Notice that the examples of the intensities found on the isohyetal map are for a 60-minute storm. This now has to
be adjusted to a particular duration that is critical for the project under construction, similar to the adjustment
made in the use of pinpoint data. Once the critical duration has been determined, the adjustment is made on the
"Standard Intensity-Duration" curve which is given in figure 1-5, page 1-9.
I60,2 years
Critical Duration
Iadj
1.0 in/hr
50 minutes
1.2 in/hr
1.5 in/hr
30 minutes
2.4 in/hr
2.0 in/hr
10 minutes
5.2 in/hr
PART B - DELINEATE A DRAINAGE AREA AND
ESTIMATE THE ACREAGE
INTRODUCTION
The terms drainage basin, drainage area, and watershed, are synonymous terms that are used interchangeably. A
drainage basin is that portion of the earth surface that contributes water in a rainfall event to the drainage basin
outlet--your drainage structure.
DELINEATION
Delineation is the process of drawing a boundary that completely encompasses and defines the area of rainfall
contribution to your drainage structure.
Once a drainage basin outlet has been selected, the process of delineation can be accomplished using the
following steps:
Locate all proposed or existing drainage structures.
Locate all existing terrain high points.
Draw flow lines away from high points. Flow lines are graphic representations drawn on a topographic
map that indicates the direction of water flow as it crosses terrain. Several rules must be followed when
drawing flow lines.
-
Flow lines must always indicate downhill flow of water.
-
Flow lines are always drawn perpendicular to contour lines. This is because water always flows
downhill at right angles to elevation changes.
-
Flow lines never cross delineation lines. This is because the delineated boundary defines the divide
or the outer boundaries of rainfall contribution.
Draw the delineated boundary or the dividing line starting at the top of a terrain high point, down a spur and
across a saddle connecting another spur and saddle. The flow arrows will actually show a pattern of water
moving in two directions (see figure 1-6). Figure 1-7 shows the delineation of a spur.
EN5465
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