Feb 11, 2020

Eroding History

A permanent vegetative solution aids in the preservation of a historical site 

erosion control

Located in Shorewood Hills, Wisconsin, just east of the University of Wisconsin campus, The Pew House remains a modest example of Frank Lloyd Wright’s harmonious design between nature and the built environment. Constrained by cost, the original homeowner could only purchase a narrow, 75-foot wide lot along the shores of Lake Mendota. In order to work with the narrow lot and the natural topography along the shoreline, Wright proposed constructing the home on pillars to limit the impact to the forested slope and create an appearance the home is “floating in the forest” above the lake. Trees and native plantings were added to the already forested slope, and a natural limestone stream and waterfall spans between the home’s pillars in a Falling Waters fashion, adding to the natural beauty of the site.

Fast forward 75 years to August of 2018. The village of Shorewood Hills has grown from the time the Pew House was constructed. Natural topography, dense urban residential neighborhoods and intense commercial development along the thoroughfare edge now present issues with storm water runoff and erosion. While lakefront living has its advantages, it also presents many challenges. For some homeowners along Lake Mendota Drive, it is a constant battle with erosion. The steep shoreline slopes are covered with a narrow band of topsoil that mask the limestone bluffs below. The key to keeping these slopes stable is vegetation. Once the vegetation is gone, the soil erodes, leading to slope failure. A typical rainfall is manageable for most sites. However, size and frequency of storm events are taking a toll on the waterfront landscapes. In August of 2018, the Madison area was ravaged by a historic level flood, receiving 12 to 14 inches of rain in less than 24 hours. Many of the residential hillsides along the Lake, including the Pew House, could not withstand the inundation of groundwater and overland flow. The torrential downpour loosened the root structures of historic Oaks and Pines, causing them to tumble and take the soil with them. Water moving toward the hillsides also opened up passages between the limestone shelves and the narrow bands of soil, leaving near vertical bluffs as a result of the devastation.

While slope stabilization was necessary for many of the property owners, the bigger challenge was how to prevent future erosion. The owner of the Pew House reached out to Landscape Architect John Gishnock III. Through his design-build landscape firm Formecology, Gishnock had spent countless hours in design, installation and maintenance of the site. Before being able to evaluate potential solutions for the slope failure, it was critical to understand what attributed to the failure and what type of failure occurred on the slope. That is where many designers and contractors fall short. They look at the problem and not the root of the problem itself. If the approach was simply to stabilize the slope without understanding why it occurred in the first place, the solution would be a temporary fix.

In early site visits and planning, it became clear that there was a substantial amount of overland flow from the street and driveway. The native plants that engulfed the front façade of the house provided ample space to redirect some of the water way from the impervious surfaces. The mixture of native woodland and savannah plants contain deep rooted plants that are ideal for slowing, filtering and infiltrating surface runoff. A simple plan of maintaining the existing landscape and adding plants provided a great start to reducing the rate and amount of runoff that reaches the shoreline slope.

A successful design would need to further slow, filter and redirect runoff away from the slope. While many neighbors facing similar issues, looked to expensive hardscape walls with complex drainage systems, the homeowner of the Pew House wanted to be true to Wright’s philosophies and continuing the “floating in the forest” look would be consistent with the architect’s original vision. This could not be achieved with large stone or concrete walls. The water feature below the house and limestone wing walls that span the edges of the front façade also presented a problem as it made the slope impossible to access with machinery and limited the access for materials. The solution would need to be low impact and limit both cut and fill. Not only would it be challenging to move materials on and off site, many of the soils in this area are infested with jumping worms, in invasive species of earthworms. Jumping worms feed ravenously on organic matter, consuming nutrients that are essential for plants. The results of their intense feeding leave the soil in a pellet type state with a structure too poor to sustain native plants and result in a take over of invasive plant species. Moving the soil or taking the soil off-site could have devastating consequences.

With the complexities of the site, Formecology turned to Edge Engineering Consultants to provide a fully engineered plan for both storm water and stabilization solutions. Solving the storm water issue was the first step in design. The proposed design included a 24-inch catch basin at the top of the slope to catch runoff from the roof and any overflow from the impervious drive. The water would then be carried to an additional catch basin at the base slope by an 8-inch pipe. The catch basin at the base of the slope is designed as an energy dissipator to slow the water and release sediment prior to reaching the lake. In addition to these measures, Formecology worked with Edge Consulting to surround the top catch basin with native plants to filter sediment and debris before the water enters the pipe.

In looking for solutions to stabilize the slope, the Edge team reached out to Envirolok as a potential permanent vegetative solution to the continued bluff erosion. Envirolok’s geobag system can be easily transported down the slope or to the site via barge. 

A mix of 80% sand and 20% topsoil were used to fill the Envirolok bags. This mixture provides engineered strength for stability and suitable media to grow and maintain vegetation. The 30-foot tall terraced slope design eliminated the need for extensive embedded courses of bags and reduced the lengths of the earth anchors that would be required for additional strength. Once the installation is completed, the system is vegetated using live plants, hydroseed or even sod. In addition to providing engineer review, the Envirolok teamwork hand-in-hand with both Edge Consulting and Formecology to provide material quantities, estimating tools and a general range in cost for construction. This step was essential for the team and client to understand the cost impact prior to finalizing the plan.

Upon completing the design, the team met with the village’s engineer for approval to use the Envirolok system. Unlike other vegetative systems that had been used in the village, Envirolok is fully engineered. This along with being a green solution was a big selling point for the village staff. Formecology was ready for final permitting and construction.

Once approved and permitted, Formecology was ready to get to work. While the Envirolok system is simple to install, the slope size and challenge of accessibility made the project seem intimidating. Envirolok introduced Formecology to seasoned Envirolok installer Pat Dixon of Dixon Shoreline Landscape. By forging a collaboration between the two landscape teams, Gishnock was able to bring the experience and expertise of the Dixon team as labor and on-site mentor for the project. Being no stranger to these challenging shorelines, Dixon used a series of mobile conveyors rather than barging the material to the shoreline.

Turning Dirt

Before starting work, Gishnock's team buttoned up the top of slope by installing a number of temporary erosion control measures to slow and divert water during construction. Along with Dixon Shoreline, the team then reinforced the toe of the slope with native boulder rip rap. Removal of the invasive vegetation was completed, and the crew went to work on installing the drain pipe and inlets. From there, slope stabilization could begin. The mobile conveyor system was set up and prepared to move the backfill material and soil bags down the slope. The direction of the conveyors could also be reversed to move any waste material up the slope and into trucks.

erosion control
The project required a plan, including slope stabilization and erosion control solutions.

Native Vegetated Mats

When the Envirolok installation was completed in late October, it was time to vegetate. Careful consideration was made in plant selection and timing of planting. Because it was late in the season, the team discussed waiting until spring to hydroseed. However, in order to provide an immediate green for the homeowner, Formecology installed native vegetated mat (NVM) over the surface of the Envirolok Bags. NVM is essentially a native prairie sod developed by Envirolok’s sister company, Agrecol Native Plant Nursery. The dense root structure and internal coir mat make it ideal for erosion control for slope applications, swales and biofiltration gardens. While not as dense as a traditional turf sod, NVM provides approximately 25 to 30 plants per square foot grown in a thin soil layer. Native species for the NVM included a mix of grasses, sedges and pollinators that will thrive on the slope conditions.  

storm water management
The project's design needed to slow, filter and redirect runoff from the slope. 

Passing the True Test

The Madison area has seen an unseasonably warm and wet fall and winter. The success of the installation and vegetation are on its way to providing a permanent vegetative solution. Through Formecology’s preventative approach to storm water management and erosion control, the Pew House and its historical grounds continue to stand the test of mother nature and reflect the original vision of architect Frank Lloyd Wright. 

erosion control
A vegetative mat was installed that includes grasses, sedges and pollinators.

 

About the author

Eric P Seidl is LEED-AP and sales manager for Envirolok LLC. Seidl can be reached at [email protected]

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