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Limitations This section indicates important limita- tions of WSP2. If a given problem demands more accuracy than these limits allow, a different method of solution must be used. Because data entry was made as easy as possible, WSP2 cannot check for many sources of input errors. For example, the capability of sorting the cross section sta- tion-elevation points precludes WSP2 from checking the integrity of station values. The surveyor, coder, and key-puncher must get correct data on the input cards. When supercritical flow occurs, WSP2 uses the elevation that corresponds to criti- cal depth for that profile at that section and continues upstream in this manner until subcritical flow recurs. WSP2 does not re- compute downstream to determine super- critical flow elevations. Critical depth is found by using the equation Q' A' g-T This relationship is valid for irregular, sin- gle-segment valley cross sections. It does not give the water surface elevation for multisegmented sections where energy is minimum. Errors of as much as 2 feet can occur, compared with the minimum eleva- tion shown on a specific energy diagram, because of the area-top width relationship. The WSP2 energy balancing procedure assumes gradually varied flow in a reach. Furthermore, since only friction loss is con- sidered, it is essential that velocity changes in a reach be small. Since WSP2 is based on the law of conservation of energy, the limits of gradually varied flow should be related to kinetic energy (velocity head) rather than directly to velocity. If velocities in a reach are less than 6 ft/sec, the maxi- mum change in velocity head would be about 0.5 foot. Certain evaluations could be significantly affected by this change, but they would normally be within the degree of accuracy of other hydrologic estimates. If velocities are between 6 and 11 ft/ sec, answers should be t:arefully examined to determine their adequacy. At a velocity of 11 ft/sec, the maximum change in velocity head would be about 2 feet. This is also the point at which critical depth computa- tions are inaccurate. Therefore, answers that include velocities of 11 ft/sec or more should be carefully verified. Important limitations to the BPR bridge analysis procedure are: (1) the channel in the vicinity of the bridge must be neariy straight; (2) cross sectional area of the stream must be fairly uniform; (3) stream gradient between the exit and approach sections must be approximately constant; (4) flow must be free to contract and ex- pand; (5) no appreciable scour can be present at the bridge; (6) flow must be sub- critical. WSP2 culvert analysis is limited to rec- tangular, circular, and standard arch shapes. A culvert that is deformed by set- tlement, partially filled with sediment, or of a different shape must be represented by one of the three standard shapes. Of all parameters estimated, a close similarity should be maintained between the actual area and the area of the culvert chosen as most representative. The mysteries of computer processing along with the form of computer output sometimes suggest more accuracy than is justified. When examining WSP2 output, consider the following limitations: 1. Accuracy of estimated input parameters. A Manning's roughness value can be esti- mated to be 0.04. Over a period of time it may range from 0.025 to 0.045. This might correspond to a variation in water surface elevation of -0.3 to +0.1 foot. 2. Precision of technical procedures ap- plied to input data. For instance, since en- ergy is balanced to the nearest tenth of a foot, a digit in the hundreth position has little meaning. 3. If an IBM computer is used for process- ing, 32 bits are used to store a number. This limits the maximum number of signifi- cant digits that can be stored to seven. Decimal-to-binary and binary-to-decimal conversion routines and the normal use of arithmetic functions further limit computer accuracy to five or six digits. 8