Laserfiche WebLink
<br />This Circular incorporates the results of the NBS research <br />on imp~oved inlets into a new culvert design procedure. The <br />research demonstrated that improved inlets, with their more <br />efficient flow characteristics and better utilization of <br />available head, may greatly improve the performance of culverts <br />operating in inlet control. Use of the design procedure of <br />Section VI will result in the inlet design and barrel size <br />most appropriate for a given combination of site characteristics. <br /> <br />While many improved inlet configurations were tested in <br />the research, only those determined to best satisfy the criteria <br />of hydraulic efficiency, economy of materials, simplicity of <br />construction, and minimization of maintenance problems are <br />presented. For example, while the use of curved surfaces rather <br />than plane surfaces might result in slightly improved hydraulic <br />efficiency at times, it was decided that the advantages were <br />outweighed by the construction difficulties involved. Thus, <br />only plane surfaces are discussed and recommended. <br /> <br />The improved inlet design charts of this publication <br />apply only to rectangular or circular barrel shapes. No other <br />barrel shapes were tested with improved inlets, and different <br />coefficients and curves would be necessary. However, identical <br />concepts are applicable to barrels of any shape. <br /> <br />As in previous FHWA publications, the design procedures contained <br />herein are based on the philosophy of "minimum performance." At times, <br />favorable hydraulic conditions will cause a culvert to operate at a <br />greater capaCity than the design would indicate. Some of these <br />favorable conditions are transient and cannot be depended upon to <br />operate continuously; thus, their precise analysis is not warranted. <br />For instance, approach velocity is neglected, as are possible negative <br />pressures within the culvert barrel, both of which would result in <br />lower headwater requirements to pass a given discharge. <br /> <br />If inlet control governs, inlet improvements can result in the <br />need for a barrel size smaller than would be required for a.conventional <br />culvert at the same site. The amount of barrel size reduction depends <br />on the site and a subjective judgment regarding the dependability of <br />the design flood estimate and the risk of damage inherent in exceeding <br />the allowable headwater elevation. If the design discharge estimate is <br />not well supported and considerable damage would result if the allowable <br />headwater elevation were exceeded, it may be wise to select a culvert <br />barrel somewhat larger than would be required to accommodate the design <br />discharge. On the other hand, if the design discharge estimate is <br />liberal or well supported by data and analysis or a headwater elevation <br />higher than the allowable would result in little or no damage to the <br />highway or the adjacent property, then the smallest possible barrel <br />size might be selected. Design techniques presented in this Circular <br />will enable the designer to evaluate the hydraulic variables and select <br />the most rational design for the particular site. <br /> <br />13-2 <br /> <br />It <br /> <br />'" <br /> <br />. <br /> <br />It <br /> <br />" <br /> <br />'" <br /> <br />It <br />