HomeMy WebLinkAboutWPO200900021 Review Comments Stormwater Management Plan 2009-05-29ALg�,��
�'IRGINZ�
COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project: SDP - 2009 - 00036, Monticello High School Minor Amendment
WPO- 2009 - 00021, Monticello High School SWM and ESC Plans
Plan preparer: Skip Notte, Dewberry and Davis, Inc.
Owner or rep.: Joe Werres, ACPS Office of Facilities Development
Date received: (Rev. 1) 26 May 2009 (plan signed 22 May 2009)
4 May 2009 (plan signed 1 May 2009)
Date of Comment: (Rev. 1) 29 May 2009
14 May 2009
Engineer: Phil Custer
The minor site amendment, SWM, and ESC plans for the Monticello High School Turf Replacement
project, received on 26 May 2009, have been reviewed. The plans cannot be approved as submitted and
will require the following changes /corrections prior to final approval.
A. Minor Amendment comments (SDP- 2009 - 00036):
4. The tops of the 6 drainage structures appear to be proposed at an elevation well below the
grade of the turf, assuming 1.5% slopes from the track and crown of the field.
(Rev. 1) The detail provided by Joe Werres drawn by General Sports Venue has been
approved and will be stapled to the site plan set.
B. Stormwater Management comments (WPO- 2009 - 00021):
1. For water quality analyses, the County uses a composition spreadsheet based on the Simple
Method to assess whether water quality measures are needed and what removal rate facilities
should be designed for. My analysis of the watershed delineated by the applicant indicates
that a facility with a 50% removal rate is required. The water quality volume is 4145cf.
(Rev. 1) At the meeting held on May I8 'h, it was our understanding that the applicant
offered to perform water quality sampling as well as flow metering to address water quality
concerns. Please provide the following note on sheets CIA and CIIB:
As a condition of WPO plan approval for this project, the applicant commits to flow
metering and water quality monitoring. A flow meter will be installed on one of the
loin. underdrain outlet pipes at a midfield stormwater structure. The frequency and
location of the water quality sampling should be determined by a qualified individual
with experience in water quality analysis. A report shall be submitted to the
Community Development office every three months after the construction of the field
for a period of one year. The report must include all raw data collected to date and a
summary prepared by a qualified individual. The report must also include rainfall data
from the nearest NOAA rain gauge.
The applicant has indicated that water quality treatment will be provided through infiltration
practices and extended detention. However, the blue book notes that infiltration practices
should only be designed when the infiltration rate of the in -situ soil is between 0.52 in/hr and
8.0 in/hr. The geotechnical field tests submitted by the applicant show that only one boring
possesses a rate in this range. So, it appears infiltration is not a viable option. Also, it does
not appear a drawdown calculation for an extended detention basin was provided. [VSMH 5-
6.2] Engineering review recommends designing a biofilter meeting the current regulations
(for a 50% removal rate, a footprint of 2.5% of the equivalent impervious area would be
needed). There appears to be sufficient room for a 2500sf biofilter at the northwest corner of
the track.
Engineering would permit the facility to be constructed a year after the installation of the field
if a flow meter was installed on the outlet pipe and a report was prepared evaluating the
hydrologic characteristics of the turf field. If the data found from the flow metering shows a
significant reduction in runoff, the construction of the biofilter may not be necessary (to be
determined by the engineering review group). This would also help provide solid background
information for the county review department when future grass to artificial turf field
enhancement projects are submitted in the years to follow.
(Rev. 1) See previous comment.
C. Erosion and Sediment Control comments (WPO- 2009 - 00021):
All comments have been addressed.
Please contact me at (434)296 -5832 ext. 3072 should you have any questions.
File: E2_mia, esc, swm_PBC_Monticello High Turf Replacement.doc
ALg�,��
�'IRGINZ�
COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project: SDP - 2009 - 00036, Monticello High School Minor Amendment
WPO- 2009 - 00021, Monticello High School SWM and ESC Plans
Plan preparer: Skip Notte, Dewberry and Davis, Inc.
Owner or rep.: Joe Werres, ACPS Office of Facilities Development
Date received: 4 May 2009 (plan signed 1 May 2009)
Date of Comment: 14 May 2009
Engineer: Phil Custer
The minor site amendment, SWM, and ESC plans for the Monticello High School Turf Replacement
project, received on 4 May 2009, have been reviewed. The plans cannot be approved as submitted and
will require the following changes /corrections prior to final approval.
A. Minor Amendment comments (SDP- 2009 - 00036):
1. Please provide the date and source of the topography.
2. Please provide in the set the County's General Construction Notes which can be found in the
latest edition of the design manual, available online.
3. Are the 6 drainage structures around the field going to be converted to manholes? If so, please
specify the tops as VDOT Standard MH -1.
4. The tops of the 6 drainage structures appear to be proposed at an elevation well below the
grade of the turf, assuming 1.5% slopes from the track and crown of the field.
5. Please provide greater detail in the section showing the trench and underdrain area. For
instance: the slope of the compacted earth soil under the gravel layer and elevations of the
bottom of trench and the beginning of the 1.5% slope.
B. Stormwater Management comments (WPO- 2009 - 00021):
1. For water quality analyses, the County uses a composition spreadsheet based on the Simple
Method to assess whether water quality measures are needed and what removal rate facilities
should be designed for. My analysis of the watershed delineated by the applicant indicates
that a facility with a 50% removal rate is required. The water quality volume is 4145cf.
2. The applicant has indicated that water quality treatment will be provided through infiltration
practices and extended detention. However, the blue book notes that infiltration practices
should only be designed when the infiltration rate of the in -situ soil is between 0.52 in/hr and
8.0 in/hr. The geotechnical field tests submitted by the applicant show that only one boring
possesses a rate in this range. So, it appears infiltration is not a viable option. Also, it does
not appear a drawdown calculation for an extended detention basin was provided. [VSMH 5-
6.2] Engineering review recommends designing a biofilter meeting the current regulations
(for a 50% removal rate, a footprint of 2.5% of the equivalent impervious area would be
needed). There appears to be sufficient room for a 2500sf biofilter at the northwest corner of
the track.
Engineering would permit the facility to be constructed a year after the installation of the
field if a flow meter was installed on the outlet pipe and a report was prepared evaluating the
hydrologic characteristics of the turf field. If the data found from the flow metering shows a
significant reduction in runoff, the construction of the biofilter may not be necessary (to be
determined by the engineering review group). This would also help provide solid background
information for the county review department when future grass to artificial turf field
enhancement projects are submitted in the years to follow.
3. It is critical to the project meeting the detention requirements that the elevations used in the
routing are constructed as proposed on site. Typically, when a pipe is laid in the trench at a
certain slope, we find that the trench is also sloped at that percentage because it is easier for a
contractor to build. Please provide more notations through out the plan and details that the
bottom of the trench is to remain flat at the 485 elevation.
C. Erosion and Sediment Control comments (WPO- 2009 - 00021):
1. Please provide in the set the County's General Construction Notes for Erosion and Sediment
Control Plans which can be found in the latest edition of the design manual, available online.
2. Please provide Dust Control symbols (DC) on the plan as well as a location for a soil
stockpile.
3. What is the reason for phased construction of the turf? Is the goal to have half the field
available for use during construction? If so, please provide another CE and access path for
Phase I. If not, please remove the phasing lines.
4. Please stipulate in the construction sequence that the trench and underdrain at the perimeter
of the site should only be installed after the majority of the field is graded and graveled. If
the trench and underdrain are installed in the first stages of construction prior to the majority
of the earthwork, a significant amount of sediment will likely choke up the soil (limiting
infiltration) and be lost through the pipe system.
5. Please provide a soil boundary map with labels in the plan set.
6. Please provide an adequate channel analysis of the downstream storm sewer network to the
pond forebay. (MS -19)
Please contact me at (434)296 -5832 ext. 3072 should you have any questions.
File: El_mia, esc, swm_PBC_Monticello High Turf Replacement.doc