Construction of On-SiteStormwater Treatment andFlow Control Facilities

Dan Cloak, P.E.PrincipalDan 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 requirementsin 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

Swaleor In-groundPlanter

Reservoir, 12" min. depth

Reverse bend trap or hooded overflow

18" sandy loam, minimum infiltration rate 5" per hour

12" open-graded gravel, approx. ½" dia.

Perforated pipe

Downspout

Building exterior wall

Cobbles or splash block

Filter fabric

Concrete or other structural planter wall with waterproof membrane

Additional waterproofing on building as needed

Drain to storm drain or discharge; bottom-out or side-out options

Planter Box

Dry Well

Sizing criterion for treatment

Planting medium

0.2 inches/hour

i = 5 inches/hour

BMP Area/Impervious Area =0.2/5 = 0.04

Application of sizing factor

LID for flow control

Can LID facilities mitigate increased peaks and volumes of flows from impervious areas?

How would we demonstrate that? What are the design criteria?

Before

After

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

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 1 2 3 4 5 6 7 8 9 10

Recurrence Interval (years)

Peak

Flo

w (c

fs)

ImperviousMitigated Post-Project SitePre-Project Site0.5Q2

IMP Reduces Impervious Runoff to Less Than Pre-Project Levels

Results: Flow Duration Control

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

% Time Exceeded

Flow

(cfs

)

ImperviousMitigated Post-Project SitePre-Project SiteQ100.1Q2

IMP Reduces Impervious Runoff to Less Than Pre-Project Levels

Sizing Factors for Flow Control

IMP Sizing FactorsIn-Ground Planter

Group A: 0.08Group B: 0.11Group C: 0.06Group D: 0.05

Flow-Through Planter

Group C: 0.06Group D: 0.05

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

Bioretention Basin

Group A: 0.13Group B: 0.15Group C: 0.08Group D: 0.06

IMP Sizing FactorsDry Well Group A: 0.05 to

0.06Group B: 0.06 to 0.09

Infiltration Trench

Group A: 0.05 to 0.06Group B: 0.07 to 0.10

Infiltration Basin

Group A: 0.05 to 0.10Group B: 0.06 to 0.16

Adjustment to annual rainfall

Group A, y = 0.0020x + 0.08Group B, y = -0.0005x + 0.11Group C, y = -0.0022x + 0.06Group D, y = -0.0022x + 0.05

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

-14 -12 -10 -8 -6 -4 -2 0 2 4 6

Mean Annual Rainfall (MAP) Relative to Martinez Gauge (in)

Sizi

ng F

acto

r

Group A soils

Group B soils

Group C soils

Group D soils

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