HomeMy WebLinkAboutWPO201400052 Calculations 2014-06-02 EROSION & SEDIMENT CONTROL NARRATIVE
COLTHURST FARM LOT 6 ,
BLOCK "B"
Albemarle County, Virginia
22 May 2014
Prepared by:
WATER STREET STUDIO, LLC.
418 E. Main Street
Charlottesville, Virginia 22902
(434) 295-8177
Project Description
The purpose of this project is the construction of a single family residence on a previously
vacant lot in the Colthurst Farm Subdivision. The footprint of the residence is approximately
3,000 square feet. There will be a paved driveway from Reynard Drive. To accommodate the
topography of the lot the house will have a lower level that exits to existing grade
approximately 10 feet below the first floor level. The house will have porches/decks adjacent
to the front and rear doors. The house will be served by public water but will have a septic
drainfield for sewer.
The project site is 1.36 acres and the work limits, or limits of disturbance, are 0.87 acres. All
the aforementioned improvements are within these limits. Previously within the work limits
there were no impervious surfaces other than the pond. With the proposed construction, the
impervious surface area would increase to 4,500 square feet.
Construction is on-going and will be completed by September 2014.
Existing Site Conditions
The lot is located at the convergence of Reynard Drive and Tally Ho Drive and is the low
point in the topography of a dozen lots on Tally Ho and Reynard Drive. Previously, the lot
was primarily wooded and it contained a small, shallow pond. The pond was fed by a very
small trickle of a stream likely caused by a spring. Overflow from the pond drained to/through
an existing 24" culvert under Reynard Drive and continued on in the form of a small stream
feeding another pond just downstream within an existing County WPO Buffer. The pond on
the lot in question has been removed and the house and yard construction started prior to
commencement of this Erosion and Sediment Control Plan and Narrative. Over the last
month the site has experienced several significant rainfall events and the County identified
the need for a more comprehensive erosion control plan, specifically regarding the
continuation of the existing small stream through the site to the existing culver under
Reynard and protection of the culvert inlet and stream from further erosion and
sedimentation.
Adjacent Property
The property is bound on the west by Reynard Drive. It is bound on the south by Tally Ho
Drive. It is bound on the north by a residential lot. It is bound on the east by a vacant,
wooded lot.
Off-Site Areas
The proposed plan does not require any disturbance outside of the shown project work limits.
Waste/borrow materials will be obtained/disposed of at a site with an approved grading
permit or on site within the work limits.
Soils
According to the County GIS, the soil types in the working limits are 36B, Hayesville Loam, 2
to 7 percent slopes, 36C Hayesville Loam, 7 to 15 percent slopes, and 37D3, Hayesville Clay
Loam, 15 to 25 percent slopes. Soils of the Hayesville series are deep and well drained
formed in the weathered products of granite, granite gneiss, and granodiorite. The soils have
moderate permeability and moderate erosion hazard. The Hayesville series are in Hydraulic
Soils Group"B".
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Critical Erosion Areas
No critical erosion areas are anticipated.
Erosion &Sediment Control Measures
A construction entrance should be installed at the beginning of construction as indicated on
the plans. Tree protection will be installed on all trees to remain within the working limits as
well as those in close proximity to the working limits. Perimeter measures such as silt fence
and sediment traps should be installed prior to clearing and grading including diversions into
traps or away from site. As final grades are established the contractor should establish
vegetation as quickly as possible by re-spreading topsoil and installing permanent seeding
and mulching. Should stockpiles be required, silt fence should be placed around the
perimeter of the stockpile. At any point, at the direction of the Engineer or Inspector,
additional measures may be required.
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).
1. Temporary Construction Entrance- E&S Std. &Spec. 3.02
Temporary Construction Entrance is proposed to define the entry and exit point to the
construction site and to provide the platform to clean construction vehicles before
leaving the site. A Temporary Construction Entrance shall be installed where indicated.
2. Silt Fence with Wire Support- E&S Std. & Spec. 3.05-1
Silt Fence is proposed to capture the sediments in the sheet flow. Silt Fence with wire
support shall be installed in a sequenced manner as shown on the plan.
3. Culvert Inlet Protection- E&S Std. & Spec. 3.08
A sediment filter located at the inlet to storm sewer culverts.
4. Stormwater Conveyance Channel - E&S Std. &Spec. 3.17
A permanent, designed waterway, shaped, sized, and lined with appropriate vegetation
or structural material used to safely convey stormwater runoff within or away from the
site.
5. Rip Rap—E&S Std. & Spec. 3.19
A permanent, erosion-resistant ground cover of large, loose, angular stone with filter
fabric or granular underlining.
6. Topsoilinq - E&S Std. &Spec. 3.29
Topsoil will be stripped from areas to be graded and stockpiled for later use. Stockpile
locations shall be located on-site and are to be stabilized with temporary vegetation.
Selection of the seed mixture will depend on the time of year it is applied. Prior to land-
disturbing activities, the contractor shall submit a supplementary E & S plan to the
owner covering the off-site stockpile area which would have to be approved by the plan
approving authority before any off-site activity commences.
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7. Temporary Seeding: E&S Std. & Spec. 3.31
All areas temporary disturbed by construction (like diversion dike etc)will be
stabilized with temporary seeding immediately following the conclusion of
clearing operations. Unless otherwise specified, permanent seeding will be done with
a seed mix approved by the Owner prior to application in accordance to E&S Std. and
Spec. 3.31 of the 1992 Virginia Erosion and Sediment Control Handbook.
8. Permanent Seeding: E&S Std. & Spec. 3.32
All areas disturbed by construction will be stabilized with permanent seeding
immediately following the conclusion of clearing operations. Permanent seeding
will be done with a seed mix approved by the Owner prior to application in accordance
to E&S Std. and Spec. 3.32 of the 1992 Virginia Erosion and Sediment Control
Handbook. The contractor shall perform soil testing and consult with the local
Agricultural Cooperative Extension representative to verify that the Virginia Erosion and
Sediment Control Manual Permanent Seeding specifications are appropriate for this
site.
9. Mulching: E&S Std. & Spec. 3.35
Areas which have been temporarily/permanently seeded, diversion dikes, fill diversion
should be mulched immediately following seeding.
10. Soil Stabilization Blankets/Matting: E&S Std. & Spec. 3.36
The EC-2 matting shall be placed where indicated on Erosion & Sediment Control Plan.
11. 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.
Management Strategies
1. Construction will be sequenced so that grading operations can begin and end as quickly
as possible.
2. Areas which are outside the construction limits and not to be disturbed shall be clearly
marked.
3. Tree protection shall be installed before any grading activities commence and before
equipment and material storage areas are established.
4. Sediment trapping measures shall be installed as a first step in grading and will be
seeded and mulched immediately following installation.
5. Matting and seeding, whether temporary or permanent, will follow immediately after
grading.
6. The site contractor shall be responsible for the installation and maintenance of all erosion
and sediment control practices.
7. After achieving adequate stabilization, the temporary erosion and sediment controls will
be cleaned up and removed and final grades established according to the grading plan
for the project.
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Sequence of Construction
1. Clearly mark limits of work.
2. Utilize existing construction entrance and install culvert inlet protection at existing culvert
under Reynard drive.
3. Install silt fence at the toe of slope of all disturbed areas as well as around all material
stockpiles.
4. Install stormwater conveyance channel to existing reynard drive culvert.
5. Continue building construction and yard grading.
6. Establish vegetation on all newly disturbed turf areas with permanent seeding, sodding,
mulching, fertilization, and liming. Amend soils where needed.
7. Once site has been deemed stabilized by County Inspector, remove all temporary
erosion control measures.
Maintenance
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:
1. The inlet protection will be checked regularly for sediment cleanout.
2. The gravel outlets will be checked regularly for sediment buildup which would prevent
drainage. If the gravel is clogged by sediment, it shall be removed and cleaned or
replaced.
3. 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 the top of the barrier.
4. The seeded areas will be checked regularly to ensure that a good stand is maintained.
Areas shall be fertilized and reseeded as determined necessary by erosion control
inspector in accordance with the Virginia Erosion & Sediment Control Handbook.
The contractor shall be responsible for the installation, maintenance, and removal of all
erosion and sediment control practices and restoration of any damaged areas during
construction and for any warranty period prior to transfer of responsibility to owner.
Vegetative Practices
After the majority of the grading program has been executed they should be redressed with
an ample depth of topsoil from the stockpile and surface roughened. Thereafter, permanent
seeding and mulch should be applied. If for some reason, construction is halted or delayed,
exposed areas are to be treated with temporary seeding and mulch until such time as
construction begins.
Permanent Stabilization
As mentioned above, permanent seeding, lime, fertilization, and mulching is to be applied to
all areas not proposed for hardscape or building improvements as soon as possible. The
specifications for the Piedmont area in the Virginia Erosion and Sediment Control Manual are
outlined below.
Permanent seeding shall be Turf-Type Tall-Fescue (100%) applied at 200-250 lbs per acre.
Liming shall be 2 tons/acre of pulverized agricultural grade limestone (90 lbs/1000 ft2).
Fertilizer shall be 1000 lbs/acre 10-20-10 or equivalent nutrients 23 Ibs/1000 ft2).
5
Sodding may be utilized as stabilization in place of seeding.
The contractor shall perform soil testing and consult with the local Agricultural Cooperative
Extension representative to verify that the Virginia Erosion and Sediment Control Manual
Permanent Seeding specifications are appropriate for this site.
Stormwater Conveyance Channel
The Stormwater Conveyance Channel section was analyzed using calculated 2-yr and 10-yr
storm runoff rates for the contributing drainage area. Calculations are included in the
Appendix.
State Minimum Standards
An erosion and sediment control program adopted by a district or locality must be consistent
with the following criteria, techniques and methods:
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.
a. The minimum storage capacity of a sediment trap shall be 134 cubic yards per acre of
drainage area and the trap shall only control drainage areas less than three acres.
b. Surface runoff from disturbed areas that is comprised of flow from drainage areas
greater than or equal to three acres shall be controlled by a sediment basin. The
minimum storage capacity of a sediment basin shall be 134 cubic yards per acre of
drainage area. The outfall system shall, at a minimum, maintain the structural integrity
of the basin during a twenty-five year storm of 24-hour duration. Runoff coefficients
6
used in runoff calculations shall correspond to a bare earth condition or those
conditions expected to exist while the sediment basin is utilized.
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 these 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 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.
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
8
(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.
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.
Certified Responsible Land Disturber(RLD) Statement
A certified RLD is required during all stages of construction, from the initial land disturbance
through final stabilization. The name of the project RLD must be provided to the County
before any land disturbance may begin. Notify the office in a timely manner if the RLD
changes during the course of the project.
9
Appendix
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RATIONAL METHOD WORKSHEET Project: Colthurst Farm
Date: 5/22/2014
Designer: AGF
Watershed ID: Channel
Drainage Area: 13.08 Acres
c-factor: 0.30 (see worksheet)
tc: 21.06 mins. (see worksheet)
Q(2): 11.15 cfs
Q(10): 15.18 cfs
Time of Concentration Worksheet Weighted c-factor Worksheet
Overland Flow Time Cover Type Area c areaxcn
Source: Seelye Chart as modified in VDOT Drainage Manual(Fig.1.5.1.1 Residential(lots 1/2 acre+) 13.08 0.30 3.92
on Page 1-13E) 0.00
Notes: 0.00
1)Length of overland flow always<=200 ft 0.00
2)Calculate a separate Tc for each ground cover condition along the flow path 0.00
c-factor= 0.30
300.00 Enter Length of Strip(ft) Sample"C"values:
4.00 Enter Slope(%or ft/100ft) 0.22 Dense grass
0.30 Enter Rational Method"C"value 0.28 Average grass
0.36 Poor grass
®Result Tc(minutes) 0.48 Bare soil
0.90 Pavement
Shallow Concentrated Flow Time Shallow Concentrated Flow Time
Source: SCS Velocity Graph as published in Virginia Erosion and Source: SCS Velocity Graph as published in Virginia Erosion and Sediment
Sediment Control Handbook,Third Edition(Plate 5-2 on Page V-12) Control Handbook,Third Edition(Plate 5-2 on Page V-12)
Notes: Notes:
1)Not intended for well-defined channels-see channel flow calculation 1)Not intended for well-defined channels-see channel flow calculation
2)Calculate a separate Tc for each ground cover condition along the flow path 2)Calculate a separate Tc for each ground cover condition along the flow pall
205.00 Enter Length of Flow(ft) 395.00 Enter Length of Flow(ft)
7.80 Enter Slope(%or ft/100ft) 8.00 Enter Slope(%or ft/100ft)
0.76 Result Tc(minutes)IF UNPAVED Result Tc(minutes)IF UNPAVED
Result Tc(minutes)IF PAVED 1.11 Result Tc(minutes)IF PAVED
Pipe Flow Time
Assumes 5 ft/s velocity in pipe
0.001 Enter Length of Flow in Pipe(ft)
0.00 Result Tc(minutes)
Channel Flow Time
Source: Kirpich Chart as modified in VDOT Drainage Manual(Fig.1.5.1.2 on Page 1-14)
Notes:
1)For small drainage basins.
2)For concrete channels,use 0.2*Tc
623.00 Enter Length of Flow(ft)
25.00 Enter Height of most remote point along flow path above outlet
Elm Result Tc(minutes)
Rainfall Intensity(Albemarle County,Virginia)
Return I Duration Intensity
Period (mins.) I (in/hr)
2 21.1 2.8
10 21.1 3.9
Acceptable values for duration are between 5 and 60 minutes. Use values outside this range at risk.
Channel Report
Hydraflow Express Extension for AutoCAD®Civil 3D®2013 by Autodesk,Inc. Thursday,May 22 2014
Colthurst Lot 6, Block B Channel (2-yr)
Trapezoidal Highlighted
Bottom Width (ft) = 2.00 Depth (ft) = 0.69
Side Slopes (z:1) = 4.00, 4.00 Q (cfs) = 11.15
Total Depth (ft) = 1.50 Area (sgft) = 3.28
Invert Elev (ft) = 528.00 Velocity (ft/s) = 3.39
Slope (%) = 2.00 Wetted Perim (ft) = 7.69
N-Value = 0.035 Crit Depth, Yc (ft) = 0.66
Top Width (ft) = 7.52
Calculations EGL (ft) = 0.87
Compute by: Known Q
Known Q (cfs) = 11.15
Elev (ft) Section Depth (ft)
530.00 2.00
529.50 1.50
529.00 1.00
528.50 0.50
528.00 0.00
527.50 -0.50
0 2 4 6 8 10 12 14 16 18
D.-- .►,
/PA
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Channel Report
Hydraflow Express Extension for AutoCAD®Civil 3D®2013 by Autodesk,Inc. Thursday,May 22 2014
Colthurst Lot 6, Block B Channel (10-yr)
Trapezoidal Highlighted
Bottom Width (ft) = 2.00 Depth (ft) = 0.80
Side Slopes (z:1) = 4.00, 4.00 Q (cfs) = 15.18
Total Depth (ft) = 1.50 Area (sqft) = 4.16
Invert Elev (ft) = 528.00 Velocity (ft/s) = 3.65
Slope (%) = 2.00 Wetted Perim (ft) = 8.60
N-Value = 0.035 Crit Depth, Yc(ft) = 0.77
Top Width (ft) = 8.40
Calculations EGL (ft) = 1.01
Compute by: Known Q
Known Q (cfs) = 15.18
RECEIVED
MAY 302014
COMMUNITY
DEVELOPMENT
Elev (ft) Section Depth (ft)
530.00 2.00
529.50 1.50
529.00 1.00
528.50 0.50
528.00 0.00
527.50 — -- -0.50
0 2 4 6 8 10 12 14 16 18
if+\
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