Startled by 2008 Dredging Model Results

Environmental Consultant Sam Johnston meets with Beach Erosion Committee members Saturday morning

Editors note:  Diane Desenberg chairs LSPOA's newly formed Beach Erosion Committee.  Diane, along with committee members Elliott Himelfarb, Bob Lifeso, Bob Thill and Ken Cox met Saturday morning at our beach with Sam Johnston of Independent Environmental Consultants.  Johnston has a masters degree in Oceanography and worked for the Florida Department of Environmental Protection Agency in Tallahassee for 8-years.  

LSPOA has hired Johnston to consult with us on the erosion concerns on north Lido Beach.  The following article from Diane continues what will be an ongoing series relating to the past, present and future of our valuable beach asset.  

With dredging of the New Pass channel (to be used to renourish Longboat Key Beaches) set for the summer of 2016, Diane did some digging... and takes a look at the protective shoal off our coastline.
It's instructive to note that the damage likely occurring to our beach today was predicted by the City of Sarasota's own dredging study nearly a decade ago.  
~BT

It sounded innocuous enough.  Reduced beach erosion at the very north end of Lido Key. Sounds almost like a benefit, no? After deeper inspection and contemplation, it looks to me like the City of Sarasota threw Lido Shores beach under the bus, in order to get the sand they wanted to renourish Central and South Lido beaches. If not under a bus, perhaps under a minivan. Given the year-long erosion we have suffered since the last dredging in March 2015, we got run over none-the-less. And the City's own Modeling Study predicted erosion of our beach. Now read that last sentence again.

Looking north toward shoal just outside of New Pass

Let me go over this in a little more detail.  In 2006, the City originally proposed to dredge the sandy shoal just south of New Pass - FOR THE FIRST TIME EVER - without any studies or modeling, whatsoever. I have to say I find this logic-defying, given the instability of our shoreline.

If I were to put a negative spin on it, I would say they showed a callous disregard for the integrity of our beach. The Florida DEP (Department of Environmental Protection) pushed back.  They said, wait a minute. You've never mined that area before. You need to provide an analysis showing the impact to the adjacent beaches. I give high praise to the Florida DEP for doing their job and identifying a huge oversight on the part of the City of Sarasota.

What was the City of Sarasota's response? They responded to many other items, including sediment flow through the area and recovery rate. Recovery rate seems to mean how long it takes for the sand removed from the shoal to be fully replaced by the natural movement of sand. They did some back of the envelope calculations based on data collected between 1974 and 1992 and determined that the sand removed from the shoal would be fully replaced in 3.6 years. That didn't sound too bad; however the data they used was pretty old.

Of greater concern was the City's total silence about the impact to the adjacent beaches. Perhaps they were hoping the Florida DEP would not notice.  But the Florida DEP did notice. First, the DEP said that 15-year old data was insufficient; a new study was needed.  Second, they directed the City to investigate alternative dredging options in order to find one with the minimum adverse impacts to the area. The DEP's primary concern was the ability of the north end of Lido Key to maintain a stable shoreline. Again, high praise to the Florida DEP.

Sketch of Area Dredged in 2009

In response, the City of Sarasota hired an engineering firm to conduct a study comparing two alternative dredging options for the sandy shoal just south of New Pass. After reviewing the study in 2008, the Florida DEP was concerned with the validity of the data used in the modeling. More specifically, the DEP felt that the ebb and flood velocity ranges used were too low. They wanted assurances that all available field data had been evaluated. They were concerned that the model did not accurately predict historical data. The DEP also questioned the validity of using a tide gage from Clearwater rather than from around New Pass. Again, high praise to the Florida DEP. But don't send your thank you notes just yet. Apparently the DEP was eventually satisfied with the engineering firm's reasons for not re-running their models and accepted the study.

What were the results of the study? Both options would extract a smaller amount of sand from the shoal using a smaller footprint than had been anticipated in the original proposal. That sounds good. Scenario 2 resulted in less erosion than Scenario 1. So, the City opted for Scenario 2. Less erosion also sounds good.

But let's take a closer look at their conclusion. The study indicated that the City's dredging could cause an area of increased shoreline retreat as sand is released from the north end of Lido Key and transported to the south. In other words, the study indicated that this dredging project could result in erosion at the north end of Lido Key. Their model shows shoreline erosion of about 9 feet/year for the first five years. The engineering study claims this is not a problem. I don't agree. Losing 45' of beach in a 5 year period is a huge problem. Here we have a renourishment project that could cause erosion on the adjacent beach. Nobody should find such an outcome acceptable.

Consultant Sam Johnston explains importance of continual monitoring

And now that they have modeled the extraction of sand, what is the timeframe for the shoal to refill? The study suggests 24-60 years!!  I guess that means "Not in my lifetime!"

One issue that the DEP did not raise concerned future dredging. The study assumed that after this dredging was complete, there would be no future dredging in the area. The City misleadingly stated that no program for maintenance of the New Pass Channel had been developed and that they were unsure of future funding for navigational dredging.

This was misleading, because the main reason for Sarasota or Longboat to dredge is to extract sand. They correctly stated that nobody was planning to dredge for maintenance purposes, but everybody had their eyes on New Pass sand for their renourishment projects. Why is this important?  This assumption might affect both shoreline erosion and long-term shoal recovery.

Sketch of Area Dredged in 2015

The study suggests that it will take 24-60 YEARS for the shoal to refill, assuming no more dredging. But, of course, there was more dredging in 2015. And there are already plans for a couple more dredgings in New Pass. Who knows when the shoal will refill. But did anybody even look into it? Did anybody look into accretion events on north Lido beach and how these would be affected by the dredging? Here's a guess: with a depleted shoal, there will be limited accretion, if any.

And that is why I began this article by saying that it looks to me like the City of Sarasota threw Lido Shores beach under the bus, in order to get the sand they wanted to renourish Central and South Lido beaches.

~Diane Desenberg