Hello all, first time posting here, but I've read and watched the site for several years. I'm looking for some help with strategy for recovering our pond's spillway which is quickly turning into a 20' deep gulley, and avoiding losing our pond entirely.
Here's the background on the issue I'm having:
1 - We built a roughly 2 acre pond in East Texas just over two years ago.
Maximum depth is around 16', average depth is about 7'. We had significant red clay on-site and harvested that for the dam, and the bottom of the pond is red clay. The dam is keyed in 8' below the pond floor, as it was placed on an old logging trail and we had to remove about 8' of compacted logs that had been buried on this spot to get down to clean clay. So, total height of the clay dam is roughly 24' and is approximately 14' across, and ~ 120' long.
There is no drain through the dam, as we wanted to reduce the risk of any long-term weakening of the dam due to the drain.
Our original spillway (see below!) was 16' wide with 14" of freeboard, lined with rip rap. Last summer, it was expanded to 24' wide with 24" of freeboard. It runs pretty much all winter/spring.
2 - We have a lot of drainage into the pond (See attached topo image) - 3 wet weather creeks draining approximately 180 acres met where we build the dam.
3 - We've had two record wet winters since building the pond, and have already had to re-build the spillway last summer after the late spring brought us 9" of rain over 3 days, including a 3" deluge in 1 hour.
The dam was topped, taking about a foot off the backside, we re-enforced the back of the dam, re-constructed the slope, and expanded the spillway to 24' wide, 24" deep, and extended to it about 275' long (see photo attached of the new spillway when it was cleaned up. We added several more trucks of rip rap, it looks like sand - but in the deep summer the dry clay turns into a powdery substance when disturbed, that goes right back into hard red clay when it gets wet.
4 - Our new spillway is gone now!
So, come this winter/spring, we had a much larger spillway. We built check dams out of old logs at the end of the spillway, before it reached the main gulley, and we had solid grass coverage of the spillway (grew through the riprap) as soon as we got it wet.
This winter/spring was even wetter than last winter/spring! It culminated in a 5" deluge last week, and the water found a weak spot in the left side of the spillway, cutting the drainage from a gentle 24' wide area to one about 4' wide near the mouth, and ending up in a 15' deep newly-formed gulley, with toppled trees. The erosion is now about 15' from the mouth of the spillway, and once it hits there the pond will drain quickly by at least 2', and from that point any additional rain will likely cut down to at or below the dam level, draining the entire pond. The photos don't really do it justice, but -all of this- happened in one 24-hour period.
So, my question is this - given the probability of more winters/spring like this, how would you approach this spillway? Should we concrete it ($$!) and then drop a truckload of boulders in the gulley at the drain point, or is there another strategy that may be more cost-effective? We'd really hate to lose this pond, as we've spent a lot on it, and it is a beautiful pond so far.
It's worth noting, that we're also open to other strategies, such as cutting into the dam during the summer when water levels are low to add a drain pipe, but the amount of water we're seeing is enough to ensure the spill way will get used 3-5 times a year, at a minimum.
I'm no expert here but the drain tube you mentioned along with adding some additional freeboard my be in order. Sounds like you have a recurring potential for big inflows of water. A properly sized drain that can handle some or all of them before the water has a chance to cut through an emergency or high water overflow sounds like a very workable project.
I'm sure one of the other more knowledgeable guys will chime in. I'm looking forward to following this one.
Looking at your pics, it is clear that the erosive force of your water flow is clearly too much for that size of rip-rap.
I am not a pond builder, but I am a geologist, so I have some experience with erosive forces.
I am missing something important from your posts. When the three waterways converge (at the current location of your pond), there must have been a larger waterway developed in the draw that heads NW. That draw should have been capable of handling the existing water flows and come to some sort of natural equilibrium. However, when I look at your pics, I do not appear to be observing a waterway that used to be capable of conveying a heavy rainfall prior to your modifications of your spillway. What am I failing to grasp?
Normally, a pond should actually reduce the erosive force of the water flow since it delays the peak discharge as the freeboard of the pond fills up, while a significant amount of water is be transmitted through the "normal" outlet. Your pond outlet is essentially all "emergency" spillway. That actually maximizes the peak discharge rate. It gets even worse if erosion cuts out some depth from the top of the dam or the top of the spillway. In that case, you have the water flows from the three waterways PLUS the impounded water in the pond that is now released due to down cutting of your containment.
One option would be to install a relatively large siphon system to serve as your normal outlet. That could be constructed with only a pipe through the freeboard of the dam, so as to not reduce your dam integrity. I would also build it at the opposite side from your emergency spillway so any erosion in that spillway does not cut our your siphon system. A properly constructed siphon can convey a LOT of water!
Were you ever on location when your spillway was at peak flow? If so, can you estimate the peak discharge? For example, 15' wide channel with an average depth of 1.5' means any type of drop structure would have to have an area 22.5 sq. feet to pass the flow.
If you add some more info, I can add some (possibly harebrained) ideas for you to assess for your situation.
I'm far from an expert, built 2 ponds. Let's look at basic math . 3" rain in one hour X's 180 Acres = 540" of water on one Acre. 2 Acre pond , 540" of water divided by 2 = 270" of water divided by 12" = 22.5' of water hitting your spillway ? I'm a bit amazed dam held at all. Any chance of building 3 more ponds , one on each wet creek, divide up water shed into 3 60's roughly , instead of 1 180 Acre ? My 2 Acre pond stays quite full in western Okla prairie , with 45 acres of water shed , then overflow drains into 2nd 2 acre pond that stays full from 1st ponds overflow , plus 6 additional acres that drain around upper pond. Hoping this made sense. I'm just amazed 2 Acre pond can handle 3" flow from 180 ACRE water shed. What will happen if you get 9" in 3 hours over night ? it'll make a tsunami look like a water park. Experts will correct my very basic math, but I think it conveys the basics. See what topping will be like during Hurricane on your dam . I so hope the experts all say, FA , it doesn't work like that , and he has plenty of pond for the water shed.
Thanks for the quick feedback! A couple of things to help w/ response:
The original waterway is pictured below. The dam goes exactly where the trail is shown, with the pond to the right of it. That was part of the reason we built the pond, as in the winter the trail was too muddy for effective travel, cutting off 90% of the property from the front 10%. The dam was used in lieu of a very large bridge / very large culvert.
Downstream of the dam, originally, about 500 yards northwest, the gulley is about 400' wide, with a trench as deep as 25' cut into it. We've had such significant rain on mother's day, that the trail that crosses the drainage intersecting the main gulley northwest of the pond was also cut for the first time in 30 years, washing out the trail entirely and turning it into a new 12' deep creek.
As for acreage, I manually calculated 80 acres going into it, Plan My Land Operation is all over the map, calculating from 0.5 acres to 300 depending on where in the pond you click. The county roads all mess with the normal drainage, forcing water down into those three creeks, so I roughly estimated the 180. I could be very high in that calculation.
The pond filled by mid October, after having been down 6' from the summer drought (3.5 months w/ no rain and > 100 degree temps), and then we had a big storm dropping 5" over 12 hours, a camera on the spillway I installed just before the storm showed 12" of freeboard during the worst of the flow. We haven't had a topping event since we widened the spillway. Given that the pond is always full by the time the big rains start, for most of the wet season 100% of the water that comes out of the drainages goes through the spillway, there is no holding capacity in the pond its self.
With respect to a syphon, a concern I have is that it is complicated and may not always work as expected until tuned properly, based on what I'm reading. That would be a concern for us, as we're usually only out there every two weeks, and most of the big storms hit when we're not there. Are they reliable enough to be built and forgotten for several years?
We're also willing to raise the dam another few feet and move the spillway uphill a bit, to install a traditional drain. Heck, we could add 8-10' to the pond (and likely about 5 acres) without encroaching on neighboring properties.
once again, I'm not a professional pond builder. I recommend you contact a professional , that can correctly calculate your watershed, and truly calculate if a 500 year storm hits your pond. if it's even a 100 acre watershed , and a 12" down pour hit's your spillway , I'm afraid your dam will be topped. I would think expansion done correctly will help reduce your risk , how much, I don't know, and neither do you. Get a professional involved. Without a solid number on a maximum potential rain, and actual size of watershed, no one , even a licensed Engineer , can give you good numbers as to pond size, spillway design , Siphon design , etc. What little I know , I would feel like I was playing Russian Roulette , in this situation . Sorry not to be of help , you need a professional , IMHO.
It helps to line the bottom of the spillway with geotex fabric and make durn sure that the leading edge is under ground so that the water doesn't get under it. That fabric will help minimize dirt being washed out from under the rip-rap. You can also use cages of stone in the emergency overflow to help act as a dam to slow down the water velocity. Talk to your counties NRCS office to get some engineering help with the watershed and spillway.
I'd also look into putting one or two 12" siphon drains if you have enough fall for them to function correctly.
Im a long way from a professional but have a little bit of experience with ponding,
180 acres of runoff into a surface acre or two pond size is a lot, 18 inches or even 2' of free board is virtually peanuts for that much runoff, now depending on how steep that drainage is and what cover is on it will determine how fast all that water gets to your pond.
I have 15 acres of pond with about 130 acres of drainage coming into it and a big rain will bring that 15 acre lake up 8 to 10 inches, which scattered over 15 acres is probably enough to fill your pond several times from scratch, granted when my water comes up like that it might be spread out over another acre or two.
My point is, you will need to get a professional to design an overflow - spillway, to handle that much water, assuming that your pond was pretty close to full to begin with, and with very little free board, you will need a design that can handle the release of all that water, three creeks worth, in a very short amount of time. it can be done but it will be a pretty expansive, expensive project.
Good Luck!
All the really good ideas I've ever had came to me while I was milking a cow.
I also have a waterbody in East Texas impacted by the recent abnormal rains. My creek fed, 18-acre lake is in the Hillister area, just south of Woodville. I have a 21 ft wide concrete primary spillway and a 70 ft wide earthen emergency spillway. I also have two 36" diameter drawdown pipes to help drawdown the lake in advance of tropical storms or other predictable extreme rain events. I am considering adding another concrete lined spillway in place of the earthen emergency spillway since it appears we are having more extreme, unpredictable rain events. Iy you are in the area, you are welcome to come take a look to see if any of the flow controls I have might work for you.
In the mean time, I am struggling to find hydrographs for the creek that feeds my lake (Spurlock Branch). I've downloaded all sorts of rain data and geographic information such as drainage basin and catch basin areas, but can't find 25-yr or 100-yr flow data for the creek. Anyone have any ideas about where I might find that?
Also, anybody know of an experienced spillway designer in the East Texas area?
Have you contacted your local NRCS office? I believe yours is located at 418 N 4Th St Stratford, TX 79084 If so, your contact person should be Bruce.Meyer@usda.gov (806) 396-5517 Ext. 3
They should be able to get you the information that you need.
That's for Sherman County. If you need infor for Tyler County, they should be able to furnish you the contact info for that county.
Sroane, I have 378 drainage acres in a creek that I'd like to make into a pond(may have to build off to side). Nrcs guy sent me a flow rate which is probably by acreage fed& by inch of rain....I need to check it again to be sure by acreage. Nrcs only engineers up to 80 acres drainage. Will try to include pic of it here.... you might adjust it for your acreage(divide by 378, then multiply by your drainage acreage(?)).
Last edited by Thirdy8special; 05/31/2403:35 PM. Reason: Add pic
