[Ord. No. 24-13956, 5-20-2024]
A. 
Post-Construction — Flood Control.
1. 
Purpose. A development's stormwater drainage system shall be designed to:
a. 
Protect natural waterways.
b. 
Convey upstream and on-site stormwater runoff to a natural watercourse or to a storm drainage facility.
c. 
Provide protection from the design storm event and address the major storm so as to prevent major property damage and loss of life.
2. 
Plans And Calculations.
a. 
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 Engineering 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.
b. 
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.
c. 
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).
d. 
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.
e. 
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.
f. 
Elevation versus discharge relationship for the basin.
g. 
Elevation versus storage relationship for the basin.
h. 
Inflow calculations and data for all required frequencies.
i. 
Hydraulic grade line computations for pipes entering and leaving the basin for all required frequencies.
j. 
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.
k. 
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.
l. 
A summary of routing calculations for all required frequencies.
m. 
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.
3. 
Design Criteria.
a. 
Projects shall be designed to detain on-site or off-site as approved and released at a rate not to exceed the allowable release rates for the 2-year, 10-year, and 100-year, 24-hour events.
b. 
The 2-year, 10-year, and 100-year, 24-hour inflow hydrographs shall be determined by using Technical Release 55 (TR-55), "Urban Hydrology for Small Watersheds" from the Natural Resources Conservation Service, formerly Soil Conservation Service (SCS). The inflow hydrograph shall be developed based on the actual flow and timing characteristics upstream of the detention facility. The Rational Method for calculating stormwater runoff may be used for watersheds up to ten (10) acres.
c. 
The rainfall distribution shall be Type II. The rainfall quantities to be used are from NOAA's National Weather Service, Atlas 14 Point Precipitation Frequency Estimate, and shall be as follows: three and twenty-nine hundredths (3.29) inches for the 2-year, 24-hour storm, four and eighty-two hundredths (4.82) inches for the 10-year, 24-hour storm, and seven and niney-four hundredths (7.94) inches for the 100-year, 24-hour storm.
d. 
The volume of detention may be provided through permanent detention facilities such as dry basins or ponds, permanent ponds or lakes, underground storage facilities or in parking lots. It is noted that when runoff volume reduction BMPs are utilized within the tributary area upstream of a flood volume detention basin, the quantified runoff reduction may be used to adjust tributary area TR55 Curve Numbers (CN) for detention basin sizing.
e. 
The engineer shall make every effort to locate the detention facility at or near the lowest point of the project such that all of the on-site runoff will be directed into the detention facility.
f. 
Flows from off-site, upstream areas should be bypassed around the detention facility to ensure that the proposed detention facility will function as designed and will provide effective control of downstream flows with development in place. If off-site flows are directed into a detention facility, the allowable release rates shall not be modified without City Engineer approval. Modifying the release rate to accommodate off-site flows may reduce or eliminate the effectiveness of the detention facility, because it will no longer control the increased volume of runoff during the critical time period of the watershed.
g. 
Detention basin volume will be based on routing the post-developed 2-year, 10-year, and 100-year, 24-hour inflow hydrographs through the detention facility while satisfying the appropriate allowable release rate. The routing computations shall be based on an application of the continuity principle, (i.e., level pool routing).
4. 
Calculation Of Runoff.
a. 
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, Chapter 400.
b. 
Either the Rational Method or the TR-55 Method for calculating stormwater runoff may be used for watersheds up to ten (10) acres.
c. 
Figures A (rainfall intensity - referenced above) and B (runoff factors) shall be utilized with the Rational Method.[1]
[1]
Editor's Note: Figures A and B are included as attachments to this Chapter.
d. 
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.
e. 
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%
f. 
The minimum percentage of imperviousness for Planned Development Districts shall be dictated by the development plan proposed for the specific district.
g. 
Special uses, such as schools, churches, etc., shall have the differential runoff computed and approved by the City Engineer.
h. 
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.
i. 
See Figure B[2] for the runoff factor to be utilized for various impervious conditions and rainfall durations.
5. 
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 10-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 (1) 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 10-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.
[Ord. No. 24-13956, 5-20-2024]
A. 
Purpose: Post-Construction — Water Quality. In order to preserve the quality of water in natural streams, it is important to provide a mechanism to remove contaminants on the site prior to water entering the natural watercourse. Typically called post-construction BMPs (best management practices), these methods identify a critical water quality volume that will need to receive a treatment to remove certain contaminants. These improvements, whether structural or non-structural, will remain in place after the construction is completed. The concepts introduced in the following Subsections are taken from the APWA MARC Manual of Best Management Practices For Stormwater Quality, October 2012. Nothing in the following Subsections shall change or replace any of the City's detention ordinances.
B. 
Requirements.
1. 
This requirement shall apply to redevelop and new development that exceeds one (1) acre or more disturbed or sites that are part of a common plan development that exceeds one (1) acre or more disturbed, the following assumptions may be made:
a. 
The water quality volume WQv for off-site areas is not required. The following equations are used to determine the storage volume, WQv (in acre/feet of storage):
WQv
=
[(P)(Rv)(A)]/12
P
=
1.14 inches of rainfall
Where:
WQv
=
Water quality volume (in acre-feet)
Rv
=
0.05 + 0.009 (I) where I is percent impervious cover
A
=
Area in acres
b. 
Measuring Impervious Cover. The measured area of a site plan that does not have vegetative or permeable cover shall be considered total impervious cover.
c. 
Multiple Drainage Areas. When a project contains or is divided by multiple drainage areas, the WQv volume shall be addressed for each drainage area.
d. 
Off-Site Drainage Areas. The WQv shall be based on the impervious cover of the proposed site. Off-site existing impervious areas may be excluded from the calculation of the water quality volume requirements.
e. 
BMP Treatment. The final WQv shall be treated by an acceptable BMP(s) from the list presented in this Chapter or other approved methods which may include bioretention, permeable pavers, or others. Reference the APWA MARC Manual of Best Management Practices For Stormwater Quality, October 2012, for guidance.
f. 
Extended Detention For Water Quality Volume. The water quality requirements can be met by providing an extended draw down of all or a portion of the water quality volume (WQv) in conjunction with other systems.
g. 
Infiltration Trenches/Basins For Water Quality Volume. Practices that capture and temporarily store the WQv before allowing it to infiltrate into the soil over a two-day period include:
(1) 
Infiltration trench.
(2) 
Infiltration basin.
(3) 
Infiltration practices will be allowed on sites where it is proven that infiltration will work. Percolation rates shall be determined for proper use.
h. 
Open Channel Practices. Vegetated open channels that are explicitly designed to capture and treat the full WQv within cells formed by check dams or other means. The drawdown period of each channel shall be less than twenty-four (24) hours.
i. 
Filter Strip. Filter strips can be provided at the edge of impervious areas where sheet flow is occurring. The edge shall be protected to avoid erosion.
C. 
Stream Channel Setbacks. A setback of twenty-five (25) feet from the top of an existing ordinary high water mark on any channel identified on the most current USGS Topographic Survey, shall be avoided. The setback shall clearly be defined on the applicant's site plan and identified in the field by staking.
D. 
Gutter Downspout Disconnect. Gutters and downspouts shall not extend to within five (5) feet of any right-of-way line or property line, and shall not be directed toward neighboring property to cause damage.
[Ord. No. 24-13956, 5-20-2024]
A. 
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 one (1) foot 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 (24) hour period unless utilized as extended detention for water quality. Minimizing erosion shall be considered for flows from inflow structure to outflow structure.
9. 
Multipurpose Basin. If the detention basin is to have other uses, the design of the basin bottom should include underdrains, engineering soils, or other methods to expedite drying of the bottom between runoff events.
10. 
Aesthetics. Designs should result in aesthetically pleasing configurations which will enhance public acceptability. Consideration should be given to adding signs for education about the purpose of the facilities.
11. 
An orifice diameter of less than three (3.0) inches will require a special internal control for orifice protection. For orifice between three (3) inches and one and one-half (1 1/2) inches diameter, an internally controlled orifice shall be used with slot width less than or equal to one-third (1/3) of orifice diameter. Less than one and one-half (1 1/2) inch orifice will not be allowed.
B. 
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. 
Fish should not be kept in Stormwater Retention Facilities.
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. City shall be notified in advance of any cleaning and emptying of facilities, and City shall be present to inspect.
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.
C. 
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.
D. 
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.
E. 
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, bioretention, permeable pavers, or swales, etc. Emergency overflow conditions shall be considered in all methods.
F. 
Shared Facilities And Regional Detention.
1. 
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:
a. 
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.
b. 
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 (F)(1)(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 (F)(1)(b)(1)(c), Establishment Of Fund, below. Developments which have a differential runoff of three (3) cfs or less for the 10-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 (F)(1)(b)(1)(b) of this Section shall be deposited to said fund. Said fund shall be deposited in an interest-bearing account.
G. 
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.
1. 
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.
2. 
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.
3. 
All such privately owned and maintained systems shall be subject to periodic inspection by the City Engineer or his/her representative. The owner in charge of maintenance shall conduct annual inspections that are reported to the City. The City of Washington, Missouri, upon failure of the responsible party to maintain the systems shall be given notice of violation and given three (3) weeks to respond to the violation and work with the City Engineer to determine a reasonable timeframe for corrections to be made. Failure to correct the deficiencies within the agreed upon timeframe shall result in a fine of not less than one hundred dollars ($100.00) per day. If the responsible party fails to correct the deficiencies and maintain the system within the time prescribed the City shall 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 therefore 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.
4. 
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.
5. 
At the time plat approval, the developer may request for dedication to the City to take over the detention facility for maintenance. This request shall be accompanied by a stormwater impact fee, payable to the Stormwater Fund, in the amount of ten (10) years of anticipated basin maintenance, certified by an engineer. Once accepted, the City will immediately begin maintenance of said facility.
H. 
Existing Basin Maintenance Request.
1. 
The City reserves the right to accept maintenance responsibility of existing detention facilities at the request of all lots served by the facility.
2. 
Requests shall be submitted in writing and signed by each owner of each lot within the subdivision to which the facility serves.
3. 
Only detention facilities that serve residential subdivision will be reviewed for acceptance.
4. 
Detention facilities that are wet, such as ponds or lakes, will not be considered as these facilities provide other sources of benefit to the owners of the subdivision.
5. 
Upon the City accepting the facility, the owner(s) of the lot to which the detention basin resides upon, shall provide the City with an easement to allow for maintenance, improvements, and storage of water.
6. 
The City reserves the right to reject any requests for dedication for City maintenance responsibility.
I. 
Storage Capacity.
1. 
The rates (pre-developed and post-developed) of runoff shall be determined for the 2-, 10- and 100-year rainfall frequencies. The storm duration shall be the 24-hour event when the SCS TR-55 method is utilized and a minimum 20-minute event when the Rational Method is utilized. Rainfall data shall be determined using the most current 24-hour rainfall data published by the National Weather Service (NWS) Technical Paper 40 (TP40).
2. 
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 Section 420.060(A)(3), 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.
3. 
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 (2-, 10-, 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.
J. 
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.