1. Design of Hydraulic Controls & Structures
CVE 341 – Water Resources
Lecture Notes 6:
CHAPTER 16
Design of Hydraulic Controls & Structures

2. 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

3. 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.

4. Bridges

5. Culverts

6. 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.

7. 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

8. 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

9. Inlet & Outlet Control Outlet Control
When flow is governed by combination of headwater depth, entrance geometry, tailwater elevation, and slope, roughness, and length of culvert
Subcritical flow
Frequently occur on flat slopes
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
Inlet Control
Flow is controlled by headwater depth and inlet geometry
Usually occurs when slope of culvert is steep and outlet is not submerged
Supercritical, high v, low d

10. 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.

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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.

18. 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.

19. 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