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Channel Maintenance Considerations
in Hydropower Relicensing
by J.P. Potyondy &
E.D. Andrews
Over 250 hydroelectric projects were licensed
and constructed on the National Forests during the 1940s and 1950s.
Although hydropower has played an important role in economic development,
many projects have caused significant unanticipated adverse impacts
to National Forest resources. During the next 10 years, many of
these projects will be relicensed by the Federal Energy Regulatory
Commission (FERC). The forthcoming relicensing will be especially
important because the standards and rules governing new operating
license were changed by the Electric Consumer Protection Act of
1986. In the future, the operation of hydropower facilities must
consider impacts to federal lands and resources when a license is
issued. The relicensing process affords the Forest Service a unique
opportunity to assess and possibly eliminate or mitigate the negative
environmental impacts of these facilities.
A thorough understanding of the impacts that
the existing and alternative flow regimes may have on the downstream
physical characteristics of the channel is essential during consideration
of a new license. Channel morphology, especially the stability of
channel banks and riparian vegetation, bed-material composition,
water table influences, and bedload transport can be important issues.
Understanding channel change is important because the condition
of the channel directly influences the biological sustainability
of aquatic and riparian ecosystems.
Existing channels below hydropower facilities
are frequently damaged due to either too much or too little water.
Although the impact to a given site is highly variable and site
dependent, a desired goal for the downstream channel is to reestablish
proper channel function and thus assure channel maintenance. On
National Forests, a key objective is to manage regulated rivers
to mimic natural geomorphic processes to the greatest extent feasible.
Proper channel function enables the Forest
Service to meet in part the "favorable conditions of water flows"
reservation purpose of the Organic Administration Act. Forest Land
and Resources Management Plans provide additional guidance for protecting
and improving resource values. These plans often identify resource
objectives that may require modifications to regulated flow regimes
to achieve multiple use purposes such as sustaining fisheries, providing
recreational opportunity, or maintaining riparian vegetation.
Channel maintenance flow regimes must meet
the following criteria to achieve the desired properly functioning
condition:
- Move bed-material to maintain a long-term
sediment balance;
- Maintain streamside vegetation and the
structural stability of streambanks; and
- Prevent vegetation encroachment in the
channel.
In summary, an analysis of flow and bedload
sediment must be conducted to assure sufficient sediment is conveyed
to maintain channel capacity and prevent adverse impacts to banks
and riparian vegetation from sediment accumulation in the channel.
The objective of geomorphic analysis during
the relicensing process, therefore, is to analyze the existing sediment
and flow regime to understand the existing condition and predict
changes to the downstream channel under a variety of alternative
flow regimes. Major factors to consider
include: (1) Channel characteristics, (2) the operational hydrology
(flow regime), and (3) the sediment regime.
Channel Characteristics
A stream classification system is beneficial
to begin to understand the physical characteristics of stream channels
involved in a relicensing project (Figure 1). Classification schemes
developed by Rosgen (1996) or Montgomery and Buffington (1997) are
examples. Mapping the spatial distribution of stream types allows
for a focused analysis of sensitive reaches.
Many hydropower projects are situated on
bedrock-controlled reaches of streams. Such channels are inherently
stable and are less susceptible to alteration. Under certain circumstances,
however, impacts on bedr ock
stream channels can be quite dramatic, e.g., where the hydroelectric
facility significantly reduces peak discharges while tributaries
continue to supply large quantities of coarse sediment.
Alluvial channels by contrast are more adjustable.
Nevertheless, gradients of adjustability exist for different stream
types. For example, sand-bed channels are inherently more sensitive
to change than gravel or cobble-bed rivers, while a different suite
of channel processes operate in alluvial channels with cohesive
clay banks.
Recognizing the major channel types and materials
is especially important for the selection of appropriate sediment
transport models. For example, the Ackers and White bedload transport
equation (Ackers and White, 1973) is appropriate for sand-bed channels
while the Parker equation (Parker, 1990) is best suited for gravel-bed
rivers. A procedure for applying the Parker equation to the design
of channel maintenance flows in gravel-bed rivers is illustrated
in Andrews and Nankervis (1995).
Operational Hydrology
The water impounding structure and operational
hydrology will typically have a significant effect on downstream
impacts (Figure 2). In general, unregulated flows (run-of-the-river
projects) will have fewer significant impacts and are more easily
analyzed for channel maintenance flows. In contrast, projects which
divert flows frequently have severe impacts. In such schemes, flow
in the natural channel is diverted off-stream to the power generating
facility with a diversion dam. These designs generally utilize flow
from the diversion for as long as possible, limited by plant hydraulic
capacity and energy demand. The impact of regulated flow systems
is directly related to the size of the dam and the operation of
the facility. Larger dams have the capacity to store almost all
of the flow while smaller structures generally store and regulate
a smaller percentage of the annual volume. Dams used for
power peaking have the potential to be most destructive and issues
related to ramping rates need to be carefully evaluated.
In all cases, the operational hydrology needs
to be understood and analyzed. The "stop-and- go" nature of the
flow regime compared to natural conditions changes flood hydrographs,
sediment transport regimes, and alters channel morphology. It is
crucial to understand when, how, and what magnitude of flows are
diverted and recognize the range of usable flows that the facility
accommodates.
Streamflow data are essential during the
consideration of a new hydropower license. If streamflow data is
unavailable, stream gages should be established as soon as possible.
Streamflow data may be available from the project proponent. Streamflow
analysis typically relies on developing existing and pre-project
flow duration curves based on mean daily flows and analyzing low
flow and peak flow frequencies. The Indicators of Hydrologic Alteration
(IHA) analysis approach and software (Richter et al., 1996; see
also STREAM NOTES, January 1999) is a tool for summarizing numerous
hydrologic flow parameters and displaying changes to the hydrologic
regime.
Sediment Regime
The several landforms, e.g., riffles, pools,
point bars, etc., that make up a floodplain and river channel are
composed of various sediment particle sizes. Typically, a hydroelectric
facility will alter the supply and transport of the relatively fine
sediment sizes differently than the coarse sediment sizes. The extent
and stability of a landform will be impacted when the transport
of the associated particle sizes is altered. The magnitude of channel
response is largely a result of coarse sediment (bedload) trapped
by the dam (Figure 3).
On the one extreme are run-of-the-river facilities
that allow passage of all of the bedload sediment in the river.
This is a common situation for small hydropower projects. The mostprobable
response in the diverted reach with water removal is a decrease
in channel size (aggradation) and an increase in surface/subsurface
fines.
At the other extreme are structures that
trap all of the bedload. The most common response with water removal
is the "hungry water" phenomenon where clean water picks up sediment
resulting in an increase in channel size (incision) and/or removal
of gravels and fines (armoring).
Between these extremes are a vast array of
complex, highly variable responses. These are due to the many possible
combinations of altered sediment (changes in volume or size) and
water discharge (changes in duration, timing, peak flows or volumes
of flow) that are possible at hydropower facilities. While general
response may be predicted, every river is unique. In the majority
of instances, the site-specific circumstances of a project must
be analyzed in detail. Numerous independent watershed variables
(e.g., rainfall, geology, vegetation type, land use) combine in
countless ways to produce different flow and sediment regimes. Numerous
dependent channel variables (e.g., width, depth, bedforms, sediment
transport) also adjust in many different combinations to any given
regime. In addition, hydropower plant operating schemes and the
physical layout of the facility and the stream environment are extremely
variable.
A final important consideration in analyzing
the sediment situation is the influence of downstream tributary
inflow of water and sediment to the by-pass reach. Generally, the
critical location is the first major tributary below the dam. Other
important factors to consider are the location of the tributary
input with respect to accretion flows, the amount and size of material
coming from the tributary, and the desyncronization of tributary
inflow with the mainstem.
It is important to recognize that it is often
impossible to restore channels to their pre- project condition simply
by changing the flow regime. Novel approaches may be necessary to
achieve satisfactory sediment balance. For example, lack of sediment
input immediately below the dam may severely limit the rebuilding
of channel banks and floodplains. Likewise,
a goal of recapturing spawning habitat may be unattainable without
artificial, annual introductions of gravel into the system.
Analysis Process
The technical analysis for hydropower relicensing
can be very complicated. The following basic steps are suggested:
1. Determine unimpacted (baseline)
conditions (flow, sediment, channel conditions) above project or
pre-project.
2. Evaluate existing (current) conditions
(below project).
3. Consider alternative post-project
conditions.
Detailed flow and sediment information is essential
to support any changes to the flow regime that might be proposed by
the Forest Service to FERC. Three years of continuous streamflow record
and at least 20 samples of bedload and suspended sediment over the
full range of flows, including those above bankfull, are typically
the minimum required during the relicensing process. This basic information
is needed to validate any sediment transport modeling, understand
local hydrology, and support flow change recommendations that may
result. In many cases, specialized technical skills in sediment transport
measurement and analysis will be required.
The need to build an adequate administrative and scientific record
which contains "substantial evidence" of the facts supporting requested
flow changes was reinforced by the Bangor Hydro decision on
FERC relicensing (Pizzi, 1997). Well designed studies that link scientific
evidence to management decisions are increasingly important for agencies
to successfully recommend and implement flow changes designed to improve
the adverse channel conditions that exist at many hydropower facilities.
References
Ackers, P. and W.R. White, 1973. Sediment
transport: new approach and analysis. Journal of the Hydraulics
Division, ASCE, 99 (HY11): 2041-2060.
Andrews, E.D. and J.M. Nankervis. 1995. Effective
discharge and the design of channel maintenance flows for gravel-bed
rivers. In: J.E. Costa, A.J. Miller, K.W. Potter and P.R. Wilcock
(editors), Natural and Anthropogenic Influences in Fluvial Geomorphology,
Geophysical Monograph 89, Amer. Geophysical Union, p. 151-164.
Montgomery, D.R. and J.M. Buffington, 1997.
Channel-reach morphology in mountain drainage basins. Geological
Society of America Bulletin 109 (5): 596-611.
Parker, G., 1990. Surface based bedload transport
relation for gravel rivers. Journal of Hydraulic Research 28(4):
417-436.
Pizzi, L., 1997. Implications of the Bangor
Hydro decision on FERC relicensing. Rivers 5(2): 103-115.
Richter, B.D., J.V. Baumgartner, J. Powell,
and D.P. Braun, 1996. A method for assessing hydrologic alteration
within ecosystems. Conservation Biology 10:1163-1174.
Rosgen, D.L., 1996. Applied River Morphology.
Wildland Hydrology, Pagosa Springs, Colorado.
John P. Potyondy is a Hydrologist
with the Rocky Mountain Research Station, Stream Systems Technology
Center, Fort Collins, CO. (970) 498-1846; jpotyondy/rmrs@fs.fed.us
Edmund D. "Ned" Andrews is a Research
Hydrologist with the U.S. Geological Survey, Water Resources Division,
Boulder, CO. (303) 541-3002; eandrews@usgs.gov
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