Potential Impacts of Sea Level Rise on Water Management

Excerpt From South Florida Regional Compact Implementation Guidance Series 2021

As the sea level continues to rise in response to the changing global climate, south Florida will become more and more susceptible to a wide variety of negative effects, primarily due to its very low topographic elevation. While the effects of sea level rise on coastal communities in south Florida are being reported worldwide, the more subtle impacts to the regional water resource management system, have recently come to light. The impacts fall into three general areas:

  1. Flood protection and drainage systems
  2. Quality of existing underground water supply sources
  3. Natural systems already impacted by human activity

The first, and most obvious impact is to the overall drainage system which in virtually every case ultimately releases excess stormwater into the ocean through the coastal estuaries. These systems are almost exclusively gravity driven. This means that water flows from higher elevations on the landscape to lower elevations simply by allowing gravity to pull the water from one drainage feature (swale, ditch, canal, pond, etc.) down to another until it finally flows into the ocean at the lowest level. When the ocean elevation rises, the elevation difference between upland areas and the ocean is lessened, which in turn lessens the gravitational pressure to move large volumes of rainfall runoff out to sea. This effect ultimately reduces the flow of water through the structure and slowing it down and increasing the frequency and duration of flooding resulting from heavy rains.GraphicThe path that stormwater takes from its beginning as runoff from heavy rains to its release into the ocean, is managed by a series of numerous physical structures made up of pipes, gates and sluices. These structures are operated (opened or closed) in conjunction with one another to release water from developed areas to avoid flooding or hold it back to enhance water storage and groundwater recharge.

Slow drainage problems are amplified when coastal structures are hit with a heavy rainfall event which is typical during land-falling tropical storms and hurricanes. In these cases, the elevation of the storm surge downstream of a structure (ocean side) can be pushed above the water level in canals upstream (land side). In these situations, the system operational personnel have no choice but to close the water control structure to avoid the inland rush of the coastal storm surge into the canal network, thereby further worsening flooding impacts.

Regional coastal structures maintain groundwater levels, which hold back saltwater intrusion into underground freshwater aquifers; they are also critical to minimizing flooding impacts. But numerous communities downstream of these facilities do not directly benefit from their operation yet face the same threat to their drainage infrastructure. The cities of Miami, Miami Beach and Fort Lauderdale are key examples of this situation.

Sea level has risen about eight inches over the past 100 years, and there are clear indications from recent tidal data that the rate of rise is accelerating. This forces immediate attention on the regions, like coastal south Miami-Dade County, where the current water control elevations are already very close to the current range of tidal fluctuations.

Most of the major water control structures along the coastline in Miami-­‐ Dade County already maintain canal elevations very close to the upper end of the normal tidal elevation range. In some cases, spring tides already exceed the normal canal elevation, which forces gate closures at least twice a day during those periods.

Similar situations exist in Broward County, where numerous communities and their local drainage systems are situated downstream of the regional water control system. These communities find themselves in the position of facing a double threat associated with flooding. They are threatened by both direct impacts of rising seas on their local drainage systems, and storm discharges from the western regional systems flowing through the canal systems in their communities. A simple solution would be to stop or reduce the releases from the western communities. However, this action would most certainly worsen flooding in those towns and cities.

Adaption strategies can be complicated and expensive but must be considered for future implementation Statewide. For more information on the response to sea level rise, visit  https://southeastfloridaclimatecompact.org.