HomeMy WebLinkAboutWPO200600064 Calculations Erosion Control Plan 2006-08-03 EROSION & SEDIMENT CONTROL NARRATIVE
POPLA1 GLEN 2
ALBEMARLE, VIRGINIA
3 AUGUST 2006
Prepared by:
TERRA CONCEPTS
224 Court Square
Charlottesville, Virginia 22902
(434) 295-4005
RECEIVED
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Project Description
The Poplar Glen 2 development is proposing 28 townhouse units on 3.64 acres. The subject
property is located adjacent to the Poplar Glen 1 site which is currently under construction
and is accessed via Stillfried Lane. At the far back (SE) corner of the University of Virginia's
Kluge Children's Rehabilitation Center campus property,Stillfried Lane casually terminates,
to serve three small parcels. A private driveway bears east from this point on Stillfried Lane
and runs to an existing,unoccupied house,which is to be demolished prior to construction.
The existing driveway will be improved and remain roughly in the same location but will
serve as the entrance to the site. Five buildings containing 28 townhouse units will be
terraced throughout the property,while the southern most portion of the site will remain
largely unaffected.
Existing Site Conditions
The existing site,which is at the foot of Lewis Mountain,is primarily wooded with slopes
ranging from 10% to 50%. There is an existing single family residence (formally used as
office space) and an existing asphalt drive. The existing drive lies between the terminus of
Stillfried Lane and the existing residence,and terminates at the residence. The existing
structure rests at the northern most corner of the property and will be demolished upon start
of construction. Approximately 2.7 acres of land adjacent to the existing drive and residence
is sparsely wooded,with the occasional specimen tree, and has slopes conducive to the
proposed development. The remaining 0.9 acres is more densely wooded and is considered
too steep for development.
All storm water drainage on the site is conveyed as surface flow. The current topography
includes a swale running diagonally across the site from the southwestern property line to
the northeast corner where storm water is picked up by an existing drainage inlet located
between the James W. Newman residence and University Apartments.
Adjacent Property
The property is bound on the south by the Julia Courtney Campbell tract. To the east lies the
University Heights apartments and to the north the John F. Matthews property. To the west
lies the Poplar Glen 1 site and The Rector and Visitors of the University of Virginia parcel.
Off-Site Areas
Off site runoff will be collected in a series of temporary drainage inlets which will convey the
storm water around the site to the existing drainage structure at the northeast corner of the
site. The temporary drainage inlets are part of the permanent storm water conveyance
system that will be converted to manholes as additional drainage inlets are installed to
convey the offsite water.
Soils
According to the Soil Survey of Albemarle County, the following loamy soils are present on
site:
Name Code Erodibility Permeability Depth Shrink/Swell Strength
Culpeper 21C severe moderate <50" moderate low
Hazel 40E very severe moderate <30" low low
Udorthents88 severe moderate n/a n/a n/a
Critical Erosion Areas
There are areas of 2:1 slopes proposed with this plan that will be protected by diversion dikes
constructed at the top of the slope.
Erosion & Sediment Control Measures
Erosion and sediment control for the site will be handled in two phases. Phase A of the
proposed erosion control plan is intended to protect the site during the initial mass grading of
the project while the building and parking areas are brought to their proposed grades but
before installation of building slabs, parking lot curb and stone base. Phase B of the plan
serves to protect the site after installation of building slab and parking lot stone base and curb
and when County Erosion Control Officer approves removal of the sediment basin.
For Phase A, a storm water conveyance system will be constructed to intercept storm runoff
from the off-site and the undeveloped portion of the property. Then tree protection and
perimeter silt fence will be installed and a temporary sediment trap with a riser structure will
be constructed in the future underground storm water detention system. Topsoil will be
stripped and stockpiled. Temporary diversion dikes will be utilized as the building/parking
areas are brought to grade to protect new 2:1 slopes and to direct a majority of the site's
runoff to the sediment trap. A construction entrance will be the erosion control measures for
the improvements to the entry road at Stillfried Lane.
For Phase B,building slabs,parking lot stone base, and curbing should be in place. The
sediment trap will be removed and standard inlet protection measures installed to minimize
silt build up in the underground detention systems. Prior to demobilization, the contractor
shall flush all of the underground detention systems to remove any accumulated silt.
Perimeter measures will remain in place until removal is approved by the County Erosion
Control Officer.
Vegetative Practices
After the majority of the grading program has been executed and, in particular, the cut and fill
slopes for the project have been established, they should be redressed with an ample depth of
topsoil from the stockpile and surface roughened. Thereafter, fertilizer, permanent seeding
and mulch should be applied. In those areas exceeding a 3:1 gradient, soil stabilization
matting is to be applied and stapled to the slope. If for some reason, parking area construction
does not commence soon after the mass grading of the site is complete, exposed areas are to
be treated with temporary seeding and mulch until such time as finished construction begins.
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. Site walls are to be constructed early in the construction schedule and the areas
immediately above and below them stabilized as quickly as possible.
6. Matting and seeding, whether temporary or permanent,will follow immediately after
grading.
7. Construction is to be sequenced such that, once cut and fill slope grades have been
established and erosion control measures applied, no further disturbance of the slopes will
be necessary.
8. All construction in and around drainageways shall be strategically scheduled appreciating
pending weather conditions to avoid work in, or disturbance of, the channels during rain
events. All stabilization products, equipment and personnel to install them shall be on-site
at the time disturbance is to commence. In the event weather interrupts work within a
drainageway, emergency measures are to taken to temporarily stabilize the channel so that
it functions without undue erosion taking place until work can progress.
9. The site contractor shall be responsible for the installation and maintenance of all erosion
and sediment control practices.
10. 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.
Permanent Stabilization
As mentioned above, permanent seeding is to be applied to all areas not proposed for
hardscape or building improvements as soon as possible. Please see the permanent seeding
specifications for this project attached to the end of this report.
Stormwater Management
All stormwater management, in terms of detention and water-quality are to be accomplished
on-site, as shown on the attached plan.
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 sediment traps and basins will be checked regularly for sediment cleanout.
2. Drainageway improvements shall be inspected after each rain event to determine if further
armoring is necessary.
3. 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.
4. Silt fence barriers and diversion dikes will be checked regularly for undermining or
deterioration and repaired as necessary. Sediment shall be removed when the level of
sediment deposition reaches half way to the top of the barrier.
5. 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 the county agent.
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 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.
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
1. (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
2. (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.
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.
SEDIMENT TRAP CALCULATIONS
RATIONAL METHOD WORKSHEET Project:
Poplar II
a 2/200
Designer: MOS
Watershed ID: ESC
Drainage Area: 2.30 Acres
c-factor: 0.60 (see worksheet)
tc: 5.98 mins. (see worksheet)
Q(2): 6.80 cfs
Q(25): 9.82 cfs
Time of Concentration Worksheet Weighted c-factor Worksheet
Overland Flow Time Cover Te _ Area c
Source: Seelye Chart as modified in VDOT Drainage Manual(Fig. Bare Packed Soil 2.30 0.60
1.5.1.1 on Page 1-13E)
Notes:
1)Length of overland flow always<=200 ft
2)Calculate a separate Tc for each ground cover condition along the flow path
c-factor= 0.60
184.38 Enter Length of Strip(ft) Sample"C"values:
4.76 Enter Slope(%or ft/100ft) 0.22 Dense grass
0.60 Enter Rational Method"C"value 0.28 Average grass
0.36 Poor grass
5.98 Result Tc(minutes) 0.48 Bare soil
0.90 Pavement
0.90 Pavement
Shallow Concentrated Flow Time
Source: SCS Velocity Graph as published in Virginia Erosion and
Sediment Control Handbook,Third Edition(Plate 5-2 on Page V-12)
Notes:
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
0.00 Enter Length of Flow(ft)
3.36 Enter Slope(%or ft/100ft)
0.00 Result Tc(minutes)IF UNPAVED
0.00 Result Tc(minutes)IF PAVED
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
537.69 Enter Length of Flow(ft)
12.00 Enter Height of most remote point along flow path above outlet
Pipe Flow Time
Assumes 10 ft/s velocity in pipe
0.00 Enter Length of Flow in Pipe(ft)
0.00 Result Tc(minutes)
Rainfall Intensity(Albemarle County,Virginia)
Return Duration Intensity
Period (mins.) (in/hr)
2 5.98 4.9
5 5.98 5.6
10 5.98 6.4
25 5.98 7.1
50 5.98 7.7
100 5.98 8.3
Intensity values calculated using Atlas 14 rainfall precipitation frequency data
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TEMPORARY SEDIMENT BASIN DESIGN DATA SHEET
(with or without an emergency spillway)
TERRA CONCEPTS, P.C.
8/2/2006
MOS
PROJECT: POPLAR GLEN 2
BASIN #: 1
LOCATION: FUTURE UNDERGROUND STORMWATER BASIN AREA
TOTAL AREA DRAINING TO BASIN: 2.30 ac
BASIN VOLUME DESIGN
WET STORAGE:
1. MINIMUM REQUIRED VOLUME: 154.10 cy
2. AVAILABLE BASIN VOLUME:
AVERAGE END AREA - STORAGE - ELEVATION CURVE
ELEVATION AREA VOLUME VOLUME TOTAL
ft sf cf cy cy
574 ko0.00
396.34 14.68
576 396.34 14.68
1,434.31 53.12
578 f IC} 1,037.97 67.80
2,859.03 105.89
580 ``I 4 1,821.06 173.69
4,544.14 168.30
582 2,723.08 341.99
6,445.49 238.72
584 3,722.41 580.72
REQUIRED WET STORAGE VOLUME: 154.10 cy
APPROX. ELEVATION: 579.70
3. TO OBTAIN REQUIRED VOLUME, EXCAVATE: 269.79 cy
4.AVAILABLE VOLUME BEFORE CLEANOUT REQUIRED: 78.20 cy
5. ELEVATION CORRESPONDING TO CLEANOUT LEVEL: 578.30
6. DISTANCE FROM DEWATERING ORIFICE TO CLEAN OUT LEVEL:
INVERT DEWATERING ORIFICE REQUIRED: 579.70
ACTUAL INVERT DEWATERING ORIFICE ELEVATION: 579.70
VOLUME AT PROPOSED ORIFICE INVERT: 154.10 cy
DISTANCE FROM INVERT TO CLEAN OUT LEVEL: 1.40 OK
ADEQUATE WET STORAGE VOLUME: YES
DRY STORAGE:
7. MINIMUM REQUIRED DRY STORAGE VOLUME: 154.10 cy
8. TOTAL AVAILABLE VOLUME AT CREST OF RISER:
CREST OF RISER: 581.60
TOTAL VOLUME AT RISER CREST: 308.20 cy
AVAILABLE DRY STORAGE VOLUME: 154.10 cy
ADEQUATE DRY STORAGE VOLUME: YES
9. DIAMETER OF DEWATERING ORIFICE:
Q= 0.19 cfs
CALCULATED D= 2.74 in
DESIGN D = 3.00 in
10. DIAMETER OF FLEXIBLE TUBING: 6.00 in
PRELIMINARY DESIGN ELEVATIONS
11. CREST OF RISER= 581.60
TOP OF DAM = 584.00
DESIGN HIGH WATER= 582.60
UPSTREAM TOE OF DAM = 574.00
BASIN SHAPE
12. LENTH:WIDTH
WET STORAGE SURFACE AREA: 1,696.82 sf
LENGTH OF FLOW: 80.30 ft
LENGTH-TO-WIDTH RATIO: 3.80
BAFFLES: N/A
RUNOFF
13. c= 0.60 (Bare packed soil)
tc= 15.00 min.
i = 3.40 in/hr
Q(2) = 6.80 cfs 5 ,7 (to /4,1 r)
14. c= 0.60 (Bare packed soil)
tc= 15.00 min.
i = 5.50 in/hr
Q(25) = 9.82 cfs
PRINCIPAL SPILLWAY DESIGN
15.
IS THERE AN EMERGENCY SPILLWAY(Y OR N)? N
WITH EMERGENCY SPILLWAY, REQUIRED SPILLWAY
CAPACITY Q(P) =Q(2) = N/A cfs
WITHOUT EMERGENCY SPILLWAY, REQUIRED SPILLWAY
CAPACITY Q(P) =Q(25)= 9.82 cfs
16.
WITH EMERGENCY SPILLWAY:
601
ASSUMED AVAILABLE HEAD (H) USING Q(2)= N/A ft
(H = CREST OF EMERG. SPILLWAY- CREST OF RISER)
WITHOUT EMERGENCY SPILLWAY:
ASSUMED AVAILABLE HEAD (H) USING Q(25)= 1.00 ft
(H = DESIGN HIGH WATER- CREST OF RISER)
17. RISER DIAMETER(Dr) = 30.00 in
ACTUAL HEAD = 0.55 ft
(SEE ATTACHED CHART)
18. BARREL LENGTH (L) = 69J6 ft
HEAD OF BARREL TROUGH EMBANKMENT= 11.65 ft
(FROM PLATE 3.14-7)
Takes into
19. BARREL DIAMETER= 21.00 in account offsite
bypass flow.
20. TRASH RACK AND ANTI-VORTEX DEVICE:
DIAMETER 42.00 in
HEIGHT 15.00 in
(FROM TABLE 3.14-D)
EMERGENCY SPILLWAY DESIGN
21.
REQUIRED SPILLWAY CAPACITY Q(E)= 0(25)-Q(P)= 26.05
22.
BOTTOM WIDTH (B)= 12.00 ft
SLOPE OF THE EXIT CHANNEL (S)= 21.40 %
MINIMUM LENGTH OF EXIT CHANNEL (X) = N/A ft
(FROM TABLE 3.14-C)
ANTI-SEEP COLLAR DESIGN
23.
DEPTH OF WATER AT PRINCIPAL SPILLWAY CREST (Y)= N/A ft
SLOPE OF UPSTREAM FACE OF EMBANKMENT(Z) = N/A :1
SLOPE OF PRINCIPAL SPILLWAY BARREL(Sb)= N/A %
LENGTH OF BARREL IN SATURATED ZONE (Ls)= N/A ft
24.
NUMBER OF COLLARS REQUIRED = NONE PROPOSED
DIMENSIONS=
(FROM PLATE 3.14-12)
PRELIMINARY DESIGN ELEVATIONS
25.
TOP OF DAM = 584.00
DESIGN HIGH WATER= 582.60
EMERGENCY SPILLWAY CREST= N/A
PRINCIPAL SPILLWAY CREST= 581.60
DEWATERING ORIFICE INVERT= 579.70
CLEANOUT ELEVATION = 578,30
ELEVATION OF UPSTREAM TOE OF DAM OR EXCAVATED
BOTTOM OF "WET STORAGE AREA"(IF EXCAVATION WAS
PERFORMED) = 576.00
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