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<br />il <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Lesson 3 <br /> <br />Culvert Design - Participant Workbook <br /> <br />3.7 HEADWATER AND TAILWATER CONDITIONS <br /> <br />HEADWATERlTAILWATER <br />CONSIDERATIONS <br /> <br />. INCREASED WATER SURFACE PROVIDES <br />NECESSARY ENERGY <br />. HEADWATER (HW) IS THE DEPTH OF WATER <br />AT THE CULVERT ENTRANCE <br />. PONDING WILL OCCUR UPSTREAM OF A <br />CULVERT AND MAY ATTENUATE FLOOD PEAK <br />. TAILWATER (TW) IS DEPTH AT THE CULVERT <br />OUTLET, MEASURED FROM THE INVERT <br />. TAILWATER DEFINED BY DOWNSTREAM <br />CHANNEL CONDITIONS <br /> <br />3.25 Headwater and Tailwater <br /> <br />A. Energy is required to force flow through the constricted opening <br />represented by a culvert. This energy occurs as an increased water <br />surface elevation on the upstream side of the culvert. <br /> <br />B. The headwater depth (HW) is defined as the depth of water at culvert <br />entrance. <br /> <br />C. In areas with flat ground slope or high fills a considerable amount of <br />ponding may occur upstream of culvert. If significant, this ponding can <br />attenuate flood peaks and could justify a reduction in the required culvert <br />size, but this is seldom applied in practice. <br /> <br />D. Tailwater is defined as the depth of water downstream of the culvert, <br />measured from the outlet invert. Tailwater is an important factor in <br />determining culvert capacity under outlet control conditions. <br /> <br />E. Tailwater conditions are most accurately estimated by water surface <br />profile analysis of the downstream channel; however, tailwater conditions <br />may be estimated by normal depth approximations. <br /> <br />3-11 <br />