Pond Boss Forums POND CONSTRUCTION Selecting a site 2 Ft of depth or 6 Ac of Run-off???

I have to make a decision and am looking for opinions, mathematical reasoning, and facets that I am not thinking of.
I need to provide the knowns of the project: 5 Ac surface area; overall average 7 ft depth (13' at dam); water source is run-off; 55 total ac drainage (5 of which is the pond itself), 38" avg annual precip. Now, using Pub 590 to calc soil hydrology and using a 2" rain (as the common factor), I would receive 10" depth (of run-off) over 5 surface Ac.
Dilemna prepatory description, part 1...
I have the option readily available to raise my dam to accommodate 24" additional depth. The surface area would not change alot because most of the shoreline is fairly steep at this depth (I might pick up another .33 Ac. backed up into a shallow draw).
Dilemna preparatory description, part 2...
I have 6 Ac directly adjacent to the pondsite, just the other side of short berm, that could feed additional run-off into the pond. Using the 2" rain common factor, it would provide .6" (of run-off)over 5 surface Ac. It is heavily wooded, and my driveway runs thru it.
Now, the Dilemna...
If I leave the normal pool where it is planned (24" short of the max avail), it will be 12" below the 6 Ac area on the other side of the berm; I can cut thru the berm and channel the additional 6 Ac of run-off into the pond as valuable feed-stock. If I raise the normal pool by 24", I cannot cut thru the berm as normal pool water will run out of the pond and fill the 6 Ac run-off area about 12" deep thru alot of it's area (It would also run over the driveway in some spots). I don't want a wetland and I don't want to re-engineer the driveway. The real decision that I need to make is:
A) do I leave the normal pool low and take on the additional (seemingly minimal???) run-off provided by the 6 Ac run-off zone thru the berm?
or
B) do I raise the normal pool by 24" and forget about the additional 6 Ac run-off?
(FOOTNOTES): If I pass on adding the 6 Ac zone of run-off to the pond, I can very easily channel it off in another neutral direction and render it useless and harmless. Also, I am making a mind-project out of this because I am working with less than ideal total run-off and want to capture as much as possible. I realize that a deeper pond will take that much longer to fill, but I am anticipating potential drought conditions...you guys taught me a very important lesson. I guess the bottom line is: What is more important...24" additional depth over 5 Ac or 24" less depth, but being able to add .6" more depth during a 2" rain? (lots more stock, or less stock but able to slightly increase the ability to re-stock)?

Brettski:

Maybe you've told us, if so I don't remember - how much history do you have on this property? Has it had problems staying full in dry spells before? 55 acres watershed for 5 surface acres is (to me) a sufficiently high ratio for maintaining depth well in a 38 in/year precipitation zone. My ratio is 11 acres to 1 in a 40 in/year climate, so you can see where I'm coming from. I've never been more than 8" low, but that's only 4 years experience.

Even if you don't know the pond history, climate history for the area would be very useful - what's the variation on that 38 inches of precipitation that can be expected? And that info should be available even if you just moved in from out of state.

I see you have three choices: 1) raise the dam, 2) add 6 acres of watershed, or 3) take no action but study and gain experience. As to the choice of whether 2 additional feet of depth or more watershed would be better in a drought, I'm sure someone has the hard experience and/or the hard math to tell you (I'm looking forward to hearing it). My uninformed opinion is I'd rather be 2 feet deeper going into a drought 'cause if it's not raining, more runoff means nothing. If it got low enough, could you maybe add the watershed during the drought to recover quicker, then either shut it off or lower the drain (dam) before flooding the driveway? That seems like it would take earthmoving, perhaps in a hurry, but I think that might be do-able under the "construction, anything hands on" label.

Brettski,

Here's a thought: Raising your dam 2 feet will add about 3.3 million gallons of water which represents about a 30% increase in total volume of water in your pond. I don't believe the 6 acres will even come close to that kind of added drought protection, nor provide an increase in fill rate to offset the value of that additional water.

Another thought: The critical number is your annual rainfall and the variability about that mean value. If you can find the standard deviation, we could do a probabilistic estimate of the likehood of drought problems with and without the extra 2 feet.

One last thought based on personal experience:
The NRCS told me my large pond would never fill and even if it did it would never stay full. It is 4 acres and has only about 3 acres of drainage feeding it. It filled within a year and stayed full for 4 years until this recent drought...and where ponds all around are drying up, I still have about 16 feet of water at the dam. I would always go for the extra dam height over the additional drainage in my area, but your area may be different...50 inches of annual rainfall is normal here.

Theo,
My history on the property is almost one year; the dam for the pond project will be constructed this summer. I like the tip for making the 6 Ac run-off a "turn it on and off" option...I don't think the ROI is practical, but I am gonna think about it. Good job thinkin' outside the box...thx.
ML,
Both you and Theo hit on a figure that I do not have; avg precip. variability. I have a good dialogue going with my NRCS agent; he should be able to provide it...I will advise. Btw, amazing little side-bar on a 4 Ac pond with 3 Ac of drainage. I would have laughed that off as impossible... :rolleyes:
Thx for the help and input.

Brettski,

I like Theo's assessment. Go deeper, use the 6ac of runoff in times of drought but install an overflow to channel excessive runoff before it affects your driveway. Its nice to have options. Like Yogi says, when you come to a fork in the road, take it.

Brettski :

I may be missing something but what about both?

Can you add a culvert to the driveway/berm such that you can 1. raise the dam to have a bigger/deeper lake and 2. make a second small pond in the 6 acre watershed and 3. have any excess water run from the big pond run back to the 6 acre watershed and then out of its outlet. If so this would give you the option of 2 types of ponds for example a trophy BG pond (small pond) and a mixed fishery (big pond). Is that an option or am I missing critical facts ?

Thx Russ and Ewest.
Ewest...
By no fault of your own, the picture is difficult to understand...blame it on the artist. About 250 ft of my driveway runs thru the center of this run-off area, and the crown of some of the drive in this section would be underwater at the "higher" normal pool. Also, the entire area is heavily wooded. I don't want, nor can afford, to go thru the necessary excavation, clearing of some timber, and raising of the driveway. My goal for this 6 ac zone is to clear it of the run-off, either into the pond or away to a neutral run-off. Like Theo, you are answering my request to think outside the box, and I am grateful...thx. (I will admit, tho, that I did entertain the thought pretty much as you described it) If I decide to raise the pond by 24" and abandon the 6 ac run-off zone, it will leave me all the other options as long-term. I can still come back and exercise any of them, including the auxillary pondsite....kind of a comforting thought as I get used to this new development.

Then I would go with the plan that leaves my options open i.e. raise the level of the existing pond . Good luck and happy planning !!

I just spent 1-1/2 hours talking with my excavator on the phone last nite. He concurs with the thought of taking the additional 24" of depth and dismissing the 6 ac run-off zone, hands down. He did, tho, throw in an interesting twist...we can use this short berm that separates the two areas as the emergency spillway. Instead of having to remove additional timber and fabricating an emergency spillway around the dam, we can spend alot less time and $ by creating the proper passage under the driveway in this 6 ac zone. Moving the E-S from the damsite area will make even more sense if we raise the water level by 24". The additional height means that the E-S path must now travel that much further and will have that much more erosion potential. He adds that in my zone, a 10 year rain will yield 4.250" of water. With my drainage area, he expects that the principal spillways will likely never even need additional help (planning a pair of 12" dia pipes for the principal). This all makes pretty good sense to me. This may be the lemonade I was looking for in 6 ac of lemons???

Not surprising he jumped on the addition of 24 inches. That represents a whole lot of pond water(over 3 million gallons) and great drought insurance policy. The emergency spillway is also another great idea. Better than locating it at the dam where slopes may be more of a problem.