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I I I I I I I I I I I I I I I I I I I overtop the roadway. Signficant detention volumes exist relative to the total storm runoff volume. Additional hydraulic analysis was perfOlmed for these cases to detlne the floodplain. Shay Ditch at 1-25 At this crossing the [-25 embankment creates an inadverdent storage volume of more than 150 acre-feet (AF) before overtopping 1-25. This storage volume is greater than the total 100-year runoff volume of 126 AF; thus, the entire 100-year runoff could be stored upstream of the embankment without overtopping 1-25 or creating transbasin flow. Be- cause of these conditions, the decision was made that the inadverdent detention storage would be considered in delineation of the floodplain. C. HYDRAULICALLY DISCONNECTED FLOW A flow split occurs when a portion of the flood flow completely sepa- rates from the main channel due to inadequate drainage facilities. [f the tlow: (I) does not return to the main channel, (2) is a signitlcant portion of the tlood peaks, and/or (3) is a potential flood hazard, then the condition requires an independent analysis. The SWMM input model was moditled to reflect the existing detention storage by adding the reservoir routing data. The maximum water sur- face elevation in the detention facility was computed for the various frequency storms. The tloodway limits were delineated based on the storage required to store the 100-year runoff with a I-foot rise in the 100-year water surface elevation. Downstream of the crossing, the peak flows used to detelmine the corresponding water surface elevations assumed that no storage exists at the interstate crossing. At all major and minor street crossings and the UPRR. the eXlstmg cul- verts are inadequate to pass the full 100-year flow under future devel- opment conditions; thus, the streets and railroad will be overtopped. At many of these crossings, the road profile and the surrounding topo- graphy are such that a pOltion of the 100-year runoff will overtop the roadway and flow into the main channel downstream while another portion of the flow will split from the main tlow and create a tloc;>dplain sepa- rate from the main channel floodplain. For example, the Holly Street crossing of Morris Creek and the Washington Street crossing of Shay Ditch create areas of hydraulically disconnected flow. South Fork Preble Creek at [-25 A similar situation occurs at the [-25 crossing of South Fork Preble Creek. The storage volume upstream of the crossing, however, will not contain the entire 100-year runoff volume without split flow occurring. Approximately 60 AF of storage is available upstream of the crossing compared to 72 AF of runoff volume during the 100-year storm. Because of the storage available relative to the total lllnoff volume and the split flow situation, the same philosophy as described above was fol- lowed. The SWMM model was moditled to reflect the storage at the crossing and to determine the corresponding water surface elevations for the various frequency storms. The floodway limits were delineated based on the storage volume required to obtain a I-foot rise in the 100-year water surface elevation. In all cases, the water sUlface elevations In the downstream channel were based on tlows that assume no hydraulically disconnected tlow. The f100dway limits at these locations assume that improvements are made that would contain the full flow in the main channel. The flood- plains created by the split tlow conditions were delineated and are shown as hydraulically disconnected flow. D. FLOODWAY The f100dway is the main channel of a stream. plus those adjacent tloodplain areas that must be kept free of obstructions in order that the 100-year runoff be carried without signitlcantly increasing the water surface elevation. Based upon UD&FCD cliteria, the f100dway was detlned as being the limit of encroachment into the floodplain where the existing energy grade line profile IS not increased more than 0.5-fee!. The HEC-2 computer program was utilized to determine the f100dway limits. IV-3 IV-4