The seasonality of extreme floods in the conterminous United States
Abstract
Flood seasonality studies of the conterminous United States typically characterize the timing of annual maximum series (AMS) floods. However, because AMS include the largest flow of each year for the period of record, they have many relatively small, within-bank flows that are not consequential for human communities. Most flood seasonality studies also characterize annual timing with circular statistics, an approach not well suited to regions with multiple flood seasons. We characterized in detail the seasonality of large, overbank floods that can disturb ecosystems and pose risks to human communities by identifying the annual timing of the largest ten AMS floods at 381 stream gages with natural, or near-natural, flow for the period 1966-2015. Basins are from 5-13,000 km2; most are <103 km2. We grouped sites into fourteen geographically-coherent clusters identified by others that correspond to climatic and physiographic regions across the country and then characterized each cluster's flood seasonality at a monthly resolution using a probabilistic method. The results show strong seasonality for large floods along the West Coast (Regions 1, 2), in the Intermountain West (4), and across the northern Plains, Upper Midwest, and western Great Lakes (5, 9, 10), reflected in unimodal monthly frequency distributions (Fig. 1). This suggests a small number of flood-generating mechanisms dominate these regions. Flood seasonality is more complex (i.e., multimodal) in the Southwest (3, 6), Gulf Coast (7, 12) and eastern United States (11, 13, 14), reflecting a greater variety of flood causes in these areas. Comparing the monthly frequency distributions of large floods for the Northeast (13, 14) and Southwest (3) with those of all floods over a specified threshold available in earlier studies (including smaller, within-bank flows) reveals that the annual timing of large floods is different. These regions also have moderate differences in seasonality between the largest two AMS floods and the largest ten, which is not the case for regions with unimodal frequency distributions. Our results demonstrate that in regions with multiple flood seasons and generating mechanisms, the different mechanisms are not equal in their flood magnitude potential. This has important implications for design, forecasting, and preparedness.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H14G..08C
- Keywords:
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- 1821 Floods;
- HYDROLOGY;
- 1833 Hydroclimatology;
- HYDROLOGY;
- 1840 Hydrometeorology;
- HYDROLOGY;
- 1855 Remote sensing;
- HYDROLOGY