Proposed Giant Tidal Gates by the Throgs Neck Bridge and their Impact on the North Shore

On February 19th, the Army Corps of Engineers will release an interim report that will lay out their conceptual plans for protecting New York City from future major storms. This will be followed by public meetings (yet to be announced) in March and April. This article will bring you up to speed on what we know so far.

Late last Summer / early Fall, we learned that the Army Corps of Engineers was proposing a number of alternatives which included the construction of massive tidal gates by the Throgs Neck Bridge, which when closed, would raise water levels behind the gates (called “induced flooding”). Despite the potential for this impact on north shore communities, no hearings or public meetings had been scheduled on the north shore. That changed when we contacted Rep. Tom Suozzi who in turn convinced the Army Corps to hold a public meeting at the U.S. Coast Guard Academy on October 23rd.

At that meeting, the Army Corps committed to studying the impact of the induced flooding on our communities and to hold future meetings in our area. The extent of their study into induced flooding was not clearly defined. We are hoping that it will be in the upcoming report.

The following is a synopsis of the October 23rd public meeting. It was prepared from notes taken by Sarah Deonarine of the Manhasset Bay Protection Committee and Eric Swenson of the Hempstead Harbor Protection Committee.

Rep. Tom Suozzi pointed out that following nitrogen, the top concern voiced at last year’s L.I. Sound Summit was sea level rise. Flooding is a real concern for north shore communities. 

The Army Corps’ Bryce Wisemiller then pointed out that post-Sandy, the Corps completed aNorth Atlantic Coast Comprehensive Study,which identified focus areas for further study. The New York and New Jersey Harbor and Tributaries (NY NJ HATS) is the largest of the focus area studies; it covers 2,150 square miles, 900 miles of shoreline, 25 counties, and 16 million people. Because the study area is so large, the Corps has two non-federal partners (they always have to have one): NJ DEP and NY DEC.  And New York City is a “major player.” Without the protections envisioned in this study, a 100-year storm, 227,000 structures would be impaired and 300 square miles would be flooded.

The Corps emphasized that this is just the beginning of the process and that they want to study and collect information from others.  They have concept plans developed for the alternatives based on best guesses and assumptions but are still looking for public comment and input. They requested that audience members send them any information that they may not have.


The Corps’ presentation started with a map that shows only the fatalities from Sandy, because those are “the resource that cannot be replaced.”

The Corps started with definitions and explaining their process: “Coastal Storm Risk Management (CSRM)” – it’s risk management, not flood control.  No matter how high the wall is, it will be overtopped eventually. They can reduce the number of flooding, divert water, store the water, stop the water with a localized measure – structural.  Non-structural is when we don’t change the way it floods, but try to get structures and people out of harm’s way (elevating houses, flood-proofing, buy-outs, etc.) – not changing the flooding regime (non-structural).  

The Corps has 6-step iterative planning process.  Evaluate alternatives:  has to be environmentally acceptable, cost has to be less than the gains. The Corp’s revised schedule is based on receiving a waiver to their “3x3x3” rule (studies must be completed within 3 years, cost no more than $3 million, and involve no more than 3 tiers of administration): 


o   January 2019 – “Interim Report” (not any sort of statutorily required document, but will include summaries of their work thus far, and will respond to all scoping comments received by 11/5/18) – concept plan/mock-up

o   Spring 2020 – Draft EIS and Feasibility Study (here they will narrow down the alternatives – as opposed to the original plan to narrow down this fall before the Draft EIS)

o   Comments can be taken on the Interim Report and the Draft EIS/Feasibility Study

o   There will be some sort of outreach event(s) after the Jan. 2019 Interim Report and after the Spring 2020 deadline – Corps does not know what format yet

o   The Corps recently added CT stakeholders to its mailing list on the project.  Stakeholder outreach list is over 3,000 and growing.


There are six alternatives. #s 2 – 4 are in-water structures. #5 is land-based structures. All alternatives are designed for a 100-year storm with increases for wave action:

1– baseline/ no action alternative (note: they will show what others are doing including other Army Corps projects in the study area).

2 & 3ais for a surge barrier just west of the Throgs Neck bridge.  Planning to the 1% storm event (in 100 years, the 1% storm would be one in 10 years).  Alternative 2 has the largest number of surge gates.  They referred to the “Study Area Proper.” This also included surge gates closing off NY Harbor from Sandy Hook NJ to the Rockaways.

3a– tide gates go further back into the estuaries.  Surge gates are very expensive.

3b – does not have as many surge gates.

3b & 4are very similar, the differences are on Staten Island and NJ

5– only on-land based measures (sea walls along the shoreline, etc.)

Storm risk gets worse for the Atlantic Ocean areas as next 100 years pass (worse than for the north shore).  Being designed to 100 year event + mid-level SLR projection + wave run-up

Surge barrier examples from around the world:  

  • Surge gate in Louisiana was $3B
  • Thames River in London, structures on the bottom rotate up, they don’t get much wave action, so it works there.  
  • Largest moving structure in the world is in Rotterdam, the Netherlands.  Sector gate comes out from the land and is sunk down.
  • As the surge gates won’t be closed for every event, high frequency events may impact areas behind a surge barrier.

Nothing is easy, given the urbanization and the coastal storm risk in the area.

NEPA Process:

They will be doing a tiered environmental impact statement (EIS), the most robust NEPA analysis. Because of the size and complexity, they will consider environmental impacts at every stage, but won’t be as in-depth until they get further along in the process.  During the feasibility study phase (conceptual), will have a Tier 1 EIS.  Then tier 2 based on more detailed designs, etc.

No plan can be constructed until permits are in place, and they can’t get permits until EIS is completed.  There will be lots of public input through this long process.  At the end of the process, a Chief’s Report will be prepared which will then be sent to Congress. Congress then decides what will happen based on the Corps reports and if the non-federal sponsors agree (New York, New Jersey and NYC). There was no mention of Connecticut. Congress has to authorize the project and fund it, which alone is probably a multi-year process.  On the current scheduled, they anticipate making a recommendation to Congress in Summer 2022. Construction may not begin until the 2030s.

Public comments on this study are due by November 5th.

Question & Answer Take-aways:

Army Corps is very concerned about “induced flooding” behind the barriers (i.e. the north shore). Induced flooding would have to be dealt with as part of that alternative – all impacts have to be mitigated.  This could shut-down an alternative.

The January report will only present information they have, no decisions/selections will happen at that time.  Right now they are throwing out a lot of assumptions and best guesses. Developed a general cost and benefit that they created, will be in the January report.  As they go through the process, these will change.

Modeling of impacts outside the alternatives will be done for each alternative.  Reflection of waves on gates is part of modeling analysis that will have to be done in the future.

Storms send surge a couple days before the storm hits, so the surge gate would be closed a day before that.  Transportation and navigation already stop for big storms, but the details for protocols have not been worked out yet.

Saddle Rock’s Mayor stated that they’ve had four 100-year storms in the past 5 years:  Irene, Sandy, something else, and the March Nor’easter. He felt that adding induced flooding on top of this could devastate his village and that places like Bayville would “become a sump”.

FEMA money has not been considered, they only identified those projects authorized shortly after Sandy.  If there needs to be a look at specific areas, that would come through non-federal partners.  ßThis was Sarah Deonarine’s (Manhasset Bay Protection Committee’s) question, she was attempting to get at the point that their alternatives would counteract federal money being spent in Manhasset Bay.

Explain the waiver to the “3by3by3” rule:  the schedule is assuming that there is a waiver. The Corps has a motto that a project has to be done within 3 years, $3m, and 3 levels within vertical team.  This process in this area is going to exceed these restrictions.  HQ is supporting the waiver to go through, but want to make sure that there are “checks” along the way.  The waiver still needs Secretary’s approval.

Did you consider other alternatives, would you consider other alternatives?  Other alternatives were considered and will be included in the iterative process, especially if there are other ideas suggested.  Right now, they’ve just been focused on the 6 shown here and studies done prior to this.  If anyone has ideas/suggestions, send them in. 

Eric Swenson (Hempstead Harbor Protection Committee) asked how far east the Corps would study in terms of induced flooding. They did not give a direct answer.

The January 2019 draft is just to say “what we’ve been doing,” and is part of being transparent.  The report will consolidate all the information they have now and the alternatives they have looked at to display what they’ve done, and identify what additional information is needed.  They acknowledge that this information is going to have to be followed up.  Spring of 2020 should be the draft report and EIS.

The study alone may cost up to $19M and take up to 6 years [half federal money, half non-federal money (NJ &NY)].

They are considering establishing a steering committee to help with outreach as well as a technical advisory group. It was pointed out that the Long Island Sound Study already has a Citizens Advisory Committee and a Technical Advisory Committee, and both are comprised of stakeholders from around Long Island Sound. They stated that they would consider using them.

Eric Swenson (Hempstead Harbor Protection Committee) also asked if the study would consider and comply with the recommendations in the Long Island Sound Study Comprehensive Conservation and Management Plan. They said that they would.

One person stated that the depth of the water at the Throgs Neck Bridge is 100 feet and asked if the tidal gates would be that tall. The Corps stated that the area where the gates would be installed is not that deep but they could not give the actual depth. The gates would have to be as deep as the water.

The Corps also stated that Long Island would be included in future public meetings. 

Here is a direct link to the Army Corps slides shown at the presentation

Stay tuned….

Leave a Reply