Stormwater Conveyance Down a Steep Slope 1

[danenviro]

Hello,

I am trying to convey stormwater from a pond outlet down a steep slope (approx. 32% slope or 17.7 degrees) and need some help with my design.

We are proposing the use of buried pipes and manholes and I am wondering if this is the best approach. Can someone suggest alternatives for both above ground and below ground conveyance. And if our proposed approach is used, what sort of manholes can be used to limit the amount of fill required and still dissipate the energy of the flow?

 

[cvg]

how high is the hill and what is your Q? How often will the pond discharge through the pipe? What kind of pipe?

 

[danenviro]

The hill is approx. 80 ft drop and the Q is 1.13 cfs (32 L/s). The discharge will be intermittent. We are using a 10 inch pipe.

 

[cvg]

well, you have very low flows and not a huge hill. However, if you wish to construct a drain with drop manholes - assuming a 4 foot drop, you would have 20 manholes or 1 every 12 feet. Hardly sounds economical to me. In addition, constructing a manhole on this steep a slope will be very difficult. To burn off more energy you might consider using a smaller pipe. With this slope you could get by with a much smaller one, maybe 6 inch or smaller if you can keep it flowing full. With either 6 or 10 you will have to screen the inlet to keep the trash out or it will plug. Usually I require at least 15 or 18 inch outlet pipe to keep clogging down to a minimum. With this slope a larger pipe will not flow full and will not help with dissipating much energy. You might be better off with a concrete lined ditch, shotcrete or broomed finish and maybe step it. I have seen these used with some success as down drains on large slopes and carrying larger flows than what you have.

 

[danenviro]

Thank you very much, cvg. What do you think, though, of a design that would have the pipes almost mimicking the slope of the hill and utilizing two manholes (spaced 85 ft apart on the slope) and a perforated manhole at the foot of the slope. Would the resulting velocities be too high?

 

[cvg]

your velocity will be somewhere between 10 and 20 fps depending on size and type of pipe and your froude number will be probably about 4. You will have a hydraulic jump at the bottom. The pipe can probably handle the velocity for short time periods. Make sure you keep sediment out of the water or you will scour the invert of your pipe. Corrugated pipe will keep the velocity lower. You may have some scour in the manhole. I don't know what a perforated manhole is.

 

[danenviro]

Once again, thank you very much cvg. You've been very helpful.

 

[Ryb01]

danenviro,

I live in a rather hilly region and have completed several of these types of designs. As cvg has said your best best would be to size your pipe smaller to ensure full flowing conditions with some kind of energy dissipation located at the downstream end if required. It may be beneficial to consider pipe materials as well in this case due to 1) higher velocities and 2) Given the slope, there may be a possibility for our pipe joints to separate. Considering a single section of HDPE or Cobra Lock PVC may be an option to consider. Placing perforated pipe at the bottom of the slope may be something you would want to look at in more depth and also consult a geotechnical engineer as this added moisture may be a cause of toe failure down the road.

Outside the scope of the storm sewer, constructability may be an issue. Given the overall vertical drop and slope of the hill, I would assume that you would assume that you would want to disturb as little as possible on the slope to maintain slope stability. This being said, regardless of soil type, you'd want to maintain as much tree'd vegetation as possible and use a long armed shovel and small compaction equipment.

Hope this helps.

 

[gbam]

Dont forget about anchoring the pipe into the slope. At that grade the pipe may slip down the slope. It is at all possible to convey this down the hill in a swale? You would only have about 250+ LF of lined/armored swale, just a thought.

 

[CarlB]

I notice you mention above-ground as a possible option.
I know of a project of similar slope that as a State park, and the Parks department did not want any trees cut (scars). And other projects dumping over the slope had caused washouts. Solution was a pipe on the surface, snaking around trees to the toe of the slope, then into a stilling basin constructed of large rocks. I believe it was 8" corrugated HDPE, and secured with a strap & duckbill anchors at intervals.

 

[rosalitambarde]

hi cvg.. i am a flood control engineer here in our place and one aspect of my job aside from being the hydrologist, i am also opt to design lateral drainage this includes designing appropriate concrete pipe to accommodate flood waters and domestic waste water..now, how can i design an appropriate concrete pipe for a community having a catchment area of 1456 ha.?thank you so much cvg. i really need your help on this matter.

 

[GabionGuy]

For an above grade conveyence, what about stepped weirs or sloped spillways?