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IML
<br />1.5
<br />1.9
<br />2.1
<br />2.4
<br />2.7
<br />3.0
<br />3.4
<br />3.7
<br />...
<br />....... ..................................
<br />...................
<br />.........�,,
<br />2.4
<br />3.1
<br />3.7
<br />4.4
<br />5.0
<br />5.7
<br />6.8
<br />7.7
<br />„ .,.
<br />,,,� , ...................
<br />r1 ........
<br />...................
<br />..
<br />.................
<br />;8 ;;
<br />,,.,
<br />........
<br />,, ,,,,,,
<br />A .
<br />,,,,,,,,
<br />3.2
<br />4.4
<br />5.3
<br />6.6
<br />7.7
<br />9.1,,,,,,
<br />11.2
<br />13.1
<br />3
<br />,,,,,,,,,,,,,,,,,,,,,,,,,,,
<br />:4
<br />. ,,......
<br />8:CS'
<br />,,,,
<br />„ ,,,
<br />11:1
<br />, ,,,,,,,,,
<br />1 .
<br />,,,,,,,
<br />1 .............
<br />,,,,,,,,,
<br />4.5
<br />6.4
<br />7.8
<br />9.8
<br />11.6
<br />13.5
<br />16.6
<br />19.3
<br />r
<br />...7.,,, ; ...
<br />,
<br />0 c
<br />i 1.
<br />1 ;r1
<br />181 .0:7
<br />.
<br />........
<br />6.2
<br />8.7
<br />10.,5
<br />12.,9
<br />14.8
<br />16.9
<br />19.8
<br />22.3
<br />The Rational Method Runoff Coefficient for each drainage area was calculated with
<br />land cover imperviousness data downloaded from the National Land Cover Database
<br />and verified with the latest aerial photographs. The average percent imperviousness
<br />was converted to a runoff coefficient (C) value decimal with 100 percent impervious
<br />equal to a 0.9 C value and zero percent impervious equal to 0.2 C-value.
<br />In order to fully integrate the system and define the overall drainage impacts, an
<br />XPSWMM model was set up with links and nodes representing the drainage areas,
<br />and the drainage structures.
<br />The Green-Ampt Method was used to calculate infiltration losses, and Harris County
<br />Region 3 loss parameters from HCFCD design manual were used. Table 4 presents
<br />the parameters.
<br />TABLE 4 - HARRIS COUNTY HYDROLOGIC REGIONY 3 LOSS PARAMETERS
<br />IIN u�m �� IUM �n6Ilun I.. Im �� IIunm lumi Im
<br />YI I�IIPIIII IIII I IIII II IIIIIIIfl ll�i �II� �III�I �iIII I ya, , .:::::...�. 1 �.....
<br />!lww!!l�l,lUi�!uu!IuIIIIII �IIIII III IIIII IIII �II��I IIIIII� IIII Ills IIII 0.385
<br />IIIIIIIIIIIIIIIIIIIIIIIIIIIII ll�i III �iIII I 'flL
<br />llln �llil III �I�III IIIIII lu' III VI II III ul II IIII III IIII 0.024
<br />For each individual drainage area, the Clark Method was used to develop a
<br />hydrograph. Time of Concentration (TC) was calculated using the Kerby-Kirpich
<br />Method, per the TxDOT Hydraulic Design Manual Chapter 4 Section 11.
<br />To„ = overland f low time (min) = K(L x N)o.467S-o.235
<br />K = 0.828
<br />L = overland f low length (f eet)
<br />N = dimensionless retardance coefficient = 0.15
<br />Page 118
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