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This guideline explains the standard criteria to be used by civil engineers for the design:
The guidelines here are general and may be modified by Seattle Public Utilities (SPU). SPU and the Seattle Department of Transportation (SDOT) must approve all drainage improvements in the right-of-way.
When available, discharge drainage to the public storm drain system instead of the public combined sewer. Direct pipe connections are preferred over curb or gutter discharge.
Please consult with SPU early to determine the best point of discharge.
Guidelines on grading in this document are provided only to illustrate concerns for SPU drainage review. See the City of Seattle Standard Plans for Municipal Construction and the Right-of-Way Improvements Manual for more information.
2.1 Curb returns
Grade curb returns at a minimum 0.5% slope in the flow line so that any low point is not:
2.2 Right-of-way behind the curb
Grade right-of-way behind the curb to the street. The standard cross section is shown on Standard Plan 400.
When SPU agrees that the standard cross section is not feasible, especially at curb bulbs and bus bulbs, typical drainage design considerations or exceptions include:
2.3 New curb bulbs
To the extent practical, minimize public storm drain extensions while grading new curb bulbs to drain.
When locating low points, consider drainage structure maintenance access. Minimize locating access where vehicles will be parked.
Survey the roadway cross section at curb to identify:
2.4 Concentrated flow
Collect flow concentrated along a gutter or flow line in a drainage structure, not left to fan out.
2.5 Inlet and catch basin staking points
Set elevations for inlets and CB grates at curb face. Include the drainage transition zone as shown in Standard Plan 260a.
Grade alleys per Standard Plan 403 and to drain to the perpendicular street. Public storm drain extensions to mid-alley low points will not be permitted, except when SPU agrees that there is no feasible alternative. When considered:
3.1 Collection points
Design considerations and exceptions may include:
Additional drainage pickups (CBs) are required at closed contour low points.
3.2 Maximum curb length
Water from less than 1,000 total lineal feet of curb on a residential street may discharge into one catch basin. This includes the length of curb for inlets which discharge into a catch basin as well as the catch basin itself. Flat residential streets may require additional drainage pickups.
3.3 Open grates
Do not locate open grates in any crosswalk or in front of curb ramps per Standard Plan 260a. From a curb ramp landing to any grate, the minimum clear distance is 1 foot. When unavoidable, a variance that is Americans with Disabilities Act (ADA) compliant will be considered.
3.4 Pedestrian and bicycle routes
Locate and relocate solid cover CBs to avoid the pedestrian and bicycle access routes. When possible, stay close to the curb to limit lane closures during maintenance and avoid the vehicle wheel path to limit noise and wear.
3.5 Standard locations for drainage grates and exceptions
For standard locations see Standard Plan 260a and b. Typical exceptions to locating structures 1½ feet from the point of curvature (PC) or point of tangency (PT) include:
Maintaining drainage structures can be a hazard to tree health. To the extent feasible, locate structures outside the full growth drip line of street trees. The minimum distance from the edge of the trunk to the edge of the structure is 5 feet per Standard Plan 030.
3.7 Gutter flow calculations
Gutter flow calculations including sag analysis is required for new arterial streets and significant revisions to arterial streets. As a starting point for an analysis, refer to the WSDOT Hydraulics Manual. Seattle has not adopted strict gutter spread standards. SPU and SDOT will review impacts for specific locations. Evaluating depth and spread of peak flows and grading of the street network beyond the project area may be required.
4.1 CBs preferred
CBs provide more reliable drainage pickup and are preferred over inlets. Typical exceptions when inlet installation is allowed instead of a CB include:
4.2 Replacing existing inlets, CBs and pipe
Inlets along new curbs must conform to Standard Plan 250 or be replaced. Whenever an inlet is replaced, the connection pipe to the CB must be replaced with new pipe. If the existing CB or CB connection pipe is determined to be defective by SPU, it must also be replaced.
4.3 Standard CB installation
Standard CB installation within the street shall be per Standard Plan 242B or 240D, which have a vaned grate and through curb opening. Typical design considerations and exceptions include:
4.4 Standard inlet installation
Install standard inlets within the street per Standard Plan 250B, which has a vaned grate and through curb opening. Use type 250A when the structure is not at a curb or curb height is less than 4 inches.
4.5 At curb bulbs
For curb bulbs, see Detail 4 (pdf). Locate a Standard Plan 242 CB no further up gradient than 1 ½ feet from the PC or PT of the curb bulb. Use a Standard Plan 250 inlet when the existing condition is an inlet and CB system and:
4.6 Closed contour low points
At closed contour low points, two independently connected CBs are required to minimize:
At most locations, the second CB is located on the opposite side of the street. See Detail 3 (pdf).
In alleys, use a Standard Plan 241 catch basin.
4.8 Elevated structures
For all elevated structures, consult with SPU early about requirements. If drains are required on a bridge, install per Standard Plan 290 with a grate per Standard Plan 265. New bridge downspouts shall have a minimum pipe diameter of 6 inches and a minimum bend radius of 4 feet.
4.9 Non-standard installations within shallow ditch and culvert system
For non-standard installations within the shallow ditch and culvert system, consult with SPU early about requirements. Typical design considerations and exceptions include:
4.10 Other non-standard installations
Other non-standard installations and modified structures may be approved if SPU agrees that grading to eliminate the need is not feasible. Structures will be individually reviewed, using the following criteria:
Inlet connection and CB connection pipe standard size is 8-inch diameter. Typical exceptions include:
5.2 Inlet connections
Inlet connections must be straight and:
Inlets must connect to a CB. Direct connection to a mainline must be made from a structure with a sump and outlet trap.
5.3 Outlet location and orientation
See Standard Plans 242, 250 and 261for allowable location and orientation of outlets from structures. Pipe must be oriented to allow tool access utilizing the length of the casting opening and traps shall be below the casting, so they can be reached.
5.4 CB connections
CB connections must be placed at a minimum slope of 2% and a maximum of 100% slope. Horizontal and vertical bends are expected, but shall not exceed 22½º in a single fitting. A straight pipe section of 1 foot minimum is required between fittings.
5.5 Pipe material
Pipe material is ductile iron (DIP). See standard construction notes “Drainage CB and Inlet Notes” (docx) and Standard Specifications for Municipal Construction. Typical exceptions include:
5.6 Connecting to the mainline pipe
Connections to new mainline pipe must be by manufactured tee unless the new mainline is:
For information on allowable connections and procedures for connections to existing mainline pipe, see SPU Core Tap Procedures. Connections to maintenance holes are non-standard and will be evaluated by SPU on a case-by-case basis.
5.7 Shallow street culvert connections
When it is not feasible to install a tee on the existing culvert above springline, connect to shallow street culverts with a junction box.
5.8 Ditch connections
Connections to a ditch must be tapered to match the ditch grading and armoring to prevent erosion may be required.
5.9 Plan submittal requirements
6.1 Standard location
Locate storm drains in the standard location, 7 feet south or west of the right-of-way centerline, as shown in Standard Plan 030. Alternate alignments require approval by SPU and SDOT.
Storm drains must be designed for full gravity peak flow with a 4% annual probability (25-year recurrence). The hydraulic grade line (HGL) for that peak flow must stay a minimum 4 feet below the rim of all drainage structures and a minimum 3 feet below the lowest elevation served by gravity on adjacent private properties.
For more information on requirements for hydrologic analysis, including tidal and lake backwater constraints, see Appendix F of the Stormwater Manual Hydrologic Analysis and Design (pdf).
PSDs must be a minimum of 12-inch diameter.
6.3 Pipe slope
Pipe slope shall generally follow the surface topography at a standard depth of cover of 6 feet. Desired minimum pipe slope is 1%. Typical exceptions include:
6.4 Minimum velocity
Minimum velocity required is 3 feet per second (fps). If velocity exceeds 20 fps, energy dissipation in the downstream MH is required to minimize hydraulic jumps.
6.5 Pipe material
Pipe material will be as approved by SPU, most typically DIP or RCP. See standard construction notes “SPU Mainline and Detention Pipe Notes" (docx) and Standard Specifications for Municipal Construction.
6.6 Plan submittal requirements
7.1 Where maintenance holes are required
Maintenance holes (MH) are required at intersecting streets for future extensions every 375 feet at:
Typical exceptions include:
7.2 Match pipe crowns
Crowns of pipe must match at MHs. For details on MHs, see the Standard Plans 204 through 212. For standard installations, use Type A unless the pipeline is too shallow.
7.3 Drop connections
Drop connections may be approved when SPU agrees that slope or depth makes matching crowns not feasible. For inside drop connections, see Standard Plan 233b.
For information on MH sizing, see maintenance hole selection. (pdf).
8.1 Detention pipe standards
Detention must be in a detention pipe conforming to Standard Plan 270.
8.2 Locating detention and flow control structures
Locate detention to minimize traffic impacts during maintenance, including:
8.3 Detention pipe material
Detention pipe material will be as approved by SPU, typically DIP or RCP. Polypropylene or steel reinforced polyethylene will be allowed if pavement restoration can wait 30 days after installation for flexible pipe testing. See standard construction notes “SPU Mainline and Detention Pipe Notes" (docx) and Standard Specifications for Municipal Construction.
8.4 Plan submittal requirements
Within the informal ditch and culvert system, the City of Seattle does not generally permit the filling of a ditch in the street right-of-way.
Culverts on fish-bearing creeks or streams, even with intermittent flows, are not addressed in this guideline.
9.1 Sizing culverts
Size roadside culverts to maintain the capacity of the existing ditch. Culverts must be a minimum of 12-inch diameter, or match an existing larger culvert size.
9.2 Culvert pipe material
Use ductile iron with Class D bedding. Alternate materials may be approved by SPU when pipe cover is greater than 2 feet.
9.3 Direction of laying culvert pipe
Lay the bell end of any pipe on the upstream side.
Related information on permits and plan preparation, including standard construction notes for Street Improvement Permits are available at the SDOT Street Improvement Permit website.
For Standard Plans of drainage and roadway system components, see the latest edition and most current amendments of the City of Seattle Standard Plans and Standard Specifications for Municipal Construction.
For information on how to make connections to existing pipe, see the SPU Core Tap Procedures.
For roadway design standards see the Right-of-Way Improvements Manual.
For Stormwater Code requirements, see the Seattle Department of Construction & Inspections (SDCI) Stormwater Code website.
This Client Assistance Memo (CAM) should not be used as a substitute for codes and regulations. The applicant is responsible for compliance with all code and rule requirements, whether or not described in this CAM.
Download print version of Client Assistance Memo 1180 (pdf).