Engineer Boards

# Unit Hydrograph problem from CERM

## Recommended Posts

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.

##### Share on other sites

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

##### Share on other sites
envirotex    761

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

##### Share on other sites
Guest

+1 on posting the question

##### Share on other sites

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

##### Share on other sites

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

##### Share on other sites

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.

##### Share on other sites

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

##### Share on other sites

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!

##### Share on other sites

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