BFC 43201 (FOC ASSIGNMENT)
1.
Explain clearly, four main application of HECRAS in Hydraulic Engineering
i.
Culvert Modelling
HEC-RAS computes energy losses, caused by structures such as culverts, in three parts. The first part consists of losses that occur in the reach immediately downstream from the structure, where an expansion of flow takes place. The second part consists of losses that occur as flow travels intro, though, and out of the culvert. The last part consists of losses that occur in the reach immediately upstream from the structure, where the flow is contracting towards the opening of the culvert.
ii.
Inline Structures
This version of HEC-RAS allows the user to model inline structures, such as gated spillways, overflow weirs, drop structures, as well as lateral structures. HEC-RAS could model radial gates (often call tainter gates), vertical lift gates (sluice gates), or overflow gates. The spillway crest of the gates can be modelled as either an ogee shape, broad crested weir, or a sharp crested weir shape. In addition to the gate openings, the user can also define a separate uncontrolled overflow weir.
iii.
Lateral Structures
HEC-RAS could model lateral weirs, gated spillways, culverts, and user entered rating curves. The modeler can insert a lateral weir only, or a separate gated spillway structure, or any combination of the four types.
iv.
Water Quality Analysis
This component of the modelling system is intended to allow the user to perform riverine water quality analyses. The current version of HEC-RAS can perform detailed temperature analysis and transport of a limited number of water quality constituents.
2.
Discuss critical depth determination.
i.
Critical depth for a cross section will be determined if any of the following conditions are satisfied:
The supercritical flow regime has been specified.
The calculation of critical depth has been requested by the user.
This is an external boundary cross section and critical depth must be determined to ensure the user entered boundary condition is in the correct flow regime.
The Froude number check for a subcritical profile indicates that critical depth need to be determined to verify the flow regime associated with the balanced elevation.
The program could not balance the energy equation within the specified tolerance before reaching the maximum number of iterations.
ii.
The HEC-RAS program has two methods for calculating critical depth: a “parabolic” method and “secant” method. The parabolic method is computationally faster, but it is only able to locate a single minimum energy. For most cross sections, there will only be one minimum on the total energy curve, therefore the parabolic method has been set as the default method (the default method can be changed from the user interface). If the parabolic method is tried and it does not converge, then the program will automatically try the secant method.
3.
Discuss energy, mass and momentum equation.
i.
Energy equation
Water surface profile are computed from one cross section to the next by solving the Energy equation with an iterative procedure called the standard step method. The Energy equation is written as follows:
Where Z1 and Z2
=
Elevation of the main channel inverts
Y1 and Y2
=
Depth of water at cross sections
V1 and V2 a1 and a2
= =
g he
Average velocities (total discharge/total flow area) Velocity weighting coefficients =
=
Gravitational acceleration Energy head loss
4.
ii.
Mass equation
iii.
Momentum equation
An engineer is to analyse flow in an open channel. The channel is designed to be constricted by placing bridge embankment at both sides of the channel. Explain the consequences due to the constriction.
Constriction are normally short river reaches where the cross-section is reduced due to bridges and piers. As a result, the hydraulic discharge is obstructed to a certain degree by flood evacuation. The occurrence of such zones may be outside or within the constriction, depending on the flow energy at the upper and lower boundary of the constriction and the flow energy within the constriction. To describe these occurring processes the critical (index crit) water depth must be introduced. 5.
A spillway discharges a flood flow at a rate of q = 7.75 m 3/s per meter width. At the downstream horizontal apron, the depth of flow was found to be 0.50 m. What tail water depth, y2 is needed to form a hydraulic jump? If a jump is formed, find its (i) Type of jump, (ii) Energy loss, (iii) Percentage of energy loss to the initial energy (iv) Power dissipated per meter width of the channel
