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<br />correlation of discharge estimates for each of the outlet pipes, For the 36-inch outlet pipe, an
<br />equivalent Manning's n value of 0,0194 was calculated; a calibrated value of 0,0215 was used,
<br />For the northern and southern 84-inch outlet pipes, an equivalent Manning's n value of 0.0358 was
<br />calculated; a calibrated value of 0,0404 was used. For the middle 84-inch outlet pipe, an
<br />equivalent Manning's n value was calculated to be 0,0358, a calibrated value of 0,0422 was used.
<br />The orifice discharge coefficient for the inlet grate was modified slightly from 3,74 to 3,75; this
<br />resulted in a value for C.A of 8,84, The calibrated parameters represent changes from the
<br />physically-based parameters of no more than 18 percent,
<br />Results of the EXTRAN model, with the final calibrated Manning's n values and orifice
<br />discharge coefficient, indicate a 100-year water surface elevation upstream of the BNRR of 4993,5
<br />ft, NGVD, which is conunensurate with the results of the effective SWMM model. The total 100-
<br />year discharge though the BNRR embankment, as computed by EXTRAN is 1,494 cfs, which is
<br />29 cfs, or 2 percent greater than the discharge of 1,465 cfs given in the effective SWMM model.
<br />As shown in Table 4,1, similar accuracy in representing SWMM results with the EXTRAN model
<br />was achieved for the 10-, 50- and 500-year events, Output for the calibrated EXTRAN model is
<br />provided in Section A of the Technical Addendum,
<br />
<br />Table 4,1
<br />
<br />Comparison of Calibrated EXTRAN Modeling Results to the Effective SWMM
<br />Model.
<br />
<br /> Maximum Ponded Maximum
<br />Return Period Water Surface Elevation Difference Release Rate Difference
<br />(Years) (ft, NGVD) (feet) (cfs) (%)
<br /> SWMM EXTRAN SWMM EXTRAN
<br />10 4989,\ 4989.2 +0,\ \,008 970 -3,8
<br />50 4992,2 4992,2 0,0 \,335 1,333 -0,\
<br />\00 4993,5 4993,5 0,0 1,465 1,494 +2,0
<br />500 4995,9 4995,8 -0,) \,677 1,758 +4,8
<br />
<br />4.1.2 Post-Proiect Condition EXTRAN Model
<br />
<br />The calibrated pre-project condition EXTRAN model was modified to reflect the physical
<br />changes to the detention system associated with the Centre A venue roadway extension project.
<br />
<br />T:\OPEN\cocsrtU 1 '..cocsrlD 1 fema-LOMR final-rpl.\\-pd
<br />
<br />16
<br />
<br />ANdERSON CONSulTiNG ENGiNEERS, INC,
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