The design of storm water systems, culverts, curb and gutter, inlets, ditches, and other drainage facilities and improvements in subdivision and individual, commercial or industrial developments shall be in compliance with the requirements of this Code and provisions of the current edition of the Illinois Department of Transportation Drainage Manual, hereinafter referred to as “the Drainage Manual” with the following modifications:
(A) Design calculations shall be submitted to the City at the time of Improvement Plan or development submittals, including but not limited to:
(1) Cover sheet titled “Drainage Calculations” with the name of the subdivision or development;
(2) Signature and seal of a licensed professional engineer in Illinois;
(3) Certification as follows signed and dated by the licensed professional engineer:
I hereby certify that this report for the drainage of (X, name of development) was prepared by me or under by direct supervision in accordance with all applicable City of Waterloo ordinances, standards, and manuals for the owners thereof. I understand that the City of Waterloo does not and will not assume liability for drainage facilities designed by others.
(4) Table of contents with each sheet numbered;
(5) Description of the project, including existing conditions, proposed features, methodology, and unique aspects;
(6) Map of existing drainage area with north arrow, scale, contours at vertical intervals no greater than five feet, boundaries, existing inlets, culverts, and storm sewers, and other topography. Existing, upstream and downstream areas beyond survey limits may be depicted with other mapping, however, sufficient topographic detail must be shown on these, as deemed necessary by the city;
(7) Proposed drainage area map with similar information as the existing map;
(8) Runoff characteristics including runoff coefficients or curve numbers for each existing and proposed drainage area and time of concentration calculations, or other applicable hydrologic characteristics depending on the methodology utilized. These should also include time of travel for existing conduits or other channelized systems affecting the development, upstream and/or downstream;
(9) Gutter capacity calculations;
(10) Inlet and bypass calculations;
(11) Conduit capacity and hydraulic grade line calculations;
(12) HEC-RAS calculations, if applicable, including a map showing sections and stationing, graphical printouts of existing and proposed sections, flow and boundary data, and tabular results clearly indicating stationing and water surfaces;
(13) Detention/retention calculations;
(14) Calculations of how Base Flood elevations were determined.
(B) Improvement plans. All proposed drainage easements, storm sewers, inlets, manholes, grate and frame details, curb and gutter details, pipe sizes, invert and lid elevations, detention storage areas, and other drainage improvements are to be identified on the plans. Drainage arrows denoting proposed flows and ditches shall be included in the plans, typically on the overall site layout showing contours and proposed and existing inlets and culverts. Elevations of proposed ditches shall be shown on the plans as necessary to ensure constructability of proposed drainage systems. The complete construction details for all drainage systems shall be shown on the plans.
(C) Determination of areas, surface characteristics, and conditions. Grading plans shall correspond to the drainage areas prepared in the report. Each drainage area shall have its own hydrologic and hydraulic calculations. Generally:
(1) Residential, single and multi-family.
(a) In a residential subdivision or area, either single family or multi-family (duplexes, condos, or villas), excepting apartment complexes, each lot’s primary drainage area shall be planned to drain towards the street, divided such that estimated roof peak defines the innermost edge of a trapezoidally shaped drainage area. Streetside corners of the trapezoidally shaped area shall terminate at the lot’s property lines.
(b) In any type subdivision or development, side and back lot line swales and ditches shall be shown on the grading plans in the Improvement Plans with proposed contours. Grading shall be such that runoff from any individual lot or tract is directed away from adjoining properties (see also side lot line easement requirements in other sections of this Code).
(c) Side lot line swales and ditches shall generally be directed away from street curbs and inlets. In the event this is not possible, the maximum allowable surface runoff discharge into the street shall be no greater than what can be accommodated by the gutter and curb inlets that serve streetside areas meeting gutter encroachment criteria.
(d) In the event side lot line swales and ditches are directed towards streets and the maximum allowable discharges noted above are exceeded, storm sewer piping and inlets of appropriate size and spacing (at that point in the street) shall be added the entire length of said lot line. Said piping shall be connected to piping along the street and shall terminate at the back lot line with either an inlet, end section, or junction (all junctions are required to be inlets) with contiguous piping system. Inlets at the back of sidewalk that have openings towards the side lot lines will not be allowed.
(e) All surface discharges directed away from streets down side lot lines shall be in underground conduit the entire length of the side lot line with an inlet at the upper end (near the street) as a cleanout, if said discharges exceed three cfs during a ten year event. All discharges from the drainage areas including street pavement, regardless of magnitude, shall be in underground conduit down side and back lot lines.
(f) For back lot lines and back lot areas, when surface runoff discharges exceed five cfs of flow or three fps velocity during a ten year event, storm sewer piping and inlets of appropriate size and spacing shall be added at the back lot lines from the point of said discharge until said discharge enters into detention/retention or other piping system.
(g) For back lot lines and areas, all incoming discharges in piping from side lot lines or other areas shall remain in piping down the back lot line until discharge enters into detention/retention or other piping system.
(h) For calculating runoff coefficients or curve numbers, each single family lot shall have a minimum roof area of 3,000 square feet and a minimum driveway area of 600 square feet contributing to impervious area. Each multi-family lot’s runoff characteristics shall be determined on a case-by-case basis, with impervious area not less than that for each single family lot.
(i) Each single family lot’s proposed impervious area runoff coefficient shall be 0.90 and the pervious area runoff coefficient shall be 0.30 using the rational method. With the SCS TR55 or TR20 methods, curve numbers comparable to those of the rational method shall be utilized. The proposed impervious area curve number shall be 98 and the proposed pervious area curve number shall be for good condition grass cover and the applicable hydrologic soil groups: 39 for hydrologic soil group A, 61 for group B, 74 for group C, and 80 for group D.
(2) Residential, apartment complexes: The impervious areas for each apartment complex with off-street parking shall include the planned rooftop area and parking areas, determined on a case-by-case basis. Impervious area shall not be less than that for each single family lot and runoff coefficients and curve numbers shall be comparable to those for single family.
(3) Commercial or industrial development. When individual lots of a subdivision are being developed as commercial or industrial tracts, or when such tracts are being developed as a separate development other than a subdivision, with or without large impervious areas such as rooftops and parking areas, the drainage areas within the tract(s) shall be established on a case by case basis with the following requirements:
(a) Rooftop areas to gutters and roof drains shall be defined and shown on the drainage maps when the building(s) have previously been designed or are being designed. Each roof drain shall have its own separate drainage area.
(b) Each inlet and piping intake, such as a flared end section, shall have its own separate drainage area.
(c) The entire proposed industrial or commercial area, except for low-lying areas near existing or proposed drainageways that will never have parking lots or structures, shall have a runoff coefficient of 0.90 using the Rational method and a comparable curve number of 98 with the SCS methods.
(4) Drainage area conditions beyond tract.
(a) The complete limits of all drainage areas affecting the subdivision or development, upstream and downstream, shall be determined and shown on drainage area maps including more complex urbanized areas that have storm sewer systems.
(b) When the development entails and/or abuts agricultural land, the drainage area maps shall be developed utilizing field surveys of the drainage areas extending beyond the development boundaries. The city may, at its sole discretion, require similar field surveys beyond the development, for non-agricultural lands.
(c) When the upstream drainage areas adjoin an existing road or street that must be improved with widening, curbing, storm sewers, and larger storm water conduits, as determined necessary by the city, as it abuts the boundary of the subdivision or development in question, these drainage areas shall be configured equal to the detail required for those within the subdivision or development being designed with sub-areas for street inlets and intakes from larger sub-areas adjoining right-of-way. Future improvements along said road or street beyond the platted boundaries shall be accounted for at the same level of detail. This may require estimations of the locations of future curbing, inlets, and storm sewers. All bypass from inlets within these areas shall be accounted for and all conduit and drainageways shall be integrated with those for the new subdivision or development such that hydraulic grade lines and flows are evaluated with the new subdivision or development.
(d) For all other upstream areas, the maximum size of any drainage sub-area shall be 15 acres for any area that is urbanized or in the process of becoming urbanized as deemed such by the city. For these areas, representative, composite runoff coefficients or curve numbers shall be calculated. For those with storm sewer systems, short times of concentration shall be utilized. For urbanizing areas, runoff coefficients or curve numbers for comparable land usage shall be applied, as well as short times of concentration in anticipation of future storm sewer systems.
(e) In situations where the upstream drainage areas pass through bridges or culverts, these structures shall be ignored as these could be upgraded in the future. Similarly, upstream ponds, lakes, and detention systems attenuation shall be ignored.
(f) Drainageways and conduit systems passing into or through a proposed subdivision or development, and those downstream, shall be modeled as flowing at or above capacity regardless of the storm events being applied within the proposed subdivision or development. The maximum size of any drainage sub-area shall be 15 acres and runoff characteristics shall be as for the upstream areas.
(g) All existing constrictions on drainageways and conduit systems downstream of the proposed subdivision or development shall be modeled to remain in place in establishing proposed drainage conditions of outfalls, lot elevations, created head of new structures (and associated freeboard), and similar feature. It is noted that this approach conflicts with establishing jurisdictional drainageways’ water surface elevations and thus would require separate calculations beyond those required for establishing jurisdictional drainageways’ water surface elevations.
(5) Runoff coefficients and curve numbers. The pre- and post-development runoff coefficients for agricultural ground including but not limited to, pasture and row crop ground, and other areas with grass or woodland cover shall be 0.30 (grass cover), using the rational method. The coefficients for gravel areas shall be 0.90 and those for all impervious surfaces including but not limited to roofs, driveways, patios, and paved areas (either asphalt or concrete) shall be 0.90. With the SCS TR55 or TR20 methods, curve numbers comparable to those of the rational method shall be utilized. Curve number values of 98 shall be used for areas with rational method runoff coefficients of 0.90, as prescribed herein. For agricultural ground described, the following curve numbers shall be used: 39 for hydrologic soil group A, 61 for group B, 74 for group C, and 80 for group D.
(6) Sheet flow. Sheet, or overland flow, as defined by various hydrologic methods shall be limited to 300 feet.
(7) Sag areas. At sag inlets along streets there shall be separate drainage sub-areas delineated either side of said inlets for checking allowable gutter encroachment.
(D) Hydrology.
(1) The hydrologic methods considered applicable for particular size drainage areas as noted in the IDOT Drainage Manual shall be utilized. The Rational Method shall not be used for sizing bridges, culverts, and other conduits intercepting runoff from municipal-jurisdictional areas and areas larger than municipal-jurisdictional areas. When a contributing drainage area to a drainage structure warrants use of what will be described as more intensive hydrologic methods such as the SCS TR-20 or TR-55, the USACoE’s HEC-1 (HEC-HMS), or the USGS Regression Equations (most current), the designer has two options for tailwater inputs for smaller systems within or adjoining the subdivision or development that do not require the more intensive hydrologic methods: (1) use the water surfaces generated by the more intensive methods in sizing described smaller systems at appropriate storm frequencies, or, (2) re-calculate the runoff that warrants use of the more intensive hydrologic methods using less intensive methods such as the rational method and use these resulting water surfaces in sizing described smaller systems at appropriate storm frequencies. Note that the aforementioned option (2) shall not cause the design of structures or conduits requiring the more intensive hydrologic methods to be based on the less intensive methods.
(2) With use of TR-20 methodology, which is encouraged as it facilitates estimation of detention/retention requirements and can be used for both smaller areas within the subdivision or development as well as those requiring created head calculations, the ISWS’s Circular 172 and Bulletin 70 data shall be utilized. The critical duration of storm shall be determined with trial runs, for a representative drainage sub-area within the new subdivision or development and this duration may be used for other similar sub-areas within the new subdivision or development. For areas beyond the new subdivision or development, unless these are similar to those within the new subdivision or development, separate trial runs for determination of the critical duration shall be made.
(E) Storm frequencies and allowable water surfaces. Storm event frequencies from the IDOT Drainage Manual shall be applied as follows:
(1) Existing and proposed ditches shall be designed for the 50-year event such that flows do not exceed defined banks. The exception is that existing and proposed ditches draining municipal-jurisdictional areas and larger areas and those intercepting commercial, industrial, or multi-family area discharges shall be designed for the 100 year event such that flows do not exceed defined banks. Ditches and swales, other than for those draining municipal-jurisdictional areas and larger areas shall be designed such that the depth of flow does not exceed nine inches at said 50 year events. Ditches and swales at boundaries where off-site sheet flow is intercepted and channelized shall be designed such that there is no increased water surface depth at any point beyond the boundary for any storm event.
(2) Proposed ditches and swales at boundaries shall direct intercepted runoff to culverts or storm sewer systems. These cannot be directed towards proposed pavement and pavement inlets near the boundaries.
(3) Culverts that do not intercept jurisdictional and municipal-jurisdictional area runoff shall be designed for capacity only for the 50-year event. Note definitions of culverts: continuous conduit systems devoid of an inlet at the upstream end. This includes the entire downstream storm sewer system that may be connected at the downstream end of the culvert.
(4) Culverts, storm sewers, or other structures such as bridges intercepting runoff from municipal-jurisdictional areas shall be designed for a 100-year event capacity such that allowable created headwater requirements are met for the same event; i.e. the increased water surface at the upstream face must be 0.50 feet or less above the natural water surface and such that there is no more than a 0.10 foot increase in the natural water surface elevation 1,000 feet upstream of the upstream face. Additionally, the allowable freeboard from design headwater to edges of pavement in the vicinity of the culvert shall be a minimum of two feet and this shall be based not on natural conditions downstream, but existing conditions - with existing downstream culverts and other structures in place. Proposed structures shall be considered existing in calculating allowable freeboard. Freeboard requirements shall be required for any structure, regardless of its position in relation to the subdivision boundary due to storage considerations, unless comprehensive unsteady flow time effects are being modeled and compared for within the entire subdivision and incoming flows. No flood easements shall be obtained to effect these requirements.
(5) Culverts or other structures draining municipal-jurisdictional areas coincident with jurisdictional floodplains shall have 0.0 created head.
(6) Gutter capacities, inlet spacing determinations, inlet capacities, and storm sewer piping systems shall be designed for the ten-year event. IDOT Drainage Manual requirements for storm sewer systems draining sag locations in pavement to be designed for 50 year events shall not be required except for roadways requiring review by IDOT. Allowable encroachment, measured from the joint of edge-of-pavement and the street-side edge of curbing into the pavement shall be as follows:
(a) For streets without parking, with one lane in each direction, encroachment shall be allowed to within ten feet either side of the roadway centerline.
(b) For streets with adjoining parking lanes, encroachment shall be limited to the parking lanes.
(c) For streets without parking, with more than one lane each direction, encroachment shall be allowed such that 11 foot driving lanes devoid of encroachment flows are maintained either side of the roadway centerline.
(7) Bridge and culvert hydraulic grade lines (HGL’s) shall be calculated for the ten and 100 year events, minimum. Storm sewer HGL’s shall be calculated for the 10 and 100 year events, minimum. HGL’s shall remain one foot below tops of inlets at the applicable design frequency. Checking the 100 year event is primarily intended for evaluating whether inhabitable shelters are adequately above 100 year water surfaces but it is also required indirectly for design of detention systems. When HGL’s for the 100 year exceed inlet tops and increases in pipe sizes cannot be made to lower HGL’s, the water surface occurring at these inlet locations on the surface shall be analyzed in greater detail by calculating surface depths in the contributing swale, ditch, topography, and gutter area. Inlet capacity calculations may or may not address the aforementioned calculations in that the adjoining topography may not have been included in the inlet capacity calculations previously.
(8) Inlets other than those in pavements shall be designed such that depth of flow at the surface does not exceed nine inches.
(9) Detention or retention facilities shall be designed to attenuate discharges from them for the ten and 100-year events.
(F) Hydraulics. The hydraulic methods considered applicable for particular size areas, as noted in the IDOT Drainage Manual, shall be utilized, except for the following requirements:
(1) Hydraulic grade lines (HGL’s) for all ditches, channels, and conduits draining jurisdictional, municipal-jurisdictional, and larger areas shall be calculated utilizing USACoE’s computer software HEC-RAS, or equivalent. In the event unsteady flows are analyzed with this software, such that higher frequency events are analyzed coincident with those for other smaller conduit and ditch systems that enter these larger ditches, channels, and conduit, throughout the development (e.g. ten-year), then HGL’s for said smaller conduit and ditch systems can be based on the higher frequency water surfaces, otherwise, the smaller systems hydraulics shall be based on the 100-year water surfaces of said jurisdictional and municipal-jurisdictional systems, i.e. 100-year tailwaters input for said smaller systems.
(2) As noted in previous sections of this Code, the most conservative interpretations will be made for existing and future backwater effects downstream and upstream and determinations generating the highest water surfaces occurring in the subdivision or development shall be used.
(3) HGL’s for all other conduits other than those for jurisdiction and municipal-jurisdictional areas shall be calculated using gravity and not pressure flow to meet freeboard requirements.
(4) Computer software functioning primarily to generate hydrographs but with some hydraulic capabilities such as routing through conduits, detention, and the like, may be utilized to address time effects throughout systems, however, it shall not be used for calculation of continuous HGL’s throughout proposed and existing drainage systems unless it can fully link individual components of the systems such that the user does not need to manually input tailwater elevations for each component.
(5) The minimum slopes for all conduits shall be 0.50%.
(6) Gutter encroachment and capacities for all sections of pavement shall be calculated and these shall be in accordance with IDOT requirements.
(7) Inlet capacities shall be calculated for all inlets and these shall be in accordance with IDOT requirements. Weir flow, and not orifice flow, shall be the basis for determining inlet capacities.
(G) Placement, appurtenances, materials, and backfilling. The following pertain to systems the city will have maintenance responsibility for, including connections to said systems within easement and right-of-way).
(1) Placement.
(a) Clean out structures such as manholes with a minimum four foot inner diameter or larger area shall be placed a minimum of 300 feet apart along all buried conduits, including box culverts.
(b) The minimum inlet spacing for commercial and industrial areas along dedicated streets, one side, shall be 300 feet, anticipating changes in developments.
(c) Curb opening inlets with no flag grates shall not be used at non-sag locations.
(d) A minimum of two grate inlets, if utilized, will be required at sag locations. These may be connected with conduit under curb but this distance shall not exceed 15 feet.
(e) No grates shall extend into pavement, beyond curbing, unless it is shown that other type inlets cannot be utilized.
(f) Drainage conduits paralleling streets shall not run under pavement and shall be beyond the back of curb.
(g) Pavement inlets connecting to main conduits beyond the back of curb shall not require manholes for connection at the main conduits if the connecting run of conduit is less than four feet. A tee in the main may be utilized. Two sag inlets can be connected with a single conduit run to the main utilizing a tee.
(h) Dual and multiple pipe culvert runs are allowed, however, to improve hydraulics, the influents or effluents of these shall be tied with a cast-in-place concrete headwalls rather than flared end sections (see appurtenances).
(2) Appurtenances.
(a) All storm sewer appurtenances or structures such as inlets and manholes maintained by the City shall be cast-in-place, concrete masonry, or precast concrete. Other type materials will not be accepted. These shall be designed and constructed in accordance with IDOT requirements where applicable, otherwise in accordance with ASTM C478 or C913.
(b) All detention facility appurtenances shall be cast-in-place or precast concrete.
(c) All manholes and all inlets with flat concrete slabs shall have access frames and lids. These shall not be constructed of concrete.
(d) All structures larger than two foot inner diameter at the base shall have standard IDOT steps cast/formed into the walls for access.
(e) Grated inlets shall be pedestrian safe in areas having pedestrian access.
(f) Trench grates shall be cast-in-place.
(g) Standard IDOT inlet types and grates shall be utilized wherever possible to facilitate future maintenance by the city. All grated inlets shall be pedestrian safe. The city reserves the right to require specific inlet and grate types through the City of Waterloo Construction Details.
(h) Horizontally offset and/or curb opening inlets with grates in the flag will be allowed if:
1. the capacities for these type inlets are calculated in accordance with the Drainage Manual’s curb opening inlets on grade procedures, including the intercept equation Q = 0.80L(a+y)3/2 where L=length of clear opening at the gutter flow line; a = depth of depression below normal gutter flow line, occurring between curb flow line and where free fall into the inlet occurs; y=depth of flow in approach gutter.
2. These are constructed per the City Construction Details with the added stipulations that the adjoining curb shall not be placed higher than the height shown on the typical section for the roadway in order to accommodate the inlet grate or flat slab top, the apron to the inlet is a minimum of eight inches thick, the flat slab does not protrude beyond the projection of the back of curb, there is a minimum of three inches of opening into the inlet, there is a wall in the inlet supporting the flat slab every 3.5 feet, and the inlet’s flat slab top is properly aligned with sidewalk such that there are no parallel joints between the flat slab and the sidewalk and such that there is no disruption in the vertical alignment of the sidewalk.
(i) Precast reinforced concrete flared end sections and poured toewalls are required at all single pipe inlets and outlets, at a minimum. Connections to non-reinforced concrete pipe, as needed, shall require cast concrete collars positioned a minimum of eight feet back from the flared end section. Pipe between the flared end section and the collar shall be IDOT Class A (concrete) pipe.
(j) Cast-in-place, reinforced concrete headwalls per IDOT standards, or specially designed with a structural engineer’s signed and sealed drawings, are required for all dual or multiple pipe culvert runs.
(3) Conduit materials.
(a) Corrugated metal pipe systems including culverts shall not be utilized for bridges, culverts, conduits, and structures.
(b) IDOT Class A storm sewer pipe shall be utilized for all storm sewers, culverts, and storm outlet structures in detention or retention facilities when circular conduits are hydraulically adequate.
(c) All circular conduits shall have an inside diameter equal to or greater than 15 inches.
(d) All conduits shall increase in inner area as a system progresses in a downward sloping direction regardless of hydraulic calculations.
(e) Soil conditions may preclude the use of precast concrete box culverts, in accordance with IDOT requirements and the geotechnical investigation.
(4) Backfilling.
(a) When trench limits of undisturbed soils extend within two feet of the back of curb, or the building edge of sidewalk, final backfill of the type shown on the City of Waterloo Construction Details.
(b) All overdig areas regardless of their limits, around appurtenances positioned as noted in (1) above shall be backfilled in similar fashion.
(c) All non-Class A pipe regardless of its location shall have aggregate bedding, haunching, and initial backfill a minimum of one foot above the top of the pipe within easement and right-of-way.
(H) Detention and retention facilities. In addition to the planning requirements of this code, more specific requirements are as follows. These shall apply to both existing facilities that will be utilized as retention or detention that were not utilized as such prior to subdivision and proposed facilities. Design of these facilities shall follow requirements herein and those of state agencies governing such, whichever is more restrictive. Features required for detention and retention facilities are applicable to either, unless noted otherwise. Geometric requirements generally only pertain to subdivision development and not development of single tracts.
(1) Retention and detention facilities.
(a) Facilities shall be designed such that there is no increase in velocity and flow from pre-development conditions at the point or points of discharge at boundaries of the tract for the detention facilities. This is for all storm events between the two-year and 100-year events. Drainage calculations shall also show conditions for the ten-year and 50-year events. Outletting of these intermediate events via appurtenances at the detention outlet structures is not strictly required but is recommended.
(b) Pre-development times of concentration shall be longer than post-development times of concentration for conversions of sheet and shallow concentrated type surface flows into those of shallow concentrated type in gutters. Post-development times of concentration through major storm sewer piping runs shall be utilized in calculating post- flows.
(c) Tailwaters downstream shall be calculated and utilized in determining the capacity of the existing or proposed facility.
(d) The 100-year water surface, that occurring above normal pool, shall be contained within the drainage easement area designated for the facility and shall not extend into buildable lot areas.
(e) A minimum freeboard of three feet is required above the 100-year water surface for tops of embankment.
(f) Banks shall not be greater than 3:1 near the high water areas and extending down into basins, except for retention facilities.
(g) A 3% sloped earthen plateau shall be provided around the entire perimeter of the 100-year high water elevation, three feet beyond side slopes required, a minimum of 12 feet wide to afford vehicular access, except in areas with paved accessways as prescribed in other sections of this code. These plateaus shall extend across any embankment acting as impoundment for the facility.
(h) All facilities of the open basin type shall have a defined emergency overflow area that shall be designed as a concrete spillway and/or walls with adequate protection against scour and frost heave. It shall be designed and detailed on the plans with said signed and sealed by a licensed structural engineer in Illinois.
(i) All perimeters of spillways, pipe intakes, standpipe structures, weirs, and other structures shall have rip rap placed alongside. Riprap shall extend fully into the lowest point of the basins either side of any embankment and to points beyond embankment where velocities are dissipated. The city reserves the right to modify proposed rip rap placement proposed on a case-by-case basis.
(j) A 20 foot minimum setback shall be required from all property lines to the detention outlet structures and/or toe of embankment at the outfall of the facility.
(2) Retention facilities.
(a) Existing ponds or lakes that are in the path of an existing ditch, stream, jurisdictional and municipal - jurisdictional drainageways, or base flood areas shall be improved to maintain existing normal pool elevations with no increase to said elevations due to increased discharge. Enlarging these with additional excavation will be required.
(b) Facilities shall be designed as detention facilities with no allowances for percolation due to the soils within the jurisdiction of this code.
(c) Normal pool elevation shall be considered the bottom for detention calculations.
(d) Facilities shall be designed to retain water to the normal pool elevation. The geotechnical report required for the development or subdivision shall also evaluate the need for basin liners to maintain said elevation.
(e) The geotechnical report required for the development or subdivision shall also evaluate the feasibility of expanding existing impoundment with elevation of earthen embankments if so required. Placement of earth on top of existing embankment will not be allowed without adequate preparation and integration with the underlying, existing embankment.
(f) Due to their nature, these facilities may or may not be modified to discharge at higher frequency events with the use of weirs, orifices, pipes, and other outlet structures. In the event that these are not modified with such, considering the need to retain storage capacity above normal pool discounting any percolation, a system of graduated spillways designed to discharge no more than the required storm events are considered the minimum modifications required.
(g) Facilities shall be a minimum of four feet in depth under normal pool, unless fish habitats are desired, for which the minimum depth of eight feet shall be required for a minimum of 25% of the bottom area to afford winter habitat.
(h) For basins eight feet in depth, 2:1 side slopes around the areas that are eight feet deep will be required.
(i) Shoreline stabilization shall be required around all facilities. This shall extend to a depth of one foot below the normal pool elevation and to an elevation of one foot above the normal pool elevation measured vertically. Riprap, cast-in-place or modular concrete block retaining walls are acceptable. When walls are utilized, their maximum height is limited to four feet. Other methods shall be reviewed by the city for approval.
(j) An outlet structure shall be designed and installed for dewatering of the pond for maintenance. Gravity dewatering mechanisms are preferred. These shall be designed and constructed with materials approved by the city on a case-by-case basis.
(3) Detention facilities.
(a) The minimum size of any outlet structure pipe shall be 12 inches. Galvanized or stainless steel orifice plates may be utilized for small development sites and subdivisions on upstream inflow pipes. These shall be adjustable with corrosion protected systems such that no cutting or disruption of the installation is necessary to make any adjustments. These shall be constructed so that they are accessible and so there is working room to perform adjustments if needed on the outside of the structure.
(b) Standpipe outlet structures, box structures within embankment with pipe and weir systems on the wet side of embankment and any other structures shall be constructed of material required by this code.
(c) A low flow channel shall be provided that is no less than a 0.40% slope. Said channel shall have a minimum two foot wide, eight inches thick concrete gutter constructed of the same mix for curb utilized in the IDOT Standard Specifications.
(Ord. 1069, passed 7-20-1998; Am. Ord. 1378, passed 6-26-2006; Am. Ord. 1527, passed 9-8-2009; Am. Ord. 1545, passed 4-5-2010; Am. Ord. 1597, passed 3-5-2012; Am. Ord. 1696, passed 3-21-2016) Penalty, see § 34-1-12