calculating the volume of sediment behind a dam using...
TRANSCRIPT
Calculating the Volume of Sediment Behind a Dam Using Basic Techniques
Cara Walter1, Desiree Tullos2
AbstractOne of the most fundamental factors influencing outcomes of dam removal is the volume of sediment stored behind the dam. General methods for calculating the sediment wedge are not well defined, require different degrees of data resolution, and result in different estimates. For example, the planimetric area method is more data and analysis intensive, as it involves identifying and calculating the volume of each area of deposition. In
contrast, the cross sectional method approximates and calculates the volume of sediment as one continuous wedge stretching from directly behind the dam to the farthest upstream extent of dam influence, using the pre-dam slope estimates to compare with surveys of the current channel bottom. Further, sediment wedge estimates may be made through volume calculations in a spreadsheet and through cut/fill algorithms within drafting programs (i.e. AutoCAD). In addition to methodological differences between estimates, differences also depend on the availability of comprehensive
surveys. A longitudinal profile stretching above the zone of influence of the reservoir to the dam face is required; however, the extent of a dam’s influence can be difficult to identify. Comprehensive surveys and definitive volume estimates are further complicated by a lack of clear definitions of what constitutes the appropriate width and location of the sediment wedge in the channel. In this presentation, we will (1) present methods for calculating sediment wedge volumes, (2) demonstrate the variability of volume estimates between some of these methods, and (3) make recommendations
for detecting the upstream extent and location of the sediment wedge for collecting sufficient surveys.
Method 1 - Cross-Sectional
1. Slope Calculation
2. Approximate Pre-Dam Elevations
Variability in Estimates
3. Calculation of Pre- and Post- Dam River Bottom Differences
Brownsville Dam
Example
Figure courtesy of Sarah Marshall
With a longitudinal profile or cross-sections that contain the elevation and distance between the farthest upstream and the downstream extent of
dam influence, slope is calculated as the difference in elevations over the longitudinal distance. Without survey data, the valley slope can be calculated using a topographic map of the area. Elevations at river mile markings or manual measurement of distance between elevations on the river can be used to calculate the valley slope and used as an approximation of the river bottom slope. However, it can be difficult to position the slope line vertically since, unlike the survey data calculation, there are no local elevations inherent in the calculation to which the slope
can be positioned.
-Channel Geometry
Three common shapes used to approximate cross-sections of pre-dam channels are triangles, trapezoids, and rectangles, with the triangle tapering to a point and the
trapezoid and rectangle tapering to a line at the farthest upstream extent of the sediment wedge.
Brownsville Dam, Calapooia River, Oregon
Cross Section 830 in Brownsville Reservoir, Calapooia River, Oregon
Station (ft)
Rel
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leva
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(ft)
Survey Data
Width of Sediment Wedge
Approximately 16’ upstream of dam
-Upstream Extent of Sediment Wedge
For any method, determining the farthest extent of deposition can be difficult and possibly subjective. Extensive channel surveys using Acoustic
Doppler Profile (ADP), LiDAR, and seismic refraction techniques can inform this determination, though reliable field indicators (e.g. change in slope, bed texture, vegetation, width) exist in the field for identifying where the reach shifts from a lotic, uninfluenced channel to the lentic, impounded environment.
-Depth of Sediment
Both in the field and in the calculations, determining the depth of sediment for each deposition area introduces variability. In the field, hand borings and seismic refraction surveys may be used to determine the depth of the sediment layer. Obvious layering in the sediment wedge or presence of
exposed bedrock in upstream reaches can guide depth estimates. Rough estimates can be made by assuming a trapezoidal wedge shape, using an estimated pre-dam slope, and calculating post-removal channel competency.
-Degree of Approximation
The number and location of points surveyed, the precision with which they were surveyed, and the amount of averaging in the calculations will all affect the precision of the volume calculated.
BackgroundDownstream movement of stored sediment following dam removal is often
of concern as it can clog municipal water supply, wastewater treatment, and agricultural pipes, impair spawning habitat, change erosional and depositional features, raise bed elevations, and impair navigation. Incision through the sediment wedge can lead to upstream channel instability and possible loss of infrastructure. With an estimation of the volume of sediment behind a dam, uncertainty regarding these geomorphic processes may be evaluated. However, estimates of stored sediment volumes may vary based on the assumptions and features of the estimation approach.
Method 2 - Planimetric Area
Depositional Areas
Flow
Brownsville DamCalapooia RiverBrownsville, Oregon
ConclusionsThe Cross Sectional method with minimal data is the coarsest approximation as it averages depth and area over the entire sediment wedge. In contrast, the Planimetric Area method is limited by the possible number of partitions for each depositional area based upon the number of data points.
For the Cross Sectional method, varying determinations of the farthest upstream extent can lead to substantial differences in volume estimates as a
result of different slopes and different vertical placements of pre-dam river bottoms. For other methods, variations in the determination of the farthest upstream extent could lead to varying inclusion or exclusion of whole depositional areas.
The importance of precision and uncertainty will vary between dam removal projects based on the size of the dam and the level of risk associated with its removal.
1 - MS Candidate
Oregon State Univ. - Biol. & Ecol. Engr.
2 - Assistant Professor
Oregon State Univ. - Biol. & Ecol. Engr. [email protected]
-Partitioning the Wedge
Using the surveyed points from each
cross section, or points to approximate them, the surfaces between adjoining cross sections or within adjoining cross sections should be partitioned, as shown in the figure to the right. The area and volume calculations can be made easier by creating parallel partitions, and therefore trapezoidal
areas.
Flow
The basis of this method is the assumption that the deposited sediment consists of a continuous wedge stretching from directly behind the dam to the farthest
upstream extent of dam related sediment deposition.
The ease and precision of calculating the area within cross sections versus calculating the area between cross sections depends on degree of approximation (which depends on the number of partitions), the program being used, and the
similarity of the two cross sections. Calculating the area between cross sections seems to be more common, and can approach the planimetric area method when a large number of data points between cross sections are available and used.
-Area Calculation: Within CS or Between CS
This method can be very data and analysis intensive, requiring
detailed surveys and use of a program such as Auto-CAD or ArcGIS. In addition, there are still the problems of definitive identification of the farthest upstream extent of the sediment, and the isolation of sediment deposition solely due
to the dam.
-Definition of Widths
The width of the pre-dam river
bottom approximation should be determined by the breaking point between the banks and the bed. It is easiest to visualize this with the trapezoid, as the sides of the trapezoid should look like a natural continuation of the bank. All three shapes should have the same width.
The bottom point of the triangle most logically falls below the thalweg.
USBR, 2003
Longitudinal Profile From Around Brownsville Dam, Calapooia River
80859095
100
-1625 -1425 -1225 -1025 -825 -625 -425 -225 -25Station (ft)
Rel
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leva
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(ft) Dam Crest
Calculated Slope
Survey Data
Theoretical longitudinal profile (red) with pre-dam slope (black).
542,000RectangleDepth Averaged
448,500TrapezoidDistance Averaged
431,900TrapezoidDepth Averaged
552,100RectangleDistance Averaged
127,100TriangleDistance Averaged
108,000TriangleDepth Averaged
Volume (ft3)Pre-Dam Cross-Sectional ShapeCross-Sectional Method
Cross Section 871 Partitioned for Area Calculation Within the Cross Section
Station (ft)
Rel
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leva
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(ft)
Partitions
Approximately 57 feet upstream from the dam