hydrographs?

Aurora09 said:

@Jbone27 PEhey! sorry for the late response but here is the problem:

A 6 hr storm rains on a 25 mi2 (65 km2) drainage

watershed. Records from a stream gaging 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 (5 cm) starting at t = 0 and the second storm

drops 5 in (12 cm) starting at t = 12 hr.

So the average precipitation was calculated as P = V/Area which gives you 4.37 cm. The rest of the solution for Part B was:

(b) To find the flow at 15 h, add the contributions from

each storm. For the 5 cm storm, the contribution is the

15 h runoff multiplied by its scaling factors; for the 12 cm

storm, the contribution is the 15 h - 12 h = 3 h runoff

multiplied by its scaling factors:View attachment 12699

let me know if you understand the solution  

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Jbone27 PE said:

Okay so I'm assuming you are just looking for an explanation on the scaling? If not please let me know. 

So to get the solution they went to the hydrograph constructed based on the 4.37 cm precip and found Q = 35 m^3/s at 15 hrs and 10 m^3/s at 3 hrs. 

Keeping in mind these values are for 4.37 cm you have to scale each value so for the 5 cm event 5/4.37 = 1.14 or 114% ( I like to think of them in percentages).  Replicated for the 12 cm event 12/4.37 = 2.75 or 275%. 

Q = 35*1.14 + 10*2.75

Q = 67.5

I feel like I just rewrote their solution but just a little more in my mindset. Point being you have runoff values based on the 4.37 event but the events you are calculating are some degree different so you have to scale for the variance.

Vice versa you could say @ 15 hrs you get 35/4.37 = 8 so you get (8 m^3/s) per cm  and @ 3 hrs you get 10/4.37 = 2.29 (m^3/s) per cm then multiplied those values by your corresponding storm events.

Hope this helps. Please let me know if you have any further questions.

Click to expand...