engineering design: process, predilections, & priorities. jennings... · engineering design:...
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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
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
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
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
Traditional Solution: Is this optimal?
• Re-shape and re-align channel to convey design flow
• Harden channel and minimize roughness
• Ecological integrity?
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