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Briefing for Blue Plains Regional Committee on Potomac Interceptor Capacity Analyses

This briefing discusses the analysis and capacity expansion proposal for the Potomac Interceptor system, incorporating the integration of models, design basis considerations, and findings from M&E 2003 results. It evaluates the impact of Fairfax's request to increase flow, provides insights on peak flow management strategies, and suggests options for enhancing capacity.

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Briefing for Blue Plains Regional Committee on Potomac Interceptor Capacity Analyses

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  1. Briefing for Blue Plains Regional Committee on Potomac Interceptor Capacity Analyses June 28, 2007 District of Columbia Water and Sewer Authority

  2. Agenda • Purpose • Model Integration • PI Design Basis • M&E 2003 Model Results • Review of Fairfax Request

  3. Add Potomac Interceptor model to model of WASA system PI Model 2003 by M&E/COG SWMM Model from Potomac P.S. to PI WASA Model Danish Hydraulic Institute Mike Urban Model Includes combined sewers & major separate sewers to D.C. line Evaluate Fairfax request to add flow to PI – two scenarios Add 6.5 mgd (avg) Add 9.5 mgd (avg) Purpose

  4. Obtained model data & calibration from M&E Potomac Interceptor DC Sewer System Model Integration - Completed • Converted to Mike Urban • Confirmed calibration • Completed integration

  5. PI Design Basis

  6. M&E 2003 Findings • Dry weather flows - 2025 RWWFM forecast • Wet Weather Flows – per metering, regression analysis • Offloads/System Configuration • Loudoun & Fairfax offloaded to Broad Run at MH-56 • WSSC Cabin John Flows >16 mgd diverted to PI • Section of UPI out of service in DC is repaired so UPI flows no longer routed to PI • Results of 5 & 10 yr storm runs • 5-yr storm: PI surcharged, but no overflow • 10-yr storm: PI not adequate • PI surcharged capacity about 144 mgd at DC line (vs. 151.17 original design basis)

  7. M&E Results: Flooding During 10 Year Storm D.C. 7

  8. M&E Results: Flooding During 10 Year Storm Flooding locations during 10 yr storm 8

  9. M&E Flows in Model 9

  10. Comments on Prior Work • Several jurisdictions predicted to have peak flows >> IMA peak allowance • Not all flow was getting to PI because total peak flow >> PI capacity  water levels exceeding ground level at inflow nodes for peak flow  this excess volume is NOT entering the modeled PI system and is lost.  These occurrences just show that problems could be expected in the according subsheds

  11. Evaluations Requested by Fairfax

  12. Summary of Model Runs Impact of peaks > IMA Add Fairfax flow Add Fairfax flow but limit peaks to IMA Addit. CSO impacts Note: No Fairfax offload to Broad Run – all flow in PI

  13. Findings • Agree with prior work (M&E/COG) • PI capacity (2025): • No Fairfax changes: • Peaks not limited = 5 yr storm • Peaks limited to IMA values = 15 yr storm + • Fairfax adds flow • Peaks not limited = 5 yr storm @ +6.5 mgd, <5 yr storm @ +9.5 mgd • Peaks limited to IMA values = 10 yr storm

  14. Options • Additional metering – improve predictions • Develop PI peak flow management strategy • Add capacity • Relief sewer or relining • Probably increase impact on D.C. CSO

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