Another Hydraflow Question

[WILDWINGS27]

I have read that the Time Interval needs to be less than 20% the time to peak. Is this correct? If so, I assume that means 20% of the time to peak of the Unit Hydrograph. Can anyone confirm my statement?

 

[saruman26]

The time interval cannot exceed more than 50% of the Time of Concentration, with minor exceptions. If the Tc is 5 minutes, the time interval can be set at a maximum of 3.

A few things to keep in mind when setting time intervals:

1) The maximum number of data points for a hydrograph is 1440, or 24 hours. If you need your hydrographs to extend beyond 24 hours (i.e. extended detention) the time interval needs to be larger.

2) Changing the time interval varies the runoff computations - work this to your advantage as necessary. Play with it and you'll see what I mean.

3) In order to route and combine hydrographs, their time intervals must be the same. Its best to use the same time interval in all hydrographs within the file. I base this upon the shortest Tc in the computations.

 

[RWF7437]

saruman26 wrote:

"The maximum number of data points for a hydrograph is 1440, or 24 hours. If you need your hydrographs to extend beyond 24 hours (i.e. extended detention) the time interval needs to be larger."

The maximum number of data points allowed in Hydrographs 2007 is, I believe 2880, not 1440. Also, this does not limit you to 24 hour storms. You can create what Hydraflow calls Custom Design Storms which are text files with the file extension name ".cds". This is not as easy to do as the documentation would have you believe because any tiny mistake in the text file creates errors in Hydraflow. The program doesn't trap these errors very well so sometimes you can, as I have, spend hours tracking them down.

Hydraflow has now been sold to Autodesk so whether or not some of these program bugs get fixed remains to be seen. Terry Stringer has, in the past been very responsive but some of us fear the sale of the program will end that good support and will almost certainly increase the cost of the program.

good luck