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<br />where the nonna1 bridge routine might be used. <br />1. Because the special bridge routine requires a trapezoidal <br />approximation of the bridge opening for low flow solutions, the normal <br />bridge routine could be used where the flow area cannot be reasonably <br />approximated by a trapezoid. (See the suggested approach for Multiple <br />Bridge Openings.) <br />2. For long culverts under low flow conditions, the standard step <br />method is the most suitable approach. Several sections can be taken <br />through the culvert to model changes in grade or shape, or to model a <br />very long culvert. <br />3. In cases where the bridge and abutments are a small obstruction <br />to the flow, the normal bridge routine can be used. <br />4. In the special bridge routine, the correction for submergence <br />in the weir flow calculations is not very reliable for high submergence <br />on weirs that are not ogee shaped. The normal bridge routine may then <br />be preferable. <br />Special Bridge Routine. The special bridge routine is capable of <br />sOlving a wide range of flow problems. The following are situations <br />where the routine is applicable. <br />1. The special bridge routine will determine the class of low flow <br />based on a trapezoidal approximation of a bridge with piers. If a bridge <br />opening can be reasonably modeled by a trapezoid, the program will deter- <br />mine when the profile goes through critical depth and what the correspond- <br />ing water surface elevation is on either side of the bridge. <br />2. Pressure flow is computed using the orifice equation. The ori- <br /> <br />27 <br />