I am from the island of Trinidad in the Caribbean. We hired a bulldozer a few weeks ago to cut some dirt roads on our 30 acre estate. ...... we also had a pond built at the same time, but now need to install overflow pipe and spillway before rain comes in mid-May..
Any sugestion on the size of overflow pipe? 12"
how wide a spillway?
Pond , embankment Pond size about 40' x 90' watershed area, ?? 4 acre, sloping Location.. Trinidad,West Indies (tropical rainstorms) Soil , heavy Clay
Sorry for the length of this response, but we've had 18 dams built, so I'll tell you what has worked for me in South Dakota.
If my calculations are correct, your pond of 3600 square feet will be less than 0.1 acre in size. Your pond has 4 acres of upland supporting it, which would be a ratio of 40 acres per acre of pond surface area. This is a good ratio for my area, which received about 15-20 inches of rain/year. However you could get that in a few days during a hurricane.
There are formulas that can calculate the size of the spillway tube based on drainage area, slope, and the amount of precipitation that would fall in a 25 or 50 year storm. The idea is to pass all this water through the tube, without needing to use the emergency spillway (which is the sod covered channel around one end of the dam). I get my calculations done by our USDA NRCS office engineers. Maybe we have some engineers on this board that could give you the accurate numbers. However I'll give you some rough estimates based on my experience, but thinking that you would need to pass 10 inches of rain in a few hours.
To make the greatest use of the pipe passing through the dam, I would install what is known as a vertical drop structure. This is a short section of vertical pipe, connected to a pipe that then runs at a slight downward horizontal angle to the backside of your dam. The vertical pipe is larger diameter than the horizontal one, ensuring that the horizontal pipe runs at close to capacity. When one just puts a straight pipe through a dam, the inlet can be completely covered with water, but due to the downward slope and faster flow of water, most of the pipe only runs at partial capacity.
You will need to have an excavator to cut a trench through the dam to install this structure. After it is installed, make sure you pack the soil in extremely well (use a motorized tamper that is gas driven, with a pad that will allow you to pack on both sides of the tube.
Take a concrete culvert of at least 30 inches diameter that is 6 ft tall. Near the bottom of this tube, cut out a 20 inch circle on one side. This culvert would be placed into the trench, sitting on a pre-poured, reinforced concrete base, 6x6 ft and 6 inches thick. Then you slide the 18 inch heavy sewer tube into the hole. I generally cut the end of the sewer pipe at a 45 degree angle, with the longer side on the top. This creates a "hood" effect, and helps maximize the water flow through the tube. Lay out all the 18 inch sewer pipe, using a relatively gentle angle, so that the end of the pipe is at least 10 ft beyond the downstream toe of the dam. Later you will want to place rock (with heavy plastic sheeting below) underneath the outlet, to prevent the water from eroding the soil at the base of the dam.
At the joint where the 18 inch tube comes out of the vertical culvert, you will want to frame up a form, so you can seal this with concrete. If your finances allow, you could actually seal the first 20 ft of the 18 inch pipe with concrete as well. That will provide the tightest seal. Below this point make sure you add in an antiseep collar as added protection. I've used rubber sheet style antiseep collars on some dams with good success. Spend a lot of time packing in good clay soil around the pipe, and you shouldn't have any leakage problems.
The top of the concrete culvert will set the normal water elevation of your pond. You will want to have your emergency spillway elevation at least 12 inches higher than this. The emergency spillway should probably be at least 15 ft wide, with a gentle slope back into the channel below the dam. Make sure this is seeded into a sod forming grass. Then the top of the dam should be another 3 ft higher than the emergency spillway elevation.
Tried to do some calculation from the 590 handbook and arrived at a peak discharge around 14 cubic feet per second.
due to time running out as rains are upon us- dirt road will soon be impassable , cost and no excavator..
we dug a trench across the dam on one side sloping down, layed BRC wire and steel and casted a concrete pad 4-5" thick last sunday.. Anti-seep collar? tried to make a concrete form sticking out in the center. You just place a rubber sheet aroung the pipe and ram dirt around? Then we are going to place 18" concrete culverts, straping them down to pad with steel rod/wire and concrete in the whole thing... lower down we are placing some half culvert with rock around..
One type of anti seep collar we've used is made from a heavy gauge rubber material, that's about the same thickness as a tire intertube. This sheet is in the form of a square, that is several feet wider/taller than the tube diameter. There is generally a 1x4 inch framework of wood around the edges to hold it in the extended form. You cut a circle in the center of the rubber square, a little smaller than the diameter of the pipe, and then push this seep collar over the end of the pipe until it is in position. On our dams we generally put one seep collar as close to the inlet as we can, making sure that it will still be completely covered by clay of the dam. Then we place a second one in the front part of the core trench. You can use a heavy grade duct tape to seal the rubber of the seep collar to the pipe itself. Then backfill slowly with good clay (proper moisture for packing, and smaller particles, so you get good fill without air pockets). I run a tamper over the material after every 6 inches of fill is added. It takes some time, but it well worth it.
I'm not sure I understand your 18" culverts and half culverts, but it sounds like if you are laying them on the concrete pad that you will need to seal all this together with concrete as well.
The screen/rack in front of your tube is a good idea, but make sure to check on it during heavy rains. It looks like you should have a good seal around your pipe with all the concrete. On one of our bigger dams we encapsulated the first half of the plastic tube within a concrete casing. On the downstream end of your tube, make sure you place some rock and rubble to absorb the impact of the water flowing through the tube, as it will speed up as it passes through.
Here a pond update, Oh the pond burst in May of last year when we got 6 inches of rain plus in 7-8 hours ...
here are the pics..
have visitors to the pond river fish(where they came from who knows),cat fish (which i put in) , river conch and even a caimon , aligator...
OH
Think of fixing back the pond , so here is what we are thinking of doing..
Filling back the hole in the embamkment
Raising the embamkment by 4-5 feet
adding another drainage pipe or 2 pipes where the embamkment burst, is this a good idea? , being 12" diameter in a drop inlet style , So we would have 18 dia , and two 12" dia pipes
making two 15" wide spillways on the two sides of the pond in the hill.
Would like to hear you comment and sugestion please
birdie, do you have any idea (in acres) of land that will accumulate rainfall that will eventually make it to your pond? Here in the states it's called the watershed of the pond. Also, what is your rainfall per month? I'm asking those questions so we can get an idea of the amount of water that will be making it to your pond during a good rainstorm.
Rainfall .. found some meteorological data(old 1950's) on the net about 55-68.5 inches a year, max in a month 12.58 inches, max in a day 3.31 inches
Last year we had the worse dry season in 50 years, also above normal rainfall in the rainy season, max of 15.75 in a month, 6inches plus in 7-8 hours with lots of flooding islandwide and burst embamkment
conditions - soil type d - clay slope 45 deg plus runoff cn 82 - woods, orchard, timber type III storm distribution
taking the watershed at 6 acres and the rainfall at 6 inches
using 590 ..leads to volume of runoff 630469 gallons
Bridie Getting in late on this one. Been out of town and away from the computer. Do you know why the dam failed in the first place? Did water go over the top or wash out from a hole in the dam? If you are not sure why it washed out ,try and find out what happened before you rebuild.
poor compaction of clay, catch 22 situation need water to compact clay but no water for miles around.. so as rains came embamkment slid and settled ..droped in height then major storm event and it over topped with no grass planted.
So with 8-9 months of rain embankment has stablized and with dry conditions now, was hoping to fix it.
there is water now in the pond 4-5 feet there is a 18" dia concrete overflow tube in place
So was raising the embamkment to have 3' of freeboard making a 15' wide spillway on undisturbed soil in the hillside got some grass to plant
but was think of adding another overflow tube where the embamkment burst, which is the natural channel , I can get abs tubes in 12, 18 and 24" dia
