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SECTION 3 – STORMWATER <br />21 <br />SECTION 3 – STORMWATER <br />3-1 STORMWATER QUANTITY MANAGEMENT <br />3-1.1 STORMWATER STORAGE POLICY <br />This manual was created to assist engineers planning and designing non-residential <br />(re)development plans. This manual addresses the minimum stormwater quantity design <br />requirements. This manual does not address stormwater quality impacts. However, minimizing a <br />site’s impact to our community waters is strongly encouraged and shall be considered when <br />reviewed by City staff. When unexpected or complex site issues arise, the applicant/applicant’s <br />designer is responsible for identifying such conditions in advance and shall obtain approval of the <br />City Engineer for approach before continuing design. <br />It is recognized that most streams, drainage channels and storm sewer systems in the City of South <br />Bend do not have sufficient capacity to receive and convey stormwater runoff resulting from <br />continued urbanization. This manual is meant to reduce negative impacts of flooding on private <br />property, local infrastructure, and natural resources. Accordingly, the storage and controlled <br />release of excess stormwater runoff shall be required for all developments and redevelopments <br />where land disturbance is greater than 2,500 square feet. <br />3-1.2 CALCULATION OF REQUIRED STORAGE VOLUME: SIMPLE METHOD <br />Sites less than 5 acres releasing by infiltration and evaporation only, that do not have a concentrated <br />outfall and that are not located in a wellhead protection area are eligible to use the Simple Method <br />to calculate the amount of required storage volume. Facilities shall be adequately sized to store the <br />entire volume from a 25-year, 24-hour rainfall without any release. All impervious areas, excluding <br />Public right-of-way shall be directed to the storage facility. The storage volume required shall be <br />calculated using the Simple Method as calculated in Table 1. The Simple Method storage volume <br />calculations shall account for off-site runoff that flows through the site. Note that the Simple <br />Method of Required Storage Volume is a special case of the Modified Rational Method, where the <br />release from the storage facility is not considered. If an Engineer wishes to consider the release of <br />infiltrated runoff in the sizing of a storage facility, the Standard Method in Section 4 shall be used. <br />Table 1: Storage Volume Required <br />Land Use <br />(1) <br />Area <br />(square feet) <br />(2) <br />Runoff <br />Coefficient <br />C <br />(3) <br />Rainfall Depth <br />25-yr, 24-hr <br />(inches) <br />Storage Volume <br />Required <br />(1)x(2)x(3)/12 <br />(cubic feet) <br />Impervious Surface 0.9 4.90 <br />Pervious (Grass) 0.2 4.90 <br />Total = cft <br />For sites greater than 5 acres or that have a concentrated outfall refer to Section 4 for Standard <br />Method design criteria. <br />3-1.3 CALCULATION OF DESIGN STORAGE VOLUME PROVIDED <br />Facilities shall be designed to store the volume required in Section 2. 6% of the calculated storage <br />volume shall be added to the total volume of the storage facility to account for siltation. Dry storage <br />facilities (detention basins, subgrade detention, etc.) shall be designed in such a manner that the