Hydraulic Modelling

Hydrologic Frequency Analysis

2008-11-25T09:24:00.000-08:00

One of the primary objectives of the frequency analysis of hydrologic data is to determine the recurrence interval of a hydrologic event of a given magnitude. The recurrence interval, which is the same as the return period, may also be defined as the average interval of time within which the magnitude of a hydrologic event will be equalled or exceeded once, on the average. Statistics is a branch of mathematics that studies the collection and organization of numerical data. Based on statistical analysis of data, inferences are made about larger data sets using the results of related smaller data sets. That part of mathematics used to predict the likelihood of the occurrence of a random even is probability and both of statistical and probability concept is frequently used in hydrologic analysis. Hydrologic frequency analysis is the approach of using probability and statistical analysis to estimate future frequencies (probabilities of hydrologic event occurring) based upon information contained in hydrologic records. Through the use of statistical methods, observed data is analyzed so as to provide not only a more accurate estimate of future frequencies than is indicated by the observed data, but also criteria for determining the reliability of frequency estimates.

The results of flood flow frequency analysis can be used for many engineering purposes such as for the design of dams, bridges, culverts, water supply systems, and flood control structures; to determine the economic value of flood control projects; to determine the effect of encroachments in the floodplain; to determine a reservoir stage for real estate acquisition and reservoir use purposes; for flood-plain zoning.

What Are Floods?

2008-11-25T08:39:00.000-08:00

Definition of Floods

There are a lot of definition and scope of view when we were talking about flood. Normally, flood occurs when prolonged rainfall over several days and intense rainfall over a short period of time that causes a river or stream to overflow and flood the surrounding area. Beside that, flood can be defined as a great amount of water over a place that usually dry or a situation of an overflow of water from it actual barrier. Flood also can be described as an area which actually at normal condition is a dry place that is later filled with water during certain season or occasion due to overflow of water from other area or from heavy rains. Floods also can be defined as a hydrological events characterized by high discharges and/or water levels leading to inundation of land adjacent to streams, rivers, lakes, and other water bodies. The US National Flood Insurance Program also defined a flood as a general and temporary condition of partial or complete inundation of two or more acres of normally dry land area or of two or more properties from an overflow of inland or tidal water, unusual and rapid accumulation or runoff of surface waters from any source or from a mudflow.

Floods are common and costly natural disasters

When rivers overflow their banks they cause damage to property and crops. Floods are common and costly natural disasters.

Floods usually are local, short-lived events that can happen suddenly, sometimes with little or no warning. They usually are caused by intense storms that produce more runoff that an area can store or a stream can carry within its normal channel. Rivers can also flood when dams fails, when ice jams or landslides temporarily block a channel, or when snow melts rapidly. In a broader sense, normally dry lands can be flooded by high lake levels, by high tides, or by waves driven ashore by strong winds. Small streams are subject to floods (very rapid increases in runoff), which may last from a few minutes to a few hours. On larger streams, floods usually last from several hours to a few days. A series of storms might keep a river above flood stage (the water level at which a river overflows its banks) for several weeks.

MIKE 11 & MIKE FLOOD

2008-11-22T01:02:00.000-08:00

MIKE 11 - Dynamic modeling of river, channel and reservoir hydraulics

MIKE 11 is an industry standard for simulating flow and water level, water quality and sediment transport in rivers, flood plains, irrigation canals, reservoirs and other inland water bodies.

Flexibility and speed ensure efficient modelling applications for all aspects of river engineering. In-built parameter optimization tools facilitate efficient model calibration and provide uncertainty assessment of results.

MIKE 11 can be applied on applications ranging from simple design investigations to large forecasting projects including complex hydraulic structure operation policies.

Through dynamic couplings to other DHI software products MIKE 11 allows you to integrate your river and floodplain modeling with models for watershed processes, detailed floodplain representation, sewer systems and coastal processes. MIKE 11 offers also links to external groundwater, and is OpenMI compliant.

Typical application areas for MIKE 11 includes:

Flood risk analysis and alleviation design;
Real-time flood forecasting;
Dam break analysis;
Optimization of reservoir and canal gate / structure operations;
Ecological and water quality assessments in rivers, reservoirs and wetlands;
Real-time water quality forecasting and pollutant tracking;
Sediment transport and river morphology;
Salinity intrusion in rivers and estuaries;
Integrated surface water and groundwater and water analysis.

MIKE FLOOD -Integrated Flood Modelling

MIKE FLOOD is a comprehensive modelling package covering all the major aspects of flood modelling. MIKE FLOOD integrates flood plains, streets, rivers and sewer/storm water systems into one package. Using this integrated approach enables the best engineering and numerical practices to be used where appropriate.

MIKE FLOOD integrates three of the most widely used hydrodynamic models namely MIKE 21, MIKE 11 and MIKE URBAN into one package. The philosophy being that the appropriate spatial resolution is applied where needed e.g. pipes and narrow rivers are modelled using one-dimensional solvers whereas the overland flow is modelled using two spatial dimensions.

MIKE FLOOD offers the following advantages as compared to traditional flood modelling techniques:

Coupled 1D/2D
Integration of hydraulic structures in 2D grids
Effective, stable and locally/globally mass conserving flooding/drying routine
Applied for riverine, urban and coastal flood mapping
Accurate and physically based simulation of flow splits
Extensive user support and documentation.

Click Here To Download MIKE 11 & MIKE FLOOD

HEC-RAS

2008-11-21T14:22:00.000-08:00

HEC-RAS is a computer program that models the hydraulics of water flow through natural river and other channels. The program is one-dimensional, meaning that there is no direct modeling of the hydraulic effect of cross section shape changes, bends, and other two- and three-dimensional aspects of flow. The program was developed by the US Department of Defense, Army Corps of Engineers in order to manage the rivers, harbors, and other public works under their jurisdiction; it has found wide acceptance by many others since its public release in 1995.

The Hydrologic Engineering Center (HEC) in Davis, California developed the River Analysis System (RAS) to aid hydraulic engineer in channel flow analysis and floodplains determination. It includes numerous data entry capabilities, hydraulic analysis components, data storage and management capabilities, and graphing and reporting capabilities.

The basic computational procedure of HEC-RAS for steady flow is based on the solution of the one-dimensional energy equation. Energy losses are evaluated by friction and contraction / expansion. The momentum equation may be used in situations where the water surface profile is rapidly varied. These situations include hydraulic jumps, hydraulics of bridges, and evaluating profiles at river confluences.

For unsteady flow, HEC-RAS solves the full, dynamic, Saint-Venant equation using an implicit, finite difference method. The unsteady flow equation solver was adapted from Dr., Robert L. Barkau’s UNET package.

HEC-RAS is equipped to model a network of channels, a dendritic system or a single river reach. Certain simplifications must be made in order to model some complex flow situations using the HEC-RAS one-dimensional approach. It is capable of modeling subcritical, supercritical, and mixed flow regime flow along with the effects of bridges, culverts, weirs, and structures.

The purpose of HEC-RAS program is for modeling water flowing through systems of open channels and computing water surface profiles. HEC-RAS finds particular commercial application in floodplain management and flood insurance studies to evaluate floodway encroachments. Some of the additional uses are: bridge and culvert design and analysis, levee studies, and channel modification studies. It can be used for dam breach analysis, though other modeling methods are presently more widely accepted for this purpose.

Download your HEC-RAS software here.

Physical & Numerical Model

2008-11-19T08:33:00.000-08:00

Once the flows entering a river network during flood are known, a way of predicting how the flood event will propagate is needed. River catchments taken as a whole is a complex system. The main system of channels may simply flow together or, as is often case in developed areas, have interconnecting channels joining various limbs of the network together. The flow within a given channel is affected by the way the water level is controlled, by variations in channel roughness, the channel cross-sections and bed levels, and the location and shape of bridge openings and other structures.

Hydraulic models are an essential tool in the design of flood alleviation works, assessing levels of service and estimation of residual flooding (John, 1991). The models are used to understand past flood events and predict possible behaviour of future events. There are broadly two types of model namely physical model and mathematical model.

The Physical Model

Physical modelling involves studying the behaviour of water in a scaled physical model of the relevant part of the catchments. Flow velocities and depths of the water in the model are assumed to be proportional to that in the prototype according to an appropriate scaling law such as Froude Number. Water levels and discharges are controlled at appropriate points in the model and the resulting flow conditions measured at points of interest. Such models have been used in the past on extensive sections river. However, because of the high cost of large physical models, the considerable space which they occupy, and the subsequent need to break them up to release that space, such models are now most often confined to specific areas of the catchments to investigate local flow patterns or calibrate control structures.

The Mathematical (Numerical) Models

The numerical models are more appropriate to the modelling of extensive reaches of river and large areas of floodplain. In a mathematical hydraulic model the equations most nearly representing conditions in the river under investigation and on the floodplain are identified and expressed in a form amenable to solution on a computer. The river characteristics and floodplain topography are given in the form of data appropriate to the model. These will comprise channel cross-section data at intervals along channels, bank and floodplain levels, details of flow controls, inflow hydrographs and observed flood levels. Example – HEC-RAS, Mike 11, EXTRAN, EXTRAN-XP and others.

Fundamental of Hydraulic Engineering

2008-11-19T05:33:00.000-08:00

The word "hydraulic" comes from the Greek word "hydraulikos", meaning water. The vital importance of water to human life is justified by the fact that hydraulic engineering is as old as civilization itself. There is much evidence that hydraulic system of considerable magnitude existed several thousand years ago. A large-scale drainage and irrigation system built in Egypt can be dated back to 3200BC. Rather complex water supply systems, including several hundred kilometers of aqueducts, were constructed to bring water to ancient Rome. Dujenyen, a massive irrigation system in Siechman, China, build nearly 2500 years ago, is still in effective use today. The abundant knowledge developed from these experiences has been found to be indispensable.

Hydraulic system are designed to deal with water at rest and in motion, and designed to control, conserve, or transport water. The planning, construction and operation of hydraulic system involves the application of fluid mechanics and others science and engineering disciplines in the design of structures, and the development of projects and systems involving water resources.

Unlike most other branches of engineering, each hydraulic project encounters a unique set of physical conditions to which it must confirm. There is no standard solution for which simple handbook answers can be assessed. Hydraulic engineering itself relies on fundamental knowledge that must be applied to meet the special conditions of each project.