(A) General.
(1) Drainage facilities shall be designed to provide a sufficient storm water drainage system for the conveyance of storm water runoff received from upstream and from the subject property with due allowance having been made for continued conveyance of storm water runoff from adjacent properties as the drainage basin develops.
(2) The drainage system including streets, bridges, culverts, open channels, detention/retention facilities and the like, shall be designed to carry all probable storm recurrence intervals up to and including the 100-year storm event.
(3) A dedicated drainage easement shall encompass all land lying below the water surface elevation generated by the 100-year storm event and resulting storm water runoff. These dedicated drainage easements shall be platted as common area in all developments.
(4) Site improvement shall provide for the grading of all building pads to an elevation where the lowest finished floor elevations shall be a minimum of one foot above the 100-year storm event. In all developments, surface water from each dwelling lot will flow away from 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 or in the depth of flooding downstream.
(6) Existing structures within and adjoining a proposed development shall be evaluated in terms of hydraulic capacity and structural soundness. Those structures found to be structurally inadequate shall be modified or completely removed and replaced by the developer. Structures found to be hydraulically inadequate for flow and/or for protecting property as a result of the changing runoff characteristics shall be replaced or modified by the developer to provide the required capacity and/or protection.
(7) All utilities located within the floodplain or any other open channel or swale shall be 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 block numbers, dimensions, right-of-way and easement lines, floodplains, street names, paved surfaces (existing or proposed), contract limits, location, size and type of inlets, manholes, culverts, pipes, channels and related structures, outfall details, miscellaneous riprap placement, two-foot 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, ground line and curb line elevations, typical sections, riprap construction, filling details, open drainage features, pipe crossing, design flow capacities and title block.
(10) Official floodplain designations and delineations of the floodplains and floodways denoting limits of permissible developments shall be shown on all preliminary plans and final plats submitted for approval wherever such plans and plats contain a floodplain segment. In any case in which official floodplains are not delineated, they shall be determined on the basis of these standards and shall be shown on all preliminary plans and final plats submitted for approval.
(11) The limits of the floodplains shown on regulatory floodplain maps are close approximations based on extrapolations made between widely-spaced cross sections. Therefore, the actual limits of the floodplain shall be determined by extracting base flood elevations from the stream profile and determining where each elevation intersects the adjacent existing topography. If a conflict exists between the overlay of the regulatory floodplain 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 Federal Emergency Management Agency (FEMA) is required when the FEMA Flood Insurance Rate Map (FIRM) conflicts with proposed lots or other proposed structures or improvements.
(12) Computations and plans to support all drainage designs shall be submitted to the City Engineer for review. The computations and plans shall be in such form as to provide the basis for timely and consistent review and will be made a part of the permanent record for future evaluation. The computations and plans shall be accompanied by the certification of a registered professional engineer licensed to practice in the state. Before final acceptance, the submitting engineer shall provide an as-built plan based on field survey data of the streets, storm sewer, detention facilities, and all other conveyances of storm water; a revised set of drainage calculations that correspond with the as-built plans; and a letter of certification stating that the as-built site complies with all governing ordinances of the city.
(13) Earth surfaces shall meet the requirements defined in the following table:
Maximum Slope (Horizontal : Vertical) | Maximum Horizontal Distance (feet) |
4:1 | 16 |
6:1 | 24 |
8:1 | 32 |
10:1 | Any distance |
(a) In a case of maximum allowable slope there shall be a shelf with a four foot horizontal distance of 8:1 (horizontal: vertical) slope or flatter separating the maximum slope areas.
(b) There shall not be in any case more than an eight foot vertical rise of an earth surface in any 32-foot horizontal distance without introduction of a wall.
(c) Walls greater than two feet in height measured from the top of the foundation (footing) to the top of the wall with a catchment area of 0.2 acres or more shall:
1. Include a six inch diameter (min.) perforated foundation drain system that outlets to a manmade or natural drainage path and be constructed in such a manner as to prevent the flow of water from causing a negative impact to other properties. The foundation drain material shall be corrugated polyethylene pipe with a smooth interior or approved equal.
2. Include a minimum of 12 inch wide stone backfill drainage system from the top of the footing to six inches below the top of wall, allowing surface stormwater to drain to the foundation drain and reduce overtopping. Separation from the stone backfill and earth backfill shall include nonwoven geotextile fabric in accordance with Oklahoma Department of Transportation 2019 standard specifications for highway construction; geotextiles for subsurface drainage purposes.
3. Alternate drainage methods shall be allowed, as approved by the City Engineer.
(d) Walls greater than four feet in height measured from the top of the foundation (footing) to the top of the wall shall include a design submittal sealed by a qualified licensed professional engineer.
(B) Drainage report. For all areas not otherwise designated in a floodway or floodway-fringe area, the developer shall be responsible for having an engineer (registered in the state) prepare a drainage report for the area of the proposed subdivision, and all areas affected by runoff resulting from development of the proposed subdivision. The drainage report shall show compliance with this section and shall clearly indicate the existing and proposed runoff from the subdivision and state whether or not peak release rates comply with these regulations.
(C) Determination of storm runoff.
(1) General. Numerous methods of runoff computation are available on which the design of storm drainage and flood control systems may be based. The rational method will be accepted as adequate for drainage areas up to 40 acres. For larger areas, the U.S. Army Corps of Engineers Hydraulic Engineering Center HEC-1 or other engineering programs or methods acceptable to the city’s Engineer shall be used. Any hydrological soil group data to be used when computing runoff shall be determined from the most current edition of the county’s Soil Survey provided by the U.S. Department of Agriculture Natural Resources Conservation Service.
(2) Rational method.
(a) Runoff coefficient “C”. The runoff coefficients shall be chosen to represent the integrated effects of infiltration, detention storage, evaporation, retention, flow routing, and interception. The “C” values are provided in the table below, Runoff Coefficients for Use in the Rational Formula.
Runoff Coefficients for Use in the Rational Formula
| ||
Land Use
|
Basin Slope
| |
Slope < 2%
| Slope > 2% | |
Runoff Coefficients for Use in the Rational Formula
| ||
Land Use
|
Basin Slope
| |
Slope < 2%
| Slope > 2% | |
Apartments | 0.75 | 0.85 |
Commercial | 0.90 | 0.95 |
Historic conditions | 0.25 | 0.35 |
Multi-family | 0.60 | 0.70 |
Parks, golf courses | 0.35 | 0.45 |
Single-family residence < 2 acres | 0.55 | 0.65 |
Single-family residence > 2 acres | 0.45 | 0.55 |
(b) Rainfall intensity (I). The average rainfall rate in inches per hour considered for a particular drainage basin or sub-basin and selected on the basis of design rainfall duration and design frequency or occurrence.
Rainfall Intensity Formula
I = a/(td + b)c
Where I = Average rainfall, intensity, inches/hour
Td = Rainfall duration (or time of concentration), minutes
a, b, c = Variable values from the following table:
2-Year | 5-Year | 10-Year | 25-Year | 50-Year | 100-Year |
a = 56.43 | a = 72 | a = 82 | a = 95 | a = 108 | a = 120 |
b = 11.5 | b = 15 | b = 15 | b = 15 | b = 15 | b = 15 |
c = 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 runoff to the point of interest. Street flow shall be considered as being in an open paved channel.
An acceptable formula for use in determining overland flow travel time is:
Time = K (L.37 / S.2)
L = Length of flow in feet
S = Average slope in feet/foot
K = Constant for character of surface
Values of K:
Average grass | 1.00 |
Bare soil | 0.60 |
Dense grass | 1.13 |
Pavement | 0.37 |
Poor grass | 0.90 |
An acceptable formula for determining channel flow time is:
Time = K (L2/S) .385
Values of K:
Bar ditch | 0.012 |
Concrete lined channel | 0.006 |
Curbed street | 0.0035 |
Sodded swale | 0.008 |
Manning’s Equation is acceptable for analyzing open channel flows in prismatic channels without backwater control and for analyzing free water surface conduit flows.
(c) Drainage area. The size and shape of the watershed must be determined. Drainage areas 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 divides must follow actual lines rather than artificial land divisions.
(D) Flow in streets.
(1) Flow in gutters which are on straight or parabolic crown pavement may be determined by using Manning’s Formula for channel flow or acceptable nomographs.
(2) Minimum gutter slope on residential streets shall not be less than 0.005 feet/foot (0.5%). Maximum gutter slope should not be more than 0.08 feet/foot (8%). In a gutter which has a radius of 100 feet or less, the design flow depth shall not exceed 70% of the curb height.
(3) No lowering of the standard height of street crown or splitting of curb heights shall be allowed for the 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 four inches for the 100-year storm event for a standard eight-inch barrier curbed street. The flow depth in the gutter of other classification of streets shall not exceed five inches for the 100-year storm event for streets with standard six-inch barrier curbs or standard six-inch mountable curbs. For streets with non-standard curbs, the maximum 100-year storm event water surface elevation in the street shall be limited to provide the same inches of freeboard between the water surface elevation and the top of the curb that is required in the first part of this division (D)(5).
(6) Where two or more streets intersect, the flow shall be intercepted by appropriate storm sewer techniques prior to the intersection of said streets such that the depth of flow within the intersection will not exceed four inches 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 or drainage 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 exceeding two and one-half acres in size.
(8) The city may set minimum finished floors or freeboard requirements on proposed structures deemed at risk of flooding such as those proposed at a “T” street intersection that has a steep slope or a large drainage basin.
(E) Enclosed storm sewer.
(1) (a) The use of the Manning Equation or the use of appropriate charts or nomographs in the design of storm sewers will be considered acceptable. Investigations of the capacity of all existing structures on the waterway shall be made.
(b) In addition, runoff from storms exceeding the design storm shall be anticipated by the developer and disposed of with minimum damages to surrounding property. The storm sewer must be accessible for maintenance.
(2) Pipes which are a part of the storm sewer shall have a minimum diameter of 18 inches.
(3) Enclosed storm drains shall be designed for open-channel flow.
(4) The slope shall be such to maintain a minimum velocity of 2.5 fps.
(5) 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; and
(c) A one-foot freeboard shall be maintained below the proposed finish grade through the storm sewer system. The submittal of energy calculations shall be required on any reach of a system which exceeds 500 linear feet, on the total system that exceeds 1,500 linear feet and/or on a reach of a system which has an elevation change exceeding ten feet.
(6) Manholes shall be located at intervals not to exceed 350 feet for pipe sizes 42 inches or less. Above 42 inches, 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.
(7) The use of one material to extend a storm sewer constructed of a different material shall not be allowed, except at manholes, junction boxes or inlets.
(8) Degree of curvature shall be as per manufacturer’s recommendations.
(9) All storm sewer outlets shall have erosion protection provided by headwalls, flared end sections, curtain walls or other approved outlet protection.
(10) All storm sewer pipe under pavement shall be “O-ringed” reinforced concrete pipe.
(11) Water-tight seals shall be required for all storm sewer pipe in the right-of-way.
(12) A minimum of two feet of cover is required for storm sewer pipe.
(13) All storm sewer shall be analyzed for the existence of tailwater conditions during the 100-year storm event. If tailwater is present, the storm sewer shall be designed to adequately compensate for this situation. The calculations or other documentation demonstrating that the tailwater check has been made shall be submitted.
(14) A drainage easement of satisfactory width to provide working room for construction and maintenance shall be provided for all storm sewers with the minimum width of the easement being 15 feet. The presence of any bedding materials, backfill and the depth of pipe also shall be taken into consideration.
(15) 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 street intersection.
(c) Inlets shall not be located in a curb radius.
(d) All hoods in a sump condition shall have eight-inch openings with the gutter lowered to accommodate the inlet. Transition section shall be ten feet 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 inlet design to ensure the ponding depth does not exceed five inches in a 100-year storm event for a standard six inches barrier curbed or standard six inches mountable curbed street. For non-arterial streets with non-standard curbs, the maximum 100-year storm event water surface elevation in the street at all low points shall be limited to provide the same inches of freeboard between the water surface elevation and the top of the curb that is required in the first part of this section. For arterial or section line streets, ponding depth shall not exceed four inches in a 100-year storm event.
(g) All storm sewer systems located at a low point (sump) shall provide for an emergency overflow channel with a designated drainage easement sufficiently sized to contain within its boundaries the 100-year storm event runoff under the assumed 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 considered bicycle safe.
(F) Open channels.
(1) All land adjoining open natural or improved storm drainage channels having an elevation below the 100-year flood elevation of the channel shall be dedicated drainage easement for the purpose of providing a zone of protection to all property owners. To facilitate future maintenance, the entire width of the channel and channel banks shall be covered by the same drainage easement. Such channels in developments shall be platted as common area and have a maintenance program carried out by the property owners’ association.
(2) All drainage easements platted for the purpose of conveying storm water above ground shall remain clear of obstructions and encroachments. Prohibited obstructions and encroachments shall include, but are not limited to, fences that block or inhibit the flow of water 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 of any work thereon. Where safety issues are identified, additional requirements may be imposed for mitigation by the Planning Commission.
(4) Whenever channel improvements are carried out, sodding, backsloping, cribbing and other bank protection shall be designed and constructed to control siltation and erosion for 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 for construction and access for channel maintenance shall be provided.
(6) For an unimproved channel, the width of the drainage easement shall be wide enough to satisfy division (F)(5) above as well as to provide enough area to flatten the slope of the bank of the channel 4:1 (horizontally:vertically) to allow for the remediation of any future bank erosion.
(7) All drainage swales, channels, berms and other drainage improvements shown on the approved 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 without causing ponding.
(9) The use of Manning’s Equation in the calculations of hydraulic characteristics of open channel flows in prismatic channels without backwater control will be acceptable. When analyzing natural channels or when analyzing any overland conveyance system that has downstream control that could affect the 100-year event water surface elevations, the use of the HEC-RAS (U.S. Army Corps of Engineers Hydrologic Engineering Center River Analysis System) modeling program or other approved water surface profile modeling program shall be required. The “n” value(s) used for channel shall be based on the individual channel characteristics. When submitting calculations, include the source for the “n” value.
(10) The minimum bottom width shall be two feet with side slopes not steeper than 4:1 (horizontally:vertically) for sodded sections and a minimum bottom width of two feet with side slopes of not steeper than 1:1 for paved sections.
(11) Velocities shall not exceed six fps for sections sodded in grass. Velocities in concrete lined or paved sections shall not exceed 15 fps. The dissipation of energy shall be required at the confluence of improved channels with natural channels through the use of energy dissipaters, stilling basins and the like.
(12) All channels altered or improved from the natural state and all drainage swales shall be sodded to prevent erosion. Sodding shall be required in the bottom of the channel and on sideslopes.
(13) Concrete flumes shall be allowed as overflow protection for storm sewer systems and in lieu of enclosed pipe to drain areas not exceeding two acres in size. All concrete flumes shall extend to the rear of adjacent lots, have scour protection at their point of discharge and shall discharge into a dedicated drainage area or channel without creating drainage or erosion problems.
(14) Concrete flumes that serve as overflow protection at a sump-position storm sewer system shall be designed for the 100-year storm event flow under the conditions that all upstream inlets and grates are experiencing 50% clogging and the sump position inlets/grates are totally clogged. The weir length of the entrance to the flume shall be sufficiently sized to convey that flow from the street gutter to the flume without allowing overtopping 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 shall be designed for the 100-year storm event flow under the conditions that all upstream inlets and grates are clogged. The weir length of the entrance to the flume shall be sufficiently sized to convey that flow from the street gutter to the flume without allowing overtopping 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 five fps shall not be allowed as an acceptable method to remove flow from the street due to the high likelihood that the flume entrance will be bypassed by the flow.
(17) The weir length of the entrance to a flume on grade shall be designed and accounted for when calculating the amount of flow intercepted by the flume.
(18) All bar ditches and driveway culverts shall be designed by the engineer to convey the 100-year storm event. The minimum acceptable driveway culvert diameter is 18 inches. The maintenance of the bar ditch is the responsibility of the property owner. It is encouraged that bar ditches in residential developments have a maintenance program carried out by the property owners’ association.
(19) Reinforced concrete box structures shall have an apron reinforced with rebar. The apron shall be tied to the box structure with rebar and have a toe wall. Adequately sized riprap shall protect the end of the apron and toe wall from scour during the 100-year storm event. Energy dissipaters also shall be used where riprap alone is inadequate to reduce discharge velocities. Sandstone boulders or blocks shall not be used as riprap or energy dissipaters. The reinforced concrete box structure shall be designed to have a minimum freeboard at its inlet from the 100-year storm event water surface elevation to its low chord equal to 15% of the box culvert rise or two feet, whichever is less, to help compensate for potential blockage from debris during flooding. The use of Manning’s Equation for sizing reinforced 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’s rise or one foot, whichever is less. The use of Manning’s Equation for sizing span bridges at street crossings shall not be acceptable.
(21) All other culverts 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 its rise of two feet, whichever is less. The use of Manning’s Equation for sizing culverts at street crossings shall not be acceptable.
(G) Sump position flood protection.
(1) In a location where the storm sewer improvements already have been constructed, the structure shall have its minimum finished floor including electrical and mechanical equipment servicing the structure, but excluding ductwork, two feet above the highest elevation of either the top of the curb elevation at the sump inlets or flume or two feet above the top of the curb elevation of the overflow flume at its entrance. In lieu of this division (G)(1), division (G)(2) below shall be allowed.
(2) If the builder so chooses, he or she shall provide to the city a hydrologic and hydraulic drainage study and model prepared by a registered professional engineer licensed to practice in the state using HEC-RAS or other program acceptable by the City Engineer. The model shall be based on surveyed data accompanied by a proposed grading plan, proposed site plan and any proposed drainage structures or alterations of existing structures. The model shall demonstrate that even though the proposed structure would not be compliant with the minimum finished floor elevation required in division (G)(1) above, the following design criteria shall be met.
(a) The proposed structure shall have at least two feet of freeboard on all sides from the 100-year storm event water surface elevation on and/or immediately adjacent to the lot and the proposed finished floor of the structure including all mechanical and electrical equipment servicing the structure, but excluding ductwork. A proposed structure with a basement of walk-out shall be reviewed by the City Engineer to determine if the two- foot freeboard requirement shall apply to it; the decision shall be based on the proposed building plans, site plan, location of the sump position and the likelihood of flooding.
(b) At no location would the 100-year storm event flood water come in contact with the proposed structure of its mechanical and electrical equipment.
(c) This modeling shall be done with runoff quantities calculated as specified previously under the conditions that all of the upstream inlets and grates are 50% clogged, while any existing inlets and/or grates at the sump position are totally clogged. The model shall demonstrate where and at what depth the flow overtops any curbs and the flowpath(s) the runoff will take via flumes, swales and overland in the vicinity of the property. Flow velocities also shall be provided.
(3) If the above are adequately demonstrated, the city shall be allowed to approve the building 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 the structure shall be safe from flooding. The city may schedule the item for discussion at a regularly scheduled Council meeting.
(4) For new developments and site plans, the sump position locations shall be designed so that when all upstream inlets and grates are 50% clogged, the sump position inlets and/or grates, or flume if no storm sewer is present, shall handle the 100-year storm event without exceeding depth-in-street design criteria. Clogging factors shall be applied to the sump position inlets and/or grates during their design. All flume entrances shall have sufficient weir length.
(5) In addition, an emergency overflow flume shall be provided for sump position inlets and/or grates. The overflow flume shall be sized to handle the 100-year storm event under 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 shall remain within the flume to the back of the lot(s). A sufficiently sized drainage easement, minimum 15 feet, shall be provided.
(6) If all of the above conditions are met, a proposed structure in a sump position shall be built as follows.
(a) Where a storm sewer system with the emergency overflow flume exists, a minimum of one foot of freeboard shall be required from the finished floor of the structure’s leading edge closest to the sump position to the top of the flume curb immediately adjacent to it.
(b) Where a flume serves as the primary method of storm water conveyance, a minimum of two feet of freeboard shall be required from the finished floor of the structure’s leading edge closest to the sump position to the top of the flume curb immediately adjacent to it.
(7) In both situations, the electrical and mechanical equipment servicing the structure shall conform to the freeboard requirement; the ductwork is excluded.
(8) The developer is encouraged to submit as-built survey data and corresponding drainage calculations to the city upon completion of the development’s infrastructure as soon as possible so that the staff can verify that the sump positions meet the design criteria. By doing 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 structure shall submit to the city a proposed grading plan signed and sealed by a registered professional engineer licensed to practice in the state. The grading plan shall include the proposed site plan including driveway, topographic data, bar ditches, channels, swales, creeks, easements, proposed grading, proposed swales, minimum finished floor elevations, and any additional drainage improvements required to ensure that the structure shall be safe from flooding.
(H) Detention facilities.
(1) Design of storm water storage facilities shall include an analysis of the storm water storage outflow hydrograph for both pre- and post-developed conditions.
(2) Modeling of and routing of outflow hydrographs downstream shall be required under criteria above and will be accomplished through the use of computer programs designed specifically for hydrology and hydraulics.
(3) The length of a stream reach requiring modeling shall be either:
(a) To a point where no increase in peak flow is demonstrated; or
(b) To a point where an increase in peak flow does not increase the depth of flooding.
(4) The storm events to be modeled shall be the two-, five-, ten-, 25-, 50- and 100-year storm events.
(5) In the event peak flow studies indicate a development will increase runoff or depth of flooding, then storage facilities or alternative methods of storm water runoff control will be required.
(6) The City Council shall review all storm water storage studies and shall implement the conditions 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 demonstrates through that an increase in downstream flooding would occur if on-site storm water detention was provided or that there would be no detrimental effect to all upstream, adjacent and downstream properties if storm water detention was not provided, the requirement for storm water storage may be waived by the City Council.
(9) An engineer submitting a request to waive the storm water storage requirement will be required to produce a detailed drainage study containing any information necessary for the city to make a determination. No limits of the extent of a study should be assumed by an engineer as none are provided.
(10) The detention facility shall be designed based upon the maximum allowable release rates. The release rates will be established based upon natural state runoff characteristics from the development site. A continuous probability or recurrence interval criterion shall be used in designing the outlet control structure. As a minimum the outlet control structure shall be designed for the two-, five-, ten-, 25-, 50- and 100-year recurrence intervals.
(11) When a combination of storage facilities are used to control runoff, the system as a whole shall be designed with discharge rates in accordance with division (H)(10) above.
(12) All storage facilities shall be provided with a paved or otherwise scour-protected emergency spillway sized to handle the 500-year event storm with one foot of freeboard and provide scour protection at its downstream end. Earth embankments shall have side slopes not steeper than 4:1 (horizontally: vertically). Proper materials shall be specified with the corresponding optimum compaction according to standards to provide stability and minimum seepage.
(13) The outlet to a detention pond should extend to the storm sewer, creek, or other suitable location covered by a drainage easement.
(14) The engineer shall take into account the flow that would be discharged through the emergency overflow spillway during the 100-year storm event with total blockage of all other outlets of the detention pond when designing the emergency spillway, and shall provide a safe means to convey the water without adversely affecting downstream properties.
(15) The storage volume of a storage facility shall be oversized 10% to allow for sedimentation.
(16) All storage ponds shall be provided with a concrete paved trickle channel from the inlet to the 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 professional engineer in accordance with accepted engineering practices to assure that dam or embankment failure will not occur. Design criteria used by the Natural Resources Conservation Service (NRCS), formerly the Soil Conservation Service, in the selection of materials and construction procedures will be acceptable.
(20) Parking lot detention may be utilized upon approval of the City Engineer. The use of parking lots for storage shall be well-designed to minimize the potential damage and/or threat to pedestrians, emergency responders and vehicles, and parked vehicles. Where parking lot detention is proposed:
(a) The maximum depth of the stored stormwater shall not be greater than six inches;
(b) The maximum coverage of the parking area by the stored stormwater shall not be greater than 35%; and
(c) Structures located on parcels with parking lot storage facilities shall have finished floors 18 inches above the 100-year storm event water surface elevation.
(21) Structures adjacent to a detention and/or retention site shall have their finished floors (excluding ductwork) set at a minimum of two feet above the 100-year storm event water surface elevation in the facility, and in all cases a minimum of one foot of freeboard shall be provided from the finished floor of the structures to the top of the projected 100-year water surface elevation in the emergency spillway calculated under the conditions that all other outlets of the pond are blocked.
(22) All detention bypass areas on the development must be identified. Increased runoff from bypass areas shall not detrimentally affect adjacent and downstream properties.
(23) All storage facilities shall be provided with landscaping which includes the planting of appropriate trees and shrubs. The use of evergreen and wetlands adaptable plantings should be incorporated to improve the water quality of the storage runoff.
(24) Outlet control facilities shall include headwalls, slopewalls, energy dissipaters and any additional outlet erosion and scour control measures approved by the city. The outlet facilities, as a visible part of any storage facility, shall include landscape plantings to improve aesthetics and provide a more pleasant and appealing appearance.
(2002 Code, § 153.089) (Ord. 230, passed 9-8-1986; Ord. 651, passed 10-26-2009; Ord. 903, passed 3-14-2022; Ord. 905, passed 4-11-2022)