detention basin invert elev 2

[newjerseype]

Where should the invert elevation of the pipe into a basin be set at? If its low, as the basin fills up, so will the pipe. Does this matter? Usually pipes are designed to flow full so I'm not saying the pipe can be used as additional storage but what are the considerations when determing the inlet invert elevation?

 

[cvg]

doesn't matter, as long as you consider the tailwater in your hydraulic analysis of the pipe

 

[DRC1]

Usually for a basin, the sump is about 2 ft below the drain pipe invert. That allows some seltlement of soils and large objects before entering the drain pipe. The sump must be cleaned occasionally.

 

[Maury]

Try to set the pipe enough above the bottom of the detention basin so that it won't readily be obstructed by debris. Ther's nothing wrong with including the pipe as additional storage. A catch basin ahead of the outlet is a good idea.

 

[bltseattle]

Setting the pipe invert near, or below the pond bottom in a sump, will help to dissipate energy and minimize side slope erosion during high flows. In a wet pond I would set it with 6-12" water above the crown. In a dry pond you could use a sump, or set it within a foot of the bottom with appropriate scour apron at the outfall. As noted by CVG, you should the design for the tailwater, eg. check the max design flow with the max WSEL (a culvert nomograph - flowing full - is a simple way to check the last pipe segment HGL). I also recommend limiting the inflow pipe to 2% max slope, using a drop at the upstream CB as needed, so that inflow velocities are between about 3-10 fps for the max design flow.

 

[newjerseype]

How exactly do you add a sump to a pond? Do you put in an inlet at the pond bottom and have your inlet pipe connect to that?

 

[gbam]

If you design the basin with a sediment trap at the inflow of the basin you can set the pipe invert at bottom of the basin. You have to consider how often the basin will be maintained. I have experience with our goverment agencies never maintaining these types of structures. Just a note.

 

[Bayou]

Our local regulations require the pipe invert to be a foot above the bottom of the basin. This will decrease the erosion potential near the pipe. Of course some protection will be needed.

 

[bltseattle]

newjerseype,
You add the sump (aka small permanent pool) to a pond by making the area where the inlet pipe comes in deeper than the rest of the pond. For example, the sump could be 2-5 feet (say pipe diameter + 2ft) deeper than the overall pond bottom and would have length and width dimensions at least 3x the inlet pipe diameter. Side slopes could be as steep as suitable for the soil characteristics(say around 2H:1V) and stabilized with rock over geotextile.

The point of a sump is that it would hold water, so that energy from incoming water is dampened by the backwater effect on the end of the inlet pipe, even when the "overall pond" is empty. Consider that if a sump is not provided, during the initial filling of the pond the incoming water will potentially scour the pond sides and bottom as the flow enters the facility.

You can tell if you need a sump by comparing the discharge velocity of the pipe to the erosive velocity for the pond bottom material. A sump is one way to reduce the velocity; another way is to control the pipe slope & design so that erosive velocities do not occur. A rock pad may also be acceptable.