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: -.." Designing for Dynamic Equilibrium in Streams occurs, leading to a decreased gradient. A new dynamic equi- librium will be attained, once the decreased gradient is in equilibrium with the decreased bed.materialload and degrada- tion ceases. If the decreased bed-material load is replenished by bank material, meander formation takes place. This leads to lengthening of the stream and, thus, to gradient decrease, until dynamic equilibrium is regained. Depending on types of bed and bank material, both degradation and meander for- mation may occur simultaneously. IMPORTANT FACTORS IN STREAM CONTROL PLANS An important requirement for effective control design is sufficient and adequate data. As stated above, within the multitude of interdependent hydraulic variables, flow and sediment discharge are most important. These should, there- fore, be the basis of the data bank. But in many situations, data may not be available and may have to be generated. Time should be set aside, therefore, for this design phase. The data should be used to test the applicability of empirical relationships developed elsewhere, because theoretically and physically sound equations are not available for adequate prediction. From past experiences, we have learned that engineering, as well as vegetative stream control measures, must be applied with greatest caution and that effective control plans require some degree of flexibility. This should allow for additions or changes of future actions, because, at best, we may be able to foresee the trend of future stream development but not the magnitude of the stream's response. In his review of the last 50 years of sediment research, Vanoni (1984) con- cluded that, "we still carmot predict with adequate certainty the velocity and sediment discharge in a channel for a given water discharge. Nor can we predict if the channel will be stable." For example, we may forecast degradation as a response to sediment load reduction or withdrawal,but the expected depth of the streambed lowering remains an esti- mate. The reason for this inability to realistically quantify lies in the fact that a bed armor may develop and stop future downcutting. Even if extensive drilling into the bed is pos. sible and shows that at a given depth, large particles, suitable for armor formation, will be exposed by the flows, the true rate of future bed erosion remains unknown, This rate will depend on frequency and amounts of the future precipita. tion input into the stream system; hence, estimation of the rate is a probabilistic, and not a deterministic, task. It should be stressed that the term "stable chalUlel" is not to be equated with "steady state." Change is the role in all natural systems, and as discussed earlier, a stable chan- nel is one in dynamic equilibrium. Vanoni, et aL (1961), have discussed different adjustment processes leading toa new equilibrium when equilibrium problems eXist. An example of unexpected bed armor development in the Missouri River was reported by Livesey (1963). Below Fort Randall Dam, engineers had estimated that bed lowering by 5 m would take place. But when the bed elevation dropped by about 1 m, further downcutting stopped. Large material had been unearthed by the flow and provided an armor. How can flexibility be incorporated into a stream control plan? Above all. we must recognize our inability to deter- J ministically and quantitative Iv forecast future stream develoP. ~ents. From this it follows that major considerations, reach- ing beyond the inunediate objectives, must be part of the plan. They are: 1) recognition that the control measures may create new critical locations (places of failure) in the stream system, 2) design of additional control plans to be used if required, and 3) - not least' - appropriation of sufficient future funding to take care of maintenance and additional work. An example of flexible planning shall be given for fishery biologists confronted with the task of enhancing fish habitat in a small stream. Let us assume the problem is to protect an endangered species from a non-native dominant fish. The measure selected by the biologists consists of the installation of a relatively high dam in the lower reach of the stream. It is estimated that an effective dam height (the distance be- tween streambed and crest of spillway) of 2 m would prevent migration of the dominant species into the upper reaches. These would be cleaned of this species after dam closure. In a small mountain stream, a 2-m high dam represents a substantial barrier for water and sediment. A location for installation must be found, therefore, to support a structure with a spillway capacity sufficiently large for the expected high flows. Otherwise, overtopping of the freeboards and with, this loss of dam or flooding may occur. The plan must also include estimates on the expected sediment wedge that will form upstream from the dam. This estimate would be derived from natural barriers where sedi- ment deposits developed by calculation of the ratio between the original channel and the sediment deposit gradients (Heede, 1976). If future sediment deposits will occur in a reach with trapezoidal cross-sections, it will be necessary to estimate the relative increase in channel bed width with sedimentation. If substantial increases are expected, future small flows may not cover the bed from bank to bank but may concentrate, form- ing a relatively narrow thalweg. Usually, meandering of the thalweg occurs in this situation, hugging the opposite banks. This hugging causes bank erosion where erodible material exists. Since it would be extremely difficult, if not impossible, to estimate the locations of future meandering, bank pro- tection measures may be required after the sediment wedge found its equilibrium condition. The new bed level will not be static, but will change between high and low flows. A "happy medium" must be struck between both, if bank protection will be required. The plan must include this future measure as a prerequisite for success. 353 ' WATER RESOURCES BULLETIN