Maximum Flood Discharge Research Articles

Scour and fill in steep, sand-bed ephemeral streams

The traditional idea that entire long reaches of alluvial stream channels in semiarid regions are scoured at high flood discharges and subsequently filled in the waning flood phase (mean-bed scour and fill) can be challenged. The alternative concept that mean-bed elevation varies but little during a flood and that both scour and fill occur concurrently at different migrating loci within a reach (local scour and fill) is also consistent with published field data. Field and laboratory investigations reported herein suggest that mean-bed scour and fill in a straight uniform channel is minor compared to local scour and fill caused by bedform migration and, furthermore, that maximum local scour and fill may occur during the waning flood phase in some instances. The field experiment, utilizing a rectilinear array of maximum-scour indicators (scour-cords), produced data for contoured plots of maximum scour and fill in an ephemeral stream bed during two floods. In the first flood, 24 cm of scour and fill was measured for a bankfull flow depth of 23 cm. In the second, maximum scour and fill was at least 66 cm for a bankfull flow depth of 34 cm. Estimates of antidune amplitudes for the two floods, based on theoretical models and laboratory and field observations, are 27 to 61 cm and 44 to 92 cm, respectively. This indicates that all scour and fill measured by the scour-cord array could have been caused by antidune migration. Laboratory experiments were conducted in an 18-m long non-recirculating flume with automated controls for rates of sediment and water input. A series of experiments in a 26.7-cm-wide sand-bed channel with rigid walls, at grade for a simulated flood patterned after those typical of ephemeral streams, showed that mean-bed scour and fill was less than 3% of local scour and fill. For these experiments, mean sand size was 0.3 mm, channel slope was 0.009, maximum water depth was 40 mm, maximum local scour and fill was 22 mm, and maximum mean-bed scour and fill was 0.6 mm. Maximum mean-bed elevation variation was thus only two sand-grain diameters. Maximum local scour and fill took place near the end of the simulated floods, when bed-form amplitudes were the greatest. Antidune amplitudes calculated for flows in the Arroyo de los Frijoles, New Mexico, are larger than published values of scour and fill for unit discharges greater than 0.5 m 3 /m · s (5 cfs/ft). Below that threshold discharge, the antidunes that formed at maximum flood discharge are smaller than the dunes that probably form during the waning flood, and calculated antidune amplitudes are less than reported scour and fill.

On the Amount of Bottom Sediments in Reservoirs

This paper presents the results of the study on the amount of bottom sediments in reservoirs.The study concerns the geology, topography, annual precipitation in the watershed, discharge of rivers flowing into the reservoir, and amount of bottom sediments in the reservoir. The characteristic features of erosion and transportation in the watershed could be found from this study.1. As regards the 52 reservoirs studied, the amount of the sediments (the mean annual amount per unit area of the watershed) has little or no relation directly with any one factor of geology, relief and annual precipitation during heavy rains in the watershed. Each of the reservoirs based upon the geological characteristics (rock distribution) of their watersheds, can be classified into the following three groups, namely, A: Reservoirs which have the watershed consisting mainly of Paleozoic and Mesozoic rocks.B: Reservoirs which have the watershed consisting mainly of acidic plutonic, hypabyssal and their metamorphic rocks (Granitic rocks).C: Reservoirs which have the watershed consisting mainlyf Cenozoic sedimentary rocks, effusive rocks or crystalline schists, or sedimentary, plutonic and effusive rocks of nearly equal distribution.2. For the reservoirs of each group included in the three groups by the characteristics of rock distribution in the watershed, the larger amount of bottom sediments is found for the reservoir with the watershed which has larger numerical value of product of those representing the relief and the annual precipitation during heavy rains.3. Moreover, from comparison of the amount of bottom sediments in the reservoirs of these three groups with each other, it is clear that the finer the watershed products, the more the sediments are deposited. The same relation is also found for the reservoirs.of the three subgroups (B1, B2 and B3) of B group which were classified based upon the climatic condition of the watershed.4. Based on the aforementioned relations between the amount of bottom sediments in the reservoirs and the several factors of the watersheds, it is concluded that for each reservoirs of A, B and C groups, the larger the numerical value of the product of those representing the relief and the annual precipitation during heavy rains, the more the clastics are eroded in the watershed and transported into the reservoirs. Furthermore, for the reservoirs with the watershed of which combined factors of relief and annual precipitation during heavy rains is nearly equal, the finer the sizes of the clastics, the more clastics are eroded in the watershed and transported into the reservoir. 5. For each of the several main reservoirs selected from the aforementioned 52, the annual variation of the amount of bottom sediments is little relatad with that of the annual discharge of inflow, whereas it is controlled by that of the yearly maximum flood discharge. Therefore, it is considered that for all the reservoirs which have their own rock distribution and relief, the amount of the clastic sediments transported by the rivers has a close relation with the flood discharge.

Fermeture de la Rance déroulement des travaux et anlyse des observations

1. Leading particulars of the problem. The main characteristics considered for the selected tidal power project site were 1) the area and volume of the estuary 2) maximum and minimum flood and ebb tide discharge and 3) estuary be features. These showed that the damming of the Rance would be a particularly difficult major operation. 2. Progress of the work. The damming scheme basically involved the creation of two areas protected by cofferdams (one, of medium size, being designed to enable the gate sluices to be built and put into service by the earliest possible date), and the gradual damming of the estuary by closing successive gaps between prefabricated piers positioned on the estuary bed. The damming zone was originally 360 m long and required nineteen reinforced concrete caissons and eighteen metal sheet piling cells. Structural stability has to be checked to within an adequate safety margin at each construction stage. 2.1. CAISSONS : The caisson foundations were laid under water with the aid of a pneumatic caisson, the features of which "and associated working conditions are briefly described in the article. The nineteen caissons were placed on their foundations at neap tide by tipping them over into position with the aid of special equipment, sketches of which are included in the article. 2.2 CELLS : After closing the gaps between caissons with special "rut-off boards," the sheet piling rolls were built up around the special jigs shown on the photographs. Various difficulties arose during." the construction work, especially due to rapidly increasing erosion during the final phase. 3. Analysis of observed results. LEVEL DIFFERENCES. - The maximum level differences depended fairly closely on the size of the flow passage and the tide coefficient (see graph 31). They were greater at flood tide than at the ebb and occurred at varying time intervals. The flow capacity of the 8m wide openings due to differences in level exceeded 1,000 cu.m/sec. EROSION. - This became marked at velocities of 6 m/sec. It's development is discussed in the light of Isbah's formula. 4. Purpose and reliability of the scale model. A 1:50 undistorted scale model was used to predict the progress of the damming work and to ascertain the loads acting on the temporary structures. A comparison between conditions predicted experimentally and those observed at the site confirmed the model's complete reliability.

STUDY ON THE PEAK DISCHARGE OF FLOOD IN RIVERS

In this paper, the rational formula which is generally used for the presumption of maximum flood discharge of river in Japan is discussed, and its three insufficient items, namely, the coefficient of runoff, mean hourly rainfall during a time of arrival of flood are studied. And then the study on the reformation of the formula is presented, for the reformation of flood discharge by a few data of rainfall and flood condition. As a result, the revised formula may be more accurate than the original one.