What is Stream Restoration?
Stream restoration is generally defined as the re-establishment of the natural channel form, function, and processes of a self-sustaining system that existed prior to watershed perturbation. Understanding the cause of impairment is a critical step in restoring a streams physical, biological and chemical functions. A successful restoration design involves a holistic approach to determine a sites potential functional uplift in order to achieve realistic goals and objectives. The practice of stream restoration is evolving and considers multidisciplinary approaches and methodologies that require regulatory input and approval. Restoration design parameters are developed as guidelines to encourage natural variability in stream morphology formed by fluvial processes, vegetation establishment, sediment supply and watershed influences.
At WLS, our design philosophy considers applied approaches to stream restoration, such as applicable reference data (analog), published hydraulic geometry relationships (regional curves), performance monitoring from past projects (empirical), and 2D hydraulic models using process-based equations to test design geometry and stability (analytical). Maximizing restoration potential can often be achieved by regrading or rebuilding unstable stream segments (reaches) and connecting to their geomorphic floodplain, installing in-stream structures, removing invasive species and planting native vegetation to help provide channel stability, bedform diversity and aquatic habitat. Below are examples of common in-stream structures, techniques and applications used in stream restoration throughout the riparian corridor.
In-stream structures are intended to mimic natural stream features using materials such as wood and rock that are often harvested and recycled as part of the construction activities.
Vane structures (‘A’ or ‘C’, ‘J-Hook’) can be constructed from a combination of both log and/or boulder materials. Vanes are typically installed along outer meander bends in areas where flow direction changes abruptly or in areas where deeper pool habitat is desirable. The location, vane length, angle, and slope are designed for the site specific conditions. Vanes function by intercepting stream flow and redirecting that flow away from the stream bank and towards the center of the channel, reducing the erosive force of water on the streambanks. Vanes also improve in-stream habitat by creating scour pools and providing oxygen and cover for aquatic habitat.
Constructed riffles provide grade control and habitat as the streambed stabilizes naturally. Riffles can be built from woody debris, gravel and cobble and form interstitial pockets that promotes life cycles of many aquatic macroinvertebrate species.
Step pools are commonly built with logs and stone to provide grade control, streambank protection and pool habitat. The step pools are typically constructed as angled sills or weirs in a series across the restored stream channel, with an excavated plunge pool immediately downstream of each step.
Floodplain improvement features such as sloughs, meander scars, depressions, and woody debris can be installed to mimic elements commonly found in natural riparian systems. These features are added to provide habitat diversity, water storage and sediment sinks to improve floodplain and wetland functions.
Revegetation is the process of replanting native species and rebuilding the disturbed soils. The revegetation strategy often includes removing any exotic species and planting early successional, as well as climax species, to establish a natural vegetation community and appropriate strata (canopy, understory, shrub, and herbaceous species). As vegetation becomes established across the riparian corridor, the roots deepen and provide long-term stability which reduces scour from erosive flows along the streambanks and floodplain. Over time, these trees create shade for reducing water temperature and provide a source of plant litter and biomass inputs to the restored stream system.
Erosion control matting made from biodegradable coconut fibers and commonly used to stabilize restored stream banks. The matting is installed on newly constructed stream banks to hold everything in place until vegetation becomes established.
Bioengineering techniques for streambank stabilization are used instead of traditional hard armoring approaches such as riprap, gabions, and concrete-lined channels. Bioengineering uses natural plant materials that include live cuttings or whips, branches, woody debris and live stakes to establish root mass within the streambanks. Our team has extensive experience selecting appropriate plant materials based on native vegetation, aesthetics and site constraints. Bioengineering techniques are often a more cost-effective solution than traditional riprap revetments.
Vegetated geolifts are most commonly used to stabilize the outside meander bends. They are installed as a series of overlapping soil lifts using erosion control matting and native soils. Live cuttings from woody native species plants are planted in the layers between the lifts. A stone toe or toe wood base is typically installed to provide additional streambank protection and to provide a foundation for the geolifts.
Live stakes are native species woody plants that are planted directly into the streambanks. Live stakes are often installed in areas of higher bank stress, such as along the outside of meander bend, and in areas along the stream channel where accelerated vegetation growth is desired.
Woody debris/ Toe wood are effective measures to ensure long-term stability against eroding bed and banks and provides a more natural and cost-effective alternative to riprap armoring. Woody debris and toe wood are often utilized in the construction of riffles, geolifts, sod mats and vegetation transplants. Woody debris and toe wood consist of recycled tree limbs, tops, and brush materials harvested on site from stream and floodplain grading activities. In addition to providing streambank stability, toe wood and woody debris enhance aquatic and terrestrial habitat as a source of detritus material and carbon storage.