10.6.4 Connectivity to Other Features Just as designing wetlands with topographic variability can increase both FRM and ecological function of wetland NNBF, smoothly linking wetland elevations to subtidal areas and uplands can also increase FRM benefits and ecological and social co-benefits as well. The bathymetry near wetland edges strongly affects the translation of wave energy, with steep or concave wetland edges subject to greater forces from wave impact than terraced or convex wetland edges where the waves may break away from the wetland edge (Tonelli, Fagherazzi, and Petti 2010). If adjacent mudflat or sand flats are too deep, wave breaking may occur directly on the wetland edge, leading to higher long-term erosion rates. Deep water adjacent to wetlands can also exacerbate erosion or preferentially accrete sediments that would otherwise deposit on the wetland surface (Ganju 2019). Thin-layer or strategic sediment placement can be used to increase the elevation of adjacent mudflats and sand flats in the same way these techniques can be used to increase the elevation of the vegetated wetland area (see Table 10.5). Table 10.5. Engineered Elements That May Be Incorporated into a Coastal Wetland Design Structural element • Vegetation planting • Geotextiles (temporary)
Purpose Stabilize sediment
Design considerations • Choice of plant species (should be native) • Seedlings damage or removal by large waves or strong currents
• Brushwood fencing (temporary) • Coir logs (temporary)
Erosion protection
• Wave or current energy • Water depth
• Oyster reefs
• Durability of element
• Geotextile containers
• Size of element or fill components
• Nearshore breakwaters—wood or rock
• Source of element or fill components
• Gabions
• Bearing capacity of substrate
• Concrete or flexible mattresses
• Salinity (for oysters)
• Rock • Open-stone asphalt • Brushwood fencing (temporary) • Coir logs (temporary) • Wooden palisades • Rock or sediment berm or bund • Vegetation planting
Retain or accrete sediment
• Wave or current energy • Water depth • Permeability of element • Geometry of element (e.g., height, length, spacing)
• Groins
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