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17.38.035 Drainage and Stormwater Management Policies and Standards.

City code section
  1. A. Stormwater must be managed in as close proximity to the development or redevelopment site as is practicable. This section is intended to prevent a net negative impact on nearby streams, wetlands, groundwater, and other water bodies.  All local, state, and federal permit requirements related to implementation of stormwater management facilities must be met by the owner/operator prior to facility use.
    1. 1. The City may enter into agreements with property owners to manage stormwater flows through methods other than onsite controls:
      1. a. In facilities where public and private property flows co-mingle.
      2. b. In offsite managed areas that are “traded” for required onsite management areas related to new development and redevelopment.  The City may require more than a 1:1 exchange on the amount of required management area.
    2. 2. All discharges from a site must be routed through a discharge point to a receiving system as approved by the City Administrator. Approval of discharge points is subject to the following:
      1. a. The discharge must be conveyed along a route of service approved by the City Administrator.
      2. b. The discharge point must comply with the following:
        1. (1) The Sewer and Drainage Facilities Design Manual and the Source Control Manual, for sanitary, wastewater, or other discharges to the sanitary or combined system.
        2. (2) The Stormwater Management Manual and the Source Control Manual, for stormwater and other discharges to the City’s storm and drainage system, groundwater, or surface water.
  2. B. The quality of stormwater leaving the site after development or redevelopment must be treated to the extent practicable to reduce the discharge of pollutants based on the following criteria:
    1. 1. Except as allowed under Subsection B.2. below, the development or redevelopment must fully treat all stormwater:
      1. a. Onsite;
      2. b. Within the original parcel from which the new parcel was created; or
      3. c. In an approved offsite facility with sufficient capacity, as determined by the City Administrator.
    2. 2. The owner of a development or redevelopment with a stormwater discharge that cannot practicably comply with Subsection B.1. above may, with written City Administrator approval, meet stormwater requirements by:
      1. a. Managing stormwater in an offsite facility designed to treat flows from the subject property and managed by the site developer/owner or another legal agent:
      2. b. Managing stormwater in an offsite facility designed to treat flows from the subject property and operated by the City; or
      3. c. Paying a stormwater offsite management fee as required by the Stormwater Management Manual.
      4. 3. Stormwater management facilities required for development or redevelopment must be designed, installed and maintained in accordance with the Stormwater Management Manual.
    3. 4. Land use activities of particular concern as pollution sources may be required to implement additional pollution controls and source controls including but not limited to those management practices specified in the Source Control Manual.
    4. 5. The City Administrator is authorized to exempt land uses, discharge locations or other areas of Portland from the requirements of this Subsection if onsite pollution reduction or pollution control is not needed or desirable due to limited pollutant loads or offsite methods of pollution control are available. All exemptions are specified in the Stormwater Management Manual and the Source Control Manual.
  3. C. The quantity and flow rate of stormwater leaving the site after development or redevelopment must be equal to or less than the quantity and flow rate of stormwater leaving the site before development or redevelopment, as much as is practicable, based on the following criteria:
    1. 1. Except as allowed under Subsection C.2. below, stormwater will be fully managed:
      1. a. Onsite;
      2. b. Within the original parcel from which the new parcel was created; or
      3. c. In a privately developed offsite facility with sufficient capacity, as determined by the City Administrator.
    2. 2. The owner of a development or redevelopment with stormwater discharges that cannot practicably comply with Subsection C.1. above may, with written BES approval, meet stormwater requirements by either:
      1. a. Managing stormwater in an offsite facility designed for the volume and rate of flows from the subject property and managed by the site developer/site owner or another legal agent;
      2. b. Managing stormwater in an offsite facility designed for the volume and rate of flows from the subject property and operated by the City; or
      3. c. Paying a stormwater offsite management fee as required by the Stormwater Management Manual.
    3. 3. Development and redevelopment must mitigate all project impervious surfaces through retention and on-site infiltration to the maximum extent practicable.  Where on-site retention is not possible, or is limited, development and redevelopment must detain stormwater through a combination of measures that prevent an increased rate of flow leaving a site during a range of storm frequencies as specified in the Stormwater Management Manual.
    4. 4. The City Administrator is authorized to exempt areas of Portland from the quantity or flow rate control requirements if flow control is not needed or desirable because there is sufficient capacity and limited impacts to the receiving drainage system.  All exemptions will be specified in the Stormwater Management Manual.
    5. 5. Any development or redevelopment that discharges to a tributary of the Willamette River, other than the Columbia Slough, must design stormwater management facilities to minimize hydromodification impacts from storm events as determined by the City Administrator.
    6. 6. Site drainage facilities must be designed to safely convey less frequent, higher flows to an approved discharge point with adequate capacity without damage to the receiving system, whether natural or constructed.
  4. D. The pumping and discharge of groundwater to a City receiving system may be allowed only after a BES discharge authorization has been obtained, as required in the Source Control Manual. The application for that authorization must demonstrate that groundwater discharges meet the associated requirements in the Source Control Manual and Chapters 17.34 and 17.39, which govern both quality and quantity of groundwater discharges.
  5. E. All conveyance systems must be analyzed, designed and constructed for existing tributary offsite stormwater and developed onsite stormwater from the proposed project in compliance with the City's Sewer and Drainage Facilities Design Manual. The general goal of these standards is to convey both onsite and offsite waters in a way that meets the capacity needs of the City conveyance system, is protective of public health and safety, and minimizes environmental impacts in the downstream receiving system.
  6. F. All stormwater management facilities, source controls, and drainage systems must comply with the standards of the Stormwater Management Manual and the Source Control Manual and may require permit review and approval before commencement of work. Public systems must be reviewed and approved by the City Administrator in compliance with the sizing and location standards in the Stormwater Management Manual. Private onsite systems must be reviewed and approved by the City Administrator for compliance with the stormwater hierarchy and other guidance specified in the Stormwater Management Manual and the Source Control Manual, and may be reviewed by the City for compliance with the plumbing code regulations in Section 25.01.020 and the state building codes.  The City Administrator may require a written certification from the design professional on completion of the project to confirm that the facility or a component of the facility has been installed in compliance with the approved plans. Installation or modification of any stormwater system or source control, whether it involves structural changes, changes to planting schemes, or the management of drainage area in addition to what was previously approved, may require a permit from or review by the City Administrator.

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