2002 Annual Report
Research
Geomorphic Characteristics of Drainage Ditches in Southern Minnesota
John Nieber, Professor
Bruce Wilson, Associate Professor
Gary Sands, Assistant Professor
Bradley Hansen, Assistant Scientist
Joseph Magner, Minnesota Pollution Control Agency
Gregory Johnson, Minnesota Pollution Control Agency
Funding Source
Minnesota Pollution Control Agency
Objective
Investigate a relationship between the geomorphic characteristics of
natural stream channels and man-made drainage channels and their associated
watersheds for locations in southern Minnesota.
Project Description
Drainage of land in Minnesota has been done since the mid-1800s to provide
arable land for agricultural production and to make land suitable for
the siting of homes, industry, businesses, and transportation facilities.
Much of the drainage has involved the construction of open drainage channels
needed to convey drainage water to watershed outlets.
Maintenance of drainage channels has proved to require a substantial
financial commitment to local governments and drainage districts. Cost
figures have not been derived yet for Minnesota, but a recent survey for
northwestern Ohio determined that the maintenance of drainage channels
averaged $400/mile/year with a total cost to the region of $1.7 million/year.
In addition to this significant financial commitment, it has been determined
that the process of construction and maintenance of drainage ditches is
the leading cause of aquatic life impairment of the channels in northwestern
Ohio. Similar conditions are expected for southern Minnesota.
The purpose of this project is to investigate possible new design and
maintenance approaches based on fluvial morphology to help to reduce the
cost of ditch maintenance and the impact on aquatic life.
Results
The study will be performed by collecting channel morphology data from
selected locations on natural streams and man-made drainage channels.
A total of 9 locations on natural streams and 27 locations on man-made
drainage channels will be investigated. The stream and drainage channels
selected will have contributing drainage areas up to 100 km2 maximum,
with a minimum drainage area of about 10 km2. The lower limit on drainage
area is set because at present the study is limited to conveyances that
have perennial flows. Channel reaches selected will have a minimum length
of one km.
Three natural stream channel reaches will be selected for each of the
slope categories including steep, moderately steep, and relatively flat.
The steep channels will be selected from streams that are directly tributary
to the Minnesota River. The moderately steep channels will be selected
from streams that are tributary to the Cottonwood, Yellow Medicine and
the Watonwan Rivers. The relatively flat channels will be selected from
streams in the Blue Earth and Leseur area. Similar categories of slope
will be used to select the drainage channel study reaches, and in addition,
the drainage channels will be selected from the categories of newly constructed,
older channel with regular maintenance, and older channel with inadequate
maintenance.
The channel morphology data to be collected include: 1) Main channel
cross-section at several points along a reach; 2) Bed profile of the channel
reach; 3) Water surface profile for prevailing flow conditions; 4) Azimuth
of the prevailing flow direction for the channel reach; 5) Depth of the
channel from the top of the bank; 6) Bed material particle size distribution;
7) Vegetation type and vegetation density within the channel; and 8) Estimated
bankfull discharge.
The channel morphology data will be correlated with selected data measures
for the contributing watersheds. Data collected for contributing watersheds
will include: 1) Contributing watershed area; 2) Landuse characteristics;
3) Soil classification; and 4) Estimated return period that produces bankfull
discharge in the channel.
This project began in the late fall of 2002. Field trips to the Minnesota River Basin region have led to the selection of potential reach study sites. Channel morphology data have been collected for a few sites, and analysis of those data has been initiated.