1Large rivers often present a river-lake-delta system, with a wide range of temporal and spatial scales 2 of the flow due to the combined effects of human activities and various natural factors, e.g. river 3 discharge, tides, climatic variability, droughts, floods. Numerical models that allow for simulating 4 the flow in these river-lake-delta systems are essential to study them and predict their evolution 5 under the impact of various forcings. This is because they provide information that cannot be easily 6 measured with sufficient temporal and spatial detail. In this study, we combine one-dimensional 7 sectional-averaged (1D) and two-dimensional depth-averaged (2D) models, in the framework of the 8 finite element model SLIM, to simulate the flow in the Mahakam river-lake-delta system 9 (Indonesia). The 1D model representing the Mahakam River and four tributaries is coupled to the 10 2D unstructured mesh model implemented on the Mahakam Delta, the adjacent Makassar Strait, 11 and three lakes in the central part of the river catchment. Using observations of water elevation at 12 five stations, the bottom friction for river and tributaries, lakes, delta, and adjacent coastal zone is 13 calibrated. Next, the model is validated using another period of observations of water elevation, 14 flow velocity, and water discharge at various stations. Several criteria are implemented to assess the 15 quality of the simulations, and a good agreement between simulations and observations is achieved 16 in both calibration and validation stages. Different aspects of the flow, i.e. the division of water at 17 two bifurcations in the delta, the effects of the lakes on the flow in the lower part of the system, the 18 area of tidal propagation, are also quantified and discussed. 19 Keywords 20 Mahakam River, coupled 1D / 2D model, SLIM, river-lake-delta system 21 3 of 45 1 Introduction 22Large rivers such as the Mekong River (Southeast Asia) hosting a river-lake-delta system consist of 23 various interconnected regions such as a river and its tributaries, lakes, floodplains, delta or estuary, 24 and adjacent coastal ocean. In such river-lake-delta systems, continuous interactions and exchange 25 of water between interconnected regions exist, under the combined effects of riverine and marine 26 forcings (e.g. river discharge, tides), mutual influences of natural processes (e.g. climatic 27 variability, droughts, floods), and human activities [1,2]. As a results, a wide range of temporal and 28 spatial scales of motion can be observed [2]. Such systems also feature complex geometries, 29 especially in deltaic or estuarine regions [2,3]. Therefore, a global system approach that is able to 30 handle the flow in the whole river-lake-delta system is required, to understand the complex flow 31 processes occurring at different temporal and spatial scales and to study related issues, e.g. transport 32 processes of sediment, morphology, ecological status of coastal waters. 33 Detailed and long-term field measurements (e.g. flow velocity, flow...

Mahakam land-sea continuum, fine-grained sediment, finite element model, coupled

Water renewal timescales, namely age, residence time, and exposure time, which are defined in accordance with the Constituent-oriented Age and Residence time Theory (CART), are computed by means of the unstructured-mesh, finite element model Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM) in the Mahakam Delta (Borneo Island, Indonesia). Two renewing water types, i.e., water from the upstream boundary of the delta and water from both the upstream and the downstream boundaries, are considered, and their age is calculated as the time elapsed since entering the delta. The residence time of the water originally in the domain (i.e., the time needed to hit an open boundary for the first time) and the exposure time (i.e., the total time spent in the domain of interest) are then computed. Simulations are performed for both low and high flow conditions, revealing that (i) age, residence time, and exposure time are clearly related to the river volumetric flow rate, and (ii) those timescales are of the order of one spring-neap tidal cycle. In the main deltaic channels, the variation of the diagnostic timescales caused by the tide is about 35% of their averaged value. The age of renewing water from the upstream boundary of the delta monotonically increases from the river mouth to the delta front, while the age of renewing water from both the upstream and the downstream boundaries monotonically increases from the river mouth and the delta front to the middle delta. Variations of the residence and the exposure times coincide with the changes of the flow velocity, and these timescales are more sensitive to the change of flow dynamics than the age. The return coefficient, which measures the propensity of water to re-enter the domain of interest after leaving it for the first time, is of about 0.3 in the middle region of the delta.

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