A Tale Of Two Bridges

By: Alan Pollock

Topics: Bridges , Infrastructure , History , Environment

The Muddy Creek Bridge (left, Christopher Seufert photo) and the Mitchell River Drawbridge (right, Carl Jacobs photo).

CHATHAM — In the span of a few weeks, officials opened two new bridges in Chatham, one over the Mitchell River and the other crossing Muddy Creek. That's pretty big news for a town that, as of a few months ago, only had one automobile bridge. Both spans are winning high praise from the public, both received substantial outside funding, and both were completed on time.

But that's where the similarities end. The two bridges are fundamentally different in design and purpose, and they came about as a result of very different public processes.

Wetlands Restoration vs. Transportation

In a very real sense, the two projects couldn't have been more different, Chatham Natural Resources Director Robert Duncanson said.

“One was a wetland restoration project and one was a bridge project,” he said. “That's the fundamental difference between the two.”

From the perspective of highway engineers, the earthen causeway that carried Route 28 over Muddy Creek was in fine shape and didn't require replacement. In fact, it was much sturdier than the timber bridges that spanned the “Wading Place,” as it was known, in years past. Rather, the new bridge was conceived as a way to increase tidal flushing under the roadway, helping clear away nutrient pollution from septic systems in the surrounding neighborhoods.

 The Muddy Creek Bridge seeks to restore 56 acres of wetlands which suffered from varying degrees of eutrophication. In past years, the buildup of nutrients like nitrogen in the waters caused blooms of phytoplankton that shaded the sun from reaching deeper waters where eelgrass and other key vegetation grows. The result had been that, particularly during the summer, the amount of oxygen in the water would drop below levels needed to support some marine life. Healthier wetlands not only provide better habitat for juvenile fish and shellfish, but they also make the shoreline more resistant to storm damage. Importantly, there are hopes that reducing the nutrient load on the waterway will reduce the need to extend sewer pipes or other expensive wastewater treatment efforts in parts of Harwich and Chatham.

Muddy Creek

 

Mitchell River

$6.5 Million

Cost

$14 Million

Around 2006

Talks Started

Before 2008

138 Days

Time To Build

648 Days

Wetlands Restoration

Primary Purpose

Replace Structurally Deficient Bridge

Earthen dike, and a timber bridge before that

Replaces

The last remaining timber drawbridge in the U.S.

The new drawbridge over the Mitchell River won't have any effect on water quality. Its purpose was to replace an aging bridge that was making it increasingly difficult for boats to pass underneath and for vehicles to cross the river. After state bridge inspectors declared the old timber bridge structurally deficient, town officials closed the bridge to trucks and buses. Deterioration of the bridge decking also created a loud “washboard effect” for vehicles passing over it.

Though ingeniously designed, the old bridge had a flaw: it didn't open completely. As a result, sailboats traveling to and from Mill Pond often scraped their masts on the opened span. Mariners using the bridge had to time their passage carefully and handle their boats skillfully; they also had to wait while bridge operators carried out the tricky task of raising the span.

Public Involvement

The first Muddy Creek bridge was built in 1850, and a series of replacement bridges followed in the succeeding decades. If the spans had any historic value, it wasn't recognized at the time; the solid causeway was seen as a practical way of keeping the important crossing open to traffic. When it was time to install a new bridge, historic preservation wasn't a concern.

Not so for the Mitchell River drawbridge. When state officials offered to replace the drawbridge using funds from the federal Accelerated Bridge Program, they showed some designs that immediately raised the hackles of preservationists. The planned steel and concrete bridges might have been practical and cost-effective, but they didn't belong.

Duncanson said drawbridges with overhead lifting arms are common in Europe, but in Chatham, “they never would've fit in.” In an effort led by the citizens group “Friends of the Mitchell River Wooden Drawbridge,” the old drawbridge was determined to be eligible for listing on the National Register of Historic Places. A detailed review process under Section 106 of the National Historic Preservation Act ensued, bringing together a host of stakeholders and prompting a complete redesign of the bridge. The process involved a seemingly endless series of often argumentative meetings.

“Let's just be honest here. It was a unique bridge,” Duncanson said. If indeed the old drawbridge was the last timber drawbridge of its kind in the country, “then I don't think that process – as long and as contentious as it was – I don't think that was unreasonable,” he said. The process helped engineers find a design that met modern standards for safety, functionality and longevity, while retaining some of the qualities of the old bridge.

The Muddy Creek bridge also involved a long planning process, Duncanson said.

“There was a lot of community involvement in the Muddy Creek project, but not so much in the design of the bridge itself,” he said. The Mitchell River drawbridge inspired preservationists because of its location in a historic neighborhood, because of its unique construction that was reminiscent of wooden boat construction, and because of its scenic location. The drawbridge was, itself, a destination, Duncanson said.

“Frankly, people are looking more at the scenery when they're passing over the Muddy Creek area,” he said.

Design And Construction

“Physically, the bridge structures are vastly different,” he said. That's why the Muddy Creek bridge required a road closure of 138 days, while the drawbridge took 648 days to substantially complete.

Once the causeway was excavated for the new channel, the Muddy Creek span was largely prefabricated off site. The steel girders that make up the span were trucked in “and basically dropped into place,” Duncanson said. The design was very straightforward, with the fixed span sitting atop concrete abutments and wing walls. The channel under the bridge is relatively shallow; it is deep enough to allow the tide to flush out the waterway, but not deep enough for large boats to pass underneath. Because of shoaling on either side of the span, canoes and kayaks are about the largest craft that will pass underneath.

Larger boats pass through the Mitchell River to Mill Pond, where there are important mooring fields and a thriving marina, so a bridge with a movable span was a necessity. It is also a federal navigation channel, so the Coast Guard had a say in the plans as well. Modern construction materials are much heavier than timber, so while the old wooden drawbridge could be hoisted with winches and cables using a fairly simple overhead pulley, a similar concept for a modern bridge would have required the massive counterweight and lifting apparatus to be suspended over the bridge deck. Instead, the unique design involves a counterweight that is hidden in a watertight bascule chamber, reducing the visual impact of the span.

Because the design was custom-made for the Mitchell River, the assembly and most of the construction was done on-site, “very similar to stick-building a house,” Duncanson said.

At two different ribbon-cutting ceremonies in May and June, officials praised the cooperative efforts of stakeholders in designing and funding both projects, saying the bridges will serve their purposes for decades to come. The reopening of the roadways marked the end of years and years of work for many. Asked if he would've changed anything about either project, Duncanson said he would have encouraged more realistic public expectations about the time frames.

“A lot of these projects take longer than you think they're going to,” he said.