Experimental and Theoretical Investigation of the Hydraulics of Storm water Drill Drop Manholes and Vertical Drains with Radial Inflow

• Author / Creator

  Banisoltan, Sahar

• Vertical drains have been always an important topic in hydraulic engineering because of their application in draining from large reservoirs such as in shaft spillways of dams. In these structures, a radial water flow is supplied to the vertical shaft. Nowadays, vertical drains have been mostly used in stormwater and sanitary manholes. In these structures, flow is affected by the momentum of inlet jet. While in flow from large reservoirs, traditional flow regimes of weir and orifice flow are expected, some unsteadiness of water depth has been also observed by researchers. In some cases, unsteadiness caused a rapid rise of water depth. This was because the outlet could not pass the design discharge at that certain head and resulted in the serious risk of overflowing of the dam. In case of stormwater manholes, there is also a risk of overflowing into the streets and therefore it is essential to reevaluate the design, if rehabilitation is required, to ensure safe operation. For example, a component of the stormwater management in urban systems; e.g. in the City of Edmonton, is the Drill Drop Manhole (DDM). DDM’s consist of two concentric manholes attached in a vertical orientation. The diameter of the upper manhole is only twice or three times larger than the lower manhole and there can be different numbers of inlets with different sizes and configurations that convey water into the upper manhole. Recognizing this fact that most of previous studies and guidelines are related to large tanks discharging into smaller outlets, this experimental study was defined. As the first stage of the study, variation of the water depth h with the discharge Q was studied and new flow regimes were determined. Hydraulic design guidelines were developed so that based on the discharge, outlet size and inlet configurations and sizes, the water depth in the upper manhole could be calculated to prevent overflowing. Among new flow regimes, a filling and emptying regime was observed that had similarities to the unsteady flow in shaft spillways. It was believed that the incoming momentum and small ratio between the tank and outlet were responsible for the unstable flow behavior. Therefore, in the second stage of this study, the effect of the incoming momentum was eliminated by providing radial flow and the diameter ratio was progressively reduced. It was observed that while the flow was at the weir flow regime, gulping occurred frequently. This initiated the formation of the orifice flow regime or an oscillatory variation of water depths. Water depths in each of these oscillatory stages were recorded and it was found that the formation of each stage was related to the different location of the control section; i.e., the top end or the bottom end of the vertical outlet. The principle of maximum discharge and vortex theories (free vortex and Rankine vortex) were used for this purpose. It has been shown that for the unsteadiness related to the maximum head, the control section is at the end of outlet and there is a critical air core to outlet diameter ratio which can explain the observed unsteadiness. Field water depth variation with discharge in one dam has been used for validation purposes. This criterion can be used to ensure a safe operation. It was found that there is a tendency for the flow to form a free vortex in which the control section is located at the throat; i.e., just below the entrance to the lower pipe. In DDM, effect of the outlet size and throat length were discussed and the condition under which the discharge was controlled by the top of the vertical pipe was analyzed. Moreover, the continuity equation was used to calculate the outlet discharge and the results were examined. Effect of tank size on discharge coefficient in weir flow regime has been discussed and compared with the recommended design values for shaft spillways. It has been shown that the aeration of nappe can affect the coefficient of discharge in these cases. Another part of this research was related to the clarification of the effect of tank size on the coefficient of discharge and determination of the required freeboard in dropshaft structures.

• Subjects / Keywords

  • Hydraulic
  • Unsteady flow
  • Drill drop manhole
  • Air core
  • Radial inflow
  • Vortex
  • Vertical drain

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3SN01C3B

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.