HYDRAULIC PROFILES IN TRENCH-TYPE WET WELLS FOR WASTEWATER PUMPING

Designers of trench-type wet wells should ascertain that sweeping velocities will occur during the cleaning cycle so that sediment or debris is successfully transported to the cleaning cycle pump. A versatile computational tool is presented for calculating the water surface profile and hence depths, velocities, and Froude numbers down the curved ramp and along the floor of a trench-type wet well. A spreadsheet is presented that uses a standard-step approach to balance the energy equation along the ramp and floor of the wet well. The user specifies all pertinent characteristics including the design flow rate, physical dimensions, and hydraulic properties and the spreadsheet predicts the water surface profile and accompanying hydraulic parameters. Some wet wells have a sloped approach pipe that is larger in diameter than the adjoining sewer pipe to increase the storage in the wet well system These approach pipes are designed to create a hydraulic jump where the sequent depth is a fixed percentage of the conduit diameter. A Mathcad™ worksheet is presented that simultaneously solves the Manning's equation prior to the hydraulic jump, the momentum equation across the jump, and the user specified ratio of sequent depth to conduit diameter to arrive at the proper diameter for the wet well approach pipe. The Excel (Office 2000 or newer required) spreadsheet and Mathcad (Mathcad version 2000 or newer required) worksheet may be downloaded from the internet file transfer site at http://www.coe.montana.edu/ce/joelc/wetwell/. Disclaimers and instructions are embedded in the worksheets.