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Development Prototype: Puget Sound Ecosystem Portfolio Model

The Beach Armoring Index

The Beach Armoring Index – scores beaches based on the potential for geomorphological and ecological changes due to scenarios of cumulative on-site and updrift armoring

The physical qualities of beaches such as beach width and profile, and substrate composition, moisture and temperature influence the distribution of numerous Valued Ecosystem Components such as eelgrass, forage fish, and native shellfish (Dethier 2006, Mumford 2007, Penttila 2007). Coastal bluff erosion is the primary source of beach sediment in Puget Sound, and this sediment source is essential for maintaining the quality of beaches and their associated habitats (Johannessen and MacLennan 2007).

A drift cell is a unit of coastline that represents a sediment transport sector from source to deposition. Drift cells have a net shore-drift direction, which is the long-term, net direction of longshore sediment transport. Within a drift cell, bulkheads or other shore armoring limit bluff erosion and reduce coastal sediment transport and supply to down drift beaches, resulting in changes to beach condition and habitat quality. Bluff-backed beaches and barrier beaches in Puget Sound appear to be most affected by armoring (Schlenger et al. 2010), and common consequences are erosion of the beach profile, reduced shallow water habitat, and substrate composition, temperature and moisture changes, which can lead to, among other things, decreased forage fish spawning, reduced shellfish production and decreased eelgrass (Schlenger et al. 2010).

The Beach Armoring Index model provides a method for quantifying the cumulative effects of updrift and on-site armoring on Puget Sound beaches. The Index assigns a beach a score based on wave exposure and shoreline armoring on coastal bluffs on-site and updrift, within its drift cell. The score indicates the potential for beach geomorphological and ecological changes, such as erosion and loss of eelgrass or forage fish spawning habitat, due to loss of sediment supply. The Index evaluates ENVISION scenarios of future shoreline armoring to assess which shorelines are at greater risk of shoreline change and habitat loss. River deltas and streams also supply sediment to beaches; at this point this index only addresses bluff sediment supply. Given these known limitations, we intend for the index to be a tool for evaluating armoring and development scenarios in Puget Sound and not a deterministic predictor of shoreline change.

Currently three variables are included in the index:

  • Fetch distance measured from the South (180°) (SFetch)
  • Percent length of updrift bluffs that are armored (P_Up_Armor)
  • Percent length of on-site bluffs that are armored (P_Armor)

The index was calculated for all bluff-backed (BLB) and barrier beaches (BAB) that are not in zones of “No appreciable drift”, as defined by the Puget Sound Change Analysis Geodatabase. The three variables were assigned scores based on their data distribution. Scores were combined to create an index with values ranging 1 to 5, with 5 being greatest potential for beach impacts. Based on expert opinion, the P_Up_Armor and SFetch variables were weighted double.

The index is calculated as: 2*(P_Up_Armor score) + 2*(SFetch score) + [(P_Armor score) if Beach type = BLB]

The model applies a network analysis method in GIS to attribute cumulative updrift and on-site armoring to a given bluff-backed or barrier beach in Puget Sound. To attribute cumulative updrift bluff armoring to a given beach, each drift cell in Puget Sound, as mapped in the Puget Sound Change Analysis Geodatabase, was defined as a network with determinate flow in the net shore-drift direction. The Geodatabase provided source data for armoring and bluffs, which were defined as shoreforms = bluff-backed beaches (BLB). Then, a Network Analysis Upstream Accumulation method was performed. From this analysis, the percent length of updrift bluffs that are armored was calculated for each beach, as well as the percent length of on-site bluffs that are armored. ENVISION scenarios of shoreline armoring were then applied to calculate changes to these variables out to 2060.

For the baseline year 2000 index score, south facing, armored shorelines scored higher than other shorelines. The southerly fetch distance is highly weighted in this index; changes to the fetch variable, such as the use of mean fetch, would greatly influence index calculations. There is also a trend toward higher index scores on the eastern shores of Puget Sound, from Possession Sound to Commencement Bay. The highest average index scores are in the South Central Subbasin (2.6) and the South Subbasin (2.1), which corresponds with higher armoring rates in these locations (Table 1). A comparison of the 2060 Unconstrained Growth to Managed Growth scenarios finds an increase in index scores on parts of Bainbridge Island, parts of the South Puget Sound Subbasin, and parts of the Whidbey Subbasin.

The ENVISION Managed Growth scenario assumed no future shoreline armoring; as a result there is no change in index scores for this scenario across decades. For Status Quo and Unconstrained Growth scenarios, future shoreline armoring rates were based on current ratios between armoring density and shoreline development densities by zoning class. The use of these ratios led to very low prediction rates for future shoreline armoring for both scenarios out to 2060. As a result, there are very few differences in index scores across scenarios and years. The greatest projected increase in armoring occurred in the 2060 Unconstrained Growth scenario for the South Central Puget Sound Subbasin – a 4.6% increase in the length of armored shoreline (Figure 1).

Table 1. Average Index Score by Scenario, Year and Subbasin
UG = Unconstrained Growth, SQ = Status Quo, MG = Managed Growth

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Figure 1. ENVISION armoring projections for the South Central Puget Sound Subbasin

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References

Dethier, M. N. 2006. Native shellfish in nearshore ecosystems of Puget Sound. Puget Sound Nearshore Partnership Report No. 2006-04. Published by Seattle District, U.S. Army Corps of Engineers, Seattle, Washington.

Johannessen, J. and A. MacLennan. 2007. Beaches and Bluffs of Puget Sound. Puget Sound Nearshore Partnership Report No. 2007-04. Published by Seattle District, U.S. Army Corps of Engineers, Seattle, Washington.

Mumford, T. F. 2007. Kelp and Eelgrass in Puget Sound. Puget Sound Nearshore Partnership Report No. 2007-05. Published by Seattle District, U.S. Army Corps of Engineers, Seattle, WA.

Penttila, D. 2007. Marine Forage Fishes in Puget Sound. Puget Sound Nearshore Partnership Report No. 2007-03. Published by Seattle District, U.S. Army Corps of Engineers, Seattle, Washington.

 

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