Climatic and Geomorphic Influences on the Spatial Scaling of Floods

Abstract:

Climate and land use changes are altering global, regional and local hydrologic cycles. These changes affect the frequency and magnitude of flood events. Severity of flooding is determined by both the type of hydrometeorological event and morphological characteristics of the watershed. Understanding how the hydro-climatology and geomorphology of a river basin network influence flood magnitude can inform flood risk assessments under a changing climate. The relationship of discharge (Q) to drainage basin area (A) can be expressed as a power function: Q = aA^b. This relationship is widely used in regional flood frequency models for prediction in both gauged and ungauged basins. This study examines how the scaling parameters may be related to the geomorphic and hydro-climatic setting of a watershed. Scaling exponents (b) and coefficients (a) are compared for floods of varying magnitude across a selection of major Atlantic Coast watersheds. These watersheds capture the hydro-climatic and geomorphic transitions (glaciated versus non-glaciated) along the Atlantic Coast. Results indicate that southern Coastal Plain watersheds have lower scaling exponents (b), and thus dissipate flood peaks downstream more effectively than northern watersheds. Due to climatic reasons, however, the 100-year and other large floods have higher relative magnitudes in southern streams than northern streams. The Potomac River basin is in a transitional region that has flood magnitudes similar to the Southern Coastal Plain watersheds, but scaling exponents similar to northern watersheds that indicate little downstream dissipation of flood peaks.