CVE 341 – Water Resources
Lecture Notes 6:
CHAPTER 16
Design of Hydraulic Controls & Structures
Design of Hydraulic Controls & Structures
Very Complex task
Incorporates many elements of engineering practice
Concerns when designing Hydraulic Structures
Safety
Efficiency
Reliability
Cost effectiveness
Environmental concern
CULVERT versus BRIDGE
A culvert is a conduit for the passage of surface drainage water under
a highway, railroad, canal, or other embankment;
A bridge is a structure carrying a pathway, roadway, or railway
over a waterway.
The span length is the main criteria to demarcate between a culvert
& a bridge
the span is less than 6.5m (or 20 ft), it is called a culvert
and it is a bridge when the span is more than the specified length
A culvert usually is either circular or rectangular in cross-section,
so it has an all-enveloping structure (i.e. a roof, 2 walls and a floor
Bridge sits on foundations on each bank (or on wing-walls),
and as it does not have a floor, it is called a bridge.
Bridges
Culverts
Key Design Considerations
CULVERT
BRIDGE
Hydraulic performance
· Safety and security
· Conveyance of sediment, debris and
trash
· Environmental acceptability
· Ease of construction
· Ease of O&M
Design flows
· Design flow depths & velocity
· Acceptable backwater
· Acceptable afflux
· Effects of blockage/ debris
· Acceptable clearance in flood for
floating debris
· Acceptable scour depths
· If river is navigable, consideration of
boat/ship strike.
We will focus on CULVERT.
Hydraulic characteristics of culverts
• A culvert consists typically
- an inlet, an outlet and between these a culvert barrel of length L and height D
- a trash screen
• The barrel is normally on a slope, So and the depths of water above the
upstream and downstream invert levels are known as the headwater depth
(HW) and tailwater depth (TW) respectively
CIRIA Report 168, Culvert Design Guide
Flow Control
• Flow control is a fundamental principle in hydraulic design and a control
point is a feature that controls the water level/discharge relationship. It is the
point in a channel or culvert which has the lowest capacity. In other words
points upstream and downstream of the control point have a greater capacity
than at the control point. In the context of culverts, both inlets and outlets
can control the flow.
CIRIA Report 168, Culvert Design Guide
Inlet & Outlet Control
Inlet Control
• Flow is controlled by headwater
depth and inlet geometry
Outlet Control
•When flow is governed by
combination of headwater depth,
entrance geometry, tailwater
elevation, and slope, roughness,
and length of culvert
•Subcritical flow
• Usually occurs when slope of
culvert is steep and outlet is not
submerged
•Frequently occur on flat slopes
• Supercritical, high v, low d
•Concept is to find the required
HW depth to sustain Q flow
•Tail water depth often not
known (need a model), so may
not be able to estimate for outlet
control conditions
TYPES OF FLOW THROUGH CULVERTS
The six principal types of flow which can occur in culverts
depending on
 the flow rate,
 the upstream and downstream water levels and
 the geometry of the culvert.
In each case, the flow is controlled either at the inlet/the outlet
or downstream of the culvert.
Type I flow – Critical depth at inlet
• In this case the culvert flows with a free surface and the flow is
controlled at the inlet. The flow accelerates as it contracts into
the culvert and the water surface passes through critical depth
slightly downstream of the inlet face of the culvert. The water
surface remains below critical depth for the full length of the
barrel and the tailwater elevation is below critical depth at the
outlet.
CIRIA Report 168, Culvert Design Guide
Type II flow – Critical depth at outlet
• The culvert flows with a free surface and the flow is controlled
at the outlet. The culvert barrel slope is less than critical slope
and the tailwater depth is less than critical depth at the outlet.
Under these conditions the outlet behaves like a weir, and
critical flow occurs
CIRIA Report 168, Culvert Design Guide
Type III flow – Subcritical flow throughout
• In this case the culvert flows with a free surface and the water
level is controlled downstream of the outlet (see Figure 2.5).
This is the type of flow which occurs most frequently in culverts
and the type for which the culvert should be designed. The
tailwater depth is above critical depth at the outlet but the
tailwater elevation is below the soffit of the culvert.
CIRIA Report 168, Culvert Design Guide
Type IV flow – Submerged outlet
• In this case both the inlet and outlet are fully submerged
and the culvert barrel flows full. The water level is
controlled at a point downstream of the outlet.
CIRIA Report 168, Culvert Design Guide
Type V flow – Rapid flow at inlet
• A high headwater depth could cause Type V flow to occur where the
flow separates at the inlet of the culvert. In this case the culvert inlet
behaves as an orifice, and critical flow occurs just inside the culvert
barrel. The water surface remains below critical depth for the full
length of the barrel, and the tailwater depth is below critical depth at
the outlet. A hydraulic jump will occur downstream of the outlet.
CIRIA Report 168, Culvert Design Guide
Type VI flow – Full flow with a free outlet
• In this case the culvert flows full throughout its length but the tailwater
elevation is below the culvert soffit at the outlet. The control point
occurs at or downstream of the outlet, and the culvert therefore
operates under outlet control.
A variant on Type VI flow is the
case where the culvert flows full
for part of its length but the water
surface then separates from the
culvert soffit.
CIRIA Report 168, Culvert Design Guide
HYDRAULIC ANALYSIS OF CULVERTS
The engineer usually undertakes the hydraulic analysis of
culverts for one of two reasons:
 to determine a suitable size for a new culvert that satisfies
the design discharge, culvert length, permissible headwater
depth and outlet velocity
 to calculate the peak capacity of an existing culvert for
which all the dimensions are known.
Culvert Design
• Most culverts operate under downstream control. This means that
the hydraulic computations proceed from the downstream in the
upstream direction.
• The design discharge and allowable headwater elevation are
initially established. Other constraints such as culvert shape,
material, aesthetics, etc., are specified.
• Assume a culvert size and check performance assuming both inlet
and outlet control. Whichever gives the highest HW elevation
controls the hydraulic performance.
• Compare culvert performance with design constraints, and select
the smallest (least expensive) size that meets the criteria.
An Easy Way to Size Culverts
There are several free software:
(for example from US Federal Highways administration
– Has been used for a long time and is very well respected
– New Graphical Interface
– Allows for multiple culverts to be specified at a variety of
locations on a map
Please see also Handout for Culvert (Hand)
Design Procedure
www.riverconveyance.net/aes/partners.html