Large Area Drainage - Over 6,900 ac

[maddogxoxo]

I am analyzing the drainage of a canyon. The total area is 6,909.92 ac. I am wondering what the best way to go about this. I have started HEC-HMS, but the flows are extremly over conservative. I only used 6 basins in my model, but the areas are still so big, that the intensity is a killer. I can break it down further into 27 subasins, but is that too many and a waste of time? Please advise.

Maddog

 

[francesca]

You should use regression equations.

 

[cvg]

larger areas do not correspond with higher intensities. Generally, making smaller areas will shorten your times of concentration which will increase your intensities. For large areas, breaking into smaller subbasins is not always the smartest idea.

Many would say that you should simplify your model to the extent possible. More complex models do not generally generate more accurate results.

Also, why do you think your flows are so "overly conservative?" Have you compared your results with another similar area or with regression equations? Have you done any calibration of your model?

 

[maddogxoxo]

The canyon I am looking at is kind of long. I had subdivided it into 6 parts lengthwise. I did that so all the flow wouldn't arrive at the discharge point at the same time. The flow estimates we are thinking should be around 500 - 900 cfs. The flows I am getting are about 8,000 cfs.

 

[cvg]

500 - 900 cfs sounds incredibly low to me. 8,000 sounds a lot more reasonable. What return period are you estimating? Also, I would not delineate "string bean" subbasins as you apparently have. Instead you should drain to an intermediate point along the canyon and then route the flow.

 

[maddogxoxo]

I am sorry, I subdivided it width wise.

 

[francesca]

Your time of concentration deals with how long it takes the flow to arrive at the point of interest. It will all arrive "at the same time" if it rains for a duration longer than your time of concentration. As cvg says, you need a combination of drainage basins and channel routing, which HEC-HMS is quite capable of analyzing.

 

[RWF7437]

First, Why are you doing this calculation? Is it for a flood plain study ? Is it to design a storm drain, a detention pond, some other reason ?
Second, how sure are you that your area is 6909.92 acres, plus or minus .01 acres; really?
Third, where is this "canyon" ? In the U.S. ?
Fourth what rainfall data do you have to work with and what is the range of error in that data ?
Fifth, who will review your calculations and what are their criteria and expectations?
Sixth, what range of storm durations, probabilities and antecedent runoff conditions do you want to consider?

These are threshold questions. Do not pass go and do not collect $200 if you haven't answered them.

good luck