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Problem Solving with InfoWorks RS 3: Analyzing Saline Intrusion

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Courtesy of Innovyze

This is the third in a series of articles on InfoWorks RS, Wallingford Software’s integrated modeling solution for river systems. The series explains how InfoWorks RS can address specific problems that face the authorities around the world that are responsible for managing rivers. 

The capabilities of InfoWorks RS have been greatly expanded since its original development as a flood modeling tool. It can now also be used for exploring many other key river management environmental issues such as sediment transport and water quality. There is no need to build a new model of the river and its catchment area to understand these issues as they use the same InfoWorks RS flow models that have been set up for the flood analysis.

The problem

Many parts of the world are experiencing increased developments and engineering works alongside their rivers. There is an impact on the catchment’s water flows when changes are made such as additional abstraction or alterations to the regime’s water storage through construction of new reservoirs and flood defense schemes.

Such developments can lead to changes the tidal zone area where salt and fresh water mixes. Changing the flow regime may lead to salt water intruding further into the river. Saline intrusion into rivers is an increasing problem in many parts of the world. Parts of the river that were previously outside the zone’s limit may face a tidal inflow of salt water extending further inland.

This has a huge impact in places that use the river water for irrigation or rely on it for water supply. The effect is not only in the immediate vicinity. Towns and cities in the tidal zone at the mouths of rivers may take their water supply intake from further upstream above the existing saline zone. Changes that move the limit of saline water may therefore compromise their intake. Some rivers now have virtually stretches downstream of barrages that have been built without any environmental controls.

A full understanding of the effects of any proposed changes to the regime is vital to assess proposals, maintain water quality and ensure compliance with legislation. However, traditional desk-study approaches were highly subjective and incomplete.

The solution

The water quality engine in InfoWorks RS provides a rapid and accurate method for modeling the effects of any proposed changes, and to analyze the flow regime and investigate whether there will be any adverse effects.

Setting up the model is straightforward. InfoWorks RS contains a choice of data import methods to ensure that it is easy to bring in the necessary data such as cross sections of the river and estuary together with roughness details. Details of structures, barriers and bridges can also be brought in automatically, minimizing the set-up time. 

The next step is to set up the boundary conditions of the model, which will typically involve entering a whole seasonal cycle covering both flood periods and low flows. The downstream boundary condition is provided by a tidal hydrograph which can be generated by InfoWorks RS using an in-built tide curve generator. This produces location-specific information from tidal harmonic constants, enabling modeling to be carried out even in the absence of tide gauge data. If however there is any observed data, calibration can be carried out to ensure that the model predictions match the current situation. This could involve changing the roughness, or adding additional cross-sections.

The baseline conditions are established by running the model to simulate a 10 or 20 year period. Any proposed changes can then be made, such as reducing the amount of upstream flow by abstraction. Re-running the simulation with then show the new salt concentrations and extent of the tidal zone. The model is quick to run, enabling the exploration of several different options for the proposed works.  

Comparison with the base data will highlight the impact of the proposed works by showing whether the salt water would intrude further inland. InfoWorks RS presents the output using a choice of graphs, tables and schematic mapping to aid understanding of the effects of the proposals. The results can also be saved in a presentation-quality format for inclusion in reports and discussions with clients and other stakeholders.  

Summary

InfoWorks RS offers objectivity and certainty in analyzing the extent of saline ingress along a river. It can test the effect of proposed developments to see whether they would adversely affect the salinity at key points along the river. Setting up the analysis is quick and straightforward, as it uses the river’s existing flood model in conjunction with tide curve generators that are built into InfoWorks RS.

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