Matching Pipe Crowns

[martin888888]

A good hydraulic practice is to always match pipe crowns in a manhole for a sanitary or storm mainline. However I was reading that you should match 0.8D elevations between pipes. Can anyone expand on that and the reasoning behind it? Thanks

 

[cvg]

matching inverts is standard in some areas, for low flows it may result in less head loss and better conveyance of sediment.

in other jurisdictions, crowns are matched which increases head losses due to the drop at each junction. it minimizes trench depth.

for design maximum flows at about 80% full flow, matching 0.8D results in a WSL which is smoother, theoretically less head loss at design flow.

in most jurisdictios, a drop is required at each manhole to account for manhole losses which makes it difficult to match anything

 

[debgallery]

I've always used a 0.2-ft drop in invert across a manhole as a rule of thumb. This corresponds to a theoretical 0.5% slope across a 4-manhole. The only time I've matched the crowns is when the pipe changes to a larger size (our municipality requires this for wastewater and I've just followed suit with stormwater).

 

[Twinkie]

I've never done anything other than .2' drop across a manhole.