Baffles and Surface Water
Discharge Design
J.P. Johns, PE - Woolpert, Inc.
SC State Rep to IECA
Agenda
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CGP Requirements
Porous Baffles
Floating Skimmers
Multipurpose Basins
Design Options
CGP Requirements (3.2.6 II)
• For common drainage outfalls that serve an area with 10 or
more disturbed acres, a sediment basin, or equivalent sediment
control BMPs, which meets the criteria identified in State
Regulations 72-307.C.(5), must be provided where attainable
until final stabilization of the construction site is achieved.
• For common drainage outfalls serving areas < 10 acres,
sediment basins are still recommended where applicable.
CGP Requirements (3.2.6 II)
• Unless infeasible, properly design, install and maintain porous
baffles, or similar control measures capable of enhancing
settling capabilities and restricting the accumulation of sediment
around the outlet structure, in all temporary sediment traps
and sediment basins to reduce velocity, turbulence, and
improve sediment trapping efficiency.
• Unless infeasible, sediment forebays, or similar control
measures capable of providing sediment trapping at inlets of
sediment basins, should be installed as practicable based on
sediment storage requirements of each sediment basin.
• Unless infeasible, each sediment basin must be equipped with
a cleanout stake indicating when the basin is to be cleaned.
CGP Requirements (3.2.6 II)
• Water Surface Dewatering.
• When discharging from sediment basins and similar
impoundments, utilize outlet structures that only withdraw water
from near the surface of the basin or impoundment, unless
infeasible. This outlet structure should be capable of conveying
the flow for the 10-year, 24 storm event.
CGP Requirements Non-Numeric Effluent
Limits (3.2.10)
• Control Stormwater volume and velocity within the site to
minimize soil erosion during construction activity.
• Control Stormwater discharges, including both peak flow rates
and total Stormwater volume, to minimize erosion at outlets
and to minimize downstream channel and stream bank erosion
during construction activity.
CGP Requirements Non-Numeric Effluent
Limits (3.2.10)
• Stormwater Volume Discharge Control must be
accomplished during construction activities to minimize erosion
at outlets and to minimize downstream channel and stream
bank erosion. This can be accomplished through the use of
various BMPs and techniques including, but not limited to, the
following:
o Surface outlets; and
o Water quantity storage within impoundments.
• Peak Flow Rate Discharge Control must be accomplished
during construction activities to minimize erosion at outlets and
to minimize downstream channel and stream bank erosion.
New Specifications
• Significant new requirements included:
o Surface Outlet and Baffle Sediment Basins
Porous Baffles
Floating Skimmers
Surface Outlet & Baffle Sediment Basin Design
• Stormwater runoff to a single
outfall which disturb 10
acres is controlled with
sediment basin.
• Design Criteria is 80% TSS
• Efficiency is calculated for the
10-yr 24-hr design event.
• The outfall / system design
must account for total
drainage area served by
sediment basin.
Surface Outlet & Baffle Sediment Basin Design
• New design requires dewatering from water surface and
implements 3 spillways:
1. Primary Riser Spillway
2. Floating Skimmer
3. Stabilized Emergency Spillway
o (pass the 100-year 24-hr storm event with 1-ft min freeboard)
Temporary vs. Multipurpose
• No perforations or weirs in Primary
Riser for Temporary Basins that do
not require peak flow reduction to
pre-development conditions.
• Orifices and weirs incorporated into
Primary Riser for Permanent
Multipurpose Basins requiring
peak flow reduction for disturbedstate peak flow rates.
o Ensure all Multipurpose Sediment
Basins are designed to meet any postconstruction water quality specifications
if required.
General Design Requirements
• Drainage Area: 5 acre min, 100 acre max.
• Drainage Area: 10 acres or more draining to one location
requires a Surface Outlet & Baffle Sediment Basin.
• Min 80% design removal efficiency for TSS.
• Sediment storage volume accounted in overall design volume.
• No side slopes steeper than 3H:1V where applicable.
• Optimum Basin length to width ratio is 2L:1W.
• Temporary Basin bottom slope of 0.5%.
• Permanent Dry Multipurpose Basins final Basin bottom slope
of 2%.
• Anti-vortex device / trash rack required for Primary Riser.
General Design Requirements
• Floating Skimmer with min dewatering time of 24 hrs. and max
dewatering time of 72 hrs.
• Min 3 Baffles installed in Basin.
• At least 1 row of Baffles placed between Primary Riser
structure and all pipes or channels discharging into Basin.
• Min embankment width at the top of the dam is 8 feet.
• Antiseep collars on all penetrations through the dam.
• Perform temporary stabilization by seeding and install
Temporary Erosion Control Blankets on Basin side slopes.
Temporary Basin Primary Riser
• Solid riser, no low flow orifice or staged orifice/weir discharges.
• Runoff only enters Primary Riser structure by overtopping and
through Floating Skimmer.
• Design max 10-year 24-hour storm peak stage elevation
approximately 6 inches to 1-foot above top elevation of
Primary Riser to ensure weir flow / surface withdrawal.
Temporary Basin Primary Riser
• Primary Riser and Outlet Barrel operate in weir flow control
and transition to pipe/barrel flow control. Orifice flow not
allowed for 10-year 24-hour storm event.
• Minimum 1.5-foot elevation difference from top of riser to crest
of emergency spillway.
• Emergency Spillway safely pass peak runoff from 100-yr 24-hr
storm event with min 1.0-ft freeboard.
• Design Emergency Spillway as a run-around conveyance that
is constructed on existing ground and not over the Basin
Dam/Embankment.
• Show all Basin dimensions and slopes on the Plans.
Baffles
• Install 3 rows of Baffles with a spacing of ¼ the basin length for
Basins greater than 25 feet in length.
• Install 2 rows of Baffles with a spacing of 1/3 the basin length
for Basins less than 25 feet in length.
• Place at least 1 row of Baffles between Primary Riser structure
and all pipes or channels discharging to Basin.
Baffles
• Baffles may consist of Porous Baffles, or Riprap Baffles.
• Install Riprap Baffles consisting of Class A or B Riprap. Do not
place washed stone on the face of the Riprap Baffles.
• Designers may use 0% dead space when using Baffles.
• Baffle Height as directed by the Design Engineer,
o Height based on the 10-yr 24-hr design water surface elevation.
o In no case will Baffle material height be higher than primary spillway
elevation of the sediment basin or sediment trap.
Baffles
• Porous Baffle TRM Material
Physical Property1
Test Method
ASTM D 6567
or Equivalent
Required Value
10% Min
35% Max
Tensile Strength2
ASTM D 6818
145 X 110 lb/ft min.
Ultraviolet Stability
(retained strength after 1000 hrs of exposure)
ASTM D 4355
80%
Light Penetration (% openings)
• Porous Baffle Coconut / Excelsior Blanket Material
Physical Property
Test Method
Required Value
Light Penetration (% openings)
ASTM D 6567
or Equivalent
10% Min
35% Max
Tensile Strength1 (machine direction)
ASTM D 6818
ASTM D 4595
145 lb/ft Min
Baffles
Baffles
Floating Skimmers
• Design Primary Riser with Floating Skimmer attached to
bottom of riser dewatering runoff volume below top elevation of
riser in a time period ranging between 24 to 72 hours.
• Not required for sediment traps.
• Include a trash guard and maintenance rope to prevent and
remove blockage from floating debris.
Floating Skimmers
• Multiple Manufacturers (many more)
o Faircloth
o IAS
o Erosion Supply Company
• Ensure manufacturer's drawdown
rates singed/sealed by SC PE.
• May require 3rd party testing
Sediment Cleanout Stake
• Install a metal post sediment clean out stake 4-feet above the
Basin bottom in the first Baffle cell upstream of the first Baffle.
• Cleanout Basin when sediment level reaches 2-foot mark on
sediment cleanout stake (50% of the sediment storage
volume).
Permanent Multipurpose Basin Option 1
• PEAK FLOW CONTROL DURING CONSTRUCTION REQUIRED
• Design and utilize 1 Permanent Riser and outlet barrel
configuration for both phases of project based on postconstruction water quality and quantity control.
o Ensures no damage or structural weakness to the primary riser, outflow
barrel and permanent dam structure resulting from installing a temporary
riser, removing it, and then installing a permanent riser.
o Maintaining the structural integrity of the outflow barrel and permanent
dam structure is of the highest importance.
o Ensures a small temporary riser is not installed to maintain peak flow
rates by forcing a large head on top of the riser that could jeopardize the
stability of the riser, barrel, and permanent dam.
Permanent Multipurpose Basin Option 1
• Primary Riser has Floating Skimmer attached to bottom of riser
(typically post construction water quality low flow orifice) during
construction phase to dewater in 24 to 72 hrs.
o Skimmer provides withdraw from water surface for majority of storm
events during the construction phase of the project.
• Design Primary Riser with orifices/weirs to provide peak flow
rate control and/or water quality control.
Permanent Multipurpose Basin Option 1
• Design Primary Riser to have trash rack and anti-vortex
device.
• Provide min of 3 rows of Baffles during construction. Install at
least 1 row of Baffles between riser structure and all pipes or
channels discharging to the Basin.
• Designers may use 0% dead space when using Baffles.
Permanent Multipurpose Basin Option 1
• Provide calculations showing 80% trapping efficiency during
construction phase of project.
• Provide calculations showing basin designed to meet preconstruction peak flow rates for the 2-year 10-year, (25-year)
24-hour storm events.
• Emergency Spillway safely pass peak runoff from 100-year 24hour storm event with min 1.0-ft freeboard.
• Post construction staged orifices, low flow orifices, or staged
weirs are installed in the Primary Riser structure prior to
construction phase.
Permanent Multipurpose Basin Option 2
• PEAK FLOW CONTROL DURING CONSTRUCTION NOT REQUIRED
• 2 spillway configurations
o Temporary Riser Spillway consisting of solid riser with no staged
discharges.
o Permanent Multipurpose Basin Riser spillway designed to reduce postdevelopment peak flow rates to pre-development peak flow rates for the
2-yr and 10-yr, (25-yr) 24-hour storm events where applicable or
designed for post-construction water quality control.
• Post-construction staged orifices, low flow orifices, or staged
weirs securely covered or sealed during construction phase.
• Uncover post-construction staged orifices, low flow orifices, or
staged weirs after the construction phase is complete.
Permanent Multipurpose Basin Option 2
• Floating Skimmers and Baffles may
be removed when construction
phase ends.
• Clean Temporary Basin of
deposited sediment and re-grade
Basin to meet Permanent Basin
contours if necessary when
construction phase ends.
• Make any necessary modifications
to the Emergency Spillway after
construction phase has ended.
Permanent Multipurpose Basin Forebays
• Forebays are required for all
Multipurpose Basins.
• Design Forebay volume (or
combined volume of Forebays)
equal to min of 10% post
construction water quality
treatment volume of
Multipurpose Basin.
• Design Forebay(s) so 75
percent of required sediment
storage volume is allocated to
the Forebay.
Permanent Multipurpose Basin Forebays
• Forebay separated from Multipurpose
Basin area by berms, barriers, or
baffles that may be constructed of
earth, stones, riprap, gabions, or
geotextiles.
• Forebay berm may incorporate drain
pipe to drain Forebay so it does not
maintain a permanent pool.
• Design Forebay so it is accessible for
easy cleanout because it will eventually
fill in with coarse particles.
• Design the access to the Forebay with
a maximum slope of 15-20% extending
from top of embankment to toe.
Temp Surface Outlet & Baffle Basin Design
• Basins can be designed showing 80% trapping efficiency
during construction phase of project using:
o SEDCAD4
o Pond Pack
o Sedimot
o Other computer models
o Spreadsheet/Graphical
o Design Aids
Temp Surface Outlet & Baffle Basin Design
• SEDCAD4
o Incorporate Baffles by input of 0% dead space.
o Incorporate Skimmer discharge by selecting “User Input
Discharge” as the spillway type. (constant rate)
Calculate required drawdown volume below primary riser crest.
Determine design flow rate required for drawdown in 24-72 hours.
Select specific skimmer(s) required to obtain design drawdown rate.
Input skimmer specific constant rate flow rate in cfs as user input
discharge in SEDCAD.
The first two “This Spillway Discharge” values at “Elev’s (ft)”
0.00 and 0.01 are required to be zero.
If more than 1 skimmer is used, add the flow rates together to
create user input discharge.
Temp Surface Outlet & Baffle Basin Design
• SEDCAD4
Skimmer Design Aids
Temp Surface Outlet & Baffle Basin Design
• Design Aids
o SCDHEC Design Aids
o New Design Aids
Temp Surface Outlet & Baffle Basin Design
• SCHEC Design Aids
Where:
qpo = Peak outflow for 10-yr
24-hr storm (cfs),
A = Pond Surface area at
riser crest (acres),
V15 = Settling velocity (fps) of
D15 eroded particle (mm).
•
A basin not located in a
low lying area and not
having a high water table,
has basin ratio equal to
2.20 E5 at 80% trapping
efficiency.
Temporary Surface Outlet & Baffle Basin New
Design Aids (Sedimot Based)
• Required storage volume for sediment basins implementing
surface withdrawal and Porous Baffles
o 2,400ft3/acre for Upper State soils
o 415 ft3 per disturbed acre sediment storage.
• 7 foot deep basin with 4 foot riser
• Baffles 4 feet high
• Sediment basins conservatively designed for min sediment
trapping efficiency requirement of 80% TSS for 10yr 24hr
storm event.
New Design Aids
Temp Surface Outlet & Baffle Basin Design
• Spreadsheet / Graphical Design (From NCDENR)
o 435 ft2 per cfs of Q10 peak inflow min at riser crest.
Surface Outlet and Baffle Sediment Basins
Surface Outlet and Baffle Sediment Basins
Questions
100% Trapping
for Vehicles