Browsing by Subject "Minnesota River"

Now showing 1 - 7 of 7
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    Changes in channel geometry through the Holocene in the Le Sueur River, south-central Minnesota, USA
    (2017-03) Targos, Courtney
    Paleochannels preserved on terraces via meander cutoffs during an incisional period record the channel geometry and thus discharge at distinct points in time throughout a river’s history. We measured paleochannel geometry on terraces throughout the Le Sueur River in south-central Minnesota, to track how channel geometry has changed over the last 13,400 years. A rapid drop in base level 13,400 yr B.P. triggered knickpoint migration and valley incision that is ongoing today. Since the 1800’s, the area has developed rapidly with an increase in agriculture and associated drainage, directly impacting river discharge by increasing water input to the river. Five paleochannels were identified on terraces along the Le Sueur River from 1m-resolution lidar data. Ground Penetrating Radar (GPR) was used to obtain a subsurface image across paleomeanders to estimate the geometry of paleochannels. Paleochannel geometry and estimated discharge were then compared to modern conditions to assess how much change has occurred. Three lines were run across each paleochannel perpendicular to the historic water flow. Each of the 15 lines were processed using the EKKO Project 2 software supplied by Sensors and Software to sharpen the images, making it easier to identify the paleochannel geometry. Paleodischarge was determined using the Law of the Wall and Manning's Equation, using modern slope and roughness conditions. OSL samples were collected from overbank deposits on terraces to determine the time of channel abandonment, and supplemented with terrace ages obtained from a numerical model of valley incision. Paleodischarge coupled with depositional ages provide a history of flow conditions on the Le Sueur River. Results show an increase in channel widths from the time paleochannels were occupied to modern channel dimensions from an average of 20 meters to 35 meters. The change was not constant through time, as all paleochannels analyzed on terraces had similar-sized channels. The best way to determine paleogeometry was using the 'best interpretation' of GPR data couple with coring data; and paleodischarge was best estimated using Manning's equation with an n value of 0.035. Results show an increase in discharge compared to paleochannels of a factor of two. Uncertainty estimates in GPR-based paleogeometry can change paleodischarge calculations by 50 %. Incremental flood frequency analyses, based on data obtained from the Red Jacket stream gage at the outlet of the Le Sueur, suggest a 1.5- and 2-year flood of 102 m3/s and 154 m3/s, respectively, which is comparable to estimations of bankfull based on current channel geometry at the Red Jacket gage, validating the methodology. Problems associated with paleogeometry estimations are primarily due to meander bend preservation in the subsurface, challenging GPR interpretation. The increase in channel geometry and discharge implies that the increase in flow associated with drainage and climate change since the area’s development has greatly impacted the Le Sueur River. This resulted in a change in channel morphometry through increased erosion along the bluffs and banks, widening channels. This increase in erosion has directly impacted the amount of sediment delivered to the rivers from banks and bluffs, increasing the fine sediment load in this turbidity-impaired river system.
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    Mixing of the Seneca and Blue Lake waste water treatment plant effluents with the Minnesota River
    (1984-11) Stefan, Heinz G.; Farrell, Gerard J.; Riley, Michael J.; Lindquist, Katherine F.; Horsch, George M.
    Eight field surveys, forty five laboratory experiments, and several types of analyses have produced information to understand and predict the mixing of the Seneca and Blue Lake WWTP effluents with the Minnesota River to a reasonable degree. Both discharges are from submerged pipes, 7 ft and 6.5 ft in diameter, respectively. The mean annual discharge rates are at present on the order of 23 to 25 cfs and at velocities on the order of .6 to .8 fps.
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    Model study of the Minnesota River near trunk highway no. 169 Bridge, Minnesota
    (1982-09-02) Parker, Gary; Martinez, Ismael; Hills, Randy
    The Minnesota Trunk Highway No. 169 crosses the Minnesota River just north of the town of Le Sueur. The bridge is immediate1y downstream of the confluence of the Minnesota River and a small tributary, Le Sueur Creek, entering from the east. The bridge was completed in 1960. The approaches were designed so as to preclude overtopping. In fact, they were not overtopped during the largest flood on record, which occurred in 1965. During severe floods, the entire floodplain carries a considerable discharge. The severe constriction at the bridge was partially alleviated by excavating the western bank and immediately adjacent floodplain to provide a floodway. The bridge spans the original channel (east side), Where the piers are deep, and the original f1oodway (west side), where the piers are shallow. During the episodic flood of 1965, the severe constriction of overbank flow at the bridge caused scour that endangered the western approach. The recession of the flood waters revealed a large bar at the mouth of the tributary. This bar had occupied the old channel and forced the river to the west, against the shallow floodway piers. At one point several of these piers were subjected to scour below their seals. Although various corrective measures were taken, the problem became progressively worse. In 1981, contingency measures were implemented, including filling of a portion of the west side with riprap and excavating a portion of the east side to provide a pilot channel. In addition, the model study described herein was commenced, in order to obtain guidance as regards more permanent corrective measures. The model study was conducted using Froude similarity with a horizontal scale of 1:200 and a vertical scale of 1:40. Crushed walnut shells were used to model the bed sediment. The model study indicated that scour resulting from the severe constriction at the bridge becomes an increasingly difficult problem as the fraction of discharge that is overbank increases. During severe floods, sediment from Le Sueur Creek cannot enter the channel due to slack water at the mouth of the tributary. Sediment collects in Le Sueur Creek, and enters the channel as a slug as flood flows recede. Additional deposition can occur when the Minnesota River is low, but the tributary is high. It was found necessary to distinguish between corrective measures operable at below- and above-bankfull stages. At above-bankfull stages, a T-spur dike on the western floodplain adjacent to the channel can pull floodplain flow away from the western approach and deflect it to the east, is near the mouth of the tributary. At lower flows, two in-channel permeable dikes can encourage deposition on the west side, and deflect the flow to the east side. The height of the permeable dike must be well below bankfull stage in order that appropriate conveyance be provided for higher flows. These, and certain other maintenance-oriented corrective measures, are proposed in order to return the channel to its original position and control the growth of the bar at the mouth of the tributary. At flows above about 60,000 cfs, severe scour is induced at the bridge due to the intense constriction of floodplain and channel flow. Increased conveyance would reduce the potential for severe scour. If this cannot be provided, extensive riprapping of pier footings is recommended.
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    Sediment Problems and Solutions for the Minnesota River
    (St. Paul, MN: University of Minnesota Extension Service, 1996) Senjem, Norman B.; Moncrief, John F.; Randall, Gyles W.; Evans, Samuel D.
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    Stability of the channel of the Minnesota River near state bridge no. 93, Minnesota
    (1982-04-01) Parker, Gary
    The Minnesota State Highway No. 93 bridge crosses the Minnesota River near the town of Le Sueur. The bridge is situated at the apex of a bend the outside of which impinges against the eastern river valley wall. Near Le Sueur, the Minnesota River is a meandering stream with actively migrating bends. The valley walls has helped stabilize the channel in the immediate vicinity of the bri~ge, where the channel has moved little in the course of a century. Just upstream of the bridge is a reach consisting of several short bends, nowhere impinging against the valley walls, that are migrating downstream and outward at a relatively rapid rate near 9 ft/year (3 m/ year). The western approach to the bridge was riprapped in order to thwart the downstream progression of a bend. As a result, the bend ravelled up against the riprap and cut itself off. The channel now impinges against the riprap at a ninety degree angle, and then flows along the base of the riprap to the bridge opening. A large scour hole exists at the point of impingement, where the approach is in danger of being washed out. In the future, successive bends can be expected to migrate into the riprap, until the channel breaches the western approach and abandons the bridge. The Minnesota Department of Transportation is presently considering replacement of the bridge deck. The actively migrating bends in the reach in question preclude relocating the bridge away from its present stable location near the valley wall. A short channel relocation just upstream of the bridge can improve alignment. It can also mitigate the possibility of a natural cutoff causing the bridge to be abandoned.
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    Using BSTEM to Estimate Sediment Erosion in Seven Mile Creek Watershed
    (2016) Hammer-Lester, Rebecca F
    Sediment has been recognized as an important water contaminant and there has been substantial research into the sources and sinks of sediment on the landscape. However, ravines are an understudied erosive landform. The flashy, intermittent flows in ravines make them difficult environments to study and manage. The goals of this study were to collect data on the hydrology, sediment, and vegetation in ravines and to use that data to model sediment loads in a steep ravine in the Seven Mile Creek watershed in south-central Minnesota. There are many ravines throughout the Seven Mile Creek watershed and the Minnesota River Basin (MRB), of which it is a small part. The MRB carries the largest sediment loads entering the Mississippi River in the state of Minnesota and these sediments are creating problems downstream including rapid infilling of Lake Pepin. In the present study the Bank Stability and Toe Erosion Model (BSTEM) was used along with substantial field data on hydrology, sediment, and vegetation to model sediment loads generated in ravines in the Seven Mile Creek watershed. The results of the study show that substantial sediment loads can be generated in ravines. Sediment loads varied from a few metric tons/reach/year to tens of thousands of metric tons/reach/year with the strongest control exerted by hydrology followed by sediment, slope, and added cohesion from vegetation. The depth to water table and depth and duration of flow exerted strong control on sediment loads. Changing the water table and depth and duration of water flow caused a relative change in sediment load of at least 193% for the whole ravine results. It is hoped that the field data and modeling results from this study can aid researchers in understanding the magnitudes of sediment loads that can be expected in ravines and that it can guide managers in placing best management practices on the landscape in order to decrease erosion and sediment delivery from ravines to larger streams and rivers.