LTC 348-2020 September 2020 Tidal Flooding UpdateDocuSign Envelope ID: 675C522A4-14A3-40C0-989C-C9FE874833D00
MIAMI BEACH
OFFICE OF THE CITY MANAGER
348-2020
NO. LTC# LETTER TO COMMISSION
TO: Mayor Dan Gelber and Members gf.[hge. Gjty Commission
FROM: Jimmy L. Morales, City Mana ~1-\1.~~ (I.A • .1,v·i,.u,s
~28CA18194070496 ...
DATE: October 1, 2020
SUBJECT: September 2020 Tidal Flooding Update
Over the course of the last month, some areas within the City of Miami Beach have
experienced several tidal flooding events. There are certain times of the year when tides
rise to a higher elevation. These tides are astronomical high tides, colloquially known as
King Tides. When King Tides rise to an elevation higher than the adjacent land, flooding
is often exhibited. Since Miami Beach is low-lying and near sea level, the City is
particularly vulnerable to King Tide flooding. Miami Beach also faces flood risk from
storms, hurricanes, and sea level rise and is implementing a holistic climate resilience
strategy to address these flood risks.
A flooded street alone can be a difficult experience; the experience is exacerbated when
the flooding affects a home or a property. The City is currently working to address
flooding across all neighborhoods in a comprehensive and interdisciplinary manner as
recommended by the Urban Land Institute (recommendations adopted in 2018) and
Jacobs Engineering (recommendations adopted in 2020). However, depending on the
cause of flooding, the solution may differ. For example, when it rains and streets flood,
the drainage system and pump stations covey the water away from the flooded
neighborhood. In areas that have older generation pump stations, or perhaps no pump
stations at all, a drainage improvement project is required to address rainfall flooding.
While these drainage improvement projects are important, they do not address all types
of flooding. Tidal flooding requires adaptation to sea level rise. This is because as sea
level rises, we must remain above the elevation of the water to stay dry - we cannot
pump the ocean.
The way we adapt our roads is by elevating these roads above the water level - in most
cases this means only a matter of inches but in more severe cases, it can be more than
a foot. The South Floridian soils are largely calcium carbonate based - essentially the
soils are comprised of limestone. Limestone is a very porous rock that readily
accommodates the flow of water. The porosity of the limestone is vital to our community.
It is this very porosity that replenishes the Biscayne Aquifer - our primary source of
drinking water. However, the porosity works in both directions (upward and downward
flow).
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The City recently constructed numerous groundwater monitoring wells to help our
experts better understand our subterraneous hydrology. The foremost conclusion we
derived from the data was that the City's groundwater elevation is directly correlated with
tidal elevations. Any time that tides rise or fall, so does our City's groundwater table. In
fact, it not only moves in lockstep with the tides, but it also exhibits the same elevation
shown at tidal gauges within the City and neighboring communities.
Structural barriers, such as seawalls, cannot stop the flow of groundwater. As tides rise,
the waters will permeate through the limestone rock below these structural barriers and
find equilibrium on both sides. Specifically addressing the tidal flooding experienced over
the past month - the highest astronomical tide recorded reached an elevation of 2 .16
NAVO. This tide would flood properties and city rights-of-way below this elevation.
The Public Works Department has spatially allocated flooding complaints from
September 2020 -- the current King Tide season. As seen in the attached map, most of
the concerns are concentrated in areas that have been programmed for neighborhood
improvement projects, which include elevating roadways. Clusters of complaints can be
seen:
• North Bay Road - programmed as part of the La Gorce Project, which was
subsequently canceled in May of 2018 by Commission action
• First Street - currently in design, pending resolution of pump station location
• Indian Creek Drive Phase 3 (north of 27/ Street) - currently in construction
• Normandy Isles - first project in the Jacobs Neighborhood Prioritization
It is also important to note that the areas that have undergone neighborhood
improvement projects did not exhibit significant complaints during this tidal season.
These include:
• Sunset Islands 3 & 4
• Venetian Islands
• Palm and Hibiscus Islands
• West Avenue Phase 1
• Indian Creek Drive Phases 1 & 2 (south of 27 Street)
• Sunset Harbour Neighborhood
• Dade Boulevard & West Avenue Bridge (from Biscayne Bay to Alton Road)
• 11Street (from Alton Road to Washington Avenue)
• Convention Center Drive (along 18 Street and 19 Street)
Additionally, as seen in the second attached map, which identifies all roads below an
elevation of 2.0 NAVO; the clusters of complaints are located within the low-lying areas,
further illustrating the correlation between road elevation and tidal flooding.
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As stewards of the public infrastructure, we understand the concerns of our residents
and we are proactive in implementing whatever temporary measures we have at our
disposal. At times, this can include deploying temporary pumps or vacuum trucks to
alleviate localized flooding. Unfortunately, during the most severe events, when flood
mitigation is most needed, these temporary measures are ineffective and unsustainable.
The only sustainable approach in the long-term is to construct drainage and roadway
improvements. Therefore, the City worked diligently to develop a stormwater plan in
2015, leveraging the expertise of AECOM (a global engineering firm), and conducted an
in-depth review of that plan in 2017, and utilizing industry experts such as the Urban
Land Institute in 2018. The City then procured the services of Jacobs Engineering in
2019, a global expert in stormwater and resilience, to incorporate the recommendations
from the Urban Land Institute in our stormwater plan. The Jacobs Engineering
recommendations were approved in 2020.
To holistically build our resilience to flooding now and in the longer term from sea level
rise, the City has enacted numerous land use regulations to ensure that private property
is elevated to meet current or future needs. Notably, 93% of all buildings in Miami Beach
are in the FEMA Special Flood Hazard Area. Existing buildings do have flood mitigation
and sea level rise adaptation options at their disposal. A private property owner can
harden its thresholds against water. Examples include but are not limited to dry flood
proofing (preventing water from entering), wet flood proofing (allowing water to flow
through the property in a manner that does not adversely impact the property), elevating
mechanical, electrical and plumbing, and first floor or building elevation. In September
2020, the Land Use and Sustainability Committee approved and forwarded a "Private
Property Adaptation" program to the City Commission.
The City is continuously working to implement drainage and roadway projects in a
commonsense manner that not only address flooding, but also include much needed
improvements to the aging water and sewer infrastructure. We call these neighborhood
improvement projects, as they comprehensively address most of the critical
infrastructure in the neighborhood.
Fortunately, this multifaceted approach will reduce the risk of flooding from an
infrastructure perspective, however, the drainage infrastructure is only one component
of flood mitigation. In addition to elevating roads, the City and private property owners
alike must raise seawalls to address storm surge and overland flow. If a road is built to
an elevation higher than the groundwater table, water would not flood the streets when
it finds its equilibrium. This approach will attenuate the effects of rainfall flooding, storm
surge, and sea level rise.
The Public Works Department will continue to document flooding concerns to ensure
that they are properly addressed in a future infrastructure project. We will also continue
to monitor conditions and, when possible, provide any temporary relief we have within
our means.
The City will continue to share flood awareness precautions for high tides through
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communication channels. The next expected high tides, based on NOAA predictions,
are estimated to be October 14-21, 2020 and November 13- 18, 2020. If you have
any questions please contact Roy Coley, Director Public Works, 305-673-7080. ..
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Average Road Elevations Road Elevations (NAVD88)
Miami Beach
Miami-Dade County, FL -- 2.00 or Less (18 Miles/ 10%) -- 2.01 or Higher (150 Miles / 90%)