# Unit Hydrograph problem from CERM

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### #1 jaa046

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Posted 15 March 2012 - 01:54 PM

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.

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Posted 15 March 2012 - 05:47 PM

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

### #3 envirotex

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Posted 15 March 2012 - 09:37 PM

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

### #4 jregieng

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Posted 15 March 2012 - 11:43 PM

+1 on posting the question

### #5 jaa046

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Posted 19 March 2012 - 12:32 PM

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

### #6 jaa046

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Posted 19 March 2012 - 12:40 PM

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. ( 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

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Posted 19 March 2012 - 01:16 PM

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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Posted 19 March 2012 - 01:40 PM

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.cl...ls?hash=mKziLTC

https://documents.cl...ls?hash=mKzigxp

### #9 jaa046

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Posted 19 March 2012 - 04:39 PM

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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Posted 19 March 2012 - 04:42 PM

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

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