MIXING INDUCED BY AN INTERNAL HYDRAULIC JUMP1

ABSTRACT The efficiency of an inverted internal hydraulic jump as a mixing and dispersion mechanism in an aquatic environment was examined. The flow considered was a two-dimensional buoyant flow from a shallow channel over a sloping bottom into a deep reservoir. It could be seen that a rapidly varied flow associated with violent turbulent mixing occurred near the point of discharge if specific discharge conditions and downstream controls were met. Downstream from the mixing zone the flow was stably stratified. The main object of the study was to find the conditions under which a turbulent mixing zone occurred and the rate of turbulent entrainment. Energy loss and length of the mixing zone were also investigated. The independent variables were the outlet densimetric Froude number, the density differential between outfall water and receiving water, the relative depths of the upper and lower layers in the stratified flow portion, the total depth, and the slope of the transition. Theoretical calculations had to be confined to a step increase in depth, but experiments in a laboratory flume showed that results obtained with slopes of 23° and 90° were quite similar. Both theory and experiments showed, for example, that dilution (entrainment rates) up to 2:1 (2 parts heated water to one part cold water) can be achieved at very little energy expense and with downstream depths approximately 8 times the outlet depth. Theoretically, any amount of dilution can be obtained, but large depths may be required. Experimental results also indicate that the length of the mixing zone was frequently ten times the value of the outlet densimetric Froude number.     

How Hydrologic Connectivity Regulates Water Quality in River Corridors

Downstream flow in rivers is repeatedly delayed by hydrologic exchange with off‐channel storage zones where biogeochemical processing occurs. We present a dimensionless metric that quantifies river connectivity as the balance between downstream flow and the exchange of water with the bed, banks, and floodplains. The degree of connectivity directly influences downstream water quality — too little connectivity limits the amount of river water exchanged and leads to biogeochemically inactive water storage, while too much connectivity limits the contact time with sediments for reactions to proceed. Using a metric of reaction significance based on river connectivity, we provide evidence that intermediate levels of connectivity, rather than the highest or lowest levels, are the most efficient in removing nitrogen from Northeastern United States’ rivers. Intermediate connectivity balances the frequency, residence time, and contact volume with reactive sediments, which can maximize the reactive processing of dissolved contaminants and the protection of downstream water quality. Our simulations suggest denitrification dominantly occurs in riverbed hyporheic zones of streams and small rivers, whereas vertical turbulent mixing in contact with sediments dominates in mid‐size to large rivers. The metrics of connectivity and reaction significance presented here can facilitate scientifically based prioritizations of river management strategies to protect the values and functions of river corridors.     

A Geomorphic Explanation for a Meander Cutoff Following Channel Relocation of a Coarse-Bedded River

The Veteran's Fishing section of the Blackledge River in central Connecticut was relocated in the late 1950s. The relocation resulted in an unstable channel despite extensive efforts to prevent erosion. Overbank erosion and meander cutoffs were investigated using detailed survey data, characterizations of sediment deposits, flow modeling, and a moment-stability analysis. Limited reworking of revetment boulders indicate that riprap bank material was immobile during a 1979 flood event responsible for the formation of the cutoff channel. A moment-stability analysis factor-of-safety value of 1.1 supports the conclusion that riprap was not directly eroded from the banks. Alluvial particles with d₉₅ values ranging up to 120 mm were deposited along a bar downstream from the cutoff channel at flows estimated to be below a 1.5-year recurrence interval flow. Development of the bar deposit resulted in locally elevated water surfaces at high flow. The resulting overbank flow across the meander neck to the adjacent downstream bend led to the creation of an upstream migrating knickpoint, the erosion of approximately 16,000-year-old sediments, and the subsequent meander cutoff. The results of the study indicate that traditional erosion-control measures cannot prevent extreme channel adjustments if the geomorphic processes that control sediment continuity also are not considered.     

DESIGN AND IMPACT ANALYSIS FOR DIVERSION AT COAL CREEK MINE1

A diversion system has been designed to carry the flow from East Fork of Coal Creek around the area proposed for mining at Thunder Basin Coal Company's (TBCC) Coal Creek mine in Campbell County, Wyoming. This paper describes the field and analysis procedures necessary to prepare the diversion design and impact evaluation, and the innovative concepts developed for the diversion system design to minimize impacts on downstream channel stability. Under the proposed diversion system design, water from the East Basin of Coal Creek will be diverted at two locations. At one location, flow will be impounded by a small dam and decanted by a pump through a pipeline into East Fork at the location of the second diversion. At this location, a training dike will be placed across the stream channel to divert flows into a diversion channel. Gravity flow along the diversion channel will deliver water to a playa area which will be converted into a detention basin by placing a small dam across its southern end. Flows up to the magnitude of the 24-hour 2-year peak flow will be passed directly through the detention basin into Middle Fork with negligible attenuation of flow rates. For less frequent events, water will be stored in the detention basin in order to prevent velocities in Lower Middle Fork from exceeding the maximum permissible velocity above which scouring may occur. Evaporation and seepage losses from the diversion system were estimated to be small and should be more than offset by the addition of water from the playa drainage basin into the Coal Creek drainage. Velocities predicted for the Lower Middle Fork after-the diversion is constructed are expected to be low enough that significant erosion of the channel is not expected to occur.     

The effect of flow path and mixing layer on phosphorus release: physical mechanisms and temperature effects

Soil phosphorus (P) concentrations typically are greater in surface soils compared with subsurface soils. Surface soils have a greater chance to interact with runoff leading to P transport to streams. The thin surface layer where P concentrates is referred to as the mixing layer denoting where water and chemicals mix during transport. The objective of this study was to evaluate the effect of hydrologic flow paths on soluble reactive phosphorus (SRP) loss at two temperatures. Laboratory flumes were built to simulate infiltration, return flow, saturation excess, and interflow, and subsequent interaction with the mixing layer. The sandy loam soil in the flumes was kept at saturation throughout all experiments, so that biochemical effects were normalized. Flow through the flumes was maintained at 3.6 mm/h for 24 to 99 h (at 6 and 25 degrees C) with water entering and exiting the flumes at different ports (to simulate different flow paths) or as low intensity rainfall. Experiments were performed with and without an artificially created P-enriched surface layer (5 mm thick, total P increased from 1010 mg/kg in the original soil to 2310 mg/kg by addition of dissolved phosphate). Results indicated that (i) SRP release was greater in soil with a mixing layer than in soil without a mixing layer; (ii) SRP release was greater during experiments at 25 degrees C than at 6 degrees C; (iii) at 25 degrees C, SRP release was greatest when water traversed the mixing layer in the upward direction (i.e., in return flow), and by flow parallel to the mixing layer (i.e., surface runoff); and (iv) at 6 degrees C, SRP release in subsurface flow following rainfall was slightly greater than in return flow and infiltration. Our results confirmed the presence of a variable, temperature-dependent desorption process when runoff water interacted with the mixing layer. Our findings have important implications for how different water flow paths in and over the soil interact with P in the soil, and what the ultimate concentration will be in runoff and interflow.     

Erosional Consequence of Saltcedar Control

Removal of nonnative riparian trees is accelerating to conserve water and improve habitat for native species. Widespread control of dominant species, however, can lead to unintended erosion. Helicopter herbicide application in 2003 along a 12-km reach of the Rio Puerco, New Mexico, eliminated the target invasive species saltcedar (Tamarix spp.), which dominated the floodplain, as well as the native species sandbar willow (Salix exigua Nuttall), which occurred as a fringe along the channel. Herbicide application initiated a natural experiment testing the importance of riparian vegetation for bank stability along this data-rich river. A flood three years later eroded about 680,000 m³ of sediment, increasing mean channel width of the sprayed reach by 84%. Erosion upstream and downstream from the sprayed reach during this flood was inconsequential. Sand eroded from channel banks was transported an average of 5 km downstream and deposited on the floodplain and channel bed. Although vegetation was killed across the floodplain in the sprayed reach, erosion was almost entirely confined to the channel banks. The absence of dense, flexible woody stems on the banks reduced drag on the flow, leading to high shear stress at the toe of the banks, fluvial erosion, bank undercutting, and mass failure. The potential for increased erosion must be included in consideration of phreatophyte control projects.     

GEOMORPHIC ANALOGIES FOR ASSESSING PROBABLE CHANNEL RESPONSE TO DAM REMOVAL1

ABSTRACT: There is a pressing need for tools to predict the rates, magnitudes, and mechanisms by which sediment is removed from a reservoir following dam removal, as well as for tools to predict where this sediment will be deposited downstream and how it will impact downstream channel morphology. In the absence of adequate empirical data, a good initial approach is to examine the impacts of dam removal within the context of the geomorphic analogies of channel evolution models and sediment waves. Channel changes at two dam breaching sites in Wisconsin involved a succession of channel forms and processes consistent with an existing channel evolution model. Sediment transported downstream after removal of other dams suggests that reservoir sediment may be translated downstream either as a distinct wave or gradually eroded away. More extensive data collection on existing dam removals is warranted before undertaking the removal of a large number of dams. However, if removal is to proceed based on current knowledge, then geomorphic analogies can be used as the foundation for sediment management and stabilization schemes.     

Nitrate removal in riparian wetlands: interactions between surface flow and soils

Riparian wetlands containing springs are thought to be ineffective at removing nitrate because contact times between the upwelled ground water and the underlying microbially active soils are short. Tracer experiments using lithium bromide (LiBr) and nitrate (NO3-N) injected at the surface were used to quantify residence times and NO3-N removal in a riparian swale characteristic of New Zealand hill-country pasture. An experimental enclosure was used with collecting trays at the downstream end to measure flow and concentration, shallow wells to measure subsurface concentrations, and an array of logging conductivity probes to monitor tracer continuously. The majority of added tracer reached the outlet more slowly than could be explained by surface flow, but more quickly than could be explained by Darcy seepage flow. There was evidence from the wells of tracer diffusing vertically to a depth of at least 5 cm into the surface soil layer, which was permanently saturated and highly porous. During dry weather 24 +/- 9% of added NO3-N was removed over a distance of 1.5 m largely by denitrification. The net uptake length coefficient for this wetland (K = 0.08 +/- 0.03 m(-1)) is slightly higher than the range (K = 0.01-0.07 m(-1)) measured in a small stream channel infested with macrophytes. Nitrate removal is expected to decrease with increasing flow. Seepage flow is estimated to have removed only 7 +/- 4% of the added NO3-N and we hypothesize that vertical diffusion substantially increases NO3-N removal in this type of wetland. Riparian wetlands with springs and surface flows should not be dismissed as having low NO3-N removal potential without checking whether there is significant vertical mixing.     

A Depth‐Proportional Intake Device for Automatic Water Samplers1

Abstract: This paper describes the construction and testing of a device for pumping water samplers that collects suspended sediment samples by moving the intake vertically to keep it at the same proportion of flow depth. The device uses a simple sprocket mechanism that can be mounted vertically on the downstream side of culverts and bridge pilings to protect against damage from floating debris during storms. Suspended sediment samples collected from an urban stream with the depth‐proportional device were compared with manual samples taken with a depth‐integrated sampler. Scatter in the relationship between pumped and manual samples (R² = 0.76) are probably explained by horizontal variability in concentrations, poor mixing associated with lateral sediment inputs from construction site erosion, the downstream orientation of the intake, and the failure of the concentration at 60% of the flow depth to match the average vertical concentration.     

Comparing the Extent and Permanence of Headwater Streams From Two Field Surveys to Values From Hydrographic Databases and Maps

Supreme Court cases have questioned if jurisdiction under the Clean Water Act extends to water bodies such as streams without year‐round flow. Headwater streams are central to this issue because many periodically dry, and because little is known about their influence on navigable waters. An accurate account of the extent and flow permanence of headwater streams is critical to estimating downstream contributions. We compared the extent and permanence of headwater streams from two field surveys with values from databases and maps. The first used data from 29 headwater streams in nine U.S. forests, whereas the second had data from 178 headwater streams in Oregon. Synthetic networks developed from the nine‐forest survey indicated that 33 to 93% of the channel lacked year‐round flow. Seven of the nine forests were predicted to have >200% more channel length than portrayed in the high‐resolution National Hydrography Dataset (NHD). The NHD and topographic map classifications of permanence agreed with ~50% of the field determinations across ~300 headwater sites. Classification agreement with the field determinations generally increased with increasing resolution. However, the flow classification on soil maps only agreed with ~30% of the field determination despite depicting greater channel extent than other maps. Maps that include streams regardless of permanence and size will aid regulatory decisions and are fundamental to improving water quality monitoring and models.     

Analysis of bank erosion on the Merced River,Yosemite Valley,Yosemite National Park,California, USA

Channel changes from 1919 to 1989 were documented in two study reaches of the Merced River in Yosemite National Park through a review of historical photographs and documents and a comparison of survey data. Bank erosion was prevalent and channel width increased an average of 27% in the upstream reach, where human use was concentrated. Here, trampling of the banks and riparian vegetation was common, and banks eroded on straight stretches as frequently as on meander bends. Six bridges in the upper reach constrict the channel by an average of 38% of the original width, causing severe erosion. In the downstream control reach, where human use was minimal, channel widths both decreased and increased, with a mean increase of only 4% since 1919. Bank erosion in the control reach occurred primarily on meander bends. The control reach also had denser stands of riparian vegetation and a higher frequency of large woody debris in channels. There is only one bridge in the lower reach, located at the downstream end. Since 1919, bank erosion in the impacted upstream reach contributed a significant amount of sediment (74,800 tonnes, equivalent to 2.0 t/km²/yr) to the river. An analysis of 75 years of precipitation and hydrologic records showed no trends responsible for bank erosion in the upper reach. Sediment input to the upper reach has not changed significantly during the study period. Floodplain soils are sandy, with low cohesion and are easily detached by lateral erosion. The degree of channel widening was positively correlated with the percentage of bare ground on the streambanks and low bank stability ratings. Low bank stability ratings were, in turn, strongly associated with high human use areas. Channel widening and bank erosion in the upper reach were due primarily to destruction of riparian vegetation by human trampling and the effect of bridge constrictions on high flow, and secondarily to poorly installed channel revetments. Several specific recommendations for river restoration were provided to park management.     

LOW FLOW SPATIAL CHARACTERISTICS IN FORESTED HEADWATER CHANNELS OF SOUTHWEST WASHINGTON1

ABSTRACT: Patterns of dry season surface flow in forested headwater channels of southwest Washington were observed during August to September 2001 and July to October 2002. In 2001, 17 channels were sampled once, and the uppermost points of continuous flow (CF) and surface water (SW) were located. In 2002, sampling was replicated three to five times at each of 21 channels. Annual and seasonal data suggested that the location of SW varied less than CF. In most channels, SW remained at or near the channel head year around. The pattern of surface flow between CF and the channel head was used to test alternative hypotheses describing dry season recession patterns: (A) surface flow consistently retreats in a downstream direction, and (B) flow comes from fixed sources along the channel, thus surface flow retreats up‐channel towards these sources. The dominant surface flow spatial pattern in streams less than 30 percent slope was increased intermittency without a clear pattern of retreat, and thus inconsistent with either hypothesis. High gradient channels (< 30 percent slope) exhibited a combination of increased intermittency, and extensive upward retreats of surface water consistent with Hypothesis B. Differences between 2001 and 2002 suggest late summer flows in small headwater basins were controlled by spring precipitation, rather than the typically greater winter precipitation.     

AN ANALYTICAL FRAMEWORK FOR EVALUATING CHANNEL MAINTENANCE FLOWS IN COLORADO1

ABSTRACT: An accounting procedure is developed which determines a flow regime that is capable of transporting an amount of bedload sediment necessary to ensure channel stability downstream. The method allows for sediment buildup in the channel within geomorphic threshold limits during low flow periods. During periods of high runoff, enough water is bypassed to transport the stored sediment. The procedure utilizes only those flows of sufficient magnitude to maintain channel stability over the long run (25–50+ years). An example is presented which determines the volume of water and frequency of release for channel maintenance purposes downstream from a hypothetical water diversion project. Of some 1,200,000 acre feet generated during a 59-year period, 86,500 acre feet was required for channel maintenance flows. Bypass flows were not required each year, but only during those years when average daily flow reached bankfull or greater. Such releases were made on 202 of the 411 days when average flows either equalled or exceeded bankfull discharge.     

Sources of salinity near a coal mine spoil pile, north-central Colorado

A small (1 km2) salt-affected stream drainage on the High Plains north of Denver, Colorado was sampled to determine the near-surface dispersion of soluble salts and metals from low-sulfur coal mining waste (spoil). Surface waters collected along the 0.8-km stream reach, and aqueous leachates of spoil and naturally saline local soil, were analyzed for chemical constituents and sulfur isotopes. In this semiarid setting with abundant carbonate-bearing surficial sediments, the limited, mildly acidic drainage from the spoil pile is quickly neutralized, restricting the mobility of many elements. However, some spoil-derived constituents were clearly traceable within the upper 0.4 km of the stream reach. Spoil leachates and surface water near the spoil pile have distinctive compositions of major anions and cations, and elevated levels of dissolved nitrate compared with downstream waters. Spoil-derived sulfate was traceable because it has generally positive values of delta34S that contrasted with generally negative values of delta34S in soil leachates and evaporite salts from the surrounding area. Spatial-chemical sampling of surface water showed an abrupt increase in dissolved U, Se, B, Li, and Mn in the lower 0.4 km of the stream reach where shallow ground water from surrounding irrigated fields contributed to surface flow. The downstream evolution of surface water chemistry and sulfur isotopic composition is consistent with mixing between spoil-affected upstream water and irrigation-return water. The methods described should be applicable at other sites in similar settings where the environmental effect of low-sulfur coal mining waste must be assessed and where access to samples of shallow ground water is limited.     

Observations of relative humidity in the near-wake of a wind turbine using an instrumented unmanned aerial system

Simulation, modeling, and limited observations have shown that wind farms have an impact on the near-surface atmospheric boundary layer as turbulent wakes enhance vertical mixing of momentum, heat, and moisture. The few observational datasets that do exist lack high spatial resolution due to their use of a limited number of meteorological sensors or remote sensing techniques. This study utilizes an instrumented small unmanned aerial system to gather high-resolution in situ field measurements in order to differentially map near-wake changes to relative humidity. Observations show that downstream relative humidity is differentially altered in the vertical, spanwise and downstream directions.     

INFLUENCES OF A FOREST ON THE HYDRAULIC GEOMETRY OF TWO MOUNTAIN STREAMS1

ABSTRACT The influence of a forest on the formation of steps in two small streams of the Colorado Rocky Mountains was studied. Steps provided by logs fallen across the channel added to flow energy reduction. The streams required additional gravel bars to adjust to slope. Average step length between logs and gravel bars was strongly related to channel gradient and median bed material size. Based on the average number of log steps per 50 feet of channel, an average of 116 percent of gravel bars were added at Fool Creek and 60 percent at Deadhorse Creek. The latter had 52 percent more logs in the channel and therefore required less bed material movement than the former. Although these are “rushing mountain streams,” most flow velocities ranged between 0.5 and 2.5 f.p.s. Exponents of a function relating rate of change of depth or velocity to discharge indicated that dynamic stream equilibrium was attained. Implications for forest management are that sanitation cuts (removal of dead and dying trees) would not be permissible where a stream is in dynamic equilibrium and bed material movement should be minimized.     

LINE SOURCE DISPERSION WITH APPLICATION TO MIXING IN RIVER CHANNELS1

ABSTRACT: Two-dimensional solutions for transient dispersion of nonconservative dispersants in uniform flow resulting from a transverse line source of variable concentration are obtained using multiple integral transformations. In general, the solutions are in integral forms, which can be efficiently evaluated using Filon's quadratures. Examples are presented for cases of practical interest. Applicability of the solution for modeling dispersion in natural river channel where the distribution of flows across the channel are nonuniform is discussed.     

LOCAL FLOW ACCELERATIONS IN RIVERS1

The geometry and formation of and the energy dissipation in flow accelerations in short steep reaches have been assessed, the latter as to its contribution to the energy expenditure in a long channel reach. An analysis of river gradients, aerial photos and additional field surveys has shown that flow accelerations can (1) originate naturally and sequentially from bars across a channel and nonsequentially from channel abandonment and channelside deposits and (2) be induced by a bankside groyne.     

CHANNEL DISCHARGE MEASUREMENT BY THERMODILUTION1

ABSTRACT: An investigation was conducted to determine the feasibility of applying thermodilution technology to discharge measurements in small open channels. A series of tests were performed in which the time-temperature dilution curves were recorded and analyzed. The independent variables included the channel discharge, the injectate drop height, the volume of tracer, and the mixing distance. Flows ranged from 0.67 cfs to 2.45 cfs with Froude numbers less than 0.30. The results indicated that the thermodilution technique is a feasible method for discharge measurement. It was determined that a heat content, 1°, of 40°C provides a design criteria in which the mixing distance was related to the flow depth and discharge in a rectangular channel. An empirical expression was derived to determine the approximate mixing distance as a function of the flow depth.     

Mixing Zone Characterization of Two Transition Terrain Streams With Tracers1

Abstract: For most wastewater discharges to streams, the effluent creates a plume that becomes less distinct as it mixes with the receiving water. Constant‐discharge tracer studies were used to characterize the plume or physical mixing zone (PMZ) at two similar transition terrain streams. At both sites, the laterally unmixed PMZs did not extend across the entire stream and mixing occurred relatively quickly. The observed plumes were significantly smaller than the regulatory mixing zone (RMZ) allowed by the State of Colorado. At Site 1 mixing occurred within a much shorter distance due to the presence of a riffle zone located a few meters downstream of the discharge point. Interpretation of field data with an analytical model suggests that the effective transverse dispersion coefficient (k_z) for the riffle zone at Site 1 (～1 m²/s) was significantly higher than the average value over the longer nonriffle section at Site 2 (～0.01 m²/s). These results imply that to achieve the fastest mixing in transition terrain streams, thereby minimizing the size of the PMZ, discharge outfalls should be located upstream and close to riffle zones.