Guest Blog: Using BioBase to determine sedimentation in the Central Arizona Project canal

by Scott Bryan

The Central Arizona Project (CAP) is a multipurpose water resource development and management project that provides irrigation, municipal and industrial water to much of Arizona.  The primary means of water conveyance is a 336-mile concrete-lined aqueduct that transports water from the Colorado River, on Arizona’s western border, across the State to Phoenix, and then southward to the aqueduct terminus near Tucson.  Each year, over 1.5 million acre-feet of water is delivered to our customers.

Since its completion in 1993, the aqueduct system has experienced increasingly severe sedimentation that creates problems within the pumping plants and in the aqueduct itself.  Because the sediments can decrease the flow capacity of the aqueduct, cause damage to pumps and internal systems, and restrict flow through critical filtration units, it is imperative that dredging operations occur periodically.

sedimentation, ciBioBase, water volume, depth, mapping, bathymetry
CAP forebay dredging in 2009

In the past, CAP performed intensive sonar based sediment studies to determine bathymetry and the amount of deposition in the forebay of each of the 13 pumping plants.  The surveys show when and where dredging operations should occur.  These surveys were contracted to outside companies with costs ranging from $40,000 to $120,000 annually.

In 2012, CAP began to use the sonar technology provided by BioBase to conduct its own bathymetry surveys in the pumping plant forebays.  Water depths are compared to historical baseline surveys and the volume of sediment in each forebay can easily be calculated.  Annual surveys allow us to compare sedimentation from year-to-year to determine loading rates and critical areas to target sediment removal.  Surveys of all 13 forebays can now be accomplished in three days rather than six months, and when compared to the expensive surveys from the past, are equally as accurate.

ciBioBase, bathymetry, water volume, depth, Lowrance, acoustics, mapping, sonar, sedimentation, dredging
Blue-scale bathymetric map of a CAP forebay.  The light blue contours show an area that is extremely shallow and is in need of sediment removal.

 

ciBioBase, sedimentation, Lowrance, downscan, sonar, mapping, bathymetry, depth, water volume
Example transect design and resultant bathymetric map coupled with the sonar log viewer.  Notice the detailed image of the forebay’s trash racks produced by Lowrance HDS DownScan
This new approach to bathymetric and sedimentation mapping saves time and money, allows us to evaluate results immediately, and makes dredging operations more efficient and timely.

Scott Bryan is the Senior Biologist for Central Arizona Project (CAP).  After receiving an M.S. in Fisheries Management at South Dakota State University, Scott worked as a research biologist for Arizona Game and Fish for 10 years, then specialized in lake and stream management for seven years at a private consulting firm in Albuquerque.  Scott’s current position at CAP includes a broad scope of work, including aquatic and terrestrial vegetation control, fisheries and wildlife management, invasive species research, and water quality monitoring.

Assessing Fish Habitat in Rivers

BioBase is not just a lake vegetation mapping tool, it also can help Fisheries managers and researchers assess, monitor, and simulate fish habitat conditions in large rivers.  We demonstrated this application on a trip to the Mississippi River Pool 2 in St. Paul, MN on 4/27/2012.  Just downstream of the Lock and Dam, we used a Lowrance HDS sounder and the automated processing of BioBase to map the bathymetry of a pool where a range of fish species often congregate (Figure 1).

Figure 1.  Bottom mapping with a Lowrance HDS-5 on Pool 2 of the Mississippi R. just downstream of the Lock and Dam on 4/27/2012.

 

The raw pool elevation on 4/27/2012 was 4.27 feet; still within the range of moderate drought according to the US Drought Monitor but 1.7 feet higher than the most recent low on 12/10/2011. Coincidentally, these drought levels follow historic flood levels just one year earlier (Figure 2). To demonstrate BioBase’s utility as a fish habitat assessment tool, we compared sizes and volumes of our mapped pool under the hydrologic conditions experienced on Pool 2 during the last year.

Figure 2. Hydrograph for the Mississippi River at St. Paul, MN (DNR ID# 20088002; USGS ID# 05331000; Data and figure courtesy of the MN DNR).


On 4/27/2012, we mapped and analyzed a 15-ft pool using the ciBioBase polygon creation tool and determined that the max depth was 17 ft, surface area was 317 m2 and the volume was 1508 m3 (Figure 3).

Figure 3.  Diagnostics of a pool of interest using BioBase’s polygon tool.

In order to reconstruct changes to this pool under the recent low flow on December 10th 2011, we used the Z-depth Offset feature iniBioBase to drop the elevation down 1.7 feet.  In Figure 4, you can see the striking difference this reduction has on the size of this pool and consequently the amount of available fish habitat.  The area on December 10th 2011 was estimated to be 3.1 m2 and volume was 9.4 m3; 100 times smaller in size and 161 times smaller in volume than on 4/27/2012. If we increase the offset by the peak flood elevation on March 30th 2011, the 15-foot hole becomes a 30-foot hole (Figure 5).

 

Figure 4. Polygon overlay in BioBase demonstrating the difference in size and volume of a 15-ft deep hole between the yearly low elevation on 12/10/2011 (pink) and during data collection on 4/27/12 (green).

 

Figure 5. Polygon overlay of drought elevations in 2012 (green and pink) overlain onto simulated peak flood bathymetry on 3/30/2011.
This demonstrates one potential application of BioBase for fish habitat studies in large rivers.  We presented three striking contrasts in fish habitat conditions within one year’s time with data that took 20 minutes to collect and an hour to analyze in BioBase. Different hydrological scenarios can be modeled in BioBase and thus could be used in predictive fisheries habitat models or to reconstruct habitat conditions over some period of time.

New Z-offset (depth offset) Feature

Some of our customers have requested the ability to make their maps even more accurate by eliminating the distance between their transducer and the bottom.  We listened! 
Depth calculations (z) using hydro acoustics are calculated from the source (transducer) to the bottom. Because a depth finder transducer is typically mounted below the water surface, depth readings are always off by the distance between the bottom of the transducer and the surface of the water . . . not anymore!   With the new z-offset feature, any user can now recalculate depths by entering this distance and reprocessing the trip.  For example, if your transducer is 6 inches below the surface, all of your depth readings should have a half foot added to them.  A 10 foot z should actually be 10.5” deep.  With a .5” z-offset, all of your depths will be reprocessed for better accuracy.  This is very important when calculating water volumes! 
The z-offset feature can also be used for calculations to high water marks or draw downs.  By using the z-offset for a 5 foot draw down scenario, our users can identify which bottom structures will be exposed as land (see below).  In addition, lake and pond managers can determine total water volumes at a high water mark by measuring this distance.  By simply offsetting all depth readings with a single z-coordinate offset, your trip will be reprocessed the way you want it.  Water volumes, blue scale, and plant biovolume will all be recalculated in your account.  Simple!
Below is an example of the z-offset in action for a simulated draw down.  We took an accurate trip from Trout Lake in Wisconsin and offest the z-coordinate by 20 feet to simulate a 20 foot draw down.  The new blue scale reflects the changes and displays the new land in green:

Water Volume Analysis

BioBase is powerful! The centralized (online) nature of our System allows us to offer new features to our customers and make them available to trips that have already been uploaded to the System. A perfect example is our recently pushed out water volume analysis feature that is included in the standard report generated from every trip.

We employ a TIN (triangulated irregular network) to provide total water volume of a water body in cubic meters. A TIN is a vector representation of a lake bottom produced with irregularly distributed lines and nodes with 3D coordinates arranged in a network of nonoverlapping triangles. Since the BioBase System already uses an X, Y, and Z coordinate system for bathymetry, we can use a TIN for water volume analysis. An advantage of using a TIN is that the points of a TIN are distributed variably using an algorithm to determine which points are most necessary for an accurate representation of the lake bottom.
Actual covered water volume is based on the surveyed area only. In addition to actual, we also offer the total water body water volume that is based on a universal kriging algorithm that estimates contours (using statistical analysis of actual data) to the shapefie boundary. TIN analysis will be more accurate with more data points gathered with you Lowrance HDS depth finder. However, we have found that there are diminishing returns when adding more data points to improving the accuracy of the water volume analysis. Here are a couple trips on a 3.2 acre lake for comparison:

Less data collected (1019 data points):
More coverage (1546 data points):
Both trips merged together:

“Actual” represents the total surface area of the water body sampled with your HDS sonar unit. Although we also show acres covered, “Total” represents total acreage of the shapefile. Total Lake Volume represents total volume using the TIN analysis with kriged estimates using the actual data points. More data points means more accurate contours and more accurate volume. More variability in the contours could mean more places for water to fit in the pond. All relative to the water body size, when data points increased from 1019 to 1546 and actual coverage increases from 59% to 90%, total volume increased by ~6k cu. m. However, when merged together (using our trip merge function) and data points increase from 1546 to over 2500, total water volume only increases by ~300 cu. m.

One way to potentially add more actual coverage to this trip would be to cross some of these with vertical zig zags. We provide the tools and allow you to decide how to maximize them.

Bathymetric and vegetation mapping is easy with the automated CI BioBase System. We’re changing perspectives on what it mean to map and who can do it.

If you have any questions about BioBase, give us a call or email us at info.biobase@navico.com.