4) A hydraulic model allows for quick modification of key variables, such as Manning's n, to develop scenarios for the determination of the most appropriate solutions on flood problems (Klotz et al., 2003).
Another tool widely used in floodplain delineation studies is the Geographic Information Systems (GIS) which offer the ideal environment for this type of work.
GIS offers hydrologists and planners the powerful capability to analyze and visually express flood control measures.
Although GIS is an excellent tool to accomplish the requirements of maintaining, acquiring, and utilizing a spatially referenced database, it is limited in its ability to perform floodplain modeling and flood damage calculations. Thus, GIS have been incorporated with hydrologic and hydraulic models to delineate floodplain areas. There are several advantages of using GIS with these models: 1) it is possible to integrate data from different sources, 2) GIS can create spatial relationships that are important in floodplain modeling studies, 3) the display and data organizing capabilities of GIS are powerful tools for visualizing insights of the physical process of storm water transport (Shrestha, 2000). Therefore, the GIS-based Flood Information System (GFIS) was developed for floodplain modeling, analysis of project feasibility, and informational support (Yang & Tsai, 2000).
Developments in fully dynamic, unsteady models have provided engineers with highly accurate hydraulic modeling techniques that result in two-and three-dimensional graphical visualizations for flood analysis. The key to graphical visualizations in dynamic modeling is the inclusion of time-series data within a spatial interface (Snead, 2000).
Figure 6.2 Steps of floodplain modeling studies (Klotz et al., 2003).
Setting project and study objectives
Study phases
Type of Model Needed
Determination of Data Needs
Defining Hydrologic Modeling Procedures
Data Input and Calibration
Production Runs for Base Conditions
Alternative Evaluations
Report Preparation
Preliminary Evaluation Feasibility Detail of
Design Hydrology
Data Hydraulic
Data Discharge
Data Other
Geometry Data Hydrology
Data Hydraulic
Data
Recommended Plan Field Study
1) Setting project and study objectives 2) Study phases
3) Field study
4) Determination of the hydrologic and hydraulic simulation types needed 5) Determination of data needs
6) Defining hydrologic modeling procedures 7) Preparing input data and calibration
8) Performing production runs for base conditions 9) Performing project evaluations
10) Preparing the report (Klotz et al., 2003).
1) Setting project and study objectives: The main objective of floodplain modeling studies is the analysis of flood damage reduction. The other objectives considered may be the analysis of flood impacts on navigation, hydropower, irrigation, water supply, environmental concerns, and permits.
2) Study phases: There are three main phases in a flood study; preliminary evaluation phase, feasibility phase, and detailed design.
A preliminary evaluation is made when it is uncertain whether there is an economic interest in further pursuing a project. Rough designs with associated costs and benefits are developed. This information is then used to ascertain if it is likely that more-detailed studies will lead to a feasible and desirable solution. Since there is limited time and money for this phase, evaluations are often based on available hydraulic data and engineering judgement.
In the feasibility phase, the scope and magnitude of the project are specified.
Here, floodplain hydrology and hydraulic analyses are performed for base conditions. For this purpose, hypothetical frequency flood profiles are determined, and then final hydrologic and hydraulic studies are performed to determine potential flood damages along the analyzed stream (Klotz et al., 2003).
The detailed design phase covers the structural and hydraulic design of a flood control scheme.
3) Field Study: Field studies are needed for all modeling studies. The hydraulic engineer should take photographs of representative reaches of the river, bridges and culvert crossings of the main channel, and the adjacent floodplains. The channel bed material should also be surveyed at different locations. Bed material grain size is an important factor in estimating Manning’s n for the channel.
The river banks should also be surveyed as well. 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 give information on changes in channel geometry. These changes may need to be included in the floodplain modeling process (Klotz et al., 2003).
Highwater marks can be obtained from interviews with local residents to collect calibration data.
4) Determination of hydrologic and hydraulic simulation types needed: This is another important step in floodplain modeling. The most appropriate method for floodplain modeling will always be a one-dimensional model, coupled with either steady, quasi-unsteady, or fully unsteady hydraulics (Klotz et al., 2003).
5) Determination of data needs: Flow and geometry related data are the main requirements for floodplain modeling. If the study reach is short and uncomplicated, only a peak discharge value may be necessary. On the other hand, a full hydrograph is required if the reach is long with several tributaries, or if there are ongoing changes in the watershed.
Channel and floodplain cross-sectional 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 include sharp breaks in the channel slope, large expansions or contractions of the floodplain width, and significant changes in land use or vegetation (Klotz et al., 2003).
Cross sections should extend across the entire width of the floodplain, if possible.
The geometry data of hydraulic structures should also be obtained either from new surveys of old structures or by getting sections of structures from the agency responsible.
6) Defining hydrologic and hydraulic modeling procedures: This step is an important part of the planning process. The hydrologic model to be used in the modeling process should be the acceptable and accurate for the project area.
The modeling procedures can be selected on the basis of the following criteria for steady flow modeling:
• Precipitation Data: Depth, temporal distribution, or mean areal precipitation.
• Infiltration modeling technique: Uniform and initial, SCS curve numbers, Green-Ampt, Holtan, Horton, or other methods.
• Runoff Modeling: Kinematic wave or synthetic unit hydrographs (SCS, Clark, Snyder, or other methods).
• Hydrograph Routing: Straddle-Stagger, Muskingum, Modified Puls, Muskingum-Cunge, or other methods.
• Calibration Data: If a flood has produced known highwater marks or if stream gage data are available, gaged rainfall data should also be obtained. Rainfall maps are prepared, using the Theissen or isohyetal techniques. 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 (Klotz et al., 2003).
For unsteady flow modeling, modeling procedures should 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, and 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. The discharge data come from gage records or from simulation with runoff models. A “warm–up” period featuring a constant flow for a specified time is typically included as an initial condition for the model operation (Klotz et al., 2003).
7) Preparing input data and calibration: Preparation of input data, debugging and model calibration require significant amounts of time and effort to enter all cross-sections, bridges, culverts, dams, diversions, and other structures affecting water surface profiles into HEC-RAS model. If storage-discharge data are needed, ineffective flow areas should be specified in the HEC-RAS cross-sectional data for hydrologic routing. Historic discharges 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 (Klotz et al., 2003).
8) Performing production runs for base conditions: Model calibration and verification processes are the most important steps in modeling studies. Actual representation of the floodplain depends on calibration, but data availability is generally the most problematic issue for modelers. Further adjustment to model parameters during the model runs may be required, based on available local data and engineering experience (Klotz et al., 2003). In the simulation of large and rare events, the following actions may be required:
• Modification of infiltration parameters to reflect more runoff
• Modification of peaking coefficients to increase the peak discharge of the unit hydrograph
• Modification on routing of travel times to reflect a faster movement of the hydrograph through a routing reach
• Reduction of Manning’s n to reflect a more efficient channel during rare flood events
• Simulation of the buildup of trash and debris during a major flood.
9) Performing project evaluations: When modeling process is completed, the engineer has several water surface profiles, indicating the flood levels from actual or hypothetical floods at any location in the study area. These water surface profiles and inundated areas are obtained by examining a number of scenarios which deal with flood reduction studies or changes in the basin. For example, land use changes such as urbanization or reservoir construction may be possible in the future. Since both the hydrologic and the hydraulic model will change in these cases, they should be run for the expected changes; and, thus, new profiles are computed. Such additional runs are recommended in order to show possible future developments that will affect floodplains. In addition to potential damage costs, a benefit/cost analysis is prepared for the future conditions as well. This effort should commence well in advance of a detailed design of flood control measures by other civil engineering disciplines, so that the overall study effort is not delayed (Klotz et al., 2003).
Although all possible alternatives are examined in initial planning activities, only adequate solutions with respect to effectiveness and costs are analyzed in detail. Pre-project versus post-Pre-project profiles and inundated areas are assessed.
Unlike the case of small-scale projects, the reduction in flooding for each scenario is expressed as an annual project economic benefit for large-scale projects.
Appropriateness of the project in economic terms depends on the benefit-cost ratio and the least cost condition. In the former, the suitable relation is the one for which the average annual costs are less than the annual benefits; while in the latter, a least cost solution is selected to satisfy a design criterion. The hydraulic engineers, economists, and project managers are work together in the project evaluation phase (Klotz et al., 2003).
10) Preparing the report: In the planning stage, a report, which includes the estimates on time and costs spent on the project, is essential. Acceptance of the
technical work generally depends on the adequacy of the technical report. Thus, sufficient time and budget should be allocated to the report which should be brief, clear, and well-written (Klotz et al., 2003).
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CHAPTER SEVEN
HYDROLOGIC AND HYDRAULIC MODELING TOOLS
Hydrologic and hydraulic modeling studies play an important role in floodplain management studies. These studies, which include the estimation of flood peaks and/or flood hydrographs and the determination of flood inundation maps, have been realized first to save human lives, and then to protect people’s property, agricultural areas, animals, etc. (Olivera & Maidment, 2000). Thus, hydrologic and hydraulic models have become essential tools in prediction of floods and the investigation of flood management alternatives. This chapter discusses the modeling capabilities of the computer programs utilized in this study.