Multiple basins

[kechha2060]

I have a 60 Ac project site with multiple basins, site laid out by architect which I have to abide by. My city requires using unit hydrograph method to size basins and store only incremental runoff. Is it Ok to compute the incremental runoff volume for the whole site and size the basins based on proportions (contributing areas)? It is one development so I believe I can assume the basin characteristics are similar.

Thanks

 

[RWF7437]

It is probably OK to size the detention(?) basins as you suggest based on the area of the drainage basins, or catchments. But why not just ask the reviewers what they will accept. Some of these people can get VERY fussy about this stuff.

good luck

 

[francesca]

You should delineate drainage areas for each pond and check that each individual pond can contain the water immediately draining into it, sizing your outlets accordingly. You need to do this, not to satisfy the city, but to make sure you aren't flooding buildings and other facilities on site.

If ponds discharge into each other, you need to compute an outflow hydrograph, route it through the conveying channel, and add it to the drainage basin hydrograph to compute the combined inflow hydrograph for the downstream basin.

 

[RWF7437]

"You should delineate drainage areas for each pond and check that each individual pond can contain the water immediately draining into it, sizing your outlets accordingly. You need to do this, not to satisfy the city, but to make sure you aren't flooding buildings and other facilities on site."

To test whether or not this is a true statement, try it both ways. Given that the precipitation data you are likely to have available has a typical statistical error of plus or minus 30%, it seems very unlikely that you would see any differences in your design either way. In 60 acres how many sub-basins and how much variability in rainfall and runoff can there be ?

Also remember, that you are not likely to be required by the reviewing agency to design for more than a few of the many possible storms. So there will always be some probability, however small, that a storm larger than any considered in your design will occur during the useful life of your pond(s) and conveyance system(s).

You can, by careful selection of design storms and a healthy dose of safety factors, reduce this possibility to a very small number; but you can never, economically reduce it to zero.

good luck

 

[kechha2060]

In hydrograph analysis which one do i select the concentration point, the inlet (catch basin) or the basin site (pipe outfall), if routing along pipe is to be ignored?

 

[RWF7437]

You've answered your own question. If pipe routing is to be ignored, then select the most downstream point in the sub-basin. If you use a hydrograph generation method, such as the NRCS Curve Number method, the result will not be significantly affected by small differences in the time of concentration. Some other methods don't use the time of concentration at all.

 

[francesca]

In a 60 acre site, even with low density residential (30% impervious), there can be a good number of basins and a large amount of variability of runoff. I have designed sites a third of that size with four different drainage basins, and that not in particularly hilly terrain. If there isn't a conscious effort to make the basins match the catchment, there is a huge liability risk, especially if the site does not all drain to a single outfall.