Dividing Offsite / Onsite DAs. SCS Methodology Question
Dividing Offsite / Onsite DAs. SCS Methodology Question
(OP)
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:
1.
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.
2.
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.
Opinions?
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:
1.
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.
2.
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.
Opinions?





RE: Dividing Offsite / Onsite DAs. SCS Methodology Question
This is often a great way to learn. One may get similar results with either method.
RE: Dividing Offsite / Onsite DAs. SCS Methodology Question
RE: Dividing Offsite / Onsite DAs. SCS Methodology Question
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
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
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
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
RE: Dividing Offsite / Onsite DAs. SCS Methodology Question
RE: Dividing Offsite / Onsite DAs. SCS Methodology Question
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
RE: Dividing Offsite / Onsite DAs. SCS Methodology Question
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
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.