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Stormwater in the FAHP

Stormwater in the FAHP. Stormwater in the FAHP. Stormwater Management Goals. 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.

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Stormwater in the FAHP

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  1. Stormwater in the FAHP Stormwater in the FAHP

  2. Stormwater Management Goals • 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

  3. Triggers for Stormwater Management

  4. What Triggers Stormwater Treatment Construct new pavement that increases capacity or widens the road prism: • Add a lane, a turn refuge, a wider paved shoulder • Build a new alignment

  5. What Triggers Stormwater Treatment Increase the CIA Adding new pavement, or Change the drainage to capture stormwater from outside the pre-project CIS

  6. What Triggers Stormwater Treatment Change the Stormwater Conveyance: • Relocate the drainage system • Change the type of conveyance: • Replace a ditch with a pipe • Add curbing, etc. • Enlarge the capacity of the conveyance

  7. What Triggers Stormwater Treatment Reconstruct pavement down to subgrade This applies to projects rehabilitating a highway, not localized repairs.

  8. What Triggers Stormwater Treatment Replace a Stream Crossing

  9. 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

  10. What Is Not a Trigger Explicit: • Minor repairs • Repaving and resurfacing that does not go down to the subgrade

  11. What Is Not a Trigger Implicit • Minor, localized increases in impervious surface, such as: • Guardrail flares • Police, bus, and mail-box pullouts • Sidewalks and separated bike/ped paths that do not include installation of curbing

  12. The Thing About Sidewalks • 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.

  13. Flow Control Trigger Discharge into a receiving water with an upstream drainage basin of less than 100 mi2, and increase the peak 10 year/24 hour discharge by 0.5 cfs or more*.

  14. Water Quality Design Storm Site Specific Storm Size from ODOT Precipitation Viewer Percentage of the 2 year 24 hour storm: 50% 67% 75%

  15. Contributing Impervious Area: • Pavement, new and old, within the project limits, and • Highway related pavement that drains into the project

  16. CIA Boundaries Project Limits

  17. CIA if stormwater treatment within project limits CIA if stormwater treatment at the bottom of the hill Project Limits

  18. 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

  19. 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.

  20. 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.

  21. BMPs • Infiltration: • Infiltration Basins • UICs • Filter Strips with no • adjacent conveyance • or receiving water • Surface Treatment • Bioswale • Bioretention (with underdrain) • Bioslope (Ecology Embankment) • Filter strip • Detention/Retention Pond • Constructed Wetland • Proprietary Filter System • (on QPL)

  22. BMPs • 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

  23. Stormwater Management Stragety • 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

  24. Off-site Mitigation: Justification • 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)

  25. Off-Site Mitigation MUST coordinate with NMFS!!! Aim for a site with • The same general ADT, • The same impervious surface area, and • The same watershed.

  26. Flow Control Range of Flows: Lower end • Western Oregon: 42% of the 2 year • Eastern/Central: 50% of the 2 year Upper End • Bank overtopping if ER is = or > 2.2 • 10 year 24 hr storm if ER is < 2.2

  27. Approval of SWMPs • Required for Flow Control for projects discharging to watersheds smaller than 100 mi2, or • Projects that do not meet the SW design standards on-site.

  28. Notification

  29. Notification • Impervious surface, both project and total CIA, with net change • Acres treated on and off site, by infiltration and surface BMPs

  30. Notification Traffic Volumes for the project area and any off-site mitigation location General Categories: Very High: >100,000 ADT High: 30,000-100,000 Medium: 10,000-30,000 Low: 2,000-10,000 Very Low: <2,000

  31. Notification • Water Quality and Flow Control Design Storm depths • Reason for exemption from flow control (if applicable) • Stormwater Manual used for design

  32. Notification • 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

  33. Notification Site plan with CIA, sub-basins, flow path, BMP locations and receiving waters

  34. Tracking/Monitoring • BMPs Installed, Inspected and Maintained • Any Stormwater Recommendation or Design reports developed and stamped by an engineer • O&M Manual

  35. Tracking/Monitoring • Photo of the outfalls from a project • Map of the project with outfalls and receiving waters

  36. Other Stuff If the highway runoff discharges to a CSO: • 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.

  37. Advance Mitigation 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.

  38. “Excess Treatment” 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!

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