Section 98-111. Surface and storm drainage.
(a) Request for approval of subdivision preliminary plat plans shall be accompanied by a comprehensive study of the surface and storm drainage system. In making this study, a modified formula may be used in computing the amount of stormwater to be carried by the drainage system. The formula to be used is as follows:
Q = CIA
In which:
TABLE INSET:
| width="50%" VALIGN="TOP">
Q |
width="50%" VALIGN="TOP">
=stormwater runoff. |
| width="50%" VALIGN="TOP">
C |
width="50%" VALIGN="TOP">
=runoff coefficient representing the ratio of runoff to rainfall. |
| width="50%" VALIGN="TOP">
I |
width="50%" VALIGN="TOP">
=of rainfall, or rate of rainfall in inches per hour. |
| width="50%" VALIGN="TOP">
A |
width="50%" VALIGN="TOP">
=area in acres. |
In the use of this formula, the following ratio of imperviousness zoning classifications shall be taken:
TABLE INSET:
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commercial.......... |
width="50%" VALIGN="TOP"> |
| width="50%" VALIGN="TOP">
Residential and industrial areas.......... |
width="50%" VALIGN="TOP"> |
| width="50%" VALIGN="TOP">
parks and undeveloped areas.......... |
width="50%" VALIGN="TOP">
5 |
Since the coefficient of runoff is a combination of these factors, together with other factors such as retention, imperviousness, etc., the following coefficients of runoff may be used in the study:
Coefficient of Runoff C
TABLE INSET:
| width="14%" VALIGN="TOP">
of Storm (minutes) |
width="86%" VALIGN="TOP" COLSPAN=10> | |||||
| width="14%" VALIGN="TOP">
Area |
width="14%" VALIGN="TOP" COLSPAN=5> | width="14%" VALIGN="TOP"> | width="14%" VALIGN="TOP"> | width="14%" VALIGN="TOP"> | width="14%" VALIGN="TOP"> | width="14%" VALIGN="TOP"> |
| width="14%" VALIGN="TOP"> | width="14%" VALIGN="TOP" COLSPAN=5>
.548 |
width="14%" VALIGN="TOP">
.645 |
width="14%" VALIGN="TOP">
.745 |
width="14%" VALIGN="TOP">
.836 |
width="14%" VALIGN="TOP">
.955 |
width="14%" VALIGN="TOP">
.975 |
| width="14%" VALIGN="TOP">
Residential |
width="14%" VALIGN="TOP" COLSPAN=5>
.356 |
width="14%" VALIGN="TOP">
.440 |
width="14%" VALIGN="TOP">
.502 |
width="14%" VALIGN="TOP">
.576 |
width="14%" VALIGN="TOP">
.680 |
width="14%" VALIGN="TOP">
.740 |
| width="14%" VALIGN="TOP">
Undeveloped |
width="14%" VALIGN="TOP" COLSPAN=5>
.237 |
width="14%" VALIGN="TOP">
.316 |
width="14%" VALIGN="TOP">
.369 |
width="14%" VALIGN="TOP">
.422 |
width="14%" VALIGN="TOP">
.522 |
width="14%" VALIGN="TOP">
.617 |
(b) The system shall be designed by accepted engineering principles for design floods resulting from rainstorms of the maximum intensity predicted for the county for at least the 25-year storm of 24-hour duration for major water management facilities, and for at least the ten-year storm of 24-hour duration for all other facilities. The system shall be designed for 20-year life, low maintenance cost and ease of maintenance by normal maintenance methods. The city may require such data from the applicant as is determined necessary to prove the adequacy of the design of the water management system.
(c) The storm drainage plan may include the use of street curbs and gutters and open ditches as transportation arteries for the stormwater, provided the length and grade of the street is not too great, thereby permitting the accumulation of excessive amounts of water in the street proper. Culverts shall be used at all points where open ditches occur at street or driveway crossings. Underground storm sewers shall be used if the area to be drained is too large for normal sized drainage ditches. The decision in this respect shall be made by the planning and zoning board upon the recommendation of the city engineer.
(d) Outfall of the subdivision surface or storm drainage system shall be one of the following:
(1) Proper connection between the new subdivision drainage system and the existing city drainage system if, in the opinion of the city engineer, the existing city system is adequate.
(2) Proper connection from the new subdivision to existing natural drainage, such as a creek, river or low swampy area adjacent to the subdivision.
(3) Proper connection from the new subdivision to an existing drainage ditch or canal of the city, county or drainage district, provided written permission has been secured from the political unit having ownership of such drainage facility.
(4) If none of the methods of disposal of the stormwaters from the subdivision outlined in subsection (d)(1) through (3) of this section are readily accessible to the subdivision, it shall be the responsibility of the subdivider to provide a suitable positive outfall for the stormwaters of the new subdivision by securing permanent easements or the purchase of right-of-way for the construction of a suitable underground storm drain through private property, so as to connect the subdivision storm drainage system with a satisfactory disposal area. In addition, the cost of obtaining and installing such drainage right-of-way and structures shall be the responsibility of the developer.
(e) Materials for storm drainage systems shall be as follows:
(1) Pipe used in the construction of storm drainage systems shall be reinforced concrete, vitrified clay or bituminous coated corrugated metal pipe and pipe arch or cast iron pipe, or corrugated plastic pipe conforming to the "Roadway and traffic Design Standards," latest edition, as prepared by the state department of transportation.
(2) Catch basins and manholes shall be either masonry or reinforced concrete class B furnished with cast iron frame grating or cover, conforming to the state department of transportation "Roadway and traffic Design Standards," latest edition.
(3) The minimum size of storm sewers or culverts shall be 15 inches. Equivalent elliptical pipe sizes may be substituted based on the approval of the city engineer.