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Integrative Floodplain Design

By
Chris Hammersmark (1), PhD, PE
Stefan Lorenzato (2)
Tom Griggs (3), PhD,
Chris Bowles (1), PhD, PE

Figure 1. Vegetation can influence both scour (note the roots exposed due to recent scour) and deposition in high energy floodplain environments. Photo courtesy Stefan Lorenzato.

Floodplains are among our most valuable lands. They derive their richness from the dynamics of the river systems that created them, providing us with fertile farm land, land easily accessible for building our communities, and abundant fish and wildlife. Despite this value, our approach to floodplains has been to “reclaim” land from the river for agriculture and more recently to rapidly expand our communities with the result that the floodplains of California’s Central Valley have experienced losses of wetlands and riparian habitats between 90% and 95%. In the past, flood management concentrated on attempts to control and limit flooding by confining the path of flood waters using levees and “improved channels” and accelerating the conveyance of water to downstream reaches. This management approach diminished the ecological quality and areal extent of floodplain habitats while increasing risk as use of floodplain lands intensifies. Because current policies recognize our floodways serve many simultaneous purposes (see for example California Water Code Section 9600 et seq.); current and future management efforts are now required to integrate the multiple possible functions of these areas in order to simultaneously achieve ecological and community stability and sustainability.

An apparent shortcoming of current efforts is the general lack of understanding about what these multi-function, integrated systems look like and how to design them to successfully achieve multiple functions. There is a need to provide practical techniques and training that translate societal demands into concrete actions that both preserve and enhance the ecological quality and function of floodplains. The Riparian Habitat Joint Venture, in coordination with cbec, inc. eco engineering and River Partners, Inc., is in the process of developing a workshop which explores: 1) the multiple functions of floodways/floodplains, 2) tools available for analysis and design of these areas, 3) ecological tolerances of native vegetation communities which occur in engineered floodways, and 4) how to synthesize this information and available tools into designs which result in improved flood safety and ecological health. This training will be beneficial to water resource engineers, land managers, landscape architects, biologists and regulators by providing an integrative and interdisciplinary approach to the design of floodplains.

Figure 2. Hypothetical example demonstrating the influence of vegetation community placement on velocity patterns during a ~5-yr recurrence interval event. Top panel shows hypothetical vegetation patterns, and the lower panel shows the resulting velocity patterns as simulated with a two dimensional hydrodynamic model. Differences are due solely to different vegetation patterns (roughness), nothing else (e.g., topographic modification). Images courtesy cbec eco engineering.

Floodplains contain a multitude of gradients acting across spatial and temporal scales. These gradients link physical, biological and chemical processes, in complex feedback mechanisms. Topography drives the hydraulics of water flow in river channels and adjacent floodplains by dictating reach slope and cross sectional area available to convey both floodwaters and entrained sediment. As flood waves travel through the floodway, varying energy gradients control the geomorphic processes of scour and deposition. These varying geomorphic processes act in conjunction with other processes to determine the composition of the floodplain substrate, including both sediment texture and percent of organic matter. Sediment texture and amount of organic matter in turn impact the moisture availability to plants as flood waters recede. Topographic features also influence the depth-to-groundwater across a site as well as the extent and duration of inundation. The extent of flood inundation controls the transport and deposition of hydrochorous seeds as well as plant material responsible for plant establishment and growth. So through controls on soil moisture, depth-togroundwater and inundation regime, topography influences the distribution of various plant species in riparian zones.

Once plants come to occupy their appropriate physical, ecological and chemical niche in the landscape, their physical structure acts in concert with the geophysical setting to further influence energy (i.e., disturbance) gradients. The presence of both living and dead plant material (i.e., large woody material) alter flow patterns and affect sediment scour, deposition and sorting (Fig. 1), and provide feedback mechanisms that further enhance topography, moisture and disturbance gradients. Longer-term temporal gradients also play an important role as succession occurs. Initially, early colonizing plant species act to encourage sediment deposition and accumulate organic material and nutrients. As succession occurs, trees begin to dominate the landscape and limit resources available to other species (moisture, nutrients, light, etc.), further changing the way floodwaters move through an area. The complex interplay between biotic and abiotic factors control the distribution of various plant species/communities, as well as the manner in which floodwaters move through the system.

Past management efforts focused on one aspect of the system: maximizing conveyance in as small an area as possible. Evolving needs have brought us back to a point where the quality of river ecosystems is again important. A variety of tools and information products exist to help inform the interdisciplinary design of floodplains, allowing for flood risk management in addition to other important ecosystem functions.

Advances in modern computing have enabled the development of sophisticated numerical hydrodynamic models capable of simulating depth and velocity patterns of flood waves. When properly developed, calibrated and validated, such models allow for the prediction of physical effects of manipulating topography, vegetation or infrastructure at a site. Hydrodynamic models can provide very powerful tools to screen various scenarios, and refine preferred designs (Fig. 2). In such models, vegetation is incorporated as “roughness,” with higher values indicating a higher resistance to flowing water. The roughness parameter incorporates more than just vegetation (e.g., substrate size, bed forms, and other factors also contribute to the roughness), and is the parameter typically modified to calibrate hydraulic models. Likewise, when potential management scenarios are simulated, the roughness parameter is altered within the model to account for changes in vegetation type and distribution. In these instances, high roughness values are typically applied in order to conservatively ensure flood risks are not elevated over the existing condition due to the proposed modifications to the system. However, careful consideration of plant biology overcomes the tendency of the models to mask the ability for plants to decrease or at least not increase to flood risk.

Through exploration of these topics, the workshop intends to enhance understanding of multi-function floodplain management. The workshop includes an introduction to various floodplain types within the Central Valley and the multitude of functions they provide; the biological and physical conditions and plant communities present in each type; the physical interactions which occur between various plant species, sediment and water during floods (Fig. 3); how biologists and engineers look at vegetation differently; and an introduction to numerical hydrodynamic models and how these models describe physical conditions and vegetation. Examples of existing floodplain conditions and various scenarios using different combinations of vegetation patterns and topography are evaluated. Drawing upon the newly developed map of riparian vegetation in the Central Valley, the flood and habitat context of sites is explored. Through interactive involvement, participants will build floodway mosaics, practicing integrative floodplain design and will leave the course with a sense of how plants can be used to achieve flood risk reduction goals while simultaneously enhancing ecosystem functions and maintaining biological diversity within the floodways of the Central Valley.


Figure 3. Physical response of a typical riparian shrub to inundation by moving floodwaters. Image courtesy River Partners, Inc.

The contents of this workshop contents were first presented at the Floodplain Management Association Annual Conference in San Diego in September 2011. Since that time the contents of the workshop have been refined, improved and expanded, and will be presented at upcoming meetings and conferences in 2012 and beyond. Please contact Chris Hammersmark for more information: c.hammersmark@cbecoeng.com.

1. cbec, inc. eco engineering, 1255 Starboard Drive, Suite B, West Sacramento, CA 95691.

2. Riparian Habitat Joint Venture & Department of Water Resources–FESSRO, 901 P Steet, Rm 411A, Sacramento, CA 95814.

3. River Partners, 580 Vallombrosa Avenue, Chico, CA 95926. c.hammersmark@cbecoeng.com