Transportation Engineering

Erosion and sedimentation control measures are required for construction areas where the ground surface will be disturbed by clearing, grading, fills, excavations, and other construction activities. Erosion and sediment controls shall conform to WES standards and their Erosion Prevention and Sediment Control Planning and Design Manual. Engineering will require an erosion and sediment control plan for projects that disturb between 800 square feet and one acre when located in the unincorporated area and outside a stormwater district. A permit is required for this work. 

For areas outside a surface water management district, NPDES 1200-C permits are required for projects that disturb an acre or more and are issued by the Department of Environmental Quality (DEQ). An applicant will be required to provide evidence of an approved 1200-C permit upon request.

1. The installation of water quality facilities shall conform to WES standards.
2. Water quality structures shall not be constructed in the public right-of-way without prior approval from Engineering.

1. Bridges and other major conveyances identified as deficient in a downstream analysis shall be designed to accommodate the 100 year storm.
2. The outfall of detention facilities may be required to accommodate the 100 year storm depending on the downstream impacts.
3. Detention structures shall not be constructed in the public right-of-way without prior approval from Engineering.
4. Conveyance facilities shall be designed to carry a 25 year storm event with the basin built with 85% impervious.
5. Infiltration testing is required for all detention ponds and other surface water management facilities proposed that utilize infiltration. In rural areas, the simplified method for infiltration testing may be used if coordinated with County staff.

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.

430.1 Acceptable Hydrology Methods (Detention Hydraulics)

Engineering accepts the Rational Method, and TR-55, in addition to WES approved methodology. Other methods will require prior approval from Engineering.

Detention pond routing shall be by the Storage Indication (Modified Pulse) method: 

{2S²/ Δt + O² = I² + I¹ +2S¹/ Δt + O¹} or approved equal.

430.2 Rational Method

The Rational Method (Q=ciA) may be used to estimate peak discharge from drainage basins of less than 300 acres. 

The Soil Conservation Service ("SCS") TR55 method may be used for drainage areas less than 25 square miles.  

Refer to the ODOT Hydraulics Manual for additional information. Some of the figures contained herein have been reproduced from the above manual.

430.2.1 Rational Method Basic Methodology

The Rational Method is a simplified model for estimating the discharge of a drainage basin based on the area of the basin, type of ground cover, and intensity of rainfall.

Q = ciA
Q = peak discharge (cfs)
c = runoff coefficient
i = rainfall intensity (in/hr)
A = drainage area (acres)

The Rational Method can result in a wide range of discharge values based on assumptions made by the Engineer. The following sections establish parameters for the variables in the Rational Method and should be used in the design of drainage systems within Clackamas County.

430.2.2 Runoff Coefficient

The runoff coefficient (C) is a dimensionless parameter based on the type of ground cover and slope of the terrain. Table 4-1 lists runoff coefficients for various conditions of ground cover and slope.
           
Where ground conditions vary throughout a drainage basin, a composite runoff coefficient can be calculated as follows:

Cavg = (C1A1 + C2A2 + C3A3 + . . . CnAn)
(A1 + A2 + A3 + . . . An)

430.2.3 Rainfall Intensity

The rainfall intensity indicates the "quantity" of rainfall and is related to the rainfall duration and the design storm. Rainfall intensity is usually represented by an Intensity-Duration-Frequency (I-D-F) curve. The IDF curves for drainage design in Clackamas County are taken from the Hydraulics Manual (see Figures 4-1 to 4-5).

430.2.4 Time of Concentration

1. The duration of rainfall is equal to the time of concentration (Tc), in minutes, where the time of concentration is defined as "that amount of time from beginning of a storm event, that it takes water from the most remote time location in the basin to reach the point being considered."
2. Determination of the time of concentration, Tc, shall be in accordance with the Hydraulics Manual as follows: Most drainage basins will consist of overland flow segments as well as channel flow segments. Overland flow can be further divided into a sheet flow component and a shallow concentrated flow component. Urban drainage basins may be further complicated by having significant pipe flow segments. The travel time is computed for each flow segment and the time of concentration is equal to the sum of the segment travel times.

3. The best method of determining overland sheet flow time is the kinematic wave equation. The equation is only applicable for travel distances less than 300 feet.

   Tc = KL0.6n0.6 / I0.4S0.3

   Where: Tc = overland flow time in minutes
   L = overland flow length in feet
   n = Manning's roughness coefficient (see Table 4-2)
   I = rainfall intensity, in/hr
   S = the average slope of the overland area
   K = 0.93

4. Figure 4-6 is a nomograph for the solution of the kinematic wave equation for overland sheet flow. In using the nomograph, the time of concentration and rainfall intensity are unknown. The solution is one of iteration or trial and error.
5. After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow. The average velocity for this flow can be determined from Figure 4-7 in which average velocity is a function of watercourse slope and type of channel. This figure was reprinted from the 1972 SCS Handbook.
6. For open channels, Manning's equation can be used to estimate average flow velocity, which is usually determined for bank-full flow.

430.2.5 Design Storm

1. A design storm defines the statistical recurrence interval of a storm event. The probability of a 25-year storm occurring in a given year is 4%. Conversely, a 25-year storm will statistically occur once every 25 years. The selection of a design storm is dependent on the balance between the cost of the drainage facility and the flood risks associated with the storm event. The design storm required for public drainage facilities within the County, but outside an existing stormwater district, shall be based on a 25-year storm event. Design storms within existing surface water management districts, shall be based on the districts requirements, but in no case shall public drainage facilities within the County's right-of-way be designed for less than the 25-year storm at basin buildout with 85% impervious. 
2. The effects of the 100 year storm event on storm drainage structures and bridges shall be evaluated as required.  
3. The effects of the 100 year storm event on developments within a flood plain shall be evaluated as required. 
4. The effect of the 100 year storm shall be evaluated when the path of the drainage could place persons or property in jeopardy.

430.2.6 Drainage Area

The discharge of a storm system is dependent on the size of the basin contributing to the flow. The design of a drainage facility should account for the entire drainage basin surrounding the affected area. The Engineer shall submit a topographic map of the entire drainage basin with the drainage calculations. This map should identify the existing and proposed drainage facilities and sub-basins considered in the design. This overall design shall anticipate and accommodate the acceptance and conveyance of surface water on or crossing the roadway or roadway system considered in the design area.

Figure 4-1. Clackamas County Rainfall Zones
Reference: ODOT Hydraulics Manual

Figure 4-2. ZONE 3 Rainfall Intensity, Duration, Frequency Curves
Reference: ODOT Hydraulics Manual

Figure 4-3. ZONE 5 Rainfall Intensity, Duration, Frequency Curves
Reference: ODOT Hydraulics Manual

Figure 4-4. ZONE 7 Rainfall Intensity, Duration, Frequency Curves
Reference: ODOT Hydraulics Manual

Figure 4-5. ZONE 8 Rainfall Intensity, Duration, Frequency Curves
Reference: ODOT Hydraulics Manual

Figure 4-6. Time of Concentration
Reference: ODOT Hydraulics Manual

Figure 4-7 – Shallow Concentrated Flow Velocity
Reference: ODOT Hydraulics Manual

The following standards are exceptions to WES stormwater standards.  These standards are directed towards the design and construction of public storm drainage facilities and for the coordination of stormwater runoff from private drainage systems into public systems.  The intent is to ensure a comprehensive engineering review and sufficient design to identify and mitigate existing deficiencies, protect the environmental health of our watersheds, and identify the capacity requirement of new system improvements, resulting in an overall benefit of reduced flooding.

If any conflicts arise between these standards and WES standards, the following shall govern.

420.1 Best Management Practices (BMP) & Low Impact Development Approaches (LIDA)

Engineering acknowledges the need for Best Management Practices (BMP) or Low Impact Development Approaches (LIDA) with development.  In addition to any LIDA standards adopted by WES, Engineering encourages designers to submit LIDA designs for review to meet the water quality and infiltration requirements outlined in WES stormwater standards.  Calculations will be required to illustrate to Engineering how a given LIDA provides water quality benefit. Private improvements in rural areas may work with DTD to provide a simplified approach to stormwater management that utilizes vegetation and infiltration if the site conditions warrant it. City of Portland's Simplified Approach for infiltration testing and typical details for stormwater management may be used in the rural area.

420.2 Acreage as a BMP

Development outside WES service districts is predominated by larger lot sizes.  A reasonable BMP outside the UGB may be the utilization of undeveloped acreage.  The applicant shall demonstrate to Engineering that water quality, detention, and/or infiltration requirements are met using the acreage BMP.

420.3 Surface Water Management Applicability

Surface water management plans in conformance with this chapter will be required for any of following:

1. When 5,000 square feet or more of new or reconstructed impervious surface is proposed within the UGB.
2. When 10,000 square feet or more of new or reconstructed impervious surface is proposed outside the UGB.
3. When grading or any new or reconstructed impervious surface is proposed or replaced within 50 feet of a perennial stream, creek, wetland, or lake, or within 10 feet of a property line.

420.4 Underground Injection Control (UIC) Devices

UICs are only permitted in Clackamas County right-of-way when registered with DEQ and maintained by an established stormwater district, unless otherwise approved by DTD Transportation Maintenance.  When UIC are proposed in County right-of-way, an approved IGA and maintenance agreement with an established stormwater district shall be in place prior to Development Permit issuance.

Chapter 4 establishes the technical requirements associated with storm water management.  

410.1 Regulatory Authority

The Engineering Division of DTD is responsible for ensuring the adequate drainage of public roadways and developed properties in unincorporated areas outside of established stormwater districts within the County.  Engineering regulates the construction of public and private roads and other site improvements to ensure adequate drainage of storm/surface water to an appropriate discharge point.

Clackamas County has multiple surface water districts: Water Environment Services (WES) encompasses the Clackamas County Service District #1 (CCSD#1) and the Surface Water Management Agency of Clackamas County (SWMACC) and Hoodland Service District. The other districts are Clean Water Services (CWS) and the Oak Lodge Sanitary District (OLSD).   

Engineering manages storm water drainage and surface water regulations for all development outside of the County's established storm water districts or outside city limits located within the County's boundary.  For the regulations in these other areas, please refer to the respective jurisdiction.

410.2 Engineering Regulations

Engineering has adopted WES stormwater standards, with the exceptions noted within this chapter. 

410.3 Erosion Control Contractor Certification Not Required

WES Erosion Control Certification shall not apply to Engineering regulations.

410.4 Fees

Fees listed in the WES rules and regulations, or their Stormwater Standards, only apply to areas within their surface water management districts.  For information on Engineering stormwater and erosion control fees outside of existing surface water management districts, see County Code, Appendix A: Fees. For surface water management fees, within another district or municipality, contact the district or municipality directly.

Grades may not exceed +/- 3% in the service area.

1. All driveways shall meet ADA accessibility requirements if the driveway intersects with a planned or existing sidewalk or other pedestrian facility.
2. Driveways should be designed with a minimum 28-foot wide approach except where the Comprehensive Plan requires narrower driveways.
3. If the design vehicle for the site requires it per Section 250.1.3 or if traffic operations necessitate additional travel lanes, driveways wider than 28 feet may be required to reduce the pedestrian crossing length or implement additional measures to reduce conflicts with pedestrians.
4. Driveways on streets with curb tight sidewalk should be constructed per Standard Drawing D600. 
5. Driveways on streets with curb and sidewalk with a landscape strip should be constructed per Standard Drawing D650. 
6. In rare cases where a development's trip generation is such that higher speed egress maneuvers from the adjacent roadway are desired or is used for a large number of truck deliveries, the County may allow or require driveways to be constructed per Standard Drawing D675.
7. Driveways constructed without curbs should be constructed per Standard Drawing D500.
8. Driveway throats (measured from the back of the public sidewalk in the UGB and from the edge of pavement outside the UGB to the nearest perpendicular drive aisle) shall have a minimum length of 20 feet but should be designed to accommodate the 95th percentile queue.  Driveway throat depths may be required to be based upon a traffic study per Section 295.
9. Driveways accessed by trucks with trailers, should require a minimum throat length of 50 feet.
10. Parking, intersecting drive aisles and designated pedestrian crossings are prohibited within the minimum throat depth.
11. If a gate is proposed on a driveway serving the public, the gate shall be placed a minimum of 20 feet from back of sidewalk or edge of pavement, whichever is greater.  If queues are likely to extend into the travel lane of the nearest roadway, then a queuing analysis shall be provided per Section 295.  Gates may require the approval of the fire district. 
12. Gates are required to have a minimum of a 20' wide unobstructed opening.  The opening width will be required to increase when the length of the anticipated vehicle increases unless the gate is set back far enough for the vehicle to be perpendicular to the gate.

320.1 Maximum Slopes and Grades

1. The plan review, permit and inspections for ADA improvements will be covered under a Development Permit when they are not related to a building permit under review.
2. The ADA facilities should be designed with a 0.5% tolerance from maximum slopes and grades.
3. General parking areas and adjacent drive aisle slopes should not exceed +/-5%.
4. Drive aisles not adjacent to parking spaces should not exceed a longitudinal slope of 10%.  Cross slopes shall not exceed 5%.

320.2 Pedestrian Walkways

1. Walkway connections to the public sidewalk shall meet ADA landing area requirements per PROWAG and be reviewed and permitted by Engineering.
2. Onsite accessible routes, when permitted through a Development Permit, shall meet the requirements of the latest version of the Oregon Structural Specialty Code (OSSC) Chapter 11 and the International Code Council (ICC) A117.1.

320.3 Vehicular Circulation and Maneuvering

All vehicle maneuvering shall be provided on site and should limit backing maneuvers in locations where pedestrians are expected.

1. For dimensions of parking spaces and drive aisles refer to Standard Drawings P100 and P200.
2. To be considered a parking space, adequate maneuvering area shall be provided for each vehicle to enter and exit said parking space.
3. All garbage and recycling facilities shall have adequate access, with onsite maneuvering and circulation for the service provider's vehicle.
4. All movements for non-passenger design vehicles should be shown on an exhibit, showing paths traced by the extremities of the vehicles, including off-tracking.  Adequate turning radii shall be provided for all loading spaces and the largest anticipated vehicles maneuvering on site and at driveway approaches.  At a minimum, the circulation of a fire truck and garbage truck shall be illustrated.

320.4 Parking and Maneuvering Area Surface & Structural Section

1. The parking and maneuvering surface (gravel or paving) shall comply with ZDO Section 1015 or with Standard Drawing R100 if not related to development. 
2. The structural section of parking and drive aisle areas shall meet or exceed Standard Drawing R100.
    

320.5 Parking Stalls

1. All automobile off-street parking quantity and dimensions shall meet the ZDO and Roadway Standard Drawing P100 or P200
2. Dimensions for on-street parking spaces shall meet or exceed the requirements of Roadway Standard Drawing P100 or P200.  The on-street parking shall utilize the street for the drive aisle.  The parking spaces shall not diminish or obstruct the required travel lanes or bike lanes.
3. All compact, carpool, disabled, and loading berth spaces shall be labeled on the plans. 
4. Accessible parking spaces shall meet the requirements of the Oregon Transportation Commission's Standards for Accessible Parking Places, latest edition.

320.6 Curbs and Wheel Stops

1. If parking lot curbs carry, direct or channel surface water, the curb should be structural curb. 
2. Alternative curbs will be considered when it is determined by Engineering that structural curb is not the most appropriate.
3. Extruded curbs shall not be used for carrying, directing or channeling surface water, or as a vehicle wheel stop.

320.7 Signage and Pavement Markings

1. All traffic control devices on private property shall be installed and maintained by the property owner.  Traffic control devices that are located where private driveways intersect a road right-of-way shall be kept in good condition. 
2. The applicant shall provide a signing and pavement marking plan to Engineering for onsite parking and circulation.  This plan shall be reviewed and approved by Engineering and the local fire service provider prior to the applicant being issued a Development Permit.
3. All compact, carpool, disabled parking spaces shall be clearly marked on the site.

320.8 Reciprocal Access Easements

Comply with Section 220.