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EM 1110-2.1416 15 Oct 93 or velocities for specific time periods may be needed, depending on the study requirements. (2) Discharge data include measured and/or synthe- sized flows along with frequency, velocity, duration, and depth information. Measured data at gages are the pre- ferred source for this category; seldom, however, does sufficient measured data exist. A typical hydraulic analy- sis requires simulated data from hydrologic models as well as information on hislorical events, usually floods. This latter data is often obtained from extensive discus- sions with local residents living along the study stream and the review of newspaper accounts and/or Corps or other agency reports. A field survey during the recon- naissance and data gathering stages of a study by the responsible hydraulic engineer is essential. b. Channel geometry. (1) Channel geometry is required for any hydraulic study. Geometric data include channel and overbank topography, stream alignment, bridge and culvert data, roughness information, changes in stream cross section shape, and alignment over time. Extensive field and/or aerial surveys supply the bulk of these data; however, cost reductions can be achieved by locating and using available data. Most rivers and streams have been stud- ied in the past. Floodplain or flood insurance reports are often available and can be valuable sources of geometric and other data. Bridge plans are usually available from state, county, or municipal highway departments. Navi- gable rivers have hydrographic surveys of the channel taken periodically. Aerial photos have been taken at regular intervals by the Soil Conservation Service since the mid-1950's providing data on stream channel changes. Even if it is decided that new surveys need 10 be obtained, the above sources provide valuable informa- tion on changes in channel alignment and geometry over time, indicating potential problems related 10 the stream's sediment regime. The keys to the usefulness of the data are the accuracy of the survey data and the locations of cross sections along the stream. Accuracy is discussed in section 3-4e and Appendix D. Additional information on the effects of survey data accuracy on computed water surface profiles can be found in "Accuracy of Computed Water Surface Profiles" (US ACE 1986). (2) The amount of survey data required depends on the study objective and type. For instance, more frequent surveys are needed for navigation projects than for flood control projects. Detailed contour mapping for urban stwlies should be obtained in the feasibility phase rather than in the design phase, whereas detailed mapping for 3-6 agricultural darnage reduction studies may often be post- poned to the post-authorization stage. For movable bed studies repeat channel surveys are needed at the same locations, separated by significant time periods, to evalu- ate a model's performance in reproducing geometric changes. Thalweg profiles and/or repetitive hydrographic surveys are needed for analysis of bed forms and the movement of sand waves through rivers. e c. Sediment. (I) The amount of sediment data needed is not always apparent at the beginning of a hydraulic study. The sediment impact assessment, as outlined in EM 1110-2-4000, is performed during the initial planning process. Sediment assessment studies are typically per- formed to determine if the project proposal is likely 10 create a sediment problem or aggravate an existing one. The results of this evaluation will dictate the need for additional data and quantitative studies during the feasi- bility and design phases. If a sediment problem presently exists, or is expected with a project in place, a sediment data collection program must be initiated so that the problem can be properly addressed in later stages of the analysis. c . ~ (2) Sediment data include channel bed and bank material samples, sediment gradation, Iotal sediment load (water discharge versus sediment discharge), sediment yield, channel bed forms, and erosion-deposition tenden- cies. Long-term sediment measuring stations are few in number, and modem methods of sediment measurement can make older records questionable. Sediment data collected at a gaging site are usually short-term. Flood control or navigation studies must address sediment to determine if there is, or will be, a sediment problem if the study proposal is implemeuted. Often, the initial sediment analysis is performed in a rather qualitative fashion with a minimum amount of data. If there appears to be a sediment problem, a data collection pro- gram should be established, at least for a short period, 10 obtain calibration data. Chapter 7 and EM 1110-2-4000 should be reviewed for further guidance on sediment data. e . < . ,. (3) The type of project often dictates the amount and type of sediment data needed. For instance, reser- voir and channelization proposals require that the entire suspended sediment load (clays, silts, sands, and gravels) be analyzed, whereas flood control channels or river stabilization projects primarily require analysis of the bed material load (mainly sands and gravels) because the finer materials (clays and silts) usually pass through the e