In July this year I found myself enjoying some lovely music at an international seminar (HATHI) attended by over 400 hydraulic engineers in Bali. To my surprise the band was composed of hydraulic engineers. But there was something else that was like music to my ears. ‘Building with Nature’ featured on the program as an innovative approach for coastal defense, with nature based solutions and inclusiveness at its heart. I am personally convinced that the Building with Nature model will contribute to ‘water resilience in a changing world’, the theme of the seminar. But how did the 400 other engineers at the conference perceive this approach, which is a real shift away from business as usual?
The Ministry of Public Works and Housing of Indonesia, co-organiser of the conference is already a partner in a large-scale Building with Nature pilot project in Central Java where millions of people are threatened as result of coastal erosion, in particular in the province of Demak.
Sea walls expensive and ineffective in muddy mangrove coasts
Until now sea walls have been placed in front of coasts to halt the erosion, said professor Han Winterwerp from Deltares, one of the project partners in Demak. But he explained that seawalls are not very suitable to stop erosion in muddy mangrove coasts. They are often expensive for this scale and also disturb the balance of incoming and outgoing sediment in muddy coasts, which may cause further erosion. Furthermore, they do not address the cause of the problem, mangrove conversion for aquaculture, infrastructure works and land subsidence caused by water extraction.
The forces of nature
To rehabilitate this environment, mangrove coast needs to be restored. Planting and replanting mangroves, however, fails on a large scale along eroding coastlines, as trees cannot thrive in the deeper waters and seedlings are readily swept away by the forces of the sea. For successful restoration we need to restore the conditions for nature to restore itself, in this case for instance the sediment balance.
Han showed how in Demak we use permeable structures to dampen the waves and to allow coastal sediment to filter through and settle in the quiet waters behind the structures. When the first zone is fully sedimented a new structure is built in front of it. Over the years this follows the pace of nature and it is repeated towards the original coastline as long as sediment is available. When the seabed is high enough the mangroves can recolonize naturally and after a few years take over the coastal protection function of the structures. The construction is done by the local community and they do not need machinery. The structure consists of bamboo poles and brushwood fill. It is tied together with thick rope.
Initially mostly technical questions started boiling up in the audience: how do you determine where to build the structures? How do you decide on the dimension of the dams? Han explained that there are a number of main criteria to be used for the design and locations, but that the exact methods are also highly site-specific. For each site an expert team needs to first develop a good understanding of the specific ecological and hydrological system.
Transforming the economy
I was pleased that the audience also asked how to prevent re-conversion of the restored mangrove belt. This is a key aspect to any sustainable Building with Nature solution. It requires transforming the economy that caused the collapse of mangroves and soil subsidence in the first place. In Demak we revitalize aquaculture, which used to be the main economic motor in the region. We do this in such a way that there is both space for mangroves and for ponds, and with techniques that use less chemicals, increase yields, and prevent soil subsidence and hydrological disturbance. Part of the extra income will finance coastal safety measures.
Orchestra of disciplines
Clearly Building with Nature cannot be done by engineers alone. The band needs to be composed of several sectors and specialists in order to orchestrate and fine-tune to site specific circumstances. This is a real shift from single-sided traditional mitigating approaches such as hard structures. But it is effective, feasible, affordable and flexible with benefits for both people and nature. Building with Nature may transform the way in which we can address erosion problems along thousands of mud-coasts across the tropics. The project team will continue to share lessons from the project in Demak and in the coming years provide training and implementation guidance to stimulate sound replication of the approach in other sites with similar problems. Let the music play!
Header image © Stefan Verschure