Bank Stability and Erosion Potential Along The Mohawk River, NY Studied Using LiDAR Imaging

By Ashraf M. Ghaly

The concept behind an airborne LiDAR scanner is to send laser pulses that hit objects on the ground and to receive and record the returns which contain all sorts of information. The information can be analyzed to reveal the types of objects, shape, height/depth, extent, etc. Coupled with a GPS system on the plane where the LiDAR scanner is mounted, the location and altitude of the scanner are used to determine similar relevant information on the ground. This results in laser images, called tiles, which, if stitched together after applying proper corrections (if necessary), will form an information-rich map of the scanned surface.

LiDAR data can be used in many engineering and non-engineering applications. Some of the engineering applications include, but are not limited to, highway engineering, land development, pipeline mapping and leak detection, power line mapping, cell tower mapping, forestry, mining, flood study, coastal erosion, etc. The state of New York (NYS) has collected LiDAR data that cover a significant area of the state. These data are made available for educational and research purposes.

Data that cover the Mohawk River basin span a number of counties and involve areas of the river where there has been significant meander migration and bank erosion, which has compromised the stability of the riverbanks. Due to the unstable nature of the soil forming the riverbanks and the considerable steepness of slopes at many locations, displacement, settlement, yielding and even failure of the foundations of some structures have been documented.

Fig. 1. Sharp bends along the Mohawk River in Schenectady and Albany Counties, NY
Fig. 1. Sharp bends along the Mohawk River in Schenectady and Albany Counties, NY. (Click for larger view.)

This study was initiated with the goal of using the LiDAR data provided by NYS to map the slopes along the banks of the Mohawk River and to detect areas where steep slopes could be critical indicators of imminent failure. The data were also used to identify sections of the river where, due to sharp bends, erosion may be accelerating, resulting in faster meander migration. Detecting these phenomena at early stage is key to the prevention of catastrophic failures that can result in loss of life and property.

The Mohawk River is over 140 miles long (NYS Department of Environmental Conservation, DEC). It flows, generally, from the west where it originates toward the east where it confluences with the Hudson River. The river's watershed covers all of Montgomery County, most of Schoharie County, much of Schenectady, Greene, Fulton, Herkimer and Oneida counties, and smaller parts of Albany, Saratoga, Delaware, Otsego, Hamilton, Madison and Lewis Counties (NYS DEC).The topography of the Mohawk River basin and its watershed is fascinating and its total area is 3,460 square miles. There are 4,086 miles of freshwater rivers and streams supplying the Mohawk River (NYS DEC).

The Schoharie Creek (1,650 river miles long), which flows northward, and West Canada Creek (1,165 river miles long),which flows southward, are two major tributaries of the Mohawk River. The path of the river itself and those of its two main tributaries are jammed with extreme bends and meanders. An example is illustrated in Figure 1, which shows a part of downriver where the Mohawk joins the Hudson. Considering the nature of the soil in many sections along the length of the river and its tributaries, it should be no surprise that erosion and meander migration are constant concerns and pose threats to banks and foundations of infrastructure.

Figure 2 shows transparent LiDAR tiles plotted using GIS, converted to a KMZ file and imported into Google Earth. To demonstrate how the LiDAR data were used to study the stability of earth slopes, a focus area is selected near Fort Hunter, NY (Figure 3). Interstate 90 crosses the Schoharie Creek at this site. Unfortunately, this was the location of a 1987 bridge failure (due to a compromised pier foundation) that claimed 10 lives. Four LiDAR tiles cover the area of this site, which is shown in Figure 4 with the aid of GeoViewer.

Fig. 2. Transparent LiDAR tiles superimposed over Google Earth
Fig. 2. Transparent LiDAR tiles superimposed over Google Earth. (Click for larger view.)
Fig. 3. Study focus is at I-90 Schoharie crossing, Fort Hunter, NY.
Fig. 3. Study focus is at I-90 Schoharie crossing, Fort Hunter, NY. (Click for larger view.)
Fig. 4. LiDAR images covering focus study area as viewed in GeoViewer
Fig. 4. LiDAR images covering focus study area as viewed in GeoViewer. (Click for larger view.)

To show the variation in elevations, a 3D LiDAR image is generated. The image shown in Figure 5 has a z-magnification (ratio between vertical and horizontal dimensions) factor of 20, which helps accentuate the variations in elevations for better visualization. Using the LiDAR tiles, the slope of the ground measured from the horizontal in degrees can be generated. There are sections along the path where extremely steep slopes of over 80 degrees can be seen.

Fig. 5. Focus study area 3D LiDAR images with z-magnification factor of 20
Fig. 5. Focus study area 3D LiDAR images with z-magnification factor of 20 (Click for larger view.)

Figure 6 shows three outlined locations where the slopes are almost vertical (80 to 90 degrees). These very steep slopes are also confirmed by the 10-m intervals contour map shown in Figure 7. Steep slopes in areas of soft or loose soils are very dangerous, as the potential for failure or landslide significantly increases. If steep slopes are found at sections with sharp bends, the problem could become extremely serious over time as the erosion effect takes its toll on the structure of the soil layer. Such sections of the waterway require special attention and protection with natural or artificial means to ensure stability.

Fig. 6. Slope in degrees of focus study area
Fig. 6. Slope in degrees of focus study area. (Click for larger view.)
Fig. 7. Contour map with 10-m intervals of focus study area
Fig. 7. Contour map with 10-m intervals of focus study area. (Click for larger view.)

Summary

The process of producing slope rasters and/or contour maps from LiDAR images as described above can be beneficial in analyzing large swaths of land with questionable or critical slopes. The advantage of such an analysis is to pinpoint the locations of steep slopes and study the formation of soil in these areas to assess the potential for failure. Soil formations, coupled with information regarding whether the detected locations are in straight or bent sections of the water stream can reveal the extent to which the danger of slope failure or hazardous erosion is present.

This type of study may not require such a sophisticated analysis if it were utilized for only a few locations or for a limited length of the waterway, but when the watershed under consideration comprises thousands of miles of streams and creeks, LiDAR imagery can render the entire operation very efficient.


Published Thursday, September 15th, 2011

Written by Ashraf M. Ghaly


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