Detention Basin too big?? TR-55 estimates?

[nutbutter]

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

 

[jgailla]

We have high groundwater and flat land and get ~10% developed area for residential and ~20% for commercial retention ponds. My understanding is that TR-55 is inherently conservative. It seems like it's always a good idea to have another engineer QC your work, even if they are less experienced.

BTW, what kind of work did you do before you got your PE.

 

[cvg]

that's kind of a difficult question to answer as the size is dependant on many variables, not the least of which is the amount of precipitation during your design storm. For instance, a 100-year storm around here would drop about 4 inches of rain, whereas a 25-year storm in Florida might drop 15 inches. Pre vs post runoff coefficients will also affect the amount of increase in the runoff. Also important is the type of basin. Is this a retention or detention basin? Is the purpose to retain the storm, attenuate to pre-design level or attenuate even more to reduce downstream flooding? Is it online or offline, ie: is it situated on the stream so that all flow passes through the basin, or is there a spillway that scalps off the peak flows and discharges into a basin located offline? Online basins need to be sized to accept the entire hydrograph up to and including the peak. They allow the receding limb to pass without much attenuation. Offline basins are sized to accept the volume of the hydrograph during the time when the pre-development peak flows are being exceeded. This might result in a smaller basin. Around here, most basins don't exceed about 4 - 5 feet in depth, so your basin is considerably deeper. Can you create your freeboard above ground? If so, this will help to reduce excavation and disposal of dirt.

 

[nutbutter]

Thanks for your advice, jgailla. I have a PE coworker with more experience QC'ing my design as I type. I worked in environmental remediation and landfills for 3 years and I switched to doing land development work about 1-1/2 years ago. Unfortunately, I haven't had the opportunity to design a detention basin on my own until now. I usually took the sites w/o detention requirements before I got my PE, because my 2 coworkers had more experience and we were swamped. Fortunately, now I've been given some opportunities.
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

 

[nutbutter]

Thanks cvg. Good advice. I'm rather new to hydrologic/stream modeling so that helps me immensely. The basin is for detaining runoff to reduce offsite flows to pre-developed levels. It is online in a sense that it will be picking up runoff from 40 additional undeveloped acres. It is not necessarily a stream per se (as there is no base flow), however, it is clearly in the path of a decent-sized drainage channel that flows during precipitation events. Does this constitute it being online or does there have to be a base flow (stream flow)?

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

 

[cvg]

base flow is not the issue. it sounds like your basin is online as it appears that all flow will enter the basin, flow through it and the outlet works will reduce the peaks to pre-development levels.

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.