Scour-inducing stilling basin flow patterns

Bed scour downstream of stilling basins continues to threaten the stability of water control structures. A 1/30 scale physical model of one such structure operated by the South Florida Water Management District was constructed at Michigan Tech to further study this problem. Detailed flow measurements were taken on a flow scenario corresponding approximately to the prototype design flow, which resulted in high scour, namely that of a high flow rate and upstream headwater depth and a low tailwater depth. Equilibrium bed scour and velocity measurements were taken using an Acoustic Doppler Velocimeter. Velocity data was used to construct a vector plot in order to identify which flow components contribute to the scour hole. Flow entering the scour region exhibited an aerated nappe and was comprised of a strong-velocity region exiting the stilling basin that induced a standing wave and split into a downward roller with a near-bed component flowing upstream inducing near-structure scour and a plunging region downstream of the structure inducing far-field scour and continuing downstream to form a vertically contracting region far downstream as the flow returned to normal conditions after the local scour effects were dissipated. In addition, slowly-rotating vertical flow-separation eddies in the corners of the structure-channel banks intersection were formed. © 2011 ASCE

Experiments identifying worst case scour conditions of gated weir stilling basins

Bed scouring at stilling basins downstream of gated spillways has been identified as posing a serious risk to the spillway\u27s structural stability. Although this type of scour has been studied in the past, it continues to represent a real threat to water control structures and requires additional attention. A 1/30 scale Froudian model of one such structure operated by the South Florida Water Management District was constructed at Michigan Tech to further study this problem. A series of experiments were performed in this model to identify the conditions that induce maximum scour. Utilizing these flow conditions as baseline on subsequent experiments aimed at studying scour-reducing measures will verify these measures has the desired effect. The four flow conditions considered in the study were: controlled submerged, uncontrolled submerged, controlled free, and uncontrolled free. Gates restricting flow were positioned at the spillway crest and at the end of the flume where the model was built to control the tailwater level. Positioned downstream of the stilling basin was a scour chamber filled with uniformly graded sand representing the channel bed. The distance check feature of an Acoustic Doppler Velocimeter was utilized during the experiments to determine when scour equilibrium was established for a particular case. Point gauges mounted on a carriage above the experimental flume were used to measure the water surface profile at equilibrium. After the scour hole reached equilibrium, the flume was drained and the channel bed contour profile was measured. Key parameters considered during this study include flow rate, headwater, tailwater, water surface profile, bed profile, and spillway gate opening. These experiments show that the worst flow condition for scour was uncontrolled free flow with high headwater and low tailwater below the spillway crest. Results of these experiments are useful for predicting the effect of various flow scenarios on maximum scour near and away from the structure along with total scour volume. © 2010 ASCE

Experiments identifying scour-inducing flow patterns at a gated weir stilling basin

Excessive scour downstream of stilling basins poses significant risk of structural failure. This problem was experimentally investigated at Michigan Technological University in a Froude-scaled, physical model of one such structure operated by the South Florida Water Management District. Detailed flow measurements were taken on a flow scenario that resulted in high scour, namely that of a high flow rate and upstream headwater depth and a low tailwater depth. Equilibrium bed scour and velocity measurements were taken using an Acoustic Doppler Velocimeter. Velocity data was used to construct a vector plot in order to identify which flow components contribute to the scour hole. It was found that downward-plunging flow upon leaving the stilling basin induces the primary scour hole. © 2010 American Society of Civil Engineers

Field-Scale Crop Seeding Date Estimation from MODIS Data and Growing Degree Days in Manitoba, Canada

Information on crop seeding date is required in many applications such as crop management and yield forecasting. This study presents a novel method to estimate crop seeding date at the field level from time-series 250-m Moderate Resolution Imaging Spectroradiometer (MODIS) data and growing degree days (GDD; base 5 ºC; ºC-days). The start of growing season (SOS) was first derived from time-series EVI2 (two-band Enhanced Vegetation Index) calculated from a MODIS 8-day composite surface reflectance product (MOD09Q1; Collection 6). Based on GDD calculated from the Daymet gridded estimates of daily weather parameters, a simple model was developed to establish a linkage between the observed seeding date and the SOS. Calibration and validation of the model was conducted on three major crops, spring wheat, canola and oats in the Province of Manitoba, Canada. The estimated SOS had a strong linear correlation with the observed seeding date; with a deviation of a few days depending on the year. The seeding date of the three crops can be calculated from the SOS by adjusting the number of days needed to accumulate GDD (AGDD) for emergence. The overall root-mean-square-difference (RMSD) of the estimated seeding date was less than 10 days. Validation showed that the accuracy of the estimated seeding date was crop-type independent. The developed method is useful for estimating the historical crop seeding date from remote sensing data in Canada to support studies of the interactions among seeding date, crop management and crop yield under climate change. It is anticipated that this method can be adapted to other crops in other locations using the same or different satellite data