Chicago Botanic Garden Lake Shoreline Enhancements

Challenge

Motivated by the need to protect rapidly eroding shoreline surrounding the garden’s 60 acres of lakes, together with the goal of improving lake water quality and aquatic habitat, the garden embarked on a multi-year process of researching best practices for shoreline restoration. The urban setting of the garden’s lakes produces a unique set of challenges, notably the lakes’ function as a compensatory floodwater storage basin and the resultant fluctuating water levels. Traditional turf grass shorelines installed when the garden was created over 40 years ago had become highly degraded with near-vertical edges that exceeded five feet.  Designers and garden staff were tasked with developing and implementing shoreline restoration approaches that would afford long-term erosion protection and improved water quality and aquatic habitat – and yet also provide these benefits within a landscape design context that garden visitors would considered desirable and visually attractive.

Solution

Together with garden staff, designers produced a Shoreline Restoration Master Plan with a tool kit of over 45 distinct solutions for shoreline enhancement.  These solutions emphasized “bioengineering” approaches that rely heavily on the ability of native plantings to anchor shoreline soils and create beneficial wetland habitat, coupled with landscape design principles (particularly the grouping of plants into modest-sized drifts) to create a widely accepted landscape appeal.  Through public-private funding partnerships and private donations, shorelines have been restored incrementally over the last 12 years, with more than three quarters of the shorelines now stabilized. Substantial improvements in water quality and aquatic habitat have been realized, along with 6.05 acres of new shoreline gardens that showcase the subtle yet complex beauty of native plantings.

• Three miles of unstable, eroding shorelines were replaced with heavily vegetated shallow water, wetland “shelves” and adjacent floodplain banks that are responsive to unique site conditions and constraints. This approach demonstrates an effective alternative to turf grass shoreline edges that typically erode and offer no habitat.
• The created shorelines integrated a combination of plants and structure to provide additional stability needed to deal with wave action and water levels that fluctuate by as much as 6 ft within the larger lake system, which also serves as a major flood storage facility for the North Branch of the Chicago River. Structures included geoweb, stone, sheetpile, benthic mesh, geotextile stone walls and boulders. Many different cross sections comprised varying reaches of the shoreline to respond to specific stabilization or programmatic needs. With the inclusion of these structures and the support they provide to the establishment of the plantings, the shorelines are predicted to hold fast indefinitely with proper stewardship. The earliest restored shorelines have been successfully performing for over 13 years.
• The shallow water shelves target a maximum 10H:1V slope for the portion of the shoreline that is situated at and below the normal water line, and a maximum 5H:1V slope for the portion of the shoreline that rises above the normal water line and is subjected to periodic flooding. These grades better reflect the historic natural habitat conditions that many of the native species are adapted to, and better allow the highly unnatural urban flooding conditions to be less stressful for the plants. If the grades are too steep, and/or flood innundation periods are too long (more than two weeks), native plants may not be able to adapt to these unstable site condition and survive long term.
• The shoreline restoration efforts at the garden have all been constructed “in the dry”.  This required dewatering of each lake basin via coffer dams and pump systems. This dewatered condition is crucial to allow soil that the vegetation needs to grow in to be placed within the new cross sections. Another benefit to the dewatering construction approach was the lake bottoms could be cleared of unconsolidated sediments that had built up from the historically eroding shorelines, allowing for additional improvements to the garden’s lake water quality.
• The placed soil consisted of 75% clay and 25% topsoil compacted to 95% proctor to insure a tight shelf that would not float away when rehydrated. Perforated Geoweb was placed such that it straddled the normal waterline and could provide additional stabilizing support for less intense but more frequent flood events that cause water levels to rise up to 6 ft above the normal waterline.
• Benthic mesh was laid over the soil shelves to create a more stable walking surface for maintenance crews (that might otherwise disrupt newly placed soils with each footstep) and to also protect the plants from fish and waterfowl herbivory.   
• 500,000 native shoreline plants were planted, representing 244 species, 100% of which are native perennials. Plants installed along the three miles of lake shoreline include sedges, rushes, forbs, and grasses that were specifically placed with the cross section to meet each species’ hydrologic requirements, as well as four season aesthetic interest and stabilization function. Tight plant spacing (12-15 inches on center) insured rapid establishment needed to protect against wave action and flooding, as well as minimized weeding required during the critical establishment years.
• Species propagation was specified to be from locally sourced genetic material to support best practices in preserving genetic diversity of the species used.
• The shorelines are surrounded with many walking paths that allow visitors and staff to get long views, as well as up-close interface with the new native habitats that now support many interesting birds, insects and other animals.

• Some  shoreline areas where unwanted or invasive vegetation was not completely removed prior to re-planting saw a return of these nuisance plants even when the native species were densely planted.  Thorough eradication of invasive or unwanted species prior to planting (along with intensive weeding during the first few years after installation) is imperative to secure robust establishment of the desired native species.
• The utilization of “workhorse” species during the initial planting stages of the shorelines greatly helped establish native plants in the reconstructed areas. Workhorse species are native perrennial species known for their ability to withstand myriad stressors experienced in these fluctuating urban lake settings, and thus can effectively protect the land/water interface from erosion relatively quickly. These species are characterized by possessing at least several of the following attributes: 1) dense, rapidly developing/spreading root systems, particularly those with rhizomatous and fibrous roots, 2) tolerance of disparate soil moisture conditions, including complete inundation during periods of flooding as well as summer heat/drought, 3) for plants placed near the land-water interface, possessing dense clumps of senesced vegetative material that serves to buffet the energy of incoming waves during the late fall, winter, and early spring seasons when live biomass is absent, 4) providing habitat for native resident and migratory wildlife, and 5) may be propagated efficiently by local plant nurseries.
• Installing the native plant plugs early enough in the growing season (such that the roots would have time to establish themselves sufficiently prior to winter dormancy and the impacts of frost heave) was found to greatly enhance plant health and establishment. For plants placed below the normal water line, use of six-inch metal sod stables helped anchor the plant plugs to the hydrosoils and afforded more resistance to uprooting caused by bottom feeding fish and wave disruption
• Installation of plant plugs (vs. seeding) is most appropriate for shorelines to mitigate erosion from washing the seed away, especially on steep slopes or areas that experience waves or riparian flows.  Plant plugs should be installed as densely as the project budget allows. Plug spacing on these restorations was 12 inches, which is recommended.
• Application of double-shredded hardwood mulch led to more consistent soil moisture conditions for the new plantings.  This dense/heavy mulch is also resistant to “floating away” during high water/flooding events.
• Availability of an irrigation system and/or watering equipment for the new plantings is imperative for achieving robust plant growth during the first three years after installation.

Project Team

2009-2012 North Lake
Lead Design: Living Habitats
Project Administration and Engineering: U.S. Army Corps of Engineers
Project Management: Chicago Botanic Garden
General Contractor: John Keno & Company
Plant Supply and Installation: Native Landscape Contractors

2004-2006 Japanese Garden Lake
Lead Design: Living Habitats
Structural Engineering: Ground Engineering Consultants
Project Management: Chicago Botanic Garden
Construction Management: Featherstone Incorporated
Earthwork: Lenzini Excavating Company
Sheetpile: Lorig Construction Company
Plant Supply and Installation: ILT Vignocchi

2003-2004 Shoreline Restoration Master Plan
Lead Design: Living Habitats
Project Management: Chicago Botanic Garden
Cost Estimating: Featherstone Incorporated

2000-2002 Section 319 A+B Shoreline Restoration
Lead Design: Conservation Design Forum (now Environmental Consulting & Technology, Inc.)
Project Management: Chicago Botanic Garden
Construction Management: Brown + Associates
Earthwork: Lenzini Excavating Company
Structural Engineering: Red Barn Engineering
Sheetpile: Lorig Construction Company
Plant Supply and Installation: Applied Ecological Services

Role of the Landscape Architect

Living Habitats was the lead designer for the North Lake Ecosystem Restoration Project, the Japanese Garden Lake Shoreline Restoration and the Shoreline Master Plan. Working in coordination with Garden staff, contractors, construction firms, design firms and government agencies, Living Habitats designed, documented and oversaw implementation of the restoration strategies throughout the shorelines’ phased reconstruction. The firm also created a tool kit containing strategies for shoreline reconstruction in urban lakes.