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<br />'e <br /> <br />: <br /> <br />,. <br /> <br />e <br /> <br />. <br /> <br />-. <br /> <br />e <br /> <br />given set of conditions. The need for making these computations may be <br />avoided, however, by computing headwater depths from the charts in this <br />circular for both inlet control. and outlet control. and then using the <br />higher value to indicate the type of control. and to determine the head- <br />water depth. i'his method of determining the type of control. is accurate <br />except for a few cases where the headwater is approximately the same for <br />both types of control. <br /> <br />Both.inlet control and outlet control. types of flow are discussed <br />brief1.y in the following paragraphs and procedures for the use of the <br />charts are given. <br /> <br />Cul.verts F1.owing With Inlet Control <br /> <br />Inlet control means that the discharge capacity of a cul.vert is <br />controlled at the cu1.vert entrance by the depth of headwater (HW) and <br />the entrance geometry, including the. barrel shape and cross-sectional. <br />area, and the type of inlet edge; Sketches of inlet-control flow for <br />both unsubmerged and submerged projecting entrances are shown in fig- <br />ures 1.A and 1.B. Figure lC shows a mitered entrance flowing under a sub- <br />merged condition with 1n1.et control.. <br /> <br />In inlet control. the roughness and length of the cu1.vert barrel <br />and out1.et conditions (including depth of ta11.water) are not factors in <br />determining cul.vert capacity. An increase in barrel slope reduces head- <br />water to a smaJ.1. degree and any correction for slope can be neglected <br />for conventional. or commOnly used cul.verts f1.owing with inlet control.. <br /> <br />In al.1. cul.vert design, headwater or depth of ponding at the en- <br />trance to a cul.vert is an important factor in cu1.vert capacity. The <br />headwater depth (or headwater HW) is the vertical. distance fran the <br />cul.vert invert at the entrance to the energy Hne of the headwater pool <br />(depth + velocity head). Because of the low velocities in most en- <br />trance pools and the difficul. ty in determining the velod ty head for <br />al.1. flows, the water surface and the energy line at the entrance are <br />assumed to be coincident, thus the headwater depths given by the inlet <br />control. charts in this circul.ar can be higher than will occur in sane <br />insta1.1.ations. For the purposes of measuring headwater, the cul.vert <br />invert at the entrance is the low point in the cu1.vert opening at the <br />beginning of the full. cross-section of the cul.vert barrel. <br /> <br />Headwater-discharge relationships for the various types of circu- <br />lar and. pipe-arch cul.verts f1.owing with inlet control are based on <br />laboratory research with models and verified in same instances by pro- <br />totype tests. This research is reported in National. Bureau of Stand- <br />ards Report No. 4444 enti t1.ed "Hydrau1.ic Characteristics of Commonly <br />Used Pipe Entrances", by John L. French and "Hydrau1.ics of Conventional. <br /> <br />5-3 <br />