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?
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.
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:
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