HomeMy WebLinkAboutSDP200800078 Calculations 2008-05-13EROSION AND SEDIMENT CONTROL
PROJECT NARRATIVE
AND
STORMWATER MANAGEMENT
REPORT
FOR
MONTALTO RESTORATION
MONTALTO
CHARLOTTESVILLE, VA
• MAY 13, 2008
PREPARED BY:
DRAPER ADEN ASSOCIATES
700 HARRIS STREET, SUITE E
CHARLOTTESVILLE, VA 22903
DAA PROJECT No. CV04110-15
4100�k
I)raper Aden Associates
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Erosion & Sediment Control Project Narrative
PROJECT DESCRIPTION: This project consists of the site grading and utility installation for the
Montalto Restoration project. The site is located off of Route 53 outside of Charlottesville, Virginia. A
total of 17.2 acres (more or less) will be disturbed during construction. No disturbance to adjacent lots
will be performed during the construction.
EXISTING SITE CONDITIONS: The site area is at the top of a hill, with runoff flowing in all directions.
The pre -developed drainage plan (Sheet C1.3) shows two principal drainage areas; DA 1 drains to the
west and DA 2 drains to the east. The site is comprised of grassy areas and trees, with a number of
buildings and driveways at the top of the hill. Critical slopes are denoted on the plans.
ADJACENT PROPERTY: This site is bounded by a conservation easement area to the east, Route 53 to
the north, and an orchard to the south and west.
OFF-SITE AREAS: No off-site construction areas will be required.
SOILS: Per the attached USDA Soil Survey of Albemarle County, soils on site are classified as follows:
- 12D - Catoctin silt loam, 15 to 25 percent slopes: Group C, K=0.32
- 71 B - Rabun clay loam, 2 to 7 percent slopes, 15-25% slopes: Group B, K=0.32
- 71 D - Rabun clay loam, 15 to 25 percent slopes, 7-15% slopes: Group B, K=0.32
CRITICAL AREAS: Critical slope areas are denoted on the plans. These areas will be protected with soil
stabilization matting during construction.
EROSION AND SEDIMENT CONTROL MEASURES: Unless otherwise indicated, all vegetative and
structural erosion and sediment control measures shall be constructed and maintained according to the
standards and specifications of the Virginia Erosion and Sediment Control Handbook. Unless waived or
approved by local administrator, the minimum standards of VESCR shall apply.
MINIMUM STANDARDS: All applicable Minimum Standards must be addressed.
Specifically:
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
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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. 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.
6. Concentrated runoff shall not flow down cut or fill slopes unless contained within an adequate
temporary or permanent channel, flume or slope drain structure.
7. Whenever water seeps from a slope face, adequate drainage or other protection shall be provided.
8. 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.
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.
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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 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. Trapped sediment and the 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:
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.
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c. If existing natural receiving channels or previously constructed man-made channels or
pipes are not adequate, the applicant shall:
(l) 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.
i. 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.
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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.
STRUCTURAL PRACTICES:
1. TEMPORARY CONSTRUCTION ENTRANCE - 3.02 A stone pad, located at points of
vehicular ingress and egress to the construction site. The purpose is to prevent or reduce the
amount of mud transported to the public road.
2. SILT FENCING - 3.05 Silt fence sediment barriers will be installed down slope of minimal
graded areas to filter sediment laden runoff.
3. INLET PROTECTION - 3.07 All storm drain inlets shall be protected during construction.
Sediment laden runoff shall be filtered before entering storm drain inlets and piping systems.
4. OUTLET PROTECTION - 3.18 Structurally lined aprons or other acceptable energy dissipating
devices placed at the outlets of pipes or paved channel sections. Installed to prevent scour at
storm drain outlets, to protect the outlet structure and to minimize the potential for downstream
erosion by reducing the velocity and energy of concentrated stormwater flows.
5. TEMPORARY SEEDING - 3.31 The establishment of a temporary vegetative cover on disturbed
areas by seeding with rapidly growing annual plants. Its purpose is to reduce erosion and stabilize
the area until permanent seeding can take place.
6. PERMANENT SEEDING - 3.32 Seeding to establish a perennial vegetative ground cover on
disturbed areas. Reduces or eliminates erosion, provides permanent stabilization on areas and
enhances landscaping.
7. SOIL STABILIZATION_ MATTING - 3.36 Protective covering on a steeply -sloped prepared
planting area to aid in controlling erosion.
8. TREE PROTECTION - 3.38 Protection of desirable trees from mechanical and other injury
during land disturbing and construction activities.
PERMANENT STABILIZATION: All areas disturbed by construction shall be stabilized with
permanent seeding immediately following finish grading. Seeding shall be done using the mixture
provided on the plans and in the specifications. Erosion control blankets will be installed on steep slopes.
Refer to ESC Technical Bulletin #4, Nutrient Management for Development Sites.
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MAINTENANCE: In general, all erosion and sediment control measures will be checked at least every
two weeks and/or after each significant rainfall. During construction, the contractor's designated
Responsible Land Disturber (RLD) will be responsible to inspect and repair damaged erosion/sediment
control measures. The following items will be checked in particular:
1. Sediment trapping features will be checked regularly for sediment and cleaned as required.
2. The silt fence barrier will be checked regularly for undermining or deterioration of the fabric.
Sediment shall be removed when the level of sediment deposition reaches half way to top of
barrier.
3. The seeded areas will be checked regularly to ensure that a good stand is maintained. Areas
should be fertilized and reseeded as needed. Refer to ESC Technical Bulletin #4.
4. Provide remedial stabilization and seeding for a period of one year after construction.
Stormwater Management Report
Permeable pavers will be utilized in low -traffic areas on site to allow infiltration of surface water into the
underlying soil and a cistern will be provided to store runoff from the roof of the main house. These low
impact development strategies will ensure that stormwater runoff from the development, as planned, will
not adversely affect adjacent or downstream properties or waterways.
Water Quantity:
Post -developed runoff will not increase in relation to pre -developed conditions, as summarized below.
Pre -developed Runoff
A (ac)
C
Tc (min)
Qz (cfs)
Q10 (cfs)
Q100 (cfs)
DA 1
8.11
0.36
17
9.52
13.20
23.63
DA 2
9.11
0.43
12
15.10
20.60
36.54
Total
17.22
0.90
5
24.62
33.80
_
60.17
Post -developed Runoff Y
A (ac)
C
Tc (min)
Qz (Cf S)
Q10 (cfs)
Q100 (cfs)
DA 1
8.07
0.38
17
10.00
13.87
24.82
DA 2
9.05
0.43
16
13.09
18.11
32.35
Roof
0.10
0.90
5
Roof Drains to Cistern
Total
17.22
23.09
31.98
57.17
7
Water Quality:
Water quality calculations per the County Design Standards Manual are provided below. Imperviousness
was counted for more than simply hard surfaces according to the land use chart provided in the Manual.
Removal rate computations were performed according to the Simple Method, as outlined in the Manual
and the Virginia Stormwater Management handbook.
For Drainage Area 1, both the pre and post -developed percent impervious cover is at or below the 16%
average land cover condition threshold. For Drainage Area 2, the post -developed pollutant loading is less
than the pre -developed loading. Thus, no pollutant removal is required for this site.
Drainage Area 1
Drainage Area 2
Imp. A
(ac)
Total
DA (ac)
Imp. %
L (Ib/yr)
_
Imp. A
(ac)
Total
DA (ac)
Imp. %
L (lb/yr)
Pre -developed
0.86
8.11
12
4.60
2.01
9.11
24
8.39
Post -developed
1.07
8.07
15
5.24
1.89
9.05
22
8.01
For Drainage Area 1, both the pre and post -developed percent impervious cover is at or below the 16%
average land cover condition threshold. For Drainage Area 2, the post -developed pollutant loading is less
than the pre -developed loading. Thus, no pollutant removal is required for this site.