DRAINAGE AREAS
After a site has been located the next step in processing a permit application or a
Request for Floodplain Information is to determine if the drainage area (DA)
above the site is greater than or less than one square mile. A site having a DA of
less than one square mile is out of our jurisdiction and will not require a permit
from the Department of Natural Resources, nor do we offer base flood elevation
information for these sites.
Drainage areas also are used in the calculation of discharge information needed
to develop hydraulic models of streams.
DRAINAGE AREA DELINEATION AND COMPUTATION GUIDELINES
Streams form the valleys in which they flow. Every river consists of a major trunk
segment fed by a number of branches or tributaries that diminish in size away
from the main stream. The tributaries define a network of channels that drain
water from a discernable, finite area that is the drainage basin, or watershed.
A drainage area above a site is defined as “the area, measured in a horizontal
plane, that is enclosed by a topographic divide such that direct surface runoff
from precipitation would normally drain by gravity into the river basin above the
specified point.” All runoff enclosed by a delineated drainage area must pass the
point of interest. Runoff outside of the delineated area cannot enter the area
upstream of the site.
Basic rules for the delineation of drainage areas are as follows:
1)
2)
3)
4)
5)
6)
Utilize the USGS topographic mapping in the Resource Center in
ABE 104
A stream or body of water should not be crossed
A basin divide will not fall in a drainage swale
All ridge lines should be followed
When in doubt, consult someone with more experience
After the area has been delineated, it should be reviewed for
accuracy
Drainage areas are determined by tracing all of the water bodies flowing into the
stream or river upstream of the proposed site. A divide or ridge surrounds every
drainage basin. A divide is defined as “the line of separation or dividing ridge
marking the boundary between two drainage systems.”
Schematic of Typical Drainage System Characteristic
Computing a Drainage Area
Pull the “purple line” quads from the Hydrology files in the Resource Center. The
quads are filed in alphabetical order. Each file will have a blue line copy showing
drainage areas delineated by the US Army Corps of Engineers and at least one
USGS topographic map showing previous delineations done by our office.
Check all of the quad copies to see if there is a previous delineation for your site
or one near your site. If so, review the delineation to see if it is correct. The site
will be marked with a dot and a circle around the dot, and labeled with the
month/year the DA was completed.
Check the blue book “Drainage Areas of Indiana Streams,” published by the U.S.
Department of Interior. This book measures the drainage areas of rivers and
streams having a drainage area of five miles or greater. This information can also
be found on our Web site.
If no previous information is available, begin your delineation by locating your site
on a quad pulled from the file. Be sure to use one that is not stamped
“Superceded.” Transfer the purple lines from the blue line print if any are located
near your site.
Next, working upstream, trace all of the streams that drain into the water body on
which your site rests. Remember that the tributaries point upstream, and the
streams and tributaries will not cross the purple lines. After tracing the streams,
you can delineate your drainage area by tracing the divide that encircles all of the
tributaries draining into the main stream.
Skills Test
The five USGS topographic maps included with this training kit have sites labeled
#1-#5. Pull another copy of each from the drawer in Technical Services and
locate the respective sites on each map. This will be your working copy. Trace
the tributaries that drain into your site and, using arrows as illustrated in the
above schematic drawing, point out the direction of flow. Next, trace the divide
that separates the drainage area from the adjacent drainage areas. Don’t forget
to pull the purple line quads from the Resource Center.
Measuring Drainage Areas
After the drainage area has been delineated, have a co-worker review and make
necessary corrections. Then measure the drainage area using a planimeter or
digitizer.
Planimeters are located in Engineering and in Technical Services. The settings
should be “AREA,” “STREAM “, and ” MILES.” The scale should be set at
“24000” – meaning a ratio of 1:24000 as specified in the Reading a Topographic
Map handout.
Unlock the curser arm, turn on the planimeter, and move the arm back and forth
to stop the beeping that will start as soon as you turn it on. Check the settings.
If not as stated above, they can be changed using the buttons as labeled in the
illustration.
Place the curser over the site, push the START button and trace the DA in a
clockwise direction. When you return to the point of beginning, press the END
button. Press START and repeat the last instructions. Trace the area three times.
At the conclusion of the third time, hit END, the TOTAL button, and the
AVERAGE button; this will give the area of the DA in square miles. Check
yourself by counting the number of Sections located in your DA to determine a
rough estimate of what the DA should be.