Fluvial Geomorphology in an Arid Environment A Case Study.ppt · Fluvial Geomorphology in an Arid ... Stability at Highway Structures” (HEC‐20, 4thEdition), Chapter 6.2, ... calibration

Fluvial Geomorphology in an Arid Environment: A Case StudyPresented by:

David T. WilliamsDTW and AssociatesCommerce City, CO

[email protected]

Co‐author:Joanna Czarnecka, EIT

Watermia Consulting, LLC 1

• Sponsor: USACE, LA District, for the Kewa Pueblo Tribe, NM

• To determine future conditions of the Creek meanders and their migration and extension rate in order to predict locations of the channel to help location future infrastructure.

1. Predict the Creek’s future planform locations.2. Assess potential for floodplain creation or abandonment and

potential for incision or aggradation.3. Document significant sites of sediment inflow that could

influence the planform movement of the Creek.4. Generally, predict changes in sediment size within the channel

bed and note future sediment loads at key locations.

Purpose of the Study

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• Sinuosity index ‐ a dimensionless term representing the ratio of the sinuous length over the straight length of the channel

• Braiding parameter ‐ a measurement of the number of bars or islands in a channel reach

• Existing and future Channel Slopes

• Meander characteristics and potential growth and movement

The Key Parameters Determined

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• Galisteo Watershed, a 669‐square mile drainage basin (428,160 acres), is located in central New Mexico

• Considered the Middle Rio Grande Basin, also termed the Albuquerque Basin

• The headwaters start in the southern Sangre de Cristo Mountains at about 9,500 feet and on the Glorieta and Rowe Mesa at 7,500 feet

• The Galisteo Watershed is part of the Rio Grande Santa Fe hydrologic unit (HUC: 13020201)

Watershed Description

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• The area study covers the lower portion of the watershed from I‐25 downstream to the confluence with Rio Grande.

Project Location Description (Galisteo Creek)

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Study area

• An ephemeral creek (defined as a stream that flows only during and immediately following a period of rainfall in the immediate locality)

• Is subject to extremes between dry conditions and flash floods

• The variable hydrologic regime creates a stream dynamic where banks tend to be unstable and new meanders can suddenly cut through established riparian areas.

Galisteo Creek Description

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• Google Earth: version 7.1.7.2606, Build Date: October 6, 2016https://www.google.com/earth/explore/products/

• Earth Data Analysis Center (EDAC) at the University of NMhttp://edac.unm.edu/image‐archive/

• New Mexico Resource Geographic Information System (RGIS is part of the EDAC), hosted and managed by the Earth Data Analysis Center at University of New Mexico

http://rgis.unm.edu/

• New Mexico Bureau of Geology and Mineral Resources, interactive maps. http://geoinfo.nmt.edu/maps/

Significant Data Sources

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Measuring meander migration, part 1

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Measuring meander migration, part 2

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1. Locate the bankline delineation points on the outside of a river bend

2. Fit a circle to the bankline demarcation points to describe the radius and orientation of the bend

3. Use consecutive historical records and the data collected in Steps 1 and 2 to estimate the historic extension and translation rates for a bend

4. Use the migration/translation and extension rates to extrapolate and estimate the future bankline locations

Steps to be performedat each bend for each historical record

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• Historic “threads” (thalweg) of the Creek were determined from the sources presented above

• Other relevant Creek features evaluated (if discernible), were average active bank widths and locations, depths, sinuosity index, braiding parameter, and channel slope

• These Creek features were similar to those described in “Stream Stability at Highway Structures” (HEC‐20, 4th Edition), Chapter 6.2, Lateral Channel Stability

• The technique for evaluation of future meander changes over time was based upon NCHRP Document 67, “Methodology for Predicting Channel Migration”, 2004. and supplemental NCHRP Report 533, “Handbook for Predicting Stream Meander Migration,” 2004

Methodology

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Main Reference Documents

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• The validation of the prediction technique utilized the document “Sediment and Erosion Guide, AMAFCA, 1994,” starting on pg 3‐66, utilizing the procedures starting on pg 3‐74

• The future lateral movement was determined by analysis of the historic lateral movement over time and projected to the maximum lateral erosion, similar to the NCHRP document. This analysis procedure is discussed in section 3.4.3 of the AMAFCA manual.

• The technique for additional validation of historic meander change was based upon HEC‐20, Chapter 6.3.2, “An Overlay Comparison Technique.” Where applicable, the methods from NCHRP 67, HEC‐20, and AMAFCA were compared for reasonableness of the results.

Methodology

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Main Reference Documents

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• There is very little site specific information on meander migration or lateral movement of channels for use in the calibration or confirmation process

• A literature search was conducted for the arid southwest to obtain regional data and to identify potential transferability of that data to the project, based upon climate, watershed altitude, drainage area, cover and soils, and other relevant physical parameters

• The literature search resulted in very little usable information, therefore engineering judgment and the AMAFCA document were the bases for evaluating the reasonableness of the results

Calibration and Confirmation of Results

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• The aerial imagery was obtained from Google Earth Pro software for the following years: 1996, 2006, 2009, 2014, and 2016

• The historic imagery for year 1935 was obtained from Resource Geographic Information System (RGIS) at: http://rgis.unm.edu/getdata/#

• Digital terrain model was supplied by Bohannan‐Huston, Inc.

• Other resources were investigated to compare the quality of available data, however the decision was made to use the data referenced due to availability and quality of data

GIS Aerial Mapping Overlays

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• Google historical imagery was used for channel and bankline alignment changes

• Reference markers were placed on Google maps at strategic locations and these markers were registered

• After images were overlaid using GIS software, images were geo‐referenced and scaled to known locations with use of these registration markers

• All the maps were then traced in GIS for the location of the “low flow channel” (termed Centerline) and channel overbanks limits (bank lines) based on visual inspection of the images


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• Quality of the images played a crucial part in this step of the process

• All six historic images were used for the initial evaluation

• After careful evaluation, it was decided that to best represent historical changes and considering the quality of the data, index three years of record imagery would be used for evaluation and predictions of the Creek

• The years chosen for that purpose were images from years 1935, 1996, and 2016


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• The channel centerline and right and left bank lines were traced over the 2017 aerial photography were geo‐referenced into the GIS ArcMap 10.5 software. This was done for all years.

Aerial Imagery with Stream Centerline Alignment and Bank Lines

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• Based on the soils map, the majority of soils within the study area are type Qa – Latest Pleistocene to Holocene Alluvium Undifferentiated, Quaternary alluvium (Stewart and Carlson geologic unit)

• This type of soil represents unconsolidated sediment and are easily eroded

• Since the Creek meander belt is almost wholly contained in the area represented by soil type Qa, the meander predictions would not be affected by encountering different types of soils that would have different strength characteristics

Soils of the Meander Belt

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• Based on the NCHRP method, best fit circles were drawn using GIS at meandering loops for all the aerial photographs

• These circles are best fit to each bend and are drawn tangential to the outer banks at the apex of the loop (see next slide)

• Each of the bank lines and circles for a given year share the same color for easy differentiation while comparing between different years

Methodology for Fitting Meandering Loop Circles

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• Below is an example of circle fitting and determination of circle center and radius of curvature, Rc, of meander bends


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• After circles were identified in the ArcGIS software and while still using separate imagery for each of the alignments, circles, and bank lines, the files were transferred to the CAD software

• AutoCAD Civil 3D was used to further evaluate the stream geomorphology

• For all three representative years, 1935, 1996, and 2016, at each bend, 27 circle centers were identified and arrows were drawn from the base circle to the future circle to represent the direction of change in the channel morphology


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• The lateral migration rate of the low flow channel, as defined by the channel centerline, was calculated based on the method outlined in NCHRP Document 67 and NCHRP Report 533

• GIS tools cited in these documents (Data Logger and Channel Migration Predictor) are no longer available but the methodology used in these references is still valid.

• The approach is similar to the HEC‐20 method

• Three separate channel alignments were prepared with drawing tools using ArcGIS software (ArcMap 10.3).

Lateral Migration Analysis

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• As an example, outlines of channel centerlines, fitting circles (with radii) and bank lines from aerial photography for year 1996.


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• Two separate time comparisons were used based on the fact that Galisteo Dam was built in 1970s and its possible influence on the meander migration rates

• The first time period captures the years 1935 to 1996 and second, the years 1996 to 2016

• Ideally, the time division between the two periods would have been near the year 1970; however, good quality data was not available near that time


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• Two tables were prepared to compare the results for pre‐Galisteo Dam and post‐Galisteo Dam conditions. A sample of the results for the period 1935 to 1996 are presented below


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• Some of the radius changes for the fitted circles are negative

• In some areas, the fitted circles produce significant overlaps of the meanders represented by the fitted circles, and connection of these circles result in drawing a severe meander or overlapping meanders

• When the sinuosity reaches approximately 3.7, the meander “folds” on itself and a cutoff would occur, resulting in a bend that is straightened out – see next side

• This produces a negative bend radius change

Negative Loop Radius Change

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Negative Loop Radius Change

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• Two time frames were chosen for future movement projections of the meanders in Galisteo Creek.

• The base year was 2016 and future years are 2041 (25 year projection) and 2066 (50 year projection).

• In these time frames, each of the bends will move laterally (and the radius of the circle will change) to a new location as predicted by the methodology.

• Only the time period between 1996 and 2016 was used for the migration projection of the meanders since the other time period included time before construction of Galisteo Dam.

Migration Projections for 25 and 50 years

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• Below is a portion of the projection table for 25 and 50 years.


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• The meanders for an areal portion of the 25‐year prediction are shown below. “R” is the radius of the circle, the other number is the movement distance of the circle center, and the light blue arrow is the direction of the meander movement.


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Final Circle Radius

Final Movement of circle center and distance

Direction of movement

• The bank lines of the meanders for a portion of the 50‐year prediction are shown below


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New Bank Lines

New Bank Lines

• Equilibrium slope is a theoretical slope that could be attained within decades or thousands of years.

• Using various methodologies, a mean grain diameter of 2 mm and a channel forming discharge of 3,218 cfs (5‐year, 6‐hour storm from HEC‐HMS), equilibrium slopes were calculated using the PBS&J Scour spreadsheet written by Leonardo Kreymborg and David T. Williams.

• The results for equilibrium slope varied moderately based on the different methods and ranged from 0.00067 to 0.00022.

• This range was lower than the existing slope.

Equilibrium Slope Calculation

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• Since the existing slope is greater (steeper) than the equilibrium slope(s), the Creek’s tendency would be to reach this equilibrium slope.

• To do this, the channel length would need to increase and would result in higher sinuosity.

• This tendency is demonstrated in the next slide, which shows that the sinuosity index generally increases with time with a corresponding decrease in channel slope.

• This reinforces the results of the analysis showing extension of the meanders (lengthening of the stream centerline) as they progress toward equilibrium.

Equilibrium Slope Analyis

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• To calculate sinuosity of channel centerlines at different times, Creek centerlines were drawn in GIS. The Python toolbar “Calculate sinuosity” was downloaded from ArcGIS website.

Current and Historic Sinuosity

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• The braiding parameter is the measure of the number of bars or islands in a channel and is defined as the number of major bars or islands per meander wavelength.

• Based upon aerial photos and field inspections, the braiding parameter was approximately one to two which is representative of a mainly single threaded meandering channel.

• This indicates the channel is not in a predominately avulsion (braiding) zone; therefore, the usage of the prediction method for meander migration is appropriate for determining future conditions.

Braiding Parameter

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• The assumption that the meander movements are unhindered by “hard” points aids in the identification of future meander locations that may endanger existing and future infrastructure.

• If a more detailed study is to be performed, the locations of dependable hard control points such as bridges, bedrock and bank rock outcrops, hardened embankments of railroads and roads, as well as bank stabilization efforts which would prevent the movement of the channel, must be identified.

• If this is the case, the advancing parts of the circle should be stopped at the limits of the hard points and adjacent locations of the meander must be adjusted accordingly for continuity.

Assumption of No “Hard Points”

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Assumption of No “Hard Points”

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• Below shows the easily identified “hard points” in comparison to the 50 year projection of the meander belt.

• The areas where little or no meander movement is predicted should be the first options for any infrastructure placement.

• The terrace immediately downstream of a stabilizing structure such as a bridge or a location of minimal meander movement is another ideal location to place infrastructure.

• The meander movement in the downstream direction is away from this terrace area and the extension of the loop is prevented by the stabilizing structure.

• The upstream terrace is to be avoided since the downstream movement of the meander would “collapse” this area.

Recommendations

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• If wetlands creation in the floodplain is an option, these should be placed where the meanders are moving away from these proposed locations (inside of the bend) and are moving at a relatively slow rate.

• However, these inside of the bend locations may fill up with sediment in subsequent flood events.

• If the meander movement rate is high, the wetland areas could be abandoned by the Creek and would not be sustainable without human intervention.

• Optimal locations for wetlands are adjacent to meander inflection points, i.e., “crossover” or “riffle” points. These locations are the most stable in a meandering stream.

Recommendations

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Questions?

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