City of Washington, MO Improvements And Design Standards

[R.O. 1992 § 420.035; Ord. No. 16-11557, 8-15-2016, effective 3-1-2017]

Sediment And Erosion Control Standards.

General. Sediment and erosion control will be accomplished by applying conservation practices that will reduce the potential for damage from these hazards. Control practices use trapping, filtering or diversion techniques to protect adjacent properties from land disturbance activities.

NPDES Stormwater Permits. Construction sites, where the area to be disturbed is one (1) acre or more, must apply for a stormwater discharge permit from the Missouri Department of Natural Resources. If required, the applicant must obtain and submit proof of an NPDES stormwater permit to the City, before plans will be approved. Permit requirements are set forth in 10 CSR20-6.200 of the Missouri clean water laws.

When Controls Are Required. Sediment and erosion control shall be implemented whenever necessary, in the opinion of the City Engineer, to prevent damage to off-site property, drainage facilities or watercourses. Generally, standard vegetative and structural practices, as specified below, that filter, divert or promote the settlement of sediment particles from storm runoff shall be provided in the following situations:

To prevent sediment-laden runoff from leaving disturbed areas.

To isolate disturbed areas from erosive surface runoff associated with significant undisturbed areas.

To protect storm drainage conveyance systems at operable inlets.

Types Of Controls.

The stormwater management plan shall be selected, installed, operated and maintained to adequately control erosion, capture sediment and prevent pollution. To achieve these goals BMPs in accordance with the concepts and methods described in either of the following two (2) documents should be utilized.

(1)

Protecting Water Quality, MDNR (Latest Edition).

(2)

Field Manual on Sediment and Erosion Control BMPs. Jerald S. Fifield, Ph.D., CPESC, Forester Press, Latest Edition.

The developer is not limited to the use of BMPs identified in the above publications. Engineering professionals are encouraged to design innovative ways to address site specific conditions.

Filter Strip. Clearing and grubbing within twenty-five (25) feet of the top of an existing stream bank should be avoided. If the twenty-five-foot filter strip is disturbed, site specific erosion controls that protect the stream bank shall be implemented.

Construction Plans.

The applicant shall submit a sediment and erosion control plan which must be approved by the City Council prior to preliminary plat approval. The plan shall include a contoured development map clearly indicating the pattern of surface water runoff, both upstream and downstream of the development, the existing and proposed grading contours and the planned sedimentation and erosion control facilities. The phasing of construction activities shall be presented. Such phasing shall identify the sequence of construction activities and the related BMPs to include clearing, installing BMP, rough grading, installing utilities, paving streets, final grading and vegetative cover.

The City Council shall have authority to require proper controls as specified herein. The City Engineer may require a revised plan to be submitted for approval at any time due to changing site conditions.

Design Criteria.

Purpose. A development's stormwater drainage system shall be designed to:

Protect natural waterways.

Convey upstream and on-site stormwater runoff to a natural watercourse or to a storm drainage facility.

Provide protection from the design storm event and address the major storm so as to prevent major property damage and loss of life.

Rainfall Frequency. All facilities shall be designed to carry a ten-year storm of a duration which produces the maximum peak flow rate of stormwater runoff. The stormwater conveyance system shall be designed to allow for a 100-year storm to pass through the subdivision without destroying or damaging property or inundating dwellings. Design information which substantiates both conditions shall be provided. See Figure A for rainfall curves for the two-, five-, ten-, fifteen-, twenty- and 100-year frequencies.[1]

[1]

Editor's Note: Figure A is included as an attachment to this Chapter.

Calculation Of Runoff.

The method of calculating and routing stormwater runoff shall be as stated herein. The drainage area shall consider all on- and off-site lands contributing to the proposed development's drainage system. Capacity for such facilities shall be based on the maximum potential water shed development permitted by the Zoning Ordinance.[2]

[2]

Editor's Note: See Ch. 400, Zoning.

Either the Rational Method or the TR-55 Method for calculating stormwater runoff may be used for watersheds up to ten (10) acres.

Figures A (rainfall intensity - referenced above) and B (runoff factors) shall be utilized with the Rational Method.[3]

[3]

Editor's Note: Figures A and B are included as attachments to this Chapter.

For watersheds larger than ten (10) acres, the SCS TR-55 Method shall be utilized. Other methods to determine peak runoff must be approved by the City Engineer prior to acceptance.

The minimum percentage of imperviousness to be used in design shall be based on the zoning district as shown in the following table:

Zoning District	Minimum Percentage Impervious
Pre-developed	5%
R-1A Single-Family	45%
R-1B Single-Family	50%
R-1C Single-Family Attached	50%
R-1D Single-Family	50%
R-2 Two-Family	60%
R-3 Multiple Family	70%
C-1 Limited Commercial	85%
C-2 General Commercial	85%
C-3 Central Commercial	90%
M-1 Industrial	90%
M-2 Industrial	90%
PD Planned Development Districts	TBD based on plan type
Parking, streets, roofs	100%

The minimum percentage of imperviousness for Planned Development Districts shall be dictated by the development plan proposed for the specific district.

Special uses, such as schools, churches, etc., shall have the differential runoff computed and approved by the City Engineer.

Special circumstances may dictate that the developed impervious area may differ from that shown in the above table. An example may be single-family lots of larger than ten thousand (10,000) square feet. Calculations prepared by a registered professional engineer may be submitted to the City Engineer for his/her evaluation to determine if a minimum impervious area which differs from that shown in the above table may be utilized. Likewise, the City Engineer may determine that the percent of impervious area for a particular development differs from that shown in the above table.

See Figure B[4] for the runoff factor to be utilized for various impervious conditions and rainfall durations.

[4]

Editor's Note: Figure B is included as an attachment to this Chapter.

Open Channels.

Open channels consist of swales, ditches or depressions, both natural and man-made, that convey water. Channels shall be protected from scour and erosion by providing a channel lining adequate to sustain the velocity of the ten-year design storm. If velocities in channels exceed five (5) feet per second during ten-year design storms, then erosion control other than vegetation shall be provided in channel construction. Total accumulation contained by man-made berms/swales shall be a maximum of four (4) cubic feet per second.

Manning's Equation shall be used for calculating the hydraulic capacity of open channels.

Acceptable improvements to channels include fabrics, gabion lining, concrete channels, concrete slope paving, cribbing, bin walls, etc. The roughness coefficient for each material shall be per manufacturer or as indicated below:

n= Amount	Channel/Pipe Type
0.012	For cast-in-place concrete, box culverts and reinforced concrete pipe (RCP) over 48 inches in diameter
0.013	For concrete pipe less than 48 inches in diameter
0.015	For vitrified clay pipe
0.020	For grouted rip rap
0.024	For corrugated metal pipe
0.033	For gabion walled channels

Channels shall have the hydraulic capacity to carry the ten-year design storm runoff within the channel bed and banks. Out-of-bank flow may be permitted on land slopes parallel to the channel where it can be shown that no erosion damage or serious property damage will result. Channels function as part of the major drainage system and shall be evaluated for the 100-year design storm to determine the impacts of runoff on adjacent property. The channel's hydraulic capacity shall be increased where adjustments to channel geometry provide significant protection to adjacent properties during the 100-year event.

Closed Conduit Storm Sewers.

Pipe sizes for closed conduit flow shall be based on the ten-year design storm runoff and minimum allowable velocities. The system shall provide for the cleaning of sediment and other deposits by maintaining a minimum velocity of three (3) fps during the ten-year storm.

Manning's Equation, the most common method of estimating the capacity and flow resistance in closed conduits, shall be utilized.

Closed conduit storm sewer systems shall convey the ten-year design storm to a point of discharge by gravity or pressure flow. In surcharge conditions, the hydraulic grade line shall be calculated to reflect losses in pipes and structures and shall not rise to an elevation greater than the three (3) feet above the top of the sewer or two (2) feet below the inlet sill, whichever is lower.

Generally, gravity flow occurs where the capacity of pipe run exceeds the design flow and the outfall point does not control discharge. Storm sewer systems may be designed for pressure flow when the hydraulic grade line is above the crown of the pipe. The decision to design a pressure flow system may be based on aesthetics, the need to submerge outfalls, economics, limitations associated with reduced pipe sizes or grade constraints in outfalling the system.

Inlets. Calculations shall be submitted to demonstrate the capacity of all inlets. Such calculations must consider the cross-slope of the pavement, depth of water at the curb face, size of opening and the longitudinal grade of street. Street inlets and inlets in parking areas shall reduce the spread and depth of flow to acceptable levels during the ten-year design storm. The acceptable level of flow for a minor access or local access street would maintain an eight-foot travel lane with a maximum one-inch depth. One clear ten-foot travel lane must be maintained for a collector street, and two (2) clear ten-foot travel lanes must be maintained for a major street. Any area inundated by water ponding at an inlet during the ten-year storm event shall be located within an easement or right-of-way. The effects of the 100-year storm event shall also be analyzed to ensure no property damage or dangerous conditions result. Inlets located on continuous grades may be designed to permit a portion of flow to bypass the structure; however, calculations for the downstream structure must consider the bypass.

Plans And Calculations.

A drainage map shall be developed from a base reproduction of the site plan or grading plan. The existing and proposed contours shall be shown, normally at two-foot intervals, for the subject property, extending off-site one hundred (100) feet or less as determined by the Department for proper design of the proposed improvements. Contour intervals other than the above shall be used as determined by the site topography. Only United State Geological Survey datum shall be used. Locations and elevations of bench mark references are available from the Engineering Department.

The location of existing and proposed property lines, streets, sinkholes, railroads, areas within the tract subject to inundation by stormwater and other significant natural features, such as wooded areas and rock formations, etc., shall be included on the map. All existing and proposed stormwater facilities, such as inlets, manholes, pipes, culverts, bridges, channels, etc., and all existing and proposed improvements required for proper design review, such as pavement, buildings, etc., shall be included on the map.

The runoff details shall be required, showing individual flows for each existing and proposed structure and cumulative flows in pipes and gutters, including "Q" and area. The map shall show all bodies of water, such as ponds or lakes (including surface area and elevation) and all waterways (including their names or the names of creeks or rivers they flow into).

Lots shall be laid out so as to provide positive drainage away from all buildings. Individual lot drainage shall be shown and coordinated with the drainage pattern for the area and designed so that runoff from one (1) lot will not adversely affect an adjoining lot. All necessary grading to direct stormwater runoff shall be located within a drainage easement.

All computations, plans and specifications related to the implementation of this Section must be prepared and sealed by a professional engineer registered in the State of Missouri.

(Reserved)

Detention And Retention Facilities.

Maintenance.

Each owner of the property being developed has the responsibility and duty to properly operate and maintain any stormwater management system which has not been accepted for maintenance by the City. The responsibility for maintenance of the system in subdivision projects shall remain with the developer until such time as the stormwater management system escrow for such development has been released. Upon release of escrow, the maintenance responsibility goes to vested property owners within the subdivision.

The final plat of the subdivision shall contain language substantially as follows: "The owner of each lot within this subdivision shall maintain the stormwater management system serving this subdivision unless the stormwater management system has been accepted for maintenance by the City of Washington, Missouri. The maintenance costs shall be shared equally with each owner of any lot served by the stormwater management system." There shall also be recorded with the final plat of the subdivision deed restrictions containing the same language.

The responsibility for maintenance in single-lot development shall remain with the general contractor and owner until final inspection of the development is approved and an occupancy permit is issued. After occupancy, the maintenance of the management system shall be vested in the owner of the project.

All such privately owned and maintained systems shall be subject to periodic inspection by the City Engineer or his/her representative. The City of Washington, Missouri, may, upon failure of the responsible party to maintain the systems, maintain the system and charge the cost thereof against the responsible party. The costs shall be certified to the City Clerk who shall cause a special tax bill therefor against the property to be prepared and to be collected by the Collector with other taxes assessed against the property. The special tax bill from the date of its issuance shall be a first lien on the property until paid and shall be prima facie evidence of the recitals therein and of its validity and no mere clerical error, informality in the same or in the proceeding leading up the issuance shall be a defense thereto. Such tax bills if not paid when due shall bear interest at the rate of eight percent (8%) per annum.

A proper access easement to such stormwater management systems shall be provided, and a minimum ten-foot wide hard surface designed to support construction traffic with a maximum twelve percent (12%) grade shall be provided within such easement. A permeable, flexible plantable concrete block pavement system is preferred in residential areas.

Storage Capacity.

The rates (pre-developed and post-developed) of runoff shall be determined for the two-, twenty- and 100-year rainfall frequencies. The storm duration shall be the twenty-four-hour event when the SCS TR-55 method is utilized and a minimum twenty-minute event when the Rational Method is utilized. Rainfall data shall be determined using the most current twenty-four-hour rainfall data published by the National Weather Service (NWS) Technical Paper 40 (TP40).

Stormwater shall be detained on site or adjacent property under agreement and metered out at the rate of an undeveloped site for the above frequencies and minimum duration to prevent possible flooding and erosion downstream. Design criteria to establish this differential runoff rate shall be as provided in Subsection (B), Design Criteria. Note that stormwater pipes shall be sized to carry the total developed tributary upstream water shed. No reduction in pipe size shall be permitted because of detention.

Detention basin volume will be based on providing adequate storage for the 100-year storm event of the required duration. Each post-developed runoff hydrograph (two- and 100-year) shall be routed through the detention facility while satisfying the appropriate allowable release rate. The routing computation shall be based on an application of the continuity principle. The discharge rate shall be based on the maximum head conditions in the detention facility.

Shared Facilities And Regional Detention. The City retains the right to require on-site detention storage in all cases in which the proposed development will generate excess runoff that adversely affects the carrying capacity of the receiving watercourse and/or adversely affects adjoining property owners. In certain cases the applicant may make use of off-site or regional detention facilities, in lieu of on-site facilities, as described below:

Off-Site facility, two (2) or more developments: if two (2) or more developments, including that of the applicant, have provided for a common system.

Off-site facility by City: if an off-site stormwater management system has been either constructed or programmed or identified for construction by the City and the applicant has agreed to contribute to or participate in the construction thereof. Such contribution shall be determined per the following Subsection (E)(3)(b)(1).

(1)

Stormwater Management Fund.

(a)

Eligibility. It is determined that no immediate adverse effects will result to adjacent property and a contribution is made to the Stormwater Management Improvements Fund. Such fund is described in Subsection (E)(3)(b)(1)(c), Establishment Of Fund, below. Developments which have a differential runoff of three (3) cfs or less for the ten-year, twenty-minute event utilizing the Rational Method will be considered as prime candidates for a contribution in lieu of on-site detention. It is the City's intention to provide regional detention storage to accommodate these smaller developments. The City Council will render all decisions concerning participation in the Stormwater Management Fund.

(b)

Contribution. The contribution shall be an amount equal to a cost estimate prepared by a professional engineer for site specific improvements necessary to provide detention as provided herein. Such estimate shall not include the cost of any land necessary for detention purposes. All cost estimates for site specific detention facilities shall be subject to review and approval by the City Engineer. The developer may request that the City Engineer provide the necessary cost estimate.

(c)

Establishment Of Fund. The Stormwater Management Improvements Fund shall be and is hereby created. Said fund shall be reserved for funding improvements to stormwater systems owned and maintained by the City and for no other purposes unless authorized in the ordinance. All contributions made by parties developing within the City in accordance with Subsection (E)(3) of this Section shall be deposited to said fund. Said fund shall be deposited in an interest-bearing account.

Other Management Techniques. Management techniques other than detention facilities may be utilized by the development, provided that the techniques proposed meet the intent of this Section and provide a benefit to the watershed that equals or exceeds the benefit that a detention facility would provide. Such techniques would include pervious pavement systems and improved vegetation conditions.

General Design Features.

Dry Bottom Basins. A stormwater detention facility, natural or artificial, which normally drains completely between spaced runoff events, may be constructed to temporarily detain the stormwater runoff so that the rate at which it is released is the same rate as before development. The following features shall be incorporated into the design of any detention basin:

(1)

Freeboard. Detention storage areas shall have adequate capacity to contain the storage volume of tributary stormwater runoff with at least two (2) feet of freeboard above the water surface.

(2)

Outlet Control Works. Outlet works shall be designed to limit peak outflow rates from detention storage areas to or below peak flow rates that would have occurred prior to the proposed development.

(3)

Outlet works shall not include any mechanical components or devices and shall function without requiring attendance or control during operation, unless specifically approved by the City Engineer.

(4)

Emergency Overflow/Spillway. Emergency structures shall be provided to permit the safe passage of runoff generated in excess of the 100-year design storm event. Antivortex measures shall be provided.

(5)

Maximum Depth. The maximum planned depth of stormwaters stored shall not normally exceed five (5) feet.

(6)

Side Slopes. The maximum side slopes for gassed basins shall not normally exceed one (1) foot vertical for three (3) feet horizontal.

(7)

Limits Of Ponding. In no case shall the limits of maximum ponding be closer than thirty (30) feet horizontally from any building and less than two (2) feet vertically below the lowest sill elevation.

(8)

Interior Drainage. The basin should be designed to drain within a twenty-four-hour period. Flows through the detention basin should be handled by paved ditch from inflow structure to outflow structure to minimize erosion.

(9)

Multipurpose Basin. If the detention basin is to have other uses, the design of the basin bottom should include underdrains to expedite drying of the bottom between runoff events.

(10)

Aesthetics. Designs should result in aesthetically pleasing configurations which will enhance public acceptability.

Wet bottom basin: a stormwater retention facility, natural or artificial, which maintains a fixed minimum water elevation between runoff events. Wet bottom basins may also be used to temporarily detain the differential runoff from the development. In addition to the general design features enumerated above for dry bottom basins, the following features should also be incorporated into the design of any wet bottom basin:

(1)

Normal Pool Depth. In order to minimize weed growth, the normal pool depth should be four (4) feet minimum.

(2)

Depth For Fish. If fish are to be kept in the pond, at least one-fourth (1/4) of the area of the permanent pool should have a minimum depth of ten (10) feet.

(3)

Facilities For Emptying. In order to ease cleaning of the pond or shoreline maintenance, the pond design should include provisions for emptying the pond.

(4)

Low Flow By-Pass. The design of any pond may include a low flow by-pass channel or pipeline to divert runoff that can be accommodated by downstream drainageways.

(5)

Side Slopes Below Normal Pool. The side slopes below the normal pool elevation may exceed the maximum side slope permitted above normal pool (3:1 slope). The design shall, however, include provisions for a safety ledge having a depth of water not greater than three (3) feet immediately adjacent to the shoreline.

(6)

Forebay. In order to minimize siltation of the pond, a forebay should be included in the design. Calculations for sediment volume and forebay sizing shall be submitted to the City Engineer.

Rooftop Storage. Detention storage may be met in total or in part by detention on roofs. Details of such design, which shall be included in the building permit application, shall include the depth and volume of storage, details of outlet devices and down drains, and elevations of overflow provisions. Direct connection of roof drains to sanitary sewers is prohibited.

Parking Lot Storage. Paved parking lots may be designed to provide temporary detention storage of stormwater on all or a portion of their surfaces. Outlets will be designed so as to slowly empty the stored waters. Depth of storage shall be a maximum of eight (8) inches.

Other Detention Methods. All or a portion of the detention storage may also be provided in underground or surface detention facilities, to include basins, tanks or swales, etc. Emergency overflow conditions shall be considered in all methods.

Plans And Calculations.

The following must be submitted for the design of a detention facility:

(1)

Elevation versus discharge relationship for the basin.

(2)

Elevation versus storage relationship for the basin.

(3)

Inflow calculations and data for all required frequencies.

(4)

Hydraulic grade line computations for pipes entering and leaving the basin for all required frequencies.

(5)

Site plan with two-foot contours showing land to be developed and adjoining land whose topography may affect the layout or drainage of a basin site and the location of streams and other runoff channels.

(6)

Basic information regarding the receiving watercourse and affected downstream structures to a distance of two hundred (200) feet from the site. Additional analysis of the receiving stream of greater distances from the site shall be performed if required by the City Engineer.

(7)

A summary of routing calculations for all required frequencies.

All computations, plans and specifications related to the implementation of this Section must be prepared and sealed by a professional engineer registered in the State of Missouri.

Material And Construction Standards.

Storm pipes shall be protected from excessive bearing pressures by placing them outside the forty-five-degree influence zone of building structures unless an engineering calculation shows the pipe material or soil condition to be adequate for the subjected load.

Pipes on slopes of twenty percent (20%) or greater shall be anchored securely with concrete anchors or equal to prevent the pipe from creeping downhill.

Pipes or structures constructed on fill shall be stable and protected against settlement by compacting fill material to ninety-five percent (95%) of the modified proctor maximum dry density, per AASHTO T180 (ASTM D1557).

Pipes thirty-six (36) inches or larger may be placed on a curved alignment utilizing alignment radii established by the pipe manufacturer.

The receiving surface where pipes discharge shall be protected from erosion by evaluating the discharge velocity for the ten-year design storm. The use of energy-dissipating devices may be necessary to reduce the velocity to acceptable levels for the receiving surface. Grouted revetment used shall be a minimum length of ten (10) times the diameter of the discharge pipe.

A manhole, inlet or junction box shall be located at changes in pipe size, grade, alignment or material.

The angle between influent and effluent pipes shall be not less than ninety degrees (90°) and the drop between inverts shall be not less than one-tenth (0.1) foot.

Manhole and inlet castings located in travel ways shall be capable of withstanding traffic loads and shall be constructed flush with the finished surface.

All materials and appurtenances for stormwater management systems shall conform to current standards of the American Society for Testing and Materials (ASTM).

10.

Manholes shall be precast or cast-in-place concrete, brick, concrete block, with concrete or brick risers and approved manhole covers.

11.

A new drainage channel or pipe shall intersect an existing drainage channel at a maximum angle of sixty degrees (60°).

12.

All trenches under roadway pavement shall be backfilled with Missouri Department of Transportation (MoDOT) Type I aggregate in six-inch layers and compacted to ninety-five percent (95%) of the modified proctor maximum dry density per AASHTO T180 (ASTM D1557).

13.

All piping shall be bedded per the manufacturer's requirements.

14.

Grated inlets will not be allowed without special approval by the City Engineer.

15.

All materials used in the construction of storm sewers shall be subject to inspection and approval of the City Engineer.

16.

Acceptable pipe material is as follows.

Reinforced concrete pipe shall conform to the requirements of the Specifications for Reinforced Concrete Culvert, Storm Drain and Sewer Pipe, ASTM C76. Strength class or classes shall be as required per design specifications of the latest edition of the Concrete Pipe Handbook as published by the American Pipe Association.

Corrugated metal pipe shall conform to the requirements of AASHTO M36, Standard Specification for Zinc Coated (Galvanized) Pipe, or the Standard Specifications for AASHTO M196 Corrugated Aluminum Pipe. Structural design requirements shall be per the latest edition of the Handbook of Steel Drainage and Highway Construction Products, as published by the American Iron and Steel Institute.

Corrugated polyethylene pipe shall conform to the requirements of AASHTO M294, Standard Specifications for Corrugated Polyethylene Pipe. All polyethylene pipe should be installed according to ASTM D2321, Standard Practice for Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity-Flow Applications.

Dual wall and triple wall polypropylene pipe shall conform to the requirements of AASHTO M330 "Standard Specification for Polypropylene Pipe" and ASTM F2881 for sizes 12" to 30" and ASTM F2764 for sizes 30" to 60". All polypropylene pipe shall be installed according to ASTM D2321 "Standard Practice for Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity-Flow Applications" and shall meet MoDOT "Standard Specifications for Highway Construction" Section 724 Pipe Culverts specifications.

[Ord. No. 22-13620, 9-19-2022]

17.

Reinforced concrete pipe and corrugated polypropylene complying with the provisions in Section 420.035(F)(16), above, shall be required under road pavement.

18.

A minimum pipe size of twelve (12) inches is required to prevent blockage.

19.

All construction details pertaining to stormwater drainage shall be in accordance with the Metropolitan St. Louis Sewer District requirements and standards for construction of sewers and drainage facilities, unless otherwise noted herein.