maximum rainfall

[saraxo]

Hi All,

I am given the first two columns and asked to find the maximum hourly precipitation in inches/hr.

The last two columns are part of the solution. I'm not really understanding their solution so would anyone please explain why the cumulative values need to be taken? Also, it seems they didn't add the cumulative values correctly either?

 

[saraxo]

Additionally, I'm stuck on this problem below as well. The first two columns are given. I was able to calculate the volume as 134.2 ac-ft for an effective rainfall of 1 inch, which I thought were the answers. But the solution doesn't make sense to me.

 

[ExhibitGuy]

Hi All,

I am given the first two columns and asked to find the maximum hourly precipitation in inches/hr.

The last two columns are part of the solution. I'm not really understanding their solution so would anyone please explain why the cumulative values need to be taken? Also, it seems they didn't add the cumulative values correctly either?

View attachment 12727

It looks like the Cumulative Inches is the Hourly Precipitation. You need to make this column since they only gave you the rainfall in 20 minute intervals.

So in the last column, the 0.27 inches means it rained 0.27 inches between 1:20 and 2:20, and the 0.52 inches means it rained 0.52 inches between 1:40 and 2:40, and so on.  Since you're trying to find the max precipitation, this means that between 3:20 and 4:20 was the maximum hourly precipitation at 1.70 inches per hour.

 

[ExhibitGuy]

Additionally, I'm stuck on this problem below as well. The first two columns are given. I was able to calculate the volume as 134.2 ac-ft for an effective rainfall of 1 inch, which I thought were the answers. But the solution doesn't make sense to me.

View attachment 12728

The unit hydrograph represents the rainfall that would occur in that particular watershed if only 1" of rain fell. At 5 hours, the unit hydrograph says there will be 125 cfs, however you measured 325 cfs. Therefore, the 1-hr effective rainfall must be 325/125 = 2.6 inches.

Then that means the runoff hydrograph for this particular storm is the unit hydrograph multiplied by 2.6:



If you calculate the volumes from this table, you should get 350 acre-feet.

 

[DoctorWho-PE]

Hi All,

I am given the first two columns and asked to find the maximum hourly precipitation in inches/hr.

The last two columns are part of the solution. I'm not really understanding their solution so would anyone please explain why the cumulative values need to be taken? Also, it seems they didn't add the cumulative values correctly either?

View attachment 12727

The cumulative inches are per hour increment... not total cumulative. So from 1:20-2:20, .27 inches fell, while from 1:40-2:40, .52 inches fell, etc, so the max cumulative hour was from 3:30-4:20, at 1.7 inches.  The math is correct.

Does that answer your question?

 

[saraxo]

@vhab49_PEHello, Sorry I'm still not seeing how the math works here. Just for example, at 3:40 - 4:00 pm, how did they arrive at 1.50  inches? Wouldn't they have to add the previous value (1.50) plus the current increment (0.60) and arrive at 2.10 inches, not 1.50?

 

[ExhibitGuy]

@vhab49_PEHello, Sorry I'm still not seeing how the math works here. Just for example, at 3:40 - 4:00 pm, how did they arrive at 1.50  inches? Wouldn't they have to add the previous value (1.50) plus the current increment (0.60) and arrive at 2.10 inches, not 1.50?

They got 1.50 inches for that one since between 3:00 and 4:00 PM it rained 0.40,  0.43, and 0.60 inches. So at T = 4:00 P.M the past 1-hour precipitation at that point in time was 1.50 inches. Therefore that past hour's intensity was 1.50 inches over 1-hour = 1.50 inches/hr

The way to look at this problem is that there was a 6-hour long period of rainfall, and someone measured the rainfall in 20 minute increments. You're trying to figure out in what 1-hour time frame was the precipitation largest? 

This explains why they don't have the first two rows filled in for the rightmost column since it hadn't rained for 1-hour yet at T = 2:00 P.M.

 

[saraxo]

@ExhibitGuy@vhab49_PEThank you both! I think I see the pattern now. Since the increments are given in 20 min, I have to add the 3 values to get the cumulative inches per hour. Then move on to the next row and repeat.

 

[saraxo]

The unit hydrograph represents the rainfall that would occur in that particular watershed if only 1" of rain fell. At 5 hours, the unit hydrograph says there will be 125 cfs, however you measured 325 cfs. Therefore, the 1-hr effective rainfall must be 325/125 = 2.6 inches.

Then that means the runoff hydrograph for this particular storm is the unit hydrograph multiplied by 2.6:

View attachment 12744

If you calculate the volumes from this table, you should get 350 acre-feet.

@ExhibitGuyediting my post: thanks I understand the problem now!!