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(A) Applicability. In a conservation-designed subdivision as defined in the County Subdivision Ordinance, as codified in Chapter 154, a density bonus is granted for each acre (or fractional acre) of land utilized for stormwater infiltration. See § 154.268 of the subdivision regulations.
(B) Infiltration credit.
(1) To obtain the maximum infiltration credit, infiltration BMPs shall be designed, installed and maintained to infiltrate runoff to the maximum extent practicable in accordance with one of the following goals:
(a) Infiltrate sufficient runoff volume so that the post-development infiltration volume shall be at least 90% of the pre-development infiltration volume, based on an average annual rainfall for the region; or
(b) Infiltrate at least 25% of the post-development runoff volume from the two-year, 24-hour design storm with a Type II distribution. Separate curve numbers for pervious and impervious surfaces shall be used to calculate runoff volumes and not composite curve numbers.
(2) Natural conditions such as soil types and slopes will affect the ability of a site to meet these infiltration goals. Projects that are shown to meet these goals will be granted full (100%) credit (i.e., one acre of credit for one acre of land dedicated to stormwater infiltration). Projects that exceed these goals or partially meet these goals will be given credit according to the following table:
Goal #1 90% Average Volume % Calculated | Goal #2 25% 2-Year Volume
% Calculated | % Credit |
Goal #1 90% Average Volume % Calculated | Goal #2 25% 2-Year Volume
% Calculated | % Credit |
> 92.5% | > 26.0% | 110% |
90.0% to 92.5% | 25.0% to 26.0% | 100% |
85.5% to 89.9% | 23.8% to 24.9% | 95% |
81.0% to 85.4% | 22.5% to 23.7% | 90% |
76.5% to 80.9% | 21.3% to 22.4% | 85% |
72.0% to 76.4% | 20.0% to 21.2% | 80% |
67.5% to 71.9% | 18.8% to 19.9% | 75% |
63.0% to 67.4% | 17.5% to 18.7% | 70% |
58.5% to 62.9% | 16.3% to 17.4% | 65% |
54.0% to 58.4% | 15.0% to 16.2% | 60% |
50.0% to 53.9% | 14.0% to 14.9% | 55% |
< 50% | < 14.% | 0% |
(C) Pretreatment required. Before infiltrating runoff, pretreatment shall be required for parking lot and/or commercial area runoff that will enter an infiltration system. The pretreatment shall be designed to protect the infiltration system from clogging prior to scheduled maintenance and to protect groundwater quality. Pretreatment options may include, but are not limited to, oil/grease separation, sedimentation, biofiltration, filtration, swales or filter strips. To minimize potential groundwater impacts, it is desirable to infiltrate the cleanest runoff. To achieve this, a design may propose greater infiltration of runoff from low pollutant sources such as roofs, and less from higher pollutant source areas such as parking lots and commercial areas.
(D) Infiltration. Infiltration from areas requiring pretreatment and regional devices (facilities with a drainage area of five acres or greater) for infiltration from residential development shall not be installed within 400 feet of a community water system well or within 100 feet of a private well.
(Ord. 10-164, passed 6-17-2010)
SEDIMENT AND EROSION CONTROL
(A) Applicability. No person shall commence or perform any clearing, grading, stripping, excavating or filling of land without having first obtained a site development permit from the Development Review Division of the County Land Use Department. Requirements for a site development permit are found in §§ 164.080 through 164.085 of this chapter.
(B) Site planning. Sediment and erosion control planning shall be part of the initial site planning process. In planning the development of the site, the applicant shall consider the susceptibility of existing soils to erosion and topographic features such as steep slopes and stream corridors which must be protected to reduce the amount of sediment and erosion which occurs. Where appropriate, existing vegetation shall be protected from disturbance during construction by fencing or other means.
(C) Planning process. In the planning process, the applicant shall also address the following.
(1) For projects that involve phased construction, existing land cover for those areas not under current development shall be addressed. If existing land cover does not consist of an appropriate ground cover then these phases shall be planted temporarily to reduce erosion from idle land.
(2) In planning the sediment and erosion control strategy, preference shall be given to reducing erosion rather than controlling sediment. In order to accomplish this, the plan must carefully consider the construction sequence of the phases so that the amount of land area exposed to erosive forces is the minimum consistent with completing construction.
(Ord. 10-164, passed 6-17-2010)
(A) Specifications for erosion control measures shall be in accordance with the Illinois Urban Manual (2002) or latest edition. Sediment and erosion control planning shall be in accordance with Procedures and Standards for Urban Soil Erosion and Sedimentation Control in Illinois (revised July, 1988) by the Urban Committee of the Association of State Soil and Water Conservation Districts (The Green Book) Chapters 1 through 5. Where the Illinois Urban Manual supersedes sections of The Green Book, the Illinois Urban Manual shall prevail.
(B) All projects that will result in the development of one acre or more, except for agricultural projects regulated solely under § 164.024, shall be required to obtain coverage under an appropriate NPDES permit and shall submit an application for a NPDES permit or a notice of intent (NOI) to be covered under a general NPDES permit. The developer shall certify to the Chief Subdivision Engineer that all required permits, plans and inspections have been prepared and maintained in accordance with the NPDES permit. Specifically, the developer shall prepare and adhere to a stormwater pollution prevention plan (SWPPP) prepared for the development project and shall meet the conditions in the permit for SWPPPs. A copy of such plan shall be provided to the Chief Subdivision Engineer prior to commencement of development activities and a copy shall be maintained on the construction site at all times.
(Ord. 10-164, passed 6-17-2010)
The runoff from disturbed areas shall not leave the development site without first passing through sediment control facilities. This requirement shall apply to all phases of construction and shall include an ongoing process of implementation of measures and maintenance of those measures during both the construction season and any construction shut down periods.
(Ord. 10-164, passed 6-17-2010)
The condition of the construction site for the winter shut down period shall address proper sediment and erosion control early in the fall growing season so that slopes and other bare earth areas may be stabilized with temporary and/or permanent vegetative cover. All open areas that are to remain idle throughout the winter shall receive temporary erosion control measures including temporary seeding, mulching and/or erosion control blanketing prior to the end of the fall growing season. The areas to be worked beyond the end of the growing season must incorporate soil stabilization measures that do not rely on vegetative cover such as erosion control blankets and heavy mulching.
(Ord. 10-164, passed 6-17-2010)
(A) In the hydraulic and hydrologic design of major erosion control measures (those whose tributary drainage area is greater than three acres) such as sediment basins and traps, diversions and the like, the design frequency shall be commensurate with the risk of the design event being exceeded.
(B) The following design frequencies shall be regarded as minimum design frequencies for the construction period.
(1) For those projects whose construction period is less than six months, then the storm event having a 50% chance (two-year event) of being exceeded in any year shall be used for design purposes.
(2) For those projects whose construction period is greater than six months but less than one year, the design frequency for major sediment basins shall be a rainfall event with a 20% (five-year event) chance of being exceeded in any one year.
(3) For those construction projects expected to last more than one year, major sediment basins shall be designed for a rainfall event with a 10% (ten-year event) chance of being exceeded in any one year.
(Ord. 10-164, passed 6-17-2010)
(A) The sediment and erosion control plan shall designate a series of practices which shall be implemented either at the direction of the permittee or the permittee’s representative on-site or at the direction of the Chief Subdivision Engineer should an inspection of the site indicate a deficiency in soil and sediment erosion control measures.
(B) As a minimum, these measures shall include the following:
(1) Sedimentation basins;
(2) Sediment traps;
(3) Diversion swales;
(4) Silt fences;
(5) Temporary seeding;
(6) Mulching; and
(7) Erosion control blankets.
(Ord. 10-164, passed 6-17-2010)
(A) Detailed construction phasing plan identifying sediment and erosion control measures to be in place for each phase shall be submitted prior to stripping the site of existing vegetation or cover.
(B) Sediment and erosion control measures to be installed initially prior to stripping existing vegetation or mass grading shall be indicated on the plans.
(C) Permanent stabilization measures shall be indicated on a separate plan.
(D) The expected two-year and ten-year runoff rates from all off-site areas draining into the site shall be identified on the plan.
(E) Methods for conveying flows through the site during construction shall be indicated. These methods must include the temporary and permanent stabilization measures to be used to reduce velocity and erosion from flow through the construction zone.
(F) A maintenance schedule of each measure used shall be indicated on the plan. As a minimum, all sediment and erosion control measures on-site shall be inspected weekly and also by the applicant’s designee or after a one-half inch or greater rainfall event and any required repairs shall be made to keep these measures functional as designed. All plan modifications shall be reviewed by the Chief Subdivision Engineer or his or her designee. A record of all repairs and modifications shall be maintained by the applicant and available for review by the Chief Subdivision Engineer or his or her designee.
(Ord. 10-164, passed 6-17-2010)
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