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I I I I I I I I I I I I I I I I I I I The 500-year flood profile is not computed because of a lack of topographic data. Therefore, no change is requested to be made to the SOD-year flood plain at this location. Also, because the loo-year flood plain is contained in the channel and the culverts, no encroachment run was performed to determine the floodway. We request that the 100-year floodway be coincidental with the 100-year flood plain. IV. SUMMARY This HEC-2 run is tied to existing cross-sections of the Flood Insurance Study (F.I,S,). AS shown below, the computed water surface elevations at the tie-in sections are very close: *Upstream tie--in: Section E Study Report This submittal F.r.S. 1982 request for Section No. 12.0 E FIRMA 12,0 Water Surface Elevation 372.13 372.15:1: 372,08 *Downstream tie-in: Section 8,0 of the F.I.S. (1977 plan and profile drawing) This submittal F. r.s. section 8.0 Section 8.0 362.32 362,0:1: Tie-in 100-year flood plain limits are plotted on sheet 4 of the drawings. According to the HEC-2 run output, the new box culverts under Sable Boulevard have the required capacity to pass the 100- year flow in Sable Ditch, The intersection of Sable Boulevard and 23rd Aven'.le is no long-er flooded in a 100-year storm event. Therefore, we request that a Letter of Map Revision be issued to change the lao-year flood plain limits at that intersection, between Section 8.0 and Section 12.0, It should be noted that Sable Ditch downstream of Section 8.0 (downstream end of the box culverts) was also improved and channelized and should be able to carry the lOa-year flood within the channel. But this will be addressed in a future request for flood plain revisions, at that time when the downstream culverts under Interstate 1-225 are improved, Prepared by: !VOZU'lfvC1.,o..f'"" I P ,::- . Hoanh Tran, P.E, Reviewed by: ~a/~?-E. Kevln Wegener, P,E. -4-