Roadway Standards Section 440 - Storm Drainage Components

440.1 Pipes and Culverts

440.1.1 Pipe Material

1. The pipe material permitted for use within the public right-of-way is as follows:
   1. Concrete, non-reinforced, ASTM C-14, Class 3 (maximum size: 18" inside diameter).
   2. Concrete, reinforced, ASTM C-76, Class III (minimum).
   3. Ductile Iron, cement lined, class 50 wall thickness for pipes sizes up to 12-inch; class 51 wall thickness for 14-inch pipe sizes and larger.
   4. High Density Polyethylene Pipe (HDPE) smooth interior, corrugated exterior HDPE sewer pipe and associated HDPE fittings shall conform to AASHTO M294, AASHTO 252, ASTM 405 or ASTM 667.
   5. High Performance Polypropylene (HP3) smooth interior, corrugated exterior HP3 sewer pipe (12" to 30") as per ASTM F2736 and smooth interior and smooth exterior HP3 sewer pipe encasing a corrugated layer (30" to 60") as per ASTM 2764. Associated HP3 fittings shall conform to ASTM D3212.
   6. PVC C900 or PVC 3034 with a minimum 3' of cover can be used for storm runs between manholes or between catch basins.  No PVC pipe is allowed as culvert pipe.
2. The minimum life span of pipe used within the right-of-way shall be 70 years.
3. The Engineer shall provide to the County certification with respect to alternative pipe materials, if allowed by the County.  Certification shall state that upon inspection the installation, cover, and backfill compaction are in conformance with the manufacturer's recommendations for installation and the product and installation procedures are sufficient for HS 20 loading.
4. The specific strength and depth of cover for pipe shall be based on the manufacturer's recommendations for the loading requirements.  Private storm drainage materials shall conform to the Oregon State Plumbing Specialty Code and requirements of the local plumbing official.  If private storm drainage materials and applications are not listed in the plumbing code, Engineering will determine the requirements. 

440.1.2 Pipe Size

1. Publicly maintained storm drains shall be a minimum of 12-inch inside diameter. 
2. Pipes shall be sized to convey the 25 year design storm flow within the County right-of-way to accommodate the existing and subject development at a minimum velocity of 3 ft/sec when flowing full.
3. The method of analysis shall be based on Manning's equation as follows:

Q = 1.49/n x A x R0.67 x S0.5
Where: Q = discharge (cfs)
n = Manning's roughness coefficient (see Table 4-2)
A = pipe area (ft2)
R = hydraulic radius (ft)
S = slope of the energy grade line (ft/ft)

Table 4-2

440.1.3 Pipe Slope

All pipes and culverts shall be designed to operate under gravity flow and be sloped to provide a minimum velocity of 3 ft/sec when flowing full.  This may be reduced to 2.5 ft/sec with approval of Engineering.

440.1.4 Pipe Cover

1. Pipe trenching, bedding, and backfill shall conform to Standard Drawings U200 through U270B.
2. The required minimum cover for pipe for all public storm drains shall be 36 inches or shall be as specified by the manufacturer, whichever is greater, as measured from the top of pipe to the finished roadway or ground grade.
3. Reduced depth of cover may be considered by the County due to topographical constraints.  Appropriate pipe material shall be used to ensure the pipe's capacity to withstand HS 20 loading.
4. Fill heights over pipes shall meet manufacturer's recommendations and are subject to the approval of Engineering.

440.1.5 Pipe Alignment and Connections

1. Pipes shall be laid to a straight line and grade with no curves, bends, or deflections in any direction.
2. All changes in pipe slope, material or alignment shall require a manhole or catch basin. 
3. Catch basin locations shall have a maximum spacing of 500 feet from the high point in the road provided that the catch basins drain in opposite directions and there is not a pipe connected for that distance.  Typical in line spacing shall be 250 feet.
4. Extensions of pipes and culverts shall be in the same line, grade, and inside diameter as the existing pipe.  Extensions shall be of like material and with a connection approved by Engineering.

440.1.6 Pipe Inspection Including Televiewing

1. All pipes shall be lamped, mandreled and video recorded. 
2. Upon completion of all storm drain construction, the storm line shall be inspected by televiewing.  The applicant shall coordinate the video recording inspection with Engineering.
3. If deficiencies are revealed by the inspection, the corrections shall be made and the televiewing shall be repeated until all work is accepted.  The cost of the televiewing shall be borne by the Applicant.

440.2 Catch Basins and Inlets

440.2.1 Catch Basin Type

Standard GB-2 catch basins shall be used in locations where vertical curb is used.  Standard catch basins shall conform to WES Standard Drawings.  Substandard catch basins shall be upgraded when connections are proposed.

440.2.2 Catch Basin and Inlet Spacing and Location

1. The spacing of catch basins or curb inlets shall be as required to limit gutter flow to less than 4 inches depth and limit water depth in a travel lane to less than 1 inch during the 10 year storm event.  The maximum length of pipe between catch basins and inlets shall be 250 feet.
2. Catch basins or inlets shall be provided just prior to curb returns where the centerline gradient is greater than 5% or where the next upstream inlet is 100 feet away or farther.  Catch basins also are required within 500 feet of the high point of the roadway profile.

440.2.3 Catch Basin and Inlet Connections

All catch basin laterals shall be a minimum 12 inches inside diameter.  Catch basin laterals shall connect to the receiving main with a manhole or another catch basin or curb inlet.  Blind tee connections are not allowed. 

440.2.4 Lateral Connections

Lateral Connections to newly installed mainlines shall be factory fittings.  "Inserta-Tee" fittings are only allowed on existing pipe when the lateral diameter is at least two nominal sizes smaller than the mainline pipe or the mainline is at least 12".    The lateral shall be installed above the spring line of the mainline.  The lateral shall have a clean out at the property line.  Connections will not be allowed if the lateral is connected to an inlet or area drain. 

440.3 Manhole Sizing and Alignment

1. The diameter of manhole required shall ensure a minimum dimension, of solid concrete manhole wall, between pipe openings of 12 inches.  The standard and minimum manhole size shall be 48 inches in diameter.  Maximum spacing of manholes shall be 250 feet.
2. Access locations shall be required at a change in vertical or horizontal alignment or a change in pipe size or material.
3. Manhole rims shall be flush with top of asphalt.  If only one lift is placed, or a portion of the overall depth of asphalt, the rim shall still be flush with the top of the asphalt constructed.  As additional asphalt thickness is added later, the rim shall be adjusted to be level with the new top of asphalt.

440.4 Open Channels and Ditches

440.4.1 Natural Channels

1. Natural channels are those which occur naturally due to the flow of water or, following construction, those manmade channels that have become vegetated and stable.
2. Natural channels shall remain in their existing, or natural, condition wherever feasible.  The preservation of natural drainage-ways shall conform to the requirements of the ZDO.  Alteration of natural drainage-ways shall not occur without approval of all agencies having jurisdiction.

440.4.2 Constructed Channels & Ditches

1. Constructed channels include those constructed and maintained by human activity and include bank stabilization at existing channels.
2. Roadside ditches shall conform to the requirements and sections for County rural roadways.  Roadside ditches shall not be constructed within the urban growth boundary. 
3. The County may consider a constructed channel along the roadside when pedestrian and vehicle safety are not compromised and sufficient road right-of-way is available.

440.4.3 Design Criteria

1. Roadway grading shall conform to clear zone requirements of Section 245 and cross section requirements of Standard Drawings C110 to C140.
2. Constructed channels and ditches shall be trapezoidal or parabolic in cross section with side slopes no steeper than 3H:1V, or 4H:1V within the clear zone, for vegetation-lined channels and 2H:1V for rock-lined channels.  Constructed channels and ditches shall be designed for a 25 year storm event.  Constructed channels and ditches within the required recoverable slopes shall meet recoverable slope requirements:  4H:1V on the fore slope (down slope) and 3H:1V on the back slope (up slope).
3. Vegetation-lined channels shall maintain a maximum velocity of 5.0 ft/sec at the 25 year event.   Rock-lined channels or bank stabilization shall be required when design velocities exceed 5.0 ft/sec.
4. Access and utility easements shall be provided for all publicly maintained open channels and ditches as required by the County.