Date of this Version
Report # MATC-UI: 371 Final Report 25-1121-0001-371
A new instrument (Photo-Electronic Erosion Pin, or PEEP) was examined in collecting field data and remotely monitoring bank erosion near bridge abutments during floods. The performance of PEEPs was evaluated through a detailed field study to determine factors affecting their records. Proper calibration of the instrument was important in obtaining accurate erosion lengths. Calibration of the PEEPs within the banks nearby the study reach provided the most accurate erosion lengths. In addition, comparison with traditional, manual methods was recommended. Bank erosion was monitored at two study sites at the Clear Creek Watershed (CCW), IA between May 2009 and December 2009 using the continuously monitoring PEEPs and more traditional methods (e.g., geodetic channel surveys and standard erosion pins). The first site was located below an agricultural headwater of the CCW at the confluence of two 1st order streams downstream of the 190th Street Bridge near U.S. Highway 151 in Iowa County. Whereas, the second site (hereafter referred to as ―Site 2‖) was located on a 4th order stream at Camp Cardinal Rd. in Coralville, Iowa near the CCW confluence (mouth) with the Iowa River. The area surrounding this reach is mainly urbanized. The monitoring period contained two significant runoff events on June 19 and August 27, 2009. The PEEPs provided a detailed time series of bank retreat during the study period. At Site 1, the flash flood of June 19, 2009 produced significant, mass failure of the channel banks, especially at the bank crest and mid-section. Bank retreats of ~ 25 cm were measured with the highest erosion rate being observed at the mid-section of the bank. The high erosion at the bank midsection over-steepened the bank height making the bank more susceptible to mass failure and slumping. At Site 2, flow was often higher than at Site 1 providing favorable conditions for more continuous fluvial erosion punctuated with irregular bank slumping. Erosion lengths up to 38 cm were detected at Site 2. The bank erosion monitoring at high resolution intervals due to the PEEPS allowed for better characterization the fluvial erosion occurring at this site. One limitation of the PEEPs was their inability to record data while submerged. The correlation between the submerged and unsubmerged data revealed that R2 was higher for PEEPs at higher elevations above the free surface; hence, the PEEPs located at the bank mid-section or crest performed better than the PEEPs near the bank toe. Despite the above limitation, the PEEPs captured well the timing and magnitude of specific erosion events at both sites. The PEEPs were able to predict accurately bank erosion near bridge abutments during the flood. The maximum error between manual and automated measurements of the exposed length of the PEEPs was observed at site 1 and this error was less than 27%. The error between the channel survey and the automated PEEP measurements was less than 14%. The successful field experiments of the PEEPs at the study sites proved that the PEEPs technology is transferable to the field. The PEEPs present several advantages by providing real-time data of erosion in terms of magnitude and frequency, which is not possible with the traditional methods where only net changes from previous measurements are known. This real-time data coupled with the automated nature of the instrument made it ideal for certain sites that are not easy to access on a continuous basis. Automated and continuous real-time data are in great need for monitoring bank erosion near bridge abutments. The PEEPs provide valuable data on the timing of individual bank erosion events, especially the time lag between the peak erosion and the peak of the hydrograph. This information can also be of great importance to the field of geomorphology, as well as to numerical modeling.