Modeling saltwater intrusion in Incomati River as a contribution to the determination of Dynamic Environmental Flow




WIOMSA 12th symposium, poster presentation, October 10th, 2-3pm


THEME VII Estuaries and their resources



School visit, Incomati Delta, Mozambique @Dominique Vinckenbosch

Authors

Clemêncio Nhantumbo (Eduardo Mondlane Univ., Faculty of Engineering)

Dinis Juízo (Eduardo Mondlane Univ., Faculty of Engineering)

Vânia Saúl (Eduardo Mondlane Univ., Faculty of Engineering)

Nordino Paluluane (Eduardo Mondlane Univ., Faculty of Engineering)

Mary Shirima (Eduardo Mondlane Univ., Faculty of Engineering)

Abstract

The aim of this study is to analyse saline intrusion in the Estuary of Incomati River as a contribution to sustain interdisciplinary discussions on environmental flows requirements in this part of the system.

Complex estuarine ecological systems that mark transition from fresh water and the open marine system do not only provide goods and services for use within estuaries themselves but, significantly influence the nature and rate of biogeochemical processes within the biosphere. River estuary saltwater intrusion is a sign of modification of river basins conditions in particular, reduced freshwater reaching the river mouth. Climate variability and change is also likely to contribute to the change in saltwater intrusion patterns of estuary through prolonged drought conditions and seawater levels rise. Coastal habitats health is influenced by the delicate balance between fresh and saltwater, determining the ecological health status of ley ecological ecosystems such as mangrove. Understanding salt intrusion as a function of tidal and flow regime, and how salt intrusion affects the ecological system, including ecosystem productivity and services within the estuarine is a starting point for establishing environmental flow, not neglecting the relevance of social, political, and current economic interests.

Incomati river basin is a transboundary system of proximately 47000 Km2, shared between South Africa (62%), Swaziland (6%) and Mozambique (32%). The estuary of Incomati river is in Mozambique and receives an average flow of between 200-400 m3/s. The lower Incomati is characterized by Macaneta wetlands surrounded by dunes. The Macaneta wetlands attract a diversity of stakeholders that practice fisheries, agriculture (mainly rainfed around the edges of the dunes), free-range livestock keeping and the gathering of a variety of natural resources, such as, reeds, wood for energy and construction and wild fruits. The fisheries can be either marine, in the bay and open sea, along the estuarine and upriver, determined by the season characteristics and gear and crafts used by the participants. Some of these fishery resources are collected on foot (crabs, shrimp, gastropods) both for consumption and to be used as bait. It is estimated that around 20% of the 200 tonnes of shrimp fished in Maputo Bay derive from the Lower Incomati and healthy mangrove systems are a vital element in this productivity. Similarly, saltmarshes are important feeding areas for juvenile fish. Macaneta is a highly productive habitat attracting a diversity of bird species with different feed needs. More than 300 species of birds have been identified in the area making it one of the ornithological hotspots in the region contributing to increased touristic interest.

A channel on the Incomati Estuary @ Clemencio Nhantumbo

While the flow regime in the Incomati River is mostly driven by anthropogenic activities upstream (e.g. irrigated agriculture, forestation, dam operation) and much less climate variability and change, the required flow to sustain the estuarine ecosystem is not yet known. To determine environmental flow requirements a multidisciplinary study is being conducted in the Incomati Estuarine, that will contribute knowledge on the effect of changes of flow regime in the ecological system. Among the different topics being studied include salt intrusion analysis is a key component that will enable to stablish the link between habit status, river flow regime and tidal effects. A saltwater model, calibrate against observed conditions, is used to estimate the saltwater incursion along the river. These results will be used to feed a hydraulic and hydrodynamic model showing the wetland flooding condition of the wetland under different water quality situation.

From previous studies of the system, it was shown that, the length of saltwater incursion is controlled by river flow for river flow values exceeding 30 m3/s, while for smaller river flows the incursion length depend on tide. Equinox tides lead to longer incursion in the estuary reaching up to 60kms inland.

For the development of this study, (1) data on water quality was collected on the wetlands and interviews were conducted to selected households in Macaneta; and (2) salinity and water density measurements were performed along the river following both low and high tides, staring from the river mouth until salinities below 1 psu. The data collected was used to develop a salthwater intrusion model for estuaries. Data on salinity and water density was collected for development and calibration of the model. Six field campaigns were conducted (1) two on the wetlands of Macaneta and (4) four on salinity and density measurements along the river. The salinity measurement campaigns, include two for wet season and two for dry season for both low and high tidal conditions.

Boats on the Incomati estuary @ Clemencio Nhantumbo

It was observed that available ecosystem goods and services is influenced by salinity (or electrical conductivity) when compared to other measured parameters, that include pH, dissolved oxygen, nitrates, and phosphates in the Macaneta Wetlands. The measured saline intrusion for wet season, for high tide performed on May 27, 2021 (tide hight = 3.56m) was 24 km and for low tide, performed on May 31, 2021 (tide hight = 1 m) was 15 km. While for dry season the measured saline intrusion for high tide, performed on September 22, 2021 (tide hight = 3.47 m) was 36 km and for low tide performed on September 18, 2021 (tide hight = 0.84 m), was 30 km. Using the field data a saline intrusion model was developed and calibrated and the results will be used to test different scenarios the impact of different flow regimes in the estuary.

This study shows that a coupled hydraulic estuarine intrusion model with hydraulic and hydrodynamic model are useful tool to experiment and evaluate water development and used scenarios in river basis (Bandeira et al., 2014).