Transportation Engineering

Clackamas County regulates the placement and ongoing requirements of utilities that are located in the public right-of-way and easements under the jurisdiction of the County.  This chapter addresses the technical requirements associated with utility installation.

County Code Title 7.03.099 should be referenced for additional important information.  Additionally, some utilities have separate agreements with Clackamas County that may modify the requirements included herein. 

Applicants shall conform to the requirements of Chapter 4 regarding soil loss and erosion control measures.

710.1 Potholing Requirements

On arterial and collector roadway, work subject to a Development Permit shall pothole for existing utility locations to verify that the design has no conflicts with existing utilities.  The County may require potholing in other situations and on other roadways depending on the type of facility and scope of the work.  Any conflicts with existing utilities including storm or sanitary will be resolved prior to excavation for installation of the utility. 

Potholing requires a Utility Permit.

710.2 Location Requirements

1. New and relocated utility installations shall follow Standard Drawing U100 for placement.
2. The utility shall be placed as far as possible from the edge of the roadway including within public utility easements as feasible. 
3. Any placement shall not impede, obstruct, or hinder operation of any emergency service, maintenance operations, pedestrian or vehicular access or travel including to or from private properties and of legally parked vehicles or permitted items within a public right-of-way.
4. Above ground utility facilities shall follow the clear zone standards of Section 245 and pedestrian facility requirements of Section 710.3. 
5. Utilities shall be installed underground per the ZDO or as required by land use requirements.
6. Any manhole lids, junction boxes, vault lids, water meters, etc. that are located in vehicular travel lanes are to be out of any wheel tracks.

710.3 Pedestrian Considerations

When considering pedestrian facilities, utilities:

1. Shall not obstruct the pedestrian facility width. 
2. Any surface access to utilities (including manhole lids, junction boxes, vault lids, water meters, etc.) shall not be installed in a pedestrian facility unless no reasonable alternative exists.  If no reasonable alternative exists, the surface access shall be flush with the pedestrian facility grade.  The surface access located in a pedestrian facility shall be slip resistant and not have holes or depressions that can cause a tripping hazard per County Code Section 7.03. 
3. Surface access for new utilities shall not be constructed within any ADA ramps or landing. 
4. If existing utilities are located where an ADA facility needs to be constructed and the utility cannot be relocated, the surface access shall be made flush with the ADA ramp or landing and an ADA exception shall be formally requested and approved.

710.4 Structures

When attachment to a structure (i.e. a bridge or a box culvert) located in a public right-of-way is involved:

1. The applicant shall provide an engineering assessment of the existing structure to add the facility including a structural analysis that illustrates the ability of the structure to carry the weight of the facility and also considering:
   1. Dead load of the facility
   2. Supports to attach to the structure
   3. Attachment method
   4. Spacing of the supports
2. No utility shall be attached to a bridge or other structure crossing a body of water prior to County Planning and Zoning review for a possible Floodplain Development Permit and shall obtain the concurrence of County Transportation Maintenance.

710.5 Pressurized Pipes

When the proposed utility involves pressure pipe line, the applicant shall provide the:

1. Design pressure of pipe;
2. Normal operating pressure;
3. Maximum operating pressure.

710.6 Vertical Clearance

Any aerial utilities shall meet the requirements of the National Electric Safety Code for vertical clearance.

710.7 Burial Requirements

710.7.1 Depth

All underground installations shall be buried a minimum of thirty (30) inches below the nearest vertical roadway surface, (i.e., from the bottom of ditch line).  Minimum depth of bury may be decreased with Engineering approval based on topographical constraints or when matching existing utilities.  If placed less than 30 inches deep, utilities shall be placed below any cross culverts and should not be placed above cross culverts.  Additional requirements may be imposed on installations not meeting the minimum depth requirements including identifying tape.  Plans must show the distance from the nearest vertical roadway surface to the top of the proposed buried cable, pipe line, or facility.

710.7.2 Warning Signage & Ability to Locate

Warning signs for buried power or communications cable, and for pipe lines carrying gas or flammable liquids, shall be placed at each crossing under the roadway, and at intervals along longitudinal installations as required by the current Public Utility Commissioner's Order and as specified by the Road Official as follows:

1. Signs shall be placed as near the right-of-way line as is practical.
2. Notwithstanding 1) above, signs for an installation within the roadway shall be placed behind any existing guardrail.

Tracer wire or other locating device is required immediately above the facility enabling the ability to locate the facility within two feet of the facility per Oregon dig laws.

710.7.3 Pedestal Placement

Pedestals installed as part of a buried cable installation are to be located as far from the traveled portion of the roadway as is practical, and preferably one foot from the right-of-way line.  The locations shall not impact driveways or ADA ramps.

710.8 Requirements and Specifications for Trench Backfill

1. Backfill materials meeting the Oregon Standard Specifications for Construction (aka "standard trench backfill") shall be required for use when trenches exceed one of the following dimensions:
   1. 100 feet in length or longer; or
   2. Greater than 10 feet in depth.
2. Standard trench backfill may be used per Standard Drawing U200 except when the following conditions exist and then CDF shall be utilized per U250 through U275B:
   1. Utility trenches are within the roadway of arterial and collector classified roadways;
   2. The affected roadway surface is newer than 5 years from the time of the last overlay, without regard to roadway classification; or
   3. Engineering deems it necessary.
3. CDF shall conform to the following specifications:
   1. Be able to excavate and produce unconfined, compressive, 28-day strengths from 100 psi to a maximum of 200 psi.
   2. Contain aggregate no larger than 3/4 inch, and for trenches less than 12 inches in width, the aggregate shall be no larger than 3/8 inch.
   3. Slump shall be 6 to 8 inches to insure flowability and will fill all voids without requiring compaction efforts.
   4. The surface of fill shall reach a strength to withstand the process of paving without displacement or disruption within 48 hours, regardless of weather conditions, temperature or moisture content of the soil where placed.  Additives such as calcium (1% or 2%), hot water and/or a pozzilith (water reducer) are acceptable means to achieve this set.
   5. Copies of the CDF batch weights must be submitted to Engineering.
   6. Alternative backfill may utilized per Standard Drawings U270A and U270B.
4. Exemption from CDF may be considered if all of the following construction requirements are otherwise met:
   1. Backfill materials meeting Class "B" backfill specifications from the Oregon Standard Specifications for Construction or its approved equal are used;
   2. Not less than 95% relative maximum density (using AASHTO T-99) is achieved;
   3. Compaction results are provided by a certified testing lab;
   4. Perimeter excavation for manholes is 10'X10' or greater to allow for sufficient mechanical compaction of the backfill;
   5. The surety repair time duration is extended an additional two years beyond the three years as specified in Section 710.9.
5. Trenches shall be protected in the following manner:
   1. Sufficient weight and size steel plating or approved equal materials, capable of carrying a minimum of H-20 loading, shall be present at the work site prior to excavation and placed over the trench to protect the public.
   2. Plating shall be positively secured from movement and shall be ramped with anti-skid coated plate ramps.
   3. Plates must extend beyond the trench wall a minimum of one foot on all sides.
   4. Lighted barricades with appropriate signage shall be placed sufficiently ahead of, and adjacent to, plating to warn all traffic.
   5. All plating and signs are to remain in place until permanent surface repair paving operations are underway.
   6. Additionally as required by Section 290.
       

710.9 Open Cuts of Paved Roadway Surfaces

1. Unless there are extenuating circumstances that require open cutting the road to install utilities or special permission is granted by the Road Official to open cut the road, a cable, pipeline, or conduit, which crosses under the roadway, other roadway connections e.g. road approaches or driveways, shall be placed in a casing bored under the surface for that purpose in accordance with the following provisions:
   1. All utility companies serving the work site vicinity shall be contacted to request line locate services.
   2. Any utility conflicts shall be resolved before initiation of construction.
   3. The applicant will be required to comply with ORS 757.
2. Installations by plowing of cable or conduits within the UGB shall not be allowed.
3. Burial of cable outside the UGB placed by the plowing method shall be limited to areas behind the ditch line or as close the right-of-way line as practical when no ditch exists.  Approval of alternate means of installation is subject to time and schedule restraints to allow for preferable soil moisture conditions, pavement surface temperatures, and other roadway characteristics. 
   1. In all cases mechanical compaction efforts shall be applied to the entire disturbed portion of the right-of-way.
   2. Restoration of gravel shoulders and drainage ditches and the verification of the function of all drainage structures must be achieved prior to completion.
4. Open cut utility installations in paved streets shall restore those streets per Standard Drawings U275 through U290.
5. Open cut service laterals, when allowed, shall be grouped together per Standard Drawing U290.
6. In addition to the requirements of Standard Drawings U275 to U295, utility cut requirements may include, but not be limited to the following conditions:
   1. Repaving may include surface grinding, base and sub-base repairs, or other related work as needed to restore the road to the minimum standards and to reduce the number of seams or eliminate pavement seams in a wheel path.
   2. Pavement restoration beyond trench patching along roadways with sidewalk may require the installation or replacement of ADA curb ramps. 
   3. Disturbance to existing ADA facilities may require replacement of those facilities to current PROWAG standards. 
   4. Additional asphalt area removal and replacement may be required to ensure the smoothness of ride characteristics to meet the Oregon Standard Specifications for Construction Section 00744.70. 
   5. As required by these Roadway Standards and/or as determined by the Road Official the requirements may include up to full-width surface paving of the roadway depending on the limits of disturbance and the condition of the existing pavement; 
   6. Limitations on the operation to protect the roadway from temperature related damage, i.e. delamination of pavement surfaces and subgrade;
   7. If the County determines that the final repaving of the street is not appropriate at that particular time for reasons relating to weather or other short term concerns, the County may grant a delay until proper conditions allow for repaving.
   8. Apply restrictions as to the size and type of equipment during freeze/thaw conditions or for saturated subgrade with a poor or very poor PCI rating; it could also be due to the existing width of the road and traffic flow or other conditions that warrant limitations on equipment;
   9. Designation of routes upon which materials may be transported;
   10. Mitigation of potential subsurface hydrologic flow along the utility or appurtenant trench; e.g. bentonite check dams;

710.10 Concrete Street Surface Repairs

Concrete roadway surface repairs shall conform to the following specifications: 

1. The entire Portland Cement Concrete (PCC) panel must be removed between the nearest expansion joint.  If the trench excavation is within two feet of the nearest joint the abutting panel must be removed.
2. Placement of the bedding material and approved granular backfill must be placed and compacted to 95% compaction.
3. Replacement of the Portland Cement Concrete panel must equal the thickness and design strength of the concrete material removed, or be of a minimum 4,000 pound, 28 day strength design mix, whichever is stronger. The concrete must be placed in conformance with industry standards and protected against freezing.  The texture of the concrete surface must be like the adjoining surfaces.
4. Perpendicular PCC joints - 18" long, 1 ¼" smooth dowels with epoxy coating, embedded 9" into both the new PCC panel and the existing, abutting panel, spaced at 12" on center
5. Longitudinal PCC joints - 16" long, #4 rebar smooth dowels with epoxy coating, embedded 8" into abutting panels (new and existing), spaced at 18" on center
6. Longitudinal joint with curb and gutter - 8" long, #4 rebar smooth dowels with epoxy coating, embedded 4" into both the PCC gutter and panel, spaced at 18' on center.
7. Concrete roadway restoration beyond along roadways with sidewalk may require the installation or replacement of ADA curb ramps. 
8. Disturbance to existing ADA facilities may require replacement of those facilities to current PROWAG standards
9. The work area must be signed and protected to detour traffic away from the repair for seven (7) days following the placement of the concrete repair unless the use of a high early additive is requested and approved in the permit.

The following process is required in order to obtain Service District approval for street illumination for all development as required above:

615.1 Approval Process

1. The applicant shall contact the Service District and have the property owner sign a "Request for Street Lighting."
2. The applicant shall also contact PGE and provide PGE with a street illumination design.
3. The Service District, in turn, places an annual assessment determined by the district rate schedule on the property tax statement of the affected properties for the operation of the street illumination.
4. The tax lots in the development will be required to form an assessment area within Clackamas County Service District No. 5 for the purpose of paying for the operation and maintenance of street illumination.
5. After a formal hearing process, a special assessment is placed on the tax roll at the District rate determined by the classification of the property and the type of illumination installed.
6. Contact Engineering at 503-742-4400 with any questions or to initiate the street illumination process for a development.

615.2 Construction & Installation

1. In areas where new underground or overhead electricity supply circuitry is required, PGE will coordinate the street illumination circuitry design with the primary power supply.  However, the applicant cannot assume that this will happen automatically, a separate request for street illumination shall still be made.
2. Depending on the circumstances of the installation, the applicant may be required to provide and install at their expense and according to the design approved by the District and PGE:
   1. Pad-vaults for transformers or overhead transformers for street illumination,
   2. Splice boxes, and/or
   3. Circuitry conduit with pull line.
3. PGE will provide the transformers and circuitry for street illumination but will bill the applicant directly for any costs that may be incurred to install these; these charges may be offset to some extent by PGE line extension allowances.
4. PGE will provide and install the street illumination poles and luminaries at no charge to the applicant.

615.3 Rates

Rates are subject to change annually.  The first assessment is usually in November following the date of installation and the assessment is pro-rated to the date of installation.

Chapter 6 addresses the technical requirements associated with street illumination. 

Street illumination is typically installed as part of a Development Permit. 

610.1 Street Illumination Required Within UGB

Street illumination installation is required of all partitions, subdivisions, commercial, industrial, and multifamily residential development within the UGB in Clackamas County by the ZDO if adequate street lighting does not already exist.

610.2 Street Illumination Design by PGE

Street illumination design and installation is subject to the approval of Clackamas County Service District No. 5 ("Service District"), working through Engineering, and the utility serving the development, Portland General Electric ("PGE").

610.3 Street Illumination are Option A

Street illumination is owned, operated, and maintained by PGE, as Option A and the applicant is responsible for the design. PGE and the District are responsible for the design approval.

610.4 Illuminating Engineering Society (IES)

Street illumination is generally designed to IES guidelines for the road classification utilizing existing infrastructure (poles, transformers, and circuitry) where available.

610.5 Fixture Approval

The Service District has final approval for all street illumination fixtures for each development.

Chapter 5 establishes the technical requirements associated with structures.

Structures not under the review of Clackamas County Building Codes will be reviewed by Engineering.  All structures proposed in the public right-of-way including walls, bridges, box culverts and stairs are subject to a Development Permit. 

Structures under the review of Engineering shall meet the following requirements:

1. All structures described herein shall be designed by an Engineer.
2. Walls installed by the County are allowed within the public right-of-way.
3. As feasible, retaining walls should be located on private property. Easements shall be provided as necessary.
4. Stairs located in the public right-of-way shall meet IBC requirements. 
5. Other structures in the public right-of-way will be reviewed per AASHTO requirements or as otherwise determined by 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.