Models for Capacity Assessment: More Than Just Pretty Pictures!

In 2007, the City of Hampton, Virginia entered into a Special Order by Consent with Virginia Department of Environmental Quality (DEQ), adjacent localities, and the Hampton Roads Sanitation District (HRSD), the regional transport/treatment authority in southeastern Virginia. A capacity assessment was required to evaluate the wastewater collection and distribution system performance under dry weather conditions and the 2-, 5-, and 10-year peak flow recurrence events with projected 20-year future growth. The complex and inter-connected wastewater collection and distribution system in Hampton presents a unique challenge which necessitated a variety of methods to determine system capacity.An all-pipes hydraulic model of the system was built, calibrated, and used to identify the locations that violated the freeboard criteria set forth by DEQ. The modeling team then investigated various ways to evaluate these problem spots and came up with a set of tools including system maps, dimensionless parameters, additional model scenarios, and a variety of pump station performance plots. The use of these additional tools allowed for four preliminary categories of the identified limitations: Collection System Limited, Pump Station, Downstream Pressure Limited, and Downstream Level Limited.The city is beginning to see a benefit of this analysis because they can now plan on replacing relevant sections of pipe in one service area rather than rehabbing them. Avoiding the trap of stopping at the first sign of results is the key to this method. With the assistance of these additional evaluation methods to categorize the source of the problems, the proposed solutions will be better informed, better suited to the issues, and better for the taxpayers' pockets.