Division III. Stormwater Management
Chapter 19.45
STORMWATER MANAGEMENT SYSTEM DESIGN
Sections:
19.45.020 General requirements.
19.45.030 Stormwater best management practices (BMPs).
19.45.040 Maintenance of BMPs and stormwater facilities.
19.45.050 Review and approval by Public Works Director.
19.45.060 Use of stormwater site plan for other phases of the development.
19.45.070 Stormwater facilities.
19.45.080 Stormwater conveyance.
19.45.120 Downstream analysis.
19.45.130 Stormwater conveyance easements.
19.45.140 Underground stormwater pipes – Location.
19.45.150 Open channel drainage systems (ditches and culverts).
19.45.160 Plans and specifications.
19.45.010 Purpose and intent.
This chapter describes the design requirements for stormwater management systems that are located within the City of Ferndale. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 901)
19.45.020 General requirements.
A. Conformance with Stormwater Management Manual and Design Standards. Stormwater management systems shall be designed and constructed consistent with the current Department of Ecology Stormwater Management Manual for Western Washington (SWMMWW) per the stormwater control ordinance and the standards as follows. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 902)
19.45.030 Stormwater best management practices (BMPs).
A. General. Best management practices (BMPs) from the SWMMWW shall be used to comply with the standards.
B. Treatment technologies must be designated general use level designation (GULD) by the Washington State Department of Ecology. (Ord. 2059 § 1 (Att. A), 2018; Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 903)
19.45.040 Maintenance of BMPs and stormwater facilities.
Maintenance of stormwater best management practices (BMPs) and facilities shall be equal to those in the SWMMWW or the LID Guidance Manual, whichever is more restrictive. A stormwater maintenance manual and covenant or plat language shall be required per Chapter 13.35 FMC. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 904)
19.45.050 Review and approval by Public Works Director.
The stormwater site plan shall be submitted to the Public Works Director for review and approval. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 905)
19.45.060 Use of stormwater site plan for other phases of the development.
The stormwater site plan submitted during one permit/approval process may be subsequently submitted with further required applications provided it is in accordance with vesting timelines per Chapter 14.07 FMC and/or the municipal stormwater permit. The stormwater site plan shall be supplemented with additional information at the request of the Public Works Director. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 906)
19.45.070 Stormwater facilities.
A. Traditional Stormwater Facilities.
1. Facilities Not Permitted in Road Rights-of-Way. Private stormwater BMPs and facilities shall not be located in dedicated public road right-of-way.
2. Accessibility of Facilities. All stormwater facilities shall be accessible for maintenance and operation with a minimum access easement width of 20 feet.
B. Low Impact Development (LID) Stormwater Facilities.
1. Design Criteria. Compliance with the most current Low Impact Development Technical Guidance Manual for Puget Sound and SWMMWW is required for use in stormwater site plans to address the minimum requirements for flow control and runoff treatment.
2. Infiltration Facilities.
a. Infiltration facilities as allowable by the site suitability criteria of the SWMMWW shall address freeze protection. Infiltration facilities shall not be closer than five feet to the traveled way of any roadway in an effort to prevent saturation of the roadway structural section.
b. Infiltration facilities which rely on perforated pipes to distribute runoff within a trench shall demonstrate that the pipes’ perforations can provide the design flow characteristics.
c. Infiltration and/or bioretention facilities with a contributing area greater than 5,000 square feet, utilized to meet flow duration standards, shall construct a nonerodible outlet or spillway with a firmly established elevation to discharge overflow. Calculate ponding depth, drawdown time, and storage volume for that reference point. The overflow must be designed to pass the 100-year developed peak flow. It shall provide controlled discharge directly into the downstream conveyance system or another acceptable discharge point.
C. Timing of Installation. During building construction, the grading of individual lots may change from that proposed in civil construction drawings; therefore, the installation timing of permanent required stormwater facilities is based on where the stormwater facility is located. Design of all facilities shall be approved at time of development approval and recorded with the approval. Deferred facilities shall be constructed per said design.
1. Residential Plat/Planned Unit Development.
a. Storm facility on common property (owned by homeowner’s association) – prior to recording;
b. Storm facility on individual lot – prior to final inspection of building permit.
2. General Binding Site Plan.
a. Storm facility on common property – prior to specific binding site plan submission;
b. Storm facility on individual lot – prior to recording of specific binding site plan.
3. Commercial development – prior to final inspection of building permit. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 907)
19.45.080 Stormwater conveyance.
A. Runoff Computation Methods. The following standard methods of runoff computation may be utilized in sizing conveyance systems:
1. Rational Method. This method may be used for sizing conveyance systems that drain smaller tributary areas (less than 10 acres) where very short intense storms tend to generate the highest peak flows. This method may be used if the attenuation effects of existing storage facilities within the tributary area are ignored.
2. TR-55/SBUH Methods. The Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service (SCS)) TR-55 method or the SBUH method may be used to size conveyance systems where tributary areas are greater than or equal to 10 acres if storage facilities are ignored. This method may be referenced in NRCS Publication 210-VI-TR-55, Second Edition (June 1986).
3. Continuous Modeling. This method may be used if a 15-minute time step is utilized. Models may include HSPF, Western Washington Hydrology Model (WWHM) and MGSFlood. Model input and utilization shall comply with guidance found within the SWMMWW. This method may be used for tributary areas less than 10 acres where there is a significant storage feature(s).
4. Hydraulic analysis for pipe systems shall be performed in two ways. The Manning’s equation shall be used for the preliminary sizing. A backwater analysis shall be used to determine the final pipe size. Both methods shall be performed with a method and to the satisfaction of the City.
5. For single event models, the following rainfall amounts shall be used in calculation of stormwater runoff, based on a 24-hour rainfall event.
|
Recurrence |
Rainfall |
|---|---|
|
2 Year |
1.90" |
|
10 Year |
2.80" |
|
25 Year |
3.25" |
|
100 Year |
4.00" |
(Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 908)
19.45.090 Design criteria.
Stormwater design shall be based upon current Department of Ecology Stormwater Management Manual for Western Washington (SWMMWW) per the stormwater control ordinance and the Standards, when applied to site-specific conditions. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 909)
19.45.100 Flow velocity.
Stormwater pipes shall be designed to assure a minimum flow velocity of three feet per second when the pipe is flowing full. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 910)
19.45.110 Design storms.
A. Conveyance systems shall be designed with sufficient capacity to convey and contain (at a minimum) the 25-year peak flow, assuming the developed condition for on-site tributary areas and existing conditions for any off-site tributary areas.
B. These systems shall further be designed to preclude adverse impacts to adjacent and downstream properties in the 100-year peak flow assuming developed condition for on-site tributary areas and existing conditions for any off-site tributary areas.
C. Overflow of the constructed conveyance system shall be contained within an on-site drainage easement, tract, or covenant.
D. Piped systems traversing slopes steeper than 15 percent and greater than 20 feet in height or are within a geologic hazard area shall be designed with sufficient capacity to convey and contain (at minimum) the 100-year peak flow, assuming the maximum developed condition (per current zoning regulation) for all tributary areas, both on site and off site. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 911)
19.45.120 Downstream analysis.
An analysis of the potential impacts such as erosion, sedimentation, flooding, water quality, low flow, and water rights on downstream features as a result of the development shall be performed by a professional civil engineer. Except where otherwise directed by the Public Works Director, this analysis shall be limited to a distance of one-quarter mile downstream. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 912)
19.45.130 Stormwater conveyance easements.
Stormwater conveyance crossing private property shall have a minimum 15-foot-wide easement with a specified dedication as per Chapter 19.10 FMC. Wider easements may be required by the City of Ferndale for deep stormwater drain pipes or unstable soil situations, or other circumstances. Drainage elements shall be located within a single lot or tract, except where the linear extent of the drain line may involve additional properties or unless otherwise approved by the Public Works Director. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 913)
19.45.140 Underground stormwater pipes – Location.
Enclosed (underground) drain pipes shall not be located on soil that is influenced by adjacent structures, as determined by a professional with geotechnical expertise. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 914)
19.45.150 Open channel drainage systems (ditches and culverts).
Open channel drainage systems, including culverts, shall meet the following requirements:
A. Include one-foot minimum freeboard from the design flow elevation to the top of banks.
B. Be nonerodible, including the side slopes.
C. Include side slopes at a 3:1 slope (desired for maintenance).
D. Where possible, ditches shall be located along or adjacent to property lines.
E. Channels crossing private property shall have a minimum easement width of the channel cross-section plus 15 feet on one side, if maintenance is required. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 915)
19.45.160 Plans and specifications.
A. General Formatting. General formatting, copying, and submittal process for civil stormwater management system plans shall be in accordance with this chapter.
B. Contents. Construction plans shall include a plan profile of the stormwater management systems prepared in accordance with the applicable provisions of Chapter 19.20 FMC; and shall contain, at a minimum, the following additional information:
1. Detailed Topography. Detailed topography of the subject property using contours at two-foot intervals, based on actual plane survey with sufficient spot elevations to delineate the perimeter and depth of specific features. On slopes greater than 10 percent, or when approved by the Public Works Director, contour intervals greater than two feet may be used.
2. Location of Existing and Proposed Drainage Improvements. Location, type, and size of existing drainage facilities and proposed improvements for handling the runoff within the site. Any proposed off-site improvements shall also be shown.
3. Easements. Location of existing and proposed drainage easements.
4. Hydraulic and Other Physical Data. Hydraulic and physical data such as grades, cross-sections, and bottom elevations of open ditches and channels, inverts of pipes, inverts and types of all structures such as manholes, catch basins, chambers, or similar structures, size, lengths, and slope of all pipes or other conveyance facilities, and invert elevations of the existing or other proposed storm drainage system that the subject drainage proposes to discharge (tie) into.
5. Cross-Sections. Cross-sections, elevations, and details of stormwater management facilities including the design volume.
6. Temporary Erosion/Sedimentation Control. A temporary erosion/sedimentation control plan showing locations and proposed measures with specifications for controlling erosion and sedimentation during and after construction. Maintenance provisions shall be included during the period until on-site stabilization has occurred. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 916)
19.45.170 As-builts.
Actual as-built drawings of any stormwater management facility or drainage system constructed must be submitted per Chapters 13.34 and 19.05 FMC. (Ord. 2007 § 1, 2017; Ord. 1978 § 1 (Exh. 6), 2016. DS § 917)