Hec-Hms terminal basin

[Topshelf7000]

Hello all:

This may be long winded, but I hope not. My vision is blurry, my hair frazzled, eyes are red all because I'm trying to figure out, what I thought would be an easy expected runoff volume, to be retained within a terminal basin. I'm pretty good with hydraulics (intermediate) but a step below in Hydrology but never did I think I would have such a problem understanding the inputs into the HMS program. I have used Hydraflow for quick and easy 24-hr hydrographs, junctions, routing and detention ponds, and multi-stage discharge structures.

I have modeled roughly a 2000 acre watershed using HEC-HMS and I thought it was pretty user friendly. However; no terminal basin was involved, just peak flow determination. Agency design criteria for basins require a 10-day storm. I would like to be able to model the watershed using simple SCS curve number (loss) and SCS unit Hydrograph (transform method) No baseflow, canopy or surface methods. This seems easy enough filling in the Basin components.

The wheels fall off once I get into the meteorologic modeling tab... for the precipitation, can I just somehow use the PPF tabular data for the 100 yr values from 15 min up to 10 days that is available from NOAA Atlas 14 online? and use the Frequency Storm option under the precip dialogue options?

I have researched online and there are so many different approaches as to what Hydro method to use for Loss methods and transformation and I know once one is selected, all the subsequent fields become dependent on that method.

I'm okay with setting up the control specifications (unless I'm missing something - just a specified duration compatible with interval and time frame of interest [10-day]) - Based on my precip data available (PPF), I've seen where this data is input under the meteor model "frequency storm"? and then precip pull down once specified.

Long story shortened, I have a 200 acre watershed that I plan on not routing to a terminal basin that I need to size. I would like to use SCS curve number and unit hydrograph and extrapolate to a 10 day duration??? is this possible?

If anybody could help me out on this - perhaps a simple watershed/basin of similar simplexity. It would be very much appreciated...

Thanks a bunch!!!

 

[SMIAH]

I'd copy/paste the precipitation data in Excel as time(15 min)/Precipitation(units) then export it to HMS.

Subbasin with SCS Curve Number and SCS Unit Hydrograp.

Components/Meteorologic Manager/New/then name it "100-yr".
Meteorogic Model, choose Specified Hyetograph for the precipitation (and click on "yes" to include Subbasins under the tab "Basins")

Components/Time-Series Data Manager/Precipitation gages/New/then name it "100-yr".
Then in Precipitation gages, use the time interval you want (15 min) and corrects units.

Fil the start and end date and copy/paste your data in the table (check graph to see if everything's ok).
Check the control to be sure they fit with your precipitation data.

Then you can size your basin with a Reservoir Creation tool and paired data (e.g. storage-discharge and storage-elevation)

Hope it helps!

 

[Topshelf7000]

Thanks Smiah for the reply: (been out in the field and saw a reply

What I have for precipitation data is the Point Precipitation Frequency Estimates form NOAA Atlas 14 (see attached). I don't have any incremental (interval) precip depths vs. time.

Can this data be input as a viable precipitation source to determine my 10-day runoff hydrograph and volume. Basically the Duration values as time and 100 year depth column and precip amount..

Thanks Again!

 

[SMIAH]

So you know that the total precipitation for 100-year/10 days event is 7,11 inches.
But you don't know the repartition of this precipitation.

If you only want to calculate runoff volume, you could set this as uniform over the 10-day period.
But if it's both volume and peak flow...then you need more data (precipitation vs time) or a repartition for this event.

For a detention pond, I'd check a 10 day event as a steady uniform precipitation over time.
You might want to check it as a triangular precipitation (?)

There could be other way around - I'd let others join this conversation!

 

[gbam]

Maybe I am simplifying it too much. When I look at retention rather than detention, I do not usually perform a hydrologic analysis (rainfall runoff model like HMS or HEC1)unless I need to route through the basin for some reason. I use the total depth of precip and compute the volume of storm. Then I compute the runoff volume by applying the R/O transformation steps. In the case of the SCS method you get a R/O depth in inches and can compute the volume over the watershed. The retention volume needs to hold your runoff volume, correct?

 

[SMIAH]

The peak flow has a certain importance on a retention pond analysis. If the rate of water entering the basin is way over the rate of water leaving the basin, it might overtop quickly.
If you only specify a steady flow entering the basin, you may not see this effect.

Then again, depends on size, data, etc.

 

[Topshelf7000]

Thanks guys - and your right, I think I may be over-complicating this. It's just that I've seen thru other programs and reports the 10-day hydrographs and volumes from HydroCad and CivilCadd etc and I was wanting to replicate using HEC-HMS.

But yes, I am thinking of interpolating the precip. data that I do have and filling in all those blank time period intervals as a linear function. With a retention facility, criteria is to analyze up to a 10-day to make sure you identify the critical long duration design storm and possible back to back event scenarios.

I'll get it sooner or later!!!

Thanks again, and I'll be back

 

[SMIAH]

IMO, for a 200 acres, you should check it with a 3-4 hour storm duration (e.g. Chicago modified storm).