typologies

Informed by precedents including prior planning operations and physical implementations of wildlife corridors, we have established a portfolio of design categories. These categories are tools to build connectivity, and dependent on site-scale contexts, may enhance rewilding efforts in urban spaces. Some of these are applicable to our case study in San Rafael which we discuss further in later parts of this project.

Barriers & alternative design strategies

In order to decide on which operations may enhance connectivity, it is first necessary to identify which barrier(s) to combat or alleviate. The most obvious of these include highways and roads that intersect habitat, harming many individuals within a population, and preventing genetic flow decreasing potential for long term persistence. Many roads are built along shorelines, blocking access to wetlands that are potential habitat for mammals, amphibians or reptiles. Land use changes and energy development, including transmission and power lines and pipelines, water diversions and damming, and transportation development, can all impede wildlife movement as well. Indirect results include the fragmentation of fish and wildlife habitat, putting animal populations at risk and increasing stress on ecosystem health. Below is a list of the most frequent barriers found in modern urban landscapes, followed by alternative design strategies which improve connectivity and contribute toward rewilding efforts.

Barrier: Levees

Levees are designed to protect shorelines and communities which sit in low-lying lands. They may contribute toward damage-control associated with flood events. These levees are often made using fill, raising land taller than the height of tide levels or projected flood levels. They may be supported with rip rap edges consisting of large rocks, and/or hard infrastructure revetments.

Design Alternative(s): Ecotone

One nature-based option which could replace abrupt levees and hard infrastructure along shorelines includes ecotone levees. These also contribute toward flood protection. They are based on the principles of ecotones, in that they create “in-between” spaces or transition zones. This includes shifting vegetation cover and height creating long-distance shores. This lengthened space absorbs wave action while providing food, nesting, and habitat opportunities for species.

Another nature-based alternative is the restoration and management of tidal wetlands. Wetlands offer protection against flooding, while also containing unique food sources and habitat for wildlife.

Barrier: Dams

Dams may impede wildlife movement. Ponds and wetlands are separated by built-up dams which block aquatic species from migrating, and may lead to repercussions in ecosystem structure (related to sediment deposits, and debris build up). This is particularly damaging to species such as salmon and steelhead who require passageway from the sea, traveling upstream to reproduce.

Design Alternative:

Openings controlled by gates and/or weirs in the dams can enable species migration for food and nesting reasons.

Barrier: Roads

Roads represent the most common barrier for wildlife movement, with freeways cutting through swaths of animal habitat. Roads are also common sites of human-animal conflict, namely collisions between cars and animals.

Design Alternative(s): Crossings

Crossings are man-made linkages that help animals navigate barriers, such as roads, within habitat corridors. They are typically installed within known high-traffic wildlife movement areas to strategically facilitate wildlife passage.

Design considerations: all crossings should be equipped with fences and or guiding structures to safely move species within and across habitat patches.

Landform Overpass

These are usually large scale projects which provide vegetation cover for passage across large roads. They may provide shelter and habitat for smaller mammals, reptiles, and insects. They may also be large enough to guide larger mammals safely, decreasing the number of automobile-animal collisions.

Bridges

These are usually smaller scale projects made of hard infrastructure, in some cases they have contributed toward moving crustaceans across roads.

Grate underpass

Grates offer protection for animals and species whose migration paths stretch across roadways. They can protect small-scale corridors by covering them, but they are also a way to guide species safely to the corridors (i.e. acting as a guiding barrier).

Tunnel underpass

Like overpasses, tunnel underpasses help animals to move safely in their wildlife habitat and offer them the mobility they need for feeding, nesting and migrating.

Barrier: Lack of Habitat

The most glaring barrier to movement and rewilding in urban spaces is a general lack of habitat space. Cities are composed of largely impermeable, inorganic material. Though some of these areas contribute toward refugia (resting) space, they do not provide adequate feeding and reproduction resources for many species.

Design Alternative(s): Patches

Lack of habitat can be addressed through the following strategies, consisting of additional green and blue infrastructure interspersed throughout urban areas, as well as in the larger scale restoration of natural systems in cities. This relates to our previous discussion of the corridor-patch matrix found in the landscape. Through increasing patch frequency, wildlife may traverse an urbanized landscape more readily. Increases to patch frequency can occur at the household, community, city or regional scale, increasing habitat space and proximity to resources.

Pond network

Pond networks offer many species food, nesting and habitat opportunities. These ponds provide opportunities to filter runoff, and enhance water quality, sequester carbon, and when paired with vegetation provide many opportunities for wildlife habitat.

Artificial & Constructed Wetlands

Artificial wetlands may be man made floating structures, using organic material attached to hard shorelines. These act as stepping stone habitats for migratory birds. Along with this, a wide range of amphibians, reptiles, and aquatic species utilize these spaces. They may be retrofitted with rock beds for urban wildlife to lay eggs (including amphibian and fish species). Along with this, constructed wetlands may be created as an alternative to hard shorelines. These may be developed through intensive revegetation strategies focused on broadening existing wetland scopes.

Reusing dredged material & island habitat

Patches along the shoreline may contribute toward urban shoreline buffer and protection. Sediment that is moved from construction work adjacent to the shoreline, along with sediment that builds up along hard infrastructure, may be managed to create islands and patches offshore. These provide habitat for aquatic organisms and potentially waterbird and seabird populations. Paired with this, oyster islands can be made of reused hard infrastructure (such as repurposed rip rap). These concrete installations allow oysters to attach to them.

Shelterbelts:

Shelterbelts may be used as linear patches, connecting larger groupings of trees. They are made of a row or multiple rows of trees, usually along fence lines in agricultural lands. They are planted mainly to protect animals or crops from damaging winds, but also provide shade in hot weather. Some design considerations paired with this patch-enhancing strategy include identifying plant species and spacing of vegetation.

Revegetation, Tree planting & management

Revegetation offers food, nesting and habitat opportunities for urban wildlife. Vegetation patches can be species-specific (providing food for specialist pollinators for example). Tree planting and management may also help to prevent spread of wildfires in urban spaces.

key takeaways

With these options in mind, we highlight design opportunities that are appropriate to our San Rafael case study. These include a grate underpass allowing amphibians and other animals to avoid Highway 101, a network of vegetated ponds, and a revegetation strategy aimed at a large parking lot. These are detailed after the San Rafael site is introduced and analyzed.

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