It is no surprise that the world’s coastlines are often associated with what is considered the most valuable in terms of property assets for cities and communities. However in many cases, coastal areas are also the most vulnerable when it comes to the impacts of climate change. The ever increasing threat of inundation by sea, changing storm tides, flooding, erosion and sand drift represent significant risk to human life, property and surrounding communities in coastal areas. In some cases, human activities such as hard engineering and dredging are also playing a role in coastal degradation. This means that effective coastal planning, preparedness to respond and recovery capacity are more essential than ever before.
As the population across the world continues to grow, the pressure on coastal areas for land use and development is also increasing. The ability to simply move development landward and ‘away from the edge’ is often no longer an option.
Coasts are important for natural ecosystems as well, and their processes vary every day. Coastal areas are home to a plethora of marine plants, animals and insects, so planning that addresses these hazards, not only needs to prioritise the safety of surrounding communities, but also protect and integrate with the natural environment.
Ultimately, we need to work towards safe and sustainable coastal zones in the long-term. But here-in lies the challenge; there can be an overwhelming number of decisions and actions for authorities to make in relation to climate change and coastal planning. As such, a mix of long, medium and short term planning can help address the uncertainties around the magnitude and extent of future climate change impacts.
Proactive versus reactive approaches
A proactive approach to coastal planning is based around a shared understanding of what can be expected in the future and with this in mind, proactively taking steps to be as resilient as possible.
A reactive approach on the other hand typically involves ad-hoc emergency actions which often require significant funding for a long period of time. For example, we’ve seen in some parts of the world in areas prone to hurricanes, that after the loss of several properties, the properties are re-built on slightly higher ground, hoping that would be enough. But in some cases subsequent hurricanes have again caused devastation, and this devastation-rebuild cycle has occurred more than 20 times.
Before and after photos of a living shoreline approach implemented
Living shorelines
The concept of living shorelines is gaining more popularity as coastal management moves away from traditional single use hard shoreline methods, such as building large concrete walls. Living shorelines use natural elements such as native vegetation or sand to stabilise coastlines, bays and tributaries. They promote growth of plants, provide nutrient pollution remediation, carbon sequestration and also increase aesthetics, and can often be more cost effective. There are some examples that demonstrate living shorelines can be more resilient than traditional methods, and as such, in many countries, legislation is starting to emerge which supports these approaches. However, in some cases a combination of living and hard shorelines is required where the area may be too narrow or water too deep for a natural solution.
The following case studies illustrate the need for tailored approaches to coastal planning.
Case studies
Cardiff Living Shoreline, San Diego, California, US
Situation:
In California it’s become clear that beaches are eroding and homes and businesses are flooding more often – in some cases more than once a month. Local governments frequently resort to traditional interventions that are costly and short term, over and over again.
On Cardiff State Beach, the iconic Pacific Highway 101 was repeatedly being damaged, undermined and flooded during various storms and flood events. And the beach – a draw card for residents and local tourism, was eroding.
In an attempt to stop this, a group of stakeholders in San Diego County including the City of Encinitas sought a more proactive, long term approach. GHD was engaged to provide construction management and long term monitoring and reporting services.
Solution:
The project resulted in a living shoreline at Cardiff, and this is serving as a pilot to assess the feasibility for further shoreline protection projects across California. The living shoreline is designed to protect and restore natural shoreline ecosystems while also providing coastline resilience and community access benefits.
As the first line of defence against waves, native cobble was reused and placed at the toe of a created dune. As second line of defence 30,000 cubic yards of sand was introduced and placed as hummocks to create a more natural and non-engineered look. Sand fences on a 45 degree angle predominant to wind direction were also used to help capture sand in the fore dune before it goes blowing into the roadway. And closer to the road, as the last line of defence is rock riprap, buried by at least two feet of sand, which ensures that habitat can still establish and grow.
Another key element fuelled by community engagement was the addition of a brand new pedestrian trail that connected the communities to the north and south, something that didn’t exist before.
Result:
The city of Encinitas now has a more resilient coastline that is no longer flooding onto the highway. Further, the plants that have been growing over the first year have created new habitat for endangered species on the coast including the Snowy Plover.
GHD is now leading a multi-disciplinary group of scientists (including the University of California, Los Angeles (UCLA) and Scripps Institution of Oceanography) under a five year monitoring plan of the living shoreline. The first year report in April 2020 revealed that no sand movement or erosion had occurred and no project maintenance was required.
Determining joint probability of rain and surge, Christchurch, New Zealand
Situation:
Christchurch City Council (CCC) manages a large area vulnerable to coastal and river events. Due to the relatively low lying area, both natural phenomena can have a direct and significant impact on the residents and economy of Christchurch. With uncertainty about present and future flood levels, CCC engaged GHD to provide more insight on extreme river and sea levels, local water level changes and vertical land movements caused by tectonic activity.
GHD’s local presence in Christchurch helped contribute knowledge on local flood modelling. And through GHD’s international reach, the project team was able to engage a Dutch consultancy to combine knowledge regarding probabilistic design and physical behaviour of tidal and fluvial systems.
Solution:
The first step in this project was to gather and analyse all relevant data and knowledge regarding river levels, sea levels, barometric pressures, rainfall events and vertical land movements. As a second step a model of various tidal harmonics was established and calibrated against the measurements. This filtered out the influence of the tide and focused data on the residual surge that impacted the water levels around Christchurch. Analysing the surge and rainfall in relation to barometric pressures provided insight on the joint probability of extreme water levels during certain events.
An extreme value analysis extrapolated decades of data into certain return periods, which could be input into designs and risk analysis for the client. Finally the combination of vertical land movements and a time averaging approach resulted in more local predictions of sea level changes. The collaboration with the client and research institutes greatly helped interpretation of the data and the impact of earthquakes and other events on the measurements.
Result:
Within a short timeframe GHD was able to provide CCC with novel data and results, improving the Council’s confidence in the level of safety associated with current policies. It also provides the Council with the tools to take the next step in risk-based asset management and safety management. More specifically, the client now has a better understanding of the natural elements influencing water levels in its rivers as a basis to update its multi-hazard risk analysis. This in turn helps the Council to efficiently decide on where to invest money for flood protection and further modelling of multi-risk scenarios.
It’s clear that many of our coastlines are already being inundated by the impact of climate change. Responding to this means understanding that the decisions made today are pivotal to future outcomes. As such, ad-hoc, reactive approaches are inefficient and ultimately ineffective. To keep our communities and environments safe and increase coastal resiliency, we need to be prepared to embrace new thinking and approaches.
About the Author
Meet Brian
Brian Leslie
Senior Coastal Scientist
San Diego, California
T: +1 858 244-6977
Email Brian Leslie
Brian has managed a variety of projects that involve shoreline protection, dredging, beach nourishment, wetland restoration and resilience to coastal hazards. Recently he has been assisting coastal communities to identify and plan for risks associated with existing and projected future hazards through the preparation of coastal vulnerability studies and adaptation plans in compliance with state guidance. Brian is experienced in the planning and design of shoreline protection projects that aim to solve complex erosion and flooding issues.
Meet Robbert
Robbert-Jan Lenselink
Senior Project Manager and Engineer (Water, Maritime & Coastal)
Christchurch, New Zealand
T: +64 3 378 0951
Email Robbert-Jan Lenselink
Robbert is a design and project manager with extensive experience in integrated approaches to projects regarding sea level rise and flood protection at various locations along coastlines all over the world. As a result of his extensive experience with construction he is able to design practical/buildable solutions which reduces risks and variations for clients. Robbert’s open personality enables him to relate to stakeholders, clients regardless their positions, cultural- or social background and knowledge of the subject.