How to Determine When a Right-of-way Project Triggers the SWMM

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Use the guidance below to determine if your right-of-way project triggers requirements of the 2020 Stormwater Management Manual.
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Step 1. Determine if the right-of-way activities are exempt.

If your project only involves the following activities in the right-of-way, then you do not need to continue through the following steps.

  • Linear utility trenching
  • Right-of-way work that can be completed under a PBOT Minor Improvement Permit
  • Pavement base repair less than 50 percent roadway width. This includes the following scenarios, provided no more than 50 percent of the roadway width is removed or repaved.
    • Curb replacement in-place (and associated pavement restoration)
    • Mainline utility trenches (lateral utility service connections are fully exempt).

Step 2. Calculate new or redeveloped impervious area in the right-of-way. Exclude all exempt areas.

See SWMM Section 1.2.1 for additional information.

  • Calculate new and redeveloped impervious areas in the right-of-way: New development and redevelopment activities that create or replace 500 square feet or more of impervious area are subject to the requirements of the SWMM unless specifically exempt. Triggered impervious area must be based on all constructed or replaced area that is not exempt. It is not calculated based on net change in impervious area. Common scenarios include:
    • Full-width roadway replacement with base repair.
    • Construction of a brand new sidewalk behind an existing curb.
    • Full-width alleyway pavement improvements with base repair.
  • Subtract exempt impervious areas in the right-of-way: Common exemptions include (refer to SWMM Section 1.2.1 for a complete list of pavement and trenching exemptions):
    • Pavement surface maintenance: This includes repaving and resurfacing within the existing footprint of the paved surface, provided no subgrade (soil) is exposed and pavement is replaced in-kind such as using the same paving material. For pedestrian-only areas (including sidewalks), unless base repair is explicitly specified such as a basement extending under sidewalk, replacement in-kind is assumed to be maintenance

Examples of improvements and the resulting impervious areas triggering SWMM requirements

Graphic shows two diagrams side by side. Diagram on left shows existing conditions. Half is planting strip, half is gravel street. Next graphic shows proposed conditions. Half shows new paved roadway. Other half contains a curb, planting strip, sidewalk, and private property.
In this scenario, count all new impervious surface in the roadway, curb, and sidewalk corridor.
Graphic shows a diagram of existing conditions next to a diagram of proposed conditions. Existing conditions include a paved roadway with no curb or sidewalk. Proposed conditions show existing roadway improved with new curb and sidewalk.
In this scenario, count all new impervious surface in the sidewalk corridor plus portions of the existing pavement in the roadway that are considered a new or changed connection per SWMM Section 1.2.2.
Graphic shows a diagram of existing conditions next to a diagram of proposed conditions. Existing conditions include a paved roadway with curb but no sidewalk. Proposed conditions show existing roadway improved with new sidewalk and furnishing zone.
In this scenario, count all new impervious surface in the new sidewalk corridor. Note that any paving within the furnishing zone must also be counted towards the triggered impervious area.
Graphic shows a diagram of existing conditions next to a diagram of proposed conditions. Existing conditions include a paved roadway, curb, sidewalk and private property/frontage zone portions. Proposed conditions are the same with only the width of sidewalk increased.
In this scenario, count the new portion of the sidewalk outside the existing footprint. The remaining portion meets a maintenance exemption (unless base repair is explicitly specified). Note that any paving within the furnishing zone must also be counted as triggered impervious area.
Diagram shows existing conditions next to proposed conditions. Existing include private property/frontage zone, narrow sidewalk, curb, and paved roadway. Proposed conditions are the same except with a wider sidewalk.
In this scenario, base repair is explicitly specified in the sidewalk expansion, so the existing sidewalk does not meet the maintenance exemption. Entire sidewalk triggers the SWMM. This also applies in scenarios where impervious area is not replaced in-kind or considered maintained by City.
Diagrams of existing and proposed conditions side-by-side. Existing includes private property/frontage zone, sidewalk, curb, and paved roadway. Proposed shows new wider sidewalk and a vegetated furnishing zone.
In this scenario, count the new portion of sidewalk outside the existing footprint. The remaining portion meets the maintenance exemption (unless base repair is explicitly specified). Note that any paving within the furnishing zone must also be counted towards the triggered impervious area.
Diagram shows existing and proposed conditions side-by-side. Existing includes private property/frontage zone, sidewalk, curb, and paved roadway. Proposed shows sidewalk moved to make room for a new furnishing zone.
In this scenario, count the entire new sidewalk width. Note that any paving within the furnishing zone must also be counted towards the triggered impervious area.
Diagram shows existing and proposed conditions side-by-side. Existing includes private property/frontage zone, sidewalk, curb, and paved roadway. Proposed shows an expanded sidewalk that extends into paved roadway and private property/frontage zone.
In this scenario, count all new impervious area and redevelopment pavement that is not replaced in-kind (for example, asphalt to concrete).

Step 3. Add areas of new connection or new route of conveyance in the right-of-way.

See SWMM Section 1.2.2 for additional information.

  • Projects that make new connections, route new drainage areas to a stormwater receiving system, or change an existing connection or discharge point must provide stormwater management for those areas if the project connects or reroutes impervious areas that are 500 square feet or more in total. This includes changing a discharge point from one SWMM hierarchy level to another or creating a new route of conveyance where one did not previously exist.
  • Adding a new curb along a roadway is considered a new connection if the resulting runoff will discharge to a new or changed disposal location per the definition in Section 1.2.2.
  • Common scenarios include:
    • Constructing a new curb to provide conveyance to a system where no stormwater conveyance currently exists.
    • Changing a roadway shed section to a crown section. A portion of the roadway drainage is then conveyed to a different area, becoming either unmanaged or entering a different public system.

Step 4. Does the sum of steps 2 and 3 exceed 500 square feet?

  • No. If your project area is less than 500 square feet, then your project does not trigger SWMM requirements. You are done.
  • Yes. Calculate and subtract your tree credits. See SWMM Section 4.2.5.
    After subtracting the tree credits, what is the project's total impervious area?
    • Less than 500 square feet. Your project triggers the SWMM  and is adequately managed with tree credits. Submit a tree credit worksheet, and you are done. 
Tree Credit Worksheet143.28 KB

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Related

2020 Stormwater Management Manual

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Construction and development
Environment
Water, stormwater, and sewer

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