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<br />BASIC DATA REQUIREMENTS <br /> <br />1 . GENE RAL <br /> <br />A major portion of the programming in HEC-2 is devoted to providing <br />a large variety of input and data manipulation options. The program <br />objective is quite simple -- compute water surface elevations at all <br />locations of interest for given flow values. The data needed to perform <br />these computations include: flow regime, starting elevation, discharge, <br />loss coefficients, cross section geometry, and reach lengths. The <br />options available for providing and manipulating input are discussed <br />in the following sections. <br /> <br />2. FLOW REGIME <br /> <br />Profile computations begin at a cross section with known or assumed <br />starting conditions and proceed upstream for subcritica1 flow or downstream <br />for supercritica1 flow. The direction of flow is specified on the J1 card <br />(first job card) by setting variable IDIR equal to one for supercritical <br />flow or zero (or blank) for subcritica1 flow. Subcritical orofiles comouted <br />by the orogram (IOrR = 0) are constrained to critical depth' or above, and <br />supercritica1 profiles (rOrR = 1) are constrained to critical depth or be- <br />low. The program will not allow profile comoutations to cross critical <br />depth except for certain bridge analysis problems. In cases where flow <br />passes from one flow regime to another as shown in Figure 2 below, it is <br />necessary to compute the orofile twice, alternatel,V assuming subcritica1 <br />and supercritical flow. Results of a subcritica1 profile, (shown as 0 in <br />Figure 2) computed for the examo1estream would plot at critical deoth <br />(above the actual water surface profile) in the steep reach of stream. <br />Results from a supercritica1 profile computation (shown as . in Figure 2) <br /> <br />MILD SLOPE <br /> <br />STEEP <br />SLOPE <br /> <br />MILD SLOPE <br /> <br />--1(--- _ . <br />- - - x--IC- <br />Dc <br /> <br /> <br />- -x-__ <br /> <br />-- --lC- <br /> <br />CROSS SECTIO <br />PROFIL S <br />o SU8CRITI AL RUN <br />. SUPERCRI ICAL RUN <br />--- CRITIC L <br />- ACTUA <br /> <br />Dc <br /> <br />JUMP <br /> <br />Figure 2. <br /> <br />Profiles Calculated for Subcritica1 and Supercritica1 Flows <br />, <br /> <br />11 <br />