Geomorphic Responses to Extreme Rainfall, Catastrophic Flooding, and Dam Failures across an Urban to Rural Landscape

Abstract

The extreme rainfall of October 2015 in South Carolina generated numerous dam failures and spawned the flood of record at most U.S. Geological Survey stream gauges. Detailed field sampling and systematic image analysis are used to document the immediate and sustained geomorphic adjustments at four failed dams within the urbanized Gills Creek watershed. That urban focus is augmented with a similar analysis at five failed dams in more rural settings where less urban infrastructure exists. We also document the magnitude and type of geomorphic adjustments throughout the Gills Creek watershed unrelated to dam failures. Despite the extreme rainfall and associated flooding, the geomorphic effects were limited and localized, manifesting primarily at dam failures but not progressing significantly downstream, especially along Gills Creek, where the combination of intact dams, frequent urban roughness elements, thick floodplain vegetation, and numerous wetlands explains the lack of significant morphologic adjustments or major deposition. Dam failures in rural settings showed greater overbank deposition but the geomorphic effects were still limited to dam proximal locations because of the thick vegetal cover below the dam. Because these dams in rural and urban settings were constructed to maximize waterfront property, low gradient sites were selected; therefore, the geomorphic effects were unlike planned dam removals where massive headcutting, knickpoint migration, and sediment evacuation tend to occur. The most significant effect of the dam failures was the shift from an initial braided channel to a single thread channel within the emptied reservoirs, which occurred quickly, often within the first few months.

2015年十月南卡罗莱州的极端降雨，导致了无数水坝溃堤与洪泛四溢，并打破了美国大部分地质调查溪流水位的纪录。本研究运用仔细的田野抽样与系统影像分析，记录城市化的吉尔斯溪流域中的四大溃堤水坝立即和持续性的地貌调适。该城市焦点，在城市基础建设较少存在的更为乡村地区的五大溃堤水坝的类似分析中被扩大。我们同时记录整个吉尔斯溪流域中与水坝溃堤无关的地貌调适程度与类别。尽管有极端的降雨与相关的洪泛，地貌影响仍然有限并且是在地化的，主要展现在水坝溃堤，但在下游却未有显着的进程，特别是未受损的水坝、频繁的城市崎岖元素、深厚的洪泛平原植被，以及无数的湿地结合所解释的缺乏显着形态调适或主要沉淀的吉尔斯溪沿岸。乡村环境中的水坝溃堤，展现更多的漫滩沉积物，但其地貌影响，则因水坝下方丰厚的植物层，而仅限于邻近水坝之处。由于这些在城市与乡村环境中的水坝是为了极大化滨水资产而建，故选定较低坡度的位置；地貌影响因而不像计画中的水坝移除般容易发生大型向源侵蚀、迁急点变迁以及沉积清除。最为显着的水坝溃堤效应，是在清空的水库中，迅速从最初的辫状河道转变成为单一水道，并在最初几个月中即发生。

Las precipitaciones extremas de octubre del 2015 en Carolina del Sur generaron numerosas fallas en los diques y produjeron la inundación más grande que se recuerde en la mayoría de los calibradores de flujo hídrico del Servicio Geológico de los Estados Unidos. Se usaron el muestreo detallado de campo y el análisis sistemático de imágenes para documentar de inmediato los ajustes geomórficos prolongados en cuatro de los diques que fallaron dentro de la cuenca urbanizada de Gills Creek. Tal perspectiva urbana se complementa con un análisis similar de cinco diques afectados en entornos más rurales donde existe menos infraestructura urbana. También documentamos las magnitud y tipo de los ajustes geomórficos a través de toda la cuenca de Gills Creek no relacionados con fallas de los diques. Pese a las lluvias extremas y las inundaciones asociadas, los efectos geomórficos fueron limitados y de localización puntual, los cuales se manifestaron primariamente en fallos en diques pero que no se extendieron significativamente aguas abajo, en especial a lo largo del Gills Creek, donde la combinación de diques intactos, frecuentes elementos de aspereza urbana, densa vegetación en la llanura inundable y numerosos humedales explican la falta de ajustes morfológicos significativos o depósito importante de derrubios. Las fallas en diques de entornos rurales mostraron una mayor deposición sobre los bancos, pero los efectos geomórficos siguieron siendo limitados a locaciones próximas al embalse debido a la espesa cubierta vegetal abajo del dique. Debido a que estos diques en entornos rurales y urbanos se construyeron para maximizar la propiedad de la zona ribereña, se escogieron sitios de bajo gradiente; en consecuencia, los efectos geomórficos se dieron de modo diferentes a los de diques de remoción planeada donde tiende a presentarse masiva erosión frontal, migración de puntos de inflexión y evacuación de sedimentos. El efecto más significativo de las fallas de los diques fue el cambio de un tipo de canal anastomosado inicial a un canal único dentro de los reservorios desocupados, proceso que se dio con prontitud, a menudo dentro de los primeros pocos meses.

Key Words:

• breached dams
• floods
• floodplains
• infrastructure
• urban hydrology

关键词:

• 破裂的水坝
• 洪水
• 洪泛平原
• 基础建设
• 城市水文学。

Palabras clave:

• diques fracturados
• hidrología urbana
• infraestructura
• inundaciones
• llanuras inundables

Acknowledgments

We thank Keith Kantack and Alexandria Chen for image analysis and the Dartmouth Women in Science Program.

Additional information

Funding

This research was funded in part by the National Science Foundation (NSF Award BCS-1615154) and by the Office of the Vice President, University of South Carolina.

Notes on contributors

Francis J. Magilligan

FRANCIS J. MAGILLIGAN is a Professor in the Department of Geography at Dartmouth College, Hanover, NH 03755. E-mail: [email protected]. His research interests focus primarily on watershed responses to environmental change, the geomorphic efficacy of extreme floods, and the hydroecological impacts of dams and dam removal.

L. Allan James

L. ALLAN JAMES is a Professor in the Department of Geography at the University of South Carolina, Columbia, SC 29208. E-mail: [email protected]. His research interests include historical and anthropogenic fluvial sedimentation; interactions between fluvial sediment, channel morphology, and flooding; and applications of geospatial analysis to these fields.

Scott A. Lecce

SCOTT A. LECCE is a Professor in the Department of Geography, Planning and Environment at East Carolina University, Greenville, NC 27858. E-mail: [email protected]. His research interests include fluvial geomorphology, the transport and storage of metals in rivers and floodplains, and hydrologic and geomorphic responses to human-induced environmental change.

James T. Dietrich

JAMES T. DIETRICH is an Assistant Professor in the Department of Geography at the University of Northern Iowa, Cedar Falls, IA 50614. E-mail: [email protected]. His research interests include the applications of low-altitude remote sensing to geomorphology, including rivers and river restoration monitoring.

John A. Kupfer

JOHN A. KUPFER is a Professor in the Department of Geography at the University of South Carolina, Columbia, SC 29208. E-mail: [email protected]. His research couples field and modeling approaches with geospatial analyses to examine the manners by which spatial patterns of ecological phenomena interact with and constrain ecological processes.