Changes In Flood Frequency Research Articles

Paleohydrology Inferred from Diatoms in Northern Latitude Regions

Several recent studies have successfully applied diatom-based paleolimnological techniques to infer past hydrological changes in arctic and subarctic regions. For example, we summarize arctic studies that attempt to determine changes in peat water content, flood frequency, river discharge, effective moisture and ice cover in northern regions. Some of the investigations are still in preliminary stages, but represent innovative approaches to study arctic and subarctic paleohydrology. New data demonstrate that lake depth, which may be related to changing hydrological conditions, is a significant variable influencing the distributions of diatom taxa in lake surface sediment calibration sets from Wood Buffalo National Park (WBNP), on the border of Alberta and the Northwest Territories, Canada, and from Fennoscandia (mainly northwest Finland). Weighted averaging regression and calibration methods were used to develop quantitative inference models for lake depth using diatom assemblages preserved in surface sediments. The predictive abilities of the transfer functions were relatively high (for WBNP r 2 = 0.70 and RMSE = 2.6 m, and for Fennoscandia r 2 = 0.88 and RMSE = 1.8 m). However, evaluating the transfer functions using jack-knifing procedures indicated lower predictive abilities, possibly reflecting the relatively small sample size and/or short gradients used in these calibration sets. Such transfer functions can be used to track overall trends in lake levels, and provide an objective assessment as to directions of changing lake levels. Any interpretations of inferred lake levels, especially those related to climate change, must be made cautiously and must include some understanding of the local, present-day hydrological system.

Flood-induced embankment failures on the River Tay: implications of climatically induced hydrological change in Scotland

Agricultural flood embankment failure frequency within the Tay drainage basin in Scotland is explored by examination of breach data (228 breaches in total) collected during an eight-year period in which a large number of high discharge flood events (with return periods of up to 120 years) occurred. The data illustrates that overtopping is the main mechanism of failure, that certain reaches and specific locations are particularly vulnerable to failure, and that a near-linear increase in number and total length of flood embankment failures occurs with percentage increase in flood peak discharge. Non-hydrological factors contributing to flood embankment instability include construction over former river channels, location on the outside of meander bends and “honeycombing” of embankments as a result of rabbit burrowing. Hydrological data suggests an increase in the frequency of high magnitude flood events since 1988 in the drainage basin and an identifiable spatial variability; catchments draining more westerly regions exhibit the greatest shift in pattern. The significance of these shifts and future possible changes in flood magnitude and frequency for flood embankment failure risk are quantified; 5% increases in flood magnitude result in increases in the number of flood embankment failures of between 13 and 26%. Similarly a 5% decrease in flood peak discharges leads to decreases of between 11 and 25%. More generally it is shown that land next to rivers with flood embankments is sensitive to modest changes in flood generating conditions.

The influence of recent environmental change on flooding and sediment fluxes in the Yorkshire Ouse basin

This study attempts to establish causes of historical variations in flood frequency and magnitude in a 119-year flood stage record at York, and assess its likely effects on fluxes of fine sediment and associated base metal contaminants. Climatic controls of flooding are evaluated through analysis of synoptic flood generation, and large-scale land use changes are also considered in terms of their influence on food frequency and magnitude and sediment availability. The late nineteenth century was characterized by low flood frequency and magnitude, but, as a consequence of upland metal mining, contaminant concentrations and downstream fine sediment delivery would have been high. A further decline in flood frequency and magnitude between 1904 and 1943, combined with the cessation of base metal mining, resulted in a decline of sediment and contaminant fluxes. Between 1944 and 1968 increases in upland drainage, changes in agricultural practices and high flood frequency and magnitude resulted in significantly enhanced sediment fluxes. Rates of flux declined between 1969 and 1977 owing to extremely low flood frequencies and magnitudes. In the last two decades there have been a series of extreme magnitude floods, which through remobilization of mining-contaminated alluvium have resulted in high pollutant metal mining loads, probably approaching those of the late nineteenth century. In an historical context it appears that the LOIS monitoring programme has been undertaken in a period of extremely high flood frequency and magnitude in the UK, and significantly enhanced sediment and contaminant metal flux. Analysis of synoptic flood generation suggests that any future increase in the frequency and vigour of cyclonic atmospheric circulations may result in a higher frequency of extreme floods and consequently increased sediment fluxes.

Ages of Late Holocene earthquakes on the central Garlock fault near El Paso Peaks, California

A trench across the central Garlock fault within a small playa adjacent to El Paso Mountains revealed buried fissures and fault scarps that provide strong evidence for five surface rupturing earthquakes within the past 5 kyr. Weaker evidence suggested the possibility of three additional events within this same time period. These observations indicate an average recurrence interval of 700 to 1200 years. Nearly all 28 radiocarbon dates on samples of detrital charcoal, wood and shell are in correct stratigraphic order. These dates constrain the ages of the individual faulting events and indicate that the recurrence interval is not periodic but is quite irregular. Individual recurrence intervals range from 190 to 1545 years (if all eight events are actual earthquakes) or from 190 to 3405 years (if only the five well‐documented events represent earthquakes). The two most recent earthquakes occurred within the past 550 years, with preferred ages of about A.D. 1790 and A.D. 1600. The A.D. 1790 event ruptured a narrower width of the fault zone, and may have been a smaller earthquake than the others. The radiocarbon dates from the trench also indicate that the sedimentation rate on the playa has fluctuated within the past 5 kyr. Low sedimentation rates (0.1 to 0.35 mm/yr) and infrequent floods (≤1 per century) prevailed from about 2000 B.C. to A.D. 100 and from A.D. 500 to 1300. During the remaining intervals within the past 5 kyr, the sedimentation rate averaged 1.0 to 1.7 mm/yr and major floods occurred two to four times per century. The changes in sedimentation rate and flood frequency may reflect climatic changes, or they may simply reflect changes in drainage patterns.

Spring ice-jam flooding of the Peace-Athabasca Delta : Evidence of a climatic oscillation

A historic record of spring ice-jam floods of the Peace-Athabasca Delta was analyzed for the years 1826–1995. The temporal pattern of flooding is non-random. The likelihood of a flood following a flood, or a non-flood following a non-flood, is greater than expected by chance. Probability analysis of flood occurrence reveals that the period 1860–1880 was a time of unusually few floods, and the period 1915–1950 was a time of unusually frequent floods. The long-term flood frequency is 1 flood in 6.25 years. Changes in flood frequency over the record reveal a pattern of oscillation described by a sine-based model that is correlated with the long-term (Gleissberg) cycle of solar activity. Monte Carlo simulation was used to test a Bennett Dam Model and a Cyclic Model. The Bennett Dam Model is unlikely to have generated the observed flood history (p=0.04). The observed flood history shows a better fit to the Cyclic Model (p=0.65). No correlations between floods and ENSO cold or warm events was detected. The most recent wet period began about 1900 and ended in the early 1960's prior to completion of the W. A. C. Bennett Dam in British Columbia. As independent corroboration of climatically-driven changes in flood frequency we present three additional lines of evidence. The pattern of annual muskrat returns (95 year record) reveals both 10 year cycles and long-term patterns that agree well with the observed flood cycle. The annual area burned in Wood Buffalo National Park is inversely related to flood occurrence. Incised channels and dendritic drainage patterns in the bed of Lake Mamawi provide probable evidence of a previous dry period in the delta. Climatic change or oscillation likely underlies the drying trend observed in recent decades in the Peace-Athabasca Delta.

Mechanisms of floodbank failure during large flood events on the rivers Tay and Earn, Scotland

Abstract This paper describes flood embankment breaches on the Tay and Earn river systems, Scotland, during a 100-year return period flood in January 1993. The location of flood embankment breaches are compared with failures mapped during another large flood event in February 1990 and other historical information on flood embankment instability. Breaches were identified using colour aerial photography taken 10 days after the January 1993 flood event and a detailed field survey of every flood embankment breach was subsequently undertaken. In total, 116 breaches resulted from the 1993 flood event, most as a result of overtopping, and extensive floodplain damage was associated with a number of the failures. Specific sections of the Tay and Earn river systems demonstrated a high incidence of flood embankment breaching. Along these sections, embankments on the outside of bends and overlying old river courses are highly vulnerable to failure. Embankments bordering tributary streams are also vulnerable to failure as a result of being perpendicular to the direction of flow across the floodplain during inundation. A number of failures, however, cannot be attributed directly to their position relative to the river or former river courses. In these cases local factors, such as scour around fence-posts or poor flood embankment construction are probably dominant. Given the repeated breaching of embankments during flood events, changing agricultural land-use practices, and possible changes in flood magnitudes and frequencies, a number of recommendations concerning flood embankment management on the Tay and Earn river systems are proposed.

River management and conservation issues on formerly braided river systems; the case of the River Tay, Scotland

Abstract Although the present-day River Tay, Scotland is generally confined to a single course and exhibits limited lateral mobility, historical maps (1747–53) reveal that during the eighteenth and nineteenth centuries a braided river channel pattern was more characteristic of a number of reaches on the piedmont valley floor. Possible explanations of the change in channel pattern include flood embankment construction, flow regulation by impoundment and possibly a change in flood magnitude and frequency, and alterations in sediment supply. Areas of former braiding isolated from the main channel by embankments or incorporated into the floodplain sediments, exhibit a greater susceptibility to erosion and are prone to flood embankment breaches. In contrast, river channel reaches still exhibiting unconfined braiding tendencies have been designated areas of high conservational value due to large expanses of gravel at different stages of vegetation colonization and high plant species diversity. It is suggested that the identification of areas of former braiding and reaches prone to braiding is useful for land use management both for improved flood protection and floodplain habitat conservation.

A physically based method for prediction of runoff frequencies

A method is described for predicting changes in flood frequencies resulting from various causes affecting runoff in a watershed. The method utilizes the curve number and the traingular unit hydrograph procedures of the United States Department of Agriculture's Soil Conservation Service to transform rainfall frequencies into frequencies of peak discharge. The theory underlying the method is discussed. Application is illustrated by an example.

Late Holocene Flood History of the Tanana River, Alaska, U.S.A.

The Tanana River basin in central Alaska drains both the north slope of the Alaska Range and the south slope of the Yukon-Tanana upland. A sequence of historic and prehistoric flood deposits of the Tanana River is preserved in a small bedrock-sheltered slough near Fairbanks. Examination of these deposits using a suite of radiometric dates, microstratigraphic observations, and granulometric statistics suggests that large changes in flood frequency occurred during the late Holocene. Three major lithostratigraphic units are observed: (1) thick cross-bedded, pedogenically unaltered alluvial silty sands which were deposited between 3,000 and 2,000 yr BP, recording an interval of large floods; (2) a series of thin silty beds and paleosols formed after 2,000 yr BP during an interval when large floods were uncommon; and (3) a sequence of sand units recording large floods during the last several hundred years. Flood frequencies appear to have changed in response to regional climate changes, with more frequent flooding occurring during times of widespread alpine glaciation and increased storminess.

Compensation flows in the UK: A hydrological review

AbstractA survey of United Kingdom reservoirs identified the development of compensation flow policy from the early 19th Century to the present day. Details on the level and pattern of releases below 261 impounding reservoirs provided the basis for assessing the spatial and temporal variation in compensation flows and the impact of impoundment on downstream flow regimes. By standardizing compensation releases by the natural pre‐impoundment mean discharge, release patterns from reservoirs with a wide range of both catchment area and climate could be compared. Analysis of the change in flood and low flow frequency following impoundment enabled the impact of reservoir regulation to be evaluated at the regional scale. For individual reservoirs the natural downstream increase in river flow provides a basis for determining the length of river reach which is sensitive to regime changes. The majority of present day compensation discharges were set to satisfy river interests which no longer apply or were based on inadequate hydrological or biological information. This has led to a wide range of release patterns and it is therefore recommended that the re‐evaluation of existing water resource schemes should include an assessment of the suitability of current release patterns for present day downstream demands and water resource requirements.