tuttle stormwater drainage regulations

Stormwater Drainage Regulations from Ord 2012-04 as Amended (Ord 2013-01, 1/2013) of 11

Stormwater Drainage Regulations

Tuttle Code of Ordinances

Chapter 21

Article II – Stormwater Drainage

Section 21-20. General

1. Drainage facilities shall be designed to provide a sufficient stormwater drainage systemfor the conveyance of stormwater runoff received from upstream and from the subjectproperty with due allowance having been made for continued conveyance of stormwaterrunoff from adjacent properties as the drainage basin develops.

2. The drainage system including streets, bridges, culverts, open channels,detention/retention facilities, etc., shall be designed to carry all probable storm recurrenceintervals up to and including the 100-year storm event.

3. A dedicated drainage easement shall encompass all land lying below the water surfaceelevation generated by the 100-year storm event and resulting stormwater runoff. Sucheasements in developments shall have a maintenance program carried out by theproperty owners association. The Planning Commission, at its discretion, may requirethese dedicated drainage easements to be platted as Common Area.

(Amended by Ord. 2013-01, 1/2013)

4. Site improvement shall provide for the grading of all building pads to an elevation wherethe lowest finished floor elevations shall be a minimum of one foot above the 100-yearstorm event. In all developments, surface water from each dwelling lot will flow awayfrom each dwelling in a controlled manner to an approved collection or disposal location.

5. Improvements shall be designed such that there is no increase in flow leaving the site orin the depth of flooding downstream.

6. Existing Drainage Structures: Existing structures within and adjoining a proposeddevelopment shall be evaluated in terms of hydraulic capacity and structural soundness.Those structures found to be structurally inadequate shall be modified or completelyremoved and replaced by the developer. Structures found to be hydraulically inadequatefor flow and/or for protecting property as a result of the changing runoff characteristicsshall be replaced or modified by the developer to provide the required capacity and/orprotection.

7. Utilities: All utilities located within the floodplain or any other open channel or swale shallbe designed to prevent infiltration of flood water and to protect against washouts.Location of the utilities shall not restrict flood flows.

8. Plans for the proposed drainage system shall include property lines, lot and blocknumbers, dimensions, right-of-way and easement lines, floodplains, street names, pavedsurfaces (existing or proposed), contract limits, location, size and type of inlets,manholes, culverts, pipes, channels and related structures, outfall details, miscellaneousriprap placement, 20 foot or better contour lines, flow arrows, title block.

9. Profiles shall indicate the proposed system (size and material) with elevations, flow lines,gradients, left and right bank channel profiles, station numbers, inlets, manholes, groundline and curb line elevations, typical sections, riprap construction, filling details, opendrainage features, pipe crossing, design flow capacities, and title block.

10. Official floodplain designations and delineations of the floodplains and floodwaysdenoting limits of permissible developments shall be shown on all preliminary plans and


final plats submitted for approval wherever such plans and plats contain a floodplainsegment. In any case in which official floodplains are not delineated, they shall bedetermined on the basis of these standards and shall be shown on all preliminary plansand final plats submitted for approval.

11. The limits of the floodplains shown on regulatory floodplain maps are closeapproximations based on extrapolations made between widely-spaced cross sections.Therefore, the actual limits of the floodplain shall be determined by extracting base floodelevations from the stream profile and determining where each elevation intersects theadjacent existing topography. If a conflict exists between the overlay of the regulatoryfloodplain map and the existing topography, a submittal of a Letter of Map Amendment(LOMA) or Letter of Map Revision (LOMR) by the Developer/Owner to the FederalEmergency Management Agency (FEMA) is required when the FEMA Flood InsuranceRate Map (FIRM) conflicts with proposed lots or other proposed structures orimprovements.

12. Computations and plans to support all drainage designs shall be submitted to the CityEngineer for review. The computations and plans shall be in such form as to provide thebasis for timely and consistent review and will be made a part of the permanent record forfuture evaluation. The computations and plans shall be accompanied by the certificationof a registered professional engineer licensed to practice in the State of Oklahoma.Before final acceptance, the submitting engineer shall provide an “as built” plan based onfield survey data of the streets, storm sewer, detention facilities, and all otherconveyances of stormwater; a revised set of drainage calculations that correspond withthe “as-built” plans; and a letter of certification stating that the “as-built” site complies withall governing ordinances of the City of Tuttle.

13. The City of Tuttle, Oklahoma, reserves the right to require (a) improvements, (b) theprovision of drainage easements and (c) the provision of agreements beyond theboundaries of the subdivision or development, to facilitate flow of water through thedevelopment, to avoid probability of lawsuits, based on damage from changed runoff inthe development, and to provide continuous improvements of the overall storm drainagesystem.

14. Property owner agreements, where required, shall be designed to protect the City frompossible lawsuits for damage caused by changed runoff conditions.

Section 21-21. Drainage Report

For all areas not otherwise designated in a floodway or floodway-fringe area, the developer shallbe responsible for having an engineer (registered in Oklahoma) prepare a Drainage Report forthe area of the proposed subdivision, and all areas affected by runoff resulting from developmentof the proposed subdivision. The Drainage Report shall show compliance with this section andshall clearly indicate the existing and proposed runoff from the subdivision and state whether ornot peak release rates comply with these Regulations.

Section 21-22. Determination of Storm Runoff

Numerous methods of runoff computation are available on which the design of storm drainageand flood control systems may be based. The Rational Method will be accepted as adequate fordrainage areas up to 40 acres. For larger areas, the U.S. Army Corps of Engineers HydraulicEngineering Center HEC-1 or other engineering programs or methods acceptable to the City’sEngineer shall be used. Any hydrological soil group data to be used when computing runoff shallbe determined from the most current edition of the Grady County, Oklahoma Soil Survey providedby the United States Department of Agriculture Natural Resources Conservation Service.

Rational Method

1. Runoff Coefficient “C”. The runoff coefficients shall be chosen to represent the integratedeffects of infiltration, detention storage, evaporation, retention, flow routing and


interception. The “C” values are provided in the table below, Runoff Coefficients for Usein the Rational Formula.

Runoff Coefficients for Use in the Rational Formula

LAND USE BASIN SLOPESlope <2% Slope >= 2%

Single Family Residence < 2 Acre .55 .65Single Family Residence >= 2 Acre .45 .55Multi Family .60 .70Apartments .75 .85Commercial .90 .95Parks, Golf Courses .35 .45Historic Conditions .25 .35

2. Rainfall Intensity (I). The average rainfall rate in inches per hour considered for aparticular drainage basin or sub-basin and selected on the basis of design rainfallduration and design frequency or occurrence.

Rainfall Intensity Formula:I = a/ (td +b)

c

WhereI = Average Rainfall, Intensity, inches/hourTc = Rainfall Duration (or Time of Concentration), minutes = To+Tf

a, b, c = Variable values from the following table:

ZONE II RAINFALL FREQUENCY

2 YEAR 5 YEAR 10 YEAR 25 YEAR 50 YEAR 100 YEARa= 56.43 a=72 a=82 a=95 a=108 a=120b=11.5 b=15 b=15 b=15 b=15 b=15c=0.810 c=0.80 c=0.80 c=0.80 c=0.80 c=0.80

The time of concentration is the time required for the entire watershed to contribute runoffto the point of interest. Street flow shall be considered as being in an open pavedchannel.

An acceptable formula for use in determining overland flow travel time is:

To=Time = K(L0.37

/S0.20

)L = Length of Flow in FeetS = Average Slope in Feet/FootK = Constant for Character Surface

Values of K:Pavement 0.37Bare Soil 0.60Poor Grass 0.90Average Grass 1.00Dense Grass 1.13

An acceptable formula for determining channel flow time is:


Time = K(L0.77

/S0.385

)Tf =Values of K:Curbed street 0.0035Concrete Lined Channel 0.006Sodded Swale 0.008Bar Ditch 0.012

Mannings’ Equation is acceptable for analyzing open channel flows in prismatic channelswithout backwater control and for analyzing free water surface conduit flows.

3. Drainage Area. The size and shape of the watershed must be determined. Drainageareas shall be determined through the use of planimetric topographic maps,supplemented by field surveys. A drainage area map shall be provided for each project.The drainage area contributing to the system being designed and drainage sub-area(s)contributing to each inlet point shall be identified. The outlines of the drainage dividesmust follow actual lines rather than artificial land divisions.

Section 21-23. Flow in Streets

1. Flow in gutters which are on straight or parabolic crown pavement may be determined byusing Manning’s Formula for channel flow or acceptable nomographs.

2. Minimum gutter slope on residential street shall not be less than 0.005 ft/ft (0.5%).Maximum gutter slope should not be more than .08 ft/ft (8.0%). In a gutter which has aradius of 100’ or less, the design flow depth shall not exceed 70% of the curb height.

3. No lowering of the standard height crown or splitting of curb heights shall be allowed forthe purpose of hydraulic design.

4. All street sections shall have a positive crown except alleys.

5. The flow depth in the gutter of arterial streets shall not exceed 4” for the 100-year stormevent for a standard 8” barrier curbed street. The flow depth in the gutter of otherclassification of streets shall not exceed 5” for the 100-year storm event for streets withstandard 6” barrier curbs or standard 6” mountable curbs. For streets with nonstandardcurbs, the maximum 100-year storm event water surface elevation in the street shall belimited to provide the same inches of freeboard between the water surface elevation andthe top of the curb that is required in the first part of this paragraph.

6. Where two (2) or more streets intersect, the flow shall be intercepted by appropriatestorm sewer techniques prior to the intersection of said streets such that the depth of flowwithin the intersection will not exceed 4” in depth for the 100-year storm event.

7. A minimum of 70% of the flow in a residential street shall be removed to a storm sewer ordrainage system prior to reaching the point of curvature of a cul-de-sac or a “T” or “L”shaped-intersection, or the end of the street or cul-de-sac for drainage areas exceedingtwo and one half (2.5) acres in size.

8. The City may set minimum finished floors or freeboard requirements on proposedstructures deemed at risk of flooding such as those proposed at a “T” street intersectionthat has a steep slope or a large drainage basin.

Section 21-24. Enclosed Storm Sewer


The use of the Manning Equation or the use of appropriate charts or nomographs in the design ofstorm sewers will be considered acceptable. Investigations of the capacity of all existingstructures on the waterway shall be made.

In addition, runoff from storms exceeding the design storm shall be anticipated by the developerand disposed of with minimum damages to surrounding property. The storm sewer must beaccessible for maintenance.

1. Pipes which are a part of the storm sewer shall have a minimum diameter of eighteen(18) inches.

2. Enclosed storm drains shall be designed for open-channel flow.

3. Minimum Grades. The slope shall be such to maintain a minimum velocity of 2.5 fps.

4. General rules to be observed:

a. Do not discharge a larger pipe into a smaller one.

b. At change in pipe size, match top of pipe.

c. A one (1) foot freeboard shall be maintained below the proposed finish gradethrough the storm sewer system. The submittal of energy calculations shall berequired on any reach of a system which exceeds 500 linear feet, on the totalsystem that exceeds 1,500 linear feet, and/or on a reach of a system which hasan elevation change exceeding ten (10) feet.

5. Manhole Locations. Manholes shall be located at intervals not to exceed 350 feet forpipe sizes 42” or less. Above 42”, manholes shall be located as determined by the City.Manholes shall be located at conduit junctions, changes of grade, changes of alignment,and changes in conduit size.

6. Pipe Connections. The use of one material to extend a storm sewer constructed of adifferent material shall not be allowed except at manholes, junction boxes, or inlets.

7. Pipe Laid on Curves. Degree of curvature shall be as per manufacturer’srecommendations.

8. All storm sewer outlets shall have erosion protection provided by headwalls, flared endsections, curtain walls, or other approved outlet protection.

9. All storm sewer pipe under pavement shall be “O-ringed” reinforced concrete pipe.

10. Water-tight seals shall be required for storm sewer pipe in the right-of-way.

11. A minimum of (2’) two foot of cover is required for storm sewer pipe.

12. All storm sewers shall be analyzed for the existence of tailwater conditions during the100-year storm event. If tailwater present, the storm sewer shall be designed toadequately compensate for this situation. The calculations or other documentationdemonstrating that the tailwater check has been made shall be submitted.

13. Easements. A drainage easement of satisfactory width for construction and maintenanceshall be provided for all storm sewers with the minimum width of the easement being(15’) fifteen feet.

14. Inlet System.

a. Inlet location shall not interfere with vehicular or pedestrian traffic.

b. Inlets shall intercept water before it reaches a pedestrian crosswalk or streetintersection.

c. Inlets shall not be located in a curb radius.


d. All hoods in a sump condition shall have 8” openings with the gutter lowered toaccommodate the inlet. Transition section shall be 10’ in each direction.

e. Where a curbed street crosses a bridge or reinforced concrete box structure,gutter flow shall be intercepted and not allowed to flow onto the structure.

f. Inlets on non-arterial streets shall be installed at all low points with sufficient inletdesign to ensure the ponding depth does not exceed 5” in a 100-year stormevent for a standard 6” barrier curbed or standard 6” mountable curbed street.For non-arterial streets with nonstandard curbs, the maximum 100-year stormevent water surface elevation in the street at all low points shall be limited toprovide the same inches of freeboard between the water surface elevation andthe top of the curb that is required in the first part of this section. For arterial orsection line streets, ponding depth shall not exceed 4” in a 100-year storm event.

g. All storm sewer systems located at a low point (sump) shall provide for anemergency overflow channel with a designated drainage easement sufficientlysized to contain within its boundaries the 100-year storm event runoff under theassumed conditions that the sump position storm sewer inlets/grates are 100%blocked, and in addition provide sufficient working room for future maintenance.

h. Any inlet grates over which a bicyclist can ride shall be of a design consideredbicycle safe, as well as ADA compliance.

Section 21-25. Open Channels

1. All land adjoining open natural or improved storm drainage channels having an elevationbelow the 100-year flood elevation of the channel shall be dedicated drainage easementfor the purpose of providing a zone of protection to all property owners. To facilitatefuture maintenance, the entire width of the channel and channel banks shall be coveredby the same drainage easement. Such channels in developments shall have amaintenance program carried out by the property owners association. The PlanningCommission, at its discretion, may require these dedicated drainage easements to beplatted as Common Area.

(Amended by Ord. 2013-01, 1/2013)

2. All drainage easements platted for the purpose of conveying stormwater above groundshall remain clear of obstructions and encroachments. Prohibited obstructions andencroachments shall include, but are not limited to, fences that block or inhibit the flow ofwater such as those made of brick or wood panels, swimming pools, driveways,mechanical and electrical equipment, storage sheds, and retaining walls.

3. Any channel improvements shall be approved by the City prior to the commencement ofany work thereon. Where safety issues are identified, additional requirements may beimposed for mitigation by the Planning Commission.

4. Whenever channel improvements are carried out, sodding, backsloping, cribbing andother bank protection shall be designed and constructed to control siltation and erosionfor the anticipated conditions and flow resulting from a 100-year frequency rainfall.

5. Drainage easements sufficient in width to provide a buffer and provide working room forconstruction and access for channel maintenance shall be provided.

6. For an unimproved channel, the width of the drainage easement shall be wide enough tosatisfy item (5) above as well as to provide enough area to flatten the slope of the bank ofthe channel 4:1 (horizontally:vertically) to allow for the remediation of any future bankerosion.


7. All drainage swales, channels, berms and other drainage improvements shown on theapproved plans shall be constructed prior to the acceptance of the Final Plat.

8. Channels should be trapezoidal in shape and on as flat a grade as possible withoutcausing ponding.

9. The use of Manning’s Equation in the calculations of hydraulic characteristics of openchannel flows in prismatic channels without backwater control will be acceptable. Whenanalyzing natural channels or when analyzing any overland conveyance system that hasdownstream control that could affect the 100-year event water surface elevations, the useof the HEC-RAS (U.S. Army Corps of Engineers Hydrologic Engineering Center RiverAnalysis System) modeling program or other approved water surface profile modelingprogram shall be required. The “n” value(s) used for channel shall be based on theindividual channel characteristics. When submitting calculations, include the source forthe “n” value.

10. Constructed Channel Geometry. The minimum bottom width shall be 2’ with side slopesnot steeper than 4:1 (horizontally:vertically) for sodded sections and a minimum bottomwidth of 2’ with side slopes of not steeper than 1:1 for paved sections.

11. Flow Velocities in Channels. Velocities shall not exceed 6 fps for sections sodded ingrass. Velocities in concrete lined or paved sections shall not exceed 15 fps. Thedissipation of energy shall be required at the confluence of improved channels withnatural channels through the use of energy dissipaters, stilling basins, etc.

12. All channels altered or improved from the natural state and all drainage swales shall besodded to prevent erosion. Sodding shall be required in the bottom of the channel andon side slopes.

13. Concrete Flumes. Concrete flumes shall be allowed as overflow protection for stormsewer systems and in lieu of enclosed pipe to drain areas not exceeding two (2) acres insize. All concrete flumes shall extend to the rear of adjacent lots, have scour protectionat their point of discharge, and shall discharge into a dedicated drainage area or channelwithout creating drainage or erosion problems.

14. Concrete flumes that serve as overflow protection at a sump-position storm sewer systemshall be designed for the 100-year storm event flow under the conditions that allupstream inlets and grates are experiencing 50% clogging and the sump positioninlets/grates are totally clogged. The weir length of the entrance to the flume shall besufficiently sized to convey that flow from the street gutter to the flume without allowingovertopping of the street curb or exceeding street water depth design criteria.

15. Concrete flumes that serve as the primary conveyance avenue in a sump position shallbe designed for the 100-year storm event flow under the conditions that all upstreaminlets and grates are clogged. The weir length of the entrance to the flume shall besufficiently sized to convey that flow from the street gutter to the flume without allowingovertopping of the street curb or exceeding street water depth design criteria.

16. A flume on grade where the gutter velocity for the 100-year storm event exceeds 5 fpsshall not be allowed as an acceptable method to remove flow from the street due to thehigh likelihood that the flume entrance will be bypassed by the flow.

17. The weir length of the entrance to a flume on a grade shall be designed and accountedfor when calculating the amount of flow intercepted by the flume.

18. Bar Ditches. All bar ditches and driveway culverts shall be designed by the engineer toconvey the 100-year storm event. The minimum acceptable driveway culvert diameter is18”. The maintenance of the bar ditch is the responsibility of the property owner. It isencouraged that the bar ditches in residential developments have a maintenanceprogram carried out by the property owners association.


19. Reinforced concrete box structures shall have an apron reinforced with rebar. The apronshall be tied to the box structure with rebar and have a toe wall. Adequately sized riprapshall protect the end of the apron and toe wall from scour during the 100-year stormevent. Energy dissipaters also shall be used where riprap alone is inadequate to reducedischarge velocities. Sandstone boulders or blocks shall not be used as riprap or energydissipaters. The reinforced concrete box structure shall be designed to have a minimumfreeboard at its inlet from the 100-year storm event water surface elevation to its lowchord equal to 15% of the box culvert rise or 2’, whichever is less, to help compensate forpotential blockage from debris during flooding. The use of Manning’s Equation for sizingreinforced concrete box structures at street crossings shall not be acceptable.

20. Span bridges shall be designed to have a minimum freeboard at the inlet from the 100-year storm event water surface elevation to its low chord equal to 15% of the structure’srise or 1’, whichever is less. The use of Manning’s Equation for sizing span bridges atstreet crossings shall not be acceptable.

21. All other culverts shall be designed to have a minimum freeboard at the inlet from the100-year storm event water surface elevation to its low chord equal to 15% of its rise of2’, whichever is less. The use of Manning’s Equation for sizing culverts at streetcrossings shall not be acceptable.

Section 21-26. Sump Position Flood Protection

1. In a location where the storm sewer improvements already have been constructed, thestructure shall have its minimum finished floor including electrical and mechanicalequipment servicing the structure, but excluding ductwork, 2’ above the highest elevationof either the top of the curb elevation at the sump inlets or flume or 2’ above the top of thecurb elevation of the overflow flume at its entrance. In lieu of this subsection, Section21-21 shall be allowed.

2. If the applicant so chooses, he shall provide to the City a hydrologic and hydraulicdrainage study and model prepared by a registered professional engineer licensed topractice in the State of Oklahoma using HEC-RAS or other program acceptable to theCity’s Engineer. The model shall be based on surveyed data accompanied by a gradingplan, proposed site plan, and any proposed drainage structures or alterations of existingstructures. The model shall demonstrate that even though the proposed structure wouldnot be compliant with the minimum finished floor elevation required in Section 21-20, thefollowing design criteria shall be met:

a. The proposed structure shall have at least 2’ of freeboard on all sides from the100 year storm event water surface elevation on and/or immediately adjacentto the lot and the proposed finished floor of the structure including allmechanical and electrical equipment servicing the structure, but excludingductwork. A proposed structure with a basement walk-out shall be reviewed bythe City’s Engineer to determine if the 2’ freeboard requirement shall apply to it;the decision shall be based on the proposed building plans, site plan, locationof the sump position, and the likelihood of flooding.

b. At no location would the 100-year storm event flood water come in contact withthe proposed structure of its mechanical and electrical equipment.

c. This modeling shall be done with runoff quantities calculated as specifiedpreviously under the conditions that all of the upstream inlets and grates are50% clogged, while any existing inlets and/or grates at the sump position aretotally clogged. The model shall demonstrate where and at what depth the flowovertops any curbs and the flow path(s) the runoff will take via flumes, swales,and overland in the vicinity of the property. Flow velocities also shall beprovided.


3. If the above are adequately demonstrated, the City shall be allowed to approve thebuilding of the proposed structure in accordance with the submitted model and plans.The City may require a drainage easement or other remedies to help ensure that thestructure shall be safe from flooding. The City may schedule the items for discussion at aregularly scheduled Council meeting.

4. For new developments and site plans, the sump position locations shall be designed sothat when all upstream inlets and grates are 50% clogged, the sump position inlets and/orgrates, or flume if no storm sewer is present, shall handle the 100-year storm eventwithout exceeding depth-in-street design criteria. Clogging factors shall be applied to thesump position inlets and/or grates during their design. All flume entrances shall havesufficient weir length.

5. In addition, an emergency overflow flume shall be provided for sump position inletsand/or grates. The overflow flume shall be sized to handle the 100-year storm eventunder the conditions that the existing inlets and/or grates at the sump position are 100%blocked and all upstream inlets and grates are partially clogged per design standards.The flume entrance shall have sufficient weir length to capture this flow and the flow shallremain within the flume to the back of the lot(s). A sufficiently sized drainage easement,minimum 15’, shall be provided.

6. If all the above conditions are met, a proposed structure in a sump position shall be builtas follows:

a. Where a storm sewer system with the emergency overflow flume exists, aminimum of 1’ of freeboard shall be required from the finished floor of thestructure’s leading edge closest to the sump position to the top of the flumecurb immediately adjacent to it.

b. Where a flume serves as the primary method of stormwater conveyance, aminimum of 2’ of freeboard shall be required from the finished floor of thestructure’s leading edge closest to the sump position to the top of the flumecurb immediately adjacent to it.

7. In both situations, the electrical and mechanical equipment servicing the structure shallconform to the freeboard requirement; the ductwork is excluded.

8. The developer is encouraged to submit as-built survey data and corresponding drainagecalculations to the City upon completion of the development’s infrastructure as soon aspossible so that the Staff can verify that the sump positions meet the design criteria. Bydoing so, delays in issuing Building Permits at these sump positions will be minimized.

9. For rural developments or site plans, the applicant for a proposed sump position structureshall submit to the City a proposed grading plan signed and sealed by a registeredprofessional engineer licensed to practice in the State of Oklahoma. The grading planshall include the proposed site plan including driveway, topographic data, bar ditches,channels, swales, creeks, easements, proposed grading, proposed swales, minimumfinished floor elevations, and any additional drainage improvements required to ensurethat the structure shall be safe from flooding.

Section 21-27. Detention Facilities

1. Design of stormwater storage facilities shall include an analysis of the stormwater storageoutflow hydrograph for both pre- and post-developed conditions.

2. Modeling of and routing of outflow hydrographs downstream shall be required undercriteria above and will be accomplished through the use of computer programs designedspecifically for hydrology and hydraulics.


3. The length of a stream reach requiring modeling shall be either (1) to a point where noincrease in peak flow is demonstrated or (2) to a point where an increase in peak flowdoes not increase the depth of flooding.

4. The storm events to be modeled shall be the 2, 5, 10, 25, 50 and 100-year storm events.

5. In the event peak flow studies indicate a development will increase runoff or depth offlooding, then storage facilities or alternative methods of stormwater runoff control will berequired.

6. The City Council shall review all stormwater storage studies and shall implement theconditions of this section of these drainage standards.

7. Storage may be accomplished by the detention or retention of water in reservoirs, parks,side channels, or ponds either on or off-site.

8. In the event a complete drainage study by a licensed professional engineer demonstratesthrough that an increase in downstream flooding would occur if on-site stormwaterdetention was provided or that there would be no detrimental effect to all upstream,adjacent, and downstream properties if stormwater detention was not provided, therequirement for stormwater storage may be waived by the City Council.

9. An engineer submitting a request to waive the stormwater storage requirement shall berequired to produce a detailed drainage study containing any information necessary forthe City to make a determination. No limits of the extent of a study should be assumedby an engineer as none are provided.

10. The detention facility shall be designed based upon the maximum allowable releaserates. The release rates shall be established based upon natural state runoffcharacteristics from the development site. A continuous probability or recurrence intervalcriterion shall be used in designing the outlet control structure. As a minimum the outletcontrol structure shall be designed for the 2, 5, 10, 25, 50 and 100-year recurrenceintervals.

11. When a combination of storage facilities are used to control runoff, the system as a wholeshall be designed with discharge rates in accordance with (10) above.

12. All storage facilities shall be provided with a paved or otherwise scour-protectedemergency spillway sized to handle the 100-year event storm with 1’ of freeboard andprovide scour protection at its downstream end. Earth embankments shall have sideslopes not steeper than 4:1 (horizontally:vertically). Proper materials shall be specifiedwith the corresponding optimum compaction according to standards to provide stabilityand minimum seepage.

13. The outlet to a detention pond should extend to the storm sewer, creek, or other suitablelocation covered by a drainage easement.

14. The engineer shall take into account the flow that would be discharged through theemergency overflow spillway during the 100-year storm event with total blockage of allother outlets of the detention pond when designing the emergency spillway, and shallprovide a safe means to convey said water without adversely affecting downstreamproperties.

15. The storage volume of a storage facility shall be oversized 10% to allow forsedimentation.

16. All storage ponds shall be provided with a concrete paved trickle channel from the inlet tothe outlet structure to transmit low flows.

17. Suitable access shall be provided to all storage areas for maintenance purposes.

18. Maintenance of storage facilities shall be provided by the Property Owners Association.


19. Earth dams or other earth embankments shall be designed by a licensed professionalengineer in accordance with accepted engineering practices to assure that dam orembankment failure will not occur. Design criteria used by the Natural ResourcesConservation Service (NRCS), formerly the Soil Conservation Service, in the selection ofmaterials and construction procedures will be acceptable.

20. Parking lot storage shall be considered the last alternative of providing requireddetention. The use of parking lots for storage shall be well designed to minimize thepotential damage and/or threat to pedestrians, emergency responders and vehicles, andparked vehicles. In no case shall the depth of ponding in a parking lot exceed twelveinches (12”).

21. Structures adjacent to a detention and/or retention shall have their finished floors(excluding ductwork) set at a minimum of two feet (2’) above the 100-year storm eventwater surface elevation in the facility, and in all cases a minimum of one foot (1’) offreeboard shall be provided from the finished floor of the structures to the top of theprojected 100-year water surface elevation in the emergency spillway calculated underthe conditions that all other outlets of the pond are blocked.

22. All detention bypass areas on the development must be identified. Increased runoff frombypass areas shall not detrimentally affect adjacent and downstream properties.

23. All storage facilities shall be provided with landscaping which includes the planting ofappropriate trees and shrubs. The use of evergreen and wetlands adaptable plantingsshould be incorporated to improve the water quality of the storage runoff.

24. Outlet control facilities shall include headwalls, slopewalls, energy dissipaters, and anyadditional outlet erosion and scour control measures approved by the City. The outletfacilities, as a visible part of any storage facility, shall include landscape plantings toimprove aesthetics and provide a more pleasant and appealing appearance.

tuttle stormwater drainage regulations

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