A Watershed Year: Anatomy of the Iowa Floods of 2008

21. Watershed-Based Flood Management

21 Watershed-Based Flood Management Douglas M. Johnston The Iowa floods of 2008 remind us that we live in an interconnected system: the watershed. Each watershed, or drainage basin, collects the runoff (and anything the runoff carries) from a region’s land area and feeds it through streams and rivers. Consideration of the entire watershed in flood planning is critical because the areas that flood are determined by watershed conditions upstream of the flood. This complexity makes flood management very challenging and helps explain why flood damages continue to increase. In this chapter, we look at how land-use decisions, local community responses to floods, and other factors can make flooding worse, and how in contrast a watershed-based approach could contribute to better flood outcomes. To date, most of our planning and response to floods has been local. Individuals and communities attempt to manage flooding through the construction of flood barriers, regulations for floodplain management, and other methods discussed throughout this book. However, the efforts of individuals and communities are limited to the extent of their property or political jurisdiction. Water, as we have repeatedly seen, does not recognize property or political boundaries. So, at any given location, the risk of flooding depends not only 194 looking back,looking forward on local conditions, but also on distant events and practices. If a community upstream increases its runoff, downstream communities face increased flood risk. And if additional agricultural land is drained, downstream areas face increased risk. The inability of downstream communities to address upstream sources of flooding creates a defensive strategy. In the face of greater quantities of floodwaters from upstream, the best a downstream community can do is to increase its local protection by building bigger reservoirs or taller levees or investing in greater emergency management and reconstruction resources (for example, expanding and relocating emergency response facilities). Measures to mitigate future flood damages are typically exercised by the local governments that are responsible for post-flood recovery. One result of this local-response emphasis is a flood-protection arms race. Many defensive measures such as levees may protect local land from upstream floodwaters but can in turn pass increased risk of flooding to downstream neighbors, forcing them to respond likewise. In watershed-based planning, the entire area that contributes to flooding in a region is considered, and flood reduction techniques are strategically applied where they are most effective. For example, upstream land areas may be modified to reduce the volume of water they contribute to the streams and rivers they feed. Instead of a community building flood barriers, it might restore or reconstruct upstream wetlands to hold back rural runoff. Alternatively, flood easementsonupstreamlandscouldbepurchased,allowingtheeasementowner to flood that land in time of need, in order to reduce flooding downstream. In this way, a community might purchase the rights to flood relatively inexpensive land in order to reduce flood damages on highly valued land. Decisions about these types of watershed-based tradeoffs require comprehensive assessment of watershed conditions, including land use, the extent of impervious surface area, urban and agricultural drainage practices, soil properties, past and predicted changes in rainfall patterns (including those associated with climate change), topography, and stream channel geometries. Studies of these and other watershed traits are crucial to estimations of floods and the effectiveness of flood reduction efforts. Watershed-based management also requires comprehensive assessment of risk, land and property values, and community/stakeholder values to decide which strategies are the most cost effective, socially acceptable, and technically or politically feasible. As complicated as these considerations may appear, we already have a number of tools in place that can serve as models for watershed-based flood regulations . The Clean Water Act and related state and local regulations, for example, watershed-based flood management 195 effectively establish water quality standards for industrial and municipal pollution sources, and more recently for nonpoint source pollution. While stormwater management is a major focus of these efforts, the regulated elements are pollutants rather than water quantity. Couldn’t we establish watershed-based standards for runoff volumes and peak discharges with the goal of reducing floodwaters? Again, some communities now assess stormwater fees to help pay forstormwatermanagement.Thesefeesarelinkedtotheamountofimpervious surface on a given property and that property’s excess contribution to runoff. If we were to apply the excess contribution standard universally in a watershed, the cumulative impact on flood reduction could be substantial. Also in our tool bag are techniques that...