- Количество слайдов: 42
Stormwater in the FAHP
Stormwater Management Goals n n Treat the runoff generated from the Contributing Impervious Area by the Water Quality Design Storm using “Preferred” BMPs Maintain the frequency and duration of the most important channel forming flows
Triggers for Stormwater Management
What Triggers Stormwater Treatment Construct new pavement that increases capacity or widens the road prism: n n Add a lane, a turn refuge, a wider paved shoulder Build a new alignment
What Triggers Stormwater Treatment Increase the CIA Adding new pavement, or Change the drainage to capture stormwater from outside the pre-project CIS
What Triggers Stormwater Treatment Change the Stormwater Conveyance: n n Relocate the drainage system Change the type of conveyance: • Replace a ditch with a pipe • Add curbing, etc. n Enlarge the capacity of the conveyance
What Triggers Stormwater Treatment Reconstruct pavement down to subgrade This applies to projects rehabilitating a highway, not localized repairs.
What Triggers Stormwater Treatment Replace a Stream Crossing
What Triggers Stormwater Treatment Major Structural Repairs of Bridges This is the stuff that is not normal maintenance Exempted are Seismic Upgrades that do not affect the deck or drainage
What Is Not a Trigger Explicit: n n Minor repairs Repaving and resurfacing that does not go down to the subgrade
What Is Not a Trigger Implicit n Minor, localized increases in impervious surface, such as: • Guardrail flares • Police, bus, and mail-box pullouts n Sidewalks and separated bike/ped paths that do not include installation of curbing
The Thing About Sidewalks n n It’s not the sidewalk, it is the curb By putting in a curb the drainage is changed, and highway runoff that may have gotten some incidental treatment now gets none.
Flow Control Trigger Discharge into a receiving water with an upstream drainage basin of less than 100 mi 2, and increase the peak 10 year/24 hour discharge by 0. 5 cfs or more*.
Water Quality Design Storm Site Specific Storm Size from ODOT Precipitation Viewer Percentage of the 2 year 24 hour storm: 50% 67% 75% Min: 0. 7 inches Max: 2. 5 inches
Contributing Impervious Area: n n Pavement, new and old, within the project limits, and Highway related pavement that drains into the project
CIA Boundaries Project Limits
CIA if stormwater treatment within project limits CIA if stormwater treatment at the bottom of the hill Project Limits
n The stormwater that gets treated comes from • The project • Adjacent highway that drains onto project pavement • Highway that discharges into a drainage system that is modified by the project, or is in the conveyance when it discharges into the treatment facility
Always? If the CIA outside of the project limits is very large compared to the project, coordinate and negotiate with NMFS. Early The intention of the FAHP is not to derail projects by placing onerous requirements on projects.
Contributing Impervious Area: FAHP explicitly limits the CIA to the highway and highway associated facilities. Run-on from adjacent properties is not part of the CIA.
BMPs Infiltration: Surface Treatment • Infiltration Basins • UICs • Filter Strips with no adjacent conveyance or receiving water • Bioswale • Bioretention (with underdrain) • Bioslope (Ecology Embankment) • Filter strip • Detention/Retention Pond • Constructed Wetland • Proprietary Filter System (on QPL)
BMPs n n Use BMPs that are highly effective at treating the broad range of highway pollutants, i. e. those that incorporate a high degree of filtration through soil or media. ODOT’s Hydraulic Manual includes the BMP Selection Tool to help identify and choose the preferred BMPs
Stormwater Management Stragety 1. 2. 3. 4. 5. 6. Unmodified roadside ROW Modified (enhanced) roadside ROW Small, dispersed BMPs Large, consolidated BMPs Do the best possible, And if all else fails, Off-site mitigation
Off-site Mitigation: Justification n n Unfavorable topography Site hazards (geologic, haz-mat, safety etc. ) Conflicting resources (Wetlands, T&E, Historic, Archeologic, Env. Justice etc. ) Excessive cost to benefit (ROW, Maintenance/Life Cycle, Construction)
Off-Site Mitigation MUST coordinate with NMFS!!! Aim for a site with n n n The same general ADT, The same impervious surface area, and The same watershed.
Flow Control Range of Flows: n n n Lower end Eastern Cascade = 56% of the 2 -year SE, N Central Oregon = 48% of the 2 -year Western Oregon: 42% of the 2 year Upper End Bank overtopping if ER is = or > 2. 2 10 year 24 hr storm if ER is < 2. 2
Approval of SWMPs Required for Flow Control for projects discharging to watersheds smaller than 100 mi 2, or n n Projects that do not meet the SW design standards on-site.
Notification n n Impervious surface, both project and total CIA, with net change Acres treated on and off site, by infiltration and surface BMPs
Notification Traffic Volumes for the project area and any off-site mitigation location General Categories: n Very High: >100, 000 ADT n High: 30, 000 -100, 000 n Medium: 10, 000 -30, 000 n Low: 2, 000 -10, 000 n Very Low: <2, 000
Notification n Water Quality and Flow Control Design Storm depths Reason for exemption from flow control (if applicable) Stormwater Manual used for design
Notification n List of BMPs used • Drainage Area (AKA Sub-basin) • Treatment Method (category): on or off site, surface discharge or infiltration • BMP type • Impervious surface area treated • Receiving Water
Notification Site plan with CIA, sub-basins, flow path, BMP locations and receiving waters
Tracking/Monitoring n n n BMPs Installed, Inspected and Maintained Any Stormwater Recommendation or Design reports developed and stamped by an engineer O&M Manual
Tracking/Monitoring n n Photo of the outfalls from a project Map of the project with outfalls and receiving waters
Other Stuff If the highway runoff discharges to a CSO: n n Documentation that the facility will accept the runoff and has adequate capacity Description of how the runoff will be treated to a level comparable to the BMPs listed in the FAHP.
Advance Mitigation n n No formal process yet. NMFS does not have a mechanism outside of a BO to designate treatment at one location as mitigation for a future project, and even that is uncertain.
“Excess Treatment” n n No formal definition… Deliberately taking stormwater from outside of a project’s CIA in order to treat it in a project’s BMP Specify a project or an area that may need the off-site mitigation Discuss this with NMFS in advance!