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Session 20A: Collection, Pump Stations and Conveyance: Collections
1:15pm - 2:45pm
Session Chair: Tyson Schlect, HDR;
1:15pm - 2:00pm
Wastewater Collection System Capacity Analysis for Future Planning
Rizwan Hamid1, Andrew Henson1, Kevin Cook1, Annalisa McDaniel2
1Aqualyze, Inc.; 2City of Seattle, Seattle Public Utilities; , ,
Wastewater system planning requires detailed analysis of existing infrastructure using the best available tools. Seattle Public Utilities (SPU) currently serves a population of over 730,000 that is expected to grow 10% to roughly 800,000 by the year 2035. The median age of the collection system is approximately 80 years. Given the projected growth, lack of hydraulic capacity may lead to sanitary sewer overflows (SSO) during peak wet-weather conditions.
Aqualyze, Inc. was the lead consultant for SPU’s Wastewater System Analysis (WWSA) project to evaluate the existing infrastructure for hydraulic capacity limitations across the service area including over 40,000 pipes (1,420 miles), using the latest dynamic hydraulic/hydrologic (H/H) model in PCSWMM. This paper presents a robust methodology used to identify capacity constrained pipes across the system.
A thoroughly constructed and calibrated H/H model provides pipe surcharging and flooded maintenance hole (MH) locations anywhere in the system. However, a simple model run presents only part of the problem and requires further analysis. The methodology used for the WWSA project involved conducting iterative model runs to identify potential downstream impacts by upsizing initially surcharged pipes and letting unrestricted flows reach the downstream system to identify potential future capacity issues. An additional unrestricted flow scenario was created such that by the end of the system-wide analysis the unrestricted peak flow for every gravity pipe in the system was established.
Using the unrestricted peak flow and the existing hydraulic capacity of the pipe (using Manning’s equation at maximum flow), all pipes with a Qpeak/Qcapacity ratio greater than 1 were flagged as under capacity. This approach not only identified downstream pipes that could potentially be under capacity, but also eliminated previously surcharged upstream pipes that had adequate capacity but were surcharged due to downstream restrictions. Under-sized pipes were upsized and costed across the system.
2:00pm - 2:45pm
Hazelwood Sewer Upgrade: Alternatives Analysis and Selecting a Construction Method
RH2 Engineering, United States of America;
The City of Oregon City identified the need to replace and upsize portions of approximately 3,000 linear feet of sanitary sewer to relieve surcharging and flooding in Warner-Parrot Road and the Hazelwood Drive area located south of downtown. During storm events in 2009, five properties had experienced basement flooding, and two of these same properties had flooding again during storm events in 2012. In 2014, the City determined that there was a need to adopt an ordinance for a moratorium on land development and building permit approval where the sanitary sewer system was flow constrained. Replacement of the sewer main downstream of the affected area was complicated by its location within the backyards of several homes with extremely limited access, as well as its proximity to Coffee Creek, which runs parallel and immediately adjacent to the sewer main in many sections.
In order to determine the best path forward, conceptual design alternatives were developed and analyzed. These alternatives included:
Replacing the sewer within its existing alignment with either open-cut construction or pipe bursting
Rerouting the sewer out of the easement and into Hazelwood Drive Right-of-Way (ROW) using open-cut construction or horizontal directional drilling
Rerouting the sewer out of the easement and into Hazelwood Drive ROW by means of a lift station
Based on the analysis, which involved evaluation of costs, maintenance, constructability, and impacts to the community, the selected alternative for design rerouted the sewer into the Hazelwood Drive ROW with a combination of open-cut construction and horizontal directional drilling (HDD) methods. HDD was selected for a segment of sewer that would have involved deep excavations in solid basalt.
This presentation will discuss the project constraints and considerations involved in the selection of a preferred alternative, as well as lessons learned during design and construction of the project.