Slackwater Flood Deposits Research Articles

Extreme hydrological events, palaeo-information and climate change

Extreme flood events have been and continue to be one of the most important natural hazards responsible for deaths and economic losses. Extreme floods result in direct destructive effects during the time of the event, and they also may be followed by a related chain of indirect calamities such as famines and epidemics that produce additional damages and suffering. Extreme hydrological events that have occurred in the historical past may also occur in the future. Knowledge about magnitudes and recurrence frequencies of past extreme hydrological events in most regions are too short to adequately evaluate potential magnitudes and recurrence frequencies of extreme hydrological events. Stationary climate in which the mean and variance do not change over time is a basic underlying assumption of standard methodological procedures for estimating recurrence probabilities of extreme hydrological events. Palaeo-archives contained in river and lake sediments, fossil plant and animal matter, ice layers, and other natural archives show that the assumption of stationary climate is not valid when the time scale is extended beyond centuries and millennia. Records of past extreme floods that occurred long before the period of instrumentation can be reconstructed from the distribution of slackwater flood deposits or from derivation of water depths competent to transport the largest rocks found in flood deposited sediment. Palaeoflood records reconstructed from the Upper Mississippi and Lower Colorado River systems in the United States confirm nonstationary behaviour of the mean and variance in hydrological time series. These stratigraphic records have shown that even very modest climatic changes have resulted in very important changes in the magnitudes and recurrence frequencies of extreme floods. A close relationship was found between the palaeo-flood record of extreme floods in the Upper Mississippi River system and a palaeo-record of stable isotopes of oxygen and carbon preserved in speleothem calcite from a local cave. The relationship suggests that isotopic records elsewhere might be calibrated to provide insight about how future potential climate changes might impact extreme flood magnitudes and recurrence frequencies there. Atmospheric global circulation models (GCMs) mainly simulate average climatic conditions and are presently inadequate sources of information about how future climate changes might be represented at the extreme event scale. Palaeo-flood archives, however, provide basic information about how magnitudes and recurrence frequencies of extreme hydrological events responded to past climate changes and they also provide a reference base against which GCM simulations can be calibrated regionally and be better interpreted to decipher hydrological information at the extreme event scale.

Paleofloods and a dam-failure flood on the Virgin River, Utah and Arizona

Flood deposits on the Virgin River in southern Utah and northern Arizona preserve a record of the magnitude and frequency of the largest natural floods that have occurred on the river since AD 900. In the Virgin River gorge, several of the natural paleoflood discharges were comparable with, or even greater than, the flood from a catastrophic dam failure in 1989 that produced an estimated peak discharge of 1730 m 3 s −1 in this reach of the Virgin River. To estimate the paleoflood discharges, the heights of slackwater flood deposits in the lower bedrock gorge of the Virgin River were compared with water-surface elevations calculated with the HEC-2 step-backwater hydraulic model. The timing of the paleofloods was determined by the ages of layers rich in human artifacts that are interbedded with the flood deposits. The estimates of both the 1989 flood and the paleofloods were carried out in the Virgin River gorge, about 60 km downstream of the dam break site. No natural floods of this magnitude have occurred within the relatively short period of the gaged record. This study illustrates how incorporating rare, extreme events from the long-term paleoflood record improves the database for flood frequency analysis. The 1989 flood serves as an analog to the effects of rare natural floods with similar magnitudes along the lower Virgin River.

Palaeoflood hydrology of the Salt river, Arizona

AbstractGeomorphic evidence along bedrock‐confined reaches of the Salt River in east‐central Arizona provides a record of the river's largest flood events. Fine‐grained flood slackwater deposits accumulated at channel margin irregularities several metres above the low‐flow channel. Discharges associated with flow events responsible for the deposits were estimated by computer flow modelling. These estimates document flood magnitudes in excess of gauged historic streamflows. Relative and radiocarbon dating suggest that a flood record in excess of 600 y is preserved in the slackwater sequences. A prominent flood scar cut into grussy hillslope soils allows the extension of the prehistoric flood record to several thousand years.A maximum discharge estimate of 4600 m3s−1 affixed to the flood scar represents the largest flood event in the record, and is given a minimum recurrence interval of 1000–2000 y. The 1952 flood is the largest historic flow event experienced along the study reach and is estimated at 2900 m3s−1. Two palaeoflood events preserved in the slackwater stratigraphy exceed the 1952 event, and are given recurrence intervals of 300 and 600 y. The latter flood event had an estimated discharge of 3200 m3s−1. It is apparent that discharge estimates affixed to these infrequent, large‐magnitude flood events approach a maximum with decreased probabilities (large recurrence intervals). This suggests that a physical limit on discharge may exist within the Salt River drainage basin and is perhaps directly related to drainage basin size.

RECONSTRUCTING PALEOFLOOD HYDROLOGY WITH SLACKWATER DEPOSITS: VERDE RIVER, ARIZONA

Fine-grained slackwater flood deposits provide a key to the paleoflood history of the Verde River in central Arizona. During large flows, the slackwater deposits accumulate in areas of sharply reduced flow velocity at channel expansions and contractions or in back-flooded tributaries. The upper surfaces of the highest deposits approximate the peak stages of the associated floods. Stratigraphic analysis, radiocarbon and archaeological dating, and correlation between slack-water sites along the study reach revealed multiple floods, including two paleo-floods that pre-date the 80-year observational record of flows on the Verde River. Discharges calculated with the HEC-2 computer flow model extended the record of rare flood events by 1000 years. The largest flow in that time period measured only 5000-5400 cms, which supports the contention of an upper limit on the maximum flood that can be reasonably expected. Revised recurrence intervals based on the extended paleoflood record were used as a basis to compare...