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jaa046

Unit Hydrograph problem from CERM

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I am having little difficulty understanding Unit Hydrograph example problem #20.4 on page 20-9 from the civil engineering reference manual can anyone help me understand it.

Thanks for your help in advance!

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I would help you but I don't have the book, thus I do not know the question.

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Please post the question and all of the accompanying detail (fluff).

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Answer to this question is 67.5 m3/s (2492 ft3/sec).

Thank you for your help!

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I am sorry but the question didnt show up on my previous post when i tried to send it as an image.

Here is the question:

A 6hr storm rains on a 25 mi2 (65 km2) drainage watershed. Records from a stream gauging station draining the watershed are shown. (a) construct the unit hydrograph for the 6 hr storm. (B) find the runoff rate at t=15 hr from a two-storm system if the first storm drops 2 in (5cm) starting at t=0 and the second storm drops 5 in (12 cm) starting at t=12 hr.

t (hr) / Q (ft3/sec) / Q (m3/s)

0 / 0 / 0

3 / 400 / 10

6 / 1300 / 35

9 / 2500 / 70

12 / 1700 / 50

15 / 1200 / 35

18 / 800 / 20

21 / 600 / 15

24 / 400 / 10

27 / 300 / 10

30 / 200 / 5

33 / 100 / 3

36 / 0 / 0

Thank you for your help.

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You do not have enough information to solve the problem. What is the rainfall from the storm event in inches? You will need that information to create the unit hydrograph.

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Nevermind ... this is a histogram problem.

These are pretty simple:

Step 1: add up all of the flow rate

Step 2: Multiple the sum from step 1 by 3 hrs

Step 3: convert hrs to second by multiplying by 3600

Step 4: Volume = Precip x Area --> convert your area to SF by multiplying by 5280 x 5280

Step 5: Divide V (step 3) by Area (step 4) and convert ft to inches --- you should get 1.77 inches

Step 6: Divide all of the CFS from the original chart by 1.77 inches and that is your unit hydrogprah

Step 7: Use the UH for the two storms

See attached file for help. https://documents.cloud.autodesk.com/Public/Details?hash=mKziLTC

https://documents.cloud.autodesk.com/Public/Details?hash=mKzigxp

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Thank you. You are right it is pretty simple. It actually made more sense with the way you did it - it was kind of confusing in the book.

Thanks again!

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No worries ... let me know if you have other questions.

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