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I want to add 100' to my existing 40 foot driveway culvert.
Plan on using 5 20' joints of corrugated 15'' double wall smooth bore plastic pipe.
How much lower than the upper end should the lower end of the culvert be?
I'm thinking 12''.
Would that be enough or should it have more slope than that that?
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12" should be good, I think the basic rule of thumb for drain pipe is 1/4" of drop per foot of length, but with larger diameter pipe you can lessen the grade. Just my opinion
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A 1 foot drop on a 100 run is only a 1% grade.
That's not much fall, and I'd be concerned that such a shallow grade could easily be negated, or in fact reversed by just a little settling of the ground.
Generally you don't want anything less than about 3% for natural drainage.
A reduced slope will also cause very slow water flow rates. If you are in an area with cold winters and or silty soil a 1% slope may not allow the water to run fast enough to adequately carry away silt buildups or prevent freezing of melt water.
Best of luck.
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Just make sure your pipe size is at least as large as the next culvert downstream from yours (assuming it is reletively nearby). That way you have some insurance that it is sized properly to handle normal flow, as well as storm events. 1% is fine for a 15" pipe. You could even flatten it a bit if you need to. Though I wouldn't go any less than .5' drop invert-to-invert. You want to maintain flow at about 3ft/sec. which will keep the pipe clean. 1% (as you intended) should be good. Also consider installing 5-10' of medium stone rip-rap at either end. This will also help to keep out silts and sediments. Good Luck.
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1*Just make sure your pipe size is at least as large as the next culvert downstream from yours (assuming it is relatively nearby).
2* 1% is fine for a 15" pipe. You could even flatten it a bit if you need to.
3*Also consider installing 5-10' of medium stone rip-rap at either end
to help keep out silts and sediments.
MacDaddy
===============
Thank you for your above reply.
It was much appreciated as it was very informative & helpful for me.
1*The 15 inch was approved on my permit.
2*I have decided on a 3" drop for each 20 foot culvert thus the lower end of the 100 foot run will be 15 inches lower than the upper end rather than the 12 inches I had originally planed on. The result is a pitch slightly greater than 1 %.
3*The state told me they may do just that in the 75 feet of ditch from the upper end of the culvert to the upper edge of my property line.
This has given me second thoughts about the whole culvert project and makes me wonder if I'm taking on the states responsibility to maintain the road way ditch by installing the 100 feet of culvert!
I have a question about the stone rip-rap - How do I prevent the stone from washing into the culvert in a real gully washer?
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1.25% should be fine. Generally, you match the proposed grade of your pipe with the existing grade of the ditch bottom (assuming the bottom has at least .5% grade). If you create too much of a drop, you will wind up regrading the ditch from the downstream end of your pipe until it daylights (hopefully still on your property). I guess what I am saying is that (1.% grade is PLENTY of pitch for a 15” HDPE pipe). Don’t make more work for yourself b/c you think you need to have it steep to convey flow. Also, if you can shallow the pitch of your pipe and create a free-discharge (little waterfall) on the downstream side, this will prevent back-silting.
I mentioned medium stone rip-rap before. I don’t know your situation. It may be overkill for a driveway culvert. These stones are about 75+lbs each and should be fine in almost all situations.
Also, make sure you have (at least) 1’ cover over the pipe, and depending on the weight of the vehicles crossing it, you should probably use select-fill under the driveway crossing. Gravel s/b fine for light traffic.
In our area, this type of work would be the homeowner’s responsibility. However, all future maintenance of the ditch would be done by the Highway Dept.
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I would also recommend looking into the depth of the ditch and placement. Is the ditch in need of dredging soon where they could take out 2" to 4" of the ditch bottom area? Is the water flowing good right now?
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