Unit Hydrograph problem from CERM

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jaa046

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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!

 
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.

 
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.

 
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

 
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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