2 Data Requirements
3.5 Computer Models
There are numerous computer models available for undertaking hydrological analysis. For complex catchments, where a number of sub-catchments are involved, routing needs to be incorporated and there is the potential for storage effects, it is generally simpler and easier to incorporate these into a computer model rather than undertake the computations manually. There are numerous computer models available, and each one can be applicable in a range of situations.
Conceptually, most hydrological models have two functions:
Conversion of rainfall into a runoff from a sub-catchment. There are numerous approaches, including Unit Hydrograph, Storage Function Method, Lawrence method etc.
Routing of the flow from the sub-catchment (from 1) along the main drainage path or river. Types of routing models are discussed in Section 3.4.5. These routing methods may also allow for the incorporation of storages such as dams and detention basins.
Many of the computer models incorporate similar sub-models for undertaking the above calculations.
Computer models are constantly evolving, and it is important for the hydrologist to remain aware of the current software, and advantages and disadvantages of each one. Some current available hydrological software include:
HEC-HMS – available for US Army Corp of Engineers. This modelling system is probably the most commonly used modelling system in the Philippines to date.
XP-RAFTS & XP-SWMM – these two modelling systems are available from xp-solutions. XP-RAFTS represents a stand-alone hydrological modelling software while XP-SWMM includes hydraulic analysis as well.
MIKE Software – Available from DHI, and incorporates hydrological analysis within the hydraulic modelling suite.
SOBEK – Available from Deltares, and incorporates the hydrological model either as stand-alone, or integrated with their hydraulic modelling software.
This is not an exhaustive list, and there are many software available. This Guide does not recommend any particular software over another. However, whatever software is utilized, it is important that key parameters for the model setup be specific in the reporting, to ensure that this can be reviewed appropriately (refer Section 3.6).
Given it wide use within the Philippines, a broad overview of the HEC-HMS software is provided below. As noted above, this does not constitute this Guideline recommending this software over an alternative available software.
3.5.1 The Use of Computer Models
Designers who use computer models have a duty of care to ensure that they are familiar with the software, including the underlying assumptions of the software and algorithms, key input data and interpretation of output data.
It is noted that many of the problems that occur with computer models are not in the development of the program itself, but rather in the application of the software.
Typical issues include:
Incorrectly specified input data.
Errors in the input data.
Application of the model beyond the scope for which it was intended.
Incorrect schematization of the model or representation of the study area.
Incorrect interpretation of the model results.
Refer to Section 4.13 for more details.
3.5.2 HEC – HMS
The Hydrologic Engineering Center’s Hydrologic Modelling System (HEC-HMS) which was developed by the U.S. Army Corps of Engineers, simulates the precipitation-runoff processes of dendritic watershed systems. It is applicable to a wide range of geographic areas for solving the widest possible range of problems.
This includes large river basin water supply and flood hydrology, and small urban or natural watershed runoff. Hydrographs produced by the program are used directly or in conjunction with other software for studies of water availability, urban drainage, flow forecasting, future urbanization impact, reservoir spillway design, flood damage reduction, floodplain regulation, and systems operation.
HEC-HMS presents the watershed runoff process as shown in Figure 3.5. It uses separate model to represent each component of the runoff process and consists of the following:
Modelling of Catchment Runoff, through:
Models that compute runoff volume
Models of direct runoff (overland flow and interflow)
Models of baseflow
Models of channel flow or routing
Figure 3-5 HEC-HMS Watershed Runoff Processes
Source: US Army, 2000
The HEC-HMS models that compute runoff volumes are listed in Table 3-6. These models address questions about the volume of precipitation that falls on the watershed: How much infiltrates on pervious surfaces? How much runs off previous surfaces? How much runoff of the impervious surfaces? When does it runoff?
Table 3-6 Runoff-Volume Models
Model Categorization
Initial & Constant Rate Event, lumped, empirical, fitted parameter SCS Curve Number (CN) Event, lumped, empirical, fitted parameter Gridded SCS CN Event, distributed, empirical, fitted parameter Green & Ampt Event, distributed, empirical, fitted parameter Deficit & Constant Rate Continuous, lumped, empirical, fitted parameter Soil Moisture Accounting (SMA) Continuous, lumped, empirical, fitted parameter
Gridded SMA Continuous, distributed, empirical, fitted parameter
The HEC-HMS models of direct runoff are listed in Table 3-7. These models describe what happens as water that has not infiltrated or been stored on the watershed moves over or just beneath the watershed surface.
Table 3-7 Direct-Runoff Models
Model Categorization
User-specified Unit Hydrograph (UH) Event, lumped, empirical, fitted parameter
Clark's UH Event, lumped, empirical, fitted parameter
Snyder's UH Event, lumped, empirical, fitted parameter
SCS UH Event, lumped, empirical, fitted parameter
ModClark Event, distributed, empirical, fitted parameter
Kinematic Event, lumped, conceptual, measured parameter
Table 3-8 lists the HEC-HMS models of baseflow. These simulate the slow subsurface drainage of water from the system into the channels.
Table 3-8 Baseflow Models
Model Categorization
Constant Monthly Event, lumped, empirical, fitted parameter
Exponential recession Event, lumped, empirical, fitted parameter
Linear Reservoir Event, lumped, empirical, fitted parameter
For modelling channel flow with HEC-HMS are listed in Table 3-9. These are the so called routing models, simulate one-dimensional open channel flow.
Table 3-9 Routing Models
Model Categorization
Kinematic wave Event, lumped, conceptual, measured parameter
Lag Event, lumped, empirical, fitted parameter
Modified Pulse Event, lumped, empirical, fitted parameter
Muskingum Event, lumped, empirical, fitted parameter
Muskingum-Cunge Standard Section Event, lumped, quasi-conceptual, measured parameter
Muskingum-Cunge 8-point Section Event, lumped, quasi-conceptual, measured parameter
Confluence Continuous, conceptual, measured parameter
Bifurcation Continuous, conceptual, measured parameter
In addition to the model of runoff and channel processes, HEC-HMS includes models for simulating a water control structure such as diversion or reservoir/detention pond.
In the HEC-HMS watershed hydrology, the response of a watershed is driven by precipitation that falls on the watershed and evapotranspiration from the watershed. The precipitation may be observed rainfall from a historical event, it may be a frequency based hypothetical rainfall event, or it may be an event that represents the upper limit of precipitation possible at a given location. Historical
precipitation data are useful for calibration and verification of model parameters, for real-time forecasting and for evaluating the performance of proposed designs or regulations. Data from the second and third categories – commonly referred to as hypothetical or design storms are useful if performance must be tested with events that are outside the range of observations or if the risk of flooding must be described. Similarly, the evapotranspiration data used may be observed values from a historical record or they may be hypothetical values.
Details of specifying and analyzing historical or hypothetical–storm precipitation and evapotranspiration with HEC-HMS are referred to HEC-HMS Technical Reference Manual.
3.6 Reporting Requirements
A hydrological report should be prepared for a project. In some cases, both the hydrology and hydraulic report can be incorporated into a single report.
The report should contain, as a minimum, the information on parameters with hyrdrological models provided in Table 3-10 and general information provided Table 3-11.
Table 3-10 Information to be Provided on Parameters with Hydrological Models Hydrological
Model Parameters to Include
Rational Formula Rainfall intensities adopted for the assessment, including either the background on the calculation or the information provided by PAGASA, showing clearly the coordinates or location of the rain gauge
Details of the time of concentration calculation, including:
– Why the method of calculating the time of concentration was adopted – Key parameters assumed for the calculation (such as flow length ‘L’) and
details of their calculation
– Calculated time of concentration value Unit Hydrograph Details on key transformation parameters
Details on any routing or lagging that is applied, and why this was adopted SCS Unit
Hydrograph As above
Curve Number (CN) and why this was adopted. Provide suitable references and information on land-uses.
General Hydrological Models
For all computer based hydrological models:
The software that was used and the version number of that software
Key runoff generating parameters. These may include:
– Catchment slopes and areas;
– Horton roughness parameters or similar
– Rainfall loss models, such as curve numbers or initial/ continuing losses
The type of routing model (e.g. Muskingum-Cunge) that was adopted and
why Details of any storages (such as dams) included in the model
Comparison of the rainfall volume (total rainfall that fell within the design rainfall event) with the runoff volume generated in the model
Table 3-11 Minimum Hydrological Reporting Requirements
Component Description
Project Description A brief description should be providing outlining:
Purpose of the Project
What the project involves
Why the hydrological analysis is required
Study Area A description of the study area should be provided, sufficient so that the reader is aware of the location of the project. This will include:
Description of the study area
Map showing locality of the study area, including key features such as road names, river names etc.
Coordinates of the project site location. If it is a linear structure (e.g. levee), then the approximate centroid of the structure will suffice.
Why the hydrological analysis is required
Catchment Details A map showing the catchment, either identified on topographical, aerial mapping or similar. The catchment map should show the overall catchment and any sub-catchments.
A description should be provided on the catchment, particularly focusing on the land-uses within the catchment, soils, vegetation, slopes and other key features.
Rainfall Data As a minimum, a summary table should be provided with the rainfall intensities for different size rainfall events and different durations. This may be the tabular information provided by PAGASA, for example. This should clearly show the location of where the rain data is representing (i.e. the rain gauge location) Hydrological
Analysis The following should be provided as a minimum:
Statement as to what hydrological analysis technique was adopted, and why.
Summary of all key input parameters for the analysis. A justification for key parameters, such as the Rational Formula ‘c’ value should be provided (refer Table 3-10)).
Results The results should be summarized in a clear and concise format. The results may include:
Peak flows for different size floods;
Discharge hydrographs presented in graphical formats;
Runoff volumes.
The results presented should be suitable for the project application.
Validation For catchments where the Rational Formula is not adopted, either the specific discharge may be used (for rural catchments) or the Rational Formula can be used for a sub-catchment, where this sub-catchment is sufficiently small to meet the criteria in Section 3.4.1. As noted above, it is not intended that the two methods will match but rather that this provides a method for cross checking the magnitude of the results.
3.7 References
DID (Department of Irrigation and Drainage, Malaysian Government), 2012. Urban Stormwater Management Manual for Malaysia, 2nd Edition, Government of Malaysia, Kuala Lumpur.
Flood Control & Sabo Engineering Center, June 2010, Technical Standards and Guidelines for Planning of Flood Control Structures, Japan International Cooperation Agency, Philippines.
Ministry of Public Works and Highways, 1984, Design Guidelines Criteria and Standards for Public Works and Highways, Philippine Government, Manila.
United States Department of Agriculture, 2012. National Engineering Handbook – Part 630 Hydrology – Chapter 15, Natural Resource Conservation Service, May.