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OHIO’S NPDES STORMWATER GENERAL PERMITFOR CONSTRUCTION ACTIVITIES
OHC00005Justin Reinhart, PE
Division of Surface Water
ASCE Spring SeminarColumbus, OhioApril 26, 2018
Goals / Outline
1. CGP background / overview2. Update changes
– Administrative– Technical
3. Some technical instruction
CGP Background
National Pollution Discharge Elimination System and ORC 6111.
Authorizes storm water discharge associated with construction activitythat enters surface waters of the state or a storm drain leading tosurface waters of the state.
Construction activities include clearing, grading, excavating, grubbingand/or filling & support activities such as concrete/asphalt batchplants, staging/material storage yards, and disposal/borrow areas.
Disturb one or more acre of total land or CPoD will be one or moreacres.
Why Construction Activities ?
“end-of-pipe concentrations [of heavy metals]exceed EPA ambient water quality criteria anddrinking water standards in many instances.”
“Coliform are present at high levels in urbanrunoff and can be expected to exceed EPA waterquality criteria…”
In 1980, the Nationwide Urban Runoff Program concludes:
“BOD5 and CODconcentrations suggestthat…urban runoff iscomparable in magnitudeto secondary treatmentplant discharges”
To Comply
1. Submit NOI 21 days prior to construction to obtain coverage(45 for Darby & Olentangy).
2. Adhere to non-numeric effluent limitations.
Storm Water Pollution Prevention Plan
• Stand alone document.• Kept on site.• Maintained throughout construction (living document)• Includes stand alone Operation & Maintenance manual.• Must be submitted & OEPA approved if in Darby & Olentangy
special watershed areas
Storm Water Pollution Prevention PlanIncludes:
– Non-structuralpreservation
– Erosion controls– Runoff controls– Sediment controls– Surface water
protection– Other pollutant controls
Storm Water Pollution Prevention PlanIncludes:
– Post-construction storm water controls
Infiltration
Extended detention
GI / LID
Post-Construction Controls
“So that receiving stream’s physical, chemical and biologicalcharacteristics are protected, and stream functions aremaintained, post-construction storm water practices shallprovide long-term management of runoff quality and quantity.”
Water Quality Volume approach1:• Majority of annual precipitation volume captured in smaller
storms.• Coupled with drawdown time for “readiness”.• 80% TSS removal goal (CZARA, 1990).• “Maximum Extent Practical”.
1 Design of Urban Stormwater Controls (2012) WEF/ASCE
PERMIT CHANGESOHC00005 Effective April 23, 2018
Fifth Generation CGP
1992 Initial permit applies to 5+ acres2003 Expanded to 1+ acre, addition of WQv criteria above
5 acres2008 Updates to sediment pond sizing2013 Require sediment pond skimmer, add permeable
pavement2018 Revisions to post-construction requirements
Administrative Changes
A. Consolidate statewide, Big Darby & Olentangy (portions) in toone CGP.
Administrative Changes
B. Require electronic submittal through eBusiness Center.
Administrative Changes
C. Clarify definition of “Operator”, “General Contractor” &“Subcontractor”.
E. Clarify inspection frequency.F. Transition from OHC4 to OHC5:
– 180 days to renew existing coverage– If under construction or received local approval, no update to new
post-construction is required.– Phases draining to existing regional control w/ WQ treatment do
not need to update to new post-construction.– Olentangy CGP to remain in effect until next May.
Administrative Changes
For questions regarding NOIs, processing, payments, NOTs,waivers, etc. contact:
Jason [email protected]
614-728-1793
Mike [email protected]
614-752-0782
Technical Changes - ESC
A. 12-inch silt barrier as a suitable substitute for silt fence forsheet flow control.
Technical Changes - ESC
B. Clarify sediment pond requirements:– Are appropriate for sites under 10 acres.– All require 48 hr minimum drain time.
Technical Changes – Post Construction
C. Small construction activities defined as sites of 2 or lessacres.
“…the SWP3 shall contain a description of the post-constructionBMPs that will be installed during construction for the site and therationale for their selection.
The rationale shall address the anticipated impacts on the channeland floodplain morphology, hydrology and water quality.”
Technical Changes – Post Construction
D. Previous WQv:
Where,WQv = water quality volume (ac-ft)Pwq = 0.75 inchesC = 0.858i3 - 0.78i2 + 0.774i + 0.04
i = impervious area ÷ total areaA = disturbed or contributing drainage area (ac)
WQv = Pwq × C × Adisturbed ÷
Source: WEF (1998)
Previous Approach
Mean runoff producing storm(0.5”) x regression coefficient(1.5) = 0.75”
Previous Approach
Volumetric runoff coefficient ‘C’
Source: Urbonas, et. al. (1989)
WEF Approach Using Up-to-Date PrecipitationInformation
Mean runoff producing event = 0.62”w/24 hr multiplier = 0.98”w/48 hr multiplier = 1.22”
Previous Approach
Volumetric runoff coefficient:
Vol Treated x BMP Removal EfficiencyApproach
• Assumes several viable BMPs that can achieve ~90% TSSremoval from volume treated (i.e., that does not bypasstreatment mechanism) – includes wet ED basins, wetland EDbasins, bioretention, permeable pavement.
• 80% TSS removal on annual basis is achieved by capturing andtreating 90% of annual runoff volume.
90% vol capt’d × 90% treatment eff ≈ 80% reduction
Modeled Approach
Source: Dorsey, J & Winston (2018)
Technical Changes – Post Construction
D. New WQv :
Where,WQv = water quality volume (ac-ft)Pwq = 0.90 inchesRv = 0.05+0.9(i)
i = impervious area ÷ total areaA = disturbed or contributing drainage area (ac)
WQv = Pwq × Rv × Adisturbed ÷
Technical Changes – Post Construction
E. WQv equation for mixed new and redevelopment sites:
WQv = ( . × Rv1) + (Rv2 − Rv1) × Pwq × Adist
where,Rv1 = Pre-development runoff coefficientRv2 = Post-development runoff coefficient
Example – Mixed WQv
A 3 acre site will be redeveloped. The current site has 1 acre ofimperviousness. The proposed development will result in 2 acresof imperviousness.
Rv1 = Pre-development = 0.05 + 0.9(0.33) = 0.347Rv2 = Post-development = 0.05 + 0.9(0.66) = 0.644
(86% increase in Rv)
Example – Mixed WQv
A 3 acre site will be redeveloped. The current site has 1 acre ofimperviousness. The proposed development will result in 2 acresof imperviousness.
WQv = ( . × Rv1) + (Rv2− Rv1) × Pwq × Adist ÷WQv = ( . × 0.347) + (0.644− 0.347) × 0.90" × 3 ac ÷
WQv = . ac−ft (43% reduction)
Example – Mixed WQv
A 3 acre site will be redeveloped. The current site has 1 acre ofimperviousness. The proposed development will result in 0.9acres of imperviousness.
Rv1 = Pre-development = 0.05 + 0.9(0.33) = 0.347Rv2 = Post-development = 0.05 + 0.9(0.30) = 0.320
(8.6% decrease in Rv)
Example – Mixed WQv
A 3 acre site will be redeveloped. The current site has 1 acre ofimperviousness. The proposed development will result in 0.9acres of imperviousness.
WQv = ( . × Rv1) + (Rv2− Rv1) × Pwq × Adist ÷WQv = ( . × 0.347) + (0.320− 0.347) × 0.90" × 3 ac ÷
WQv = . ac−ft (68% reduction)
Technical Changes – Post Construction
F. Water Quality Flow (WQF) added for design of flow-throughpractices:
where,WQF = water quality discharge rate (cfs)C = rational runoff coefficienti = rainfall intensity (in/hr)A = drainage area (ac)
Example - WQF
A 1 acre parking lot, with a time of concentration of 8 minutesdrains to a flow-through BMP, the required treatment flow iscalculated as:
WQF = C × i × A
WQF = 0.90 × 2.04 (in/hr) × 1 ac
WQF = 1.84 cfs
Technical Changes – Post Construction
G. Revised list of “standard practices” approved for general use.
Infiltration Trench & Infiltration Basins
• Ksat between 4 and 0.5 in/hr• < 20% Clay• 20:1 impervious limit
Underground Storage Systems
• Can be:– “Open” systems that infiltrate to
native soils– “Closed” systems lined or solid-wall– Gravel filled or structural storage
• Require pretreatment designed tothe following standards:
Open systems 80% TSS removalClosed systems 50% TSS removal
Technical Changes – Post Construction
H. Alternative practices protocol updated.– Lab or field testing to show 80% TSS removal.– Given particle size distribution.– NJDEP / WaTAPE.
Technical Changes – Post Construction
I. Added runoff reduction practices & credits.
(May also be used for Darby groundwater recharge mitigation.)
Example – Runoff Reduction
WQv = Pwq × Rv × Adisturbed ÷WQv = 0.9" × 0.50 × 2 ac ÷
WQv = . ac−ft
The WQv for a 2 acre site with 1 acre of impervious surface iscalculated as:
BMP
Example – Runoff ReductionWQ treatment credit can be taken if impervious surface disconnectionand a grass swale is used in lieu of the traditional drainage system:
BMP
Example – Runoff ReductionThe WQv using runoff reduction practices is calculated as:
BMP
sw = 0.2" × 0.50 × 1.5 ac ÷ = . ac−ft
= × 0.5 ac = 0.020 ac-ft
WQv = 0.075 − 0.020 − 0.013 = 0.042 ac−ft
1.5 acres to grass swale
