[Ord. No. 777 §26-142, 3-23-1994]
Drainage Area Plan. A plan of the drainage area at a scale of one (1) inch equals one hundred (100) feet with two (2) foot contour intervals using U.S.G.S. datum for areas less than one hundred (100) acres or a plan of the drainage area at a scale one (1) inch equals three hundred (300) feet with five (5) foot 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 in this Article.
Plan And Profile. A plan and profile of all proposed improvements at a scale of one (1) inch equals fifty (50) feet horizontal and one (1) inch equals five (5) feet vertical shall be submitted. This plan shall include the following:
Location, sizes, 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 centerline of the drainage improvement.
A list of the kind and quantities of materials.
Typical sections and reinforcement of all boxes and channels.
Location of property lines, street paving, sanitary sewers and other utilities.
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 the endangerment of property or life, the problem 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 the requirements of Section 400.1230(C).
[Ord. No. 777 §26-143, 3-23-1994]
General Design Requirements.
All bridges shall be designed to accommodate a 100-year frequency rain. Box culverts, pipe culverts, channels and ditches shall be designed to accommodate a 100-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 25-year frequency rain.
Channel improvement shall be:
Such to prevent erosion while maintaining natural channels where possible. Velocities shall be low enough (less than five (5) fps) to prevent scouring. A series of detention structures, restrictions, etc., are recommended. Concrete lined channels and pipes shall be used when scouring velocities cannot be controlled.
Specific Requirement For Various Improvements.
Bridges and culverts. Bridges, box culverts or concrete pipe culverts shall be provided where continuous streets or alleys cross watercourses. The structures shall be designed to carry HS-20 loadings. Culverts and stormwater drainage pipes under public streets shall be Class 3 or 4 reinforced concrete pipe (RCP) with a minimum diameter of fifteen (15) inches or sized based upon a 25-year stormwater flow from the watershed, whichever is greater. Box culverts shall be designed for 100-year stormwater flow.
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.
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.
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 (6) inch free board must be provided. An eighteen (18) 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 (5.0) feet per second depending on existing soil conditions. Such ditches shall have a minimum side slope ration of 3:1. Encroachment of buildings and improvements on natural or designated drainage channels for the channel's flood plains is prohibited. Such flood plains are areas of land adjacent to an open paved channel or an open sodded ditch that may receive a flood condition from a 100-year frequency rain. The limits of such flood plains shall be indicated on drainage improvements plans.
Riprap shall not be used for erosion control on public rights-of-way. Concrete or a geo-fabric shall be used for erosion control at stormwater or culvert pipe outlets and at such other locations as determined or directed by the City Engineer or City Public Works Director.
[Ord. No. 777 §26-144, 3-23-1994]
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 which requires the greatest exercise of engineering judgment because of the many area characteristics which affect the runoff 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 area. 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. (See Appendix C, Drawing No. 1 to this Chapter)
Maximum intensity of rainfall of a given expectancy is greater for a short period of 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 called the time of concentration. This may include overland flow time and channel or gutter flow time. Nomographs which may be used for determining time of concentration are reproduced in this document. Once the time of concentration is known, the design intensity rainfall may be determined from the rainfall intensity curves included in this document.
Cross Reference — As to flow time chart, see Appendix C Drawing No. 2.
[Ord. No. 777 §26-145, 3-23-1994]
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 (see Appendix C Drawing No. 3 Manning's Formula):
Design Tabulations. For systems of storm sewers with inlets in various locations, the time of concentration at any point will be time of concentration at the most remote inlet upstream, plus the flow time in the storm sewer to the point under consideration. Computations for systems lend themselves readily to tabulation showing the drainage area, time of concentration runoff and capacity of each inlet and section of sewer under consideration. This data is to accompany the improvement plans.
Street Flow. Street flow shall be limited by pavement encroachment and depth of flow as indicated in the following table: