PERMFILE61158

• Permit No: C1981016
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Doc Name: Hydrology calculations
• Section_Exhibit Name: APPENDIX XXI Section XXI.9
• Media Type: D
• Archive: No

the ascend wren it is. There ie no difiereace in procedure for ESH sad TH:
Equations used in.the exaicples are listed is table 21.11.
• Example 21.5.-construct an ESH for a class (b) structure With a draia-
age arra of 1.86 square nile3, tics of coacent:ation of 1.2j hours,
CN of 82, and location at l.stitude_, lorgit:ade_
1. Determine the 6-hour deem storm rainfall amount, P. Fcr this '
structure class the ESH rai:~faL1 slaonat ie taken from ES-1020, ahaet
2 of 5. For the given loch+.ion the map shcve that P = 9.4 inches.
2. Determine the areal rairSi,1 amount. The areal rainfall is th.
same as in step 1 because the dra:nsge area ie not over 10 square ciies.
Step 2 of example 21.6 ehaie the process.
3. Make the duratlon ad.~uatneat of rainfall eaount. No adjustment is .
made because the t12e o_° coacentration is not over six hours. Ste? 3 .;
of example 21.6 show; the process.
4. Determine the ru?off amoun*. Eater figure ZO:Itvith P'. 9.4
inches and CN a 2 aid find Q 7.21 inches. ~ ~~;
5. Determine the hydro6s'sth fenny. Enter figure ~~3;'°(ES 1011) with ..;
CN a~2 and at P = 9. read hydrograph featly 2. ,.
6. Determine the duration of ~ccees rain.°all~T~o. Enter figure 2 :4~ ~•
(ES-1012 with P = 9. inch.a and at i;N ~ G2 road by interpoL3ti.on "'
• that To = 5.37 hours. -
7. Cocroute the initial va2L_ of TD. By equation 21.4 this is 0.711..Cj)
= O.~hours.
8. Compute the To/2~ ratio. This ie 5.370.88 = 6.10.
9. Select a revise3 To~'Ib ratlo fma table 21.16: This table sbo'as
the bydrograph fariliea and ra;,ios fo- which dimensionless hydro- •_ .
graphs are given in table 21.17. Ent°r table 21.16 vlth the ratio
from step 8 and select the tabulated ratio nearest it. For th:; ex-
ample the selected ratio, (TofTp)rev., is 6.
10. Compute Rev. TD. T'.~ie ie r, revised Tp used because of +.he ch:.aae;-
in ratio. By equation 21.5, Rev. Tp 5.pj7/6 = 0.895 boo=s.
11. Compute qD. E}r equation 21.6 this ie 484(1.86)/0.895 = 1005 c°s.`.-
12. Com-ute qo. Using the Q from step 4 and the qp frcm s*.ec 11 31ves
Q(qp ~ 7.21 1006) = 72j3.26 cos. Round to 7250 cfs.
13• Com-ute toe tree for vhich~hYdre rash rates will be -;~;t;i.
In equation 21.7 use Rev. Tp frcm step 10 and the entries _,z t~ ;,,!p?
column of the selected h~dro3^aph in table 21.17. The camyuted tiaes
are shown in column 2 of table 21.12.
•