Underfit bridges in Frankfort
Extreme events too much for aging infrastructure
The Halloween storm caused quite a bit of damage in the Mohawk Watershed, especially on West and East Canada creeks. I recently wrote that there was concern that some of the dams would fail or were impaired. Just last week Governor Cuomo requested a “major disaster declaration” so that FEMA could step in. Many small tributaries to the Mohawk River had considerable damage from flooding. This post is about bridges in Frankfort NY that were already aging and were further damaged in the Halloween storm.
One of those tributary creeks with record flooding in the Halloween storm is Moyer Creek, which flows north into the Mohawk through the town of Frankfort. This is one of many small creeks in the Mohawk Watershed that are essentially adjusting to a new hydraulic regime that involves more water and more extreme rainfall events. They are having trouble adjusting.
What we find is that some of this infrastructure is underfit in that it was not designed for the large volumes of water carried by the stream in these big events. In addition to more water, the accumulation of sediment and changes in channel geometry also exacerbate flooding. A good ongoing example is the underfit CSX bridge over Sauquoit Creek.
An underfit and impaired bridge that is having trouble with big floods and high volumes of sediment (from Herkimer NY Rising plan).
This issue in Frankfort re-emerged because the Times Telegram recently reported on bridge impairment and bridge closure. They noted that the Hilltop Bridge is closed and the Main Street bridge has been “Red Flagged” by the NYS DOT. They interviewed Frankfort Mayor Richard Adams who noted: “The Hilltop bridge is done. It’s not going to be open again unless we get some grant money to fix it. The foundation and the footers were compromised.” But there is an important back story here.
The Main Street Bridge has been a concern for years, and this concern has been acute since the 2013 flood. The 2013 event was an extreme event with an estimated 10-15 inches of rain in the month of June 2013, roughly half of which (5.5-8.5 inches) fell between 11 and 14 June. Recall that the rainfall total for the 2019 Halloween storm was 2-7 inches. These unusual extreme events are stressing the creeks and rivers.
The NY Rising plan for Herkimer (2014), which was focussed on evaluating best options for flood mitigation, noted:
During the summer 2013 flooding, the bank failure caused the emergency evacuation of over 30 residents from a senior living facility and some families from a nearby low income neighborhood, resulting in over 200 evacuees. After the storm, the NYS DOT conducted emergency action to re-stabilize the Moyer Creek channel but it did not address the entire bank failure area (p. 136)
After the 2013 event, the NYS Department of Transportation (NYSDOT) and NYS Department of Environmental Conservation (NYSDEC) retained Milone & MacBroom, Inc. (MMI) to do an assessment of 13 watersheds in the greater Utica area in the upper part of the Mohawk Watershed. The work involved flood mitigation analysis, hydrologic assessment, and hydraulic modeling. Thirteen reports resulted from this effort, and they suggest flood mitigation strategies for individual sites. The reports include Bellinger Brook, Big Creek, East Canada Creek, Fulmer Creek, Maltanner Creek, Moyer Creek, Mud Creek, Nowadaga Creek, Oriskany Creek, Otsquago Creek, Sauquoit Creek, Steele Creek, and West Canada Creek. These are fascinating reading, especially if you live in one of these basins.
Moyer Creek flows through the steep Frankfort Gorge, through the relatively flat town of Frankfort, and then into the Mohawk River. The change in gradient is important because sediments tend to erode from steep areas and deposit in the flatter areas. Moyer Creek has a small drainage basin of about 20 square miles that is rural (forested and agricultural). The accumulation of sediment in the lower part of the river results in aggradation or building up of the stream channel.
Thus there are two issues that Moyer Creek and neighboring creeks face. One is an increase in the tempo and magnitude of flooding. Another is an increase in sediment, including bedload (sand and gravel), and this causes problems where sediment accumulates and collects. To a certain extent the two are related because large rainfall events may cause significant liberation of sediment due to bank erosion.
The 2014 Milone and McBroom report on Moyer Creek (herein MMI 2014 or the MMI report) notes that the main stream channel has been dredged and the dredge material has been dumped along the stream edge (more on this below). In the section of the River that goes through the town (Frankfort), the banks of the creek are lined with rock walls immediately upstream of the Main Street Bridge, effectively reducing channel width. Flooding and damage here is compounded by a dam just downstream of the Main Street Bridge. The dam has trapped sediment and slows water, and this is undoubtedly a driving force behind ice jams that frequently flood this area (see MMI report and FEMA report noted below).
Is the current configuration of the stream channel sufficient for the amount water it needs to handle? No, it is not. An estimate was made by MMI on peak discharge events for the creek. FEMA did the same thing in 2004 for the Flood Insurance Rate study. The more recent analysis by MMI shows peak discharge for the 100 yr flood as 2,900 cubic feet per second (cfs) but the FEMA analysis for the 100 yr flood done 15 years ago was 1850 cfs. Thus the MMI calculation indicates that the 100 yr flood is now nearly 60% larger than what FEMA calculated in 2004 (in part using a different method). The new result uses the StreamStats program and more recent discharge data that is partly based on field measurements.
Image from Google Maps showing the Main Street bridge in Frankfort NY (flow is from left to right). Downstream, to the upper right, is a small timber crib dam that blocks sediment. The entire channel is confined (with a retaining wall - dotted line) on either side of the bridge (image from April 2003).
The 2004 FEMA map shows that Main Street bridge constricts the stream channel, which was thought to lead to problems with ice jams. The timber crib dam has trapped sediment and the sediment has reduced the effectiveness of the channel (MMI report). Both dam and the creek-lining retaining walls are owned by the Canal Corporation, so they will need to cooperate in a solution.
MMI calculated the bankfull width of the creek, and then determined the actual width of a number of bridge crossings. The Main Street Bridge is 34.0 ft wide, but they suggest that the river has a calculated bankfull width of 51.8 ft. Ideally a bridge span should be at least as wide as theoretical bankfull conditions, hence the Main Street Bridge is underfit - it is not up to the job because it is too small. Other bridges in the town of Frankfort were shown to be underfit for the job they need to do; this is a profound and potentially expensive finding.
Analysis of Moyer Creek and the Main Street Bridge in Frankfort by Milone and McBroom (Fig. 9 from MMI 2014 - Moyer Creek Analysis). An issue is the dam downstream that has trapped sediment so that aggradation has elevated the river channel. Widening of the bridge span to 70 ft and removal of that dam would reduce the channel elevation (red), and also the flood elevation.
An important but subtle finding of the MMI report is that sediment aggradation is a major problem and dealing with that sediment requires a plan. They note that just dredging the channel is not a proper solution and this approach can actually worsen flooding (and bank collapse). One final and key recommendation of the MMI report is the adoption of a sediment management plan. They note that the flood mitigation strategy should:
Adopt Sediment Management Standards – Large volumes of coarse-grained sediments will continue to be transported into Moyer Creek during high flow events regardless of what actions are taken to control sediments in the upper reaches and tributaries. These sediments will be deposited in the lower reaches, reducing channel capacity and contributing to flooding. When excavation of depositional areas is necessary, it should be undertaken in a manner that maintains channel stability, avoiding over-widening and/or over-deepening the channel. Development of sediment management standards is recommended to provide guidance to contractors and local municipal and county public works departments on how to maintain proper channel sizing and slope as well as the application of best practices.
This is an excellent evaluation of sediment accumulation in a Mohawk tributary. All too often we hear that dredging and sediment removal is a key to solving flood problems. Here we learn that the approach needs to be more strategic and thoughtful. Extreme events are liberating large volumes of sediment that are accumulating in low flat areas (including the Mohawk). Sediment aggradation reduces channel capacity, and this in turn makes flooding worse.
Thus there is a combination of factors that may be driving flooding and infrastructure damage in these steep creeks.
In April 2019 it was announced that Frankfort was awarded a $4.9 million Bridge NY grant to replace the West Main Street Bridge.
Just in time.
This and other Notes from a Watershed are available at: https://mohawk.substack.com/