STATE OF THE ESTUARY: Underappreciated Effects of Sea-Level Rise on Groundwater Levels

Photo by Jay Huang

When most think of the possible impacts of sea level rise, they think of coastal flooding and the growing risks to shore-based infrastructure — but there’s another sea level rise-related threat that is much less talked about.  As sea level rises, so too will groundwater levels in coastal aquifers, and some recent studies have concluded that in some coastal areas, as much or more land could flood as a result of rising groundwater tables than will flood directly from rising tides.

Since most infrastructure such as roads, buildings, and pipelines were built with historical groundwater levels in mind, this could lead to costly fixes for coastal communities.  Mitigation of the effects by retrofitting existing facilities may not be physically or economically feasible.

The effects of a rise in groundwater tables due to sea level rise is of particular concern for the Bay Area, which has many low-lying areas with relatively shallow depth to groundwater levels.  At the 2019 State of the Estuary conference, Dr. Reid Fisher, Principal Engineering Geologist at Cal Engineering & Geology, gave a presentation on the potential effects of rising groundwater tables in low-lying areas of the Bay Area and the potential impacts to infrastructure.

When most people think about sea level rise, they tend to focus on the threat from the ocean or the water side, but it’s actually much more complicated than that, began Dr. Fisher.

“Everything sort of functions the way we want it to, and a large part of that is because we built and planned things according to an assumed level of groundwater,” he said.

He presented a graphic (lower, left), noting that the San Francisco Bay is off to the right, developed areas are to the left, and it show the types of things we’ve built, some above grade such as roads, and others below grade, such as stormwater channels and pipelines.  There may or may not be a levee.  The groundwater slopes down and meets the Bay.  However, with a sea level rise scenario, the Bay and ocean level rises not just temporarily but permanently, causing a rise in groundwater tables that can affect both the above grade and below grade features.

He presented another graphic depicting the same scenario in a different way, noting the blue line shows the water table.  “If we’re walking around on the land surface inland, things are dry, and unless we get close to the coastline, water level is not a threat.  However, sea level and rising water tables can result in inundation or other effects from groundwater rising and getting things wet that we didn’t anticipate.”

A recent research paper looked at the effect of sea level rise on wastewater treatment plants around the San Francisco Bay.  Wastewater treatment plants are usually located at the topographically lowest part of the communities they serve and if sea level rise comes up even a small amount, it impacts the ability of those systems to serve their communities.  The graphic shows with increments of sea level rise, what increments wastewater treatment capability will essentially be lost.

To give a local example, Dr. Fisher said he picked a random part of the Bay Area to look at the potential effects, choosing San Rafael in the northwest quadrant of the San Francisco Bay which has a lot of low lying areas.  The current situation is shown on the slide on the left; the slide on the right is with three feet of sea level rise, showing the large areas that would be inundated, which includes US 101 and the developed areas.

Zooming in further shows the usual things that exist in a shoreline commercial area: highways, roadways, parking lots, and a largely commercial area with buildings built on slab on-grade foundations along with all the usual networks of utilities and similar things.

In any given developed area, there may or may not be any sort of perimeter levee protection-type feature.

If you were to contemplate raising a levee here, there’s often very limited room,” said Dr. Fisher.  “There’s very limited space between existing developed areas and the ground you’d think about, and raising these features involves huge increases in footprint unless you go to things like flood walls.”

Traditional design of floodwalls and levees involves looking at a transient rise in groundwater level on the water side such as a bay, coast, or river, and modeling the stability of the levee with the water high on one side and then a sloping piezometric surface or water table that drops to something acceptable on the land side.

But when you have really a permanent condition of water level rise, that’s no longer going to be the case,” said Dr. Fisher.  “The water is going to rise on the land side, so even if you managed to construct a stable feature here, there is going to be a completely different set of problems on the land side.”

IMPACTS TO PAVEMENT

In the example area, there is a lot of pavement, parking lots, and roadways, and even Highway 101 in the distance.  It’s not just worrying about flooding of the roadway; it’s the more insidious process of saturation from beneath as the water table rises and the resultant loss of performance of paved areas.

Dr. Fisher explained that the way pavement works is it’s designed to distribute loads downward.  The load is imposed at the top and the layers underneath the roadway distribute the loads to the point where they are spread out sufficiently enough that things hold up.  There may be a skin at the surface, such as asphalt, but it’s just a protective skin that helps to keep moisture out of the underlying materials.  The workhorse layer is the aggregate base that’s carrying and distributing loads so that it’s can be borne by the subgrade or natural soils.  He pointed out that even here, you can see inherent in the design is some measure of surface shaping to shed water.

There are numerous manuals which describe pavement design, with a lot of time spent considering how water gets into and underneath pavement – from above, from the side, and from the water table and vapor from beneath, and a lot of design details addressed to remove water from that system.  The slide shows typical scenario of a crowned roadway so that’s designed to shed water into some low area that removes water from the system.

It may contain layers designed to pass water freely and again remove it from the system, but if you can’t remove the water from the system effectively, then this becomes increasingly saturated and you have problems,” he said.  “In our target area, much of the roadways we’re thinking about are at a very low elevation so even a slight increment of sea level rise is going to have a great effect in saturating the soils in the area.  The economic effect from the flooding of a roadway is a lot less over time than the economic effect of saturating the underlying material, so there can be massive amounts of money spent in redesigning and reconstructing roadways and pavements in order to address that.”

Dr. Fisher pointed out that raising roadways introduces another set of problems, which is the reduced overhead clearance, with potential impacts to overcrossings and overhead utilities.

BELOW GRADE STRUCTURES

Below grade structures are things such as elevator pits, utility vaults, basements, buried, tanks and pipelines.  Any essentially air-filled void below the ground surface and below the water table is going to have a buoyant effect if it’s not outright flooded.

If you have a utility vault with electrical and mechanical equipment, that will flood, so our design process in the future is going to have to anticipate what we’re going to have to change to address that,” Dr. Fisher said.  “There are a lot of facilities that are air filled and will experience buoyant effects that we didn’t anticipate, so you can imagine an air-filled large pipeline all of sudden feels a buoyant effect.  The slide shows a swimming pool that popped up out of the ground once it was empty because of the buoyant effect.  Fuel tanks are vulnerable as are basements.”

SLAB-ON-GRADE FLOORS

Slab-on-grade floors are common, especially in commercial areas and they are certainly vulnerable to saturated ground.

Water proofing is kind of a misnomer as nothing is ever really water-proof,” said Dr. Fisher.  “A structure with a basement, if it’s saturated, the water is trying to enter through the walls and through the floor, but even without saturation, capillary action and moisture vapor tries to get through walls, so both of those processes are going to affect basements and slab on grade structures.”

The picture on the left is a conceptual design that is currently used in designing slab surfaces.  “This building is designed so that the slab was above the exterior grade with exterior grade sloping away so that surface drainage runs off and out of the picture, the idea being that there’s some sort of a barrier at the base of the slab, and then free draining material beneath,” he said.  “All those assume that there’s enough separation between the slab surface and the water table to remove water from the system.”

Basements below grade are more difficult.  Oftentimes, there is a drain that is designed to intercept, collect, and then convey the water away.  “Even so, this detail shows water proofing on the outside of that wall, on the inside, beneath the slab free draining material, and even then they’ve assumed that you have to use mold resistant sheetrock because it’s there’s going to be moisture regardless.  If you have sea level rise and a rising water table, it’s going to only worsen the picture so the performance of things that are set up this way are going to suffer and we’re going to have to think about how to redesign or retrofit feasible within economic reach.”

STORMWATER RUNOFF

Stormwater projects most often rely on surface gradient and surface fall.  The slide shows a schematic of the East Bay drainage network, headed southwestward towards the bay margin.  Historically stormwater was often collected and discharged to the Bay, but more progressive approaches have been implemented to make the shoreline a much healthier environment, such as detention basins.

These structures are vulnerable to the assumed baseline that was used in design, so if the base level rises, there’s reduced fall in the system so we have to ask ourselves how long will a structure like that continue to function and how could we, if we’re designing it now, how could we design it now to build in the most flexibility for the future?

CONCLUSIONS …

What it comes down to, for retrofitting, for design and planning of these things, there are a lot of nitty gritty aspects of design that we will just have to be mindful of,” said Dr. Fisher.  “They all underscore the basic problem of what is our assumed design condition?  We’re wrestling with a sea level rise curve that we don’t know the final outcome of, so what do we choose for design level and then how do we build into our planning and design process a means to adapt, if and when we get to that point?

During the question and answer period, the question was asked, what are we going to do about this?  And unfortunately there is no real answer to that at this point.

FOR MORE INFORMATION …

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