NCEES Sample Exam - Errata for WR (question #516)

WAWRE · Apr 5, 2018

Hi Everyone,

Longtime listener, first time caller. The NCEES Errata page does not have anything posted for the E&WR depth sample exam. Searched the forums with no luck.

The answers to question #516 are in the wrong units (see attached.) Unless I'm missing something here. Is there a contact at NCEES for reporting errata?

Nothing helpful on NCEES website, either:

https://ncees.org/exams/exam-preparation-materials/exam-prep-errata/

KOKOMO777 · Apr 5, 2018

I used the equation A = (Duration x Q)/2; Q peak is (cf/sec-in). Solve for Q = (2*A)/Duration

BradyD · Apr 6, 2018

I agree that the NCEES exam solutions don't exactly explain themselves that well, but this solution is correct. 1 ac-in/hr = 1 cfs (1.008 cfs really, but it's commonly used as 1:1).

As a side-note, McCuen (the reference for this problem) was my surface water hydrology professor, and he drilled 1ac-in/hr = 1 cfs into us. He was a great teacher.

I'm taking this exam next Friday as well - good luck!

WAWRE · Apr 7, 2018

Oh wow! I should have known that. I'm embarrassed, but happy to be making a note of that in my book. Good luck to all!

alexnelson8 · Feb 9, 2019

Hello everybody. I appreciate the feedback the feedback for this problem, but I still have no idea how you are solving this problem. I do know that ac-in/hr is equivalent to cfs, but this problem only states area and duration. There is no info pertaining to storm intensity. I do believe this problem contains errors.

The problem reads as:

A unit hydrograph for a 200-acre watershed has a triangular shape and a duration of 4 hours. The peak flow rate (cfs/in of precipitation) for runoff (ignoring infiltration through the soil) is most nearly:

How did the solution determine v=200 acre-in?

How does acre-in/hr convert to cfs/in, it converts to only cfs.

I have officially wasted far too much time on this problem now.

I would think a problem like this would use

Q(peak) = 0.756A / t

Badger · Feb 9, 2019

Not my subject, but an acre-ft is an acre covered with water 12 inches deep, so an acre-in is an acre covered1inch deep.

So from the solution 4hr x Q/2 = 200 acre-in so then Q = 100 acre-in/hr x (43560 SF/1 acre) x (1 hr/3600 seconds) x (1ft/12 in) = 100.833 cfs /in of precip. or 100 cfs/in.

Does that make sense?