Following a 1,000-year flood event, the repair design for this project required unique surveying & structural & civil engineering solutions
Monday, August 20, 2018, began like any other day in the city of Middleton, Wisconsin. That quickly changed when between 11 and 13 inches of rain fell over the watershed in a 12-hour timeframe. This 1,000-year recurrence interval storm caused flooded streets and buildings and damaged critical city infrastructure. Extensive erosion that occurred in numerous places damaged roads, bridges, trails and utilities.
Pheasant Branch Creek conveys a 24-square mile watershed from Middleton and surrounding communities to Lake Mendota. This watershed was centered on some of the most intense precipitation from the August 2018 storm event. The channel passes under Century Avenue and enters Lake Mendota through Baskerville Harbor, a section of channel with steel bulkheads on both sides of the channel. The volume and velocity of water passing through the channel created extensive scour and led to the failure of the sheet pile bulkhead that holds the creek banks. The repairs to Baskerville Harbor required a multi-disciplinary approach to address the surveying, structural and civil engineering needs of the project.
Employing Static LiDAR
The most significant surveying challenge for this project was trying to capture the extensive flood damage in a way that could best be understood by the engineering team. In order to provide a strong spatial understanding of the chaotic areas of debris and shoreline erosion, a departure from a typical conventional surveying process (i.e., robotic total station and/or GPS) was needed.
It was decided to deploy static LiDAR (tripod-mounted 3D laser scanning). Unlike conventional surveying technology where field crews capture one coded point at a time (i.e., edge of pavement, top of wall, etc.), static LiDAR technology allows a survey field crew to set up a 3D laser scanner in multiple strategic locations throughout the site (mainly driven by line-of-sight constraints) and capture millions of points within minutes. This collection of points is known as a point cloud. Unlike points from a conventional surveying process, these points are not coded, but the sheer density of the points in the point cloud enables an office survey technician to later discern objects within the point cloud and map them accordingly. This approach enabled engineering and surveying staff to identify specific areas of interest, extract relevant points from the data set and map those features with detail that would provide the most value to the engineering effort.
Survey technicians also published a Leica TruView data set that was easily shared and viewed by the engineering staff to provide additional context to the mapping deliverables. Leica TruView is a web application that allows users to easily view, pan, zoom, measure and markup point cloud data without needing to be an expert in laser scanning, CAD or 3D.
To augment the above-referenced survey, additional underwater channel bed elevations were captured using conventional total station technology and probing by boat.
Failed Bulkhead Presents Unique Structural Challenges
The Baskerville Bulkhead was designed and installed in the early 1970s. The bulkhead successfully supported and protected the banks of Pheasant Branch Creek for nearly 50 years. The bulkhead was designed as a cantilever sheet pile supported by the embedment of the steel sheet pile into the bed of the channel. The extensive scour from the 2018 event removed the critical bed material needed to support the wall and sections of the wall failed and fell into the channel. The bulkhead is essential for protecting homes that are immediately adjacent to the channel.
Repairing the sheet pile wall presented some unique challenges. Evaluation of the damages was extremely difficult. Several sections of wall were missing entirely, some areas were hidden by wooden boardwalks, and much of the damage was below more than 15 feet of murky water. Scour and deposition of material was also highly variable. Evaluation of the existing conditions required on the ground, on the water and probing of the bed to evaluate the site conditions.
Once the site conditions were known, raSmith’s structural engineers evaluated the existing wall and worked to design the necessary repairs. Not only did the project require repair of sections that were missing, it also needed to restore support to standing sections of the wall that could be salvaged. Ultimately, replacement steel sheet pile sections were designed and selected to fill in the missing portions of wall.
Since the material shapes and joints from the 1970s materials were no longer being made, raSmith designers needed to select current material cross-sections and joint connections that would fit with the existing wall. To make the panel connections at each end, the structural design opted to utilize some of the undamaged 1970 joints salvaged from the bottom of the channel and weld them to new panels.
This enabled welding to take place above water and the new panels could be securely attached to the old panels without difficult and expensive underwater diving. Structural engineers also worked with the contractor to select readily available cap materials and connections for the timber fender. The structural design determined the amount of embedment required to stabilize the wall, which provided necessary information for raSmith’s civil engineers to design toe scour protection for future events.
Restoring & Protecting
The site investigation revealed problems beyond the failed bulkhead. For example, near the Century Avenue Bridge, several utilities including a sanitary force main, water main and sanitary sewer crossed the channel. This infrastructure was encased in concrete; however, the scour from the flood event exposed the concrete and was starting to threaten the support of the critical facilities.
To support and protect the utility crossing, materials were selected to restore the bed elevations necessary over the utilities. To prevent future scour near the utilities, steel sheet piling was installed across the channel just downstream of the crossing. This sheet pile will assist in containing the scour blanket below the channel bed to hold and prevent scour from exposing the utilities.
Site access was one of the biggest challenges faced on this project. The bulkhead was surrounded by private property, and the channel width was barely wide enough to move the necessary equipment to place material and drive sheet pile. The project team worked with adjoining property owners to secure two locations to access the work site and bring materials closer to the installation location. The project team needed to coordinate with the city’s parks department for modifications to a public boat launch needed to launch the barges and pile driving equipment.
Using the recommendations from the structural team, raSmith’s civil engineers worked to design the necessary repairs to fill in scour areas to restore support to the bulkhead. The design team also designed a flood–resilient solution that would protect the city from future flood events. To accomplish this, a scour blanket was sized using Army Corps of Engineers design criteria to provide the needed support and protection. As part of the design, thickness and rock size needed to be considered in order to ensure the necessary water depths for boating.
Successful projects, such as this one, require innovative collaboration between disciplines, in this case, surveying, structural engineering and civil engineering. Using innovative tools such as LiDAR enabled designers to be more efficient and design better solutions. Coordination was also critical to ensure that the design functions properly for all of the disciplines and meets all project requirements. The restoration of the Baskerville Harbor bulkhead provides the city of Middleton with the assurance that critical utilities and the channel are protected for many years to come.