Technical data

Chapter 3 Performing Hydrologic Studies

Key Concepts

■

Establish an intensity duration frequency (IDF) curve file (.idf

extension) for your project location.

■

Determine the size of the drainage area (A), the runoff coefficient

(C), the adjustment factor, the time of concentration (Tc), the

rainfall frequency, and the rainfall intensity. This can all be

calculated or selected using commands from the Hydrology menu.

■

Slopes and elevations across a site can be extracted from a surface

model. You can also build a surface and model the watershed

before calculating runoff by using the AutoCAD Land Development

Desktop Terrain Model Explorer.

■

Refer to the AASHTO (American Association of State Highway and

Transportation Officials) Model Drainage manual for more

information regarding the Rational Method.

To calculate the peak discharge using the Rational Method

Steps

Use

to look up

1 Determine the specific watershed characteristics

and design criteria, including watershed

location/area, soil type, land use, and sheet,

shallow, and channel flow parameters.

Create your intensity duration frequency (IDF)

curve file (.idf extension) from applicable rainfall

data for your project location.