Research

Optimized deltaic diversion scenarios

Project overview

Hundreds of millions of people live on river deltas around the world, representing a rich diversity in culture and generating thriving economies. Governments have sought ever more drastic measures to prevent flooding and protect society and its infrastructure on deltas. But, these policies can harm the natural environment and lead to loss of precious land.

This project developed a novel analysis tool that seeks to protect millions of people living on urban river deltas, while preserving the environmental and commercial viability of these landscapes. We used the numerical model of Moodie et al., (2019) (research project page) and a novel cost-benefit formulation to determine the optimal location for deltaic channel diversions. We found that to maximize diversion effectiveness, structures need to be placed farther upstream than existing designs and structures, which tend to be near the coast.

This research has been supported by an NSF Graduate Research Fellowship from 2016–2019 and an international NSF Coastal SEES grant.


Figure 1: Yellow River delta shortly after an avulsion relocated the main channel from the north to the east. Image: Landsat, 1978.

Introduction

By restricting river channels on deltas, we have limited the delivery of sediment to the coast where it is needed to sustain land in the face of rising sea level. This is problematic, because delta lands lessen flooding by diminishing storm surge intensity. The irony here is that by preventing the river from flooding naturally, we have exacerbated land loss, and in the long run, have made society more susceptible to catastrophic floods, including from rivers and ocean storms.

Recently, diversions have been engineered on deltas globally, with the intention of building land (for example, Figure 1). However, diversions are often placed very near the coast, because it costs less and minimizes the impact on major population centers. But, these downstream locations are ineffective at maintaining the natural function of delta land building, which requires significant lobe-switching channel relocation to disperse sediment widely.

Methods

In the first component of this study, we used the numerical model of (Moodie et al., 2019) to simulate delta evolution following diversions. We varied diversion location with respect to the coast and the delta backwater region, to determine how system hydrodynamics influence the time until conditions requiring another avulsion/diversion are reached. We found that diversions located within the backwater region cause an upstream-migrating wave of bed degradation, which in turn increases the time until avulsion setup conditions are reached—this is a major boon for diversion planning.

Figure 2: Conceptual summary depicting three potential diversion scenarios. The natural avulsion location ($L_A$) is determined by the region of maximum sedimentation within the delta, near the upstream extent of the backwater length ($L_b$). Diversion (1) cost is low, but does not lower the channel bed at the natural avulsion location, nor increase the time until a natural avulsion, and so it provides very little societal benefit. In contrast, diversion (2) has a moderate cost, but creates an upstream-migrating scour wave that lowers the channel bed in the region of maximum sedimentation, significantly increasing the time to a natural avulsion, and so provides high societal benefit. Diversion (3) bypasses the region of maximum sedimentation entirely, and so it provides high societal benefit, but diversion costs are too high to be justifiable..

In the second component of this research, and a major contribution of this work, we developed a dimensionless framework that explores how location influences the cost and benefits of diversions on an idealized delta. From this framework, we derived a societal net benefit for natural landscape management (i.e., natural avulsion; $\lambda_{\Pi,\textrm{natural}}$) and for a managed delta (i.e., artificial diversion; $\lambda_{\Pi,\textrm{diversion}}$). Together, these benefit formulations give the sustainability number $\mathcal{S}$:

\[\mathcal{S} = \frac{\lambda_{\Pi,\textrm{diversion}}}{\lambda_{\Pi,\textrm{natural}}},\]

where a value greater than unity indicates a delta management strategy providing a net benefit to society. We explored how the sustainability number depends on various parameters and informs delta sustainability.

Insights

Many studies have said that society, in particular major cities like New Orleans, are doomed, and that engineering efforts leveraging nature-based solutions can’t coexist with large population centers. We’ve shown with our optimizable framework that having cities nearby actually makes it even easier to justify doing these large projects, because they protect communities. From the framework, we showed that an optimal location for diversions is approximately halfway between the delta apex and coast.

In sum, our work suggests that diversions need to be placed farther upstream than existing designs and structures.

Publications generated by this research

  1. Moodie, A. J., & Nittrouer, J. A. (2021). Optimized river diversion scenarios promote sustainability of urbanized deltas. Proceedings of the National Academy of Sciences, 118(27). doi: 10.1073/pnas.2101649118

References

  1. Moodie, A. J., Nittrouer, J. A., Ma, H., Carlson, B. N., Chadwick, A. J., Lamb, M. P., & Parker, G. (2019). Modeling Deltaic Lobe-Building Cycles and Channel Avulsions for the Yellow River Delta, China. Journal of Geophysical Research: Earth Surface, 124(11), 2438–2462. doi: https://doi.org/10.1029/2019JF005220

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