HomeMy WebLinkAboutSDP200600059 Assessment - Environmental Minor Amendment 2006-06-06 Mill Creek Office, Site Plan Amendment
County of Albemarle, Virginia
Erosion and Sediment Control Narrative
6 June 2006
Project Description
This site will consist of office and storage facilities, including an outdoor storage area. Drainage of the site is through
proposed storm drain facilities. Onsite water quality facilities will be provided. Offsite stormwater detention was provided
with the original Mill Creek Industrial Park.
Existing Site Conditions
The site is primarily wooded, with some open grass areas.
Adjacent Areas
The site is located within the Mill Creek Industrial Park. The parcel to the north is vacant. The lot to the south is
developed. To the west is Stoney Ridge Road; to the east is Southern Parkway and Five Springs Road.
Offsite Areas
No offsite areas are affected, with the exception of the entrance and connection to existing water line across Stoney Ridge
Road.
Soils
Soils on the site consist primarily of 13D— Catoctin Very Stony Silt Loam. (SCS Soil Survey of Albemarle County, Virginia)
Critical Erosion Areas
No significant critical erosion areas exist on the site.
Erosion and Sediment Control Measures
Unless otherwise indicated, all erosion and sediment control practices will be constructed and maintained according to
minimum standards and specifications of the Virginia Erosion and Sediment Control Handbook (1992).
Minimum Standards
Installation of all erosion and sediment control measures shall conform to the 1992 Virginia Erosion and Sediment Control
Handbook. Critical measures required on this project are summarized below:
1. Permanent or temporary soil stabilization shall be applied to denuded areas within seven days after final grade is
reached on any portion of the site. Temporary soil stabilization shall be applied within seven days to denuded areas
that may not be at final grade but will remain dormant for longer than 30 days. Permanent stabilization shall be
applied to areas that are to be left dormant for more than one year.
2. During construction of the project, soil stockpiles and borrow areas shall be stabilized or protected with sediment
trapping measures. The applicant is responsible for the temporary protection and permanent stabilization of all soil
stockpiles on site as well as borrow areas and soil intentionally transported from the project site.
3. A permanent vegetative cover shall be established on denuded areas not otherwise permanently stabilized.
Permanent vegetation shall not be considered established until a ground cover is achieved that is uniform, mature
enough to survive and will inhibit erosion.
4. Sediment basins and traps, perimeter dikes, sediment barriers and other measures intended to trap sediment shall
be constructed as a first step in any land-disturbing activity and shall be made functional before upslope land
disturbance takes place.
5. Stabilization measures shall be applied to earthen structures such as dams, dikes and diversions immediately after
installation.
6. Sediment traps and sediment basins shall be designed and constructed based upon the total drainage area to be
served by the trap or basin.
Mill ...�zk Office, Site Plan Amendment
Erosion and Sediment Control Narrative
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6 June 2006
7. Cut and fill slopes shall be designed and constructed in a manner that will minimize erosion. Slopes that are found
to be eroding excessively within one year of permanent stabilization shall be provided with additional slope
stabilizing measures until the problem is corrected.
8. Concentrated runoff shall not flow down cut or fill slopes unless contained within an adequate temporary or
permanent channel, flume or slope drain structure.
9. Whenever water seeps from a slope face, adequate drainage or other protection shall be provided.
10. All storm sewer inlets that are made operable during construction shall be protected so that sediment-laden water
cannot enter the conveyance system without first being filtered or otherwise treated to remove sediment.
11. Before newly constructed stormwater conveyance channels or pipes are made operational, adequate outlet
protection and any required temporary or permanent channel lining shall be installed in both the conveyance
channel and receiving channel.
12. When work in a live watercourse is performed, precautions shall be taken to minimize encroachment, control
sediment transport and stabilize the work area to the greatest extent possible during construction. Nonerodible
material shall be used for the construction of causeways and cofferdams. Earthen fill may be used for the
structures if armored by nonerodible cover materials.
13. When a live watercourse must be crossed by construction vehicles more than twice in any six-month period, a
temporary vehicular stream crossing constructed of nonerodible material shall be provided.
14. All applicable federal, state and local regulations pertaining to working in or crossing live watercourses shall be met.
15. The bed and banks of a watercourse shall be stabilized immediately after work in the watercourse is completed.
16. Underground utility lines shall be installed in accordance with the following standards in addition to other applicable
criteria:
a. No more than 500 linear feet of trench may be opened at one time.
b. Excavated material shall be placed on the uphill side of trenches.
c. Effluent from dewatering operations shall be filtered or passed through an approved sediment trapping
device, or both, and discharged in a manner that does not adversely affect flowing streams or off-site
property.
d. Material used for backfilling trenches shall be properly compacted in order to minimize erosion and promote
stabilization.
e. Restabilization shall be accomplished in accordance with these regulations.
f. Applicable safety regulations shall be complied with.
17. Where construction vehicle access routes intersect paved or public roads, provisions shall be made to minimize the
transport of sediment by vehicular tracking onto the paved surface. Where sediment is transported onto a paved or
public road surface, the road surface shall be cleaned thoroughly every four hours and at the end of each day.
Sediment shall be removed from the roads by shoveling or sweeping and transported to a sediment control disposal
area. Street washing shall be allowed only after sediment is removed in this manner. This provision shall apply to
individual development lots as well as to larger land-disturbing activities.
18. All temporary erosion and sediment control measures shall be removed within 30 days after final site stabilization or
after the temporary measures are no longer needed, unless otherwise authorized by the local program authority.
Disturbed soil areas resulting from the disposition of temporary measures shall be permanently stabilized to prevent
further erosion and sedimentation.
19. Properties and waterways downstream from development sites shall be protected from sediment deposition, erosion
and damage due to increases in volume, velocity and peak flow rate of stormwater runoff for the stated frequency
storm of 24-hour duration in accordance with the following standards and criteria:
Bent Tree Design, PLC
Mill creek Office, Site Plan Amendment
Erosion and Sediment Control Narrative
Page 3
6 June 2006
a. Concentrated stormwater runoff leaving a development site shall be discharged directly into an adequate
natural or man-made receiving channel, pipe or storm sewer system. For those sites where runoff is
discharged into a pipe or pipe system, downstream stability analyses at the outfall of the pipe or pipe system
shall be performed.
b. Adequacy of all channels and pipes shall be verified in the following manner:
(1) The applicant shall demonstrate that the total drainage area to the point of analysis within the channel
is one hundred times greater than the contributing drainage area of the project in question; or
(2) (a) Natural channels shall be analyzed by the use of a two-year storm to verify that stormwater will
not overtop channel banks nor cause erosion of channel bed or banks; and
(b) All previously constructed man-made channels shall be analyzed by the use of a ten-year storm to
verify that stormwater will not overtop its banks and by the use of a two-year storm to
demonstrate that stormwater will not cause erosion of channel bed or banks; and
(c) Pipes and storm sewer systems shall be analyzed by the use of a ten-year storm to verify that
stormwater will be contained within the pipe or system.
c. If existing natural receiving channels or previously constructed man-made channels or pipes are not
adequate, the applicant shall:
(1) Improve the channel to a condition where a ten-year storm will not overtop the banks and a two-year
storm will not cause erosion to the channel bed or banks; or
(2) Improve the pipe or pipe system to a condition where the ten-year storm is contained within the
appurtenances; or
(3) Develop a site design that will not cause the pre-development peak runoff rate from a two-year storm
to increase when runoff outfalls into a natural channel or will not cause the pre-development peak
runoff rate from a ten-year storm to increase when runoff outfalls into a man-made channel; or
(4) Provide a combination of channel improvement, stormwater detention or other measures which is
satisfactory to the plan-approving authority to prevent downstream erosion.
d. The applicant shall provide evidence of permission to make the improvements.
e. All hydrologic analyses shall be based on the existing watershed characteristics and the ultimate development
of the subject project.
f. If the applicant chooses an option that includes stormwater detention he shall obtain approval from the
locality of a plan for maintenance of the detention facilities. The plan shall set forth the maintenance
requirements of the facility and the person responsible for performing the maintenance.
g. Outfall from a detention facility shall be discharged to a receiving channel, and energy dissipators shall be
placed at the outfall of all detention facilities as necessary to provide a stabilized transition from the facility to
the receiving channel.
h. All on-site channels must be verified to be adequate.
Increased volumes of sheet flows that may cause erosion or sedimentation on adjacent property shall be
diverted to a stable outlet, adequate channel, pipe or pipe system, or to a detention facility.
j. In applying these stormwater runoff criteria, individual lots or parcels in a residential, commercial or industrial
development shall not be considered to be separate development projects. Instead, the development, as a
whole, shall be considered to be a single development project. Hydrologic parameters that reflect the
ultimate development condition shall be used in all engineering calculations.
k. All measures used to protect properties and waterways shall be employed in a manner which minimizes
impacts on the physical, chemical and biological integrity of rivers, streams and other waters of the state.
Bent Tree Design, PLC
Mill yak Office, Site Plan Amendment
Erosion and Sediment Control Narrative
Page 4
6 June 2006
Management Strategies
1. Construction should be sequenced so that grading operations can begin and end as quickly as possible.
2. The job contractor shall be responsible for the installation and maintenance of all erosion and sediment control
practices.
3. Sediment trapping measures shall be installed as a first step in grading and shall be seeded and mulched
immediately following installation.
4. Areas which are not to be disturbed shall be clearly marked by flags, signs, etc.
Temporary Stabilization
Measures
1. Temporary Construction Entrance - E&S Std. & Spec. 3.02
A Temporary Construction Entrance shall be paved with a wash rack to reduce the amount of mud transported onto
paved roads. Sediment is to be directed to the sediment trap or sediment basin. Wash water must be carried away
from the entrance to an approved settling area to remove sediment. All sediment shall be prevented from entering
storm drains, ditches or watercourses.
2. Silt Fence - E&S Std. & Spec. 3.05
Silt Fence shall be installed as shown on the plans to intercept and detain sediment from disturbed areas.
3. Storm Drain Inlet Protection - E&S Std. &Spec. 3.07
Storm drain inlet protection shall be installed at all inlets where shown on plans to prevent sediment from entering
storm drainage systems prior to stabilization.
4. Temporary Sediment Trap— E&S Std. & Spec. 3.13
Temporary Sediment Basins shall be installed in the location specified on the plan.
5. Temporary Slope Drain — E&S Std. &Spec. 3.15
A Temporary Slope Drain shall be installed to carry the discharge from the upper Sediment Trap.
6. Outlet Protection — E&S Std. & Spec. 3.18
Outlet Protection shall be installed at the discharge of the Temporary Slope Drain until the Temporary Slope Drain
has been removed.
7. Temporary Seeding - E & S Std. &Spec. 3.31
Temporary vegetative cover is to be established on rough graded areas that will not be brought to
final grade for a year or less. Such areas include denuded areas, soil stockpiles, dikes, sides of sediment basins,
temporary road banks, cut slopes, etc. All areas which are to be temporarily seeded shall be seeded in accordance
with Minimum Standards No. 1, and shall be seeded with annual rye grass at 1-1/2 lbs/1,000 sq. ft. (including
liming, mulching, fertilizing as required).
8. Dust Control - E&S Std. &Spec. 3.39
Dust Control practices shall be employed to prevent surface and air movement of dust from exposed soil surfaces
and reduce the presence of airborne substances which may be harmful or injurious to human health, welfare, or
safety, or to animal or plant life.
Permanent Stabilization
Measures
1. Permanent Seeding: E&S Std. & Spec. 3.32: All areas disturbed by construction, except areas designated
for temporary seeding, will be stabilized with permanent seeding immediately following finish
grading. Unless otherwise specified, permanent seeding will be done with Kentucky 31 Fescue and a nurse crop
of annual rye (including liming, mulching and fertilizing as required) according to E&S Std. and Spec. 3.32 of the
1992 Virginia Erosion and Sediment Control Handbook.
Bent Tree Design, PLC
Mill creek Office, Site Plan Amendment
Erosion and Sediment Control Narrative
Page 5
6 June 2006
2. Mulching - E&S Std. &Spec. 3.35: Mulch shall be applied to all seeded areas in accordance with E&S Std. & Spec.
3.35. Organic mulches such as straw, wood chips (from trees cleared on site), bark chips and shredded bark may
be used. Fiber mulch may be used at times other than dry summer months or late fall, and it may be used to
anchor straw mulch. The use of chemical mulches and soil binders will be allowed. Rates of application shall
conform to Table 3.35-A of the VESCH. Mulch shall be firmly anchored and checked often for erosion. Any erosion
noted shall be quickly rectified and additional mulch applied.
Stormwater Runoff Considerations
An onsite water quality swale is proposed to provide water quality treatment for the proposed site plan. Stormwater
detention is provided with an existing facility constructed with the Mill Creek Industrial Park.
Calculations
Drainage computations have been submitted to the County for approval.
Maintenance Schedule
All erosion and sediment control measures shall be maintained and repaired as needed until all disturbed areas have been
permanently stabilized. Erosion and sediment control measures shall be inspected at least twice weekly, after each
rainfall and at least once daily during prolonged rainfall.
Maintenance and repair shall be in accordance with the methods outlined in the Virginia Erosion and Sediment Control
Handbook at the end of each standard and specification section for each control measure specified.
In general, all erosion and sediment control measures will be checked daily and after each significant rainfall. The
following items will be checked in particular:
a. Should sediment basins and/or silt traps be required, they will be cleaned and maintained according to the
Virginia Erosion &Sediment Control Handbook.
b. All gravel outlets will be checked regularly for sediment buildup that will prevent drainage. If the gravel is
clogged by sediment, it shall be removed and cleaned or replaced.
c. All silt fence barriers will be checked regularly for undermining or deterioration of the fabric and repaired as
required. Sediment shall be removed when the level of sediment deposition reaches half way to the top of
the barrier.
d. All seeded areas will be checked regularly to see that a good stand is maintained. Areas should be fertilized
and reseeded as needed.
Bent Tree Design, PLC
TEMPORARY SEDIMENT TRAP DESIGN
calculations based on a rectangular basin
1)Enter Project Name: Mill Creek Office
2)Enter Project No 0512
3)Enter trap identification number 1
4)Enter additional trap identification(e g. centerline stationing): Entrance
5)Enter area draining into trap(AC)(<3.0): 0 87
storage required(CV): 117
storage required(CF) 3148
wet storage required(CF). 1574
dry storage required(CF): 1574_
6)Enter depth of excavation(ft)(<=4.0): 3 00
7)Enter side slopes of excavated wet storage area(_:1). 1.00
area of top of wet storage(1st try)(SF): 617
1st try width of wet storage(ft): 17
1st try length of wet storage(ft): 36
8)Enter a new value for width: 10.00
9)Enter a new value for length 85.00
top area based on new width&length(SF): 850
bottom area(SF): 316
average area(bottom, top)(SF): 583
volume based on average area(CF): 1749
volume required(CF) 1574
okay? YES
10)Repeat steps(8)and(9)until volume requirement is exceeded and dry storage 1/w=>2.
DRY STORAGE OPTIMIZATION
11)Enter H(0),which is also D(2)(ft)(0.5<=H(0)<=4.0). 1.40
12)Enter side slopes of dry storage volume L 1) 2.00
width of basin(ft) 16
length of basin(ft): 91
dry storage 1/w: 5.81
volume based on average area(CF): 1584
dry storage volume required(CF): 1574
okay? YES
FINAL BASIN DESIGN INFORMATION
TRAP 1 AREA= 0.87 Ac.
ELEVATION L W D Z
BOTT 79 4
3.0 1 0
WET 85 10
1 4 2.0
DRY 91 16
WEIR 5.2 2.5