Modelling a Pond without offsite drainage

[abfofanah]

I have been trying to model a pond. The pond's only source of water is from direct precipitation. The program I am using requires a time of concentration(Tc)to route the hydrograph through the pond. Without an offiste drainage area, is it okay for me to come up with a Tc within the pond?

 

[TerryScan]

If the only source of water is direct precipitation, the Tc is zero. I suppose one could get a Tc from the rain falling on the pond slopes above the water level which may get you a few minutes.

 

[sam74]

typically there is a minimum allowable Tc of 5 minutes for all applications I have used (I have also seen 6 min. and 8 min. minimums). A lot of software defaults to this if your calculated Tc is less than that.

 

[gbam]

What exactly are you modeling? If you are looking for volume increases,would you not just take the precipitation over the area? Then balance your water, ie infiltration, evaporation etc... vs precip.

 

[BRIS]

I agree with Gbam - where does TC come into this - you need to plot a water balance of inflow (precipitation) and outflow (evaporation, infiltration etc) over time using appropriate rainfall data. The rainfall data you use will depend on what you are trying to do. If you are looking for the maximum pond level then take the rainfall depth duration data for the relevant return period there will be no evaporation. This is simple arithmetic!!

 

[abfofanah]

Thanks for your help. The model I am using is HEC-HMS. The only way I know how to define the pond area and curve number is by using the subbasin option.The subbasin option also requires a Tc. And for a pond, of say, 200 acres without offsite drainage, using a zero Tc (not allowed by the program)or using a very small Tc which may lead to a big peak discharge is what I have been struggling with.

Again,I appreciate your help. Thanks.

 

[BRIS]

abfofanah Can you explain what you are trying to do - are you tring to predict the maximum water level rise and/or discahrge hydrograph from the pond ? - both are very simple calculations if you only have direct precipitation.

 

[abfofanah]

I am trying to design outlet structures for several ponds which are interconnected.

 

[gbam]

There are other programs out there that are more suited for this application. Interconected pond routing (ICPR) is just one. I do not beleive that HMS is the correct tool for this application. If you dont have time to learn a new program then code a spreadsheet to do your computations.