time of concentration 1

[DenverJ]

I wanted to know if this is an acceptable practice.

I have a small watershed (1.4 Ac) with a proposed gravel road in the middle. I am using a ditch turnout/level spreader to divert runoff from the roadside ditch to a forested area. I would like to run my Tc for the watershed from an upslope area down to the ditch, through the ditch, across the level spreader and through the forested area to my Point of Interest.

My question: Is it acceptable to enter the Tc as sheet flow below the level spreader?

 

[Twinkie]

This is really in a grey area, and you should probably discuss with the municipal engineer to whom you are submitting. In theory I would say no. In practice, depending on the particulars of the design, maybe.

 

[DenverJ]

Thanks Twinkie.

I figured that might be the case. I am designing a well pad with an access road. I have one watershed in the pre-construction condition and I am splitting the watershed in two for the post; one for the pad which is being treated by a BMP and one which contains the roadway with the roadside ditch and turnout. With the BMP for the pad I am reducing the runoff for all storm events above the 2-year event. For the 2-year event I have a 0.05 cfs increase which appears to be coming from the road. I am sure the level spreader below the turnout will account for this increase. I was just trying to avoid splitting the watershed again to model the level spreader given the small increase. In years past we wouldn't worry about such small increases.

 

[cieg22]

My gut is saying shallow-concentrated, but I'm a bit out of practice.

 

[psmart]

I agree with both replies: Sheet flow would be more appropriate below the level spreader, but you should discuss the details with your reviewing agency.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

I've said it before and here it comes again - why would you calculate or report a storm runoff to the nearest 0.05 cfs? Suggest you report to the nearest whole integer and your problem disappears.

 

[DenverJ]

cvg

I guess the answer is that the program defaults to two decimals and we've never really looked into whether that can be changed. Looking last night I saw how I can do that. Now I just need to talk my head engineer into letting me do it.

 

[gbam]

Agree with cvg round that puppy up to nearest integer. I cannot remember exactly what the tolerance is for hydrology; I recall /-40-50% roughly. I dont recall the web link but do a google search for Voodoo Hydrology. maybe some else has it.

Back to your question, definately chat woith the reviewer but go in with an sound approach.

 

[TerryScan]

Keep in mind, if one is doing water quality event modeling (1-2 yr storm events), the entire runoff for 1.4 ac. can easily be less than 3 cfs. If a regulator wants to see a hydrograph of this, rounding to 1 cfs probably would not produce what they are looking for.

 

[LincolnPE]

For a 1.4ac site displaying runoff to the hundredth may seem to appear disingenuous to some, but still doable... Rounding to the nearest integer is rounding too far for this small site.

Displaying runoff to nearest tenth...is just right.