(A) It is recognized that the streams and drainage channels serving the drainage needs of the City of Crawfordsville may not have sufficient capacity to receive and convey stormwater runoff resulting from continued development. To address this condition, the storage and controlled release rate of excess stormwater runoff shall be required for any development, redevelopment and new construction located within the City of Crawfordsville. All drainage in the developed area must be confined and maintained on site through perimeter structures or appurtenances including swales and inlets, detention or retention storage basins.
(B) No improvement location permit will be issued by the city for construction or extension of any proposed or existing building until the required drainage plans have been approved in writing by the Superintendent of Stormwater, except for the following exemptions:
(1) Extension or replacement of any existing building that does not increase the existing rate of runoff.
(C) The release rate of storm water from development, redevelopment, and new construction may not exceed a two-year pre-developed rainfall event rate for a ten-year post-developed storm, or a ten-year pre-developed outlet rate for a 100-year post-developed storm.
(D) The developer must submit to the city detailed computations of runoff before and after development, redevelopment, or new construction that demonstrates that runoff will not be increased. Hydrograph techniques and computer modeling methods used to determine stormwater runoff shall be proven and accepted methods, and will be subject to approval of the Superintendent of Stormwater.
(E) These computations must show that the peak runoff rate after development for the 100-year return period storm of critical duration must not exceed the ten-year return period pre-development peak runoff rate. The critical duration storm is that storm duration that requires the greatest stormwater storage. The top of the embankment must provide a two-foot freeboard above the elevation of the 100-year routed pool level. In addition, the emergency spillway must be designed so that it becomes operational at or above the elevation of the routed 100-year storm. The 100-year storm must be routed through the pond with the primary spillway inoperative.
(Ord. 2-2010, passed 3-8-10)