Investigating hydrodynamic within the Komo Estuary and surrounding mangrove forests.
The Gabon estuary is advantageously located under SWOT fast-sampling orbit, which will benefit from data at both high temporal and spatial resolution. The estuary is approximately 60 km long and 14km wide at the mouth where it connects to the Gulf of Guinea. The capital of Gabon, Libreville, is located along the northern shore of the estuary. The Komo and Ebe Rivers drain into the estuary with peak flows between November and December and March and May. The tidal range can reach up to 1.7m, producing strong tidal currents. The estuary contains the Pongara National Park, a protected area dominated by the tallest mangrove forests in the World. Within the park, several tributaries are impacted by the tide and are characterized by brackish water.
The hydrodynamic interactions between the fresh and salt water have yet to be studied. Salinity gradients control the structure of mangrove forests. To our knowledge, there is no currently available data on the water level along the Komo estuary. In 2016, only 3 gauges were installed for 1 year to cover the AFRISAR campaign to monitor water level changes that may impact how remote sensing instrument detect ground in mangrove forests. These data do not suffice to calibrate and validate hydrodynamic models. SWOT is expected to provide long time-series of water surface elevation measurements across the entire estuary during both the cal/val and the science phases. It is still unclear SWOT measurements will be able to resolve processes within the Komo tributaries which are smaller channels, sometimes without wind (potentially dark water), and bordered by mangrove trees reaching 60 meters in height.
Our larger goal is to understand how the hydrology controls the structure of mangrove forests and contribute to carbon export to oceans. To achieve this goal, we seek to calibrate and validate a hydrodynamic model using in situ and SWOT data. There is a need for in situ measurements of water surface elevation along the Komo Estuary and its tributaries. The in situ measurements will provide fast sampling—every 15 minutes— over a period of about 1 year. SWOT will provide daily measurements during its cal/val phase that can be used to spatially extrapolate in situ-measurements across the entire region, thus improving modeling of overflow into mangrove forests. The in situ data will also serve to calibrate and validate SWOT data in terms of its accuracy but also in terms of the impacts of layovers (due to mangrove tree shadow effects) and dark water (due to smooth water surface conditions resulting from lack of wind).
Doppler Current Profilers (ADCP, boat-mounted) to estimate discharge measurements and derive rating-curves with water levels.
There is no current monitoring of the area, however, the in situ gauges will serve three purposes: 1-calibrate and validate SWOT data; 2-provide water level measurements with fast sampling intervals of 15’ and; 3- validate our tide and river discharge retrieval algorithms. In order to calibrate numerical hydrodynamic models in estuarine regions— strongly impact by tides—the 15’ sampling is required. We designed a non-stationary harmonic analysis that can be used with long SWOT time-series (~3 years) to resolve tidal and discharge contribution. The in situ data and calibrated numerical model will serve to validate the SWOT retrieval.
Estimating fluxes of water across the land-ocean-sea-continuum (LOAC) is critical to evaluate the export of carbon and nutrient to the ocean, the deposition of sediment along the shore and to assess the vulnerability of wetlands and ecosystems in the face of sea level rise, increased storm activity and freshwater diversion projects. It is also critical to improve hydrodynamic models in these regions to support navigation, hazard contingency plans and rescue efforts. While gauging of estuaries and channels is commonly used to manage coastal wetlands, availability of these in situ measurements is in decline globally. But even the most extensive gauge networks cannot capture overflows to and within the wetlands. Our experiment will demonstrate the potential of SWOT as an alternative to expensive gauges networks to calibrate hydrodynamic models and retrieve seasonal freshwater discharge. These calibrate models and discharge estimates in estuaries and river deltas can be used to improve our understanding of the water and carbon cycles, thereby providing a powerful tool to monitor salinity gradients in these regions and their contribution to ocean carbon.
The Komo Estuary mission is funded by NASA ROSES.
Principal investigators: Marc Simard (Jet Propulsion Laboratory, California Institute of Technology), Jean Hervé MVE BEH (General Director of the Ministry of the Environment of Gabon), Nicaise RABENKOGO (CENAREST-IRSH-DREC).
Contact point for the study site: Marc Simard (marc.simard@jpl.nasa.gov)
