Transcript Construction of On-Site Stormwater Treatment and Flow Control Facilities Dan Cloak Environmental Consulting

Construction of On-Site
Stormwater Treatment and
Flow Control Facilities
Dan Cloak, P.E.
Principal
Dan Cloak Environmental Consulting
Topics
 Why
do we have permanent on-site
facilities to treat stormwater?
 Facility designs and how they work
 Operating characteristics and
errors in design and construction
 Draft checklist for facility
inspection
NPDES requirements
in a nutshell
Minimize imperviousness
 Control pollutant sources
 Treat stormwater prior to
discharge from the site
 Match peaks and durations to
pre-project conditions (HMP)
 Maintain treatment and flow-control
facilities in perpetuity

Low Impact Development
Stormwater treatment
and flow control
 Minimize
imperviousness
 Disperse runoff
 Use Integrated
Management Practices
(IMPs)

Residential Development
Large hillside project
Swale
or
In-ground
Planter
Planter Box
Reservoir,
12" min. depth
Reverse bend
trap or hooded
overflow
Building
exterior wall
Downspout
Cobbles or
splash block
18" sandy loam,
minimum
infiltration rate
5" per hour
Filter fabric
Concrete or other
structural planter wall with
waterproof membrane
12" open-graded
gravel, approx.
½" dia.
Perforated pipe
Additional
waterproofing on
building as
needed
Drain to storm drain or discharge;
bottom-out or side-out options
Dry Well
Sizing criterion for treatment
0.2 inches/hour
BMP Area/Impervious Area =
0.2/5 = 0.04
Planting medium
i = 5 inches/hour
Application of sizing factor
LID for flow control
After
Before
 Can
LID facilities mitigate
increased peaks and volumes
of flows from impervious areas?
 How would we demonstrate that?
 What are the design criteria?
HSPF analysis of unit-acre runoff
33 years hourly rainfall
 Pre-project condition
 100% impervious condition
 Hydrologic soil groups A, B, C, D
 Swales, Bioretention Areas,
In-ground and Flow-through Planters



Underdrain with flow-restrictor in C&D soils
Dry wells, infiltration trenches and basins
Results: Control of Peak Flows
1.20
Impervious
Mitigated Post-Project Site
Pre-Project Site
0.5Q2
1.00
IMP Reduces Impervious Runoff
to Less Than Pre-Project Levels
Peak Flow (cfs)
0.80
0.60
0.40
0.20
0.00
0
1
2
3
4
5
6
Recurrence Interval (years)
7
8
9
10
Results: Flow Duration Control
1.00
Impervious
Mitigated Post-Project Site
Pre-Project Site
Q10
0.1Q2
0.90
0.80
Flow (cfs)
0.70
0.60
0.50
IMP Reduces Impervious Runoff
to Less Than Pre-Project Levels
0.40
0.30
0.20
0.10
0.00
0.00
0.05
0.10
0.15
0.20
0.25
% Time Exceeded
0.30
0.35
0.40
Sizing Factors for Flow Control
IMP
Sizing Factors
IMP
Sizing Factors
In-Ground
Planter
Group A: 0.08
Group B: 0.11
Group C: 0.06
Group D: 0.05
Dry Well
FlowThrough
Planter
Group C: 0.06
Group D: 0.05
Infiltration Group A: 0.05 to 0.06
Trench
Group B: 0.07 to 0.10
Vegetated/
Grassy
Swale
Group A: 0.10 to 0.14
Group B: 0.14 to 0.21
Group C: 0.10 to 0.15
Group D: 0.07 to 0.12
Infiltration Group A: 0.05 to 0.10
Basin
Group B: 0.06 to 0.16
Bioretention Group A: 0.13
Basin
Group B: 0.15
Group C: 0.08
Group D: 0.06
Group A: 0.05 to 0.06
Group B: 0.06 to 0.09
Adjustment to annual rainfall
0.14
0.12
Sizing Factor
0.10
0.08
0.06
Group A soils
Group B soils
Group C soils
Group D soils
0.04
Group A, y = 0.0020x + 0.08
Group B, y = -0.0005x + 0.11
Group C, y = -0.0022x + 0.06
Group D, y = -0.0022x + 0.05
0.02
0.00
-14
-12
-10
-8
-6
-4
-2
0
2
Mean Annual Rainfall (MAP) Relative to Martinez Gauge (in)
4
6
Key Operating Characteristics
Runoff from the intended tributary
area must flow to the facility.
 The surface reservoir must fill to its
intended volume during high inflows.
 Runoff must filter rapidly through
the soil layer.
 Filtered runoff must infiltrate into the
native soil to the extent possible.
 Remaining runoff must be captured and
drained to daylight or a storm drain.

Tributary Area

Drainage
area
includes
portions of
roof and of
parking lot
Ensuring flow to the facility
Runoff may
enter by
sheet flow or
be piped.
 Roof leaders
can be piped
directly or
spill across
pavement

Distribute flow evenly
Distribute flow evenly
Surface reservoir must fill
Surface reservoir must fill
Surface reservoir must fill
Surface reservoir must fill
Surface reservoir must fill
Surface reservoir must fill
Runoff must drain rapidly





Typically no native
on-site material to be
used
Imported material to
be a mix of sand and
organics
Minimum infiltration
rate 5"/hour
Aim for 10"/hour at
installation
On-site bucket test
No filter fabric
Promote Infiltration



Protect excavation
from fine sediment
and compaction
Re-excavate if used
for sediment control
during construction
Rip the bottom soils
to promote
infiltration
Underdrain
 Perforated
pipe
 Solid pipe nearest 2' to connections
 Bedded in gravel layer
 Connected to storm drain or
daylight
 Cleanouts
 Omit in sandy soils
Recap
Runoff from the intended tributary
area must flow to the facility.
 The surface reservoir must fill to its
intended volume during high inflows.
 Runoff must filter rapidly through
the soil layer.
 Filtered runoff must infiltrate into the
native soil to the extent possible.
 Remaining runoff must be captured and
drained to daylight or a storm drain.

Additional Features
 Check
dams
 Moisture barriers/cutoff walls
 Outflow-limiting orifices on
underdrains
 Specified depths or volumes for
surface reservoirs or gravel layers
 Check plans for facility
cross-sections and details
Checklist

Pre-construction
Review facility details
 Review required inspections


Site Preparation
Erosion and sediment controls
 Runoff diverted from facilities
 Facility areas marked
 Pipes and appurtenances
delivered and inspected

Checklist

Excavation and Grading
Grade breaks and elevations correct
 Bottom of excavation elevation and
dimensions correct
 Native soil at excavation bottom ripped
 Embankment/wall elevations and widths
correct and level.
 Drainage material is Class 2 perm
 Bucket test on imported soil mix achieves
8-12 inches per hour

Checklist

Facility Installation
Location size and depths of appurtenances
 Roof leaders and impervious areas drain to
facility as intended
 Underdrain, inlet, outlet, orifice, cleanouts,
and overflow are located and installed
correctly
 Overflow at correct elevation
 Moisture barrier or cut-off walls installed
correctly

Checklist

Facility Installation
Underdrain base elevation correct and
drain bedded properly
 Irrigation system on separate zone
 Gravel (Class 2 perm) depth correct
 Soil media installed and passes bucket test
 Soil top elevation and surface reservoir
depth correct
 Inlets and overflows correctly located

Checklist

Landscaping and planting
Vegetation complies with approved
planting plan
 Woody vegetation won’t block inlets
and overflows

Checklist

Final check
Areas draining to facility are cleaned
(pavement) or stabilized (landscape)
 Inlets installed and operational
 Configuration, size and depth of facility
correct
 Vegetation established and irrigation
operating
 Overflow installed and free of debris
 Runoff reaches the facility
