Chapter 18.16 STORM DRAINAGE

The city of St. Helens Municipal Code, and the Development Code (SHMC Title 17, Ordinance 2875), have established the requirements for the design of facilities intended to protect the public health, safety, and welfare from damage due to flooding. Beyond that level of protection, additional measures are specified in this chapter that are intended to minimize any potential flooding damage and allow for efficient operation, repair, and maintenance of the storm drainage system.

Provisions must be made for gravity drainage of roofs and foundation (footing) drains for all new buildings and structures in accordance with the Uniform Building Code (UBC) and Council of American Building Officials (CABO). For multifamily, residential, commercial, or industrial developments, these drains shall be piped directly to the storm drain system. In single-family residential developments, these drains shall be piped to the street gutter or directly to the public storm drain system. The connection to the street gutter must be through a three-inch plastic pipe set in the curb during construction or bored through an existing curb (see standard drawings). In single-family residential developments where topography prevents connecting foundation and roof drains as required above, drains for each lot shall be directly piped to the public storm drain system; pipe and easement requirements shall conform to SHMC 18.08.150.

These requirements shall apply to all storm drainage facilities in existing and proposed public right-of-way, public drainage easements, and tracts of common ownership in the city. Storm drainage systems include, but are not limited to: inlets, pipes, ditches, creeks, rivers, wetlands, and storm water quality and quantity facilities. (Ord. 2875 Appx. § 310, 2003)

18.16.010 Scheduling.

As specified in SHMC 18.24.010. (Ord. 2875 Appx. § 310.1, 2003)

18.16.020 Preservation, restoration, and cleanup.

As specified in SHMC 18.24.020. (Ord. 2875 Appx. § 310.2, 2003)

18.16.030 Interferences and obstructions.

As specified in SHMC 18.24.030. (Ord. 2875 Appx. § 310.3, 2003)

18.16.040 Permanent survey monuments.

As specified in SHMC 18.24.040. (Ord. 2875 Appx. § 310.4, 2003)

Article II. Trench Excavation and Backfill

18.16.050 Trench excavation and backfill.

As specified in Chapter 18.24 SHMC, Article II. (Ord. 2875 Appx. § 320, 2003)

Article III. Pavement Restoration

18.16.060 Pavement restoration.

As specified in Chapter 18.24 SHMC, Article III. (Ord. 2875 Appx. § 330, 2003)

Article IV. Drainage Report

18.16.070 Drainage report.

The drainage report shall be prepared by and bear the seal and original signature of a professional engineer registered in the state of Oregon and shall contain the following information:

(1) Project name, developer’s name, address, and telephone number, project engineer, and date.

(2) Vicinity map.

(3) Project description: size and location of project, address or tax lot number, zoning, proposed land use, proposed site improvements, and any special circumstances.

(4) Existing conditions: hydrological conditions, topography, land use, off-site drainage to property, natural and constructed channels, sensitive areas, wetlands, creeks, ravines, gullies, steep slopes exceeding 20 percent, springs or other environmentally sensitive areas on or adjacent to the project site.

(5) General soils conditions present within the site, using SCS soil classifications.

(6) Points of discharge for existing drainage from the project site.

(7) References to relevant reports such as flood studies, ground water studies, wetland designation, storm water master plans, sensitive area designation, environmental impact statements, water quality reports, or other relevant documents. Where such reports impose additional conditions on the proponent, those conditions shall be included in the report.

(8) Soils report(s), where applicable.

(9) Hydrologic analysis.

(10) Basin maps, showing boundaries of the project, any off-site contributing drainage basins, on-site drainage basins, and approximate locations of all major drainage structures within the basin(s).

(11) Describe the drainage basin(s) to which the project site contributes runoff, and identify the receiving waters for each of these drainage basins.

(12) Describe the land cover resulting from the proposed project; describe the potential storm water quantity and quality impacts resulting from the proposed project; describe the proposal for the collection and conveyance of site runoff from the project site, for the control of any increase in storm water quantity resulting from the project, and for the control of storm water quality.

(13) Description of upstream basins, identifying any sources of runoff to the project site. This should be based on field investigation. Any existing drainage or erosion issues upstream that may have an impact on the proposed development should be noted.

(14) Downstream analysis.

(15) Hydraulic design computations, supporting the design of the proposed storm water conveyance, quantity and quality control facilities, and verifying the capacity of existing and proposed drainage facilities. These computations may include capacity analysis required either as part of the proposed drainage design or as part of the downstream drainage investigation, and flood routing computations required for the design of detention/retention storage facilities, for wetland impact analysis, or for floodplain analysis. Maintenance and operation manual: required for privately owned and maintained storm water quantity and quality control facilities. (Ord. 2875 Appx. § 340, 2003)

Article V. Drainage Plans

18.16.080 Drainage plans.

It is the responsibility of the design engineer to ensure that engineering plans are sufficiently clear and concise to construct the project in proper sequence, using specified methods and materials, with sufficient dimensions to fulfill the intent of the design guidelines contained in this title.

All engineered drainage plans shall be stamped and signed by a professional engineer registered in the state of Oregon. The drainage plan can be submitted as part of the construction drawings and shall contain the following:

(1) At least one sheet will contain a plan view of the entire project site. In the event the project site is sufficiently large that detailed drainage plans on any given sheet do not encompass the entire project site, then a sheet containing the plan view of the entire site must serve as an index to subsequent detailed plan sheets.

(2) Plans shall include a topographic map showing existing conditions for the site, including:

(a) Existing topography for the site.

(b) Adjacent streets, including street names.

(c) Existing utilities, including franchised utilities located above or below ground and drainage facilities that transport surface water onto, across, or from the project site. Existing drainage pipes, culverts, and channels shall include the invert or flow line elevations.

(d) Existing environmentally sensitive areas (e.g., ravines, swales, steep slopes in excess of 20 percent, springs, wetlands, creeks, lakes, etc.). For natural drainage features, show direction of flow, drainage hazard areas, and 100-year floodplain boundary (if applicable).

(3) Plans for proposed drainage improvements shall include the following:

(a) Finished grades. Show the extent of cut and fill by existing and proposed contours, profiles, or other designations.

(b) Proposed structures including roads and road improvements, parking surfaces, walkways, landscape areas, etc.

(c) Proposed utilities, showing exact line and grade of all proposed utilities at crossing with the proposed drainage system.

(d) Setbacks from environmentally sensitive areas.

(e) Proposed drainage structures, including pipes, open channels, culverts, ponds, vaults, biofiltration swales, detention/retention facilities, outfalls, riprap treatment, energy dissipaters, etc.

(f) Plan and profile of drainage conveyance facilities will include the following information: pipe sizes, pipe types and materials, lengths, slopes, location and type of structures, invert elevations in and out of structures, and top elevations of structures.

(g) Indicate any proposed phasing of construction.

(4) A detailed grading plan will be provided for all open storm water quantity and/or quality control facilities. This plan shall include the following:

(a) Existing ground contours and proposed ground contours at a minimum of a two-foot contour interval. Slopes steeper than six horizontal to one vertical shall be identified.

(b) Location of top and toe of slope.

(d) Location of all drainage structures as well as any other piped utilities in vicinity.

(e) Flow route of the secondary/emergency overflow system.

(f) Maintenance access, as applicable.

(5) Cross-sections shall be provided for at least the following:

(a) Detention/retention ponds, wet ponds, and sediment ponds. This cross-section shall graphically illustrate:

(i) The design maximum water surface for the 10-year and 25-year storm.

(ii) The proposed dead storage water surface.

(b) Proposed ditches and swales, including vegetated swales and privately maintained drainage or cut-off ditches. (Ord. 2875 Appx. § 350, 2003)

Article VI. Water Quantity Standards

18.16.090 General.

All development on sites within the McNulty Creek Drainage Basin that are one-half acre or greater in area shall be required to provide on-site detention. For sites smaller than one-half acre in area or where storm detention would have an adverse effect upon the receiving storm drainage system, as determined by the city engineer, a system development charge will be assessed in lieu of a constructed facility. Detention for sites within the Milton Creek Drainage Basin or other basins shall be provided when proposed development will cause increased flows that could overwhelm downstream facilities in a large storm event. A complete drainage report is required for all proposed developments greater than one-half acre in area addressing the existing and proposed conditions and any detention requirements.

Storm detention facilities shall be designed to provide storage using a 25-year event, with the safe overflow conveyance of the 100-year storm. Calculations of site discharge for both the existing and proposed conditions shall be required. Approved software programs for calculating storm conditions include the King County Hydrograph (HYD), Hec-1, Hydraflow, SWMM, or other approved software.

If a site is proposed to be constructed in phases, the first phase shall have a storm water quantity facility designed and built to accommodate the ultimate development of the site if only one facility is planned for all phases. (Ord. 2875 Appx. § 360.1, 2003)

18.16.100 Hydrologic analysis.

This section presents acceptable methodology for estimating the quantity and characteristics of surface water runoff, as well as the assumptions and data required as input to the methods. These methods should be used to analyze existing and design proposed drainage systems and related facilities.

(1) Rational Method. The rational method for analyzing small drainage basins is allowed with the following limitations:

(a) Only for use in predicting a conservative peak flow rate to be used in determining the required capacity for conveyance elements.

(b) Drainage subbasin area cannot exceed 25 acres for a single calculation.

(d) Rainfall intensities shall be from the Oregon Department of Transportation Rainfall Intensity-Duration-Frequency Curves, Zone 8.

(2) Unit Hydrograph Methods. To obtain a realistic and consistent hydrologic analysis for each development site, the hydrograph analysis method for drainage planning and design shall be used. The physical characteristics of the site and the design storm shall be used to determine the magnitude, volume and duration of the runoff hydrograph. The Santa Barbara Urban Hydrograph (SBUH) will be the primary acceptable unit hydrograph method.

The “HYD” computer program, developed by King County, Washington, in its “Surface Water Design Manual,” January 1990, uses these methods to generate, add and route hydrographs.

(3) Runoff Parameters.

(a) The physical drainage basin characteristics listed below shall be used to develop the runoff hydrograph:

(i) Area.

(ii) Curve number.

(iii) Time of concentration.

(b) Selection of Area. To obtain the highest degree of accuracy in hydrograph analysis requires the proper selection of homogenous basin areas. Significant differences in land use within a given basin must be addressed by dividing the basin area into subbasin areas of similar land use and/or runoff characteristics. Hydrographs should be computed for each subbasin area and superimposed to form the total runoff hydrograph for the basin.

(c) Selection of Curve Number. The National Resource Conservation Service (NRCS, formerly referred to as the Soil Conservation Service (SCS)) has developed “curve number” (CN) values based on soil type and land use. The combination of these two factors is called the “soil-cover complex.”

The soil-cover complexes have been assigned to one of the four hydrologic soil groups, according to their runoff characteristics. The following are important criteria and considerations for selection of CN values:

(i) Many factors may affect the CN value for a given land use. For example, the movement of heavy equipment over bare ground may compact the soil so that it has a lower infiltration rate and greater runoff potential.

(ii) CN values can be area-weighted when they apply to pervious areas of similar CN (within 20 CN points). However, high CN areas should not be combined with low CN areas (unless the low CN areas are less than 15 percent of the subbasin).

(d) Time of Concentration. Time of concentration (Tc) is the time for runoff to travel from the hydraulically most distant point of the watershed to the point where the hydrograph is to be calculated. Travel time (Tt) is the time it takes water to travel from one location to another in a watershed. Tt is a component of Tc. Tc is computed by summing all the travel times for consecutive components of the drainage conveyance system. Tc influences the shape and peak of the runoff hydrograph.

(i) Sheet Flow. “Sheet flow” is flow over plane surfaces. It usually occurs in the headwater of streams. For sheet flow up to 300 feet, the kinematics solution below can be used to directly compute Tt:


Tt =	(0.93L0.6 x n0.3)
	(I0.4 x S0.3)

Where:

Tt = travel time (min)

n = Manning’s effective roughness coefficient for sheet flow

L = flow length (ft)

I = rainfall intensity in inches per hour

S = slope of hydraulic grade line (ft/ft)

Sheet flow shall not be used for distances exceeding 300 feet.

(ii) Shallow Concentrated Flow. After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow. The average velocity for this flow can be determined from Figure 3.1 (see the standard details), in which average velocity is a function of watercourse slope and type of channel. Figure 3.1 was taken from the 1972 “Soil Conservation Service Handbook.”

(iii) Channel Flow. When concentrated flow reaches an identifiable conveyance system (pipe, ditch, stream, etc.) it becomes channel flow. A commonly used method of computing average velocity of flow, once it has measurable depth, is the following equation:

V = (1.486/n) x R2/3 x S0.5

Where:

V = velocity (ft/s)

n = Manning’s roughness coefficient

S = slope of flow path (ft/ft)

R = Hydraulic radius, area/wetted perimeter.

(Ord. 2875 Appx. § 360.2, 2003)

18.16.110 Water quantity facility design.

(1) Mitigation Requirement for Quantity. Each new development is responsible for mitigating its impacts on the public storm water system. One of the following techniques may be used to satisfy this mitigation requirement:

(a) Construction of permanent on-site storm water quantity detention facilities; or

(b) Enlargement or improvement of the downstream conveyance system.

(2) Criteria for Requiring On-Site Detention. Some criteria for requiring on-site detention facilities include, but are not limited to:

(a) There is an identified downstream deficiency, and detention rather than conveyance system enlargement is determined to be the more effective solution.

(b) There is an identified regional detention site within the boundary of the development.

(c) There is a site within the boundary of the development, which would qualify as a regional detention site under criteria or capital plan adopted by the city.

(d) Water quantity facilities are required by city-adopted storm water management master plans.

(3) Hydraulic Design.

(a) Detention design shall be assessed by dynamic flow routing through the basin. Documentation of the proposed design shall be included in the drainage report. Acceptable analysis programs include:

(i) HYD.

(ii) HEC-1.

(iii) HEC-HMS.

(iv) SWMM.

(v) HYDRA.

(vi) Others as approved.

(b) Peak release rates shall not exceed predevelopment rates for the 10-year, 24-hour storm.

(c) A pond overflow system shall provide for discharge of the 100-year storm event without overtopping the pond embankment or exceeding the capacity of the emergency spillway.

(d) Provide an emergency spillway sized to pass the 100-year storm event. Emergency spillway to be located in existing soils when feasible and armored with riprap or other approved erosion protection extending to the toe of the embankment.

(4) Design Criteria. The following are minimum requirements for detention facility design. For more complete guidelines to design criteria, refer to the King County, Washington, “Surface Water Design Manual.”

(a) The facility can be a combined water quality and quantity facility provided it meets all relevant criteria.

(b) Maximum interior side slopes up to the maximum water surface shall be three horizontal to one vertical.

(d) Maximum exterior side slopes shall be two horizontal to one vertical, unless analyzed for stability by a geotechnical engineer.

(e) Retaining walls may serve as pond walls if the design is prepared and stamped by a registered professional engineer and a fence is provided along the top of the wall. At least 25 percent of the pond perimeter will be vegetated.

(f) Overexcavate to allow one-half foot of dead storage for sediment deposition.

(g) Minimum freeboard shall be one foot from the 25-year design water surface elevation.

(h) Provide an approved outlet structure for all flows. (Ord. 2875 Appx. § 360.3, 2003)

Article VII. Water Quality

18.16.120 General concepts and principles.

As an area is developed, impervious area and surface runoff increase. This runoff collects and transports pollutants to downstream receiving waters. Pollutants of concern include:

(1) Suspended solids (sediment).

(2) Heavy metals such as lead, copper, zinc, and cadmium.

(3) Nutrients such as nitrogen and phosphorus.

(4) Bacteria and viruses.

(5) Organics such as oil, grease, hydrocarbons, and pesticides.

Pollution reduction facilities can greatly improve the quality of runoff collected and released to our waterways. Several water quality facility concepts are described further in this article. (Ord. 2875 Appx. § 370.1, 2003)

18.16.130 Dual-use facilities.

Many facilities may be designed and used as dual-use facilities, which are used to meet both pollution reduction and flow control requirements. Common examples are a pond for flow control with a swale at the bottom for pollution reduction, or a wet pond with additional capacity for flow control. Dual-use facilities are encouraged. The design standard for such facilities is that the dual-use facility must meet the design standards for both pollution reduction and flow control. (Ord. 2875 Appx. § 370.2, 2003)

18.16.140 Pollution reduction facilities.

(1) Constructed Wetlands. Constructed wetlands, like natural wetlands, remove pollutants through sedimentation, filtration, and biologic processes. Wetlands typically have shallower water depths than ponds. Wetlands also provide plant and animal habitat.

(a) General Requirements. The detention time of the storm water volume shall be no less than 36 hours. A design team comprising a hydrologist, wetlands specialist, wetlands plant specialist, and an engineer may be needed to develop a successful wetland pollution reduction facility. A water balance analysis should be performed with the design of the facility.

The configuration of a constructed wetland shall be tailored to each site, rather than limited to one design. Major elements of a wetland can include channels or trenches, shallow marshes, and deeper ponded areas. These elements shall be combined to take advantage of the site topography. All wetland design shall address habitat, planting, and aesthetic issues.

General design requirements include, but are not limited to:

(i) Soils requirement: C, D (A and B with liners).

(ii) Maximum maintained side slopes: four horizontal to one vertical.

(iii) Where wetland vegetation is to be planted, side slopes shall be no steeper than five horizontal to one vertical. Wetland plant selection shall be consistent with anticipated soil hydrology. Other side slopes shall be no steeper than four horizontal to one vertical.

(iv) Flow velocity through the wetland shall average less than 0.01 feet per second for the design storm event. If natural slope does not allow for this velocity, berms shall be used to create ponded benches.

(v) Flow through the wetland shall be distributed as uniformly as possible across the marsh and ponded section.

(2) Storm Water Filters. Storm water filters include grassy swales and sand filters. Filters work by settling and straining water, which allows the capture of sediments and the pollutants that adhere to them. Swales also remove pollutants through nutrient uptake and soil absorption.

(a) Grassy Swales. Grassy swales are vegetated open channels that trap pollutants through filtration. General design requirements include, but are not limited to:

(i) Shall serve an area less than 10 acres in size.

(ii) Maximum maintained side slopes shall be four horizontal to one vertical.

(iii) The swale cross-section and grade shall be designed to convey the required storm event at:

(A) Maximum design depth of 0.33 feet, unless swale is part of dual-use facility.

(B) Maximum design velocity of 1.0 feet per second.

(D) Minimum longitudinal slope of 1.5 percent, maximum slope of five percent. For slopes greater than five percent, check dams shall be used.

(E) Designed using a Manning “n” value of 0.25.

(F) Four horizontal to one vertical or flatter side slopes in the treatment area.

(G) Minimum length of 100 feet, maximum bottom width of eight feet.

(iv) A minimum of one foot of freeboard above the standard storm design water surface shall be provided for facilities not protected by high-flow storm diversion devices.

(v) Woody or shrubby vegetation shall not be planted in the active treatment area of the swale.

(vi) The swale shall incorporate a flow-spreading device at the inlet. The flow spreader shall provide a uniform flow distribution across the swale bottom. In swales with a bottom width greater than six feet, a flow spreader shall be installed at least every 100 feet.

(vii) To minimize flow channelization, the swale bottom shall be smooth, with uniform longitudinal slope, and with a minimum bottom width of two feet. Check dams may need to be installed to reduce flow channelization.

(viii) Grasses shall be established as soon as possible after the swale is completed. The native seed mix described below is recommended, but not required:


NATIVE SEED MIX
Blue Wild Rye	47%
Native Red Fescue	40%
Tufted Hairgrass	10%
Western Mannagrass	2%
American Sloughgrass	1%

(ix) Biodegradable erosion control matting appropriate for low velocity flows shall be installed in the flow area of the swale prior to allowing water to flow through the swale.

(b) Sand Filters. “Sand filters” are a layer of sand in a sedimentation chamber used to trap pollutants. The water runs into an underdrain system that conveys the filtered storm water to the discharge point. The sand filter consists of an inlet structure, sand bed, underdrain piping, and basin liner. The design of sand filters is based on Darcy’s Law:


A = Q / (k x i)	where	A = area of sand filter
		Q = peak flow rate from hydrograph
		k = sand permeability (= 3.5 feet/day)
		i = hydraulic gradient
i = (h + L) / L	where	h = height of water column over sand filter
		L = thickness of sand filter

General design requirements include, but are not limited to:

(i) Maximum area to be served is 80 acres.

(ii) Sand filters are appropriate for any soil classification.

(iii) Maximum maintained side slopes are three horizontal to one vertical.

(iv) No drainage shall be allowed directly to the filter; it must first go through a catch basin, inlet, sedimentation manhole, or similar large debris collection device.

(v) The sand filter shall infiltrate the entire design storm without overflow.

(vi) The drawdown period for sand filters shall not exceed 24 hours.

(vii) The inlet structure shall spread the flow of incoming water uniformly across the surface of the filter medium during all anticipated flow conditions. This flow shall be spread in a manner that prevents roiling or otherwise disturbing the filter medium.

(viii) The filter bed medium shall consist of clean medium to fine sand with no organics, frozen pieces, or other deleterious materials. Sand used as a filter medium shall be certified by a certified testing laboratory as meeting or exceeding the following gradation:


Sieve Size	Percent Passing
3/8″	100
#4	95 – 100
#8	80 – 100
#16	45 – 85
#30	15 – 60
#50	3 – 15
#100	<4

(3) Storm Water Ponds and Settling Basins. Ponds include wet ponds and extended wet ponds. They work primarily through settling of pollutants; some biological processes also help remove pollutants.

(a) Wet Ponds. “Wet ponds” are constructed ponds with a permanent pool of water called pool storage or dead storage. Pollutants are removed from storm water through gravitational settling and biologic processes.

Design requirements include, but are not limited to:

(i) Wet ponds can serve areas from five to 50 acres in size.

(ii) Requires group C or D classified soils (A and B with liners).

(iii) Maximum ground slope shall be eight percent.

(iv) Maximum maintained side slopes shall be three horizontal to one vertical.

(v) The pond configuration, as well as the inlet and outlet locations, shall maximize water travel time through the facility.

(vi) The maximum depth of the permanent pool shall be six feet.

More specific design criteria can be found in the King County, Washington, “Surface Water Design Manual.”

(b) Extended Wet Ponds. “Extended wet ponds” are constructed ponds that have both a permanent pool of water and extended detention.

The general requirements and design parameters are the same as for wet ponds in subsection (3)(a) of this section, with the following additional criteria:

(i) Permanent pool volume shall be no less than 50 percent of the design storm volume.

(ii) The remainder of the volume shall be released through an orifice.

(4) Infiltration Facilities. Infiltration facilities include infiltration trenches and infiltration basins. These facilities rely on the percolation of storm water into the existing soils for water quality improvement. As such, infiltration facilities are dependent on the ability of the native soil to drain the storm water. Ground water protection issues need to be evaluated when considering infiltration facilities.

The Oregon Department of Environmental Quality (DEQ) has identified drywells, sumps, and other infiltration-type facilities that inject untreated storm water below the ground surface as “Class V Injection Wells” under the federal Underground Injection Control (UIC) Program. Since the UIC Program states that these types of wells have a direct impact on ground water, storm water pollution controls will apply. More information about the UIC program can be obtained from DEQ.

(a) Infiltration Trenches. An “infiltration trench” is a shallow trench in permeable soil that is backfilled with sand and coarse stone and lined with filter fabric. The trench surface may be covered with grating, stone, gabion, sand, or a grassed cover with a surface inlet.

General design requirements include, but are not limited to:

(i) The maximum area to be served shall not exceed 15 acres.

(ii) Maximum ground slopes shall be five percent for surface trenches, 16 percent for buried trenches.

(iii) Maximum maintained side slopes shall be three horizontal to one vertical.

(iv) The infiltration trench shall infiltrate the entire design storm without overflow.

(v) Infiltration facilities shall not be accepted in soils with a tested infiltration rate of less than 0.50 inches per hour.

(b) Infiltration Basins.

(i) The maximum area to be served shall not exceed 50 acres.

(ii) Maximum ground slope shall be five percent.

(iii) Maximum maintained side slopes shall be three horizontal to one vertical.

(iv) The infiltration basin shall infiltrate the entire design storm without overflow.

(v) Infiltration facilities shall not be accepted in soils with a tested infiltration rate of less than 0.50 inches per hour.

(vi) There shall be no less than three feet of undisturbed depth of infiltration medium between the bottom of the facility and any impervious layer (i.e., solid rock, high ground water levels, impervious clay, etc.). (Ord. 2875 Appx. § 370.3, 2003)

Article VIII. Engineering

18.16.145 Generally.

All storm water system elements shall be designed and constructed in accordance with all applicable rules and regulations of the city, and any city interpretations thereof, and with all applicable federal, state, and local statutes and rules. (Ord. 2875 Appx. § 380, 2003)

18.16.150 Extension of public storm sewer systems.

Public storm sewer systems shall be extended to the most distant upstream parcel boundary(ies) to accommodate current and future storm flows entering the property. Except as otherwise provided, the extension of the public storm water systems to serve any parcel or tract of land shall be done by and at the expense of the property owner, developer, or applicant. The city may require that a storm pipeline that serves or may serve more than one property be a public system. (Ord. 2875 Appx. § 380.1, 2003)

18.16.160 Surveying.

The owner’s engineer or surveyor shall be responsible for establishing the location of the storm sewer system by means of construction stakes offset along the centerlines prior to commencement of construction. (Ord. 2875 Appx. § 380.2, 2003)

18.16.170 Railroad crossings.

Crossing of railroad rights-of-way shall be done in a manner that conforms to the requirements of the railroad having jurisdiction. If any bonds and/or certificates of insurance protection are required, they shall be furnished by the contractor or owner to the railroad company concerned naming the city as an additional insured.

Actual permits or easements for such crossing shall be obtained by the owner and all the terms for such permits or easements shall be met by the owner and contractor. (Ord. 2875 Appx. § 380.3, 2003)

18.16.180 Hydrologic analysis.

The hydrologic analysis shall be consistent with the guidelines outlined in this chapter. The engineer may use various computer models or formulas for the hydrograph analysis but the city may verify the design flows and volumes based on King County’s SBUH program “HYD.” (Ord. 2875 Appx. § 380.4, 2003)

18.16.190 Hydraulic analysis.

The method of hydraulic calculations shall be consistent with the guidelines outlined in this chapter.

(1) System Design Considerations. Site development improvement projects shall address on-site and off-site drainage concerns, both upstream and downstream of a project, including but not limited to the following:

(a) Modifications to the existing on-site storm drainage facilities shall not restrict flows creating backwater onto off-site property to levels greater than the existing situation.

(b) Storm drainage facilities shall be designed and constructed to accommodate all future full build-out flows generated from upstream property based upon the most recent approved city and/or county comprehensive land use plan and applicable storm water master plan.

(d) If the projected increase in surface water runoff leaving a proposed development will cause or contribute to damage from flooding to existing buildings or dwellings, the developer must construct an on-site detention facility.

(2) Review of Downstream System. The design engineer for each development constructing new impervious surface of more than 5,000 square feet shall submit documentation of the downstream capacity of any existing storm facilities impacted by the proposed development.

(3) Conveyance System Hydraulic Standards. The conveyance system shall be designed to convey and contain at least the peak runoff for the 25-year design storm. Open channel systems shall be designed for minimum one-foot freeboard from top of bank provided no structures are impacted by the design water surface elevation.

(4) Catch Basin System Standards. Catch basins and area drains collect water from an adjacent ditch, gutter line, or pavement and convey the water to a storm sewer system or culvert. Inlet systems shall be designed to catch and convey a 25-year storm event. For methodology for locating catch basins, refer to the ODOT “Hydraulics Manual.”

(a) Spacing. Maximum spacing between catch basins shall be 300 feet.

(b) Standard System. All catch basins shall be sumped. The main storm line shall not pass through any catch basin unless approved by the city. No more than three catch basins may be connected in series before connecting to the main storm line. A ditch inlet or field inlet may be connected in the same manner as a catch basin.

(c) Series System. Unsumped catch basins are allowed, provided a sumped manhole is constructed below the unsumped catch basins before the flow enters the main storm line. No more than three unsumped catch basins may be constructed above a sumped manhole. The main storm line may not pass through the catch basins or sumped manhole(s). No ditch inlet or field inlet may be part of a series of unsumped catch basins. (Ord. 2875 Appx. § 380.5, 2003)

18.16.200 Storm manhole and pipe design standards.

For pipe systems which convey flows from or through water quality sensitive areas, a local representative of Oregon Department of Fish and Wildlife (ODFW) or other applicable state or federal agency shall be contacted to determine if fish passage is required and to identify site-specific design criteria. All culverts shall be designed for fish passage in accordance with ODFW guidance for fish passage unless otherwise exempted by ODFW and the city.

(1) Manhole Design.

(a) Manholes shall be provided at least every 500 feet, at every grade change, and at every change in alignment. Unless an exception is approved by the city, manhole lids shall have a minimum of six inches of clearance from the edge of a curb and/or gutter.

(b) All manholes shall be a minimum of 48 inches in diameter.

(d) Detail(s) shall be submitted with the plans where pipes into or out of a manhole are larger than 24 inches or where more than four main line connections are made.

(e) Connections to an existing manhole, elevation of the existing ledge, location of steps, and elevations of existing inlets and outlets shall be submitted with the plans.

(f) All manhole bases shall be properly channelized. There shall be a minimum of eight inches separating connections as measured from the outside diameter of the pipe.

(g) Standard or oversize gutter or curb and gutter catch basins will not be allowed in lieu of manholes in any system.

(h) A manhole may have a maximum free fall of two feet.

(2) Water Quality Manholes. Water quality manholes shall be as shown in the standard detail drawings.

(3) Pipe Size. The design size shall be based on hydraulic calculations provided by the design engineer. The minimum diameter of public storm pipe is identified as follows:

(a) Pipe from the catch basin to the main line in the public right-of-way shall be a minimum 10-inch diameter pipe.

(b) Main line pipe shall be a minimum 12-inch diameter pipe.

(c) Storm pipes located out of a public street right-of-way, with no reasonable need to be extended, and with roof drains and/or area drains connected, shall be a minimum 10-inch diameter pipe.

(4) Location of Pipe. When storm drain pipes are located within a local public street right-of-way with curbs, the storm pipe shall be located between the curbs but no closer than five feet to either curb unless an exception is approved by the city.

Storm pipes in easements shall be located in the center of the easement unless an exception is approved by the city. The centerline of a storm pipe shall be at least seven and one-half feet from an easement side line.

(5) Alignment. Public storm pipe shall be laid on a straight alignment and at uniform grade unless an exception is approved by the city.

(6) Grade. All storm lines shall have sufficient slope to maintain a minimum flow velocity of three feet per second when flowing full.

(7) Steep Slopes. Storm pipes on slopes in excess of 20 percent shall be secured with approved anchor walls.

(8) Pipe Cover. Minimum pipe cover shall be in compliance with this section unless an exception is approved by the city.

In paved areas, pipe cover shall be measured from the bottom of the subbase to the upper surface of the pipe barrel. In pavement areas, the pipe bell shall not intrude in to the subbase. In areas without pavement, pipe cover shall be measured from finish grade to the upper surface of the pipe barrel. Minimum cover requirements are as follows:


Type of Pipe	Cover (in)
Nonreinforced Pipe	36
Ductile Iron	18

(9) Headwalls. Pipe end protection shall be required where pipe material other than ductile iron is exposed in design of an outlet or inlet pipe or where required to stabilize slope.

(10) Trash Racks/Debris Barriers. Trash racks/debris barriers are required over all pipe inlets over 12 inches in diameter and outlets over 18 inches in diameter. The engineer shall submit the trash rack/debris barrier system design to the city for approval. (Ord. 2875 Appx. § 380.6, 2003)

18.16.210 Inlet design standards.

(1) Inlet and Catch Basin Capacity. All inlets and catch basins shall be designed to accept a 25-year storm event. Grates shall, as far as practical, be designed to avoid failure due to accumulation of debris.

(2) Design Criteria.

(a) Precast and poured in place catch basins and gutter inlets are allowed.

(b) All catch basins shall be constructed with an 18-inch minimum sump unless part of a series catch basin system with a sumped manhole.

(d) The spacing of catch basins shall be determined by the capacity of each catch basin to pass a 25-year storm event. Maximum spacing shall be 300 feet. In addition, catch basins shall be provided just prior to curb returns on streets with a centerline gradient of three percent or more and a street gutter drainage run of 100 feet or more.

(e) Catch basins shall be a maximum depth of six feet from the top of grate to flow line of the lowest pipe invert.

(f) The maximum length of pipeline between the inlet and main line structure shall be 40 feet for 10-inch pipe and 60 feet for 12-inch pipe.

(g) Blind tee connections to the main line are not allowed.

(3) Area Drains and Ditch Inlets.

(a) The standard area drain and ditch inlet shall be as shown in the standard details, unless an exception is approved by the city.

(b) A main storm line shall not pass through an area drain or ditch inlet. (Ord. 2875 Appx. § 380.7, 2003)

18.16.220 Constructed channel design standards.

Open channels may be constructed to convey runoff to the existing public storm system. Open channels are not allowed in the public right-of-way. They may be used as cut-off or diversion ditches along lot lines to prevent runoff from spilling on to an adjoining lot. Open channels must be privately maintained.

(1) Design Criteria.

(a) All constructed channels located on private property must be within an easement.

(b) Open channels that ultimately drain in to the public storm system must show line, grade, and a typical cross-section on the construction drawings.

(c) Constructed open channels shall be sized to pass the required flows and have side slopes no steeper than two horizontal to one vertical.

(d) Channels and connections shall be designed to prevent scouring. All pipe connections shall match side slopes and incorporate a headwall. (Ord. 2875 Appx. § 380.8, 2003)

18.16.230 Culvert design standards.

Culverts provide for passage of water under or through obstructions placed across streams and drainageways. Culverts shall be designed to pass the required flows without compromising public safety or causing new or additional flooding.

Culverts within FEMA floodplains shall be reviewed and approved by the local FEMA-designated authority.

For culverts which convey flows from or through water quality sensitive areas, a local representative of Oregon Department of Fish and Wildlife (ODFW) or other applicable state or federal agency shall be contacted to determine if fish passage is required and to identify site-specific design criteria. All culverts shall be designed for fish passage in accordance with ODFW guidance for fish passage unless otherwise exempted by ODFW.

(1) Design Criteria.

(a) Culverts will be designed to safely pass the 25-year flow.

(b) For culverts 18 inches in diameter and larger, the embankment around the culvert inlet shall be protected from erosion by lining around inlet with rock or other protection. The lining shall extend upstream from the culvert a minimum of five feet.

(c) For culverts 12 inches in diameter and larger, the receiving channel of the outlet shall be protected from erosion by rock lining, bioengineering, or other approved energy dissipater.

(d) For culverts 18 inches in diameter and larger, the inlet and outlet shall require grating. (Ord. 2875 Appx. § 380.9, 2003)

18.16.240 Outfall design standards.

Outfalls shall be above the mean low-water level unless an exception is approved by the city.

All outfalls shall be provided with a rock splash pad or other approved erosion control measure. Outfalls will be designed to prevent scouring at the outfall discharge and provide velocity reduction prior to discharge to the receiving channel. (Ord. 2875 Appx. § 380.10, 2003)

Article IX. Technical Specifications

18.16.245 Generally.

The technical specifications contained in this chapter, together with the Oregon Department of Environmental Quality, the Federal Environmental Protection Agency, and the American Public Works Association standards and any other applicable requirement(s) of the city, shall govern the character and quality of material, equipment, installation, and construction procedures for gravity flow portions of public storm sewer systems. (Ord. 2875 Appx. § 390, 2003)

18.16.250 Manholes and structures.

Refer to SHMC 18.24.140. (Ord. 2875 Appx. § 390.1, 2003)

18.16.260 Catch basins and inlets.

(1) Materials.

(a) Aggregate, cement, and concrete shall meet the requirements set forth in SHMC 18.16.250.

(b) Frame and grate shall be as shown in the standard details.

(2) Workmanship.

(a) Excavation and backfill will conform to the requirements of SHMC 18.24.060.

(b) Bedding. The contractor shall remove all water and debris from ditch area and provide eight inches minimum layer of compacted three-fourths-inch minus crushed rock for a base.

(c) Cast-in-place catch basins shall have a minimum of six inches of concrete between the compacted gravel and the lowest invert. The forms used for cast-in-place catch basins shall be tight and well braced. The storm pipe material being used shall extend into the poured concrete of the catch basin. All corners shall be chamfered. Immediately after placement, the concrete shall be consolidated with an approved vibrator. The top surface shall be screed and exposed surfaces troweled to a smooth finish free from marks or irregularities. After forms are removed, the contractor shall patch any defects in the concrete with approved material.

(d) Precast. After the base is prepared, the contractor shall set the precast catch basin to the proper line and grade. The storm pipe material being used shall connect to the precast catch basin.

(e) Contractor shall clean the ends of all pipes and sections that come in contact with the catch basin. All inverts, stub outs and sections shall be installed according to the details using a nonshrinking grout, making sure all sharp edges or rough sections are removed, to prevent obstruction of the flow. (Ord. 2875 Appx. § 390.2, 2003)

18.16.270 Pipe and fittings.

All pipe and culverts shall have a minimum design service life of 75 years per Oregon Department of Transportation standards.

(1) Materials. Materials shall be the following types or approved equal:

(a) High density polyethylene (HDPE), SDR-26, ASTM F-667, D-1245.

(b) Ductile iron pipe (DIP).

(2) Pipe Installation. Refer to SHMC 18.24.160. (Ord. 2875 Appx. § 390.3, 2003)

18.16.280 Testing and acceptance.

All gravity storm systems shall pass a deflection test for plastic pipes. All details of testing procedures shall be subject to approval of the city.

(1) Deflection Test for Flexible Pipe. Storm systems constructed of flexible pipe materials shall be deflection-tested. The test shall be conducted by pulling an approved mandrel through the completed pipeline. The diameter of the mandrel shall be 95 percent of the nominal pipe diameter. The mandrel shall be a rigid, nonadjustable, odd-numbering leg (nine legs minimum) mandrel having an effective length of not less than its nominal diameter.

Testing shall be conducted after the line has been completely balled and flushed out with water, and compaction tests have been completed and accepted.

The contractor will be required to locate and repair any sections failing the test and to retest the section. (Ord. 2875 Appx. § 390.4, 2003)