Recovery Time and General Pond Design in HydroCad

[T Williams]

thread162-348455
I received the link for the Forum from Peter. I am referencing a previous thread to make sure I am communicating with fellow HydroCad users. First, I would like to get to know some other HydroCad users to bounce ideas off of and to get some advice occasionally.
I apologize if this is too long of a description in advance.
I am working on a small site plan. 0.63 Acres. My impervious areas exceed the thresholds and so I have to file a 10-2 Self Certification with St Johns River Water Management District (SJRWMD).
I spoke with the District Engineer and was informed due to the nature and small size of the site I do not have to model Pre & Post Development discharge rates and volumes. I only have to verify Volume of Storage and Pond Recovery Time. My pond is a 2' deep sodded Retention Swale. There is a small wier for discharge, but again I am not modeling the discharge rates or volume. This is an existing Site that already has 10,816sf of impervious with no retention on site. The proposed Impervious is only 2,000 sf more. My Pond retains 4620 cuft which is a little over a 2" rainfall (So I know my Post Development rates and volumes lower than Pre)
I am very close to the coast and I generally make my as shallow as possible so they will percolate (infiltrate) quickly.
I had Soil Borings and Permeability Tests done on Thursday and have not got the report back yet but just did one very close by and the permeability will be close to that location so until I get the report I am using the values obtained from the other site. I did witness the borings so I do know the Aquifer base was 8' below grade.
Top of pond is 15', Bottom of Pond 12.5', Aquifer 15' - 8' = 7', Pond sides slope 3/1, Unsaturated Vertical Hydrolic Conductivity (FPD) 4.0, Saturated Horizontal Hydraulic Conductivity (FPD) 12.2
OK - 2 Questions
How do I fill the pond, or can I fill the pond, with no catchments feeding the pond (No infill)(just full).
Which Conductivity do I use; Can I use both; Is the water infiltrating through the bottom of the swale or the sides or both?
I can share a copy of this file with someone who might be able to give me some tips.
TW

 

[psmart]

You can model a pond without an inflow hydrograph by setting the Starting Elevation on the pond's Advanced tab.

There are several options that determine how the exfiltration area is calculated. This will determine if you have flow trough the sides, bottom, etc. For details see

http://www.hydrocad.net/exfilt.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[T Williams]

Thank you Peter, I will try that and let you know how it works.

 

[T Williams]

Ok I am getting Hint: Longer time span advised for full volumes. I have my Start time at 0.00 and End Time at 24:00. Should I increase End time to 72:00?
I get lost quickly, and that is another reason I am trying to find someone I can speak with and maybe share screens. Peter you are great but I feel like I am to green to bug you (but I will if you are available)
T Williams

 

[T Williams]

Getting lost now.
Do I need to specify a rainfall?
Where on the node report does it show when pond is empty? I tried adding storage above 15' and putting a wier at 15'. I will have a 1' beam on the outer perimeter of the Retention Pond to contain and direct any water over the 15' Elevation to the wier. The Shape of the Storage is Pyramidal (Rectangular with walls sloping 3/1) but at 14.5' to 15' I have a couple of tributory swales that feed the main swlaes and I used the area of these tributories and added to the area of the pond at 14.5' & 15'.

 

[psmart]

The rainfall depth must be specified, but it doesn't effect the model unless you have a subcatchment.

You can see the drawdown on the tabular hydrograph.

Set the time span as long as you need to see the entire drawdown.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[T Williams]

Hi Peter
I think I found my error. I went to edit my pond and on the advanced tab somehow I had entered a Base Flow of 4 cfs, so my model thought there was a continuous inflow and it made my tables and graphs go over a hundred feet up. I cleared that field and now my report looks right. On my hydrograph table (I set my time to 72 hrs) I show the proper amount of storage at time 0.00 Elevation 15'(4,700 cuft)and at time 14.00 hrs the storage is down to zero at the 12.5' elevation (bottom of pond). So my recovery time is 14 hours is that correct?

 

[psmart]

Sounds right.

Peter Smart
HydroCAD Software

http://www.hydrocad.net