Detention Basin too big?? TR-55 estimates?
Detention Basin too big?? TR-55 estimates?
(OP)
Hello,
I have fairly little experience in hydrologic modeling and design of detention basins. I need some advice as to whether my design sounds reasonable or not.
I'm sizing a detention basin with a total watershed area of 54 acres almost entirely undeveloped. My project is developing 14 acres of that watershed area, so ultimately it will be about 14 developed and 40 acres undeveloped.
I was told by a more experienced engineer that a good rule of thumb for basin volume (storage) is 10,000 ft^3 per developed acre. So with that figure in mind, my basin should be sized about 140,000 ft^3.
After routing my 100-year, 24-hour TR-55 hydrographs (P = 7.7 in) through the basin the total storage required is 181,000 ft^3. That seems fairly reasonable, however, the pond seems to be very large to me after drawing it out and adding 2 ft of freeboard. At the top of my basin, the area is about 1.15 acres. Does that seem too large? I have no frame of reference because I've never designed a detention basin before. For that reason, I was extremely conservative in my hydrologic modeling with TR-55.
I just got my PE and this'll be the first project that I'm sealing so I won't mind being a little conservative. The basin is 9.4' from invert to top of basin (2 ft of freeboard) and 1.15 acres at top of basin. Does the size seem reasonable to you? Way too big? Is 10,000 ft^3 per developed acre a good estimate? Also, is TR-55 inherently very conservative? I was told that it was, however, I used conservative numbers anyway.
Any help would be much appreciated.
Peace,
Steve
I have fairly little experience in hydrologic modeling and design of detention basins. I need some advice as to whether my design sounds reasonable or not.
I'm sizing a detention basin with a total watershed area of 54 acres almost entirely undeveloped. My project is developing 14 acres of that watershed area, so ultimately it will be about 14 developed and 40 acres undeveloped.
I was told by a more experienced engineer that a good rule of thumb for basin volume (storage) is 10,000 ft^3 per developed acre. So with that figure in mind, my basin should be sized about 140,000 ft^3.
After routing my 100-year, 24-hour TR-55 hydrographs (P = 7.7 in) through the basin the total storage required is 181,000 ft^3. That seems fairly reasonable, however, the pond seems to be very large to me after drawing it out and adding 2 ft of freeboard. At the top of my basin, the area is about 1.15 acres. Does that seem too large? I have no frame of reference because I've never designed a detention basin before. For that reason, I was extremely conservative in my hydrologic modeling with TR-55.
I just got my PE and this'll be the first project that I'm sealing so I won't mind being a little conservative. The basin is 9.4' from invert to top of basin (2 ft of freeboard) and 1.15 acres at top of basin. Does the size seem reasonable to you? Way too big? Is 10,000 ft^3 per developed acre a good estimate? Also, is TR-55 inherently very conservative? I was told that it was, however, I used conservative numbers anyway.
Any help would be much appreciated.
Peace,
Steve





RE: Detention Basin too big?? TR-55 estimates?
BTW, what kind of work did you do before you got your PE.
RE: Detention Basin too big?? TR-55 estimates?
RE: Detention Basin too big?? TR-55 estimates?
I like doing things green anyway, so an over-sized detention basin that incorporates some BMPs (filter strips, infiltration, etc.) can't hurt since the site plan allows for it.
Peace,
Steve
RE: Detention Basin too big?? TR-55 estimates?
Anyway, I'm fairly confident in the design. The majority of the rest of the site is cut, so I think there won't be a problem with the cost of fill getting too expensive (as long as the site soil is sufficient for fill use).
Peace,
Steve
RE: Detention Basin too big?? TR-55 estimates?
For an offline basin, the lower flows go past the basin and when depth of flow reaches a higher level it spills over and flows into a basin. This effectively knocks off the peak of the storm.