BC Adapts: Design of Raised Shorelines

BC Adapts: Design of Raised Shorelines


Increases in sea level rise may create a need
to raise shorelines at existing developments – whether by dikes, floodwalls, or raised
beaches. Raised shorelines are not a panacea. The dramatic failure of dike and floodwalls
in New Orleans during Hurricane Katrina illustrates the consequences of a false sense of security. In BC, several dike failures have led to a policy at the Provincial level that encourages
homes within a dike ring to be brought up to flood proof levels. To design shorelines
that will be resilient to sea level rise, we need to understand the basics of what drives
coastal flooding. Storm surge is a key element separate from sea level rise – created when large
storms with low air pressure, and high winds, result in higher than normal water levels
at a coast. The power of storm surge is quite remarkable. This log was moved here by the
wave action. We’ve also had storms that have moved these one ton planter boxes out into
the parking lot. Storm surges have also gone over the road, closing highway 101 and flooding
homes across the street where the residents had to be evacuated and rescued by the fire
department using boats. We often visit the waterfront in fair weather, and during normal tides. But it’s a different place during spring high tides. They happen several times
a year. Add a storm, and the combination of storm surge and wind increases wave run-up
on the beach. When we take the same storm conditions and steepen the shoreline with
riprap, significant wave splash results. Wave overtopping risk is further increased by a
vertical wall. We’ll need to raise our shorelines to allow for sea level rise. In Trail Bay at the District of Sechelt, where wave overtopping has been a concern, the District has been weighing its options. In looking at the solutions a real concern we have with
the sea wall is the height it would have to be and the impact that a sea wall would have
on the adjacent properties. It may have to be as high as two meters from the current
elevation that we’re standing at which would in effect block views, both pedestrians along the esplanade here
and from the adjacent properties such as the restaurant beside us and the homes further
along. The alternative is doing a green shore approach which would be a soft structural
approach which would only end up being about 18 inches higher than the existing grade. Using sand and gravel to raise or construct a beach is often called beach nourishment, and can be
associated with green shores. It’s a very common application in resort locations, often
paid for by tourism revenues in places like Florida and the Gulf states. It works, but
requires careful engineering. We built this shoreline nine years ago to protect an eroding shoreline, an eroding bluff that had cultural features in it. One of the things we wanted
to do to was try and maintain access to the beach, to maintain the beach because obviously
it’s quite well used. And we wanted to provide something that would look relatively natural.
Designing a constructed beach or a constructed shoreline requires you to understand or to
know about what’s happening with the waves, what the water levels are, and the types of
materials, the sizes of materials that you need to use. Here in this example we have
about a one inch minor gravel-sand material that is consistent with what was here before
we actually put this structure in, and we used what was here before as our design analog.
And that really is the best way to approach coming up with a size for this material is
to go to a shore line, take some samples, measure the sizes of material and use that
as a guide for the kinds of materials you need to use because the material that you
find on a shoreline will already have been exposed to the waves and water level that occur at
that side. A prospective design for one of the constructed beaches is how long you want it to last. It is a constructed
feature that you need to maintain so it has a design life. Typically you want to be looking
at somewhere in the 10-20 years if possible. You can go for a shorter design life and place
less material. You can go for a longer design life and place more material. You can go for
a shorter design life and place finer material. You can go for a longer design life and place
coarser material. Those are all aspects that can be considered when coming up with this
sort of design. The City of Vancouver installed beach nourishment
in the late 1980s. Now, 30 years later, there are signs of erosion. Perhaps it’s time to investigate
beach nourishment again as a maintenance measure. Design of any constructed shoreline – raised
beach, dike or floodwall, requires site-specific information and analysis. Information is needed
on the history of storms, as well as long-term records of the winds, waves and water levels
that affect the site. We need to know the nearshore bathymetry – the depth of water
and shape of the offshore area. We also need detailed information on the shoreline topography,
both before and after the proposed development or shoreline improvement. We must understand
habitat issues and the potential to incorporate habitat into our solutions. As well as understanding
historic tides and storm surge, we need to use an allowance for sea level rise for the
life cycle of the development that includes consideration of relative uplift or subsidence
of the land. Coastal engineers and geomorphologists use
this information with a number of tools such as computer models, to simulate the way shorelines
interact with local wave conditions, currents, water levels, and sediment movements. With
good quality data and accurate models, we can calculate the amount of wave run-up, splash
and overtopping, or the beach erosion and accretion, and how these might increase with
sea level rise. We can compare how several different shoreline alternatives will perform
under different sea level rise scenarios to create adaptation pathways or maps that offer
options for managing an uncertain future. This type of analysis is needed whether the
option is a raised beach, a wetland habitat, a floodwall, a dike, or combinations of these. On the land side of a dike, its common to see drainage channels which collect rainwater
from inside the dike, and bring it to pump stations or tidal gates where the water escapes
to the sea during lower tides. Behind the dykes in communities there can be issues with rainfall, how to get rid of the water. With groundwater coming up higher. As the ocean levels go up higher you will see your groundwater in the areas behind your dyke coming up higher. With ground subsidence – if you have soft soils you will see the houses and everything slowly sinking. All those have to be managed by the communities. Flooding of New Orleans during
Hurricane Katrina involved failure of a dike and floodwall due to poor foundation conditions.
As dikes are upgraded, we need to stabilize the ground under them, allowing for both increased
flood levels, the added mass of the dike and the seismic risk of earthquake. Adding material to the dike for repair during a storm is a common practice. For this purpose, the crest
of the dike includes maintenance access for inspection and service vehicles. The service
road also acts as a popular recreation trail on many dikes, providing multiple public benefits.
However, unless the trail is raised to compensate for sea level rise, the pedestrians on it will become
increasingly at risk to wave overtopping during severe storms. All these factors, plus sea
level rise, lead to a future height for the dike crest that is substantially higher than
existing. The increased height has a dramatic impact on the dike footprint. Where should the dike expansion go? Pushed to the upland side, requiring private property acquisition and
impacts on drainage? Or to the ocean side, creating significant environmental impacts on habitat.
A 2012 report predicted the range of cost at $9.4B for bringing existing flood protection
infrastructure up to current standards in the BC Lower Mainland, including new dike
heights to protect against sea level rise up until Year 2100. As well as the structural
and seismic costs, the report illustrates the urgency to reserve shoreline property
now to provide space to adapt to future climate change. The costs are not just financial.
Dikes are almost always fronting sensitive aquatic habitat. Without a dike, wetlands
would move gradually up-slope in response to sea level rise. With the hard line of a
dike, coastal wetlands get squeezed between rising seas and fixed shorelines. To help
shoreline wetlands adapt, can we move dikes back in some locations, or gradually trap
sediment to raise the marshes in tandem with gradual sea level rise? Careful site-specific
design is needed for all types of raised shorelines – dikes, floodwalls, or beach nourishment.
Risk of failure and cost of maintenance can both be reduced by close attention to design
calculations and the availability of good quality site-specific and long-term data.
Design for raised shorelines is most effective and efficient at the scale of a waterfront
bay or neighbourhood, with full integration of habitat and land use considerations. Raised
dikes will be a part of adapting to climate change where existing dikes or high value
existing development offer no other feasible choice for protection for protection against
rising seas. However, governing agencies are likely to be very cautious of new dike installations
in undeveloped floodplain areas, due to the major sustained investment required to adapt
to centuries of sea level rise.