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Dividing Offsite / Onsite DAs. SCS Methodology Question

Dividing Offsite / Onsite DAs. SCS Methodology Question

Dividing Offsite / Onsite DAs. SCS Methodology Question

I have PreDev Drainage Area 1 (DA1) that goes to Study Point 1 (SP1).  It has an onsite component and an offsite component.  I have to reduce the peak 5yr post runoff rate to the pre 2yr rate, but only for runoff generated from the project property.  (Basically I'm not responsible for reducing runoff from the neighbor's property.  It is flow-through.)

How do I model this in HydroCAD?  I want to stay true to the TR55/SCS method.

Here are the two options I have come up with:

Make separate drainage areas (subcatchments) with the same Tc.  The logic behind this is to essentially treat the offsite and onsite areas as one, except that I want to quantify the offsite contribution.  Think of it as taking an extra step to do the CN weighting.  The two subcatchments would go directly to Study Point 1, a Reach in hydrocad.

Make separate drainage areas with unique Tc times.  The offsite area Tc path would end at the upper property line and then I would Tt the offsite flow down to Study Point 1, maybe using a Reach.  The ONLY problem I see with this is that Tt times are usually used when a subarea dumps into a linear drainage feature like a pipe or swale.  In my case, the offsite area sheet flows onto my property, and then concentrates into concentrated flow all the way to the study point.


RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

How about modeling both methods and comparing the results.  It sounds like it is not so complex that it would take much time to do.  

This is often a great way to learn.  One may get similar results with either method.

RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

I normally do method 2 with an artifically increased travel time (higher mannings works).

I would be interested to find out the results of your two ideas.  Please let us know what you determine.  Thanks.

RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

It's highly unlikely the offsite area sheet flows into the reach as sheet flow rapidly turns into shallow concentrated flow -- within 300' on a graded surface and sooner on natural terrain.

You want to model it as close to what is physically occurring as possible.  If the offsite component has a longer Tc then use that.  I would personally separate them if only to demonstrate that you're achieving your required goal of reducing onsite post-developed flow to less than pre-developed.  As this will require some type of detention/retention, your Tcs will be different for offsite and onsite unless your offsite component routes through your detenion/retention facility.

RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

#1 doesn't make sense.  You have two distinct subareas, so treat them separately, route one through the basin, then recombine with the Bypass.  This is the whole point of detention, to offset the peaks.

Unless Tt is significant, I would either add it to th Tc of the Bypass, or just ignore it.  But a reach would be valid, assuming it is accurately described.

Engineering is the practice of the art of science - Steve

RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

LHA (and others)
Is that true if the drainage areas are distinguished only by a property line?  The whole area is farmland that runs uninterrupted to one study point, and there is no divider like there would be if we had DAs that were separated by a road or ridge, and flowed to a central stream.

Method 1 would basically be trying to separate out each part of a whole watershed.  Think of it as determining the amount of runoff from the pervious part of your watershed separately from the lawn area.

The key here is me knowing how Tc and Tt affect the characteristics of a watershed's runoff hydrograph.  The TR55 manual is bare-bones and doesn't go into the theory.

RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

Could you not construct a swale to bypass the offsite flow around your detention area?

RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

Yes, but if I can do it with accounting I don't have to make a parallel structure just for bypass.  I can let it intermingle with site-generated water.

RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

That's true, but your detention basin would have to be bigger.

If you bypass the water, your accounting becomes a pre- and post-developed flow calculation for your site water only.  If you don't bypass the off-site water, then you have to separate the offsite water for accounting purposes only and then show that the post-developed condition, which is a different storm (5 yr as opposed to 2 year) meets the requirement that the on-site water component of the flow is detained to the pre-developed 2-year storm rate.

To sum up, you'll need to calculate:

1: On-site pre-developed 2-year hydrograph
2: On-site post-developed 5-year hydrograph
3: Off-site 5-year hydrograph
4: Combined (2 + 3) 5-year hydrograph routed through detention basin

... and finally, show that (4 - 3) <= 1

RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

Let me try this --> One concentration point for entire area, correct?  Compute predeveloped flow rate at POC.  Developed confitions will have at least two POCs, one in the basin and then at original POC.  Compute the seperate flow rates, route one through basin and then to the original POC and combine with the remaining watershed.  Compare predeveloped with post developed to confirm reduction or equivalent flow rates.  

RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

I tried it two ways:

1.  Two subcatchments with identical Tc.  One for onsite and one for offsite.  Both subcatchments pointed to a reach which was Study Point 1.  This configuration produced about a 10% higher flow rate than method 2.  The peaks of the runoff hydrographs were slightly off in time from each other.  I thought if they had equal Tc, then the hydrographs would be timed the same, but have different magnitudes.  Wrong.

2.  One subcatchment with Tc, areas, and cover conditions all equal to those in method one.  Essentially I combined the two subcatchments from method 1 into one subcatchment.  This produced the lesser runoff rate.

I have read in a Penn State professor's course notes that in cases where cover conditions vary widely over a given watershed, that splitting the watershed and combining runoffs is advised instead of weighting all the CNs together.  For example, if you have a parking lot and a meadow in one watershed, you split each into its own separate area, compute the CN, generate runoff hydrographs, and then combine hydrographs.  This is most similar to Method 1 above and would seem to legitimize it a little bit.

At the end of the day, I decided to go with method 1.  I'm not sure how this will play out in balancing with the post development runoff, but I guess I will find out.

RE: Dividing Offsite / Onsite DAs. SCS Methodology Question

I might go back a little to why do you have the reg.'s in the first place.  Holding the 5 yr to the 2 yr sounds like some sort of stream incision protection regulation.  These stream protection reg.'s are horrible to write and worse to implement.(If this isn't the case just skip the rest)

Since the the upper section part ins't your property I would push to ignore it.  I would see if the regulators let you just ignore the inflow and just treat your shed in isolation.  I know it doesn't sound real and "true" to what is going on, but these regulations are more just a hope and a prayer than true.  

While I like the notion of these regulations they can often be worse to streams than doing nothing.  I was supposed to help come up with stream protection detention requirements for an area with stream slopes of 1% and with over 20 feet of a "bedrock" strength material.  Our only real problem was deposition and yet we were worrying about incision.  Denver gave a presentation where they had pretty much given up protection be detention alone and required armoring streams.  Also by holding the 5 yr in an actually incising situation can just speed up problems.  A lot of areas that have problems have the start of erosion at just over the 1 yr event.  Since erosion is a work process, typically your increase in power to cause erosion from a 2 to 5 is less than linear.  Therefore you can increase the amount of work by increasing the duration by making the 5 yr act like a very long 2 yr.  This is also fake too but it makes for horrible to write and implement regulations.

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