NUMERICAL MODELING OF THERMAL STRATIFICATION IN A RESERVOIR WITH LARGE DISCHARGE-TO-VOLUME RATIO1

ABSTRACT: A numerical model study of thermal stratification in a high discharge-to-volume reservoir is described. Predicted temperature profiles are compared with field data for two different years. The model accurately predicts the date of fall turnover, and predicts degree of stratification and depth of the thermocline within about 20% for both years simulated. A parametric study of stratification mechanics for a high flow reservoir indicated that diffusion was the predominant heat transport mechanism in the hypolimnion, while surface effects dominated the epilimnion. Flow effects for the particular case studied, in which all inflows and outflows occur in the epilimnion, did not significantly affect stratification behavior.     

WATER QUALITY STRATIFICATION IN SHALLOW WASTEWATER STABILIZATION PONDS1

ABSTRACT: The physical limnology of three modern wastewater stabilization ponds serving a small community in Minnesota was investigated over a 1-year period (July 1989 - October 1990). Water temperatures and associated meteorological parameters were recorded continuously; underwater light, dissolved oxygen, pH, and Secchi depth were measured intermittently (about weekly). Measurements of nutrients and planktonic species were made by other investigators. Water quality stratification dynamics were studied by analyzing variations of water temperature, dissolved oxygen, and pH distributions with time and over depth. Intermittent stratification and mixing of the shallow waste stabilization ponds (1–2 m deep) were documented and related to weather. The strong response of the ponds to seasonal and daily weather variations was observed. Three types of pond stratification conditions have been identified: (1) completely mixed during consecutive day and night, (2) stratified during the day and well-mixed during the night, and (3) continuously stratified during day and night. A diurnal cycle of stratification dynamics was first noticed in late April and persisted through summer and into fall. Differences in light attenuation and hence temperature stratification and DO distribution between pond 1 (primary), pond 2 (second primary) and pond 3 (secondary) in the wastewater treatment system were documented and related to different waste loading conditions. Temperature stratification affects chemical, microbial, and planktonic processes in the ponds. Results presented in this paper can be used to provide guidance for water quality sampling in monitoring of pond performance. Information on true mixing conditions is also needed to gain better understanding of important factors affecting pond operation, and for process simulations and reactor modeling of waste stabilization ponds.     

CHANGES IN STRATIFICATION IN ONONDAGA LAJKE,NEW YORK1

ABSTRACT: The calibration of a mixed-layer stratification model to the complex stratification region of Onondaga Lake is documented. The short- and long-term impacts of the closure of an adjoining alkali plant on the stratification regime of Onondaga Lake are evaluated with this model from the perspective of natural variations associated with meteorological variability. Chemical stratification prevailed in the lake during the operation of the facility as a result of its discharge of ionic waste. A predicted likely short-term impact of the closure, that was subsequently observed, was the failure of the lake to turn over in the spring immediately following the closure. Spring turnover did not occur regularly during the operation of the facility; but turnover can be expected to occur regularly in the future. Other projected changes in average stratification conditions include: 1) a 45% shorter period of stratification, 2) a 3m deeper upper mixed layer, and 3) a 30% lower maximum density gradient. Substantial variability in the stratification is predicted as a result of meteorological variability, indicating that comparison of characteristics for individual years during and after the operation of the facility could be misleading. The changes in the stratification regime are expected to affect water quality. In particular, certain features of the oxygen resources of the hypolimnioa are expected to improve (e.g., delayed onset of anoxia).     

Longitudinal‐Vertical Hydrodynamic and Turbidity Simulations for Prediction of Dam Reconstruction Effects in Asian Monsoon Area1

Abstract: This research investigates possible impacts of enlarged water body according to dam reconstruction on the hydrodynamics and water quality of the reservoir using a laterally averaged, two‐dimensional hydrodynamic and transport model, CE‐QUAL‐W2. The lake was formed by the artificial dam in 1983 for agricultural water supply and is currently under consideration of reconstruction so as to expand the volume of reservoir for flood control as well as water supply in downstream areas. To calibrate and validate the model, field‐collected data were compared with model predictions for water level fluctuations and water temperature during the years of 2001 (from January to December) and 2003 (from March to November). The model results showed a good agreement with field measurements both in calibration and verification. Utilizing the model, impacts of dam reconstruction on the thermal hydrodynamics and turbid current were predicted. From the model results, dam reconstruction limited the depth of thermal stratification below 10 meter and formed steep temperature gradient between epilimnion and hypolimnion. The restricted thermal stratification persisted up to the end of September. This result indicated that thermal stratification would become stronger during summer and stay longer after dam reconstruction. In addition, the restricted thermal stratification caused vertical circulation of water mixing lower than 10 meter and isolated the upper water layer from the lower water layer which increased the volume of hypolimnetic water with low temperature. The vertical circulation near the surface also mitigated propagation of density plume within the depth of 10 m which would remain the hypolimnetic water clean.     

SPATIAL VARIABILITY IN SELECTED PHYSICAL CHARACTERISTICS AND PROCESSES IN CROSS LAKE,NEW YORK1

ABSTRACT: Short-circuiting of the inflow occurs from the Seneca River across the southern portion of Cross Lake, New York. Spatial differences in thermal stratification, transparency and light attenuation, and net sedimentation are documented for the lake. The depth of the epilimnion was generally deeper, and the temperature gradient in the metalimnion was at times lower, at a deep-water location proximate to the short-circuiting zone, than at a position more remote from the inflow. Transparency was generally lower, and light attenuation greater, proximate to the short-circuiting zone, indicating the contribution of attenuating components from the river. The net sedimentation rate in the short-circuiting zone was greater and temporally more irregular than for a deep-water position remote from the river inflow. The spatial differences in the physical characteristics and processes in Cross Lake are attributable to the magnitude and position of the inflow from the Seneca River.     

LIMNOLOGICAL PROPERTIES OF A ROCKY MOUNTAIN HEADWATER RESERVOIR1

ABSTRACT: Hyalite Reservoir, Montana, was studied to determine properties of this small, montane, headwater, deep-release reservoir relative to reservoirs at lower elevations. While retention times for waters were as brief as 12 d, the mean residency of 40 d from mid-March to mid-December was within the range reported for other reservoirs. No significant through-reservoir gradients for suspended sediments were observed, contrasting to observations for most reservoirs. Thermal stratification, evident during the first part of the summer, was disrupted in August by cool, dense tributary inflows and strong wind-induced mixing. Dissolved oxygen concentrations paralleled temperature patterns in the reservoir; lowest average values for both occurred in waters sampled nearest the outlet. Total phosphorus averaged greater than twice the total nitrogen concentrations; greatest average concentrations for both were found in the near-bottom waters nearest the outlet. Enrichment of nitrogen concentrations in outflow over inflow waters is hypothesized to occur through nitrogen fixation by Aphanizonwnon flos-aquae. Despite the relatively high quality of waters from tributary inflows, an algal bloom, chlorophyll a concentrations, and primary productivity estimates suggested that the reservoir was mesotrophic. Circulation of waters within the reservoir was primarily influenced by wind-induced mixing, thermal gradients, and currents produced by the deep-water outlet.     

STRATIFICATION VARIABILITY IN THREE MORPHOMETRICALLY DIFFERENT LAKES UNDER IDENTICAL METEOROLOGICAL FORCING1

ABSTRACT: Synoptic water temperature measurements were taken in three temperate lakes located within 25 km of one another to study the effects of morphometry (and changes in weather) on seasonal and short-term thermal stratification characteristics. Two of the lakes had nearly the same surface areas and two had nearly identical mean depths; all were exposed to identical weather conditions. The dominance of weather over morphometry on the water surface temperature response was illustrated by the synoptic measurements in two different years. Stratification structure was also found to be dominated by weather for sufficiently deep lakes. Surface area effects were most subtle but explainable as sheltering effects. The onset of stratification was not, as traditionally described, a simple, gradual response of a lake to the annual solar radiation cycle. Rather it depends on a series of alternating heating, cooling, and mixing cycles similar to annual and diel cycles but with a period of approximately five days. These were in direct response to the passage of major weather systems and displayed no apparent time lag. No comparable synoptic water temperature data set could be found in the literature.     

Effects of warm water inflows on the dispersion of pollutants in small reservoirs

The effects of the warm water discharged by a nuclear power plant (NPP) into a small reservoir are studied. A case study is presented (José Cabrera NPP-Zorita Hidráulica Reservoir) with experimental data of the reservoir stratification and predicted data of the dispersion of radioactive pollutants from operative or accidental releases. The vertical and longitudinal temperature profiles, electrical conductivity and transparency of the reservoir water were measured for an annual cycle. The results indicate that the continuous warm water discharge from the NPP causes permanent and artificial reservoir stratification. The stratification is significant within 1500 m upstream and 1000 m downstream from the warm water outfall. The pollutant dispersion has been predicted by using a flow model based on N(T) perfect-mixing compartments in series with feedback. The model parameter, N(T), is calculated from the longitudinal diffusion coefficient. The prediction of pollutant dispersion by means of this model shows that the stratification slows down the vertical mixing in the whole water body, and reduces the reservoir volume that is effective for the dilution and dispersion of pollutants. This means that, in the case of a radioactive pollutant release, the reservoir radioactivity level could increase significantly.     

A TECHNIQUE FOR ELIMINATING THERMAL STRATIFICATION IN LAKES1

The design, installation, and operation of a compressed air system to eliminate thermal stratification in lakes is described. During a year-long field test on a 19 ha lake, this system effectively eliminated thermal stratification, increased dissolved oxygen, and kept part of the lake ice-free during winter. This system has the capability for use in larger reservoirs and, used for control or elimination of stratification in lakes, is a promising tool for alleviating problems caused by thermal stratification.     

RESPONSE OF PHYSICAL AND CHEMICAL PARAMETERS TO ELIMINATING THERMAL STRATIFICATION IN A RESERVOIR1

ABSTRACT The effects of maintaining a 19 ha Colorado montane reservoir in a thermally destratified condition for one year were evaluated. Water temperatures were kept nearly vertically agd horizontally isothermal throughout the year. The weighted mean temperature of the lake was 1-4°C colder in winter and 1-2°C warmer in summer than normal. Deep water in summer was up to 6°C warmer than typical hypolimnion temperatures, but summer surface temperature was unaltered. Without destratification dissolved oxygen depletion develops in summer and winter, but by eliminating stratification, oxygen was kept near saturation throughout the year. Alkalinity, pH, conductivity, and total residue were not significantly affected. Seston decreased which was probably due to declines in planktonic diatom populations. Increases in iron and manganese did not occur in deep water during destratification. Calcium concentrations increased slightly. Magnesium and most anions (chloride, nitrate-N, and silica) were not greatly altered, but sulfate concentration was reduced. Artificial destratification, as a reservoir management tool, will be very useful in altering chemical problems; particularly increasing oxygen and decreasing iron and manganese concentrations.     

Recycling Irrigation Reservoir Stratification and Implications for Crop Health and Production

Recycling irrigation reservoirs (RIRs) are an emerging aquatic ecosystem and water resource of global significance. This study investigated the vertical distribution of water temperature, dissolved oxygen (DO), and pH in eight RIRs at two nurseries each in Virginia and Maryland from 2011 to 2014. Monomictic thermal stratification was observed from April to October in all RIRs, despite their shallow depths (0.75‐3.89 m). The strongest stratification had a top‐bottom temperature difference of 21.53°C. The top‐bottom temperature difference was positively correlated with water column depth, air temperature, and daily light integral (p < 0.05). Wind speed did not impact the thermal stratification, likely due to their relatively small surface areas. Thermal stratification affected the vertical distribution of DO and pH. The top‐bottom differences in DO and pH were greater during stratification periods than nonstratification periods. Water pH in all RIRs was higher at the top than at the bottom with the greatest difference of 4.16 units. Discovery and characterization of thermal stratification in RIRs helps understand water quality dynamics in this novel ecosystem and promote safe and productive water reuse for irrigation. Specifically, water withdrawal depths should be adjusted according to variations in temperature, DO, and pH during the stratification and nonstratification periods to mitigate pathogen risk and improve water treatment efficacy and crop production.     

EFFECTS OF VARIABLE DISCHARGE SCHEMES ON DISSOLVED OXYGEN AT A HYDROELECTRIC STATION1

ABSTRACT: The effects of variable discharges during the summer on the dissolved oxygen (DO) content and water temperature upstream and downstream of the Conowingo Hydroelectric Power Station were investigated. The DO dynamics are controlled primarily by meteorological factors that are independent of the mode of hydrostation operation. DO stratification occurred during the summer in Conowingo Pond, but thermal stratification was not observed. The magnitude and duration of off-peak discharges including a run-of-the-river operation did not affect DO stratification in Conowingo Pond; little vertical mixing occurred. However, strong winds and/or high river flows temporarily destroyed DO stratification. The run-of-the-river operation or off-peak continuous discharge schemes did not provide better DO conditions downstream of the hydrostation than the peaking operation with intermittent off-peak releases. A statistical model predicted that a DO of 5 ppm occurs 0.6 miles downstream of the powerhouse when the natural river flow is consistently greater than 15,000 cfs and water temperature is less than 80°F. A mean daily DO of at least 4 ppm was predicted to occur over 80 percent of the time during the 92-day summer period. Farther downstream (1.3 miles from the powerhouse) a mean daily DO of at least 4 ppm was predicted to occur 90 percent of the time in summer.     

Management and Limnology Interact to Drive Water Temperature Patterns in a Middle Rockies River‐Reservoir System

This study seeks to improve understanding of temperature patterns in reservoir outflows. We examined water temperatures in an irrigation storage reservoir, Island Park Reservoir, and its outflow, Henry’s Fork of the Snake River in eastern Idaho. Our objectives were to (1) quantify the extent to which daily temperature ranges in the reservoir outflow deviated from other reaches of the Henry’s Fork, and (2) test whether the reservoir’s net volume change during the summer — expressed as the volume of water remaining in the reservoir on September 1 — predicted mean summer temperature in the outflow. Two years of temperature data showed dampened diel temperature cycles in the reservoir outflow. Model selection with 17 years of climatic, hydrologic, and reservoir management variables found mean summer temperature in the outflow was best predicted by September 1 reservoir volume and average summer air temperature. Two years of weekly reservoir thermal profiles indicated large changes in reservoir volume eliminated cool hypolimnetic water and encouraged mixing, allowing warm epilimnetic water to be discharged into the outflow. Increases in future drought frequency and severity and increases in summer air temperatures could increase the frequency of occurrence of high mean summertime water temperatures in the outflow. Our study provides important information for local managers by quantifying influences on outflow temperatures and the downstream river ecosystem.     

EFFECTS OF CLIMATE CHANGE ON HYDROLOGY AND WATER RESOURCES IN THE COLUMBIA RIVER BASIN1

ABSTRACT: As part of the National Assessment of Climate Change, the implications of future climate predictions derived from four global climate models (GCMs) were used to evaluate possible future changes to Pacific Northwest climate, the surface water response of the Columbia River basin, and the ability of the Columbia River reservoir system to meet regional water resources objectives. Two representative GCM simulations from the Hadley Centre (HC) and Max Planck Institute (MPI) were selected from a group of GCM simulations made available via the National Assessment for climate change. From these simulations, quasi-stationary, decadal mean temperature and precipitation changes were used to perturb historical records of precipitation and temperature data to create inferred conditions for 2025, 2045, and 2095. These perturbed records, which represent future climate in the experiments, were used to drive a macro-scale hydrology model of the Columbia River at 1/8 degree resolution. The altered streamflows simulated for each scenario were, in turn, used to drive a reservoir model, from which the ability of the system to meet water resources objectives was determined relative to a simulated hydrologic base case (current climate). Although the two GCM simulations showed somewhat different seasonal patterns for temperature change, in general the simulations show reasonably consistent basin average increases in temperature of about 1.8–2.1°C for 2025, and about 2.3–2.9°C for 2045. The HC simulations predict an annual average temperature increase of about 4.5°C for 2095. Changes in basin averaged winter precipitation range from -1 percent to + 20 percent for the HC and MPI scenarios, and summer precipitation is also variously affected. These changes in climate result in significant increases in winter runoff volumes due to increased winter precipitation and warmer winter temperatures, with resulting reductions in snowpack. Average March 1 basin average snow water equivalents are 75 to 85 percent of the base case for 2025, and 55 to 65 percent of the base case by 2045. By 2045 the reduced snowpack and earlier snow melt, coupled with higher evapotranspiration in early summer, would lead to earlier spring peak flows and reduced runoff volumes from April-September ranging from about 75 percent to 90 percent of the base case. Annual runoff volumes range from 85 percent to 110 percent of the base case in the simulations for 2045. These changes in streamflow create increased competition for water during the spring, summer, and early fall between non-firm energy production, irrigation, instream flow, and recreation. Flood control effectiveness is moderately reduced for most of the scenarios examined, and desirable navigation conditions on the Snake are generally enhanced or unchanged. Current levels of winter-dominated firm energy production are only significantly impacted for the MPI 2045 simulations.     

Simulated Limnological Effects of the Shasta Lake Temperature Control Device

We estimated the effects of a temperature control device (TCD) on a suite of thermodynamic and limnological attributes for a large storage reservoir, Shasta Lake, in northern California. Shasta Dam was constructed in 1945 with a fixed-elevation penstock. The TCD was installed in 1997 to improve downstream temperatures for endangered salmonids by releasing epilimnetic waters in the winter/spring and hypolimnetic waters in the summer/fall. We calibrated a two-dimensional hydrodynamic reservoir water quality model, CE-QUAL-W2, and applied a structured design-of-experiment simulation procedure to predict the principal limnological effects of the TCD under a variety of environmental scenarios. Calibration goodness-of-fit ranged from good to poor depending on the constituent simulated, with an R ² of 0.9 for water temperature but 0.3 for phytoplankton. Although the chemical and thermal characteristics of the discharge changed markedly, the reservoir's characteristics remained relatively unchanged. Simulations showed the TCD causing an earlier onset and shorter duration of summer stratification, but no dramatic affect on Shasta's nutrient composition. Peak in-reservoir phytoplankton production may begin earlier and be stronger in the fall with the TCD, while outfall phytoplankton concentrations may be much greater in the spring. Many model predictions differed from our a priori expectations that had been shaped by an intensive, but limited-duration, data collection effort. Hydrologic and meteorological variables, most notably reservoir carryover storage at the beginning of the calendar year, influenced model predictions much more strongly than the TCD. Model results indicate that greater control over reservoir limnology and release quality may be gained by carefully managing reservoir volume through the year than with the TCD alone. RID=" ID=" Author to whom correspondence should be addressed. e-mail: John_Bartholow&commat;USGS.gov     

LEAF LITTER BREAKDOWN IN A RECENTLY IMPOUNDED RESERVOIR1

ABSTRACT: The impoundment of Richard B. Russell Lake resulted in the inundation of 3490 ha of forested area or 33 percent of the total area of the lake. Estimates of the total inundated leaf litter biomass were combined with a leaf litter decomposition study to determine the nutrient load and dissolved oxygen demand to the reservoir. Hickory leaf bags broke down most rapidly at the 3-m and 28-rn depths, followed by short-needle pine, white oak, a hardwood litter mixture, beech, and red oak. Leaf bags incubated at the 3-m depth exhibited significantly higher breakdown rates than those at the 28-m depth for most leaf types, due to differences in dissolved oxygen and temperature. Respiration rates of litter were also higher at the 3-m depth. Most leaf types accumulated nitrogen and phosphorus and lost organic carbon after an initial leaching period. Richard B. Russell Lake exhibited extensive anoxia and the buildup of total organic carbon, nitrogen, and phosphorus during summer stratification. Leaf litter breakdown accounted for 64 percent of the organic carbon increase but acted as a sink for nitrogen and phosphorus. The dissolved oxygen demand of the litter accounted for over 50 percent of the demand incurred in the lake.     

SIMULATED EFFECTS OF ALTERED SPILLWAY RELEASES ON THERMAL STRUCTURE AND KOKANEE GROWTH IN A COLORADO RESERVOIR1

ABSTRACT: In‐reservoir thermal and ecological effects of releasing some flows over the surface spillway at Blue Mesa Reservoir, Colorado, rather than routing all releases through the hypolimnetic outlet were evaluated using a calibrated and validated one‐dimensional thermal model (CE‐THERM) with a set of ecological models. Thermal model output indicated that surface water temperatures were influenced primarily by atmospheric conditions, but the release of warmer water over the spillway resulted in a thinner epilimnion and cooler metalimnetic water temperatures. Ecological model predictions indicated that spillway releases and associated temperatures resulted in lower growth rates for young‐of‐year (YOY) kokanee salmon (Oncorhynchus nerka) in the reservoir by up to 9 percent when compared with growth rates under baseline operations with no releases over the spillway. Kokanee growth rates were reduced under spillway release scenarios because lower temperatures not only affected metabolic rates, but limited the productivity of the zooplankton as well. Thus, altering the release regime with spillway discharges could have deleterious effects on Blue Mesa's YOY kokanee. However, in other reservoirs, distributing discharges among different elevations may provide managers with a mechanism to regulate temperatures to benefit species of concern that are facing challenges imposed by environmental conditions such as global warming.     

AN INTERACTIVE SIMULATION OF PUMPED STORAGE RESERVOIR SYSTEMS1

ABSTRACT: A combination pumped storage reservoir system was simulated by modifying the WRE deep reservoir model. Each of the two reservoirs was described by a copy of the WRE model program, the two programs were converted into subroutines and were called upon alternately by a main program. Operationally, the contributing reservoir, i.e., the reservoir from which flow was discharged, was simulated for one execution interval (1 hour), followed by simulating the receiving reservoir for the same execution interval. The main program directed the discharge temperature of the contributing reservoir for each execution interval as input for simulating the receiving reservoir. The two subroutines were run in this interactive mode for a simulation period of one year. Two simulations, labeled “Pump” and “Net,” were effected and differed basically in the distribution of flow volumes exchanged between the reservoirs. In the ‘Pump’ simulation the total hourly flow volumes were distributed into appropriate horizontal layers of the lakes as determined by temperature-density relationships, i.e., the pumped discharges were distributed into the upper lake and the generation discharges were distributed into the lower lake. In the ‘Net’ simulation only the net daily discharges (daily difference between pump-back and generation at the upper dam), distributed uniformly over 24 hours, were mixed into the horizontal layers of the two lakes. Both simulations produced annual thermal regimes that were apparently within reason for the geographical area and the nature of the input data. However, neither accurately reflected a generalized conditions for the reservoirs because the Pump simulation reflected conditions in the forebay and tailrace of the Upper dam while the Net simulation reflected conditions of the remaining parts of the reservoirs.     

TIMBER HARVEST IMPACTS ON SMALL HEADWATER STREAM CHANNELS IN THE COAST RANGES OF WASHINGTON1

ABSTRACT: We evaluated changes in channel habitat distributions, particle‐size distributions of bed material, and stream temperatures in a total of 15 first‐or second‐order streams within and nearby four planned commercial timber harvest units prior to and following timber harvest. Four of the 15 stream basins were not harvested, and these streams served as references. Three streams were cut with unthinned riparian buffers; one was cut with a partial buffer; one was cut with a buffer of non‐merchantable trees; and the remaining six basins were clearcut to the channel edge. In the clearcut streams, logging debris covered or buried 98 percent of the channel length to an average depth of 0.94 meters. The slash trapped fine sediment in the channel by inhibiting fluvial transport, and the average percentage of fines increased from 12 percent to 44 percent. The trees along buffered streams served as a fence to keep out logging debris during the first summer following timber harvest. Particle size distributions and habitat distributions in the buffered and reference streams were largely unchanged from the pre‐harvest to post‐harvest surveys. The debris that buried the clearcut streams effectively shaded most of these streams and protected them from temperature increases. These surveys have documented immediate channel changes due to timber harvest, but channel conditions will evolve over time as the slash decays and becomes redistributed and as new vegetation develops on the channel margins.     

ANALYSIS OF WIND- AND SHIP-INDUCED SEDIMENT RESUSPENSION IN DULUTH-SUPERIOR HARBOR1

ABSTRACT: Suspended solids and ammonium concentration profiles measured at five locations in Duluth-Superior Harbor during July-October 1985 were analyzed to quantify wind and ship effects on sediment resuspension and resulting harbor water quality. Wind components from the SE quadrant correlated strongly with depth-averaged suspended solids concentrations that were unaffected by ship passage or thermal stratification. Winds from that quadrant have the largest fetch in the harbor. The highest correlation (r²= 0.93) was with the 6-hour average of the ESE wind velocity component. Multiple linear regression analysis of data from post-ship passage concentration profiles yielded numerical estimates of settling velocities of 0.08 to 0.25 cm s^?1, typical of ship-resuspended sediments, and vertical eddy diffusivities of 4 to 13 cm² s¹. The results suggest that ambient vertical eddy diffusivities in the harbor are less than 4 cm² s^?1 in the absence of ship passages and with winds less than 5 m s^?1 (10 knots).