CulvertW20

Introduction

CulvertW20 is a computer programme designed to primarily assist Engineers and Designers undertake the hydraulic assessments and performance required for the sizing of culvert conduits under road embankments.

The programme includes:

1.      Hydraulic Analysis of Culvert/Conduits

2.      Backwater analysis of outlet channels downstream of a culvert crossing

3.      Weir analysis for weir/road overtopping

Designers may create as many Projects as required.

Each Project may comprise the following components, in combination or separately:

�         Catchment plus Channel

�         Catchment plus Culvert

�         Catchment plus Weir

�         Catchment plus Culvert plus Weir

�         Catchment plus Channel plus weir

�         Catchment plus Culvert plus Weir plus Channel

Every Design Project must have a Catchment and the catchment flow must be greater than Zero (0), Otherwise there is no flow in a culvert, over a weir or in a channel and calculations are irrelevant.

The number of design project and library components is relatively unlimited. Data File size on most hardware used today is effectively irrelevant. Notwithstanding this it is suggested that the designer maintain reasonable control of the number so that the potential for unnecessary duplications is minimised and to reduce time to load/save data.

Libraries of Data

CulvertW20 provides for the creation of libraries of common use components required for the hydraulic assessment of a particular or multiple Projects.

These Libraries of data are:

�         Catchment Library

�         Culvert/Conduits Library

�         Outlet Channel Library

�         Weirs Library

This program feature permits designers to enter data for numerous design options and combinations of data.

Libraries simplify the data entry requirements by permitting designers to have the same data available for a hydraulic component that can be used in multiple Design Projects.

The benefits of this feature are:

�         Eliminates having to duplicate data for every Design option

�         Reduces/minimises potential for errors in data entry, particularly for Channels that have multiple cross-sections with many points defining the section and weirs�

Example of benefit:

�         Channel data for an outlet channel only needs to be entered once and then imported into multiple designs, where catchment, culvert and/or weir data may vary

�         Weir data, as the profile of the weir/road formation remains constant for many designs being assessed.

Project Screen

The Project screen provides for the inclusion of design data in Libraries into the hydraulic assessment.

The screen also provides for the Designer to set a value for the Outlet Water Surface level downstream of the Crossing/Project.

If a channel analysis has not been included in the Project calculations the designer may specify a Tail Water Level to be used in calculating hydraulics of Culvert/Weir.

If a channel analysis has been included in the Project calculations the Tail Water Level determined from the analysis of the hydraulics in the outlet channel is used for hydraulic calculations of the Culvert/Weir and any value entered by the Designer is ignored.

Accuracy of Calculations

Complex theoretical calculations using the energy equations and backwater analysis can predict the profile of the hydraulics of the flow in the upstream channel, within the culvert, over a weir and downstream of the culvert. The analysis assumes the designer may confidently predict and/or determine and/or develop well known and definitive values for the physical environment.

In practice the built and natural environment, the catchment and/or inflow to the culvert, the construction of the culvert and the outlet tailwater predictions in the channel downstream of the outlet are not fully quantifiable and will impact the ultimate theoretical calculations.

The accuracy of all the calculations is significantly impacted by two critical design parameters, namely,

�         Design Flow��

No matter what method is adopted to calculate the flow in the system there is an inherent inaccuracy. This inaccuracy in the quantum of the flow is sufficient to generate potential inaccuracy in the ultimate calculation results.

�         Tailwater Level

Determination of the design tailwater level immediately downstream of the outlet can involve many and varied assumptions. These can be the standing level of the receiving water body, backwater analysis in a manmade or natural outlet channel and other design assumptions for any calculations.

In CulvertW20 all calculations are carried out at a level of accuracy comparable to Excel, using 96bit (12 byte) Decimal accuracy (29 significant digits). Data entered by designers is generally to three decimal places. Consequently, there will always be some calculation inaccuracy due to conversions to and from text (string) values and decimal numeric values.

Results of hydraulic calculations for culverts and weirs are comparable to those obtained using software such as HY8.

Minor variations in final values will generally be the result of rounding of calculations, and minor differences in some data used for calculations.

Most results are plus/minus 5mm in level, with numeric rounding to 2 decimal places indicating 10mm in different between CulvertW20 and HY8.

Sensitivity Assessment

It is suggested that Designers should consider the potential inaccuracies in data and assumptions made as to design values and inputs into the calculations.

Consideration should particularly be made to potential inaccuracy in the inflows to the system and the determination of the downstream tailwater level.

A prudent course is to undertake an assessment of the risks and potential impacts on the system by considering possible/potential variations in the values to the design inputs.

REFERENCES

�         U.S. Department of Transportation (1965) nomographs

�         Publication No. FHWA-NHI-12-029, Hydraulic Design Series Number 5, Hydraulic Deign of Highway Culverts (Third Edition), HDS5

�         ACPDM - Culvert Pipes, Boxes, Ellipses and Arches � American Concrete Pipe Design Manual (refer: American Concrete Pipe Association - Library of Congress catalogue number 78-58624)

�         CPAADM - Hydraulic Design Manual, Hydraulics of Precast Concrete Conduits Pipe and box culverts, Concrete Pipe Association of Australasia

�         Engineering paper by Dr M. J. Boyd to the 21st IAHR Congress, Melbourne, Aust., August 1985.

�         HY-8 User Manual (v7.6), HY-8 Culvert Analysis Program