Geomorphic Effectiveness Research Articles

Geomorphic and hydrologic aspects of monsoon floods on the Narmada and Tapi Rivers in central India

High-magnitude floods during the monsoon season are considered to be India's recurring and leading natural disaster. Such large flood are extremely important events, not only in terms of human impact, but also from the standpoint of geomorphic effectiveness and geomorphic work. The Narmada and Tapi Rivers in central India are characterized by one of the most intense flood regimes in the seasonal tropics. Modern, historical and palaeoflood records indicate that large floods are relatively common hydrologic and geomorphic events on the Narmada and Tapi Rivers. A catastrophic flood on Tapi in July 1991, that occurred from heavy rains and a dam break, represents one of the highest unit discharges yet recorded in any part of central India. Flood competence studies indicate that the flood power of the two rivers is comparable with some of the largest historic and prehistoric floods reported from other parts of the world. The rivers have preserved geomorphic and sedimentologic records of palaeofloods for the last two millennium, and in Choral River a 5000 year old record has been located. Examination of the synoptic conditions associated with the flood generating low-pressure systems reveals that all but a few of them are the result of Bay of Bengal depressions. The flooding in such large rivers profoundly challenges flood-hazard management, because of limited instrumental records and large spatio-temporal variation. It is therefore imperative that the design engineering community in India incorporates the geomorphic, sedimentologic and botanic information in the planning and design of water resource development projects.

Flood hydrology and geomorphic effectiveness in the central Appalachians

AbstractThis paper compares hydrologic records and geomorphic effects of several historic floods in the central Appalachian region of the eastern United States. The most recent of these, occurring in November 1985, was the largest ever recorded in West Virginia, with peak discharges exceeding the estimated 500‐year discharge at eight of eleven stations in the South Branch Potomac River and Cheat River basins. Geomorphic effects on valley floors included some of the most severe and widespread floodplain erosion ever documented and exceeded anything seen in previous floods, even though comparable or greater rainfall and unit discharge have been observed several times in the region over the past 50 years. Comparison of discharge‐drainage area plots suggests that the intensity and spatial scale of the November 1985 flood were optimal for erosion of valley floors along the three forks of the South Branch Potomac River. However, when a larger geographic area is considered, rainfall totals and discharge‐drainage area relationships are insufficient predictors of geomorphic effectiveness for valley floors at drainage areas of 250 to 2500 km2.Unit stream power was calculated for the largest recorded flood discharge at 46 stations in the central Appalachians. Maximum values of unit stream power are developed in bedrock canyons, where the boundaries are resistant to erosion and the flow cross‐section cannot adjust its width to accommodate extreme discharges. The largest value was 2570 W m−2; record discharge at most stations was associated with unit stream power values less than 300 W m−2, but more stations exceeded this value in the November 1985 flood than in the other floods that were analysed. Unit stream power at indirect discharge measurement sites near areas experiencing severe erosion in this and other central Appalachian floods generally exceeded 300 W m−2; reach‐average values of 200‐500 W m−2 were calculated for valleys where erosion damage was most widespread. Despite these general trends, unit stream power is not a reliable predictor of geomorphic change for individual sites. Improved understanding of flood impacts will require more detailed investigation of interactions between local site characteristics and patterns of flood flow over the valley floor.

Magnitude-Frequency Relationships of Alluvial Landforms of the Cumberland Plateau, Tennessee

Southeastern Geographer Vol. XXIX, No. 2, November 1989, pp. 106-117 MAGNITUDE-FREQUENCY RELATIONSHIPS OF ALLUVIAL LANDFORMS OF THE CUMBERLAND PLATEAU, TENNESSEE* Michael W. Mayfield INTRODUCTION. A long-held tenet of many fluvial geomorphologists is that most "geomorphic work" is accomplished by frequent floods (recurrence interval of 1/2 to 1 year) rather than by infrequent floods of great magnitude. This school of thought is based upon research by Wolman and Miller who examined the amount of work done by streams in relation to prevailing flow rates. Wolman and Miller suggested that the geomorphic effectiveness of extreme events and commonly occurring events can be compared "in terms of (1) the relative amounts of 'work' done on the landscape and (2) in terms of the formation of specific features ofthe landscape." (J) They argued that the suspended load of a stream is an accurate measure of the amount of geomorphic work accomplished by a stream, while channel geometry and changes in individual fluvial landforms are examples of specific features formed in the landscape. Later research has tended to agree with Wolman and Miller in that events of extremely high magnitude did not move large volumes of sediment nor modify fluvial landforms to a degree comparable to their extreme magnitude. (2) To be specific, the Wolman-Miller principle holds that frequently occurring flood events have the greatest importance in accomplishing overall geomorphic work and in modifying fluvial landforms because the greater power of extreme events is more than compensated for by their rarity. More recently, studies have shown that some streams draining areas which produce occasional flood events of extreme value transport such large volumes of sediment during those events that ". . . the infrequent hydrologie event is most significant. . . ." (3) Baker demonstrated that * This research was supported by a summer research grant from the University of North Carolina at Greensboro. I would like to thank Carol Crutchfield, Rob Stephens, Ashby Lynch, and George Wallace for assistance in the field and John Morgan for making numerous suggestions for improvement of an earlier version of the manuscript. Dr. Mayfield is Assistant Professor of Geography and Planning at Appalachian State University in Boone, NC 28608. Vol. XXIX, No. 2 107 the second element of the Wolman-Miller principle (formation of specific features) "requires modification for application to stream channels in certain climatic and physiographic settings." (4) He concluded that in regions with climatic conditions which favor infrequent floods of great magnitude, separated by long periods of quiescence, the Wolman-Miller principle is not applicable. Harvey reached similar conclusions based on research in an area of semi-arid climate dominated by cloudbursttype storms in Southeastern Spain. (5) The purpose of this research project is to evaluate the effectiveness of frequently occurring floods in modifying fluvial landforms in relatively small (250-1500 sq km) upland basins in an area with a humid climate. Thus, the applicability of the second aspect of the WolmanMiller principle, the modification of fluvial landforms, is evaluated. No attempt is made to investigate the amount of sediment transported by events of various recurrence intervals. Previously described exceptions to the Wolman-Miller principle have been in regions of arid climate with infrequent, but extreme, flood events. THE STUDY AREA. The Cumberland Plateau of Alabama, Tennessee, and Southern Kentucky is a true tableland, with broad undulating uplands only slightly to moderately dissected by streams incised 100 to 300 m below the plateau surface. The Cumberland Plateau surface truncates various Pennsylvanian sandstone and conglomerate units at elevations of 450 to 600 m. (6) The area receives 1200 to 1650 mm of precipitation per year, resulting in 600-800 mm of runoff annually. (7) Despite gentle slopes, dense forest cover, low drainage density, and permeable soils, the streams of the Cumberland Plateau produce floods of unusually high magnitude for the region. (8) The 25-year flood for Whites Creek is equal to 3.35 m3/km2, and the equivalent value for the Emory River is 1.73 m3/km2. (9) Such high flood values are attributable to a dearth of storage; soils are quite thin on the uplands and the bedrock is largely impermeable. (10) Alluvial deposits along Cumberland Plateau streams consist almost entirely of coarse cobbles and...

Avalanche impact landforms on Ben Nevis, Scotland

Abstract A 75 m long boulder rampart at 910 m altitude crosses a bedrock step in Coire na Ciste, Ben Nevis, and dams a small tarn (Lochan Coire na Ciste). The rampart and tarn occur at a break of slope at the foot of an extensive avalanche chute, and are interpreted as avalanche impact landforms, the first to be identified in Britain. The tarn is inferred to have formed through excavation of debris by repeated avalanche impact and the rampart by the accumulation of debris thrown up by impacting snow avalanches. Aprons of loose, angular debris that extenddownslope of the rampart and at the foot of an adjacent avalanche chute are considered to represent deposition in avalanche runout zones. Abundant recently‐deposited boulders on the rampart and debris aprons indicate recent accumulation and suggest that the geomorphic effectiveness of avalanching snow on Scottish mountains may be (at least locally) much more effective than recent accounts have suggested.

Relative scales of time and effectiveness of climate in watershed geomorphology

AbstractPeak rainfalls and peak runoff rates per unit area are comparable over a worldwide spectrum of climates. However, while the magnitude of the external contribution of energy or force in diverse regions is similar, the impact on the landscape varies markedly between regions. Absolute magnitudes of climatic events and absolute time intervals between such events do not provide satisfactory measures of the geomorphic effectiveness of events of different magnitudes and recurrence intervals. Although geomorphic processes are driven by complex sets of interrelated climatic, topographic, lithologic, and biologic factors, the work done by individual extreme events can be scaled as a ratio to mean annual erosion and the effectiveness of such events in forming landscape features can be related to the rate of recovery of channel form or mass wasting scars following alteration by the extreme event. Thus, a time scale for effectiveness may relate the recurrence interval of an event to the time required for a landform to recover the form existing prior to the event.River channels in temperate regions widened by floods of recurrence intervals from 50 to more than 200 years may regain their original width in matters of months or years. In semi‐arid regions, recovery of channel form depends not only upon flows but upon climatic determinants of the growth of bottomland vegetation resulting in variable rates of recovery, on the order of decades, depending upon coincidence of average flows and strengthened vegetation. In truly arid regions the absence of vegetation and flow precludes recovery and the width of channels increases in drainage areas up to 100 km2 but remains relatively constant at larger drainage areas.Area as well as time controls the effectiveness of specific events inasmuch as the likelihood of simultaneous peak discharges or rainfalls and large areas is less, particularly in arid regions where events spanning areas of more than several thousand km2 are extremely rare if experienced at all. To some extent a decrease in area in a humid region is comparable with a regional change from humid toward more arid climate reflected in the increase in importance of episodic as contrasted with more continuous processes. Exceedingly rare floods of extreme magnitudes, estimated recurrence intervals of 500 years or longer, may exceed thresholds of competence otherwise unattainable in the ‘normal’ record resulting in ‘irreparable’ transformations of valley landforms.Denudation of hillslopes by mass wasting during relatively rare events can also be related to mean rates of denudation and to recovery of hillslope surfaces after scarring by different kinds of landslides. Measured recovery times described in the literature vary from less than a decade for some tropical regions to decades or more in temperate regions. Recurrence intervals of high magnitude storms which trigger mass wasting range from 1 to 2 years in some tropical areas, to 3 or 4 per hundred years in some areas of seasonal rainfall and to 100 or more years in some temperate regions. The effectiveness of climatic events on both hillslopes and rivers is not separable from gradient, lithology or other variables which control both thresholds of activity and recovery rate.