4.1 Ten Steps of Floodplain Modeling
Most floodplain modeling studies follow a clearly defined path toward a solution. The following sections describe ten steps comprising this path, as listed in Table 4.1. The reader should note that more steps may become necessary in complicated studies.
Steps 1 through 8 establish base (existing or preproject) hydraulic conditions, such as the water surface profiles for different floods and maps showing the areal extent of flooding for these events. Preparation of flooded-area maps in Step 8 can mark the end of a technical study if hydraulic information is only required for land-use planning purposes, such as for a flood insurance study. However, studies that include quantifying the effect of alternative flood mitigation measures involve considerably more technical effort, as presented in Step 9, "Performing Project Evaluations." Step 10, "Preparing the Report," entails documenting the hydraulic analyses in a technical report. Figure 4.1 shows the sequence of the steps.
With the exception of Step 9, the steps documented in this chapter are common to most studies. Step 9 covers the development of reduced water surface profiles and/or reduced economic damages resulting from a proposed structural change on the watershed, such as adding a reservoir, channelization, or a levee project. Nonstructural solutions, such as floodproofing, flood forecasting, or relocations, usually don't require development of additional water surface profiles because they rarely affect the flood levels of the pre-project profiles. However, reduced economic damages would accrue, making the project evaluation step necessary, even for nonstructural solutions.
The floodplain modeling planning process should include a written technical outline of the work to be performed in the hydraulic analysis. A defensible time and cost estimate for the overall project really can't be developed without such a document. A poor or inadequate technical analysis is likely if the engineer guesses a study time and cost without analyzing the technical details required for the analysis. This poor outcome is even more likely if the engineer is simply given a budget or total cost from a funding authority that has little knowledge of the required work. A technical outline, which should be performed early in the project, greatly aids the engineer in justifying the time and costs necessary for the floodplain modeling effort.
Step 1: Setting Project and Study Objectives
The project and study objectives may not be solely the decision of the hydraulic engineer, but rather they may be determined in consultation with other members of the project team and funding authority. For most floodplain modeling efforts that involve evaluating changes to the stream or watershed, the overall project objective is flood damage reduction. Additional project objectives that may be considered are navigation, hydropower, irrigation, water supply, environmental concerns, and permits.
After project objectives have been identified, the study objectives can be listed. Some examples of study objectives are
Step 2: Study Phases
The three main phases of a study are preliminary evaluation, feasibility, and detailed design. These levels may be discussed in three separate technical reports for large, expensive, and complex projects, such as for the design of a major dam or levee. For less complex or controversial projects, such as a minor stream relocation for a culvert project, the three study levels are usually combined into one report.
Preliminary Evaluation Phase.
A preliminary evaluation is made when it is uncertain whether there is an economic interest in further pursuing a project. The engineer performing the hydraulic study may work with other members of the team to develop rough designs with the associated costs and benefits. This information is then used to ascertain if it is likely that more-detailed studies will lead to a feasible and desirable solution. In general, there is limited time and money for the preliminary evaluation phase, so evaluations are often based on existing hydraulic data (such as from an available flood insurance study) and the judgment of experienced engineers. Detailed floodplain modeling during a preliminary evaluation is the exception rather than the rule.
If the study does not involve a full economic (benefit-cost) analysis, a least-cost alternative analysis to satisfy local criteria (such as the minimum size bridge opening to pass the design peak discharge without causing significant stage increases) is often performed. The preliminary evaluation may provide only rough costs for a few options to determine if the available funds for the project are adequate for the lowest cost alternative that still meets project performance requirements. A detailed technical outline should be developed during the preliminary evaluation phase to determine a time and cost estimate for the major floodplain modeling work that will take place in the feasibility phase.
Feasibility Phase.
The objective of the feasibility phase is to determine the scope and magnitude of the project. The bulk of the floodplain modeling is normally done during the feasibility phase-floodplain hydrology and hydraulic analyses are performed, leading to the establishment of base, or predeveloped, conditions. This portion of the work results in the hypothetical frequency flood profiles (such as the 100-year flood profile) that show the depth and areal extent of flooding for various events. Next, final or postproject hydrology and hydraulics studies are performed to determine the potential flood damages along the study stream. Figure 4.1 illustrates the process of determining economic benefits of a project. 
Figure 4.1 Economic benefits of a proposed flood reduction structure.
The differences between the base and postproject profiles are an indication of the potential benefits that could be obtained from a proposed flood reduction structure, such as a reservoir. Such a structure could reduce the base water surface profiles, resulting in less potential damage to properties along the stream.
The project benefits for each option studied are compared to the costs associated with possible structural measures, using established economic analysis procedures. Each different structural measure (such as reservoirs, levees, or channelization) is modeled, and the measure's impact on base condition flood profiles is determined by the hydraulic engineer and evaluated by an economist in terms of reduced flood damage. Economic analysis leads to a determination of the "best" or "optimal" economic plan. The best plan is usually the one that gives the maximum net benefit, based on the following relationship:
Maximum net benefit = (base damages - postproject damages) - project costs (4.1)
These benefits, damages, and costs are calculated on an average annual basis.
Detailed Design Phase.
The detailed design phase concentrates on the structural, foundation, and hydraulic design of the features of the selected plan. It can include, for example, designing the pumping plants and collector ditches for a levee project, along with the inlet/outlet structures for any culverts through the levee. For channelization projects, the engineer would design the stream junctions, drop structures, side drainage, bridge and culvert features, and scour protection. For reservoir projects, the engineer might design the spillway shape, stilling basin features, and downstream channel protection. The engineer may perform detailed sediment transport studies employing numerical models to evaluate the effect of the project on the stream's sediment regime. The frequency and amount of dredging may also be studied to maintain desired channel capacities or to determine if sedimentation will adversely affect the levee capacity or reservoir storage.
Step 3: Field Reconnaissance
One cannot appropriately model a reach of river without conducting site visits; the hydraulic engineer needs to visit the study site in person as often as practical. Field trips will likely be conducted throughout the course of the study and during all three study phases. While in the field, the engineer should photograph representative reaches of the river under study and all bridge and culvert crossings of the main channel, the adjacent floodplains, and any relief openings. Digital cameras are especially useful for reconnaissance.
A valuable feature of HEC-RAS is the ability to link digital images, such as those taken with a digital camera or scanned from a photograph, with certain cross sections. Bridge geometry can be linked with a picture of the bridge, allowing the modeler to view the actual structure and compare it to the geometric input.
The channel bed material should be inspected at different locations to ascertain whether it is predominantly silt, clay, sand, or gravel. Bed material grain size is an important factor in estimating Manning's n for the channel. Bed material size is also an important parameter for bridge scour analysis. The application of prediction equations, such as Cowan's for Manning's n (discussed in Chapter 5), requires the estimation of several channel parameters in addition to the bed material. Therefore, the modeler should address the following questions during the field inspection:
The bankline should also be closely observed. Leaning or fallen trees in the channel, exposed root wads in the channel bank, large deposits of material, scour around bridge footings, and vertical banks with sloughed material at the toe are all important observations. These factors indicate that a stream may be experiencing rapid changes in channel geometry and stream slope caused by major scour and deposition. This situation could require the changing channel geometry to be included in the floodplain modeling process. If problems are found, it is important to identify the source of the problem. It could be that a past channelization produced a headcut that is unraveling the channel as the erosion moves upstream. Channel distress could also be due to upstream urbanization or other land-use changes that have greatly increased the discharge associated with any rainfall event.
A field reconnaissance should provide an opportunity to collect calibration data. Highwater marks obtained from interviews with local residents are invaluable. Local newspapers typically carry past flood stories in their archives, which can also be a valuable source of flood data and highwater marks. Calibration considerations are further discussed in Chapters 5 and 8. The opportunity to observe the stream during a flood event is especially important. A video recorder should be used if the opportunity to observe an actual flood arises. Field reconnaissance is important for all study phases, but especially in the preliminary evaluation and early in the feasibility study.
Step 4: Determining the Type of Hydrologic/Hydraulic Simulation Needed
Chapter 3 discusses hydrologic and hydraulic computer programs. For floodplain hydraulic modeling, the most appropriate method will almost always be a one-dimensional model coupled with either steady, quasi-unsteady, or fully unsteady hydraulics. A steady (with peak discharge possibly determined from a simple equation) or quasi-unsteady (with peak discharges developed from a hydrologic computer model) solution should be used unless one or more of the exceptions listed in Section 3.4 apply. The majority of floodplain hydraulic modeling studies employ the steady flow model in HEC-RAS with peak discharge computed from an equation, statistical analysis of gage records, a previous study, or a hydrologic computer program. These techniques will be further discussed in Chapter 5. The use of a hydrologic computer program also depends on the complexity of the watershed and the methods to be evaluated. Table 4.2 lists the procedures usually selected for various flood mitigation methods. Many of the items listed in Table 4.2 are discussed in detail in Chapters 9 through 11. If a multidimensional model is deemed necessary, study planning should allow additional time and expense for training the engineer in its use and application. Significant additional data acquisition is also necessary for a multidimensional model.
Step 5: Determining Data Needs
This section provides some understanding of how much data is available or is required during the study planning process (Chapter 5 discusses data needs and availability in more detail). Data needs vary, depending on the methods to be analyzed and the procedures employed. In the majority of floodplain modeling projects, the data needed consist primarily of discharge and geometry information.
Discharge Data.
For study planning purposes, the questions that need to be answered to properly model the situation include the following:
If the study reach is short and uncomplicated, only a peak discharge may be necessary. For these situations, a peak discharge may be estimated through a regionally derived regression equation (see Chapter 5). If the reach is long with several tributaries, or if there are ongoing changes in the watershed (such as urbanization and upstream reservoirs), a full hydrograph is often necessary to capture the effects of these features.
For analyses of moderate to large watersheds and the modeling of many miles of stream, both a hydrologic simulation program, such as HEC-HMS, and a steady flow hydraulic program, such as HEC-RAS, are usually needed. When full unsteady flow modeling is required, complete discharge hydrographs are needed to perform unsteady flow computations throughout the stream system. Chapter 5 discusses discharge data in more detail.
Geometry Data.
Planning activities need to determine the availability and quality of existing survey data. If necessary, appropriate costs should be developed for the collection of survey data that is adequate for the level of accuracy and confidence that is desired in the hydraulic output.
Channel and floodplain cross-section data must be collected at a sufficient number of locations to accurately define the water surface profile. Stream locations that have an effect on flood elevations should also be surveyed or estimated-these locations could include sharp breaks in the channel slope, large expansions or contractions of the floodplain width, and significant changes in land use or vegetation.
Cross sections should extend across the entire width of the floodplain, if possible. Roadway and low chord profiles of each road crossing should also be obtained, either from field surveys or by getting bridge sections from the agency responsible for the bridge. Although plans may be available, in some cases the plans will be very old and occasionally based on a local datum that might not be transferable to the National Geodetic Vertical Datum (NGVD). For old bridge plans that are not in an acceptable datum, new surveys should be performed to determine both accurate bridge geometry and current channel elevations through the bridge opening.
When the approach roadway to the bridge or culvert is located on a significant embankment, spot elevations or a full cross section of the floodplain adjacent to the embankment should be obtained. One source for such supplemental data are USGS topographic maps, most of which have been digitized and are available in a Geographic Information System (GIS) format for use as a base map. If the mapping scale is inadequate, the engineer may recommend orthophoto contour mapping or digital elevation maps (DEMs). Recent advances in survey and computer technology allow RAS cross sections to be obtained in the correct format directly from a computer-generated DEM. Chapter 5 discusses topographic data in more detail.
Step 6: Defining Hydrologic Modeling Procedures
As part of the planning process, the engineer should tentatively select the modeling procedures to be employed, thus leading to a more accurate time and cost estimate. This step will also include investigating which methods have been found to be most acceptable and accurate for the hydrologic area being studied.
For steady flow modeling situations that require the development of flood hydrographs, the selection of modeling procedures can include the following:
- Precipitation - Depth, temporal distribution, or mean areal precipitation
- Infiltration modeling technique - Uniform and initial, SCS curve numbers, Green-Ampt, Holtan, Horton, or other
- Runoff modeling - Kinematic wave or unit hydrograph (SCS, Clark, Snyder, or other)
- Hydrograph routing - Straddle-stagger, Muskingum, Modified Puls, Muskingum-Cunge, or other. The routing selection may require information generated by the hydraulic computer program, particularly for the Modified Puls routing method. Chapters 8 and 14 discuss routing techniques.
- Calibration data - If a flood has produced known highwater marks or if stream gage data are available, gaged rainfall data should be obtained. Rainfall maps are prepared using the Thiessen or isohyetal techniques. Either of these techniques may be used to estimate the average storm rainfall on a watershed and are described in any hydrology textbook. If discharge gages are available, the recorded flood events should be obtained from the agency in charge or from a reliable web site. If several actual storm-flood events are available, all should be used in the calibration and verification process. Chapter 8 discusses the calibration and verification steps.
For unsteady flow modeling, modeling procedures also include the following:
- Boundary conditions at upstream and downstream locations and for each major tributary. Such data as stage and/or discharge hydrographs, rating curves, normal depth, lateral inflows, gate opening settings are needed at each boundary location.
- Although a single hydrograph is often sufficient, some studies require a full period of record for unsteady flow routing. Several years to several decades have been simulated in some studies. The discharge data could come from gage records or from simulation with runoff models. A "warm-up" period featuring a constant flow for a specified time is typically also included as an initial condition for the model operation.
Step 7: Performing Data Input and Calibration
Most of the effort in a floodplain modeling project occurs during data preparation, input to the model, and debugging and calibrating the model. This effort involves coding all geometric data into HEC-RAS, including bridges, culverts, dams, diversions, and other structures affecting water surface profiles. Ineffective flow areas (discussed in Chapters 5 and 8) should be included in the HEC-RAS cross sections if storage-discharge data are needed for hydrologic routing. Historic discharge and highwater marks can be used to calibrate the output from HEC-RAS. Following calibration, a series of steady flow discharges is used to obtain a storage-discharge relationship for each routing reach in the hydrologic simulation. Following calibration and verification of the model input, an independent technical review of the output should be made for quality assurance/quality control (QA/QC) purposes.
Chapters 5 through 8 further discuss these hydraulic activities.
Step 8: Performing Production Runs for Base Conditions
Following the input and the model calibration and verification processes, determination of whether the model adequately represents the actual reach of river being simulated is based on the calibration events. Unfortunately, the engineer seldom has real data for the actual events that represent the rare floods that he or she is tasked with modeling. Further adjustment to model parameters during the production runs may be required, based on available local data and the hydraulic engineer's knowledge and experience. If the calibration event(s) are small to moderate floods, the simulation of large and rare events may require the following:
- Modification of infiltration parameters to reflect more runoff
- Modification of peaking coefficients to increase the peak discharge of the unit hydrograph or hydrograph peak
- Modification of routing travel times to reflect a faster movement of the hydrograph through a routing reach
- Reduction of Manning's n to reflect the more efficient channel during rare flood events
- Simulation of the buildup of trash and debris on bridges during a major flood
These considerations are presented in more detail in Chapter 8. The proposed water surface profiles for flood events of various frequencies should be given at least a cursory independent review for QA/QC purposes. A more detailed review should be performed for a flood insurance study or similar floodplain information report, since profiles and flooded area maps will be the main technical output of the hydraulic effort.
Step 9: Performing Project Evaluations
After a model has been sufficiently calibrated and the base condition flood profiles have been developed, the engineer has a numeric representation of the water surface elevations of actual and hypothetical floods at any location along the study reach. These water surface profiles and corresponding flooded area maps form the basis for further evaluation of the effects of different flood reduction scenarios. If the basin hydrology and/or hydraulics are expected to change in the future (such as from upstream urbanization or reservoir construction), the hydrologic and hydraulic models need to be run for the expected future changes as well. A new set of water surface profiles representing future conditions, without any proposed project, should be developed for one or more future condition scenarios. These additional profiles can then be used by an economist to develop potential flood damage costs for these future conditions and to conduct the benefit/cost analysis for the proposed flood reduction methods. This effort should commence well in advance of detailed design based on the hydraulic results by other civil engineering disciplines, so that the overall study effort is not delayed.
To analyze the selected methods, the hydrologic and/or hydraulic data sets for base and future condition scenarios can be adjusted to simulate the addition of each structural modification, such as detention storage, levees, channel modification, and flow diversions. The simulations can be achieved by modifying the storage-outflow (routing) relationships in the hydrologic model to simulate reservoirs and/or the geometry in the hydraulic model to simulate levees and channel modifications. Chapters 8, 11, and 12 discuss these activities.
The initial hydraulic planning activities, however, should include the most likely solutions to evaluate with detailed hydrologic and hydraulic modeling. Experienced engineers and planners can inspect the study watershed and often eliminate options that would obviously be ineffective or more costly than other solutions, so these would not need to be analyzed in detail. The evaluation of the project includes the comparison of preproject versus postproject profiles and flooded areas.
For most large projects, especially those undertaken by the federal government, the reduction in flooding for each flood event is translated into an annualized project economic benefit. If the average annual project benefits exceed the average annual project costs, the project is economically justified. If benefit-cost analysis is not applied, then a least cost alternative solution to satisfy a design criterion is selected. Local drainage improvements, bridge construction, and most stormwater detention structures fall into the least-cost option category. The hydraulic engineer, economist, cost estimator, and project manager typically conduct the project evaluation and method selection. An independent technical review of at least the recommended method and why other options are less desirable should be made for QA/QC purposes.
Step 10: Preparing the Report
The report should be considered in the planning stage to determine the time and cost estimates involved in its preparation. The best technical analysis will be poorly received if the resulting technical report is inadequate, but a reviewer will frequently accept the technical work if the report is perceived as a quality product. Consequently, planning activities should include adequate time and budget to prepare a clear, concise, and well-written report of the hydraulic activities. Too often, nearly all the time and budget is spent on technical activities, leaving inadequate resources to prepare the final report. In actuality, the report should be written as the technical work progresses. Some engineers even draft the report prior to the technical work, thereby obtaining a better understanding of how the work needs to progress. The tables, figures, and numeric data are left blank during the initial draft, but the flow of the report indicates how the technical work should proceed. The detailed hydraulic technical outline could also be used in lieu of writing the report first. The final draft report should be independently reviewed for QA/QC purposes.
