Determining the rate of cut bank erosion on the Boardman River
Marcia Talicska
Introduction
A large cut bank on the Boardman River, in Traverse City, northwestern lower Michigan, was
identified as an area of intense erosion (fig. 1). The Boardman River meanders through the
city of Traverse City and discharges in West Grand Traverse Bay. Little or no erosion
control has occurred along the cut bank, regardless of the high rate(s) of erosion. To
date, only several large slabs of cement have been placed in one small area near the
river's edge, possibly to maintain the embankment. Without erosion control the river may
continue to wear away the cut bank until it undercuts Wadsworth Street.
Site description
Near the intersection of West Front and Wadsworth Streets, the river meanders sharply,
forming a point bar and cut bank (fig. 2). Several feet of grass land separate the edge of
the sidewalk on Wadsworth Street from the steep embankment (average slope of 48o) leading
down to the river. The soils along the cut bank are generally sandy. Field methods were
used to determine the primary constituents of the soil (described below). Trees and shrubs
on the cut bank are leaning toward the river, indicating the embankment is eroding rapidly
(fig.3).
Survey history
A survey of the Boardman River was performed by the City of Traverse City in 1914 to
publish a plat of area (fig. 4). In 1975 the position of the Boardman River was
resurveyed. When the 1914 survey (dotted line) is superimposed on the 1975 survey (solid
line) the distance (scale 1"= 100') between the two lines can be used to determine
the rate of erosion (Lawler, 1993) occurring in the intervening 61 years (fig. 5). The
average distance between the two surveyed lines yields an erosion rate of (62.5 feet/ 61
years) = 1.02 feet/ year.
Problem statement
The purpose of this study is to determine a current rate of erosion for the cut bank near
the intersection of West Front Street and Wadsworth Street, and to determine how much time
remains before (if) the river undermines the roadway. Determining a rate of erosion from
1917 to the present (1996) required that a new survey would be necessary to establish the
current position of the river. A reference point, the edge of the Wadsworth Street
sidewalk, was established to correlate horizontal distances measured in the field to a
fixed structure on the map.
Methods
Equipment
Several pieces of equipment were required to survey the river: a 100' measuring tape, a
wooden expandable tape holder, a map of the area, chalk, pencil, and paper.
Procedure
From the northern edge of the sidewalk beginning on Wadsworth Street, chalk was used to
mark the distances between transects that were 50' apart as follows: 100', 150', 200',
250', and 300' (fig. 6). The transects were chosen at 50' intervals in order to span the
entire cut bank.
The 100' measuring tape was connected to the wooden expandable tape holder which was
extended 25 feet. A first person, Sarah Rasmussen, held the measuring tape directly on the
chalk mark on the sidewalk. A second person held the wooden expandable tape holder
vertically, from the point where the river met the soil embankment (fig. 7). Horizontal
distances from the edge of the sidewalk to the rivers edge could then be determined at all
five transects (100', 150', 200', 250' and 300').
Results
The horizontal distances from the edge of the sidewalk on Wadsworth Street to the river is
given in Table 1 for each of the five transects. The distance from the edge of the
sidewalk to the 1914 survey is given in Table 2. The amount of erosion occurring between
1914 and 1996 was determined by subtracting the distance between the sidewalk and the
rivers edge from the 1914 survey from the distance from the sidewalk to the rivers edge
for the 1996 survey, along each of the five transects (Table 3).
Conclusions
The average rate of erosion for the cut bank of the Boardman River near Wadsworth Street
is (65.6 feet/ 82 years) = 0.8 feet/ year. Drawing in the new position of the river bank
for 1996 indicates that the rate of erosion upstream from the cut bank (150' transect) is
(86 feet/ 82 years)= 1 foot/ year. The rate of erosion downstream from the cut bank (250'
transect) is much slower than upstream (51 feet/ 82 years) = 0.6 feet/ year. Drawing the
new position of the rivers edge on the map shows areas of erosion and deposition (fig. 8).
The average rate of erosion from 1914 to 1975 (1.02 feet/ year) agrees well with the rate
of erosion from 1914 to 1996 (0.8 feet/ year), indicating that erosion has continued at a
relatively constant rate. If erosion control is not initiated the river will undercut
Wadsworth Street in approximately 54 years.
Table 1. The measured horizontal distances from the sidewalk to the rivers edge based on the 1996 survey.
Transect Distance from the edge
distance of sidewalk to the river
________________________________________________________________________
100' 64'
150' 20'
200' 31'
250' 30'
300' 31'
average 35.3 feet
Table 2. The measured horizontal distances from the sidewalk to the rivers edge based on the 1914 survey. Transect Distance from the edge
distance of sidewalk to the river
___________________________________________________________________________ 100' 137'
150' 106'
200' 93'
250' 81'
300' 87'
average 128.8 feet
Table 3. The distance eroded from 1914 to 1996 using the sidewalk as a reference point.
TABLE 3
Transect 1914 - 1996 = Distance between
distance the 1914 survey and the 1996 survey
_____________________________________________________________________________
100' 137' - 64' = 73'
150' 106' - 20' = 86'
200' 93' - 31' = 62'
250' 81' - 30' = 51'
300' 87' - 31' = 56'
average 65.6 feet
Calculations
Slope
Over a distance of 3' the embankment dropped 1.25', 1.42', and 1.67', at three locations.
1.25'/ 3' = .41
1.42'/ 3' = .47
1.67'/ 3' = .56 for an average of .48 or 48 degrees
Field Methods for determining soil composition, (see attached text from The Bridge, 1992).
Gradation: well graded medium to small
Fines: 1/16" in the sample
Plasticity: The soil was shiny and wet in the surface when kneading indicating sand and silts were present.
Toughness: The soil would not roll or thread, but fell apart indicating silts and sand dominate the soil.
Dry Strength Test: The fragments crushed easily with compression of the fingers.
Hand Washing: The fines on the hand washed off without rubbing, indicating silts and sands are present.
Dispersion: Several minutes after adding the water to the sample approximately 5/16" of sand settled out of the water. At that time the water was dark and very cloudy. After forty minutes (fig. 7) 6/16" of silt settled out of the water. At this time the water was gray and slightly transparent.
After the above test were performed the soil was determined to be a sand with silt
fines.
Determining the approximate number of years it will take the Boardman River to undercut Wadsworth Street
The distance from the new 1996 dock line to the edge of Wadsworth Street for each of the five transects.
Transect Distance to Wadsworth Street
100' 62'
150' 31'
200' 43'
250' 37'
300' 43'
average of the distances: 43'
Determine the number of years it will take the river to erode 43' at an average rate of 0.8 feet/ year:
0.8 feet/ year = (43 feet/ ? years) = 54 years
References
Lawler, D.M. 1993. The measurement of river bank erosion ad lateral channel change. Earth
Surface Processes and Landforms. 18: 777-821.
US Department of Transportation. 1992. Soil classification in the field. The Bridge. 6:
1-7.
Figure 1. Boardman River located in Traverse City in lower northwestern Michigan.
Figure 2. Cut bank and point bar formed by a sharp meander in the Boardman River.
Figure 3. Pictures of trees and shrubs located on the embankment of the river.
Figure 4. The 1914 plat of the Boardman River.
Figure 5. The 1914 dock line superimposed on the 1975 survey of the Boardman River.
- - -1914
-----1975
Figure 6. Transect distances marked on the Wadsworth Street sidewalk.
Figure 7. Depiction of the method used to measure the current dock line.
Figure 8. Position of the new dock line drawn in comparison to the two old dock lines.