High-resolution assessment of riverbank erosion and stabilization techniques with associated water quality implications

Figures & data

Figure 1. With (a) the location of the Blackwater catchment and (b) digital elevation model of the Blackwater catchment, river network, and locations of Sites 1–4.

Table 1. General site characteristics for the four field-scale sites within the Blackwater catchment.

Site	Site Size (ha)	Riverbank Area (m^2)	Stream Channel Stretch Length (m)	Upstream Catchment Area (m^2)	Downstream Catchment Area (m^2)	Predominant Land Use	Riverbank Vegetation
1	6.5	1755	364	45.83	46.63	Livestock grazing and silage	Grassed waterways, shrubs (<0.5 m height)
Riverbank A	NA	937	–	–	–	–	–
Riverbank B	NA	818	–	–	–	–	–
2	4.7	3031	462	47.01	49.25	Livestock grazing and silage	Grass, shrubs (<0.5 m height), trees (<10 m height)
Riverbank A	NA	1452	–	–	–	–	–
Riverbank B	NA	1579	–	–	–	–	–
3	6.7	2667	525	215.97	216.77	Livestock grazing and	Grass, shrubs (<0.5 m height), trees (<10 m height)
Riverbank A	NA	1891	–	–	–	–	–
Riverbank B	NA	776	–	–	–	–	–
4	5.0	1484	339	104.62	104.98	Livestock grazing	Grass, small trees (c. 1–2 m height)
Riverbank A	NA	698	–	–	–	–	–
Riverbank B	NA	786	–	–	–	–	–

Table 2. Sub-field soil Olsen P range determined by 35 m gridded sampling and the risk of runoff transfer of sediment and P in Hayes et al. (2023).

Site	Soil Olsen P mg L^−1 Range	Potential Risk for P and sediment transfer by runoff
1	8.8–112.6	High risk due to elevated soil P content with high hydrological connectivity and extensive modelled surface runoff channels
2	17.7–133.6	Low risk despite elevated soil P content due to predominantly intra-field drainage
3	10.6–43.3	Medium risk as some elevated sources of soil P content but not all modelled surface runoff channels are hydrologically connected to waterways
4	7.5–55.4	High risk due to elevated soil P content and high hydrological connectivity and extensive modelled surface runoff channels

Figure 2. With (a) installation of riverbank stabilization techniques at Site 3 in September 2020, (b) close-up of coir matting, tree stems, and cobblestone backfill used to support the structure, and (c) rock armouring along riverbank B for Site 2 in November 2021.

Figure 3. (a-d) Bulk density sampling locations at Sites 1–4 displayed over summer 2020 drone imagery and upstream and downstream grab sampling points. The predominant flow direction is indicated by an arrow.

Table 3. Averaged results for bulk density and total extractable P for each site determined from six riverbank cores collected per site.

Site	Average Bulk Density g cm^−3	Range of Bulk Density values g cm^−3	Average Total Extractable Phosphorus mg L^−1	Range of Total Extractable Phosphorus values mg L^−1
Site 1	1.17	0.95-1.32	27.7	5.20-49.9
Site 2	1.20	1.09-1.26	21.9	13.5-39.1
Site 3	1.09	1.05-1.14	22.9	7.30-35.8
Site 4	1.08	1.06-1.12	7.29	2.80-12.4

Table 4. Average annual riverbank erosion, sediment, and P loading rates calculated for Sites 1–4 from 2014–2020.

Site	Average Annual Erosion Rate (cm yr^−1)	Average Mass Wasting Inputs of Sediment (kg yr^−1)	Average Total Extractable Phosphorus Inputs (mg yr^−1)
Site 1	1.84	1.92	53.23
Riverbank A	0.40	1.54	42.55
Riverbank B	1.44	0.38	10.68
Site 2	0.21	0.35	7.62
Riverbank A	NA-Deposition Occurring	NA-Deposition Occurring	NA-Deposition Occurring
Riverbank B	0.21	0.35	7.62
Site 3	79.19	128.14	2934.55
Riverbank A	50.28	103.68	2374.30
Riverbank B	28.91	24.46	560.25
Site 4	3.14	2.47	17.85
Riverbank A	2.34	1.79	12.90
Riverbank B	0.80	0.68	4.95

Figure 4. Identified erosion zones using image differencing and zonal statistics in ArcGIS displayed over the summer of 2020 drone imagery of Site 4.

Table 5. Average annual riverbank erosion, sediment and P loading rates calculated for Sites 1–4 from 2020–2021.

Site	Average Annual Erosion Rate (cm yr^−1)	Average Mass Wasting Inputs of Sediment (kg yr^−1)	Average Total Extractable Phosphorus Inputs (mg yr^−1)
Site 1	8.56	8.89	246.25
Riverbank A	4.99	5.47	151.52
Riverbank B	3.57	3.42	94.73
Site 2	38.07	69.64	712.07
Riverbank A	16.31	28.39	621.74
Riverbank B	21.76	41.25	903.37
Site 3	9.78	16.72	382.97
Riverbank A	6.95	14.33	328.09
Riverbank B	2.83	2.39	54.88
Site 4	1.99	0.79	5.76
Riverbank A	1.24	0.47	3.43
Riverbank B	0.75	0.32	2.33

Table 6. Percentage decrease in average annual riverbank erosion, sediment, and P loading rates calculated for Site 3 from 2014/2020 in Table 4 compared to 2020/2021 calculations in Table 5.

Site 3	Change in Average Annual Erosion Rate (%)	Change in Average Mass Wasting inputs of Sediment (%)	Change in Average Total Extractable Phosphorus Inputs due to Riverbank Erosion (%)
Entire Site	87.64	86.95	86.18
Riverbank A	86.16	86.18	86.18
Riverbank B	90.21	90.23	90.20

Figure 5. Comparison between the spatial variation in the magnitude of erosion and deposition as a rate per pixel in ArcMap for Site 3 before and after the introduction of riverbank stabilization techniques. Darker colours represent higher erosion and deposition occurrence and lighter colours represent lower erosion and deposition occurrence.

Figure 6. Time series plots of observed discharge in m³ s⁻¹ (top), Total Suspended Sediment (TSS) in mg L⁻¹ (middle), and Total Phosphorus (TP) in µg L⁻¹ (bottom) at the downstream sampling locations of Sites 1–4.

Table 7. Average annual upstream and downstream water quality for TSS (mg L⁻¹) and TP (µg L⁻¹) for Sites 1–4 from August 2020-August 2021.

Site	Average upstream TSS (mg L^−1)	Average downstream TSS (mg L^−1)	Downstream percentage of upstream TSS (%)	Average upstream TP (µg L^−1)	Average downstream TP (µg L^−1)	Downstream percentage of upstream TP (%)
Site 1	49.85	39.15	79	245.31	267.46	109
Site 2	38.15	102.77	269	276.00	179.92	65
Site 3	22.69	19.69	87	214.00	206.92	97
Site 4	14.85	30.31	204	192.85	240.62	125

Hayes, E., Higgins, S., Geris, J., Nicholl, G., & Mullan, D. (2023). Weighted risk assessment of critical source areas for soil phosphorus losses through surface runoff mechanisms. CATENA, 225, 107027. https://doi.org/10.1016/j.catena.2023.107027

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Data availability statement

No data is publicly available due to anonymity granted to landowners participating in this research.