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is the tabulated value times the ratio of the actual'slope (see step 5 below) to 0.0001. WSP2 interpolates or extrapolates velocity head and critical discharge on a linear basis. Step 4.-Calculate flow rate (see chapter 14 of NEH-4 for csm adjustments) for the profile being considered. The csm (cubic feet per second per square mile) adjust- ment is made on a drainage area basis so that each water surface profile closely matches a flood profile. WSP2 interpolates from the table developed in step 3 to deter- mine the elevation at which flow rate is critical. Step 5.-Figure 1 shows how the energy principle is used in WSP2. Energy is con- sidered balanced when the trial elevation plus velocity head for that elevation (from table developed in step 3) at the upstream section is within 0.1 foot of the energy level at the downstream section plus losses. Only friction losses are considered in the WSP2 program. They are found by Mann- ing's equation (S = (Q/ KD)') using Q and KD at the upstream section. The rate of friction loss is S" and the total loss is then S, times the length (L). The critical eleva- tion from step 4 is used first in the trial- and-error energy balance procedure. Step 6.-lf the initial upstream energy level (using critical elevation) is more than the downstream energy level plus friction loss, WSP2 assumes supercritlcal flow and takes critical elevation as the answer. If the reverse is true, WSP2 assumes sub- critical flow, chooses a higher elevation, and recomputes the energy balance. The program iterates until an elevation is found at which the energy equation will balance within 0.1 foot. For profiles with nearly equal discharges it is possible to get more flow at a lower elevation than at a higher elevation on a rating table. A reversal of as much as 0.2 foot is possible within the 0.1-foot accuracy limit of the energy balance equation. Note that only the total energy elevation at the downstream section is needed to balance energy at the upstream section. The section rating table contains infor- mation at bankfull and zero-damage eleva- tions. Zero-damage elevation is the lowest point in the damage segments. Bankfull elevation is the lowest of all first and last points defining channel segments. Dis- charge and end-area at these elevations are found by interpolation. Valley Section Location Valley sections can serve many needs (geologic, engineering, economic, hydrau- lic, etc.), and all of them should be con- sidered when selecting the location. For hydraulic purposes, valley sections are sur- veyed at points along the valiey length and need to be representative of several param- eters, such as flow area, wetted perimeter, and roughness. WSP2 considers only energy losses due to friction and uses the rate of friction loss at the upstream section as the rate through- out the reach. Therefore, valley sections should be located as follows. Divide the valley length into reaches that have nearly constant parameters that affect hydraulics and locate the valley section near the up- stream end of the reach. In addition to these sections, locate valley sections about 50 to 100 feet both upstream and down- stream from road-type restrictions. Survey sections perpendicular to the direction of flow and not necessarily straight across the valley. Road Restriction Analysis WSP2 analyzes a road restriction by determining (1) water surface elevation at the downstream face of the opening through the road embankment (labeled tail- water on the computer printout); (2) head loss due to the restriction (labeled HL in the output; HL plus tailwater is headwater); and (3) water surface elevation at the ap- proach section. Each step is explained below. Step 1.-The value for tailwater is found by balancing energy between the exit valley section and a new section manufactured by the program at the downstream face of the bridge or culvert. The reach length between the new section and the exit section is the channel length on the road input card. The shape of the new section is the same shape 3