Engineering Design:

Process, Predilections, & Priorities

Greg Jennings, PhD, PE

Senior Water Resources Engineer

Stantec Consulting

Retired Professor

NC State University

Raleigh, NC, USA

Thanks to Paul Frank, PE, CED

Outline

• What is Engineering?

• Traditional engineering and the design process

• Changing paradigms for engineering

• Multi-disciplinary projects: Teamwork

• New Design Model

… the profession in which a knowledge of the mathematical

and natural sciences gained by study, experience, and

practice is applied with judgment to develop ways to utilize,

economically, the materials and forces of nature for the

benefit of mankind (ABET)

What is Engineering?

• Planning

• Design

• Construction

• Evaluation

• Communication

Who is an Engineer?

Quotes from normal people:

“very smart person who builds things”

“person who charges lots of money to fix problems”

“human calculator”

“socially awkward person with bizarre sense of humor”

http://www.youtube.com/user/donmcmillancomedy

Who is an Engineer?

NC General Statutes Chapter 89C

A person who, by reason of special knowledge and use of

the mathematical, physical and engineering sciences and

the principles and methods of engineering analysis and

design, acquired by engineering education and engineering

experience, is qualified to practice engineering.

Who can practice Engineering?

NC General Statutes Chapter 89C

Any person who shall practice, or offer to practice, engineering

or land surveying in this State without first being licensed in

accordance with the provisions of this Chapter, or any person,

firm, partnership, organization, association, corporation, or other

entity using or employing the words "engineer" or "engineering"

or "professional engineer" or "professional engineering" …

… shall be guilty of a Class 2 misdemeanor.

Traditional Engineering: Focus on Infrastructure

• Physical structures that form the foundation for development

• Facilities designed by engineers to serve public good and

ensure public safety

Engineering Design Process

1. Identify Problem

2. List Known and Unknown Quantities

3. Compile Equations and Criteria

4. Determine Assumptions and Margins of Safety

5. Develop Solution(s)

Design Process Case Study: L. Shades Creek

• Sewer line adjacent to creek

• Streambank erosion during high flows

• Potential sewer line exposure and structural failure

1. Identify Problem

• Streambank erosion due to excess shear stress and lack

of soil resistance during high flows

• Need to separate creek from sewer line to prevent

infrastructure damage and public health impacts while

preserving ecological integrity

2. Known and Unknown Quantities

Known:

• Existing channel and floodplain morphology

• Rainfall and streamflow records

• Erosive velocities & shear stresses

Unknown:

• “Stable” channel and floodplain morphology

• Rainfall – runoff relationship

• Flow – stage relationship

• Soil resistance with natural forest buffer

Project Specs

1,900 feet stream length

30-60 feet riparian buffer

10 stormwater outfall channels

Sewer crossing

Greenway trail on East bank

Gravel/cobble – high bedload

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3. Equations and Criteria

• Hydraulic model for open channel flow

• Bed/bank shear stress equations

• Rainfall – runoff model

• Soil must resist erosion under design flow velocity and

shear stress conditions

4. Assumptions and Margins of Safety

• Design flow 100-year recurrence

• Assume Manning n value range (0.03 to 0.05)

• Select bank slopes (3H:1V)

• Select native trees/shrubs with optimal root density

5. Solution

• Re-shape and re-align channel and floodplain to direct

energy away from sewer line

• Grade streambanks and add native plants to enhance

soil strength

• Add in-stream boulder vane structures to reduce near-

bank shear stress

1. Channel morphology &

floodplain connection

2. Hydrologic & hydraulic

analysis

3. In-stream structures

4. Habitats & vegetation

5. Site & watershed

conditions

6. Monitoring, maintenance,

education

Project Components

Engineering Design: William McLemore, PE

Grading: Re-align channel and excavate floodplain benches to dissipate energy during high flows

ER = 1.6

W/d = 19

K = 1.2

Rc/W = 2-3

Entrenchment Ratio = Wfpa / Wbkf = 60/38 = 1.6

Width to depth Ratio = Wbkf / dbkf = 38/2 = 19

Entrenchment Ratio = Wfpa / Wbkf = 60/38 = 1.6

In-Stream Structures (11): Boulder & Log Vanes

• Grade Control

• Bank Protection

• Sediment Transport

• Habitat Enhancement

Boulder Vanes (J-hooks)

• 3-5 % arm slopes

• 20-25 degree arm angles

• Boulder footers & non-woven geotextile

• 0.5 ft drops over j-hook inverts

Log Vanes

• 2-4 % arm slopes

• 20 degree arm angles

• Sealed with woven geotextile & backer logs

Stormwater Outfall Channels (10)

• Vegetated bio-swales (low slope)

• Rock step-pools (high slope)

Construction Practices

• Track equipment

• Spill management plan

• Staged construction phases to limit exposure

Temporary Erosion Control

• Soil prep, seed, straw

• Biodegradable matting (coir, 700g)

• Wood stakes

Vegetation – Streamside Forest

• Native plants

• Grasses, shrubs, trees

• Live stakes, bare roots, containers

Natural

Succession

Partridge Pea,

Chamaecrista

fasciculata

July 2010

August 2011

Traditional Solution: Is this optimal?

• Re-shape and re-align channel to convey design flow

• Harden channel and minimize roughness

• Ecological integrity?

Rhein River

Changing Paradigms for Engineering

• Increased awareness of ecosystem and natural resource

values

• Assignment of value to ecosystem services

• Recognition of failures when long term ecosystem

processes are not addressed

• Increased regulatory pressure

Design and management of sustainable ecosystems that

integrate human society with the natural environment for the

benefit of both

Ecological Engineering

Definitions of Ecological Engineering

• Study and practice of fitting environmental technology with

ecosystems self-design for maximum purpose (Odum 1960)

• Environmental manipulation by man using small amounts of

supplementary energy to control systems in which the main

energy drives are still coming from natural sources (Odum

1963)

• Complex ecologies… engineered to undertake work on behalf

of human communities (Todd 1994)

• Facilitated development of whole, complex, resource efficient

ecosystems designed to maximize specific ecosystem goods

& services

• Restore damaged ecosystems

• Develop new sustainable ecosystems that have

human and ecological values

Goals of Ecological Engineering

Provisioning – food, energy, industry

Regulating – climate, waste, nutrients

Supporting – water quality, pest control

Cultural – recreation, inspiration

Preserving – species diversity

Ecosystem Services

Principles of Ecological Engineering

• Ecosystems are self-designing

• Ecosystem structure & function are governed by forcing

functions

• Elements are recycled in ecosystems

• Homeostasis requires accordance between biological

function & chemical composition

Mitsch & Jorgensen, Ecological Engineering

Self-Design

The reorganization, substitution and shifting of an ecosystem

(dynamics and functional processes) whereby it adapts to the

environment superimposed upon it. (Mitsch & Jorgensen,

Ecological Engineering)

Ecological Engineering: Design Principles

1. Natural ecosystems as models

Mimic crucial aspects of both structure and function

Solar powered, materially efficient and effective

2. Guided self-design

Abiotic structure designed to encourage desirable biotic function

“Multiple seeding”

3. Build in feedback loops

Negative stabilize, maximize efficiency

Positive amplify desirable attributes

4. Biodiversity at all scales (genetic ecosystem)

5. Heterogeneity and variability are good

6. Conservation ethic

Differences in Engineering Approaches

Traditional Engineering Ecological Engineering

Imposed Organization Self Organization

Standards Flexibility

Repeatability Uncontrolled Experiment

One-way Process Iterative Process

Reliability Natural Variability

Predictability Uncertainty

Factor of Safety Statistics

For Human Good For All Good

New Definition of Infrastructure

• Facilities designed by engineers to serve public good,

preserve and enhance natural resources, and ensure

public safety

• Requires multiple objectives

• Requires multiple disciplines

Multi-Disciplinary Teams

• Engineer: Practical application of concepts, delivering

project on time and budget

• Ecologist: Interpret natural systems and essential inputs

• Landscape Architect: Conceptual landscape-scale

interactions

Ecosystem Design Process

• Visualize the end from the beginning

• Must ask right questions from the beginning

• Flexibility in design and operation

• Big picture understanding of system function

• We cannot control nature; we cannot over-estimate our

ability to manage natural system

New Design Model

• Learn together from all disciplines

• Appreciate what can and cannot be controlled in nature

• If uncertain, use nature as our guide

Engineering Design:

Process, Predilections, & Priorities

Greg Jennings, PhD, PE

Senior Water Resources Engineer

Stantec Consulting

Retired Professor

NC State University

Raleigh, NC, USA

Thanks to Paul Frank, PE, CED