French Drain Discharge Capability

[BigH]

Friends:

It has been a long time since anyone has logged on to our forum. We have been busy on site – the diversion tunnel is underway – lining is almost done. Being in a mountain terrain, there are possibilities, as one can guess, of landslides – oops, severe slope distress! I’ve attached a picture for is that I know about!

But the question at hand concerns temporary stream control for construction of a 40 m high rockfill embankment. Stream runs through the foundation base – something like 140 m or so width at bottom and is in incised valley about 20 m wide. Elevation drop is something like 10 m over 200 m or so. Stream flows at about 1 cumecs; suggest that it might be upwards to 3 at peak monsoon rain downpours. See picture attached.;.

Culverts for the embankment are set up about 17 m which would be equivalent to the backside of the embankment and is approximately reservoir level (in two years). No allowance made by contractor in bid for temporary stream diversion. He wants to put in French drain in stream – lay clear rockfill along the base as an added blanket. Proj Mgr questions the 2 year use of the French drain – although it will only be about 3 to 4 months before the culvert is put in and then it will work. Stream is clear – jungle upstream so no real erosion problems foreseen. But Proj Mgr fears clogging anyway.

Cedergren in his book on seepage does talk about drains like this but his “largest” grain size is only in the order of 2 inches or so. Question is – has anyone had experience with discharge quantity of rockfilled French drain – say 1m x 1m in size? We anticipate that the actual drain will be about 2m by 3 m high in dimension – about 6m2. It is my estimation that given 30% voids (conservative) we would have about 1.5 m2 voids. k of the rockfill – say 100 cm/sec (1 m/s) so I would suggest that we have enough for normal flow of 1 cumec even if we had only turbulence loss of 40% or so. We don’t want “buildup” of water in stream behind during period before permanent culverts put in (3 mo). We are in monsoon now.

Does this seem reasonable – or experience dictate the need for a bigger French drain? I would think that the 18 m wide by 2 m high (at 30%) voids would more than handle any added flow.

See attachment for a few snaps of the valley – and the start of the blanket drain.

Thanks and

 

[jheidt2543]

BigH:

Long time no hear!

The problem you are presenting is beyond any of my experiance, but I'll make a few comments:

1. I can't imagine how a contractor could bid a job like this without thinking about how to handle the existing stream flow.

2. I can't see how the plans & specs would not address the question of flow diversion during constrcution.

3. Those two points being made, where does the French Drain drain to? I'm assuming that it would act like a "job built culvert" allow flow thru the bottom of the embankment. Once the embankment is completed, how do you plan to seal up the French Drain?

4. What happens if, during the 3-4 months it takes to get the culverts in, the French Drain does get clogged with debris? Is there a contingency plan or "safety valve" to save the work done to date?

5. Is it possible or practical to set up some kind of trash catcher up stream?

6. The photos make it hard to judge scale, but the stream flow looks pretty small. Is there enough time before the Monsoon season to get the culvert in? It seems like a bid gamble to me. If it works you're a genius and if it doesn't you're a goat.

7. Why can't the culvert be installed first, at the lower elevation?

 

[BigH]

Jim - thanks for your comments.
1. While there was a "site visit" - the site was heavily forested and doubt that he would have trudged through the forest; but point taken.
2. Diversion of temporary water control is contractor's responsibility - by the design.
3. Don't plan to "plug up" up the embankment afterwards. The embankment will be made of rockfill - drainable. Still, we have "paved" part of stream coming into the culverts. Culverts actually for purpose so embankment isn't "called" a dam.
4. Again, I am not overly worried - worse that happens is that "upstream" there is a buildup of head in the stream valley.
5. Stream is small - 1.5 m wide normally about 0.5m deep - we are in monsoon already.
6. Contractor can do so.
7. Proj Mgr (overall development) wants to do this. matter of cost.

I'm an optomist on this - many are pessimists. We can always have contractor dig down a channel if flow no good - and build a "swale" on the way up.

Thanks, Jim.