Transactions on Ecology and the Environment vol 17, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541
Evaluation of software for the hydraulic analysis
of highway culverts
Vassilios A. Tsihrintzis* & Jose Luis Oliveros^
"Department of Environmental Engineering,
Democritus University of Thrace, Xanthi 671 00, GREECE,
Email: tsihrin@platon.ee.duth.gr, tsihrin@otenet.gr
* Department of Civil and Environmental Engineering,
Florida International University, Miami, FL 33199, USA
Abstract
Several methods have been developed for the analysis and design of highway
culverts, including those by the US Federal Highway Administration, the US Corps
of Engineers, state and local agencies and private companies. This study has
collected, described, evaluated and presents the most popular methods and software
used today in the USA. Description and evaluation include: technical capability,
ease in use, cost and other factors. Major part of the evaluation is comparison
between predicted and measured velocity, discharge and upstream/downstream
water surface elevations for several culverts located at a study area in South Florida,
aiming at further evaluating the technical capability of the various methods and
software. It was found that the pieces of software that receive the overall highest
score were the WSPG, HEC-2 and SWMM-EXTRAN.
1
Introduction and background
Hand calculation nomographs, tables and procedures to analyze and design
culverts have been developed by the US Federal Highway Administration'
(FHWA). This method was used for many years until the implementation
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Computer Methods in Water Resources XII
of computers and software to the engineering field. Computers offer
advantages over hand calculation methods, including: variables can be
easily changed, results can be obtained faster, and alternatives can be tested
to optimize the design. Furthermore, calculations may be more accurate
decreasing round off errors, and the output can be saved and printed in a
format that is easy to read and can serve as part of the final documentation
(Oliveros^). Finally, computers today are inexpensive tools which increase
efficiency and effectiveness in highway drainage design (Richards^).
Because of these advantages, different pieces of software have been
developed for engineering applications in culvert analysis and design,
something that brought up the need to comparatively evaluate computer
program performance. Previous studies on this have been very few. For
example, a previous culvert software comparative study by Khine/ who
tested the US Geological Survey (USGS) Culvert Program, the FHWA HY8 and the US Corps of Engineers HEC-2, came to the conclusion that the
results from these three programs are close to each other because the
equations used are essentially the same. Nevertheless, the suggested values
of the various coefficients are different resulting in slightly different results.
Also, if the type of flow is full flow with free outlet condition, the results
from the hand calculations should be used instead of HEC-2 and HY-8.
In this study, an extensive critical evaluation of software was
performed and comparative rating is provided. In order to evaluate software
and methods, it was also necessary to collect field data such as flow,
velocity, headwater and tailwater elevations, culvert geometric
characteristics and invert elevations. The following computer programs
were tested: WSPG/ CulvertMaster,* StormCad/ Culvert for Windows,*
HydroCalc/ HY-8," HYCLV," HEC-2,^ THYSYS^ SWMM-EXTRAN,"
andXP-SWMM/s
2
Study area and methodology
As mentioned, field data was collected to evaluate the technical capability
of the culvert software. This was done at Taylor Slough in the Everglades
National Park (ENP) in South Florida, at various culverts crossing State
Road 9336. This road, which is the main road crossing ENP, spans across
Taylor Slough, interrupting its free water flow. Twenty three water
conveyance structures were constructed under the road to facilitate the
movement of water along the slough. Among these 23 structures, there is
the Taylor Slough Bridge (Structure 19) and 22 culverts. For the purpose of
Transactions on Ecology and the Environment vol 17, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541
Computer Methods in Water Resources XII
547
this research, three culverts have been selected. A box culvert (Structure
18), 8 feet wide and 4 feet deep, located on the east side of the bridge and
two 36-inch circular culverts (Structures 17 and 22) (Sikkema et al.'*). The
reason for the selection of these structures is that they are located at the
deeper part of the slough conveying most of the flow (i.e., approximately
86% of the total flow goes through structures 16 to 22, including the
bridge). Therefore, these culverts carry flow for relatively longer time
during the year compared to the other hydraulic structures. Culverts 17 and
22 have the same diameter (36-inch), but the second one has significantly
more sediment deposition than thefirstone, which makes the reduced cross
section an important point to test the various computer programs.
The field data collection task was divided into two parts: (1) structure
surveying; and (2) hydraulic data collection (i.e., upstream and downstream
water surface elevation, flow velocity and sediment deposition height).
Extensive field work was performed to acquire the geometric data for the
three structures. The length and the invert elevations of the inlet and outlet
of the culverts were estimated using standard surveying techniques (i.e.,
tape measuring and leveling). As a reference point was taken a second order
benchmark located on the east side of Taylor Slough Bridge. The length of
each culvert was measured by placing the measuring tape across the road
along the pipe. All numbers and procedures were recorded in a field book.
The hydraulic data collection comprised two parts: water level
measurements and water velocity measurements. Culvert 17 (circular) and
18 (box) do not have a water surface elevation gage. Therefore, the
inlet/outlet water surface elevations were obtained by measuring the vertical
distance from the crown of the culvert to the water surface (above or below
the crown). If the water surface was above the crown, the measured vertical
distance was added to the corresponding inlet/outlet invert elevation, the
inside height and the thickness of the culvert to obtain the water surface
elevation. On the other hand, if the water level was below the crown, the
measured vertical distance was subtracted from the inside height and added
to the corresponding inlet/outlet invert elevation. Also, a rod was used to
measure the inside diameter/depth of the culverts to determine sediment
height. Culvert 22 has water surface elevation gages at both the inlet and
outlet. Therefore, the water level was measured directly. All measured water
surface elevations were checked against the water level continuously
recorded at Taylor Slough Bridge which is done using a continuous batterypowered level measuring device (Omnidata Potentiometer/Pulley Water
Level Sensor, Model ES-310 C). This device is connected to a Campbell
Wiring and Control Module (Model CR10WP) containing a Campbell
Storage Module (Model SM 192) (Sikkema et al.'*).
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Computer Methods in Water Resources XII
Water velocity measurements were made at the inlet of the structure
when detectable flow occurred. Most data was obtained during the wet
season (May through October) when considerable flow occurs, although
some measurements were also made during the dry season (November
through April). For velocity measurements, a Marsh-McBimey Model 201
electromagnetic flowmeter with the appropriate rod, was used. Velocity
measurements were performed using standard USGS techniques (Rantz et
al.^) using the 0.6 or 0.2 and 0.8 water depth criteria. When measuring the
circular culverts the velocity sensor was set at 0.6 of the water depth (one
point only). However, when measuring water velocity in the box culvert, the
cross section was divided into three lateral subsections of approximately
2.67 feet each, the sensor was set at 0.2 and 0.8 of the water depth if that
was greater than 2.5 feet (six points total), or at 0.6 of the water depth when
the water depth was less than 2.5 feet (three points total). The effectiveness
of this method has been evaluated by Tsihrintzis et al.^
For the box culvert, the distances from the top of any deposited
sediment to the soffit of the culvert were also measured for each mid-section
bay and used with the span widths to calculate the effective cross section.
The area of a given subsection within the box culvert was calculated using
its width and the average subsection depth (Sikkema et al.^). Velocity was
computed as the average of two subsection measurements (0.2 and 0.8 of
the depth), or as one measurement at 0.6 of the depth. Subsection discharges
were then added to compute total discharge and cross-sectional average
velocity.
The two circular culverts selected for the study were 3 feet in
diameter. The cross-sectional area, taking into account any deposited
sediment, was derived based on geometry, and the average velocity based
on the velocity measurement that was taken at the 0.6 of the water depth
measured from the water surface. If the water level was higher than the
soffit of the culvert, the velocity measurements were made at a position of
0.6 of the culvert diameter measured from the soffit of the culvert and
discharge was computed as for the box culverts (Sikkema et al.^). Finally,
an average Manning's n was used in the study based on the wetted
perimeter comprising concrete and sediment. For the computation,
equations presented by Chow/* based on the wetted perimeter to the 1.5
power, were employed.
Transactions on Ecology and the Environment vol 17, © 1998 WIT Press, www.witpress.com, ISSN 1743-3541
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Results and conclusions
The computer programs tested in this paper could be divided in two main
groups: (1) the software designed specifically for culvert analysis and
design, and (2) software developed for storm drain energy grade line and
hydraulic grade line computation, that could also be used for culverts. From
these two groups, there are programs that run in WINDOWS Mode or DOS
Mode. From experience, it is known that software operating in WINDOWS
are easier to use and faster to learn. The results predicted by groups (1) and
(2) software are absolutely comparable because the hydraulic principles
used are similar.
However, there were observed general disadvantages for WINDOWS
type of software. First, they are less flexible when the user is inputting data,
i.e., it allows the user to use just the values pre-defined by the software in
almost all data input windows (for example, cross-section shape and size,
minor loss options, entrance and outlet configurations, Manning's n
coefficient, culvert material, etc). Second, the system requirements to run
any DOS Mode operating software are easily satisfied by any PC computer
manufactured today. On the other hand, the WINDOWS Mode operating
software usually needs a fast Computer Processing Unit (CPU) and a good
Random-Access Memory (RAM) size (sometimes up to 16 megabytes may
be required). Finally, the manuals for software running in DOS are usually
more complete and comprehensive explaining theory and step-by-step input
format. On-line help, which is a characteristic of WINDOWS operating
software, usually does not provide the necessary information, and theory
that the user may need is often missing.
Input files were prepared for the 11 pieces of software tested, to
describe the geometry of the culvert as surveyed and the flow conditions.
A total 38fieldexperiments and simulations were performed. Output from
each software was mainly the following: upstream water surface elevation
and velocity. The evaluation of the technical capability of each piece of
software in predicting water surface elevation and velocity was done by
plotting predicted quantities versus measured ones and fitting a straight
regression line through the data. The slope of the straight line and the
correlation coefficient are measures of the goodness of predicted data. In
fact, the closer the slope is to 1, the closer the predicted values are to the
measured ones. Values of the slope less than 1 imply overall underprediction and values of the slope greater than 1 imply over-prediction. The
correlation coefficient has also to be as close to 1 as possible, indicating
scattering of the data around the regression line.
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Computer Methods in Water Resources XII
When plotting the headwater data for the three culverts, the best
prediction slope was given by THYSYS (0.9829) and the lowest value was
reported by SWMM-EXTRAN (0.9688). The other software gave: WSPG:
0.9758, CulverMaster: 0.9737, StormCad: 0.9703, Culvert for Windows:
0.9228, HydroCalc: 0.9727, HY-8: 0.9282, HYCLV: 0.9786, HEC-2:
0.9695, XP-SWMM: 0.9732. The correlation coefficients ranged from
0.8123 to 0.9322. Velocity prediction slopes were close to 1.0 for WSPG
(0.9970) and HEC-2 (0.9695) when taking into account all culverts. For the
rest of the software the slopes were about 0.9700. The exact values were the
following: CulverMaster: 0.9689, StormCad: 0.9663, Culvert for Windows:
0.9690, HydroCalc: 0.9616, HY-8: 0.9690, HYCLV: 0.9896, THYSYS:
0.9680, SWMM-EXTRAN: 0.9676, XP-SWMM: 0.9410. The correlation
coefficients ranged from 0.9962 to 0.9998.
Other comments from the comparison include: SWMM-EXTRAN
does not take into account minor losses. Therefore, adjustments need to be
made in the input file to make up for that deficiency. Tsihrintzis et al.^
describe a method to do this. Most software, except WSPG and HEC-2, can
not analyze a culvert with irregular section. When there is sediment
deposition, the cross-section of the culvert is not a circle or a box anymore.
WSPG and HEC-2 can analyze irregular sections described by X-Y
coordinates (up to 99 coordinates for WSPG and 100 for HEC-2) in order
to minimize calculation errors. As opposed to HY-8, which just can take up
to 19 coordinates to define a section. WSPG and HEC-2 predictions were
among the best ones. On the other hand, HEC-2 inputfilesare laborious and
take a long time to put together a culvert irregular section. SWMMEXTRAN shows increasing error in continuity when the tailwater elevation
is lower than the crown of the culvert. HYCLV did not run for circular
culverts when the tailwater elevation was lower than the crown of the
culvert, and its buried pipe option seems to work for buried depths greater
than 0.13 feet. Some of the studied programs do not perform calculations
with zero or adverse slopes, which is the case of HydroCalc (slope range =
0.0001 to 1.0000), Culvert for Windows, CulvertMaster and HYCLV.
HydroCalc and HY-8 do not produce output calculation report as others do.
HY-8 outputfilescan be saved, but HydroCalc input/outputfilescan not be
saved. HY-8 seems to give erratic predictions when the given tailwater is
very low. It gives as output a default value for headwater predictions when
very low tailwaters are used. It was also observed, that storm drain software
gives as good or better output than special culvert analysis and design
software because some of the storm drain programs allow the user to enter
irregular sections due to sediment deposited and because the hydraulic
principles are the same.
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551
These discussions lead to an overall ranking of the software, based on
the following categories: easiness in input preparation, presentation of
output, accuracy in velocity and headwater predictions, capability to
analyze irregular or various types of sections, adverse/horizontal slope
capability, manual completeness/technical support, various additional
options and features, price and system requirements. The ranking in each
category was done as 1 (fair), 2 (good) and 3 (excellent). The scores were
added and the highest overall score (including technical capability as a
category) was obtained by: WSPG, HEC-2 and SWMM-EXTRAN. They
were followed by StormCad and THYSYS. The last software in the ranking
were HydroCalc and HY-8. Although SWMM-EXTRAN and XP-SWMM
are based on the same theory, XP-SWMM ranked lower because of the lack
of detail in the user's manual and the additional system requirements.
Nevertheless, their predictions were close to each other except for velocity.
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