(A) General requirements.
(1) Purpose. The purpose of this chapter is to reduce the hazard to life, health, safety and property from stormwater runoff, erosion and sedimentation and to promote the conservation and sound utilization of the water resources for the benefit of the residents of the city.
(2) The Plan Commission shall not approve any development, which does not make adequate provision for storm or flood water runoff channels or basins.
(3) The stormwater drainage system must be separate and independent of any sanitary sewer system.
(4) All stormwater conveyance components of the stormwater management system, such as storm sewers, grassed waterways, sediment basins and/or retention ponds, shall be constructed at the same general time as the initial street construction.
(5) This chapter shall be used in conjunction with Greensburg Design Standards Manual.
(6) Engineering plans, computation and specifications should be presented in an orderly manner using standard engineering practice and be in compliance with technical and administrative standards contained in this chapter.
(7) Minor stormwater conveyance systems such as collection swales, small ditches, street gutters, storm sewer inlets, storm sewer pipes and entrance drive culverts shall be designed to convey the peak discharge from the 10-year frequency storm event in open channel flow conditions (pipes) or within the channel banks (swales and ditches).
(8) Major stormwater conveyance systems such as creek or large ditch culverts, stream enclosures, etc., shall be designed to convey the peak discharge from the 25-year frequency storm event within the channel banks.
(9) All stormwater conveyance systems shall include a provision to address the safe conveyance of the peak discharge from the 100-year frequency storm event through the use of secondary or emergency overflow conveyances such that flood waters do not inundate building structures or unreasonably disrupt transportation or other critical operations.
(10) Drainage facilities shall be such that all habitable structures are free from a 100-year flood.
(11) Land development and other drainage activities should attenuate runoff rates to protect downstream properties from the impacts of development. The design should address both quantitative and qualitative impacts to the overall watershed. Great attention shall be given to the capacity and condition of the downstream receiving waterway in terms of its ability to accept runoff from the developed or otherwise altered upstream property.
(12) Land development and other drainage activities shall be designed to reasonably minimize the degradation of the natural environment and, where possible, improve the water resource benefits.
(B) Applicability. This chapter regulates improvements or changes to stormwater systems. These activities may be related to land development, redevelopment, transportation or other land or watercourse disturbing activities including, but not limited to:
(1) Subdivision development.
(2) Changes in ground contours.
(3) Increases in the runoff rates or volumes.
(4) Changes to the drainage patterns.
(5) Changes to the land surface that decreases the rate at which water is absorbed.
(6) Creates or changes a stormwater facility.
(7) Involves construction, enlargement or location of any building on a permanent foundation.
(8) Creates an impoundment or affects the performance of an existing impoundment.
(C) Stormwater drainage plans. The stormwater drainage plans for a development shall include the following items:
(1) Plans shall be sealed by a registered professional engineer or registered professional surveyor licensed in the State of Indiana.
(2) The plans shall be of adequate scale to clearly depict the proposed activities, as well as the existing contour and features of the site.
(3) The plans shall include a north arrow and a site vicinity map that clearly shows the location of the project site.
(4) The plans shall include the location and datum of the benchmark used for the plans. The benchmark datum shall represent the actual elevation above sea level.
(5) The plans shall clearly show the existing and proposed ground contours at one foot intervals and shall be based on site specific detailed surveying. Proposed contours can be shown as spot elevations if the density is sufficient to clearly illustrate surface grading and drainage patterns.
(6) The plans shall include information concerning the off- site watershed areas that affect the proposed site. Although the off-site information does not need to be given the level of precision as the on-site information, it shall clearly indicate the drainage area, drainage patterns and other pertinent hydrologic and hydraulic parameters needed to address the drainage system performance for predevelopment and post-development conditions.
(7) The plans shall show the locations of all existing and proposed stormwater facilities such as culverts, inlets, storm sewers, ditches, creeks, rivers, ponds, lakes, and swales. The data for each facility shall include the physical dimensions of the facility such as rim elevations for inlets, invert elevation, material, size, end treatment and length for pipes; normal water level and surface areas for ponds and lakes; flowline elevation and channel geometry for open channels; and surface grades and slope for paved areas. The features included on the plans shall clearly indicate the existing and proposed drainage patterns for the proposed development.
(8) The plans shall denote existing and proposed property boundaries, right-of-ways and easements.
(9) The plans shall denote the boundaries and base flood elevations of any FEMA special flood hazard area and/or floodplain and floodway boundaries derived from a site specific engineering study.
(10) In addition to information concerning the drainage system itself, the plans shall also include the elevations of other structures, such as buildings, utility facilities and roadways that may be impacted by the drainage system.
(11) The plan shall include a pre-development and post- development on-site and off-site drainage area delineation map. The watershed drainage area for each entry point of stormwater runoff into the proposed drainage system shall be delineated.
(12) In addition to the drainage plans, a stormwater design report shall be submitted with the plans, which shall contain technical information in support of proposed development plans. The report shall contain a narrative of the pertinent aspects of the proposed project, as well as detailed calculations and a summary of key results demonstrating compliance with this chapter. The level of detail will vary for each development, but shall be proportional to the size and scope of the development.
(D) Stormwater detention facilities. Detention facilities shall be designed using the following guidelines to limit the peak discharge from the development.
(1) For a development which drainage areas (tributary to stormwater detention facilities) equal to or greater than 10 acres, the following conditions apply:
(a) The resulting 100-year frequency design storm peak discharge for the entire contributing watershed area shall not exceed the 5-year frequency design storm peak discharge for pre-developed conditions.
(b) The resulting 2-year frequency design storm peak discharge for the entire contributing watershed area shall not exceed 50% of the 2-year frequency design storm peak discharge for pre-developed conditions. A standard storm hydrograph routing method shall be used to demonstrate that these conditions are met. The storm duration that maximizes the peak discharge for the pre-developed condition and the storm duration that maximizes required detention volume shall be used for the standard hydrograph routing method of detention design. In no case shall a storm duration less than 30 minutes be used either in peak flow or detention facility design calculations.
(2) For developments with drainage areas (tributary to stormwater detention facilities) less than 10 acres, the following conditions apply:
(a) The resulting 100-year frequency design storm peak discharge for the entire contributing watershed area shall not exceed the 10-year frequency design storm peak discharge for pre-developed conditions.
(b) The resulting 2-year frequency design storm peak discharge for the entire contributing watershed area shall not exceed 50% of the 2-year frequency design storm peak discharge for pre-developed conditions. A standard storm hydrograph routing method which balances the inflow design storm hydrograph inflow rates, detention pond outflow rates, and detention pond storage volume for each incremental time period over the design storm duration shall be used to demonstrate that these conditions are met. The storm duration that maximizes the peak discharge for the pre-developed condition and the storm duration that maximizes required detention volume for post-development conditions shall be used. In no case shall a storm duration less than 30 minutes be used either in peak flow or detention facility design calculations.
(3) The 2-year and 5-year/10-year frequency design storm peak discharge calculations for pre-developed conditions shall be determined using a standard storm hydrograph method based on watershed areas, soil characteristics, and land usage prior to development and application of corresponding runoff curve numbers, times of concentration, and other basin parameters.
(4) The 2-year and 100-year frequency design storm peak discharge calculations for post-development conditions shall be determined using a standard storm hydrograph method based on watershed areas, soil characteristics, and land usage after completion of development and application of corresponding run-off curve numbers, times of concentration, and other basin parameters.
(5) The 100-year peak discharge (and inflow hydrograph) shall be based on land use conditions representing fully developed conditions, using corresponding runoff coefficients, travel times, and other basin parameters.
(6) A standard storm routing method which balances the inflow design storm hydrograph inflow rates, detention pond outflow rates, and detention pond storage volume for each incremental time period over the design storm duration shall be used to demonstrate that adequate storage volume has been provided for each storm water detention facility to meet the conditions of divisions (D)(1) or (D)(2) above.
(7) Stormwater detention facilities shall not be overtopped as a result of inflow from the 100-year frequency design storm event for the storm duration that maximizes required detention volume. For detention facilities which incorporate an embankment, a minimum freeboard of two feet between the maximum routed flood pool elevation and the top of the embankment shall be provided. For excavated detention facilities, which do not incorporate an embankment, a minimum freeboard of one foot between the maximum routed flood pool elevation and the top of the detention facility shall be provided. A minimum freeboard of two feet shall always be provided between the maximum routed flood pool elevation and finished floor elevations of any building structures subject to overbank flooding of the detention pond. The top of the detention facility is defined as the lowest point along the perimeter of the detention facility where uncontrolled overflow would occur.
(8) Emergency spillway system. The purpose of the system is to safely convey flows that exceed the capacity of detention facility to the downstream drainage system in a manner that minimizes risk to the structural integrity of the facility itself and adjacent improvements or property. The emergency spillway system shall be provided to discharge flow resulting from pool elevations greater than the 100-year frequency design storm pool elevation. The spillway shall have a minimum size adequate to pass the routed 100-year frequency design storm peak discharge (assuming no storage is available within the detention basin and the primary outlet is plugged and nonfunctional) without overtopping the detention facility embankment. The elevation of the emergency spillway shall not be placed no lower than the routed 100-year frequency design storm pool elevation.
(9) Erosion protection shall be provided for the primary outlet and emergency spillway so that the detention facility embankment will be adequately protected.
(10) The minimum allowable size for the primary outlet conduit, from the detention facility, shall be 12 inches. If further restriction of the outlet conduit is required, the restriction shall be made to the inlet end of the conduit.
(11) In those instances where the discharge velocity from the primary outlet or emergency spillway is greater than five feet per second, energy dissipation may be required.
(12) Wet detention ponds shall be provided with a safety ledge that limits the depth at normal pool to one foot or less for the first ten feet out from the edge of water. At least 40% of the pond area at normal pool shall be provided with a normal pool depth of at least eight feet. Adequate erosion control shall be provided along the banks within the zone of water level fluctuation through the use of one or a combination of rip-rap, hydrophilic vegetation, shallow bank slopes, retaining walls, erosion control/turf reinforcement blankets or other suitable means.
(13) Detention facilities, which are designed to have dry bottoms, must be designed to include underdrains, to drain the bottom of the detention facility, so that the facility can be maintained. The bottom of the facility shall also be designed to have longitudinal and traverse grades of at least 1% to the outlet, so that the facility will empty, leaving no ponded water.
(14) The maximum steepness for grassed side slopes along detention basins shall be 3 horizontal to 1 vertical. The maximum steepness for grassed side slopes in residential development shall be 5 horizontal to 1 vertical.
(15) Earthen embankments that constitute a levee with a depth of water exceeding three feet against the embankment during the critical (for peak water surface elevation) 100-year design event shall be designed as a small dam. The embankment material shall be compacted to standard proctor density of at least 95% and an impervious cutoff trench shall be provided along the length of the levee portion of the embankment to prevent underseepage. Other structural measures may be required on a case by case basis depending upon the potential hazard associated with a failure of the embankment.
(16) Methodology for developing peak discharges, discharge hydrographs, and flood routing calculations shall be in accordance with acceptable engineering practice. Calculations based on the Soil Conservation Service procedures, the Corps of Engineers procedures, or the Bureau of Reclamation procedures are considered acceptable. All other procedures must be approved by the City Engineer.
(17) Peak discharge calculations, based on standard storm hydrograph methods, shall be submitted for each watershed area draining from the development site for the 2-year and 5-year/l0-year frequency design storms for both pre-development and post-development conditions. The calculations shall include contributing watershed areas (including any off-site contributing watershed areas), runoff curve numbers, times of concentration, and other basin parameters necessary to develop peak discharge.
(18) At a minimum, the following information shall be submitted for each stormwater detention facility:
(a) Calculations which show stage-discharge (rating curve) and stage-storage characteristics. Indicate the storage volumes provided at normal pool elevation, 100-year frequency design storm pool elevation, and top of detention facility.
(b) Detention facility inflow hydrograph for the 100- year frequency design storm and routed outflow hydrograph.
(c) Plan view drawings showing:
1. Location and size of the detention facility.
2. Top elevation of detention facility.
3. Locations and crest elevations of any embankments.
4. Normal pool elevation and area.
(d) Typical sections of each detention facility which include normal and 100-year frequency design storm pool elevations.
(e) Detail drawings of outlet control structure(s) showing type of structure, size, length, invert elevations, and any inlet control and/or outlet protection measures.
(E) Gutter and inlets. Drainage inlet grates and curbed gutter lines within proposed development shall be designed to meet the following minimum standards:
(1) The 10-year frequency peak discharge (Q10) shall be the design storm event. If a hydrograph generation method is utilized, the 15-minute and 30-minute storm durations shall be checked to determine the peak gutter spread.
(2) Inlets shall be spaced such that the gutter line shall convey the Q10 such that one-half a lane of traffic in each direction on a two lane road or one full lane of traffic in each direction on a four lane road is not flooded by gutter spread.
(3) Inlet grates, whether on pavement or grassed area, shall convey the Q10 with the grate assumed to be 50% blocked. The maximum depth of ponding around the grate shall be limited to six inches unless it can be shown that additional ponding will not threaten surrounding structures or property.
(4) Sumped inlets to be located in particularly sensitive locations should be provided with a safe overflow path in case of complete grate blockage.
(5) Stormwater collection systems shall be designed to prevent runoff from overflowing into the public right-of-ways or adjacent properties.
(6) Inlets shall be provided so that surface water is not carried across or around any intersection, nor for a distance of more than 400 feet in the gutter.
(F) Storm sewer systems.
(1) The development shall be provided with an adequate stormwater sewer system whenever curbs and gutters are installed and whenever the evidence available to the Commission indicates that natural surface drainage is inadequate. When the surface drainage is adequate, easements for such surface drainage shall be provided. Curbs and gutters along streets are required; however, if the Commission finds it necessary to waive this requirements, a shallow swale with its low point at least three inches below the elevation of the subgrade of the pavement shall be installed. In this case, one of the following types of improvements shall be furnished at driveway crossings:
(a) A corrugated metal pipe, at least 12 inches in diameter and 14 feet in length, to be placed where required for each driveway; or
(b) A properly dipped or swaled concrete pavement, 24 feet in length, 6 feet in width, and 6 inches thick, designed so as not to create a hazard to the underparts of automobiles, at the entrance of each driveway.
(2) It is the responsibility of the developer to keep all major watercourses, drainage systems, etc., not under the jurisdiction of any public agency, open and free flowing.
(3) Storm sewer systems shall be designed to comply with the following minimum standards:
(a) The 10-year frequency peak discharge (Q10) shall be the design storm event. The storm sewer shall be capable of conveying the (Q10) in open channel flow conditions (pipe not under pressure).
(b) The hydraulic grade line shall be checked for the (Q100) event. Any location where the hydraulic grade line elevation exceeds the inlet rim elevation or adjacent ground surface grade elevation (open pipe inlets), the drainage design report shall discuss the handling of surcharged stormwater from the system. Surcharged stormwater from the (Q100) event shall either be safely stored over the structure or overflowed to a safe location.
(c) All open end section inflow points larger than 15 inches in diameter shall be affixed with a "child-protection" grate designed to prevent accidental entry of children into the storm sewer system during high water events. The grate should have bar spacing small enough to protect children while minimizing debris accumulation.
(d) All storm sewer systems shall be designed to provide a minimum full flow velocity of at least 2.5 feet per second for the (Q10) design storm event.
(4) The minimum size for storm sewers shall be 12 inches.
(5) Refer to the city Design Standards Manual, Section 02721 Storm Sewers, for information pipe materials and installation requirements related to storm sewers.
(G) Culverts and bridges. Culverts and bridges shall be designed to comply with the following minimum standards:
(1) Stream crossings that are within the jurisdiction of the Department of Natural Resources Floodway Permitting program, or within an INDOT right-of-way shall be designed to comply with the most strict of the DNR standards, INDOT standards or the minimum standards contained herein.
(2) Open culverts or bridges shall be designed to convey the 25-year frequency peak discharge (Q25) in an open channel flow (not under pressure) condition.
(3) Open culverts or bridges shall be designed to convey the 100-year frequency peak discharge (Q100) such that the maximum depth of road overflow is 12 inches over the centerline crown of the road and the surcharge in the water surface profile upstream of the culvert does not increase the 100-year flood elevation on upstream or adjacent properties over that of existing conditions by more than six inches. The Commission may limit the allowable backwater profile surcharge and/or road overflow depth if the minimum standards contained in this chapter do not adequately protect life and property. An example of this case would be the existence of buildings or other vulnerable improvements adjacent to or upstream of the site that are subject to inundation by the 10-year flood under existing conditions or would become vulnerable to inundation by the 100-year flood if the six inch surcharge occurred. In these cases the allowable surcharge is zero inches.
(4) Open culverts or bridges that are designed to allow roadway overflow during the 100-year design event shall also be checked for the 10-year, 25-year, and 50-year frequency storm events to assure that the surcharge in the water surface profile upstream of the culvert does not increase the flood elevation on upstream or adjacent properties over that of existing conditions by more than three inches for these small storm events. The Commission may limit the allowable backwater profile surcharge and/or road overflow depth if the minimum standards contained in this chapter do not adequately protect life and property.
(5) Open culverts and bridges shall be provided with end treatments such as headwalls, wing walls or end sections as needed to protect embankments and outlet channel from erosion, the pipe from damage and to provide a smooth transition of stormwater into and out of the structure.
(6) Culverts and bridges shall be designed with adequate scour energy dissipaters at the inflow point, outflow point, around abutments and round piers. Scour protection shall be designed using standard engineering practice to protect the site and structure from the Q100 design event.
(7) Culverts and bridges that are replacement structures for existing crossings where the road profile is not being altered may be designed to maintain the current hydraulic capacity and flood profile surcharge as the existing crossing. The Commission may impose additional standards if this standard would result in a continuation of an unacceptable hazard to life or property.
(8) The method required to compute the hydraulic characteristics for culverts and bridges can vary depending upon the complexity of the application. Simple Mannings equation computations are acceptable for small culverts draining less then five acres of watershed. Larger culverts draining more than five acres of watershed shall be analyzed with either a culvert design method such as the HY8 program or standard backwater programs such as HEC2 HECRAS or WSPRO. Applications that involve potential tailwater controls from downstream constructions (such as another culvert) shall be analyzed in a manner that takes the tailwater control into account.
(H) Open channels. Open channels shall be designed to comply with the following minimum standards:
(1) Collection swales and ditches within proposed projects shall be designed to convey the 10 peak discharge (Q10) without overtopping the banks.
(2) Large ditches that drain more than 50 acres shall be designed to convey the Q100 without overtopping the designed system. The channel may overflow if an overbank conveyance system is available to safely contain the Q100 flow. Existing ditches or natural streams that pass through the site do not need to be upgraded to design capacity unless the proposed project seeks to divert, enclose or otherwise significantly alter the conveyance.
(3) Open channels shall be designed with a minimum longitudinal slope of 1%.
(4) Subsurface drains are required for any grass swale which drains three acres or more.
(5) Open channels shall have a maximum vegetated side slope of three horizontal to one vertical. Rip rap or other channel armoring methods are acceptable with a maximum side slope of two horizontal to one vertical. Retaining wall designs using proper materials are also acceptable.
(6) Open channels shall be designed to adequately protect the system from erosion during the design storm event.
(I) Subsurface drains.
(1) A subsurface drainage system shall be designed to be placed along both sides of the streets in a development and wherever else within the development that it is determined to be necessary. The purpose of the subsurface piping system is to provide drainage for the street subbase and to create an outlet for foundation drains, and sump pump drains from residences. The subsurface drainage system shall discharge to the storm sewer system or to the surface drainage system upon approval from the city. No subsurface drainage system connections will be permitted to the sanitary sewer system.
(2) Capped, plugged, or exposed connections, terminated at the property line, shall be provided for each subdivided parcel along the subsurface drainage system. The location of each connection termination shall be adequately marked so that it can be field located when the corresponding parcel is developed.
(3) Subsurface drains are required for any grass waterway, which drains three acres or more.
(4) Minimum size subsurface drains at the street curb and back yard swales (grassed waterways) serving more than one lot shall be six inches in diameter.
(J) Stormwater drainage elements. Stormwater drainage facilities shall be located in the street right-of-way, where feasible, or in perpetual, unobstructed easements. The following easement widths shall apply:
Area or Situation | Easement Width |
Storm Sewer (up to 24") | 20 feet |
Storm Sewer (24" or larger) | 25 feet |
Grassed Waterways (including equivalent sized lined channels) | Width of channel plus 15 feet |
Subsurface Drain | 20 feet |
Open Outlet Channel (including equivalent sized lined channels) | 20 feet from top of each bank |
Retention pond, detention basin, and/or permanent sediment basin | Elevation of the emergency spillway design flow, plus 20 feet horizontally |
(Am. Ord. 2007-07-18, passed 8-6-07)