§ 16.30.110. Storm water control.

A.

Purpose. The purpose of this section is to diminish threats to public health, safety and welfare caused by the runoff of excessive storm water from new development. Among the adverse impacts sought to be avoided are the inundation of damageable properties, the erosion and destabilization of downstream channels, and the pollution of valuable stream and lake resources. More specifically, these are intended to:

1.

Assure that new development does not increase the drainage or flood hazards to others, or create unstable conditions susceptible to erosion;

2.

Protect new buildings and major improvements to buildings from flood damage due to increased stormwater runoff;

3.

Protect human life and health from the hazards of increased flooding on a watershed basis;

4.

Lessen the burden on the taxpayer for flood control projects, repairs to flood-damaged public facilities and utilities, correction of channel erosion problems, and flood rescue and relief operations caused by increased stormwater runoff quantities from new development;

5.

Protect, conserve, and promote the orderly development of land and water resources;

6.

Preserve the natural hydrologic and hydraulic functions of watercourses and floodplains and to protect water quality and aquatic habitats; and

7.

Preserve the natural characteristics of stream corridors in order to moderate flood and stormwater impacts, improve water quality, reduce soil erosion, protect aquatic and riparian habitat, provide recreational opportunities, provide aesthetic benefits, and enhance community and economic development.

B.

Methods.

1.

Design Methodologies. The developer may design major and minor conveyance systems for areas up to ten acres using the rational method. For developments greater than ten acres, the developer shall use one of the runoff hydrograph methods set out below. Notwithstanding the above, the developer shall design any detention basin using a runoff hydrograph method.

2.

Runoff Hydrograph Method. The developer will provide runoff hydrographs using the following assumptions concerning rainfall and antecedent moisture. The developer will use one of the following models to generate all hydrographs: HEC-], TR-20 or TR-55.

3.

Rainfall. The developer shall base the design rainfall events upon Bulletin 70 from the Illinois State Water Survey, Bulletin 71 from the Midwest Climate Center, or upon a continuous simulation model acceptable to the municipality. The developer shall design the conveyance systems and detention basins using rainfall distributions that correspond to the following critical durations as set out in Table 37 of Bulletin 70, or in accord with the SCS Type II distribution as built into the TR-55 Model.

Rainfall Distribution	Critical Duration
1st Quartile Point	0-12 hours
3rd Quartile Point	13-24 hours
4th Quartile Point	25 or more hours

 

4.

Antecedent Moisture. Computations of runoff hydrographs which do not rely on a continuous accounting of antecedent moisture conditions shall, at a minimum, assume a conservative wet antecedent moisture condition.

C.

Drainage Design Parameters.

1.

Storm Sewers.

6;Frequency—ten year return period

Duration—two hours

2.

Detention/Retention Basins.

Frequency—one hundred year return period

Duration—twenty-four hours

3.

The detention volume required shall be based on the one hundred year return period storm from the fully developed area (post developed) less the outlet release rate. The release rate shall be based on the two-year return period from the area prior to development.

4.

Detailed storm water computations for areas up to and including two hundred acres may be based on the Rational Formula or through the use of recognized computer programs. It is recognized that the design of a drainage system has many solutions.

5.

An earthen detention basin shall include, but not be limited to, the following appurtenances:

a.

Inlet structure with end-section, geo-textile fabric and rip-rap.

b.

Outlet structure similar to inlet structure.

c.

Maximum side slope of 4:1.

d.

Emergency overflow facility.

e.

Side slopes shall be covered with geo-textile fabric, rip-rap and/or gravel. Coverage shall extend all the way around the perimeter it shall be placed three feet beyond the toe of the slope as well as three feet beyond the top of the slope. Fabric shall be secured with pins. Geo-textile fabric shall be in accordance with ILDOT specification Section 913.18. Rip-rap and/or gravel shall be between three to six inches across or classified as six-inch hand lain rip-rap in accordance with ILDOT specifications.

(Ord. 02-13 § 1 (part), 2002).