Sill Height Research Articles

Present situation and cause analysis of diversion dam failures in Japan

Diversion dams are settled in river channels to dam water. They obstruct flood flow and also affect sediment movement on riverbeds. Dam structures break due to local scouring of the riverbed caused by local flow around them. The causes of such failures should be studied by investigating how flow meanders over alternate bars during floods. Diversion dam failures are classified into three types according to cause. The first type of problem is caused by the difference between the riverbed height and the sill height. If the present riverbed is higher than the sill, the mobile gates do not close easily due to sand deposition on the sill. If the riverbed is lower than the sill, the riverbed in the downstream area of the weir is scoured, and the weir may be damaged. The second type is caused by the relation of the weir position to the alternate channel bars. The bar area is usually higher than the sill, and sand deposition on the sill causes problems for mobile gate operation. The riverbed below the weir at the pool section is scoured because the bed height is lower than the sill height. The third type is caused by the migration of alternate bars. A low water course changes due to migration of alternate bars after a flood. The sand deposition area from the upstream reach blocks the intake entrance. Before a diversion dam structure is designed, the situation and the probable changes in riverbed form should be investigated.

Protection of bridge piers using a sacrificial sill

This experimental study explores the effectiveness of using a sacrificial sill as a pier scour countermeasure. Experiments were conducted under both clear-water and live-bed conditions. Many interdependent parameters affecting the sill performance were systematically examined. The results show that the sill is efficient in reducing the equilibrium clear-water scour depth, reaching a maximum percentage reduction of pier scour depth, %RDP, of approximately 50% under the most favourable condition. The optimum state occurs under the following conditions: (a) the sill is vertical; (b) the sill height is larger than 0·3 times the depth of flow; (c) the clear distance between the pier and sill is 4·4 times the pier width; and (d) the angle of attack is zero. A large angle of attack or flow skewness is detrimental to the performance of the sill. However, this shortcoming may be overcome by installing a sill with a wedge angle of 270°. The efficiency of the sill decreases under live-bed conditions, with the %RDP reducing to approximately 20% at a velocity ratio, U/Uc = 2·6.

Protection of bridge piers using a sacrificial sill

This experimental study explores the effectiveness of using a sacrificial sill as a pier scour countermeasure. Experiments were conducted under both clear-water and live-bed conditions. Many interdependent parameters affecting the sill performance were systematically examined. The results show that the sill is efficient in reducing the equilibrium clear-water scour depth, reaching a maximum percentage reduction of pier scour depth, %RDP, of approximately 50% under the most favourable condition. The optimum state occurs under the following conditions: (a) the sill is vertical; (b) the sill height is larger than 0·3 times the depth of flow; (c) the clear distance between the pier and sill is 4·4 times the pier width; and (d) the angle of attack is zero. A large angle of attack or flow skewness is detrimental to the performance of the sill. However, this shortcoming may be overcome by installing a sill with a wedge angle of 270°. The efficiency of the sill decreases under live-bed conditions, with the %RDP reducing to approximately 20% at a velocity ratio, U/Uc = 2·6.

Protection of bridge piers using a sacrificial sill

This experimental study explores the effectiveness of using a sacrificial sill as a pier scour countermeasure. Experiments were conducted under both clear-water and live-bed conditions. Many interdependent parameters affecting the sill performance were systematically examined. The results show that the sill is efficient in reducing the equilibrium clear-water scour depth, reaching a maximum percentage reduction of pier scour depth, %RDP, of approximately 50% under the most favourable condition. The optimum state occurs under the following conditions: (a) the sill is vertical; (b) the sill height is larger than 0·3 times the depth of flow; (c) the clear distance between the pier and sill is 4·4 times the pier width; and (d) the angle of attack is zero. A large angle of attack or flow skewness is detrimental to the performance of the sill. However, this shortcoming may be overcome by installing a sill with a wedge angle of 270°. The efficiency of the sill decreases under live-bed conditions, with the %RDP reducing to approximately 20% at a velocity ratio, U/Uc = 2·6.

Ressaut contrôléar seuil dans un canal profilé en U

The control of a hydraulic jump by both thin-crested and broad-crested sills in a U-shaped channel is examined theoretically and experimentally. Under certain simplifications, the theoretical development results in explicit relations for the relative sill height, by including the effect of approach velocity. This also leads to a similar condition for the control by a sill in the rectangular channel, by extension of the Forster and Skrinde approach. A particular study is presented to define the conditions required for the minimum height of a broad-crested sill. Finally, the relative length of jump is quantified by accounting for the relative depth of the approaching flow.

Spatial and temporal patterns of light attenuation among lakes of the Mackenzie Delta

SUMMARY 1. The seasonal dynamics of light attenuation, and the relative roles of total suspended solids (TSS), dissolved organic carbon (DOC) and chlorophyll as light attenuators among two sets of lakes in the Mackenzie Delta, were assessed during the open‐water periods of 1998 and 1999.2. The first set consisted of 40 spatially discrete lakes where the frequency of flooding with river water was controlled by sill height (‘sill‐set lakes’). The second set consisted of a chain of six lakes connected to a main river channel (frequently flooded, all with same frequency), but where riverine influence was controlled by the distance from the channel connection point (‘chain‐set lakes’).3. As the flooding frequency of lakes decreased (sill‐set), and as the distance from the channel connection point increased (chain‐set), lake water became increasingly transparent and the stability (decreasing temporal variability) of underwater light increased.4. The effect of flooding on transparency was greater in years with a high minimum summer water level. However, the effect of river flooding on lake water transparency was damped more by an increase in the frequency and duration of flooding than by an increase in distance from the channel connection point.5. The index of scattering was linearly related to TSS over the common range of concentrations in both sets of lakes. The specific attenuation coefficient for TSS (and scattering) increased substantially from the most turbid to the most transparent waters.6. During the summer, DOC provided an approximate index of water colour in the sill‐set lakes but not in the chain‐set lakes, where the gradient of DOC ran counter to the gradient of water colour. The specific attenuation coefficient for water colour was roughly constant among both sets of lakes.7. Calculations of partial attenuation show that, during the spring flood peak, TSS is the dominant attenuator among most lakes, other than those with high sills or positioned far from channel connection points. During the lengthy summer period of open water, however, water colour appeared to be the most important light attenuator among almost all of the lakes in the central delta, with chlorophyll a of only minor importance.8. Lakes of the Mackenzie Delta may be quite sensitive to changes in climate and ultraviolet‐b (UV‐b) radiation in the circumpolar arctic because of the role of DOC as an attenuator of photosynthetically active radiation and UV‐b irradiance and as an energy source for microbial foodwebs in this system.

The influence of water transparency on the distribution and abundance of macrophytes among lakes of the Mackenzie Delta, Western Canadian Arctic

SUMMARY 1. Macrophyte abundance and distribution was assessed in a chain of six interconnected lakes (all with the same flooding frequency) in the Arctic, where increasing distance from the Mackenzie River channel resulted in a gradient of water transparency (‘chain‐set’ lakes), and in a group of 26 spatially discrete lakes where increasing frequency and duration of lake flooding with river water (controlled by sill height) also resulted in a transparency gradient (‘sill‐set’ lakes).2. Among the chain‐set lakes, above‐ground macrophyte biomass increased from 0 to 1000 g m−2 with increasing water transparency. Among the sill‐set lakes, the transparency gradient among the lakes was less well defined and the relations with biomass were more varied. A decrease in flooding was associated with increasing water transparency and an increasing biomass of macrophytes from about 0 to over 2000 g m−2. For a specific flood frequency, however, the effect of flooding was much greater when lakes were directly connected to a river channel than when floodwaters flowed first through an intervening lake. Among infrequently flooded lakes the effect of flooding on water transparency and biomass was negligible.3. Among relatively clear lakes in both sets of lakes, biomass increased with increasing water transparency and decreasing lake depth. Among relatively turbid lakes, however, biomass increased with the combined effect of increasing water colour (decreasing water transparency) and increasing lake water depth. The increases in biomass with increasing water colour (coloured dissolved organic matter) and increasing depth, which together result in reduced light at the bed, may be explained by reduced exposure to ultra violet light.4. An average light attenuation of 1.3 m−1 (Secchi depth about 1 m) over the growing season appears to represent a threshold water transparency which, in combination with water depths early in the growing season, is consistent with a light supply on the bed required for growth of the common macrophytes in lakes of the Mackenzie Delta. However, a comparison with other systems indicates that macrophytes among lakes of the Mackenzie Delta grow deeper, for a given level of transparency, than is reported in lakes at lower latitude, despite the lower sun angles and increased reflectivity of water surfaces in the arctic.5. A complete accounting of water transparency (at PAR and UV wavelengths), lake depth, summer sun angle and duration of sunlight may be necessary to explain patterns of macrophyte growth among lakes across a full range of latitudes.

Real Scale Investigation Of Interaction BetweenA Supercritical Flow And A Bottom Sill.1: Physical Aspects And Time-averagedPressures On Sill

Results of field scale experiments concerning the interactions between a supercritical flow and a vertical bottom sill are presented. Experiments are performed in a channel (width B = 5.01 m) downstream of the weir of Sernio (Adda River, Northern Italy) using a vertical sill with adjustable height s (0.5 m, 0.6 m and 0.7 m). Significant cinematic and geometric parameters of flow have been measured for different discharge regimes. The fluctuating local pressure on the upstream face of the sill have been recorded by means of sensors, connected to a digital acquisition system. The experimental tests show that, in accordance with previous laboratory experiments of Guadagnini, Larcan, Orsi [l], the type of forced hydraulic jump is a function of both the supercritical flow Froude number Fr, and the ratio, s/hl, between sill height S and depth of approaching supercritical flow hl. Classification of forced jump, derived under laboratory conditions, is also recognized at the field scale.Due to scale effects, some differences are recognized and discussed.

Coefficient of Discharge for Free Flow Sluice Gates

This paper aims to investigate the effect of sill shape located under the sluice gate on the coefficient of discharge for free gate flow conditions. Both sill shape and sill height are studied. Shapes tested include both polygonal and non polygonal sills. Of the shapes tested, circular sill is the most effective shape, and triangular sill is the best among the polygonal sills. Besides the shape, sill height plays an important role on the coefficient of discharge. Using dimensional analysis and non linear regression analysis, an equation is developed to predict the coefficient of discharge for both silled and non silled free gate flows. The equation includes flow parameters, gate opening, sill height, and sill shape factors. The equation developed agrees well with the experimental results as well as published data of other investigators.

Splitting of a free jet flowing over a saddle sill

If a steady jet with two free streamlines ascends a sill while entirely remaining on one sloping side, then the rising motions must be compensated by transverse sinking motions in order that the cross‐stream integral of relative vorticity vanishes at all downstream sections. The maximum sill height which allows such a flow is computed assuming slow downstream changes in a quasi‐geostrophic barotropic model; a 1½ layer baroclinic model is also considered. If the critical sill height is exceeded, it is suggested that the jet will split with only part of it crossing the sill on the original side. This might explain why the Antarctic Circumpolar Current bifurcates at the Falkland Plateau, where one branch forms the Malvinas Current and the other branch turns eastward across the Atlantic. The theory is related to previous laboratory observations in which bifurcation occurs.

Incipient Jump Conditions for Flows over a Vertical Sill

The incipient jump condition is defined as the critical condition for the formation of a surface roller immediately upstream of a sill. The incipient jump conditions can be classified in two categories. In one case, the incipient jump occurs by increasing the sill height from the state of the splashing flow. In the other case, raising the tailwater level from the splashing flow condition yields the incipient jump. The mechanisms for both cases of the incipient jump conditions are explained. Also, the effect of the channel width on the incipient jump condition is discussed. The application of the momentum equation yields a prediction for each incipient jump condition. For wide channels, in which two-dimensional breaking occurs, the agreement between the predicted and experimental results is satisfactory. The formation of a forced jump is required for the design of a stilling basin.

The morphology of the nostril sill

The nostril sill is a structure which has been overlooked in both anatomical and surgical texts. An understanding of the normal anatomy will help in reconstruction of the nostril both in congenital and acquired conditions, including cleft lip. In order to investigate this feature 100 normal Caucasian adults, with equal sex incidence, have been assessed by direct measurement and photography. Three major variants of form were identified with significant differences in sill height and angulation seen. A true nostril sill was found in only 62% of the study population and sex differences were noted among the variants. A relationship was noted between each nostril sill variant and nostril type. The implications for reconstruction are discussed.

Rectangular Lateral Weirs in Circular Open Channels

A theoretical expression for the outflow through a rectangular lateral weir located in a circular open channel is derived using the results of an existing theory of lateral efflux through a two-dimensional conduit outlet. The development of the present model is similar to a procedure that earlier yielded the theoretical expression for the outflow through a rectangular lateral weir located in a rectangular open channel, on the basis of the free streamline theory. In the present case, the theoretical weir discharge coefficient is expressed as a function of parameters relating the geometry of the rectangular lateral weir and the circular open channel and a velocity parameter, which is a function of the dimensionless flow depth, the dimensionless weir sill height, and the Froude number of the approach flow. Experimental results are presented to verify the proposed model.

Discharge Coefficient of Rectangular Side Weirs

The characteristics of flow over side weirs are taken into account to determine the discharge coefficient for subcritical flow conditions under the assumption of constant‐specific energy. The main‐channel discharge, length of the weir crest, and sill height of the weir are treated as the controlled variables. The discharge coefficient Cd is found to depend on the upstream Froude number F1 and, contrary to earlier findings, also on the ratio of sill height to upstream flow depth s/y1. Multiple regression analysis is conducted to establish the relationship of Cd with both F1 and s/y1 taken together. The relationship is of linear form and provides an easy means of estimating Cd.

Rectangular Measuring Flumes for Lined and Earthen Channels

Good water management in irrigated agriculture requires that irrigation water be accurately measured. Presented herein are design procedures and rating tables for modified broad‐crested weirs (which act hydraulically as long‐throated flumes) that have rectangular control sections. The ratings are presented in terms of discharge per unit width so that rectangular broad‐crested weirs of a variety of sizes can be used. This makes it easier for the designer to adapt the flume to the site rather than vice versa. These weirs or flumes can be placed in lined and unlined canals of any shape. The rating tables are presented for rectangular lined canals with given values of sill or crest height. An adjustment procedure is given to develop rating tables for other approach area and sill height combinations. The head loss across the weir required to maintain a unique head discharge relationship is also presented. Finally, a design example is given for a rectangular lined canal. With this information, a rectangular measuring flume can be designed for a wide variety of site conditions.

Fluid dynamic implications for massive sulphide deposits of hot saline fluid flowing into a submarine depression from below

Some massive sulphide deposits are thought to have formed from dense hot saline brine solutions that have ponded in a local depression of the sea floor. After the source has been flowing for some time the level of the warm fluid in the depression reaches the height of a weir (or sill) and thereafter the ponded fluid spills over the weir at a flow rate equal to that of the inflowing source fluid. A double-diffusive fluid interface forms at the height of the weir and a steady-state balance is achieved where the fluxes of heat and salt across the ‘diffusive’ interface are equal to the corresponding advective fluxes due to the source fluid being warmer and more saline than the fluid that spills over the weir. In this paper we consider the case where the source is below the height of the sill. The salient feature of our fluid dynamical model is that it provides a mechanism whereby the temperature of the fluid in the depression may be substantially reduced (by double diffusion across the sharp fluid interface at the sill height), but the salinity of the fluid is only slightly less than that of the source fluid. Both the low temperature and high salt concentrations are conducive to the precipitation of sulphide sediments. Another important result is that the source fluid need not be denser than the surrounding seawater for the occurrence of ponding in the depression. In fact, taking a temperature difference of 300°C between the source fluid and seawater, we show that a salinity excess in the source of just 5 × 10 −3 (or just 0.1 M NaCl) will enabble a ponded layer of fluid to exist in the vicinity of the vent and so help to explain the confined lens-like property of a massive sulphide deposit. We also discuss application to the Red Sea deep brine pools and develop a model of a diffusive thermohaline interface that explains how the interfaces in the Red Sea persist in a quasi-steady state with quite large values of the stability ratio R p .

Supercritical Flow Over Sills

Experimental investigations have been carried out to study the supercritical flow over a square section sill for all flow situations, except the jump condition. The supercritical flow has been categorized into two distinct types depending upon the ratio of the sill height to upstream depth and the Froude number. This classification also corresponds to the splashing and nonsplashing conditions. The stagnation, wake, and reattachment pressures have been investigated. The flow conditions have been explored when the downstream face pressures and the bed pressures become negative. An almost constant drag coefficient value, obtained from the present experiments, substantiated the drag characteristics of bluff bodies in open channels.

Supercritical Flow over Sills at Incipient Jump Conditions

A laboratory investigation has been conducted to study the conditions mainly responsible for the stabilization and prediction of the hydraulic jump in a rectangular, horizontal-bed channel with a square-shaped bottom sill at the incipient jump condition (i.e., at the verge of formation of the jump, however, without it actually forming). A theoretical analysis based upon the momentum and continuity principles has been carried out. A formula for the design of the sills, when the ratio of the sill height to the upstream depth is smaller than 1.25, is suggested. A relationship between the drag experienced by the sill and the upstream Froude number is also given. The theoretical formulation has been checked by laboratory experimentation.