivision will not be impacted by the
flooding of Aajker Creek during the 100-year storm event. Please refer to the Aajker Creek
Flood Analysis immediately following the Baxter Creek Flood Analysis.