ERDC/CHL CHETN-VII-5
December 2003
of total flow through this reach was increased due to the reduction in floodplain and distributary
conveyance. Based on the sparse data in Figure 5, the increase would be about 30 percent over
normal pool transport.
At this point it might be going a bit too far to say definitively that the drawdown absolutely affected
bed-load transport in the study reach. This is because the ISSDOT method is still in development,
and the number of data points very minimal. Therefore two other methods of determining if the
drawdown caused a net increase in transport will be explored. One is to analyze and compare past
historical bathymetric data that the District has collected with recent data collected during the
drawdown. Another method is to use standard analytic computations as in sediment transport
functions.
Sediment Budget and GIS Analysis:
Sediment budget:
In addition to the measurements made by the U.S. Army Engineer Research and Development
Center and used in the ISSDOT computations, St. Paul District personnel have been measuring
sediment and hydraulic parameters on the river for years. These data were collected as part of habitat
improvement projects and navigation channel maintenance activities. From this data an extensive
sand budget was developed. See Hendrickson (2003). This sand budget was developed for Pools 1
through 10 using available information on sediment transport at U.S. Geological Survey (USGS)
gauging stations, long-term channel dredging data, studies of sediment deposition, and hydraulic
data.
The transport of sand-size sediment was of particular interest because of the expense associated with
navigation channel dredging and because sand is the geomorphically dominant sediment size on the
Upper Mississippi River. That is to say, that major planform changes on the river are associated with
sand deposition in deltas or in natural levees, or sand erosion due to erosion of natural levees and
islands. Sand is transported both as bed-load sediment and as suspended sediment depending on
local hydraulic conditions, so both modes of transport must be accounted for.
The results of the sand budget are shown in Table 1 of Hendrickson (2003). The table lists data for
many locations on the Upper Mississippi River and is four pages long, thus it is not reproduced here.
In the column titled "Sand Budget (tons/year)," the value of 201,172 is given for a location at
Brownsville, MN. This number represents the estimated bed material load in tons per year at this
location, which is the same location of the ISSDOT study area. For this section of river, this would
be the sand that moves along the bed in sand waves and the suspended sand of the same size
fractions. Because of the small channel slope (due to the pooling effect created by the locks and
dams) and medium sand size, it appears that the majority of bed material transport occurs as bed
load; that is, as sand moving in the sand waves. If 100 percent of the sand moved in the sand waves,
then a mean daily transport rate through this reach would be about 551 tons per day. Even if only
50 percent of the bed material load moved in the sand waves, then the mean daily transport rate
would be about 275 tons per day. These fall within the range of values of ∆q predicted by ISSDOT
for trip 4 as ∆t gets small. This would also be the case for trip 2 if its line were extrapolated. Once
again, from a research point of view, is it just a coincidence that ∆q approaches some estimated
value of q as ∆t gets small?
8
Previous Page
