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# Determination of Size of Drainage Area2

## Determination of Size of Drainage Area

(OP)
I would like a rough idea of how the AREA of a drainage area for water flowing from a steep slope (2 to 1 ratio) would be calculated. Are there minimum area requirements that must be met for grading plans to be approved?

planimeter?

### RE: Determination of Size of Drainage Area

Are you asking how to delineate a drainage basin?

Certainly there are any number of things related to drainage that must be met for a grading plan to be approved.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

### RE: Determination of Size of Drainage Area

The drainage shed is determined by finding the total tributary area that drains to the area in question.  The slopes shouldn't be a factor in determining this area.

For large areas one might use the USGS topographic maps.  You would investigate upstream areas and draw a line delineating one drainage shed from another.  Imagine where a drop of water would drain, towards your area or away from, as an example.

A customary rule of thumb is the modified rational formula can be used for determining storm run offs for 200 acres or less.  Larger areas use the unit hydrograph analysis.

In urbanized areas, you might need to do a fair amount of research of as builts to determine if storm water is intercepted before reaching your site, for example.

### RE: Determination of Size of Drainage Area

Just remember that water always flows normal to the contour lines.

Start from the limits of your project and see where the flow lines lead you to determine the total drainage area.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

### RE: Determination of Size of Drainage Area

I wonder how many people know what a planimeter is?

### RE: Determination of Size of Drainage Area

Hard to find them anymore.  Used one to establish area-volume curves for High Ross Dam when it was being designed.

Haven't used one for 40 years now, but haven't forgotten how either.  Kinda like riding a bike or using a slide rule.

Interesting how they can both add and subract, no?

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

### RE: Determination of Size of Drainage Area

I have one at home - haven't used it in 20+ years. I'd probably need a refresher course since I suffer more and more from CRS.

### RE: Determination of Size of Drainage Area

When doing hydraulic analyses here, we use StreamStats by USGS to get drainage areas and discharges of anywhere you click on the map.  We've delineated a couple of these areas by hand and they are on the button.

It doesn't appear that every state has StreamStats but maybe you'll get luck and they'll have your state:

http://water.usgs.gov/osw/streamstats/ssonline.html

http://water.usgs.gov/osw/streamstats/ssinfo.html

### RE: Determination of Size of Drainage Area

I used one in 2005... I bet they're still using it at my old company. It was digital.

### RE: Determination of Size of Drainage Area

bridgebuster: Proud to say I have run an earthwork on a 500+ acre project with one of those contraptions at the age of 26.  I am now 33 and find it funny how many people can't even look at a topo map.  I was always that person in the office trying to memorize everything the older guys would explain to me.

Funny how people would steal ipods in the Engineers' drawers, but there are all of these HP48GX's and other expensive equipment (Leroy) sitting around like nothing.

B+W Engineering and Design
Los Angeles Civil Engineer and Structural Engineer
http://bwengr.com

### RE: Determination of Size of Drainage Area

brandonbw - thanks; you're restoring my faith in the younger generation.

### RE: Determination of Size of Drainage Area

Brandon -

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

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