A. General Information: This chapter applies to hydrologic design for detention basins and other storm drainage facilities (storm sewers, swales, ditches, culverts, bridges) within the city limits and the one and one-half (11/2) mile extra territorial zoning jurisdiction.
Hydrologic design calculations are required and shall be reviewed by the city of Carbondale for all site plans submissions, developments, redevelopments, and subdivisions, and the like.
B. Referenced Standards: Design standards for hydrologic design shall comply with this code and with the provisions of the latest edition of the "IDOT Drainage Manual" otherwise stated herein.
C. Rainfall Data: Rainfall duration and frequency distributions shall be taken from Illinois state water survey (ISWS) bulletin 70 or ISWS circular 172 or most recent ISWS equivalent, unless the use of other local data is approved in writing by the city engineer.
D. Rainfall Recurrence Interval: The design rainfall recurrence interval shall be set by the design application as follows:
1. Detention: 10-year inflow, 5-year outflow.
2. Emergency overflow routing: 100-year.
3. Bridges: 100-year.
4. Underpasses: 50-year.
5. Swales, ditches, and culverts: 25-year.
6. Streets and storm sewers: 10-year.
E. Design:
1. Choose an applicable hydrologic design method according to the "IDOT Drainage Manual" figure 4-001. More complex hydrologic design methods, such as TR-20 or HEC-1, may always be substituted for less complex methods when selecting appropriate design methods.
2. Hydrologic design calculations shall include contour map(s) clearly showing the design drainage area(s), stormwater facilities, storm sewers, swales, ditches, culverts, bridges and receiving stormwater facilities. Critical spot elevations, invert elevations, pervious and impervious areas shall be clearly indicated. See subdivision regulations for other subdivision and site plan submittal requirements.
3. Submittals shall include design calculations, drainage basin areas and surface types, rainfall data used, critical storm duration, generated site specific hydrographs (i.e., predesign, postdesign, detention, postdetention, etc.) and justification of duration selection.
4. See section 15.4.4.5 of this chapter for detention routing and design requirements. (Ord. 2013-20)