An empirical assessment of which inland floods can be managed

Abstract

Riverine flooding is a significant global issue. Although it is well documented that the influence of landscape structure on floods decreases as flood size increases, studies that define a threshold flood-return period, above which landscape features such as topography, land cover and impoundments can curtail floods, are lacking. Further, the relative influences of natural versus built features on floods is poorly understood. Assumptions about the types of floods that can be managed have considerable implications for the cost-effectiveness of decisions to invest in transforming land cover (e.g., reforestation) and in constructing structures (e.g., storm-water ponds) to control floods. This study defines parameters of floods for which changes in landscape structure can have an impact. We compare nine flood-return periods across 31 watersheds with widely varying topography and land cover in the southeastern United States, using long-term hydrologic records (≥20 years). We also assess the effects of built flow-regulating features (best management practices and artificial water bodies) on selected flood metrics across urban watersheds. We show that landscape features affect magnitude and duration of only those floods with return periods ≤10 years, which suggests that larger floods cannot be managed effectively by manipulating landscape structure. Overall, urban watersheds exhibited larger (270 m(3)/s) but quicker (0.41 days) floods than non-urban watersheds (50 m(3)/s and 1.5 days). However, urban watersheds with more flow-regulating features had lower flood magnitudes (154 m(3)/s), but similar flood durations (0.55 days), compared to urban watersheds with fewer flow-regulating features (360 m(3)/s and 0.23 days). Our analysis provides insight into the magnitude, duration and count of floods that can be curtailed by landscape structure and its management. Our findings are relevant to other areas with similar climate, topography, and land use, and can help ensure that investments in flood management are made wisely after considering the limitations of landscape features to regulate floods.

Recent Changes in Stream Flashiness and Flooding, and Effects of Flood Management in North Carolina and Virginia

“…We classified each watershed by its dominant land cover, by conducting a hierarchical cluster analysis based on the k-means procedure using percentages of forest, urban, and agriculture for five time periods that spanned from 1992 to 2011. From this analysis, we derived four watershed types: forested (89-98% forest), semi-forested (50-80% forest), rural (28-53% agriculture; 32-64% forest), and urban (40-100% urban) (Mogoll on et al, 2016).…”

“…We distinguished between large (≥5-year) and small (<5-year) floods, as the former are driven by climatic events wherein the landscape plays a negligible role, whereas the latter can be significantly altered by landscape features (Hawley and Bledsoe, 2011;Mogoll on et al, 2016). Thus, small floods ranged in size from 80% of a 1-year flood to less than a 5-year flood; large floods included all discharges greater than this range.…”

“…Conversion from farmland to forest in the Piedmont region has also impacted streamflow (Brown et al, 2005;Kim et al, 2014). The multiple changes across NC and VA landscapes suggest the response of stream flashiness and flooding to humaninduced changes is spatially heterogeneous, especially for the small floods that may be manageable (Mogoll on et al, 2016). Investigating this heterogeneity may provide insight to flood managers (Vogel et al, 2011).…”

“…Flood magnitude is the amount of discharge passing a fixed location, and flood count is the number of floods exceeding a certain flow threshold over a certain timeframe. Flood responses to landscape changes (anthropogenic or natural) are greatest for small floods, as large floods are driven primarily by large-scale climatic events (Hawley and Bledsoe, 2011;Mogoll on et al, 2016). Flashiness index measures the rate of change in stream discharge, and has been used to assess anthropogenic disturbance over time (Baker et al, 2004).…”

Economic Evaluation of Hydrological Ecosystem Services in Mediterranean River Basins Applied to a Case Study in Southern Italy

“…Both parameters are considered when estimating the values of the universal soil loss relation [10]. The latter is usually used for estimating the soil amount removed by water runoff [11]. Soil use also influences the main terms of the water balance, i.e., canopy interception, evapotranspiration, infiltration, soil water storage, and surface outflow [12][13][14].…”