Chapter 18.25
STORMWATER MANAGEMENT SYSTEMS
Sections:
18.25.020 Stormwater design documentation.
18.25.030 Temporary stormwater and erosion control.
18.25.040 Permanent stormwater management systems.
18.25.010 General guidelines.
Plans for temporary and permanent stormwater management shall be provided for every street improvement and land development proposal that disturb 1,000 square feet or more.
In addition to adhering to the standards provided below, stormwater designs should identify the potential overland flow path in the event the stormwater infrastructure becomes plugged or fails, and provide safeguards to minimize property damage.
Runoff from streets and other impervious surfaces may not be directed to a natural drainage channel, without prior approval of public works. If approved, runoff directed to natural drainage channels shall be pretreated using a pollution control structure or biofiltration method. Riprap or other energy dissipation facilities may be required at stormwater discharge points to prevent erosion. It is the responsibility of the owner to obtain all necessary regulatory permits. [Ord. 479 § 1 (EDS § 5.1), 2016].
18.25.020 Stormwater design documentation.
Hydraulic and hydrologic reports and other stormwater design documentation documents shall be prepared in accordance with this section. [Ord. 479 § 1 (EDS § 5.2), 2016].
18.25.030 Temporary stormwater and erosion control.
Management of stormwater during construction shall adhere to the Oregon Department of Environmental Quality’s Construction Stormwater Best Management Practices Manual.
A plan for temporary slope stability and erosion control, along with a plan for permanent slope stability, erosion control and plant establishment, is required as part of the construction submittal. A site design permit must be obtained from public works before any site work, including clearing and grubbing, begins.
A geological study, prepared by an Oregon licensed geotechnical engineer, engineering geologist, or geologist, is required to be submitted to and approved by public works prior to grading and/or proposed new development on slopes exceeding 25 percent. The study shall address overall slope stability, before, during, and after disturbance and/or development. Geological recommendations shall include methods for maintaining slope stability and managing stormwater during construction. All geological recommendations shall be incorporated in the applicant or developer’s grading permit application, and approved by the geologist or engineer upon completion of construction. It is the responsibility of the owner to obtain all other necessary regulatory permits. [Ord. 479 § 1 (EDS § 5.3), 2016].
18.25.040 Permanent stormwater management systems.
(1) Flow Calculations and Sizing Methodology. Runoff calculations and pipe sizing performed by a registered engineer are required, and should be based on anticipated build-out. All future anticipated impervious surfaces in the development such as streets, walkways, driveways, roofs, patios, etc., should be accommodated when designing stormwater improvements.
Hydrologic calculations for drainage basins/watersheds less than 200 acres shall utilize the rational method. Hydrologic calculations for drainage basins/watersheds greater than 200 acres shall utilize the unit hydrograph method. Storm drain design shall utilize a 25-year, 24-hour duration return interval. Design for storm drain improvements for state highways (improvements associated with state highways must have ODOT approvals) or arterial roads shall utilize the 50-year, 24-hour duration return interval. In cases where roadway overtopping is likely, a 100-year, 24-hour return interval shall be utilized for design. Refer to the city of Coos Bay stormwater master plan and the ODOT Hydraulics Manual for calculating peak flows utilizing the rational method and unit hydrograph method. Selection of methodology is at the discretion of the design engineer. Calculations should be confirmed using a second analysis method, also selected at the discretion of the designer.
(2) Inlets, Catch Basins and Stormwater Pretreatment. Inlets and catch basins in the public right-of-way shall be constructed per standard details. Inlets and catch basins on private property shall comply with plumbing code requirements.
Pretreatment of stormwater destined for conveyance in a public storm drain system is recommended. Pretreatment may be accomplished through the use of biofiltration swales, catch basin inserts, or other methods approved by public works which are designed to remove oil and grease, trash, debris, sediment, and other pollutants associated with development. Any catch basin inserts proposed for use on public facilities or located within public right-of-way will require approval of public works.
(3) Storm Drain Manholes. Manhole design shall conform to the following:
(a) Storm drain manholes shall be provided at least every 500 feet, at each change in pipe grade or alignment, and at each junction of two or more lines. Manhole lids shall have a minimum of six inches of clearance from the edge of curbs and shall not be in wheel paths of the traveled way.
(b) All manholes shall be a minimum of 48 inches in diameter.
(c) Inside drop and water quality manholes shall be at least 60 inches in diameter with at least 42 inches of clear space.
(d) Storm drain manholes shall meet the requirements of standard details in Chapter 18.35 CMC. Channelization is required in storm drain manholes.
(4) Storm Drain Pipes. The city allows the use of the following materials for storm drain piping:
(a) Pipe meeting ASTM D3034 SDR 35 specifications.
(b) High-density polyethylene (HDPE) pipe with fused joints. Joints shall be de-beaded.
(c) Dual wall corrugated HDPE pipe with smooth interior wall (for example, ADS N-12 piping for diameters of 24 inches and less; and ADS SaniTite for diameters of 30 inches and larger).
Storm drain piping shall otherwise meet the requirements of CBMC 18.20.020, except that the minimum storm drain pipe size shall be eight inches.
Green locate wire and tracer tape shall be installed as shown in standard details.
(5) Connection of Private Drains to the Public Storm Drain. Private drains may be connected directly to the public stormwater system, if approved by public works. Public storm drain pipes six-inch diameter and larger shall be connected at a nearby manhole or catch basin rather than directly to public stormwater pipelines. All connection points shall be approved by public works.
For private developments adjacent to residential streets, small private drains for individual lots may be piped under the sidewalk and through the curb, for discharge to the street gutter; provided, that there is two inches of freeboard from the water surface elevation (25-year, 24-hour storm event) to the top of curb and eight feet of the traveled way is dry. For highways and arterials, drainage under the sidewalk will be allowed on a case-by-case basis. Owner is responsible for any damage that occurs to sidewalk as a result of the drain. Gutter discharge points shall be approved by public works. Discharge points cannot adversely impact adjacent neighbors or cause flooding/ponding within the public right-of-way. The owner may be required to employ low impact development devices to capture runoff and reduce the volume and duration of runoff that enters into the gutter as a result of the private development. Public works may require connection to the public stormwater system where gutter discharge has the potential for detrimental effects.
(6) Retention and Detention Facilities. Proposed improvements cannot adversely impact downstream drainages. If there are adverse impacts, the downstream system shall be mitigated or the increase in volume and durations on site shall be mitigated.
(a) Detention Flow Control Structures. Structural detention facilities such as tanks, vaults and oversized pipes shall be designed to prevent surcharge of downstream piping systems during storm events.
(i) The following criteria apply to detention basins:
(A) When required, on-site stormwater detention facilities shall be designed to capture runoff so the post-development runoff rates from the site do not exceed the predevelopment conditions, based upon a two-year through 25-year, 24-hour return storm. Volume and duration of predevelopment conditions will be considered.
(B) Detention basin shall have an emergency spillway designed to pass the 100-year storm event.
(C) In soils where groundwater may induce flotation and buoyancy, measures shall be taken to counteract these forces with concrete ballast, earth backfill, concrete anchors, or other counteractive measures. Calculations demonstrating stability shall be submitted to public works.
(ii) Flow control structures must be approved by public works and must meet the following design criteria:
(A) Refer to the ODOT Hydraulics Manual for orifice and weir flow control equations.
(B) Weir must be accessible (e.g., for backhoe, dump truck, and personnel) for maintenance.
(C) The control structure shall be designed to pass the 100-year storm event as overflow (emergency spillway), without causing flooding of the contributing drainage area.
(7) Operation and Maintenance Plans. A plan outlining the scope of activities, schedule and responsible parties for periodic inspection and maintenance of permanent stormwater facilities shall be prepared and included with the construction submittal. This plan shall cover vegetation management, structural repairs, sediment removal and other activities necessary for the continued long-term function of the stormwater facilities.
(8) Storm Drain System Testing. All new storm drain pipes and manholes must be tested for water tightness prior to city acceptance using a water exfiltration test procedure.
In addition, new storm drain pipes must be thoroughly flushed and inspected by video, after trench backfilling is complete and prior to city acceptance. This includes video confirmation that the line is free from sags, irregular joints and connections, obstructions, restrictions or other anomalies that may cause solids to accumulate. [Ord. 479 § 1 (EDS § 5.4), 2016].