Thesis Proposal, WIP
From GeoMod
Contents |
Introduction
This study will attempt to identify geomorphic patterns river systems where exchange with the groundwater system has altered the form of the river. A multi-method approach will be used to study the effect of groundwater/river water exchange to ensure accuracy and integrity of the study (mention the methods-Lurbano). Once groundwater exchange locations are identified, bedforms and larger scale features will be statistically analyzed and compared to non-exchange river sections. A meandering river system will be used for the study area for ease of comparison of large scale features, such as meander bends, and for ease of sampling in lower velocity waters (We will potentially be using both meandering sections and straightend sections that want to meander for the reasons you mention here. You also need to mention something about using the sandbox model and field measurements to examine issues of scale and test the ability to simulate things in the sandbox - Lurbano). The ultimate goal is a better understanding of the processes at work between the surface and subsurface environment.
Literature Review
Groundwater and geomorphology
Previous work has shown that an apparent correlation can be made between river geomorphology and groundwater movements. A Texas study by Larkin and Sharp (1992) found that streams with a significant base flow tended to have a sinuosity greater than 1.5. Lower gradients where also associated with base flow dominated systems. The reason for this behavior was not investigated.
Measuring groundwater/river exchange
Previous studies done on the exchange of groundwater and surface water have mainly studied fluid transport. Although the geomorphic effect of this exchange is often overlooked, the methods used to determine amount of exchange are very useful. Becker (2004) describes three approaches to estimating the amount of groundwater flux and the problems involved with each approach. (do the following sentences refer to the approaches described by Becker (2004)? If they do you should say something about all three - Lurbano). Traditional means of stream gauging do not account for other possible sources of added river water, such as runoff, and are made inaccurate by any kind of precipitation. Temperature based methods of acquiring discharge rate can be accurate, independent of precipitation, but only measure rate at a single point. For means of this study, the temperature method will be used for identification of discharge location (Why is this appropriate for this project given the limitation mentioned in the previous sentence? You need to justify your choice here or say you'll talk about it later Lurbano).
A study by Wells (2002) Lurbano in Western New York compared the accuracy of the stream gauging method with the temperature gradient discharge method (Wells, 2002) - delete Lurbano. It was found that a temperature flux existed in cobble sized sediment to a depth of 0.5 meters while in bedrock substrate, the flux was only present to a depth of 0.1 meters. The flux in temperature was due to daily solar heating of stream water, and subsequent cooling at night. Deeper water should have less of a flux in temperature. The discharge rates found from stream gauging were an order of magnitude higher than those found by temperature gradient. It is possible that the temperature sample location was not in a peak discharge area. (Explain the relavence of this study to your project design) Lurbano
River geomorphology in the field area
(incorporate this into the next section)Lurbano
The Wolf River, a river in Shelby County similar to the Loosahatchie River, was chennelized and straightened in the early 1960’s. Flow velocity subsequently doubled and the river channel incised to varying degrees along the length of the river (Van Arsdale, 2003). Similar effects can be expected to hav occurred along the Loosahatchie.
Description of Study Area:
Loosahatchie River, Shelby County, Tennessee
The purpose of this study is to qualitatively analyze the effects of groundwater exchange on river systems. The Mississippi Embayment offers an ideal location because ... Lurbano. The embayment itself is shaped like a south plunging syncline Lurbano composed of alternating layers of sand and clay shallowly dipping towards (the axis of the syncline (delete-a concentric center line-delete)) Lurbano. This axis line trends approximately below the Mississippi River. (Insert a sentence about the depositional environment. ie. transgressive-regressive sequences of coastal facies from repeated flooding of the embayment.)Lurbano In Shelby County, Tennessee, continuous sand and shale outcrops trending in a north/south direction outcrop across the width of the county. The sand layers are known nationwide for their excellent groundwater transport ability. The shale/clay layers in the region are also known to be of varying thickness with thin spots and windows throughout. The rivers in Shelby County tend to cross these outcrops perpendicular to strike direction, flowing from East to West towards the Mississippi River. This allows for the study of river processes as the river (progresses from a groundwater exchange zone to a zone of no exchange where aquifer properties are well known there is a significant change in ground/surface water exchange in this zone, not a zone of no exchange. Refer to the Urbano et al 2006 paper)Lurbano. The Loosahatchie River was chosen primarily for its near East/West orientation and relative straightness along its length in Shelby County. In 1968, the Loosahatchie River was channelized by the Corp of Engineers to reduce flooding and increase drainage potential. Prior to channelization, the Loosahatchie River was highly sinuous with only a general East/West orientation. It has subsequently begun to revert back to a meandering state (reference VanArsdale 2003)Lurbano.
Problem Description
The Corp of Engineers channelized many meandering rivers across Tennessee in the 1950’s and 60’s. Since then, these river channels have begun to revert back to a meandering state. Along with increased sediment load, destruction of habitat, loss of organic carbon sources for aquatic life, loss of floodplain soil renewal, channelizing rivers costs money. It now appears that channelized rivers may need to be re-dug every thirty to forty years. By destroying natural streambeds and exposing aquifer recharge sediment directly to free flowing runoff sources, the potential for groundwater contamination also greatly increases. (A better understanding of the relationship between the aquifer system and the stream channels would permit the analysis of the geomorphology of channelized rivers for the determination of the potential for increased contamination Lurbano).
Several models exist to explain the propagation and migration of river meanders. None of these models are able to account for the existence of seemingly regular meanders in tandem with very irregular meanders (which models. Need references Lurbano). The symmetry encountered in many meandering systems seems to imply that an external factor to the river system is at work. Irregularity can be accounted for by consideration of variant riverbed geology or sediment deposition, but why regular bends and irregular bends occur in similar geologic settings comes into question. Nor can any model convincingly account for how the process of meander bends is initiated. The traditional view analyzes single reaches or bends and analyzes finite properties of the bend (give sample reference Lurbano). The alternative view analyzes the meanders in a collective sense, taking into account general trends and property change throughout the series of meanders. Fergason (1979) breaks this series approach into three main components; a scale variable, sinuosity, and degree of irregularity. The benefit of this approach becomes apparent when analyzing property change at various locations along the river length. The traditional approach still retains value when analyzing finite properties of an individual reach.
Proposed Solution
The effect of groundwater exchange on the geomorphology of river systems has the potential to answer the question of meander initiation as well as meander regularity or irregularity. It is important to first note that the confining layers in the Mississippi embayment are not of uniform thickness, nor are they always continuous. The possibility exists for thin locations and windows throughout the confining clay layers. Several possibilities exist for potential impact. 1. Discharge to the river form the groundwater system is not uniform along the riverbed. Locations of peak discharge may exist with a decreasing gradient of discharge radiating out around them, or possibly windows in the riverbed may exist allowing groundwater discharge solely through these windows. 2. The up-flow of groundwater in gaining streams may aid in destabilizing streambed grains, aiding in entrainment and resisting deposition. The minimum energy needed to entrain sediment is found by means of calculating the critical shear stress of bed material.
Ï„c = k(Ï?s – Ï?)g D - or approximately - Ï„c = 0.73D
where: Ï„c = critical shear stress D = Particle size
This potentially creates deeper sections, or pools, at areas of higher groundwater discharge. River water volume and velocity increases at these pools and the traditional meander propagation model developed by Einstein and Poole(1964) then applies.
3. Groundwater discharge through riverbeds may create a reduced surface or bed friction. This resistance factor can be found using Manning’s equation:
n = k(R2/3*s1/2)/v
where: n = resistance factor k = 1(SI units) R = hydraulic radius s = slope of energy gradient v = mean velocity
River water flowing through these areas increases in velocity. The velocity increase then promotes increased bank erosion in these sections.
Methodology
The problem will be approached by means of three separate methods. The primary method will involve direct measurement and observation of field conditions along the Loosahatchie River which is an a posteriori approach. The direct results of channelization and meander initiation can be studied and documented. A sand box model will act as the second method of investigation. This a priori approach will allow a physical scientifically controlled environment in which contributing factors to meander propagation and migration can be tested. Computer models offer a third approach where calculations and initial theoretical conclusions that are time dependent can be tested in a realistic time frame. The computer model results will then be compared to other physical models and field observations for accuracy.
Field Observation Methodology:
Since the thickness of the confining units that outcrop throughout Shelby County have variable thickness and outcrop location, it becomes necessary to first locate areas of groundwater discharge. The primary method that will be used utilizes the temperature difference in groundwater and surface waters. Since subsurface water equilibrates with the 58° F surrounding lithology, groundwater maintains a constant ground temperature. For the Shelby County area, this is approximately 58° F. During summer months, surface waters in lakes and rivers averages well above this, and during winter months surface water temperature can be well below the 58° F ground water temperature. Gaining streams receive water from groundwater sources, therefore areas in the river that deviate in temperature from the surface water average towards the ground water average can be seen as areas of groundwater discharge. For the purposes of this study, this deviation in temperature will be used as a proxy for groundwater discharge location. To locate discharge zones, a panel of temperatures at varying locations, depths, and cross sectional distances will be analyzed.
1. A general temperature and depth profile will be made of the study area. A temperature data logger will be attached to the end of a one meter pole and set to sample at 2 second intervals. A GPS unit will also be set to take continuous special readings at the same 2 second interval. A boat will then travel downstream towing the pole with data logger at a 1 meter depth along the entire study area. Depth will be taken at a set interval as well. The temperature data from the data logger will then be correlated to the position data from the GPS unit to from a temperature profile of the study area. This will give a general idea of areas of groundwater discharge and allow for the testing and accuracy of the data logger/GPS setup. Depth will be correlated with GPS location as well to generate a rough depth profile. 2. From the temperature profile, an area containing a discharging location will be selected, as well as a non discharging location. Each location will contain a full meander bend pair. Using a ground penetrating thermocouple probe, sub-riverbed temperatures will be measured at a depth of 5 cm below bed surface along a cross sectional profile running perpendicular to river bank. These sub-bed temperatures will be taken at a one meter interval across the cross section. This cross sectional measurement method will be applied at riffle points, outer meander bend locations, and transition zones from riffle to meander and meander to riffle. A river water temperature profile will also be created. At the same one meter interval across the same cross sections, water temperature and depth will also be measured at 10 cm above river bed, mid river depth, and at 10 cm below water surface. This will generate a river water temperature cross section that will be compared to sub-riverbed temperatures. Areas of high discrepancy between river water and sub-bed temperature will be treated as high groundwater discharge areas. A down stream line can then be created by correlating all cross section profiles to find the downstream path of highest groundwater discharge. 3. A 10 cm long ground penetrating grounded J-type thermocouple will then be attached to the end of the sampling pole. Grounded J-type thermocouples have an accuracy of +/- .5 degrees Celsius at the temperature range in question and a response time of 0.5 seconds. A skid plate will be attached directly above the probe to prevent penetration of more than the 10 cm probe length. A standard temperature data logger will then be set 10 centimeters from the penetration probe. A second standard temperature data logger will be set at a depth of 10 cm below river surface. All data loggers and GPS will be set to a sampling interval of 2 seconds. The probe will them be drug across the stream bed along the down stream line of maximum groundwater discharge. A downstream profile can then be created of sub-bed temperature, epi-bed temperature, and subsurface temperature. These profiles will then be correlated to locate exact locations and relative rate of groundwater discharge.
Depth and location of high discharge will be correlated to identify if high discharge areas coincide with deeper pool sections. If there is no correlation, than the increased erosion at discharge points theory is disproved.
4. Flow rate will be measured at the beginning, middle, and end of the known aquifer discharge study area. Rates will be obtained using a ****************. Flow rates will also be measured in the non discharging river study area. Using Manning’s equation, the resistance of stream bed will be calculated for the discharging and non discharging study areas. The sets of data will then be processed statistically using F-test and t-test methods. This will reveal any statistical correlation that may exist. If the two data sets are statistically similar, the possibility of friction reduction in discharging areas can be eliminated.
Sand Box Model Methodology: Goal: To construct a scaled sand box model capable of modeling ground water/surface water interactions. Water will percolate up from below a porous medium to simulate an aquifer discharging into a fluvial network. The box model will allow for adjustable aquifer head pressure as well as discharge area. The box will be capable of handling multiple layers of varying porosity and composition. Box Construction: Dimensions: 1.0 m * 1.0 m * .1m The box will be constructed of an outer wooden box frame with a fiberglass coating for waterproofing. One of the 1 m ends will have a drainage lip with a central notch 1 cm in depth. The other 1 meter end will have a reservoir with a variable depth drain spout to allow for multiple head levels. The bottom of the box will have 1.5 cm risers spaced in 10 cm intervals in grid format. A 1 cm mesh screen will be placed on the risers with an additional .2 mm mesh screen on top of it. A schematic drawing is attached. Testable hypothesis utilizing Sand Box: 1. Ground water discharge creates low spots in river systems due to destabilization of riverbed and therefore increasing erosion rates at this point. 2. River water velocity increases in areas of ground water discharge due to reduced bed friction 3. Variations in thin confining layer thickness creates windows through which ground water discharges at higher rates. These higher rates increase erosion rates of surrounding streambed, widening the river near the window. 4. Groundwater discharge spikes occur where aquifer outcrops are separated by a thin confining bed. 5. River channels migrate towards aquifer discharge outcrops. Procedure for testing hypothesis: 1. An even layer of sand, 4 cm thick, will be placed in the box. An overland flow component will be created in the box by means of tube supplying constant water volume to the surface of the sand. Natural waterways will be allowed to form and the water channel depths will be checked for varying depth. An artesian water pressure will then be applied to the ground water in the box and natural waterways will be allowed to form. The channels will be checked at regular time intervals for varying depth. The results form no ground water pressure will be compared to those of positive ground water pressure for variance. 2. An even layer of sand, 4 cm thick, will be placed in the box. An overland flow component will be created in the box by means of tube supplying constant water volume to the surface of the sand. Natural waterways will be allowed to form. A drop of dye will be added to the upstream end of the box and timed as it moves across the box. Water volume of the current situation will be logged as well. An artesian groundwater component will be added. A second drop of dye will be added to the upstream end of the box and timed once again as it moves across the box. The water volume will be checked again and dye velocity will be corrected for the additional water volume added by groundwater. Results from each run will be compared for variance. 3. An alternative method will be to create a 10 cm thick layer of sand. An artificial channel, 8 cm deep will be created. Overland flow will be created and water velocity cross sectional profile will be checked using dye tests. An artesian component can then be added and checked again at varying locations in the cross section and compared to the previous test. 4. An initial layer of sand will be randomly applied in the box to an average of 2 cm thickness. A layer of dry powdered clay will be randomly applied to the surface of the sand to an average of 1 cm thickness. An additional layer of sand will then be randomly applied on top of the clay layer. A surface flow will be initiated to create natural waterways. Path and channel depth will be documented. An artesian flow will then be created from below. Channel path and depth will then be documented at an even interval and compared to the non-artesian state. 5. Sand and dry clay powder will be placed in the box in such a way as to simulate a dual aquifer system separated by a 1 cm thick confining bed. A surface flow will then be created to allow a natural waterway to form. Artesian flow will then be applied to the system. Flow rates along the waterway will then be documented at even spacing to locate any high discharge areas. 6. A plastic sheet will be placed in the box covering half the screen to block flow laterally from the artesian source. An even layer of sand, 4 cm thick will be placed in the box. An overland flow component will be created in the box by means of tube supplying constant water volume to the surface of the sand. Natural waterways will be allowed to form. The artesian source will then be initiated. Only one lateral half of the box will receive ground water discharge. The channels will then be allowed to change positions naturally and pre-artesian river position will be compared to artesian positioning.
Expected Results 1. The box will form a meandering river system of low sinuosity. Cannel width will be approximately 1 cm and channel depth will be .2 cm. When groundwater flow is added, sinuosity will increase and channel depth will become less uniform and from several low spots where groundwater discharge is randomly higher. 2. It is expected that the dye travel time will be faster with the artesian component added. 3. Water channels will migrate towards random thin spots in the clay layering. This will create a more sinuous river with a more random meander. 4. Flow volume increase will be highest at the point where the two aquifers converge. It is possible that the river may try to follow the clay confining layer outcrop for this reason. 5. The river will migrate to the groundwater discharging half of the sand box. Given enough time, the entirety of the river channel will be in the artesian half of the box.
Computer Model Methodology: An appropriate model must be able to accurately simulate groundwater discharge in normal river conditions. The program Visual Python will be for its versatility and ease of programming. A two dimensional model will be created to represent a cross section of river, including floodplain and subsurface geology. Properties such as critical shear stress, flow volume and velocity, water density, sediment load, and bed and bank cohesiveness will be taken into account. The initial profile will represent the profile of a stream immediately following channelization. Groundwater discharge areas will then be added to various locations along the profile. The profile will then be allowed to entrain, erode, and deposit sediment based on the governing equations of river flow dynamics. Channel location will be monitored as the time step progresses. Sub layers of clay of varying thickness can then be added below the riverbed. Bank stability can also be made to simulate riprap, concrete encasement, or other forms of bank stabilization. The model will also be able to calculate suspended load, bed load, and flow velocity. Results of the model will be verified with field measurements of similar setting.
Scientific Importance of Study
The straightening of rivers has caused a host of problems, many of which we may not be aware of yet. The ability to predict a rivers future path would allow planning engineers to produce a more environmentally sound and structurally stable solution. Knowledge of the dynamic processes at work in such exchange systems would also aid in contaminant transport prediction in groundwater systems. The possibility also exists for measuring river flow volume by satellite if a true temperature gradient in peak discharge locations can be identified. Geomorphic surface forms may allow interpretations of subsurface geology. Applications of this would have an impact on hydrology, the petroleum industry, and even extra-planetary exploration.
Work Schedule
Field Data Acquisition
April 9 Scout the river • Find suitable boat loading points • Locate apparent meander cut banks and point bars • Locate suitable study area • Record depths at various distances downstream to get an idea of general depth range • Create a mid-water downstream temperature profile o Purpose is to test equipment o Generates a “general� idea of the effectiveness of the equipment o Identify unforeseen needs for taking temperature samples o Is the stream temperature significantly different than the groundwater Temp?
April 16 Measurement • From up stream end of study area, measure flow volume at beginning, middle, and end of study area • Create a sub-streambed temperature cross sectional profile along bend, riffle zone, and the up and downstream transition zones from bend to riffle • Create an in stream temperature profile. Record temperature and depth just above streambed, mid-depth, and at surface across the river in 1 meter intervals
April 23 Downstream Profile • Record sub-streambed temperature at 2 second intervals along the highest discharge line found on day 2 • Record river temperature 10 cm above streambed at 2 second intervals along the highest discharge line found on day 2
Sand Box Modeling
March – July March – Initial Setup, testing of materials Run model with no groundwater element
April – Test groundwater element Tests involving partial lateral groundwater discharge
May – June Tests involving various layers or layer pinch-outs
July – Analysis of sandbox data
Computer Model
April – July April – compile needed equations for model Initial design and begin writing model program
May – Finish writing model program Initial test runs
June – run model changing various elements and locations of groundwater
July – Compile results
Budget
Item means of acquisition Source of Funding Amount needed GPS Unit use DES owned unit (3) waterproof temperature data loggers use GWI units Boat use DES or other 100 ft of polybraided rope purchase personal purchase $15 15 ft, 1 in diamerter PVC pipe purchase ? $5 thermocouple data logger purchase ? $100 data logger interface dock purchase ? $30 J type thermocouple, 10 cm shielded purchase ? $30 30 feet, type J thermocouple wire, vinyl coated purchase ? $10 Type J thermocouple coupling purchse ? $5 Sand Box supplies 8 X 10 foot plywood purchase grant $30 water pump purchase grant $35 tubing purchase grant $15 fiberglass resin purchase grant $25 plumbing fittings purchase grant $20 screws purchase grant $5 resevior bucket purchase grant $10 Total $335 (optional) Infrared Camera Rental rental ? $400 Total $735
Sources:
• Scott A. Wells, Robert L. Annear, Report for 2002OR2B: Temperature Effects of Streambed Heating, Water Resources Research institute Annual Technical Report, 2002, pgs 3 – 10.
