An Assessment of Bank Erosion Along Mahoning Creek

Throughout the summer of 2004, Lycoming College Clean Water Institute interns hiked the full 8 ½ miles of Mahoning Creek in Danville, Pennsylvania. The interns assessed the creek from its emergence along PA route 642 to its confluence with the Susquehanna River in Danville. All occurrences of erosion were documented using a form identical to the one found in the appendix. In all, 308 disturbances were observed along Mahoning Creek and are presented in Table 1 and Figure 1. Of these, 26 were bridges (8.44% of total disturbances) and 38 were pipes flowing into the creek bed (12.34% of total disturbances). There were 21 instances of rip rap (6.82% of total disturbances) along the banks and 1 man-made dam impeding water flow (<1% of total disturbances). There were 69 deposition bars (22.40% of total disturbances) throughout the creek bed. There was 1 area (<1% of total disturbances) containing sediment pollution from a nearby construction site. There were 17 tributaries (5.52% of total disturbances) observed, four of which are major tributaries. There were 135 instances of erosion documented along the creek (43.83% of total disturbances), 68 of which were on the right banks (50.37% of total erosion sites), while 67 were on the left banks (49.63% of total erosion sites). The positions of the banks (right or left) were established while facing downstream.

The potential for bank erosion was determined by a combination of bank height, bank angle, density of roots present, and the particle size of the bank substrate. These factors are rate for High, Moderate, or Low erosion potential and rare explained further in Tables 2-16.

The 135 erosion potential sites were determined based on the following analysis:

Erosion Potential based on Bank Height

Erosion potential based on bank height for total erosion sites, and right and left bank erosion sites are presented in Tables 2-4. A bank up to 6 feet high was considered to have Low erosion potential. Banks 6 to 9 feet high were considered to have Moderate erosion potential, and any bank over 9 feet high had a High erosion potential. Of the 135 banks assessed for bank height, 83 were considered to have Low potential (61.48% of total banks), 42 were Moderate (30.88% of total banks), and 10 were determined to have High erosion potential (7.41% of total banks). Of the 68 right banks, 42 were Low (61.76% of total right banks), 21 were Moderate (30.88% of total right banks), and 5 were of High erosion potential (7.35% of total right banks). Of the 67 left banks, 41 were Low (61.19% of total left banks), 21 were Moderate (31.34%of total left banks), and 5 were of High erosion potential (7.46% of total left banks).

Erosion Potential based on Bank Angle:

Erosion potential based on bank angle is presented in Tables 5-7. A bank with an angle up to 45 degrees is considered to have Low erosion potential. A bank from 45 to 90 degrees is considered to be of Moderate erosion potential, and an undercut bank (one over 90 degrees) is considered to have High erosion potential. Of the 135 banks assessed, 32 were of Low erosion potential based on bank angle (23.70% of total banks), 52 were of Moderate erosion potential (38.52% of total banks), and 51 were of High erosion potential (37.78% of total banks). On the right banks, 16 of the 68 banks assessed were of Low erosion potential (23.53% of total right banks), 24 were of Moderate erosion potential (35.29% of total right banks), and 28 were of High erosion potential (41.18% of total right banks). On the left banks, 16 of the 67 banks assessed were of Low erosion potential (23.88% of total left banks), 28 were of Moderate erosion potential (41.79% of total left banks), and 23 were of High erosion potential (34.33% of total left banks).

Erosion Potential based on Root Density:

Erosion potential based on the root density of the bank is presented in Tables 8-10. A bank of Low erosion potential is one at least 60% covered by vegetation. A bank of Moderate erosion potential is one with 30% to 60% of vegetative cover, while a bank less than 30% covered by vegetation is of High erosion potential. There were 7 of the 135 total banks assessed that were considered to have Low erosion potential (5.19% of total banks), 84 of the total were of Moderate erosion potential (62.22% of total banks), and 44 of the total were of High erosion potential (32.59% of total banks). On the right banks, 5 of the 68 were of Low erosion potential (7.35% of total right banks), 42 were of Moderate erosion potential (61.76% of total right banks), and 21 were of High erosion potential (30.88% of total right banks). On the left banks, 2 of the 67 were of Low erosion potential (2.99% of total left banks), 42 were of Moderate erosion potential (62.69% of total left banks), and 23 were of High erosion potential (34.33% of total left banks).

Erosion Potential based on Particle Size:

Erosion potential based on the particle seize of the bank substrate is presented in Tables 11-13. Banks composed mainly of bedrock or boulders are considered to have Low erosion potential. Banks made up of basketball-sized rocks to pebbles are considered to have Moderate erosion potential, while banks made of sand or clay have High erosion potential. Of the 135 banks assessed, 2 were of Low erosion potential (1.48% of total banks), 63 were of Moderate erosion potential (46.67% of total banks), and 70 were of High erosion potential (51.85% of total banks). On the right banks, 2 were of Low erosion potential (2.94% of total right banks), 34 were of Moderate erosion potential (50.00% of total right banks), and 32 were of High erosion potential (47.06% of total right banks). On the left banks, there were no banks with Low erosion potential, 29 with Moderate erosion potential (43.28% of total left banks), and 38 with High erosion potential (56.72% of total left banks).

Erosion potential based on Length of Site as Compared to Bank Height:

Erosion potential based on the length of each site as compared to the height is present in Tables 14-16. The sites were divided into Low, Moderate, and High erosion potential based on bank height and then classified into the following categories: 0-50 feet in length, 51-100 feet in length, 101-250 feet in length, 251-500 feet in length, and 501-1000 feet in length. There were 58 erosion potential areas that were 0-50 feet in length (42.96% of total banks). Of these 58 banks, 44 had Low erosion potential based on bank height, 10 had Moderate erosion potential, and 4 had High erosion potential. There were 50 erosion potential areas that were 51-100 feet in length (37.04% of total banks). Of these 50 banks, 23 had Low erosion potential based on bank height, 22 had Moderate erosion potential, and 5 had High erosion potential. There were 25 erosion potential areas that were 101-250 feet in length (18.52% of total banks). Of these 25 banks, 16 had Low erosion potential, 8 had Moderate erosion potential, and 1 had High erosion potential. There were 2 erosion potential areas that were 251-500 feet in length (1.48% of total banks). The two areas were both of Moderate erosion potential. There were no erosion potential areas 501-1000 feet long observed. Of the 58 erosion potential areas 0-50 feet in length, 28 were found on the right bank (41.18% of total right banks). Of these 28 banks, 21 had Low erosion potential based on bank height, 6 were Moderate, and 1 was High. There were 30 erosion potential areas 0-50 feet in length found on the left bank (44.78% of total left banks). Of the 30 areas, 23 were Low erosion potential areas based on bank height, 4 were Moderate, and 3 were High. Of the 50 erosion potential areas 51-100 feet in length, 26 were found on the right bank (38.24% of total right banks). Of these 26 areas, 13 had Low erosion potential, 10 were Moderate, and 3 were High. There were 24 erosion potential areas 51-100 feet in length found on the left bank (35.82% of total left banks). Of the 24 areas, 10 had Low erosion potential, 12 were Moderate, and 2 were High. Of the 25 erosion potential areas 101-250 feet in length, 13 were found on the right bank (19.12% of total right banks). Of these 13 areas, 8 had Low erosion potential, 4 were Moderate, and 1 was High. There were 12 erosion potential areas 101-250 feet in length found on the left bank (17.91% of total left banks). Of the 12 areas, 8 had Low erosion potential and 4 were Moderate. Of the 2 erosion potential areas 251-500 feet in length, 1 was found on the right bank (1.48% of total right banks) and 1 was found on the left bank (1.4% of total left banks). Both areas were Moderate erosion potential based on bank height.

Conclusion

There were five sites (sites 30, 132, 163, 173, and 193) that showed evidence for a high probability for erosion. Site 30 had low erosion potential for bank angle, and high erosion potential for bank angle, root density, and particle size. Site 132 had high erosion potential for bank height and bank angle, and moderate erosion potential for root density and particle size. Site 163 had high erosion potential for bank height and root density, and moderate erosion potential for bank angle and particle size. Site 173 had moderate erosion potential for bank height and root density, and high erosion potential for bank angle and particle size. Site 193 had high erosion potential for bank height and root density, and moderate erosion potential for bank angle and particle size.