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Section 5: Hydrology Study Data Requirements

Strictly speaking, the term data refers to measurements or observations, and the term information refers to results of analysis or synthesis of data. Both data and information are needed for hydrologic studies, and the terms are used interchangeably here. To determine what data are needed, the designer must determine which hydrologic analysis method(s) will be used.

The major task of a hydrology study is to compute design flow. There are conceptual methods and empirical methods for computation of design flow.

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Hydrology Analysis Methods

Conceptual methods in this category simulate, with a mathematical model, channel flow and watershed runoff processes. Movement and storage of water through the watershed are simulated at varying time and space scales, with varying degrees of complexity, omitting, including, or combining elements, depending on the model used and the requirements of the study.

Conceptual methods that TxDOT designers may use include the Rational method (loosely classified as a conceptual method here) and the hydrograph method.

Like conceptual methods, empirical methods also use a mathematical relation that predicts the design flow, given properties of the watershed, channels, rainfall, or streamflow. However, the relationship does not represent explicitly the physical processes. Instead, the relationships are derived with statistical analyses. (Some analysts even refer to empirical methods as black box methods because the presentation of the process is not visible and obvious.)

Empirical methods that TxDOT designers may use include flood frequency analysis of streamflow observations and regression equations. With flood frequency analysis, the empirical relationship predicts the design flow from statistical properties of the historical streamflow in the watershed. With regression equations, the design flow is predicted with an equation that has been developed by correlating flows observed with watershed, channel, and rainfall properties.

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Data Requirements Vary with Method Used

Data and information required for hydrologic analysis vary from method to method. The conceptual methods require somewhat detailed information about the watershed and channel properties, whereas the empirical methods require streamflow data to establish the relationships and only limited data on watershed and channel properties to use the derived relationship.

Specific requirements for the different methods are called out in later sections of this Chapter, but broad categories of data required include the following:

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Geographic and Geometric Properties of the Watershed

All hydrologic analyses for TxDOT studies require collection of data about the geographic and geometric properties of the watershed. These data include, but are not limited to, the following:

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  • Geographic location of the point at which design flow must be computed.
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  • Location of the boundaries of the watershed from which runoff contributes to flow at the point of interest. This information will, for example, govern selection of design rainfall intensities that will be used with the rational method if that is selected for design flow computation.
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  • Properties of the watershed within those boundaries. These properties include area, slope, shape, and topographic information. This information is needed, for example, to develop a model with which to simulate overland flow, as shown in Figure 4-10 whereby water ponded on the surface moves across the watershed into channels.
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Land Use, Natural Storage, Vegetative Cover, and Soil Property Information

Data that describe the watershed properties are needed for the conceptual models, and to a limited extent, by certain empirical models.

A conceptual model of watershed runoff, with components as illustrated in Figure 4-10, represents processes of infiltration and overland flow. To do so, the model must be configured and calibrated with knowledge of the properties of the watershed that will affect infiltration and overland flow. Those include:

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  • Land use in the watershed. Especially important in this is gathering information about the distribution of impervious and pervious cover in the watershed. Rain that falls on impervious surfaces, such as parking lots and rooftops, will run off as overland flow. Rain that falls on a pervious surface may infiltrate, entering the soil layers, and not running off immediately or at all. The rate of this infiltration is related with land use, as well.
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  • Natural storage in the watershed. Water that ponds in natural depressions, lakes, and similar features in a watershed will not run off or may runoff with some delay and with reduced rates. The location of, capacities of, and behavior of storage must be identified if this is to be represented in computations of design flows.
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  • Vegetative cover and soil property information. Rates of infiltration depend on properties of soils in the watershed and upon the presence of vegetation. For example, water ponded on sandy soils may infiltrate at four or five times the rate of water ponded on clay soils. And crops planted on clay soils will increase the rate of infiltration there. Thus, the designer must gather information on the cover and soils. That information should define the spatial variations across the watershed.

These data are needed with conceptual models that do not seek to represent in great detail the physical processes. For example, with the rational method, a runoff coefficient relates runoff rate and rainfall rate. That coefficient is related to land use within the watershed. And knowledge of land use, particularly knowledge of presence or absence of impervious area, is critical for assessing the applicability of regression equations.

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Description of the Drainage Features of the Watershed

Channels, ponds, reservoirs, culverts, and other natural or constructed drainage features in a watershed affect the runoff from the watershed. Thus data that describe those must be collected.

For a conceptual model, data about the features are needed to make a decision about which model to use and configure the model appropriately. For example, with a hydrograph method, data describing channels are needed to select, calibrate, and use a routing method that accounts for the impact of a channel on the design flood peak.

For an empirical model, data on drainage features is needed first to enable wise decisions about which model(s) to use, and second, to estimate model parameters. For example, flood frequency (stream gauge) analysis procedures require that the streamflow records be without significant regulation. To determine if this is so, the designer must have information on regulation in the watershed, including descriptions of ponds, reservoirs, detention structures, and diversions in the watershed.

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Rainfall Observations and Statistics of the Precipitation

Conceptual models simulate conversion of rainfall to runoff by simulating some or all of the processes illustrated in Figure 4-10. Thus, to use a conceptual model, rainfall data are required. These data include both observations of rainfall at gauges in the watershed and statistics on rainfall from which design storms are developed.

With observations of rainfall at gauges, models can be calibrated and tested to ensure that they truly represent the behavior of the watershed.

With statistics of rainfall depths, a design storm can be developed, and the required design flow can be computed following the design storm assumption. This assumption is that “if median or average values of all other parameters are used, the frequency of the derived flood should be approximately equal to the frequency of the design rainfall” (Pilgrim and Cordery 1975).

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Streamflow Observations and Statistics of the Streamflow

Streamflow observations at or near to the location of interest are the designer’s best index of how a watershed will behave under conditions existing in the watershed at the time of observation of the flow. These data serve the following purposes:

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  • Calibration of statistical model. If available, long records of annual maximum streamflow permit flood frequency analysis and design flow determination.
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  • Calibration and verification of conceptual model. Shorter records of runoff from individual floods permit calibration and verification of conceptual models of the rainfall to runoff transformation, if corresponding records of rainfall are available. In this process, model parameters are estimated, runoff from observed rainfall is computed, and the computed flows are compared to the observed. Parameters are adjusted if the fit is not acceptable.
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  • Assessment of reasonableness of results. Records of annual maximum flows at a site for limited periods permit assessment of reasonableness of predicted design flows. For example, if a record of annual maximum flows for 12 years at a site includes six peaks that exceed the predicted 10% chance design flow, a designer can apply the binomial statistical distribution to determine that the probability is only 0.0005 that this could happen. This is so unlikely that it raises doubt about the estimated 10% chance design flow.
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