As
the first of a three-part series on stream restoration intended to realize the
disparity between design and construction, and how this contributes to failure,
this article provides the designer’s perspective on which challenges pose the
greatest risk to the success of a stream restoration project.
Restoration
may be defined as the return of natural function lost, usually resulting from
associated watershed management such as development and agricultural practices.
Depending on the number of lost functions restored, or the level of improvement,
restoration may also take the simpler form of rehabilitation or stabilization.
Restoration, rehabilitation, and stabilization of streams are performed for a
variety of reasons. The primary driver for this type of work is the Clean Water
Act (CWA). The objective of the CWA is “to restore and maintain the chemical,
physical, and biological integrity of the nation’s waters,” and, for this
reason, it prohibits the discharge of dredged or fill material into wetlands,
streams, and other waters of the United States, unless a permit issued by the US
Army Corps of Engineers or approved state under CWA Section 404 authorizes such
a discharge.
Under
the CWA, there are two major mechanisms that promote improved stream health: 1)
Section 404 of the CWA, which provides for the use of compensatory mitigation to
offset unavoidable damage to wetlands and other aquatic resources, and 2) the
National Pollutant Discharge Elimination System (NPDES) stormwater permit
program, which establishes requirements for municipal separate storm sewer
system (MS4) nonpoint discharge. Permitted jurisdictions are sometimes prompted
to perform watershed improvements, including stream and wetland restoration, as
part of a larger strategy to implement total maximum daily loads
(TMDLs).
Stability
Versus Instability
Characteristic
of most natural systems that reflect landscape conditions, stable streams exist
in a state of dynamic equilibrium (Strahler 1957 and Hack 1960). This refers to
an open system that, despite continuously receiving a varying inflow of
discharge or sediment, maintains a stable hydraulic geometry (cross-sectional
dimension, horizontal alignment [pattern], and vertical alignment [profile]) by
transporting portions of this material within and beyond a mensurable reach. One
fundamental axiom of stability is that the channel form is largely a function of
the physics between discharge and the ability of a stream to move sediment,
resulting in a equilibrium relationship, Qsds ~
QwS0 (Lane 1955), where Qs represents sediment
quantity for a given discharge Qw, and where ds serves as
the associated sediment size characterization (often the mean particle size,
d50), and where S0 is the stream slope. If any of the four
variables are changed, then changes to one or more of the other variables will
occur to restore equilibrium. More simply, the channel form is a function of
channel hydraulics, which in turn is a function of channel
form.
Typically,
restoration sites reflect the disequilibrium of a rapidly changing watershed,
and active efforts to restore the channel form directly yield impacts
(intentional or sometimes inadvertent) to the channel hydraulics and ability to
transport sediment. This simplified relationship becomes more complicated when
we attempt to account for the effect of vegetation. Regardless of attempts by
engineers to accurately predict channel response following the construction
phase of a restoration project, it is not unusual for the stream to experience
unanticipated minor adjustments such as deposition on the inside of point bars
or minor scour in areas of structure placement. The vulnerability of a
restoration project is usually at its highest immediately following
construction, and despite probability, that is usually when the stream will
experience a major storm event and an increased likelihood of failure. Failures
are often accompanied by cost overruns and adverse generalizations compounding
the difficulty of advocacy.
A
failure may be defined as the inability either to perform primary project
objectives (i.e., bank stabilization, water-quality improvements, riparian and
aquatic habitat enhancements) or to otherwise avoid creating additional impacts
that result in unstable geometry attributable to one or more typical problems.
Typical problems may include, but are not limited to, increasing velocity and/or
shear stress resulting in excess competency and bank/bed scour; decreasing
energy slope and channel depth resulting in reduced capacity and excess
aggradation; and failure to account for multidimensional flow properties, such
as lateral and vertical turbulence resulting in bank/bed armor structural
instability. Failures may occur within an actively restored reach or throughout
nearby upstream and downstream reaches, and they may take on a number of forms.
While some failures may be immediately visible, others can take years to mature
into potentially fatal conditions.
Why
Failures Occur
Once
a failure is recognized, the cause should be identified before attempting to
correct the problem. This can often be a difficult process, as the designer and
contractor have varying perspectives, and the client is pulled by strong
arguments that attempt to divert fault. In many instances, the failure is a
function of both the design and construction, and discerning accountability
becomes a fruitless effort. Progress toward resolution begins only after the all
parties involved come to realize the benefit of collaborative problem
solving.
Design
Errors
Design
failures may occur at any point during a project process, from poor selection of
a treatment site to poor design philosophy/methodology, incomplete construction
documents, and weak implementation.
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Photo: City of Charlotte
Figure 2. Before restoration, jute matting served as the only bank protection. |
Early
in the design process, one common error is the failure to establish practical
project expectation (i.e., to identify the balance between project objectives
and site potential). Before an appropriate site is selected and associated
restorative treatments are prescribed, the project objectives should be clearly
in alignment with the site potential. For example, attempting to achieve
significant water-quality improvement along a single 500-linear-foot reach of
highly urbanized stream channel is doomed from the start. Although the
cumulative effect of multiple efforts such as this may be significant, the value
of the single project may not be, cost-wise, beneficial. However, project
objectives that better match the potential for such a site may include reducing
a quantifiable source of sediment. Although stabilizing actively scouring banks
with armor and bioengineering techniques may not eliminate erosive forces, such
treatments may minimize the bank erodibility and be the most effective “fix” in
terms of cost and schedule. This level of stabilization may not be considered
restoration and will not correct the hydraulics, but will rather accommodate the
current problem. Sometimes the momentum and excitement of performing stream
restoration distracts stakeholders from realizing that the best means for
achieving the desired level of water-quality improvement may not be practical or
achievable by addressing a single site, and that retrofitting or implementing
new best management practices (structural or nonstructural) elsewhere in the
watershed may achieve desired results.
Beyond
site selection, common stream restoration design failures include those
associated with the quality of design documents. Acting in concert with
stormwater interests, the stream mitigation elements of the Clean Water Act have
stimulated the science and permitting industry, resulting in non-engineered
design. While in many states, the application of engineering principles
(hydrology, hydraulics, fluid/soil mechanics, surveying, etc.) is reserved for
the licensed professional engineer, other states have allowed these practices to
occur unregulated. Some design failures are attributable to the learning
environment of this “emerging science” and attempts to stretch application in
the interest of gaining knowledge. While this may be valuable to the industry,
it may not always meet the need or budget of the municipal client. It should be
noted that because of the complexity of working with dynamic systems driven by
unregulated forces, a qualified design team incorporates various expertise,
including those with foundations in science, engineering, and public policy.
However, the actual design and accountability of proposing large-scale earthwork
(plans, specifications, contract documents, etc.) should lie on the shoulders of
the licensed engineer. Because of this “pioneering” nature of stream
restoration, inconsistencies in design and individual designer standards have
yielded mixed results when applied to conventional construction
settings.
Professional
accreditation aside, erroneous design philosophy also contributes to design
failures through poor applications based on inaccurate assumptions and/or
failure to incorporate adequate design criteria (i.e., consideration of a small
range of discharge events). Consideration of a single index flow (bankfull
discharge) may be appropriate for the design of a first-order stream impacted by
historic agricultural and silvicultural activities, but it is important to note
that stable channel morphology is actually a function of a wider range of flows.
For the more complex urban condition, a natural channel design approach (Rosgen
1994) serves as a good starting point for the desired range of conditions, but
further consideration of process reveals that the combined flow and sediment
regime have permanently been altered, and a reference condition for a natural
system may not apply well. The key in these situations is to realize the
tendency of the natural system (when subjected to such wide-scale watershed
disturbance), understand the departure from stability associated with the
existing condition, and predict the ultimate watershed conditions and
corresponding stable stream process. This is often accomplished by amending a
natural channel design approach with additional analytical tools, such as
sediment transport modeling, stability calculations, and hydraulic modeling for
a range of flows spanning from base flow to flooding events of low recurrence.
Consideration of the process associated with a range of flows allows the
engineer to develop a more thorough knowledge and to make practical design
adjustments to prevent failure that may otherwise be
overlooked.
Construction
Errors
On
occasion, the contractor’s lack of conventional and innovative knowledge
(including the basic understanding of physical processes), equipment, and/or
skills may cause the engineer and the client to conclude that a contractor is
inexperienced or simply does not know what he or she is doing. Left unattended,
this situation almost surely leads to the failure to construct a restoration
project per design plans, ultimately resulting in project failure. This
situation can be avoided or minimized by incorporating adequate
prequalifications into the contract documents, providing for construction
assistance by qualified engineering technicians, and utilizing contract
mechanisms (such as stop work and the threat of taking bond) to prevent the
situation from reeling out of control.
Because
the actual construction of a project usually is a function of various factors
that precede the contractor’s involvement (site selection, earthwork/structural
design elements, design documents, etc.), it is both difficult and unfair to
place construction errors directly on the shoulders of the contractor. It is
always a good idea to coordinate elements of the design (types of bank or
in-stream treatments, sequence of construction, staging and access, and erosion
and sediment control) with client contractors regarding constructability and
value engineering. This prevents extreme changes (addendum, redline, or
otherwise) to the plans and unforeseen cost and schedule
burdens.
In
the case of low-bid contracting, errors related to lack of basic knowledge can
be addressed through mandatory pre-bid meetings where project expectations can
be clearly identified through an engineering presentation. Prequalifying
contractors or soliciting bids by invitation only may prevent anomalous bids
that skew the client’s perceived project cost. Occasionally, certain attempts at
saving money end up costing more in dollars, resources, and/or schedule.
Providing “good versus poor” examples to establish common expectations always
results in a tighter range of bids more reflective of the costs and prevents the
exceptionally low underbid. This type of information-sharing helps the
contractors bid effectively and prevents miscommunication of the design intent
inferred through the contract documents. A similar, but more technical, training
presentation by the engineers to the awarded contractor operators on how to best
accomplish the design intent, detailing the specified materials and methods of
construction, can prevent false starts and impacts to budgets and schedules.
Because good contractors are smart and know exactly what their operators, crews,
and equipment are capable of, this communication milestone may also serve as an
opportunity to value-engineer the proposed restoration project above and beyond
that performed during design. This two-way dialogue allows the
contractor/engineer/client team to establish communication protocols before the
construction commences. In addition to providing comprehensive and well-composed
contract documents, another tool to reduce low-bid construction failures is to
incorporate a time-and-material method of payment that relieves the contractor
of concerns about overages and allows the contractor to perform work (or a
portion thereof) under the “direction” of the engineer.
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Photo: City of Charlotte
Figure 3. Both reaches of Hope Park Branch experienced failures. |
Communication
during construction is equally as important as quality construction documents.
As a client-oriented service provider, the contractor is in the business of
identifying ways to reduce project costs and stay on—or ahead of—schedule. While
the field-savvy operator may have some valuable insight, the approval process
for field change should be clearly outlined in the contract documents and
written to include multiple levels of review for ultimate engineer acceptance
through signature. If the original design required detailed efforts to apply
standards and prudent practice by accountable engineers, then so should any
field adjustments or redline changes to the design. Without an explicit process,
the communication can quickly become muddled, with mixed messages delivered
among the contractor, engineer, and client.
Addressing
a Failure
Because
failures are to some degree unavoidable, the method and means by which we
prevent and address failures is just as important as evading failures
completely. The effectiveness of any solution is only as good as the problem
assessment. While some potential failures may be better understood if left alone
and observed over the short term, other problems pose a greater risk and require
immediate attention. Once a fatal flaw is recognized, it is usually not
financially practical to correct the fundamental problem. In this case, the
fundamental problem is not the isolated failures, but rather, the overall
failure of the design or the failure in construction. If the project is
constructed in strict accordance with the design documents and field changes
occurring only under the approval of the engineer, it is usually very difficult
to place the entire burden of failure on the contractor. Once the failure is
identified, the entire team must work collectively to resolve the matter,
maintaining the client’s interest in cost. Well-written technical specifications
(provided by the engineer) that present explicit guidance on the process and
authority to approve field changes can facilitate resolution in a situation such
as this. It is important to consider the value of timely resolution based on
clear preferences versus the additional cost of remaining in a holding pattern
indefinitely while communication dwindles.
Obvious
to most, failure prevention measures are worth tenfold their cost. In addition
to incorporating a scrutinous level of detail in the contract documents, other
strategies may be considered. The potential for contractor rework or additional
work must be accounted for in the design of the access, staging, and sequence of
work. This minimizes additional site disturbances and costs and helps remediate
the failure. In addition, a cost contingency—often between 5% and 25%, depending
on the design assumptions—should be incorporated into the overall project
budget. Although this may not cover the cost altogether, it will maintain
project momentum, allow for timely response to failure, and occasionally prevent
incurring unnecessary demobilization/remobilization costs. Effectively
anticipating problems in the field and addressing them with timely communication
is usually accomplished through frequent site inspections by the engineer or
another qualified client representative. Sometimes the client may perceive this
level of involvement as excess project cost burden, but its value is better
understood only after construction problems escalate to being
serious.
Case
Studies
The
city of Charlotte, NC, has been in the business of water-quality improvement
since 1993 through the work of its Storm Water Services Division. Performing
projects either as part of the Umbrella Mitigation Bank or under other
initiatives of the water-quality-improvement program, Storm Water Services has
completed tens of thousands of feet of stream restoration and enhancement.
Before the Umbrella Mitigation Bank was established in 2004, stream improvement
efforts focused mostly on streambank stabilization to reduce sediment loads in
local streams. Since that time, the stream restoration program has evolved to
become responsive to changes in the industry and adaptive to the urban
watersheds within which the program is constrained.
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Photo: City of Charlotte
Figure 4. A portion of the stream buffer was cleared for access during sewer line maintenance. |
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Photo: City of Charlotte
Figure 5. Bankfull benches were constructed too stream to become entrenched. |
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Photo: City of Charlotte
Figure 6. To prevent erosion, additional energy dissipation was needed. |
The
city’s Umbrella Mitigation Bank was the first municipal mitigation bank in North
Carolina and the only mitigation bank to work entirely within urban watersheds.
As an early pioneer of urban stream restoration, Storm Water Services has
encountered its share of problems. Not only were the projects technically
challenging, but also project managers, construction managers, and inspectors
had to overcome administrative challenges that were not typically found on flood
control or stream stabilization projects. Stream designers had little experience
working in urban watersheds, and construction firms that bid on the work were
not skilled in the art and science of stream restoration. These challenges led
to failures on some of the early projects constructed by Storm Water Services;
two such projects, Hope Park Branch (completed in the spring of 2003) and
Edwards Branch (completed in the fall of 2004), have been selected as case
studies to highlight some of these issues. Both projects are located within
Charlotte city limits.
Hope
Park Branch Stream Restoration.
Hope Park Branch is a small stream within the Briar Creek sub-basin of the lower
Catawba River. The project is bisected by a major thoroughfare roadway, creating
two reaches on land owned primarily by Mecklenburg County Park and Recreation
and Charlotte-Mecklenburg Schools. The watershed is entirely built-out, with
small single-family lots, large multifamily housing units, commercial
facilities, two small parks, and two school sites contributing drainage. This
creates a flashy hydrograph that leads to high shear stress for a short duration
during the typical Charlotte thunderstorm.
Hope
Park Branch was the first major stream restoration project completed by Storm
Water Services, and the problems encountered have been the result of both
inexperience and bad luck. Immediately following construction, the project was
hit by several large storms before the riparian vegetation had a chance to take
root. With a heavy jute matting serving as the only bank protection, the storms
caused bank and structure failures along both reaches of the project (Figures 1
through 3). Upon closer examination, it was discovered that several field design
changes were made during construction, resulting in a channel that was too deep
and structures that were built improperly. Through several major reconstruction
efforts, these issues have been corrected, but the cost of repairs nearly
doubled the original project estimate.
Hope
Park Branch is considered a “reach restoration,” meaning it is a small reach
that is restored within a larger contributing watershed. The downside of
restoring only a reach within a larger watershed is that the project owner does
not have control over activities that take place upstream of the restored reach.
This has been problematic for Hope Park Branch, as the upstream sediment supply
is being sequestered within the restored channel and on the floodplain benches.
This has caused the banks to build and the stream to become slightly entrenched,
thereby limiting floodplain access and reducing storage. The contributing
watershed is now being re-evaluated for stabilization opportunities to reduce
the sediment load reaching Hope Park Branch.
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Photo: City of Charlotte
Figure 7: The lower section of Edwards Branch |
The
stability issues mentioned above have taken a considerable amount of time and
money to address, illustrating the importance of post-construction monitoring
and maintenance to ensure project success. Storm Water Services now inspects
newly constructed projects after any major storm event. These inspections have
also revealed disturbances to the planted vegetation by mowing, dumping,
vandalism, foot traffic, and invasive species encroachment. Because this project
was constructed on park and school property, students and park users have
created foot trails within the buffer to travel from the parks and schools to
their neighborhoods. Park maintenance crews have mowed the buffer down several
times, and during a recent sewer line maintenance event, a portion of the buffer
on the lower reach of the project was cleared for access (Figure 4). These
issues illustrate the importance of coordination with sister agencies and
landowners during all phases of a project and following construction. Invasive,
exotic species have also posed a problem, and a long-term management strategy
has been employed to ensure the success of the native buffer
vegetation.
Edwards
Branch Watershed Improvement Project.
Edwards Branch is a medium-sized stream, also within the Briar Creek sub-basin
of the lower Catawba River. Like Hope Park Branch, the watershed is entirely
built-out, with small single-family lots, several large multifamily housing
units, several large commercial sites, a park, a cemetery, and a school site
contributing drainage. Two tributaries enter the main stem of Edwards Branch,
and restoration activities have taken place throughout the watershed. In
addition to stream restoration, project features include two constructed
wetlands, one wet pond, a dry extended detention basin, and buffer
enhancements.
Issues
encountered on this project are similar to those encountered on Hope Park
Branch. Bankfull benches were constructed too high, causing the stream to become
entrenched (Figure 5). Structures were modified or eliminated from the plans,
lowering the energy dissipation capacity of the new channel. Property owners and
park maintenance crews mowed the buffer in several areas, causing localized bank
failures. Invasive species encroachment threatens the survival of the planted
vegetation, and poor soils have caused some areas to remain sparsely
vegetated.
One
notable difference between Hope Park Branch and Edwards Branch relates to
sediment. Whereas Hope Park Branch is sediment laden, Edwards Branch is sediment
starved as a result of the stabilization of streambanks throughout the
watershed. Because this “hungry” stream no longer performs the work of carrying
a high sediment load, additional energy dissipation was necessary to prevent
erosion of streambanks and around structures (Figures 5 and 6). Field design
changes and elimination of structures during construction caused failures in the
lower section of Edwards Branch (Figure 7), and a major reconstruction effort
has recently been completed to correct these issues.
Storm
Water Services has learned many lessons through the construction of these and
other projects, and numerous processes have been implemented to reduce the
likelihood of failures in the future. Project managers, construction managers,
and inspectors receive training so that they understand the details of the
projects they are managing, and design consultants are held responsible for
their designs. Although the construction contracts are still awarded to the
lowest bidder, all contractors must prequalify in order to bid. Storm Water
Services conducts follow-up inspections of the projects after each major storm
event, and any failures or areas of concern are addressed before they become
problematic. Several on-call services contracts allow repairs to be completed in
a timely manner, leaving time for plant installation. Annual communications with
property owners have reduced the number of easement violations along restored
reaches, and easements have been strengthened to ensure protection of the buffer
in perpetuity. Invasive species are controlled annually, and willing property
owners receive training on how to identify and remove invasive species in their
easements. While these policies and processes may not prevent a case of bad
luck, they go a long way in improving the quality of future stream restoration
projects.
Closing
Thoughts
From
the designer’s standpoint, the structure of the project team is largely a
function of the design documents and resultant bid. As a responsible consultant,
promoting client awareness on the process of “doing” the work is just as
important as promoting awareness on technical elements, but it may often have
more resounding implications. In the interest of the client, prequalifying the
design engineering consultant may actually be the best first step to engaging in
a restoration project and preventing a compounded learning curve.