[R.O. 1993 § 545.090; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
Drainage Area Plan. A plan of the drainage area at a scale of one (1) inch equals one hundred (100) feet with two-foot contour intervals using USGS datum for areas less than one hundred (100) acres or a plan of the drainage area at a scale of one (1) inch equals three hundred (300) feet with five (5) feet contour intervals for larger areas. This plan shall include all proposed streets, drainage and grading improvements with flow quantities and direction of flow at all critical points. All areas and subareas for drainage calculations shall be clearly distinguished.
Hydraulic Data. Complete hydraulic data showing all calculations shall be submitted. A copy of all nomographs and charts used in the calculations shall be submitted if other than those included herein are utilized.
Plan And Profile. A plan and profile of all proposed improvements at a scale of one (1) inch equals forty (40) feet horizontal and one (1) inch equals four (4) feet vertical shall be submitted this plan shall include the following:
Locations, size, flow line elevations and grades, type of pipe, channels, boxes, manholes and other structures drawn on standard plan-profile sheets;
Existing and proposed ground line profiles along the center line of the drainage improvement;
A list of the kind and quantity of material;
Typical sections and reinforcement of all boxes and channels;
Location of property lines, street paving, sanitary sewers and other utilities, both public and private.
Field Study. A field study of the downstream capacity of all drainage facilities and the effect of additional flow from the area to be improved shall be submitted. If the effect is to endanger property of life, the problems must be solved before the plan will be given approval.
Stormwater Flow Quantities. Stormwater flow quantities in the street shall be shown at all street intersections, all inlet openings, and at locations where flow is removed from the streets. This shall include the hydraulic calculations for all inlet openings and street capacities. Street flow shall be limited according to Table 1 at the end of Article II.
Sinkholes Or Karst Areas. Sinkhole or karst areas shall be clearly defined. If any portion of the stormwater from an area is to be drained into a sinkhole, all information available shall be obtained, and the capacity of the sinkhole shall be studied, and this study shall be submitted to determine the effect of the drainage and pollution on groundwater and streams.
Additional Information. Any additional information deemed necessary by the Building Official and/or Wastewater Superintendent for an adequate consideration of the storm drainage effect on the City of Aurora and surrounding areas must be submitted.
[R.O. 1993 § 545.100; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006; Ord. No. 2010-2871 § 1, 4-27-2010]
General Design Requirements.
All bridges shall be designed to accommodate a one-hundred-year frequency rain. Box culverts, pipe culverts, channels and ditches shall be designed to accommodate a one-hundred-year frequency rain at all locations having a drainage area in excess of one (1.0) square mile. Locations having a drainage area of less than or equal to one (1.0) square mile shall be designed to accommodate a twenty-five-year frequency rain.
Channel improvements types shall be as follows:
Improvements with a capacity of up to and including 100 CFS shall be (a) open with a channel lining that is shown to be capable of resisting erosive forces; or (b) closed conduit;
Improvements with a capacity above the 100 CFS up to and including 250 CFS may be open and (1) concrete paved invert; or (2) concrete lined; or (3) closed conduit;
Improvements with a capacity above the 250 CFS shall be open and (1) concrete lined; or (2) have a 100 CFS low-flow paved invert.
Specific Requirements For Various Improvements.
Bridges And Culverts. Bridges, box culverts or concrete pipe culverts shall be provided where continuous streets or alleys cross watercourses. The structure shall be designed in accordance with City specifications for material and to carry HS-20 loadings in all cases.
Closed Storm Sewers. Closed storm sewers shall either be reinforced concrete box or pipe of approved type designed for HS-20 loadings. Reinforced concrete pipe or reinforced concrete boxes must be used within two (2) feet of the back of the street curb and under paved areas. All storm sewers having trench walls within two (2) feet of the back of the street curb shall be backfilled with granular material. The use of corrugated steel, zinc-coated pipe and extra strength clay pipe will not be permitted within two (2) feet of the curb or under pavement areas. All pipe material shall meet the requirements of the latest revision of the City of Aurora standard general conditions and technical specifications for public works construction.
Grades for closed storm sewers shall be designed so that the velocity shall not be less than three (3) feet per second nor exceed twelve (12) feet per second. All other structures such as junction boxes or inlets shall be in accordance with the standard drawings adopted by the City of Aurora.
Closed storm sewers shall extend to the furthest downstream point of the development with consideration given to velocities and to providing discharge energy dissipaters to prevent erosion and scouring along downstream properties.
Open Paved Concrete Channels. Grades for open paved channels shall be designed so that the velocity shall not be less than three (3) feet per second nor exceed twelve (12) feet per second. Such concrete channels may be of different shapes according to existing conditions; however, a channel with a flat bottom and 4:1 to 5:1 side slopes is the most desirable type and shall be used whenever possible. The thickness of channel paving shall depend on conditions at site and size of channel; however, a minimum thickness of six (6) inches is required. A six-inch freeboard must be provided. An eighteen-inch toe wall is required at both the outlet and inlet ends of the channel.
Open Ditches (Earth Channels). Ditches shall have a gradient that limits the velocity within one and one-half (1.5) to five (5) feet per second depending on existing soil conditions. Such ditches shall have a minimum side slope ratio of 3:1. The designer's attention is directed to the fact that the subdivision ordinance prohibits encroachment of buildings and improvements on natural or designated drainage channels or the channel's floodplains. Such floodplains are areas of land adjacent to an open paved channel or an open sodded ditch that may receive a flood condition from a one-hundred-year frequency rain. The limits of such floodplains shall be indicated on drainage improvements plans.
[R.O. 1993 § 545.110; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
The rate of runoff concentrated at any point shall be determined by the Rational Formula:
Runoff Coefficient. The runoff coefficient "C" is the variable in the rational formula least susceptible to precise determination and the one (1) that requires the greatest exercise of engineering judgment because of the many area characteristics which affect the coefficient. Among the factors to be considered in influencing the runoff coefficients are the following: present and future zoning; terrain; local ponding or depressions; the amount of pavement; roofs, turf, and other areas having different degrees of imperviousness.
The selection of a coefficient should take into consideration the probable ultimate development of presently undeveloped areas. Suggested values of runoff coefficients are included in the following table:
Rainfall Intensity. The average frequency of rainfall occurrence used for design determines the degree of protection afforded by a drainage system. Maximum intensity of rainfall of a given expectancy is grater for a short period of the time than for longer periods. Therefore, it is assumed that the maximum runoff will occur as soon as all parts of the drainage area under consideration are contributing. The length of time from the beginning of rainfall until runoff from the most remote point in the drainage area reaches the point under consideration is a channel or gutter flow time. Nomographs which may be used for determining time of concentration are reproduced at the end of this Chapter. Once the time of concentration is known, the design intensity rainfall may be determined from the rainfall intensity curves developed from United States Weather Bureau data.
[R.O. 1993 § 545.120; Ord. No. 99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
The size of closed storm sewers, open channels, culverts and bridges shall be designed so that their capacity will not be less than the runoff computed by using the Manning Formula: