(A) Watercourse protection.
(1) Legal drains within the Town are under the jurisdiction of the Hendricks County Drainage Board. The Indiana Drainage Act has established seventy-five (75) foot easements to either side of legal drain tile lines and seventy-five (75) feet from the top of the bank on each side of open legal drains. Any stormwater discharge or work to be completed within a legal drain easement must obtain approval from the Hendricks County Drainage Board.
(2) Waterways within the Town may be under the jurisdiction of the IDEM or the United States Army Corps of Engineers (ACOE).
(3) Work within a floodway is under the jurisdiction of the Indiana Department of Natural Resources (DNR).
(4) Any relocation of a watercourse shall require a plan approved by the Development Services Department, and it shall not discharge water onto downstream adjoiners in any location different from its existing location.
(5) Alterations shall be designed to accommodate all on- and off-site upstream drainage areas. Design capacities shall be determined as described in this chapter.
(6) Entities owning property through which a watercourse passes, or such an entity’s lessee, shall keep and maintain that part of the watercourse in accordance with this chapter and all applicable state and federal statutes and regulations. In addition, the entity or lessee shall maintain existing privately owned structures within or adjacent to the watercourse so that such structures will not become a hazard to the use, function, or physical integrity of the watercourse. The entity or lessee shall not place or construct a privately-owned structure or other impairment within or adjacent to the watercourse such that is an impairment or a detriment to the flow of water within the channel.
(B) Determination of stormwater runoff rates.
(1) Stormwater runoff rates shall be calculated in accordance with Chapter 2 of the Hendricks County Stormwater Technical Standards Manual. Composite Runoff Coefficients shall be calculated and account for connected impervious surfaces such as downspouts connected to stormwater conveyance system; typical composite runoff coefficients shall not be used.
(2) Stormwater drainage systems shall be designed to accommodate the following:
(a) All upstream off-site drainage.
(b) Existing drain tiles from upstream.
(c) Diffused water flow from upstream properties.
(d) Storm sewers from upstream.
(e) The drainage collection system from within the project or property.
(C) Detention/retention and pond requirements.
(1) General requirements.
(a) Storage volume.
1. Storage volumes shall be computed using a computer model that can generate hydrographs based on the NRCS TR-55 time of concentration and curve number calculation methodologies.
2. Storm durations shall be used that maximize the peak flow for the pre-developed condition and maximize detention storage volume for the post-developed condition. The storm duration shall be equal to or greater than the site time of concentration, but shall not be less than thirty (30) minutes.
3. When determining curve numbers for post-developed conditions, the initially determined hydrologic soil group for disturbed areas should be changed to the next less infiltrating capacity category (i.e. A to B, B to C and C to D).
(b) Detention release rates.
1. Detention control devices shall limit the discharge to a rate such that the post developed release rate from the site is no greater than 0.2 cfs per acre of development for the 0-10 year return interval storms and 0.4 cfs per acre of developed area for the 11-100 year return interval storms.
2. If the downstream receiving channel or pipe is inadequate to accommodate the post-developed flow, then the release rate must be further reduced.
3. The minimum allowable orifice size is four (4) inches.
4. For sites where the pre-developed area has more than one (1) outlet, the release rate should be computed based on pre-developed discharge to each outlet point. The computed release rate for each outlet point shall not be exceeded at the respective outlet point even if the post developed condition would involve a different arrangement of outlets.
(c) Runoff from upstream tributary areas (off-site areas) may be bypassed around the facility without attenuation. If it is to be routed through the facility, a separate outlet system or channel must accommodate those flows. Unless the pond is being designed a: a regional detention facility, the primary outlet structure shall be sized and the invert elevation of the emergency overflow weir determined according to the onsite runoff only. Once the size and location of the primary outlet structure and the invert elevation of the emergency overflow weir are determined by considering on-site runoff, the one hundred (100)-year pond elevation is determined by routing the entire inflow, on-site and off-site, through the pond.
(d) The detention facility shall be designed in such a manner that a minimum of ninety percent (90%) of the maximum volume of water stored and subsequently released at the design release rate shall not result in a storage duration in excess of forty-eight (48) hours from the start of the storm unless additional storms occur within the period. In other words, the design shall ensure that a minimum ninety percent (90%) of the original detention capacity is restored within forty-eight (48) hours from the start of the design one hundred (100)-year storm.
(e) An emergency overflow/spillway shall be designed for the release of exceptional storm runoff or in emergency conditions should the normal discharge devices become totally or partially inoperative. The overflow facility shall be of such design that its operation is automatic and does not require manual attention. At a minimum, emergency spillways shall be capable of handling one hundred twenty-five percent (125%) of the inlet peak discharge resulting from the one hundred (100)-year storm event from the entire contributing watershed in the post-developed condition.
(f) All ponds (including both dry and wet ponds, and ponds that are not used for stormwater quality or quantity management) shall incorporate applicable requirements included in the Town’s standard detail for “Detention/Retention Ponds”.
(g) All detention/retention facilities (including underground detention) shall have an upstream BMP designed in accordance with § 151.23 and installed for each inlet pipe, to collect sediments, debris, floatables, and other material prior to entering the detention/retention facility. Forebays are not acceptable. Position BMPs outside right-of-way and not more than fourteen (14) feet from a drivable surface for vac-truck and maintenance vehicle access.
(h) Underground detention facilities shall have sufficient observation wells or inspection ports, at least ten (10) inches in diameter, to allow access for inspection, maintenance, and regular cleaning operations.
(i) Designers and developers are encouraged to consider methods of runoff reduction such as reducing impervious surface area, disconnecting impervious areas, promoting infiltration, stormwater capture and reuse, and other low-impact development methods in order to reduce requirements for on-site stormwater detention.
(j) Any regulations of the State Board of Health concerning standing water must be adhered to in the planning and use of temporary ponding facilities or other retention and detention devices.
(k) Stormwater detention/retention on roadways or driving lanes is not acceptable.
(l) For storm sewer pipes discharging into a pond, the lowest pipe invert at the first upstream storm sewer structure shall not be lower than the normal pool elevation of the pond.
(m) he outlet location must be approved by the Town of Brownsburg. The outlet control structure and piping shall be designed to operate simply and effectively with minimal maintenance. The outlet structure shall be positioned in an accessible location for maintenance.
(n) A safety ramp and ingress/egress easement from a nearby road must be provided for all ponds. The safety ramp shall be minimum twenty (20) feet wide and have a maximum exit slope of6H:lV.
(o) Detention ponds located within a floodplain must be protected from the base flood elevation and shall not be designed to hold flood waters. Detention ponds may not be used for compensatory flood storage.
(2) Pond safety signs.
(a) Provide sign with minimum sign area of one (1) square foot and maximum sign area of four (4) square feet.
(b) Sign must be constructed of a permanent material, either wood or metal.
(c) At minimum, pond safety signs must have the words “danger”, “no swimming”, “stay office” and must have “no swimming” and “stay office” symbols. Warning may also prohibit trespassing, wading or other recreational activities that would involve a person entering a pond.
(d) The sign color will be determined by the homeowners’ association, property owner or developer.
(e) Install signs no more than five hundred (500) feet apart. If pond has less than three thousand (3,000) square feet of water surface area, a minimum of two (2) signs are required. If pond has greater than three thousand (3,000) square feet of water surface area, a minimum of four (4) signs are required. A sign must be placed at all public access areas to the pond.
(f) Each developer of the construction area and subsequent homeowners’ association or property owners shall be responsible for the cost of maintenance and replacement of pond safety signs.
(D) Storm sewer design standards.
(1) Storm sewer requirements.
(a) Storm sewers, inlets, and manholes, shall accommodate the ten-year (10-yr.) (10% chance/year) peak runoff and maintain the hydraulic grade line (HGL) below the crown of the pipe. Flood routing shall be provided in accordance with § 151.21(I). If an overflow route is not available, then the storm sewer system shall be designed for the one hundred-year (100-yr.) (1% chance/year) peak runoff HGL below the ground surface and less than six (6) inches of ponding at inlets.
(b) The hydraulic capacity of storm sewers shall be computed using Manning’s Equation. Where storm sewers discharge into submerged outfalls, a backwater analysis shall be performed.
(c) Minimum pipe size shall be twelve (12) inches.
(d) Minimum full-flow velocity of two (2.0) feet per second is desirable to prevent sedimentation in the pipe. Maximum full-flow velocity of ten (10.0) feet per second is recommended. The minimum Manning’s n roughness value shall be twelve thousandths (0.012), regardless of material.
(e) Storm sewer pipes shall have a minimum of twenty-four (24) inches of cover, measured from the crown of pipe to the ground surface. Shallower depth of cover may be considered by the Town under conditions where engineering design will alleviate concern for surface loadings and frost heave.
(f) A minimum vertical separation of eighteen (18) inches shall be provided between storm sewers and other utilities (sanitary sewers, water mains, and the like).
(2) Storm structure requirements.
(a) Storm sewers shall be straight between storm structures (inlets and manholes).
(b) Manholes shall be provided at the following locations:
1. Where two (2) or more storm sewers converge;
2. Where the pipe size changes;
3. Where a change in horizontal alignment occurs;
4. Where a change in pipe grade occurs;
5. At intervals not to exceed four hundred (400) feet for pipes less than forty-eight (48) inches in diameter and not to exceed five hundred (500) feet for pipes forty-eight (48) inches in diameter and larger.
(c) A minimum drop of one tenth (0.1) foot shall be provided in storm structures
(d) When changing pipe sizes in a storm structure, the crowns of pipe shall be matched unless HGL modeling shows that another arrangement would be as effective.
(e) All storm grate castings shall be pre-cast with a pollution prevention message permanently attached. The message shall be included on the site development plans and comply with the Town of Brownsburg Construction Standards.
(f) Exposed pipes greater than eighteen (18) inches in diameter shall be equipped with an animal guard or trash rack. Trash racks are typically installed at the inflow end of pipes and animal guards at the outflow end.
(3) Inlet sizing and spacing.
(a) Inlets shall not be located in the path where a pedestrian or bicycle is likely to travel.
(b) Inlet grate openings shall be designed to pass the ten-year (10-yr.) (10% chance/year) peak flow with fifty percent (50%) of the inlet area clogged with a maximum six (6) inches of ponding depth.
(c) Inlets shall be spaced to prevent water spread over six (6) feet into a travel lane of a street. Bypass flow from upstream inlets shall be incorporated into inlet calculations. Manufacturer’s literature should provide the inlet casting’s flow-intercept-efficiency coefficient. The United States Department of Transportation Hydraulic Engineering Circular-12 (HEC-12) methods may be used.
(d) Stormwater may not be stored on any pavement with pedestrian traffic. Stormwater may be stored on remote areas of pavement with a maximum water depth of six (6) inches.
(e) Flood routing shall be provided in accordance with § 151.21(I).
(4) Connections to the storm sewer system.
(a) Sump pumps which receive and discharge groundwater and other stormwater shall be connected to the storm sewer system where possible, preferably at an inlet or manhole structure. Sump pumps may also be connected to subsurface drains in accordance with § 151.21(F).
(b) Footing, perimeter, and other subsurface drains shall be connected to the storm sewer system where possible, preferably at an inlet or manhole structure. Footing and perimeter drains may also be connected to subsurface drains in accordance with § 151.21(F). Footing, perimeter, and other subsurface drain connections to the sanitary sewer are prohibited.
(c) Roof downspouts, roof drains, and other roof discharge piping shall be directlyconnected to a storm sewer system, preferably at an inlet or manhole structure. Downspouts and roof drains connections to the sanitary sewer are prohibited.
(d) Floor drains and swimming pool drains shall not be directly connected to the storm sewer system. Floor drains and swimming pool drains shall be connected to an approved outlet and comply with the requirements of other Town ordinances established for control of illicit discharges and connections.
(E) Open channel design standards.
(1) General requirements.
(a) Any relocated open channels shall require stabilization measures to prevent sloughing from layers of seasonally saturated silts and sands.
(b) Roadside ditches shall be altered to meet the needs of “strip” housing along that road so that driveway crossings may be installed with adequate cover to prevent frost heave and to be coordinated on grade with each other. They shall be constructed with side slopes flat enough to allow easy mowing and maintenance, even though this may require grading beyond the road right-of-way into the lawns.
(2) Open channel requirements.
(a) Open channels, roadside ditches, swales, and culverts shall accommodate the twenty-five-year (25-yr.) (4% chance/year) peak runoff without overflowing the banks and shall accommodate the one hundred-year (100-yr.) (1% chance/year) peak runoff within the drainage easement, provided however:
1. Open channels with a carrying capacity greater than or equal to thirty (30) cubic feet per second at bank-full stage shall be designed in accordance with the National Resources Conservation Service Part 654 Stream Restoration Design National Engineering Handbook, Chapter 10: Two-Stage Channel Design and be capable of accommodating the twenty-four (24) hour, fifty year (50-yr.) (2% chance/year) peak runoff without overflowing the banks;
2. Culverts shall be capable of accommodating a fifty-year (50-yr.) (2% chance/year) peak runoff when crossing under a road which:
a. Carries a higher volume of traffic than other adjacent routes or has the potential for carrying higher volumes;
b. Is part of a pattern of arterial routes for the entire urban area;
c. Is oriented primarily to moving traffic rather than to serving abutting land use.
(b) Flood routing shall be provided in accordance with § 151.21(I).
(c) Any alteration of an existing open outlet channel traversing a property shall be sized as described above or for the capacity of the existing channel, whichever is greater, with provisions made for the one-hundred- year (100-yr.) (1% chance/year) frequency storm so that flooding of adjacent structures, buildings, or properties will not occur. The approval of the DNR, IDEM, and ACOE must be obtained and their requirements must be followed for all areas within their jurisdiction.
(d) The hydraulic capacity of open channel systems shall be computed using Manning’s Equation. Backwater analysis shall be performed to determine the one hundred-year (100-yr.) (1% chance/year) water surface elevation along open channel systems.
(e) Channel lining shall be designed to accommodate the ten-year (10-yr.) (10% chance/year) peak discharge. The channel lining material shall be selected such that the design shear stress is less than the maximum permissible shear stress for the lining material.
(f) For grass-lined channels, the side slope shall be no steeper than three (3) horizontal to one (1) vertical.
(g) For armored channels (turf reinforcement mat, riprap, and the like), the side slope shall be no steeper than one and one half (1-1/2) horizontal to one (1) vertical.
(h) Open channels with a bottom width greater than or equal to fifteen (15) feet shall have a riprap or paved invert to convey low flows and prevent meandering.
(i) Minor drainage collector swales in rear yards and between homes shall not exceed four hundred (400) feet in length and shall not be used to convey off-site drainage areas.
(j) Subsurface drains shall be required if the channel slope is less than one percent (1.0%). The minimum channel slope shall be three tenths of a percent (0.3%). A minimum channel velocity of two (2) feet per second is desired to prevent deposition within the channel. All rear yard swales shall have subsurface drains.
(3) Culvert requirements.
(a) Culverts shall be sized in accordance with the Federal Highway Administration’s HDS-5 Hydraulic Design of Highway Culverts.
(b) Minimum pipe size shall be twelve (12) inches. Minimum full-flow velocity of three (3.0) feet per second is desirable to prevent sedimentation in the pipe. Maximum full-flow velocity often (10.0) feet per second is recommended. The minimum Manning’s n roughness value shall be twelve thousandths (0.012), regardless of material.
(c) Culvert pipes shall have a minimum of twenty-four (24) inches of cover, measured from the crown of pipe to the ground surface, unless shown otherwise in Town of Brownsburg Construction Standards.
(d) Outlet protection and energy dissipaters shall be used wherever the velocity of flows leaving a culvert exceed the erosive velocity of the downstream channel.
(F) Subsurface drains design standards.
(1) Subsurface drains are required for any grass-lined open channel which drains three (3) acres or more.
(2) Subsurface drains are required along all streets constructed in the Town to lower the seasonally high water table in Brookston, Crosby, and Miami soil types.
(3) Tees must be provided along the subsurface system to provide an outlet for footing drains and sump pumps from each lot.
(4) Subsurface drains shall be a minimum of six (6) inches diameter for areas receiving subsurface flow only. Subsurface drains which may receive surface (roof drains, yard drains, and the like) and subsurface flows shall be a minimum of twelve (12) inches diameter. Underdrain capacity calculations shall be provided to ensure underdrains are properly sized for the subsurface and surface flows.
(5) Subsurface drains shall have a minimum of twenty-four (24) inches of cover unless shown otherwise in Town of Brownsburg Construction Standards.
(G) Low impact development design standards.
(1) Low Impact Development (LID) is an approach to land planning and engineering design which manages rainfall at the source to reduce stormwater runoff and manage runoff timing, emphasizes conservation and natural features, and protects water quality by controlling pollutants.
(2) Designers and developers are encouraged to consider methods of runoff reduction such as reducing impervious surface area, disconnecting impervious areas, promoting infiltration, stormwater capture and reuse, and other LID methods in order to reduce requirements for on-site stormwater detention.
(3) LID features shall conform to current industry guidelines and will be reviewed on a case-by-case basis. Green infrastructure and stormwater BMPs shall be designed in accordance with the Hendricks County Stormwater Technical Standards Manual or the Indiana Stormwater Quality Manual. All design calculations and justification shall be submitted for review.
(H) Standards for adequate outlet. Stormwater and drainage from the site must be discharged into an adequate outlet or outlets so as not to adversely affect other landowners. The criteria for outletting is as follows:
(1) Open channels shall outlet into an existing open channel, provided that stream bank and channel erosion shall not be aggravated and that the hazard of flooding shall not be increased.
(2) All storm sewers shall outlet into an open channel, storm sewer pipe, detention basin, or retention pond.
(3) Subsurface drains shall outlet into an open channel, storm sewer pipe, detention basin, retention pond, or existing tile drain, provided that the existing drain has adequate capacity, is not deteriorated, and can be maintained.
(4) Detention/retention ponds shall outlet into an open channel or storm sewer pipe.
(I) Flood routing.
(1) Stormwater ponding and overflow path routing shall be evaluated for the peak one hundred-year (100-yr.) (1% chance/year) storm event.
(2) Peak runoff flows shall be calculated for all contributing drainage areas, on-site and off-site, in their proposed or reasonably anticipated land use.
(3) The stormwater collection system (storm sewers, open channels, etc.) shall be assumed full (at capacity) from a previous storm event at the beginning of the flood routing analysis.
(4) The overflow path/ponding resulting from the flood routing event shall be clearly shown as a hatched area on the plans. Plans shall include cross sections along the flood route path. Cross sections shall include the existing surface, proposed grading, and the maximum water elevation for the peak one hundred-year (100-yr.) (1% chance/year) storm event. The flood routing path and ponding areas shall be placed in a right-of-way or drainage easement in accordance with § 151.21(J).
(5) The overflow path/ponding may be modeled as a successive series of natural ponds and open channel segments. The calculations for determining the one hundred-year (100-yr.) (1% chance/year) flood routing elevations may be based on hand calculation methods utilizing normal depth calculations and storage routing techniques or performed by computer models such as United States Department of Agriculture (USDA) Soil Conservation Service (SCS) Technical Release 20 (TR-20); ACOE Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS), and ACOE HEC Flood Hydrograph Package (HEC-1) in combination with ACOE HEC River Analysis System (HEC-RAS).
(J) Drainage easements.
(1) Drainage easements must be provided for maintenance of the stormwater management system of publicly-owned systems and privately-owned ponds, detention/retention basins, water quality BMPs, and LID practices. Easements are not required for other private systems.
Area or Situation | Easement Width |
Area or Situation | Easement Width |
Storm sewer (smaller than 24-in.) | 20 feet, centered over sewer |
Storm sewer (24-in. and larger) | 25 feet, centered over sewer |
Grassed waterways (including equivalent sized-lined channels) | Width of channel plus 15 feet, centered over waterway |
Subsurface drain | 20 feet, centered over drain |
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) |
Underground detention facility | Detention facility footprint plus 15 feet in every direction (horizontally) |
Stormwater BMP (above or below grade) | BMP footprint plus 15 feet in every direction (horizontally) |
Flood routing path/ponding area | Width of flood path/ponding area |
(2) Access easements must be provided from a public roadway to the drainage easement, for access to stormwater management facilities.
(3) Structures, fences or landscaping may not be placed in an easement in a way that will impede the free flow of stormwater.
(4) Drainage easements shall be maintained by the property owner.
(K) Grading and building pad elevation policy.
(1) No construction of buildings within the Town shall be permitted within twenty-five (25) feet of a stormwater conveyance or within twenty-five (25) feet of the side slopes of open channels without plans prepared by registered engineers, land surveyors or architects, certifying that there is adequate room for maintenance of the drainage facility.
(2) No building or structure shall be separated by the one hundred-year (100-yr.) (1% chance/year) elevation of stormwater detention facilities by less than twenty-five (25) feet.
(3) The lowest adjacent grade (including walkout basement floor elevation) of all residential, commercial, or industrial buildings must be located a minimum of two (2) feet above the one-hundred-year (100-yr) (1% chance/year) flood elevation, the one hundred-year (100-yr.) (1% chance/year) pond elevation, or the emergency overflow weir elevation, whichever is higher. The Lowest Adjacent Grade for all residential, commercial, and industrial buildings shall be set a minimum of one (1) foot above the noted overflow path/ponding elevation resulting from flood routing as described in § 151.21(I) and included on the Final Plat.
(4) Positive drainage shall be provided over the entire lot. Protective slopes shall slope away from building foundations and water supply wells for a minimum of six (6) inches of fall per ten (10) feet horizontal, except where restricted by property lines. Maximum gradient of protective slopes shall be two and one half (2-1/2) inches per foot or twenty-one percent (21%) for a minimum of four (4) feet away from all building walls, except where restricted by property lines.
(L) Adjoining property impacts policy.
(1) Land disturbance, and alterations to the site shall provide for the discharge of all stormwater runoff from upstream off-site areas as well as the stormwater runoff from the areas being developed or altered to one or more acceptable outlets having capacity to receive upstream on-site and off-site drainage (as determined by the Development Services Department).
(2) No activities conducted as part of the land disturbance shall be allowed to obstruct the free flow of flood waters from an upstream property. Any existing field tile or other stormwater facility encountered during the construction shall be incorporated into the proposed stormwater collection system or otherwise tied to an acceptable outlet. Design calculations shall be provided to demonstrate that the downstream system has sufficient capacity to accommodate flows from existing facility. Calculations shall by certified by a professional engineer or land surveyor licensed in the State of Indiana.
(3) Where an adequate outlet is not located on the site or the outfall from a site’s stormwater system flows onto or through property not owned by the developer prior to reaching a regulated drain or natural waterway, off-site drainage improvements may be required. Off-site improvements may include, but are not limited to, extending or replacing storm sewers; clearing, dredging, and removing obstructions to open channels or natural waterways; and replacement of undersized culvert pipes.
(Ord. 2017-28, passed 10-12-17)