16.15.040 Post-Construction Performance Standards for Stormwater Management.
A. General Post-Construction Performance Standards. Post-construction stormwater BMPs shall adhere to the following performance standards.
1. All stormwater BMPs shall be maintained in accordance with the approved stormwater system maintenance agreement as described in BC 16.15.050. The design of stormwater facilities shall incorporate maintenance accommodation and long-term maintenance reduction features in accordance with the latest version of the COSM.
2. Overland flood routing paths shall be used to safely convey stormwater runoff to an adequate receiving area or stormwater BMP as defined under BC 16.15.010(B)(3), such that the runoff is contained within the drainage easement for the flood routing path and does not cause flooding of buildings or related structures. The peak water surface elevation along flood routing paths shall be at least one foot below the finished grade elevation at the structure. When designing the flood routing paths, the conveyance capacity of the site’s stormwater conveyance systems shall be taken into consideration.
3. Velocity dissipation devices shall be placed at discharge locations and along the length of any surface waterbody outfall to provide nonerosive flow velocity from the structure to an adequate receiving stream or channel, so that the natural physical and biological characteristics and functions of the receiving stream or channel are maintained and protected.
4. Stormwater drainage in excess of the predevelopment rates or volumes shall be retained on the lot of origin and not trespass onto the public right-of-way or private property except as provided below:
a. If the City determines that retaining the design storm amount of stormwater on the site of origin would pose a threat to public safety or adjacent properties. If the developer chooses to direct all or part of the runoff off-site and there is enough capacity in the conveyance system, the runoff or a specified portion thereof shall be directed to an off-site drainage facility approved by the City.
b. When runoff from non-City-owned property in excess of the predevelopment rates or volumes is directed to or allowed to flow to City-owned property or right-of-way, the owner(s) of the lot(s) of origin shall compensate the City for the costs it incurs for constructing, operating and maintaining the additional stormwater drainage and treatment capacity.
c. Access to and maintenance and operation of all stormwater facilities on private property shall be as required by the most current version of the COSM.
d. Residential, commercial, institutional or industrial developments may apply stormwater management standards described in this chapter to a common plan of land development as a whole if the development is appropriately master planned, and has formal arrangements for stormwater drainage across multiple properties. Prior to final plat approval, individual residential lots in new subdivisions shall not be considered separate land development projects, but rather the entire subdivision shall be considered a single land development project.
5. The City may require drainage easements where stormwater discharges must cross an adjacent or off-site property before reaching an adequate conveyance.
6. Stormwater facilities within residential subdivisions that serve multiple lots and/or a combination of lots and roadways shall be on a lot owned and maintained by an entity of common ownership, unless an alternative arrangement is approved by the City. Stormwater facilities located on individual lots shall be maintained by the lot owner or, at the discretion of the City, be placed within an easement and maintained by an entity of common ownership.
7. Hydrologic parameters shall reflect the ultimate land development, shall be used in all engineering calculations, and shall follow required procedures outlined in the COSM. The stormwater design shall provide for treatment of runoff from the entire land development, to the extent practicable.
8. If runoff in excess of the predevelopment rates or volumes from a land development will flow to a municipal separate storm sewer system (MS4) or other publicly owned storm sewer system, then the applicant shall obtain authorization from the system’s owner to discharge into the system. The City may require the applicant to demonstrate that the system has adequate capacity for any increases in peak flow rates and volumes.
9. All stormwater facilities and conveyance systems shall be designed in compliance with all applicable State and Federal laws and regulations, and all applicable erosion and sediment control and floodplain regulations.
10. The design of stormwater BMPs shall consider public health, safety and general welfare. These considerations include, but are not limited to:
a. Preventing flooding of structures and roadways;
b. Preventing standing water in facilities, manholes, inlets and other structures in a manner that promotes breeding of mosquitoes;
c. Preventing attractive nuisance conditions and dangerous conditions due to velocity or depth of water, and/or access to orifices and drops; and
d. Preventing aesthetic nuisances due to excessive slopes, cuts and fills, vegetation mortality, and other conditions.
11. All stormwater BMPs shall be designed to the standards of the most current version of the COSM, unless the City grants the applicant a waiver or the applicant is exempt from such requirements.
12. All new or refurbished stormwater inlets placed as part of a new development project shall be marked with a permanent “Do Not Dump” and/or “Protect Our Waters” or similar stamp or embossed image that has been approved by the City.
13. All new development projects shall prepare and submit to the City as-builts for all site impervious surfaces and permanent stormwater facilities.
14. All stormwater management designs shall preserve the natural hydrologic functions, stream channel characteristics, and groundwater recharge of the pre-developed site, to the extent practical. This shall be accomplished by:
a. Treating runoff at the source;
b. Disconnecting impervious surfaces;
c. Preserving or enhancing natural flow paths and vegetative cover;
d. Preserving or enhancing natural open spaces and riparian areas;
e. Applying other measures that replicate pre-development hydrologic conditions.
The City may exercise discretion in the application of this standard, especially in cases of infill development, redevelopment, or other unique circumstances.
15. If hydrologic, geologic, topographic, or land use conditions warrant greater control than that provided by the minimum control requirements, the City may impose additional requirements deemed reasonable and necessary to control the volume, timing, rate and/or quality of runoff in excess of the predevelopment rates or volumes. In these cases the City may restrict the use of certain stormwater BMPs, require pretreatment above the minimum standards in the COSM, and/or require a pollution source control plan. Example conditions that may warrant greater control include, but are not limited to:
a. Stormwater generated within the drainage basins of problem flooding areas as noted on the City’s capital improvement project prioritization list;
b. Stormwater discharges that are conveyed with nonstormwater discharges;
c. Stormwater in areas underlaid by naturally impermeable layers;
d. Stormwater discharged in important groundwater management areas such as wellhead protection areas; and
e. Stormwater discharged to surface water outfalls or directly to waterbodies such as the Deschutes River or Tumalo Creek. [Ord. NS-2176, 2012]