OOS 44-5
Riparian vegetation structure and flood management
The extreme richness of biological resources in floodplains has always attracted people. Today many communities lie within highly flood-prone areas at the center of controversies in which flood risk reduction is pitted against other management goals. For decades, flood risk reduction and food production have driven land management near floodplain communities. Recently, the decline in natural resource quality and persistence of flood risk have led to the notion of Integrated Flood Management. IFM seeks to balance food and safety for people with needs for stable ecosystems that provide a wide array of ecosystem services. Recent new policy in California requires consideration of ecosystem restoration, habitat improvement, water quality protection, recreation, and aesthetic value along with flood safety and agricultural production. Under this multi-benefit framework flood modeling can be used to improve riparian restoration in floodways, and evaluate vegetative structure used to reduce flood risk. To accomplish this, the traditional floodway paradigm of smooth-sided channels must be replaced with a mosaic of features and designs that provide a matrix of vegetation and hydraulic conditions.
Results/Conclusions
Computer models serve as a primary tool for floodplain assessment and design, but until recently only included vegetation to the extent that it contributes to hydraulic roughness and generates drag forces on flood flows. Historically, a single roughness value was often used to describe entire floodplains. Advances in modeling now allow the use of multiple roughness values in models, based on specific vegetation types, flow rates, and flow depths. Recent work examining floodplain vegetation structure and hydraulic response to flows has opened new design options. I will illustrate the potential benefits of considering plant structure in managing floodways using (1) a recent flume study conducted at the University of California, Davis demonstrating structural responses of floodplain adapted plants, and (2) a restoration project on a mainstem, deep-water floodway on the Feather River in the Sacramento Valley, California. In the flume study, roughness was observed to vary according to species, plant size, water velocity and depth. In the Feather River restoration project, flood modeling for both vegetation and river flows allowed comparison of design alternatives of multi-benefit, flood-safe restoration plantings.