Abstract:The characterization of stormwater runoff on urbanized surfaces by means of comparison between experimental data and simulations is a strict requirement for a sustainable management of urban sewer systems. A monitoring campaign was carried out within a residential area in Puglia (Southern Italy) in order to collect and evaluate quantity and quality data. A strong correlation was observed between COD (Chemical Oxygen Demand) and TSS (Total Suspended Solid) concentrations, whose values exceed water quality standards. TSS was used for calibration of Storm Water Management Model (SWMM) which was then validated with reference to the pollutograph's shape and the peak-time. The first flush phenomenon occurrence was also investigated by looking at the distribution of pollutant mass vs. volume in stormwater discharges, using the so-called "M(V) curves". Results show that on average the first 30% of that washed off carries 60% of TSS and provides important information for the design of efficient systems for first flush treatment.
We present experimental results on the formation of streamwise asymmetric, migrating bedforms resulting from the transition of successive surface solitary waves (SSW) interacting with an initially flat sandy bottom. We analyze seven cases, with differing ratios between SSW height and water depth. For each experiment, we generated 400 SSWs having the same features, investigating their effects on the bed, in terms of near‐bed velocity and erosional patterns. Our work proves that SSWs can generate asymmetric bedforms, similar in shape to dunes. The triggering process occurs in the region where the reverse flow induced by bottom boundary layer separation produces local erosion. The action of successive SSWs over the newly formed discontinuities gives rise to bedforms. Differently from steady flows, dune load decreases for larger wave‐induced bottom shear stress.
Nonlinear internal solitary waves (ISWs) propagating through a two-layer stratified system, in the presence of a shear background current, are theoretically investigated. We implement a new version of the Miyata–Choi–Camassa model with mobile layers (MCC-ML), by considering an asymptotic, uniform velocity distribution for each layer. To investigate the typical geophysical flow conditions observed in the coastal oceans, we focused on theoretical predictions for a density ratio between the two layers set to 0.99. A rigid-lid at the top of the theoretical domain is considered since it represents a good approximation under the Boussinesq condition. By varying the ratio of the undisturbed layer thickness from 0.1 to 10, we considered ISWs with both positive and negative polarities, when the background fluid is at rest. For increasing velocity differences between the two layers, ISWs tend to broaden (steepen) when the background velocities assume the same (opposite) direction of those induced by the wave. We show that the polarity conversion can be easily predicted since it directly depends on both stratification features and ambient velocities. The shear current affects also the wave celerity: for increasing background shear, upstream-propagating solitons reach a critical condition for which the wave celerity is equal to zero. We found that this occurrence is associated with a well-defined value of the wave amplitude. For even larger background shears, the waves are observed to change their direction of propagation. By linear analysis, we finally obtained the limiting background shear current for which the MCC-ML model does not provide any solution.
The dynamics of lock-release Intrusive Gravity Currents (IGCs) generating Internal Solitary Waves (ISWs) are investigated by threedimensional large eddy simulations. We set the numerical, laboratory-scale domain in order to release a uniform fluid in multi-layer, stratified ambient, exciting pycnocline displacements. By adopting different initial settings, we analyzed the influence of the ambient stratification on both IGCs and ISWs features. We present the main flow dynamics and the time evolution of IGC and ISW front and trough positions, respectively. During the simulations, the ISW is allowed to reach the vertical wall at the end of the domain, and it undergoes reflection. We then analyzed the interaction between the IGC and the reflected ISW: the wave is observed to accelerate as it is pushed upwards by the intrusion, which, in turns, flows below the ISW, decelerating. By analyzing instantaneous velocity fields and flow rates, we found that during this interaction, the ISW increases its celerity in response of the reduced area available for its propagation, partially occupied by the intrusion, and because the velocity field in the IGC interface surroundings acts to facilitate the ISW passage.
Triggering and evolution of internal solitary waves (ISWs) generated by intrusive gravity currents (IGCs) propagating into a stratified ambient fluid is analyzed by laboratory experiments. After the release of a fluid of uniform density, intermediate with respect to the upper (lower-density) and lower (higher-density) layers in the channel, the IGC develops and flows downstream, intruding into the pycnocline. Near the IGC leading front, the compression of the upper layer generates ISWs: they gradually separate from the current that propagates slower. Shoaling downstream over a uniform sloping boundary, solitons break and partially reflect. We investigate the dynamics of the interaction between the reflected ISWs and the incoming IGC. During the engage, an increase in the ISW celerity occurs, leading the celerity of the reflected waves to be even larger than the incident wave. Our analysis shows how both ISWs and IGCs can significantly change their features as they experience a change of the density structure in the water column. This is expected to occur, for example, in stratified small-scale basins, where river plumes intrude the seasonal thermocline. The radial ISWs, originated by IGCs, can then be reflected by the adjacent bottom bathymetry, spreading against the intrusive current from which they are generated.
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