Outlet Structure Research Articles

Designing outlet characteristics for optimal wetland performance

Central to the hydrological and botanical design of constructed wetlands for stormwater quality management is the proper control of their hydrologic regime. The hydrologic regime, defined as the probabilistic distribution of inundation depth in the wetland, reflects the inherent variability of storm inflows to the wetland and is directly influenced by the discharge characteristics of the outlet structure of the wetland. The influence of three typical outlet structures on the hydrologie regime of a hypothetical wetland was investigated using a continuous simulation approach. The three outlet types investigated were that of a culvert, a riser and a siphon. Simulations found that all three devices provides equally effective control of the ability of the wetland to capture and detain storm runoff but can have different influence on the hydrologie regime of the wetland. The siphon-controlled wetland was found to exhibit a more even distribution of inundation depth compared to the other two outlet types.

BOD5 removal in facultative ponds: experience in Tanzania

This paper discusses the removal of BOD5 in facultative ponds under tropical conditions. Data was collected from pilot and field ponds at the University of Dar es Salaam and was compared to other ponds in Dar es Salaam. Results showed deviations of performance, in some cases, from assumptions made during the design. The permissible organic loading rate for ponds in Dar es Salaam was 450 kg BOD5/ha/d. To reduce concentration of algae in the final effluent, a sunken outlet structure was suggested.

BOD5 removal in facultative ponds: Experience in Tanzania

This paper discusses the removal of BOD5 in facultative ponds under tropical conditions. Data was collected from pilot and field ponds at the University of Dar es Salaam and was compared to other ponds in Dar es Salaam. Results showed deviations of performance, in some cases, from assumptions made during the design. The permissible organic loading rate for ponds in Dar es Salaam was 450 kg BOD5/ha/d. To reduce concentration of algae in the final effluent, a sunken outlet structure was suggested.

Hydrologic design of a wetland: advantages of continuous modeling

A continuous hydrologic model for constructed or depressional wetlands, intended as a design tool to supplement event-based hydrologic methods, uses reservoir routing methods and is driven by daily rainfall and watershed inflows. Simulated daily hydrology provides annual and monthly water balances, hydroperiod distributions, flood frequency distributions, soil exceedance values for hydrologic plant suitability and retention time distributions. The model was applied to a standard stormwater wetland design. The hypothetical wetland performed as designed, providing good flood reduction and an adequate supply of water even in dry years. The analysis of drawdown regimes showed that for this wetland-watershed system, the water supply was so reliable from the watershed (423 cm/year ± 130 cm/year), that, without water level management, 40% of the wetland would likely develop into an open-water pond with low species diversity and an additional 40% would only support late-season emergents. Retention time distributions showed that an orifice outlet design would retain and provide longer treatment (98% of all runoff stays > 9 days) than would a weir outlet structure (60% of all runoff stays > 9 days).

Prefabricated drop spillways for outlets of reclaimed gullied lands

A concept was developed of prefabricated drop spillways for outlet structure of reclaimed gullies. The prefabricated drop spillways were designed, successfully constructed in reinforced cement concrete of 50 mm thick slabs, installed and tested under field conditions. The prefabricated structure efficiently handled peak discharges up to 60% of its designed capacity. The corresponding peak dischange 0.116 m 3/s was produced by 72 mm storm in 1990 which occurred with the highest rainfall intensity of 80 mm/h in the season. The prefabricated spillway of 0.20 m 3/s discharge capacity and 800 mm vertical fall (costing Rs. 1900/unit) was 23% cheaper than in situ constructed conventional structure of brick, sand and cement of equivalent size (costing Rs. 2450/unit). Use of prefabricated spillways for individual terraces is likely to reduce cost on account of outlet construction by Rs. 200 per terrace. Further, by the technology of prefabrication it is possible to construct outlets with the construction of terraces simultaneously. The simultaneous construction of outlets with land development saves fields from reverting back to gully erosion.

Runoff Detention for Flood Volume or Erosion Control

A simple design aid is presented for quick preliminary sizing of flood‐control detention basins and outlet structures. The design objective is to control the postdevelopment flood volume rather than the peak discharge. Alternatively, the method can be used to control downstream channel erosion caused by post‐development flows. The detention basins considered have a single outlet, which can be of either orifice or weir type. The design aid is based on the predetermined solutions to the reservoir‐routing equation. The hydrologic storage equation is used for this purpose. To evaluate the channel‐erosion tendencies, a generic bed‐load formula is employed. All the equations are written in dimensionless form, and the solutions are obtained in terms of a set of governing dimensionless parameters. The results are generalized using the concept of hydrologic similarity, and they are presented in chart form. These charts can be used to determine the required stage‐storage relationship for a detention basin and the size of the outlet structure. Sample applications are included to illustrate the use of the charts.

Storm Runoff Detention for Pollutant Removal

A design aid is presented to size detention basins and outlet facilities for removal of particulate pollutants from storm runoff. The single‐design‐event approach is employed. The basins that are sized using the proposed method detain the runoff for a specified period of time to achieve the desired level of treatment. At the same time, the basins empty within a specified time period before the next design runoff event occurs. The method is based on the reservoir‐routing technique, which couples the hydrologic storage equation with an expression describing the hydraulics of the outlet structure. Orifice‐ and weir‐type outlet structures are considered. The equations are written in dimensionless form and solved using a finite‐difference computer model. The numerical results obtained in terms of the governing dimensionless parameters represent predetermined solutions to the reservoir‐routing problem. The results are generalized based on the concept of hydrologic similarity, and presented in chart form. As illustrated through a sample application, these pond charts are useful in determining the stage‐storage relationship for the basin, the size of the outlet structure, and the required storage volume.

Mitigating the impacts of stream and lake regulation in the flathead river catchment, Montana, USA: An ecosystem perspective

Abstract1. Seventy‐two per cent of the Flathead River catchment (22 241 km2) is federally designated and protected as wilderness or national park. Thus, the catchment remains one of the more pristine areas of its size in the temperate latitudes of the world.2. Discharge in the downstream reaches of the river system outside the protected areas is regulated by three dams for flood control and hydropower production. These dams have blocked natural migration of native fish from Flathead Lake (496 km2) and isolated populations in sub‐catchments. Temperature and erratic flow fluctuations have altered phenologies of river zoobenthos and fish, and in dam tailwaters aquatic biodiversity is drastically reduced in comparison to unregulated segments.3. Ecological problems caused by changing water quality conditions, altered land‐use patterns and introductions of non‐native biota are interactive with the impacts of stream and lake level regulation, thereby emphasizing the complexity of this river–lake ecosystem.4. Mitigation of the effects of regulation is compromised by differing management priorities and regulatory mandates of County, State, Tribal, and Federal agencies responsible for natural resource management within the catchment. Moreover, economic and ecological interests outside the Flathead influence the way flows are regulated within the catchment.5. The most pervasive influences of stream and lake regulation can be ameliorated by retrofitting the hypolimnial release dam with a selective depth outlet structure to allow temperature control, and by controlling changes in flow rates to create a more natural hydrograph in the tailwaters of the large dams. Allowing fish passage by construction of fish ladders is problematic because upstream passage will commingle native species that were isolated upstream by construction of the dams with non‐native species that were introduced subsequently below the dams. Cascading food web interactions elicited by invasions of non‐native biota may offset any advantage to native stocks gained by passage and/or augmentation with hatchery stocks.6. Mitigation must be adaptive in the sense that unanticipated effects and interactions with other management objectives can be documented and alternative action can be implemented.7. This case history of the effects of stream and lake level regulation, and the approaches to management reviewed in this paper, should serve as a lesson in river conservation.

HGL Elevation at Pipe Exit of USBR Type VI Impact Basin

Model tests on a Soil Conservation Service principal spillway with a U.S. Bureau of Reclamation Type VI impact basin for the outlet structure provided an opportunity to establish a relationship to predict the hydraulic grade line (HGL) elevation at the pipe exit as a function of the tailwater (TW) elevation. The relationship is \IHGL\N/\ID\N = 0.85 + 0.76(\ITW\N/\ID\N); \ITW\N/\ID\N > 0.65 or \IHGL\N/\ID\N > = 1.31, where \IHGL\N is the HGL elevation relative to the pipe-exit invert; \ID\N is the pipe diameter; and \ITW\N is the tailwater elevation downstream of the basin relative to the pipe-exit invert. This relationship permits accurate determination of the head for flow computations. The equation is only valid at design discharge for impact basins designed according to \ITechnical Release No. 49,\N U.S. Dept. of Agriculture, Soil Conservation Service. Because the results showed that a \ITW\N/\ID\N = 0.65 had no adverse effect on the HGL elevation, a minimum \ITW\N/\ID\N = 0.65 is recommended for design because the tailwater reduces water surface roughness and bed erosion.

Model Study for Non-Standard Principal Spillway

Model tests were conducted on a non-standard principal spillway to evaluate the performance of the proposed drop inlet and impact basin and to evaluate the riprap size and placement downstream of the impact basin. An undistorted 1:24 model scale was used. The prototype pipe was 2.13 m (84 in.) in diameter. The outlet structure was a USBR Type VI impact basin. The design discharge was 43.9 m/s (1550 cfs). The spillway was a 1 3, two-stage, covered drop inlet with baffle trash racks and four low-stage orifices that occupied half the length of each side. The proposed design performed poorly with flow in the orifices. The trash rack was modified, performed very well, and was recommended for design. With the drop inlet in a berm [with the berm 0.610 m (2.0 ft) below the orifice invert elevations] the entrance loss coefficients were 0.28 and 0.26, respectively, with and without orifices. Locating the drop inlet in a berm will require riprap around the drop inlet entrance to prevent erosion of the berm. The outlet impact basin performed very well. There was minor scour over a considerable area downstream of the basin, but the riprap design is adequate.

Single‐Outlet Detention‐Pond Analysis and Design

A desktop method is presented to analyze or size detention ponds and outlet facilities. Different initial conditions are allowed. When used for design purposes, the method determines the size of the outlet facility for a given pond. However, if the pond is inadequate to meet the design criteria set, the method determines the required stage‐storage relationship. The method is based on the usual reservoir‐routing technique that couples the hydrologic‐storage equation with an expression describing the flow through the outlet structure. The equations are rearranged in terms of several dimensionless parameters. A computer model is used to solve the governing equation using a finite difference scheme. The numerical results obtained in terms of systematically chosen combinations of the governing dimensionless parameters are generalized based on the concept of similarity. The generalized results are presented in chart form. Sample applications of these charts are illustrated.

Outlet Structure Design for Conduits and Tunnels

This paper describes the analysis and experimental work leading to the development of design criteria for an outlet structure. The recommended design consists of a transition and stilling basin with straight diverging sidewalls. The rate of sidewall divergence is expressed as a function of the width ratio and Froude number. Design values were determined from model tests carried out for both round and square conduits. Design criteria were based upon observations on the effective position of the hydraulic grade line at the outlet, head losses in the transition, depth and velocity distribution in the transition, stability of the hydraulic jump, downstream velocity distribution and scour tendencies in the discharge channel. A major portion of the paper is devoted to details of the recommended design procedure, including the rationale behind the various recommendations. An example is worked to illustrate the procedure, and the performance data for one of the test models is included.

Damming the Naryn River channel at the Tashkumyr hydroelectric station

The experience of damming the mountain river Naryn at the site of the Tashkumyr hydrostation under conditions of a narrow rock canyon with an unusually high location of the inlet sill of the water outlet structure in the presence of considerable drops on the embankment, exceeding 10 m, permits making the following conclusions: 1. Under the confined conditions of a complex mountain relief with a limited length of the damming stretch related to the layout of the structure hampering the organization of additional accesses to the river, the pioneer one-embankment method with dumping of the material into the water primarily from two banks is recommended as the main method for damming the channel of a mountain river. This method requires, in comparison with the two-embankment, smaller expenditures of means and times for preparation and conduction of works on damming and provides a reliable embankment even with unusually large drops. 2. When reaching the maximum unit power of the flow in the gap it is quite effective to use an upstream hydro development (hydrostation) for reducing the discharges to minimum values. Temporary disconnection of the hydrostation for reducing the discharges being released and, as a consequence, reducing the volume of damming material and shortening the time of conducting damming works, as a rule, is always justified. 3. Prototype investigations established that the method of laboratory investigations of damming in the presence of considerable drops on the embankment, just as for small drops, is completely reliable and allows with sufficient accuracy hydraulic substantiation and selection of a reliable and economical damming method with determination of the quantity of damming material.

Measurement of water discharge in short sections of conduits by means of orifices

This paper describes a method and instrumentation for measuring the flow rate and volume of water in the concrete outlet structure of a hydroelectric power reservoir. Pressure measurements are taken through a series of holes in the pipe. The method is free of the disadvantages normally associated with measuring the sediment-laden flow for which the structure is designed. The metering system is designed to work in conjunction with a flow regulator to control the water level of the reservoir and also to clear sedimentation from behind the dam.

Zum Austritt der Chorda tympani aus der Basis cranii externa

Current textbooks of anatomy consider the chorda tympani to leave the external surface of the base of the skull through the petrotympanic fissure (of Glaser). In contrast to this view, Krause in 1905 described the sphenopetrosal fissure as outlet structure. The present reexamination of the extracranial course of 33 chordae tympani confirms the latter conception. Thus, the prevailing opinion has to be revised. Clinical impacts are discussed.

MULTI-STAGE OUTLET DESIGN OF STORMWATER RETENTION FACILITIES

Current strategies used in the design of stormwater retention facilities, as determined from a survey of consultants, are described. The effectiveness of these design strategies in meeting stated runoff quantity objectives are evaluated. The limitations of the current procedures are demonstrated and an alternate design method is proposed. The alternate method considers both a high and low frequency event for the design of the outlet structure, and is capable of meeting design objectives more closely.

Design Criteria for Cooling-Water Outlet Structures

This paper describes some laboratory studies utilizing buoyant and nonbuoyant tracers to investigate the effects of the initial conditions on the lateral mixing of cooling water or wastewater discharges into rivers. The paper begins with a short phenomenological description of the dispersion of cooling-water side discharges into rivers from which a concept for quantifying the effect of the outfall parameters on the near-field mixing is then deduced. By means of a virtual-source concept and two bulk parameters describing the stability of the near-field density stratification and the strength of an assumed reference source, the results can be presented in a straightforward graphical form. These graphs provide a first estimate of design criteria for an outlet structure which, e.g., is to be designed to maximize the rate of near-field mixing for a given cooling-water discharge.

Les ouvrages de prise et de rejets de la centrale nucléaire de Gravelines

Click to increase image sizeClick to decrease image sizeGravelines power station is situated on the coast just beyond the western end of the new outer harbour. Gravelines itself lies on the North Sea between Dunkirk and Calais. The power station will eventually consist of four 900 MW pressurized-water and four 1,350 MW generating plants. The total once-through cooling-water requirement will be 400 m3/s. Water inlet and reject structure layout was dictated in large measure by the presence of the new outer harbour. The structureses were designed by the Laboratoire National d'Hydraulique in collaboration with the Dunkirk Harbour Authority. The intake structure was sited at the edge of the outer harbour so that deep water not exposed to large waves would be available close by. Design was based on model tests, the principal objective being to minimize currents in the immediate vicinity of the oil tanker jetties. The outlet structure backs up against the breakwater of the outer harbour and discharges off the beach by the generating plant. Reject at this point minimizes recycle from outlet to inlet but exposes the structure to waves. The outlet structure was also studied on a scale model. The design of the bank protection is similar to that chosen by the Dunkirk Harbour Authority for the new outer harbour, viz. (a) rockfill laid on fascines and polypropylene fabric and (b) superstructures faced with bituminous concrete. The floor protection will be reinforced or repaired as required by erosion and the commissioning of new plant. Inlet and outlet structures are linked to the power station by canals. The canal floors consist of rockfill. Walls are constituted by rockfill embankment, sheet-piling or concrete. The second part of the article deals with intakes and outlets located on beaches not comprising any kind of harbour installation. Theoretical studies were undertaken to identify valid designs for the cooling water structures required by power stations near beaches similar to the one at Gravelines, i.e. without breakwaters, etc. Two schemes appeared feasible. The first, based on Gravelines, consists of two "artificial harbours", one each for the intake and outlet. This keeps the structures separate and enables water to be pumped from calm, offshore water. Cooling water in the second scheme is withdrawn via one or more towers a considerable distance out to sea. It is then fed to the power station through an underwater tunnel system. The reject structure is similar to the Gravelines outlet.

Modified USBR Impact Basin

This note presents details of design modifications to a U.S. Bureau of Reclamation impact basin. These modifications to the basin are intended for its possible application as a canal structure. The pipe outlet structure’s original design is presented as well as the modified design. The point velocities of each scenario at the basin’s exit are compared.

Soil and Geologic Engineering Investigation, Design, and Construction of San Joaquin Reservoir

This article discusses the San Joaquin Reservoir project in Orange County, California. The principal features of the project are a compacted earth‐sedimentary rock fill dam, a fully lined reservoir, an emergency spillway system, a storm water diversion system, an inlet pipeline, and an outlet structure and pipeline. In order to line the reservoir, the natural canyon had to be shaped using either cut or fill over most of the reservoir area. The unified‐design concept used in this project was the long‐term monitoring of the embankment instruments. In addition to their use during construction, these instruments provide a basis for determining the postconstruction and operational seepage pressures at various locations within the dam and reservoir lining, as well as the project settlement characteristics. This ability to monitor dam performance, so as to detect the occurrence of any unforeseen major embankment problem, allows the project design to operate on a more realistic and economical basis.