Geomorphic effects of a 10-year storm on a small drainage Basin in the Texas Panhandle

Bedded Permian salt in the Palo Duro Basin of the Texas Panhandle is being considered for isolation of nuclear waste. Studies underway to evaluate the geomorphic processes affecting any waste repository that may be sited in the region include studies of the geomorphic events associated with individual storms. On 26 May, 1978, thunderstorms occurred near Canyon, Texas, which received 130mm of rain, and on a small instrumented watershed at Buffalo Lake, 16 km to the southwest, which received 71 mm of rain. Rainfall at Buffalo Lake was concentrated in a period of 3 hours, representing a return period of approximately 10 years and a maximum 30-minute intensity of 64 mm/hour. Erosion-pin fields, topographic surveys, and stakes for headcut monitoring had been established 2 to 3 months before the storm in a 640-m long tributary canyon to Tierra Blanca Creek downstream of Buffalo Lake. Canyon slopes of moderately to slightly calichified sands and gravels of the Ogallala Formation are capped by well-indurated caliche on the canyon rim. Erosion-pin fields showed average net erosion of 2·4 cm (0° to 9° slope) to 2·7 cm (10° to 19° slope) and a single-pin maximum value of 6·2 cm in the 20° to 29° slope class. Headcuts 1 to 2 m deep in alluvial-colluvial material on the floor of the canyon migrated as much as 12 m upstream. Canyon floor deposition occurred as a series of elongate bars, 16m long and longer, located at the mouth of tributary gullies, downstream from scours, and at slope breaks. Grain sizes ranged from boulders up to 70cm in intermediate axis deposited in the upper canyon to fine to very fine sand deposited as a sheet up to 25cm thick beyond the mouth of the canyon at Tierra Blanca Creek. In an adjacent canyon a sequence of irregular beds of caliche gravel, mixed sand and gravel, and fine sand up to 3 m thick is exposed. Comparison of this sequence with deposits resulting from the 26 May, 1978, storm suggests that the coarse fraction of the alluvial sequence is deposited by repetitive major storm events. Cobble and boulder units 30 to 70cm thick can evidently be deposited in a few hours. Under the continental climate of the Texas Panhandle, erosion, deposition, and stream incision are taking place primarily in discrete steps related to episodes of intense rainfall.     

After horton

The divergent and yet related problems of post-Hortonian studies of drainage density and channel network geometry are viewed against the difficulties of defining first-order channels and basins. It is proposed that the junction of an unbranched perennial (or blue-line) channel with another perennial channel be taken as the starting point for definitions and that the entire contour-crenulation network tributary to that point be considered the first-order stream. It is shown that the concept of network diameter may be used to describe the networks so delimited and that it appears to provide a useful starting point for interregional comparisons. Finally, an analysis of Blyth and Rodda's (1973) data on channel lengths and discharge indicates that network diameter may be as closely related to discharge as is channel length itself.     

Tectonic mechanisms associated with P–T paths of regional metamorphism: alternatives to single-cycle thrusting and heating

Metamorphic pressure (P)–temperature (T) paths are commonly used as tools to interpret the tectonic history of orogenic belts, those deformed belts of rocks that record past activity along active plate margins. Many studies and reviews relating P–T path development to tectonics have focused on thrusting–thermal relaxation cycles, with special emphasis on collisional processes. Other studies have assumed that P–T paths resulted from a single tectono-metamorphic event that accounted for the entire burial–exhumation history of the rocks. In many cases, such assumptions may prove invalid.This paper speculates on the relationship of tectonic processes other than thrusting–heating to P–T path development. The processes discussed herein include subduction initiation, triple-junction interactions, initiation and shut off of arc volcanism, subcontinental delamination, and hot spot migration. All of these processes may leave a signature in the metamorphic rock record. Examples are presented from a number of localities, most of which are from the Pacific Rim. Although thrusting–heating cycles have influenced metamorphic evolution in many orogenic belts, the potential impact of other types of tectonic mechanisms should not be overlooked.     

Quantification strategies for human-induced and natural hydrological changes in wetland vegetation, southern Florida, USA

An accurately dated peat profile from a mixed cypress swamp in the Fakahatchee Strand Preserve State Park (FSPSP, Florida, USA) has been examined for pollen and spores. The near-annual resolved pollen record shows a gradual shift from a wet to a relatively dry assemblage during the past 100 years. Timing of drainage activities in the region is accurately reflected by the onset and duration of vegetation change in the swamp. The reconstructed vegetation record has been statistically related to pollen assemblages from surface sediment samples. The response range of the FSPSP wetland to environmental perturbations could thus be determined and this allows better understanding of naturally occurring vegetation changes. In addition, the human impact on Florida wetlands becomes increasingly apparent. Superimposed high-frequency variation in the record suggests a positive correlation between winter-precipitation and pollen productivity of the dominant tree taxa. However, further high-resolution analysis is needed to confirm this relation. The response range of the FSPSP wetland to environmental perturbations on both annual- and decadal-scales documented in this study allows recognition and quantification of natural hydrological changes in older deposits from southwest Florida. The strong link between local hydrology and the El Niño Southern Oscillation makes the palynological record from FSPSP highly relevant for studying past El Niño—variability.     

The significance of morphometric analysis for obtaining groundwater potential zones in a structurally controlled terrain

Watershed development and management plans are more important for harnessing surface water and groundwater resources in arid and semi-arid regions. To prepare a comprehensive watershed development plan, it becomes necessary to understand the topography, erosion status and drainage patterns of the region. This study was undertaken to determine the drainage characteristics of Pageru River basin using topographical maps on a scale of 1:50,000. The total area of the Pageru River basin is 480 km². It was divided into X sub-basins for analysis. The drainage patterns of the basin are dendritic and include a sixth order stream. The quantitative analysis of various aspects of a river basin drainage network characteristics reveals complex morphometric attributes. The streams of lower orders mostly dominate the basin. The development of stream segments in the basin area is more or less affected by rainfall. The elongated shape of the basin is mainly due to the guiding effect of thrusting and faulting. The erosional processes of fluvial origin have been predominately influenced by the subsurface lithology of the basin.     

Crack propagation trajectories for rocks under mixed mode I-II fracture

Propagation of a crack in engineering materials including rocks can cause failure. Knowledge of the stress state under which a crack can propagate, and the trajectory it may follow during its growth are thus very important for the stability of rock masses/materials and for the safe design of structures in/on rocks. In this paper, the crack initiation angle and subsequent crack propagation path are experimentally investigated for limestone rock specimens. This investigation was conducted under various mixed mode I-II loading conditions, including pure mode-I and pure mode-II.This study includes conducting diametrical compression tests on notched Brazilian disk specimens. Moreover, the effect of confining pressure and temperature on crack initiation and propagation were also studied. The experimental results were compared with theoretical predictions of crack initiation angle. The results showed that limestone behaves in brittle fashion, and the effects of confining pressure and temperature on failure trajectories were not significant. Generally, the crack initiation angle can be predicted by the maximum tangential stress criterion. However, for notched Brazilian disk with high value of crack orientation with respect to loading direction, crack does not propagate from the tip of the crack. This important observation indicated that the tensile-strength failure can become more critical than the fracture-toughness failure.     

High-altitude Plio–Quaternary fluvial deposits and their implication on the tilt of a horst, western Anatolia, Turkey

This study investigates the origin and regional tectonic implications of high-altitude Plio (?)–Quaternary fluvial deposits developed over the Bozdağ horst which is an important structural element within the horst–graben system of western Anatolia, Turkey.A total of 23 deposits occur near the modern drainage divide comprising fluvial to occasionally lacustrine deposits. The deposits are all elongated in N–S direction with a width / length ratio of 1 / 10. The largest of them is of 13 km in length with a maximum observable thickness of about 100–110 m. Morphological, lithological, deformational characteristics of these deposits and the drainage system of the area all suggest that the deposits were formed due to uplift and southward tilting of the Bozdağ horst. This tilting which is estimated as 1.2° to 2.2° caused accumulation of the stream load along channels flowing from south to north. All the deposits were later dissected by the same streams with the exception of one deposit which still preserves its original lake form. These deposits are of Quaternary age, which corresponds to the latest N–S directed extensional tectonic phase in the region.     

A comparison of drainage networks derived from digital elevation models at two scales

Automated generation of drainage networks has become increasingly popular with powerful analytical functions in geographic information systems (GIS) and with the increased availability of digital elevation models (DEMs). This paper compares drainage networks derived from DEMs at two scales, 1:250 000 (250K) and 1:24 000 (24K), using various drainage parameters common in hydrology and geomorphology. The comparison of parameters derived from the 250K DEMs with those from the 24K DEMs in 20 basins ranging from 150 to 1000 km² in West Virginia shows that the goodness-of-fit between parameter estimates based on the DEMs varies. Results clearly show that superior estimations are produced from the 24K DEMs. Better estimates can be obtained from the 250K DEMs for stream length and frequency parameters than for gradient parameters. However, the estimation of the mean gradient parameters based on the 250K DEMs seems to improve with increasing terrain complexity. Finally, basin size does not strongly affect the accuracy of parameter estimates based on the 250K DEMs.     

40Ar/ 39Ar and K–Ar geochronological age constraints for the inception and early evolution of the Izu–Bonin – Mariana arc system

K–Ar and ⁴⁰Ar/³⁹Ar dates are presented for locations in the Izu–Bonin – Mariana (IBM) forearc (Ocean Drilling Program (ODP) sites 786 & 782, Chichijima, Deep Sea Drilling Program (DSDP) sites 458 & 459, Saipan), and Palau on the remnant arc of the Kyushu–Palau Ridge. For a number of these locations, the ⁴⁰Ar/³⁹Ar plateau and ³⁶Ar/⁴⁰Ar versus ³⁹Ar/⁴⁰Ar isochrons give older ages than the K–Ar results. The most important results are: (i) at site 786, initial construction of the proto-IBM (now forearc) basement occurred at least by ca 47–45 Ma, consistent with the age of the immediately overlying sediments (middle Eocene nannofossil Zone CP13c); the younger pulse of construction dated at ca 35 Ma by K–Ar could not be confirmed by ⁴⁰Ar/³⁹Ar analysis; (ii) ⁴⁰Ar/³⁹Ar ages for the initial construction of the Mariana portion of the IBM system are as old as those of the Izu–Bonin portion, for example at site 458, initial construction commenced at least by ca 49 Ma and at ca 47 Ma at Saipan (Sankakayuma Formation); and (iii) a combination of K–Ar and ⁴⁰Ar/³⁹Ar ages indicate continued boninite magmatism in the Izu–Bonin forearc (and remnant arc at Palau) until ca 35 Ma. Subduction inception including boninite series rocks along most of the exposed length of the IBM system, clearly preceded by some 5 million years the Middle Eocene (ca 43.5 Ma) change in Pacific plate motion. Boninitic series magmatism persisted at locations now exposed in the forearc for ～ 15 million years after arc inception concurrently with low-K tholeiitic series eruptions from a subaerial arc system, established at ≥ 40 Ma, on the Kyushu–Palau Ridge. For the Mariana portion of the IBM system, reconstruction of the proto-arc places this activity adjacent to the concurrent but orthogonally spreading Central Basin Ridge of the West Philippine Basin. It is possible that a combination of subduction of a young North New Guinea Plate beneath newly created back-arc basin crust may account for some of the features of the Mariana system. It is clear, however, that the understanding of the processes of subduction initiation and early IBM arc development is incomplete.     

洞庭湖流域洪水模拟与综合治理

在洞庭湖水灾现状和特征分析的基础上,利用水平衡方程对洞庭湖及四水水系的水情作了进一步的模拟分析。初步估计了流域的泄流系数Fo,保水系数Fs和蓄水系数FR。得到洞庭湖随洪峰入湖流量而变化的水位日增量和湘、资、沅、四水流域随集中降水量而变化的水位总增量。进而对洞庭湖水患的防治提出了综合的治理方略：（1）增加蓄洪面积,预田蓄洪;（2）增大泄流系数,有控泄流;（3）减少入流流量,有效蓄水;（4）加固堤防工程,有备无患。减少入流流量,有效蓄水,就是在洞庭湖上游的四水流域,除了增加水库型集中蓄水外,还要人工地增加降水滞流量,也就是在全流域增加分散性蓄水机能,进行研究水田蓄水、旱土蓄水、草地蓄水及林地蓄水等的蓄洪限度、实施技术及蓄洪以外的经济效益,以及流域人工增加降水滞流量的巨大潜力。本文分析认为,只有对洞庭湖及其流域的水旱灾害实施综合治理,才是根治洞庭湖水患的有效方略,将对湖南社会经济可持续发展起重要作用。