4.4.1 Pipe Systems

Pipe systems are networks of storm drain pipes, catch basins, manholes, inlets, and outfalls designed and constructed to convey surface water, including stormwater runoff. The hydraulic analysis of flow in storm drain pipes typically is limited to gravity flow; however, in analyzing existing systems it may be necessary to address pressurized conditions. A properly designed pipe system will maximize hydraulic efficiency by utilizing proper material, slope, and pipe size.

4.4.1.1 Design Criteria

All pipe material, joints, protective treatment, and construction workmanship shall be in accordance with WSDOT/APWA Standard Specifications, and AASHTO and ASTM treatment as noted under "Allowable Pipe Materials."

The pipe materials and specifications included in this section are for conveyance systems installed according to engineering plans required for Kitsap County permit approval. Other pipe materials and specifications may be used by private property owners for drainage systems they construct and maintain when such systems are not required by or granted to Kitsap County.

Allowable Pipe Sizes

See Table II-4.1 for allowable pipe sizes for pipe systems to be maintained by Kitsap County. For special cases where written approval is provided by the County Road Engineer, 8‑inch-diameter pipes may be allowed within the roadway right of way. Eight (8)‑inch-diameter pipe may also be allowed for privately maintained systems.

Allowable Pipe Materials

The following pipe materials are allowed for use in meeting the requirements of this manual. See WSDOT/APWA Standard Specifications for Road, Bridge, and Municipal Construction 7‑02, 7‑03, and 7‑04 for detailed specifications for acceptable pipe materials.

1. Plain and reinforced concrete pipe.

2. Corrugated or spiral rib aluminum pipe.

3. Ductile iron (water supply, Class 50 or 52).

4. Corrugated polyethylene pipe (CPEP)1  CPEP and fittings shall be manufactured from high density polyethylene resin which shall meet or exceed the requirements of Type 111, Category 3, 4, or 5, Grade P23, P33, or P34, Class C per ASTM D3350. In addition, the pipe must be lined and shall comply with all material and stiffness requirements of AASHTO M294..

5. Polyvinyl chloride (PVC)2  PVC pipe is allowed only for use in privately maintained drainage systems. PVC pipe shall be SDR 35 or thicker and meet the requirements of ASTM D3034. pipe.

6. Solid wall polyethylene (SWPE; also known as high-density polyethylene [HDPE] pipe or high-density polyethylene pipe [HDPP]) pipe3  SWPE pipe is normally used outside of Kitsap County right of way, such as on steep slope installations. Connections to Kitsap County road drainage systems are allowed for pipe diameters of 12″ or greater. SWPE pipe shall comply with the requirements of Type III C5P34 as tabulated in ASTM D1248, shall have the PPI recommended designation of PE3408, and shall have an ASTM D3350 cell classification of 345534C. The pipe shall have a manufacturer's recommended hydrostatic design stress rating of 800 psi based on a material with a 1,600‑psi design basis determined in accordance with ASTM D2837-69. The pipe shall have a suggested design working pressure of 50 psi at 73.4°F and SDR of 32.5..

Allowable Pipe Joints

1. Concrete pipe shall be rubber gasketed.

2. Corrugated metal pipe (CMP) shall be rubber gasketed and securely banded.

3. Spiral rib pipe shall be "hat-banded" with neoprene gaskets.

4. Ductile pipe joints shall be flanged, bell and spigot, or restrained mechanical joints.

5. CPEP joints shall conform to the current WSDOT/APWA Standard Specifications for Road, Bridge, and Municipal Construction.

6. PVC pipe shall be installed following procedures outlined in ASTM D2321; joints shall conform to ASTM D3212, and gaskets shall conform to ASTM F477.

7. SWPE pipe shall be jointed by butt fusion methods or flanged.

Pipe Alignment

1. Pipes shall be laid true to line and grade with no curves, bends, or deflections in any direction. Exception:Vertical deflections in SWPE and ductile iron pipe with flanged restrained mechanical joint bends (not greater than 30°) may be allowed on steep slopes, provided the pipe drains.

2. A break in grade or alignment, or changes in pipe material shall occur only at catch basins or manholes.

Changes in Pipe Size

The following criteria apply to changes in conveyance pipe sizes (not including detention tanks):

1. Increases or decreases in pipe size are allowed only at junctions and structures.
2. When connecting pipes at structures, match any of the following (in descending order of preference): crowns, 80% diameters,4  Match point is at 80% of the pipe diameter, measured from the invert of the respective pipes. or inverts of pipes. Side lateral connections5  Side laterals include any 8-inch or smaller pipe connected to the main conveyance system at a catch basin, or manhole, as allowed under this manual. In addition, 12-inch and smaller pipes that serve a single inlet point (e.g., roadway simple inlets, footing drains, and lot stubouts including manifold systems serving multiple residential lots) are also included. Excluded from this definition are inlet pipes which contribute 30% or more of the total flow into a catch basin, or which collect or convey flows from a continuous source. 12 inches and smaller are exempt from this requirement.
3. Drop manholes may be used for energy dissipation when pipe velocities exceed 10 feet per second (fps). External drop manholes are preferred where maintenance access to the upstream pipe is preserved by use of a tee section. Internal drop structures may be approved only if adequate scour protection is provided for the manhole walls. Drop structures shall be individually engineered to account for design variations, such as flow rates, velocities, scour potential, and tipping forces.
4. Downsizing pipes larger than 12 inches may be allowed provided pipe capacity is adequate for design flows.

Structures

Table II-4.1 lists typical drainage structures with corresponding maximum allowable pipe sizes.

1. Catch basin (or manhole) diameter shall be determined by pipe orientation at the junction structure. A plan view of the junction structure, drawn to scale, will be required for submittal with the site improvement plans (Volume II, Chapter 1) when more than four pipes enter the structure on the same plane, or if angles of approach and clearance between pipes is of concern. The plan view (and any sections if necessary) shall ensure a minimum distance between pipe openings of 8 inches for 48‑inch and 54‑inch catch basins, and 12 inches for 72‑inch and 96‑inch catch basins. The minimum distance between pipe openings shall be of solid concrete wall.

2. Evaluation of the structural integrity for H‑20 loading, or as required by the Kitsap County Road Standards (latest edition), may be required for multiple junction catch basins and other structures.

3. Catch basins shall be provided within 50 feet of the entrance to a pipe system to provide for silt and debris removal.

4. All SWPE pipe systems (including buried SWPE pipe) shall be secured at the upstream end. Where connecting to a structure, the downstream end shall be placed in a 4‑foot section of the next larger pipe size. This sliding sleeve connection allows for the high thermal expansion/contraction coefficient of this pipe material.

5. Through-curb inlets (see Figure II-4.3) are required on all catch basins located at roadway sags and in low points of cul-de-sacs.

6. The maximum slope of the ground surface for a radius of 5 feet around a catch basin grate or solid lid should be 5 horizontal to 1 vertical (5H:1V) to facilitate maintenance access. Where not physically feasible, a maximum slope of 3H:1V shall be provided around at least 50% of the catch basin circumference.

Table II-4.1. Allowable Structures and Pipe Sizes.

Catch Basin Type	Maximum Pipe Diameter
	CMP, Spiral Rib, SWPE, PVC and Ductile Irona	Concrete, CPEP
Inletb	12″	12″
Type 1b	18″	12″
Type 1Lb	24″	18″
Type 2 – 48″ diameter	30″	24″
Type 2 – 54″ diameter	36″	30″
Type 2 – 72″ diameter	54″	48″
Type 2 – 96″ diameter	72″	72″
Notes: Generally, these pipe materials will be one size larger than concrete due to smaller wall thickness. However, for angled connections or those with several pipe on the same plane, this will not apply. A maximum of 5 vertical feet is allowed between finished grade and invert lowest elevation.

Figure II-4.3. Through-Curb Inlet.

Pipe Design Between Structures

1. Minimum velocity at full flow shall be 3 feet per second (fps). If site constraints result in velocities less than 3 fps at full flow, impacts from sedimentation in the pipe system shall be addressed with larger pipes, closer spacing of structures, sediment basins, or other similar measures.

2. Maximum velocity: 30 fps, except that there is no maximum velocity requirement for ductile iron pipe or butt welded SWPE.

3. Minimum slope for installed stormwater pipes shall be 0.5%.

4. SWPE tightlines down steep slopes are self-cleaning and do not require structures for maintenance.

5. The following maximum inlet spacings are recommended:

   1. For roadway slopes less than 1%, maximum inlet spacing shall be 150 feet or less.

   2. For roadway slopes from 1% to 3%, maximum inlet spacing shall be 200 feet or less.

   3. For roadway slopes greater than 3% or design flows greater than 3 fps, maximum inlet spacing shall be 300 feet.

   4. For structures with solid lids, the maximum spacing shall be 300 feet.

   5. The maximum design water surface elevation in inlet structures shall be 1 foot below the top of the structure.

Pipe Cover

1. Pipe cover, measured from the finished grade elevation to the top of the outside surface of the pipe, shall be 2 feet minimum unless otherwise specified or allowed below. Under drainage easements, driveways, parking stalls, or other areas subject to light vehicular loading, pipe cover may be reduced to 1 foot minimum if the design considers expected vehicular loading and the cover is consistent with pipe manufacturer's recommendations. Pipe cover in areas not subject to vehicular loads, such as landscape planters and yards, may be reduced to 1 foot minimum.

2. Pipe cover over concrete pipe shall comply with Table II-4.2. For other pipe types, the manufacturer's specifications or other documentation shall be provided for proposed cover in excess of 30 feet.

   Caution: Additional precautions to protect against crushing during construction may be needed under roadways if the road bed is included to meet minimum cover requirements. Damaged pipe shall be replaced.

3. For proposed pipe arches, the manufacturer's specifications or other documentation shall be provided for proposed cover.

Table II-4.2. Maximum Cover (feet) for Concrete Pipe-Compaction Design A.

Pipe Diameter (inches)	Plain	Class II	Class III	Class IV	Class V
12	18	10	14	21	26
18	18	11	14	22	28
24	16	11	15	22	28
30		11	15	23	29
36		11	15	23	29
48		12	15	23	29
60		12	16	24	30
72		12	16	24	30
84		12	16	24	30
96		12	16	24	30
108		12	16	24	30
Compaction Design A refers to the WSDOT Standard Plan for Pipe Bedding (B‑55.20‑02)

Pipe Clearances

A minimum of 6 inches vertical and 3 feet horizontal clearance (outside surfaces) shall be provided between storm drain pipes and other utility pipes and conduits. When crossing sanitary sewer lines, the Washington State Department of Ecology criteria shall apply.

Pipe Compaction and Backfill

Pipe compaction and backfill shall be in accordance with the WSDOT Standard Plan for Pipe Bedding (B‑55.20‑02).

Pipe System Connections

Connections to a pipe system shall be made only at catch basins or manholes. No wyes or tees are allowed except on roof/footing/yard drain systems on pipes 8 inches in diameter or less, with clean-outs upstream of each wye or tee. Additional exceptions may be made for steep slope applications of SWPE pipe, as deemed prudent by geotechnical review.

Pipe Anchors

Table II-4.3 presents the requirements, by pipe material, for anchoring pipe systems, and Figure II-4.4 and Figure II-4.5 show typical details of pipe anchors.

Table II-4.3. Maximum Pipe Slopes and Velocities.

Pipe Material	Pipe Slope Above Which Pipe Anchors Required and Minimum Anchor Spacing	Maximum Slope Allowed	Maximum Velocity at Full Flow
CMP, Spiral Rib, PVC	20% – (1 anchor per 100 LF of pipe)	30%c	30 fps
Concrete or CPEPa	10% – (1 anchor per 50 LF of pipe)	20%c	30 fps
Ductile Ironb	20% – (1 anchor per pipe section)	None	None
SWPEb	20% – (1 anchor per 100 LF of pipe, cross-slope installation only)	None	None
Notes: fps = feet per second LF = linear feet These materials are not allowed in Geologically Hazardous Areas as defined in Title 19 KCC. Butt-fused or flanged pipe joints are required; above ground installation is recommended on slopes greater than 40%. Maximum slope of 200% is allowed for pipe materials with no joints (one section), with structures at each end, and with proper grouting.

Figure II-4.4. Pipe Anchor Detail.

Figure II-4.5. Corrugated Metal Pipe Coupling and/or General Pipe Anchor Assembly.

Debris Barriers

Debris barriers (trash racks) are required on all pipes 18 to 36 inches in diameter entering a closed pipe system. Debris barriers shall have a bar spacing of 6 inches. Refer to Figure II-4.6 and Vol II–4.5 Culverts for debris barrier requirements.

Figure II-4.6. Debris Barrier (in road right of way).

Outfalls

Outfalls shall be designed as detailed in Vol II–4.4.2 Outfall Systems.

Other Details

In addition to the details shown in Figure II-4.1 through Figure II-4.6, standard construction details are available in the APWA/WSDOT Standard Plans for Road, Bridge and Municipal Construction.

4.4.1.2 Methods of Analysis

This section presents the methods of analysis for designing new or evaluating existing pipe systems for compliance with the conveyance system capacity requirements.

Design Flows

Design flows for sizing or assessing the capacity of pipe systems shall be determined using the hydrologic analysis methods described in Vol II–4.2 Conveyance System Design Flow.

Inlet Grate Capacity

The methods described in the WSDOT Hydraulics Manual may be used in determining the capacity of inlet grates when capacity is of concern, with the following exceptions:

1. Use 100‑year design flows as computed per Vol II–4.2 Conveyance System Design Flow.

2. Assume grate areas on slopes are 80% free of debris; "vaned" grates, 95% free.

3. Assume grate areas in sags or low spots are 50% free of debris; "vaned" grates, 75% free.

Conveyance Capacity

Two methods of hydraulic analysis using Manning's equation are used sequentially for the design and analysis of pipe systems. First, the Uniform Flow Analysis method is used for the preliminary design of new pipe systems. Second, the Backwater Analysis method is used to analyze both proposed and existing pipe systems to verify adequate capacity. Each method is described further in Appendix F.

Use of the Uniform Flow Analysis method to determine preliminary pipe sizes is only suggested as a first step in the design process and is not required. Results of the Backwater Analysis method determine final pipe sizes in all cases. The director has the authority to waive the requirement for backwater analysis as verification.