Agenda - Environmental Policy Board - 04/18/2016

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4 Results Erosion Severity Most of the riverbank inventoried had limited erosion (Table 2). Approximately 78% of the riverbank was categorized as either slight (40%) or moderate (38%) erosion severity. This corresponds to a lateral recession rate of 0.0 — 0.2 ft/yr. In contrast, 11% of the shoreline was categorized as severe and 10% as very severe (Table 2). These categories have lateral recession rates of 0.3 —>0.5 ft/yr. Table 2: Summary of erosion severity. Erosion Severity Length (miles) % SI ight . 2.32 40% Moderate 2.21 38% Severe 0.64 11% Total 5.78 100% Table 3: Severe and very severe erosion on public and private lands. Overall, shoreline categorized as either severe 'Ownership or very severe is distributed 'Public relatively evenly between 'Private public and private ownership 'Total I (51% and 49%, respectively). a Length (miles) 0.27 0.37 0.64 1 % 42% 58% 100% 1 Length (miles) 0.37 0.24 0.60 1 % 61% 39% 100% However, when looking at the severe and very severe categories individually, the breakdown is not as evenly balanced (Table 3). Public land has a lower percentage of the total severe sections (42%) and a higher percentage of the total very severe sections (61%). Estimated Soil Loss The total length of Table 4: Estimated soil loss by erosion severity. riverbank Erosion Severity Length (miles) Estimated Soil Loss (tons/yr) % categorized with Severe 0.64 1174 23% severe or very severe erosion is Total I 1.241 51481 100% relatively equal between the two categories (Table 4). However, because of the higher lateral recession rate in the very severe sections (i.e. 0.75 ft/yr), those sections represent the majority (77%) of the estimated soil loss. Stabilization Considerations The goal of most riverbank projects is to correct or prevent excessive erosion or undercutting through bank stabilization. Stabilization of eroding riverbanks is highly site -specific; there is not a simple solution that can be applied across all sites. For example, factors such as position along the river (e.g. outside bend), river dynamics (e.g. flow and flood elevations), and site accessibility must be considered individually for each project. That being said, stabilization approaches generally fall into two categories: hard armoring and bioengineering. Hard armoring uses physical structures to protect the riverbank; riprap is used commonly for hard armoring. Riprap does not necessarily need to extend to the top of the slope to be effective and can be inter -planted with native species to soften its appearance. Often times, hard armoring the toe of the slope (i.e. the very bottom) up to a moderate height (e.g. the 2-year flood elevation) is sufficient for stabilizing the rest of the bank.