Delta flood and drainage

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This page is a core topic for River Deltas as developed by the River Delta Adaptive Management Strategy

Agriculture and settlements in deltas commmonly depend on a set of flood defenses, drain ditches, and tide gates to manage ground and surface water levels. Restoration can either enhance or undermine these systems, affecting the communities and economies working in delta agriculture. Social acceptance of restoration may depend on our ability to use restoration to predict and enhance flood and drainage functions on adjacent lands.

Two sub-topics are proposed for exploration. Being able to predict the effects of restoration on farm systems is a critical component of restoration feasibility. Second, being able to describe how resilient marsh helps enhance flood defences may prove useful for management of sea-facing dikes. Finally, using hydrodynamic modelling to describe the flood benefits of increasing storage and conveyance through levee setbacks may have value, not only in deltas, but also in Floodplains.

Sub-topics	Efforts

Summary

Our inability to accurately predict changes in flooding and drainage of agricultural land as a result of restoration actions prevents us from describing the benefits of restoration to adjacent landowners and community stakeholders.

Curent hydrologic models can incorporate sea level rise predictions, change in snowpack and precipitation, and storage to describe flood frequency and duration. There are also storm protection models to describe risks to people living in river delta environments (see Floods and Drainage Methods).

Current infrastructure was designed to handle historical hydrology, while radically decreasing habitat services. A future infrastructure could be designed to manage changing hydrology while increasing habitat services.

Flooding relief and improved drainage are two of the many socioeconomic benefits that river delta restoration projects could deliver to river delta communities. Removing levees and levee setbacks could be designed to incorporate flood and drainage benefits.

From a flood-management perspective, there are three ways a project may reduce flooding vulnerability in the community: 1) reducing inundation periods, 2) increasing channel-flow conveyance, and 3) increasing floodplain capacity. In addition, improvement in ground water drainage between tidal cycles can improve the productivity of agricultural lands. The ability to gain these benefits dependds on the site. Site analysis and post project monitoring can be used to measure these benefits, and to verify the potential for improved resilience to climate change (Brophy and Van de Wetering 2011).

Because tidal flood interacts with river flood, and affects drainage, future sea level modeling, in addition to hydrodynamic modeling, is often associated with predicting and measuring changes in flood storage and drainage.

Notes

• Fisher Slough Restoration in the Skagit Delta increased flood storage and conveyance on Carpenter Creek--Weinerman et al 2012 documents economic benefits from restoration. The project provided a significant renovation of drainage infrastructure. Improvement in drainage
• Wiley Slough Restoration in the Skagit Delta produced controversy and potentially economic losses for neighboring land owners by reducing adjacent field drainage.
• Fir Island Farm Restoration in the Skagit Delta is completing feasibility work to evaluate potential effects on drainage, and is completing additional pre-restoration ground water monitoring to avoid unverifiable claims of injury from restoration actions.
• Port Susan Restoration in the Stillaguamish Delta gained local support by providing a flood gate to reduce flooding on Florence Island during an over levee river flood event.
• The Tidegate Fish Initiative in the Skagit Delta links drainage repair and upgrades to restoration actions using a credit trading system as a way to resolve impacts to endangered Chinook salmon.
• The Farm, Fish and Flood Initiative in the Skagit Delta, is coordinating planning between agricultural preservation, restoration, and flood management, to define a suite of projects with the maximum community benefits.
• The Snohomish Sustainable Lands Strategy is working in the Stillaguamish Delta and Snohomish Delta to identify a suite of actions that provide multiple benefits for fishery restoration and agriculture.

Potential Ideas for Future Work

• Predict functions from differing configurations of drainage, storage, tide gating, and restoration to maximize both field drainage and habitat function in a delta setting
• Develop and reduce the costs of robust standard methods to predict the reduction in frequency and/or duration of flood events resulting from restoration projects.
• Develop and reduce the costs of robust standard methods to predict the effect of climate change parameters (sea level rise, reduced snowpack storage, reduced precipitation, etc) on future flood frequency and duration
• Develop and reduce the costs of standard method for predicting how tidal reconnection affects field drainage and groundwater salinity at times important for agricultural stakeholders