HomeMy WebLinkAboutDRB130286 Storm Water LetterMARCIN ENGINEERING LLC
August 15, 2013
Joe Batcheller
Town Planner
Community Development
Vail, Colorado 81657
RE: Mitigation of Storm Water for Vail Racquet Club Townhomes
Dear Joe:
Marcin Engineering was requested to review the existing site improvement area and to provide storm
water mitigation recommendations along with grading and drainage improvements for the Vail Racquet
Club Townhomes.
The existing previously developed site runoff area is approximately 0.4 acres, and consists of
approximately 100% impervious area (0.4 acres). With the current design and existing storm system the
majority of the runoff is collected by an existing 2' diameter storm inlet in the center of the parking lot
and piped directly and discharged on the bank of Gore Creek to the west of the parking lot. Currently, the
direct discharge has minimal water quality treatment prior to entering the receiving stream. This direct
discharge along the bank also causes bank erosion. See attached exhibit.
Using the rational method for small site drainage basins, the runoff flow was calculated for 2 -year and 5-
year storms. The runoff coefficient was assumed to be .95, which is the maximum for both the roofs and
asphalt surfaces on site that contribute to the runoff in the improvement area discussed according to
Water- Resources Engineering. The rain fall intensities were taken from the Town of Vail Code CH 13-
10-9 Design Criteria. The design team chose to analyze small storm events such as the 2 -year and 5 -year
storms due to the fact that larger storm events will have increased runoff amounts and will help dilute the
pollutant concentration rather than small events where the pollutant concentration is higher in the runoff.
The area was assumed to be 0.4 acres, shown in exhibit attached. The results were as follows:
2 -year Storm
CIA =Q
C =0.95
I =1 in/hr
A =0.4 acres
95 * 1 *.4 = 0.38 cfs
5 -year Storm
CIA =Q
C =0.95
I =1.4 in/hr
A =0.4 acres
95 *1.4 *.4 = 0.53 cfs
After analyzing the existing site conditions and constraints, Marcin Engineering recommends the most
significant BMP (Best Management Practice) to achieve storm water quality control measures for this
application will be regarding and reinstalling new asphalt that sheet flows across the improvement area
and enters into a rip - rap/bio swale(see proposed typical detail). This design will allow for the significant
removal of sediments and pollutants from this area of concern by naturally flowing the runoff over rock
cobble and grass landscaping to help encourage infiltration of storm water into the groundwater table. The
design incorporates a proposed drainage rip -rap swale leading into a natural grass bio Swale to the south
of the parking lot, which will collect the runoff that is not infiltrated on site and allow for better water
quality treatment prior to entering into Gore Creek. Other implementations that can be included would be
to install a small dry retention ponding area integrated with the drainage Swale into the landscaping area
for additional runoff volume catchment.
PO Box 1062, Avon, CO 81620 (970) 748 -0274
Marcin Engineering believes these improvements will ultimately improve the storm water quality for this
application compared to the existing conditions. If you have any questions or concerns feel free to contact
me at 970 - 748 -0274.
Sincerely,
MARCIN ENGINEERING LLC
Thomas S. Marcin, PE, PLS
PO Box 1062, Avon, CO 81620 (970) 748 -0274