Impervious areas - connected or not connected? 4

[BillsSis]

Say I have a pre-construction site which has a total area of 3.8 acres. In this 3.8 acres lies two homes (about 1400 sf - one floor), two separate garages (not connected to the homes and about 450 sf each) and a small shed. The rest of the acreage is grass, in good condition. When the runoff hits the roofs of the buildings, it'll sheet flow until it runs off the roof, where it'll land in the grass, then flow for several hundred feet before discharging onto roadway (over the curb). When I compute the weighted CN value, would these impervious areas be considered connected or un-connected? If un-connected, how to figure the percentage of this impervious area over the total impervious area?

The whole area (all 3.8 acres) is in a Hydrologic Soil Group B Soil.

The flow path from the most distant point to the discharge point of the area is approx. 600'. This path does not appear to include the roof runoff from the buildings.

If this is considered un-connected, wouldn't the percentage of impervious areas be 100%? Or am I missing something?

Thanks!

 

[Kevin_Engineer]

Only impervious surface that is hydraulically connected directly should be considered a CN of 98 (assuming this is your impervious #). All other non-directly connected impervious areas should be considered a CN of whatever surface they join.

If you think about it a different way, a roof that drains onto the grass and sheets is going to develop a different amount of runoff than a roof that drains to the road/driveway. Instead of behaving like impervious surface, the water acts like a heavy rainfall landing on the grass, and the grass CN properties will apply.

 

[BillsSis]

Thanks, Kevin. Your answer makes sense. I think I'm thinking too hard about this; I tend to do this!

 

[TerryScan]

According to TR-55 documentation, a CN adjustment is ONLY made if the impervious is less than 30% of the total area. The document states that for greater impervious area, "the absorptive capacity of the remaining pervious areas will not significantly affect runoff."

In the example given, if one correctly applies the rules with the areas given, it is doubtful that there will be any CN adjustment.

 

[Kevin_Engineer]

TerryScan,

I would recommend taking a 1,000-foot view of the drainage before considering curve numbers. For instance, what is the slope of the impervious surface draining to pervious areas? Does it fall to the ground, losing its energy and subjecting it to any level of infiltration? Does it flow across a very large area of pervious with lots of time to absorb into the soil, or does it travel a short distance? 30% can represent any size of land, so I would recommend using "engineering judgement" when determining if the surface should be classified as CN 98 or lower.

 

[TerryScan]

One always needs to employ "engineering judgement". In my experience, If one were questioned on this judgement, regulators will typically rely on regulatory documentation. Since we are talking about NRCS methods, I cited an NRCS documentation regarding unconnected impervious. "All other non-directly connected impervious areas should be considered a CN of whatever surface they join." does not apply if one follows the methods described in the documentation.

 

[Kevin_Engineer]

Absolutely correct! All TR-55 related documentation applies here, and regulators will for sure check against it if that ever came into question. If you're submitting to an authority that will look at it through that lense for review, it is important to cross your t's and dot your i's using their assumptions for calculating runoff.

 

[peter246]

Thanks Bill for a great question, and thanks to Terry and Kevin for the great follow up! I've been at stormwater for a while and haven't quite grasped this subtlety 'till now. Having the documentary backup, along with engineering judgement and consideration of regulatory requirements is key

 

[BillsSis]

All,
I had to read each of the answers three or four times to understand the intent! The total of the impervious areas (roofs and driveways) came out to be 6.6% of the total drainage area, or 0.25 acres. So....since that is less than 30%, an adjustment would have to be made.

By the way, the very little that runs off the driveways would drain up to 200' over grass before outfalling to the road, while the runoff from the roofs would drain an average of 225' over grass (or pervious) surface before outfalling to the road. I would think that part of the roof / driveway runoff would have a likely chance of permeating into the ground, on its way to the road.

Thanks Terry and Kevin for great, thought-provoking answers!