Structural solutions to flood problems change the hydrology or hydraulics for a por-tion of the watershed under study. Some examples include dams and reservoirs, detention ponds, channel modifications, diversions, and levees. Diversions of flow, such as by dams, reservoirs, and detention ponds, change the downstream hydrology by diverting or storing some of the floodwater during a flood, thereby reducing the downstream peak discharge and delaying the time of peak discharge. Channel modi-fications, such as levees, result in a change to the water surface elevations. A flood-plain model is first developed to determine the base conditions for the stream or watershed. Structural measures are then incorporated into the model and analyzed to determine their effect on flood levels.
Dams, Reservoirs, and Detention Ponds. For studies of dams, reservoirs, and detention ponds, hydraulic floodplain modeling determines the water surface eleva-tion–discharge relationship (a tailwater rating curve) just downstream of the struc-ture. This relationship is used for a separate hydraulic analysis and design of the structure, often including physical model tests for the final design of a large dam. The spillway and low-flow conduit capacity at the dam may also be evaluated with HEC-RAS, computing pool elevations for selected values of discharge. The effects of the reservoir pool can be determined with the program by computing water surface pro-files upstream of the dam and reservoir. The program is also useful for developing the reservoir storage versus outflow relationship that is used in routing the inflow hydrograph through the reservoir. Chapter 8 presents routing operation usage with HEC-RAS, and Chapter 12 further discusses dams and reservoirs.
Channel Modifications. Increasing the size, slope, or depth of the channel or decreasing its roughness can lead to a reduction in flood levels because of the addi-tional channel capacity provided by the project. This can easily be simulated using a hydraulic model. Channel modifications can also have negative effects, which can be demonstrated with a model. One example is increased flood discharges downstream of the project due to the increased velocity in the more efficient, modified length of channel. Additional effects can include erosion and/or deposition in the modified channel, upstream migration of the erosion (a headcut) due to increased velocities, and sediment deposition downstream of the modified channel. Chapter 11 addresses these issues in detail.
Diversions/Split Flow. Redirecting all or a portion of flood flows to a different flow path or to detention facilities has become a fairly common flood reduction solu-tion. The diversion structure may go into operation after a certain river level has been reached, with progressively higher flows diverted through gate openings or via spill-way overflow. Analyzing flow around a large island or other obstruction may also require a type of diversion analysis, usually referred to as split flow modeling or divided flow analysis. Few numerical programs allow the engineer to properly evalu-ate split flow and diversions. HEC-RAS incorporevalu-ates a looped network to analyze split flow around an island or other obstruction, and has lateral weir and lateral rat-ing-curve options to perform the diversion analysis. Split flow and diversion analysis can be performed for either steady or unsteady flow simulations. Chapter 12 presents information on split flow and diversion modeling.
Section 1.4 Chapter Summary 11
Levees. Levees are earthen barriers that prevent floodwaters from flowing onto a protected floodplain, as illustrated in Figure 1.5. Concrete floodwalls are also included in this category. The required height of the levee and the effect of the levee on flood events can be determined by a numerical model. By preventing the flood from occupying the floodplain, a levee can cause increased flood heights for a certain distance along and upstream of the levee. This increase is obviously quite important and must be properly analyzed to determine the extent of any adverse effects. Simi-larly, the loss of floodplain storage behind the levee can result in an increased down-stream peak discharge. These effects on flood levels may require hydraulic and hydrologic analysis to ascertain the magnitude of any changes, possibly including the use of unsteady flow modeling. Chapters 11 and 12 address levee effects and appro-priate modeling procedures.
1.4 Chapter Summary
Floodplain hydraulic analysis is a relatively recent engineering effort. Physical model-ing and hand computations used in the middle of the twentieth century have given way to complex computer programs run on powerful desktop computers. With the availability of todayʹs faster and more sophisticated hydraulic analysis programs, one engineer can do the work not only better but more quickly than three or four engi-neers could just a few decades ago. The use of modern hydraulic programs and geo-graphic information systems (GIS) or computer-aided design and drafting (CADD) techniques can yield far more data for the model and a more-accurate hydraulic anal-ysis than was dreamed possible in the 1970s. The increased availability of computer programs for floodplain modeling has allowed detailed analyses of a wide range of structures, including bridges, culverts, road embankments, dams, levees, channel modifications, and diversions.
Only a skilled and knowledgeable hydraulic engineer can construct the model, ana-lyze the data, and ensure that the flood simulations are reasonable and representative of the floods occurring on the study watercourse. The engineer must be well trained in hydraulics and understand the basic concepts, equations, and computation proce-dures inherent in the numerical calculations. Chapter 2 starts the student or new engi-neer on the road to understanding the governing equations and computation processes.
Figure 1.5 Levee with discharge pipes along the Mississippi River.
This chapter discusses open channel flow and defines the many variables used in an open channel flow analysis. As is presented in this chapter, it is possible to classify the flow occurring in an open channel on the basis of many criteria, including time, depth, space, and regime (subcritical or supercritical). The governing equations for open channel flow are discussed, along with the classification of profile shapes.
Finally, the chapter outlines the common procedure for open channel flow analysis:
the standard step method.
2.1 Terminology
An open channel is any flow path with a free surface, which means that the flow path is open to the atmosphere. Open channels can be classified as prismatic or nonpris-matic. A prismatic channel has a constant cross section and often has a constant bed slope for long lengths of the channel. Man-made channels (such as storm sewers, drainage ditches, and irrigation canals) are typically assumed to be prismatic, although they do have occasional changes in cross sections or slope to accommodate topographic conditions or changes in their discharge rate, as illustrated in Figure 2.1a.
A nonprismatic channel varies in both the cross-sectional shape and bed slope between any two selected points along the channel length. Natural channels (rivers and creeks, such as the one shown in Figure 2.1b) are nonprismatic. Unless indicated otherwise, prismatic channels are assumed for examples in this book. Figure 2.2 shows cross sec-tions of several classificasec-tions of channels that are operating under open channel flow.
The theory and procedures of open channel hydraulic analysis were originally devel-oped from experiments on fluid flow in pipes or conduits. Flow in a pressurized pipe, however, is not representative of open channel hydraulics. In open channel flow, C H A P T E R