<p>Making a positive impact on the planet motivates us to take on and solve global water challenges. Through our projects, we support the UN Sustainable Development Goals together with clients and partners from all over the world. From developing offshore wind farms, to safeguarding cities from flooding, protecting shorelines from erosion and enriching biodiversity in water basins. </p>

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Nature-based solutions supporting climate resilience in the Shabelle River basin

Exploring how nature-based solutions can provide effective flood and drought mitigation for Somalia 
 
Over the past decades, Somalia has been experiencing increasingly frequent and severe floods. With environmental degradation being considered one of the main contributors to this change, nature-based solutions (NbS) have a high potential for mitigating flood risk in the area. Using hydrologic and hydraulic mathematical modelling to analyse options such as reforestation or sand dams, UNEP-DHI Centre on Water and Environment assessed this potential within the Shabelle River basin and found that a variety of NbS are applicable in Somalia. These findings will support the National Flood and Drought Task Force of Somalia in planning future flood and drought interventions in the basin. 
 
Challenge
Many ephemeral rivers, or ‘wadis’, in Somalia are prone to flash floods. Flooding occurs when heavy rains lead to a sudden increase in river flow, followed by a quick recession, and can cause loss of lives and property in downstream settlements. The severity and frequency of flooding from the Shabelle Rivers has been increasing in the past few decades, and it is thought that increased sediment deposition in rivers (brought about by activities such as deforestation which degrade soil structure), and subsequent raising of the riverbed, is the major contributor to this change. NbS, aimed at restoring degraded riverine ecosystems, have the potential to mitigate flooding in the area. With some of these solutions simultaneously enhancing groundwater recharge, their implementation could serve the dual purpose of flood and drought management, increasing community resilience to climate extremes. 
 
Solution
In the implementation of the project, UNEP undertook research into the potential of nature-based solutions for mitigating flood and drought hazards. The UNEP-DHI Centre team provided the technical inputs necessary to support this research, which can assist Somali decision-makers in planning and scaling up nature-based approaches to flood mitigation.
 
UNEP-DHI’s study identified flood-targeting NbS already tested in the Somali context and assessed their effectiveness using hydrological and hydraulic models developed with MIKE HYDRO River (now known as MIKE+ Rivers). For flash flood mitigation in ephemeral wadis, peak flow in response to heavy rainfall was compared between a baseline scenario and scenarios employing reforestation, terracing or water retention structures such as sand dams and weirs. For sediment-related riverine floods, a hydraulic river model with sediment transport components was used to assess the effectiveness of different NbS in decreasing the mass of sediment entering the river and in decreasing incidence of flooding. 

Navigating water resources challenges in the Senegal River Basin

Integrated hydrological and hydraulic studies enhance water management and resilience to climate change&nbsp;
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The Senegal River basin, located in West Africa, spans four countries and covers 1.6% of the continent. The river basin faces significant challenges due to uncontrolled tributaries causing droughts, floods and erosion, directly impacting the lives of local communities and the health of the environment. Additionally, existing water management systems, designed using historical climate data, may not adequately address the increasing frequency and severity of climate events. By utilising DHI’s integrated hydrological and hydraulic modelling, the Organisation for the Development of the Senegal River (OMVS) has adopted a modern approach to water resource management. This enables more accurate understanding of floods and droughts, helping local communities adapt to future uncertainties and safeguard their livelihoods.
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Challenge
Uncontrolled tributaries are streams or rivers flowing into a main river without regulation or management. Influenced by natural factors like rainfall and topography, their flow can fluctuate due to weather events. Human activities such as deforestation and agriculture can further impact their flow and health. This can lead to challenges like flooding, erosion and water quality issues, making effective monitoring and management essential for the river system's overall health.
In the Senegal River basin, these challenges affect both residents and the environment. For local communities, these issues have led to water scarcity, affecting agriculture and livelihoods while limiting access to clean water, exacerbating food insecurity and poverty. Environmentally, these challenges result in habitat degradation, loss of biodiversity and decreased soil fertility, highlighting the urgent need for integrated water resources management approaches to address the impacts on both human and ecological communities.&nbsp;
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Solution
For nearly two decades, DHI and OMVS have built a strong partnership that has resulted in an enhanced understanding of the hydrological dynamics in the Senegal River Basin.&nbsp;
For this project, DHI conducted hydrological and hydraulic studies of the Senegal River's uncontrolled tributaries using DHI’s Global Hydrological Model (GHM), focusing on evaluating water resources, understanding the dynamics of water flow and erosion phenomena, and identifying interventions for sustainable management. Using DHI’s GHM, runoff has been computed for the entire river basin to assess the water availability in key uncontrolled tributaries.
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The solution involved gathering data on geographic, climatic and hydrometric conditions, creating thematic maps to illustrate the basin's characteristics and using hydrological modelling to simulate water movement and distribution. The aim was to establish a foundation for informed decision-making that addresses both the immediate and long-term needs of the basin's ecosystems and communities while considering the impacts of climate change and human activities.
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Results
The solution has established a basin-wide hydrological and hydraulic model for the Senegal River's basin. OMVS can now improve strategic planning and better anticipate floods and droughts, enabling targeted prioritisation of interventions in the uncontrolled tributaries of the basin. The enhanced water management aims to improve water availability, reduce flood and erosion risks, and increase agricultural productivity to the benefit of the quality of life of the local community. Additionally, this approach has facilitated the stabilisation and restoration of natural habitats, benefitting aquatic ecosystems.

 Supporting sand management along Adelaide’s shoreline

Applying shoreline morphology modelling supported by 40+ years of coastal monitoring to evaluate management options
 
Since the 1970s, the Government of South Australia’s Department for Environment and Water (DEW) has actively managed Adelaide’s shoreline through renourishment efforts. Despite recent large annual backpass campaigns, the shoreline between Adelaide Shores boat harbour and the West Beach Surf Life Saving Club (SLSC) continues to experience localised erosion. To address this issue, DEW has commissioned DHI to conduct a coastal processes modelling study. This study aims to enhance understanding of the coastal dynamics at West Beach and to explore alternative long-term management strategies.
 
Challenge
The Adelaide Metropolitan coastline comprises a discrete littoral sediment cell that extends along 30 km of the semi-protected eastern shoreline of Gulf St Vincent in South Australia. Sediment within the littoral cell is moved in a net northward direction with no significant supply of sediment provided from the south or offshore to replenish the beaches. The chronic sediment supply deficit combined with the impacts of historical coastal developments pose an ongoing challenge to the management of Adelaide’s Metropolitan coastline.
 
Solution
Providing models to assess management options
 
A comprehensive assessment of the coastal processes and observed erosion in the West Beach sediment cell was undertaken. This evaluation combines analysis of existing data with a state-of-the-art numerical modelling framework to predict long-term shoreline evolution.
 
Driven by MIKE Powered by DHI software, we developed hydrodynamic, wave and shoreline morphology models and calibrated them based on available measurements. In total, six management scenarios comprising of combinations of soft management options (nourishment, backpass systems) and hard structures have been simulated. For each scenario, the morphological evolution of the entire West Beach shoreline has been simulated for 7.5 years providing comparative assessments of the pros and cons of each scenario.
 
Results
The solution provided the Government of South Australia with:

An updated description of coastal processesUtilising historic data and advanced modeling techniques, the solution provided a comprehensive understanding of coastal processes in the region, facilitating more accurate assessments of coastal behaviour over time.
Comparison of management options and associated morphological impactsThe solution offered valuable insights into how different management approaches might affect coastal morphology, enabling more informed decisions about which strategies are likely to be most effective in mitigating coastal erosion and other challenges.
Data to support future decision-making for coastal managementReliable data is now available to support ongoing and future decision-making processes related to coastal management.


River mouth training in the Bay of Plenty

Designing twin training walls for the dynamic Opotiki Harbour Entrance
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Opotiki is a township with a strong vision of increasing the social and economic well-being of its community. Researched initiatives have shown that Opotiki is perfectly positioned for a large-scale aquaculture venture—a success that can only be brought alive through a transformation of the harbour entrance. Once a well-used trading route, local authorities are looking to make the harbour easier to navigate by constructing two parallel river training walls at the entrance. To help Opotiki District Council (ODC) achieve this, DHI provided design expertise and also assessed many significant factors which could impact the components of the ambitious project.
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Challenge
The coastal township of Opotiki sits on New Zealand’s eastern coast, in the North Island’s Bay of Plenty. Prior to the mid 1960s, the Opotiki harbour entrance had a long history of use by coastal shipping vessels which visited the township primarily as a trading port. Since then, the ability to navigate through the entrance has been adversely affected as waves routinely transporting sediments into the entrance formed an unreliable and often unsafe passage between the river and the sea. The confluence of the Waioeka and Otara rivers, approximately 1.5 kilometres above the harbour entrance, adds further complexity to the dynamic nature of the unique coastal environment.
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The most serious incident to date was when the entrance was closed for 64 days in 2007. Regular users of the route found it challenging and dangerous to navigate the passage five days out of 30 in a month on average. The ODC rallied central and local government, iwi and community groups, and commissioned a series of investigations to improve access to the harbour. We were engaged to find a solution by providing a conceptual design for the river training works while assessing environmental impacts at the same time. After considering many alternatives, our findings concluded that twin training walls at the entrance would provide the most appropriate stabilisation. One of the most important elements involved determining whether the large scale infrastructure project would adversely affect flood levels on the township.
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Solution
Different training wall configurations were assessed for a range of river flows and coastal conditions. DHI studied many aspects of the walls, including their ability to maintain a&nbsp;navigable entrance, the expected navigation conditions and possible scour in their vicinity. Our team also assessed the impacts on salinity within the harbour, sedimentation elements and backwater effects. On mitigating flood impacts, our experts recommended a confluence realignment as a feasible solution.
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DHI's solution was to construct two short straight breakwater training walls with a new entrance to the east of the existing one. The walls, approximately 500 metres long and 120&nbsp;metres apart, will also have scour protection elements included, without which the walls could be undermined during a large flood when deep scour may occur in between.&nbsp;
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With the application of one-dimensional (1D) and two dimensional (2D) models utilising MIKE Powered by DHI software, our team successfully developed a robust solution for ODC.
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Results
Ensuring the safe navigation of the harbour entrance while avoiding upstream flooding was principal within the project, as was understanding the many various coastal morphological impacts at play.
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From our modelling, we established that a re-engineered harbour entrance could alleviate the vessel access issues, allowing for more stable and improved entrance conditions. Although long term morphological impacts for flooding mitigation required further investigation, a realignment channel would somewhat reduce flooding impacts. The project, when undertaken, will greatly open up opportunities for the coastal settlement of Opotiki, creating potential aquaculture ventures and raising the wellbeing of the local community on manylevels.

Protecting coastal communities around San Francisco Bay

Using 2D modelling to assess flood risks and improve San Francisco Bay’s floodplain maps
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Flooding can significantly impact coastal homes and businesses. However, information on flood-prone areas around San Francisco Bay was outdated or lacking. The United States Federal Emergency Management Agency (FEMA) needed to assess flood risks for the entire Bay Area. DHI's coastal models provided reliable data for flood risk analysis, helping local governments improve their flood management and equipping local residents with the information needed to better protect themselves from floods.
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Challenge
Located on the West Coast of the United States, the shores of the San Francisco Bay are home to more than seven million people. With billions of dollars of infrastructure located adjacent to the hundreds of miles of shoreline, an accurate assessment of the flood risk is vital. Extreme water levels caused by storms and other factors can have serious implications for the people and businesses that call the Bay Area home.
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The Bay Area hasn't had a comprehensive study since the early 1980s. To better understand coastal flood hazards for the entire San Francisco Bay, FEMA requested a regional Flood Insurance Study (FIS) to assess flood risks in the area.
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Solution
Using MIKE 21, DHI developed a system of regional scale coastal models in order to predict the following for the entire San Francisco Bay:

Extreme tide (astronomical tide plus storm surge)
Swell entering from the Pacific Ocean
Sea waves generated from local winds in the Bay

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Normally, we would apply a highly detailed study to a focused study area. However, this project required the same level of detail for the entire 250 mile-long Bay Area shoreline. To allow us to identify the storms in advance, we operated the models in hindcast mode to produce continuous results of water levels and waves covering 31 years in the North and Central Bay and&nbsp;54 years in the South Bay.&nbsp;
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Following this, we used the regional model results to establish maximum 1% (100-year) and 0.2% (500-year) Still Water Levels (SWLs) – the maximum water level that will occur – and wave height conditions. The DHI team then calibrated and validated the models using National Oceanic and Atmospheric Administration (NOAA) tide gage measurements from around the bay as well as various wave measurements from inside and outside the bay.
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Results
The results of the regional modelling study were key components for the development of FIS floodplain maps for the entire Bay Area. FEMA and its study contractors are using the regional model results as boundary conditions for further detailed coastal hazard analysis. This will also be the basis for most future studies. The detailed coastal hazard analysis could have far reaching implications for the businesses and people who live and work around the Bay. The results could lead to a modification in flood risk designations for coastal communities, which could in turn lead to changes in flood insurance rates, land-use planning around the Bay and floodplain management practices.
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By having a clearer idea of where flooding is most likely to occur, communities surrounding the San Francisco Bay can take the appropriate measures to ensure their safety.

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