Design of Hydraulic Controls & Structures 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 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

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