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

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

Results: Flow Duration Control

Sizing Factors for Flow Control IMPSizing Factors In-Ground Planter Group A: 0.08 Group B: 0.11 Group C: 0.06 Group D: 0.05 Flow- Through Planter Group C: 0.06 Group 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: 0.13 Group B: 0.15 Group C: 0.08 Group D: 0.06 IMPSizing Factors Dry WellGroup A: 0.05 to 0.06 Group B: 0.06 to 0.09 Infiltration Trench Group A: 0.05 to 0.06 Group B: 0.07 to 0.10 Infiltration Basin Group A: 0.05 to 0.10 Group B: 0.06 to 0.16

Adjustment to annual rainfall

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

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