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 Purpose  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)

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

5 PI Design Basis PI Flow At DC Line, MH-2 (mgd) Year Annual Rainfall (National)Annual Avg.Peak Hour Peak Hour/ Avg Flow Ratio Burns & MacDonald 1961 PI Design BasisN/A Actual Data

6 M&E 2003 Findings  Dry weather flows 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 original design basis)

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

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

M&E Flows in Model Ann. Avg. Flow (mgd) Peak Flow (mgd)IMA / LCSA Flow (mgd) Jursdiction Meter Name5-yr, 24-hr Storm10-yr, 24-hr StormAnn. AvgPeak WSSC CJ Dulles (to PI) Muddy Branch Watts Branch Rock Run Unmetered WSSC Total LCSA Broad Run Offload LCSA Total Fairfax Sully Road # Sully Road # Rock Hill Road Sugarland Run Great Falls Scotts Run Pimmit AT&T B Fairfax Subtotal (Fairfax Offload)(4.89)(14.22) Fairfax Total Other Dulles Airport DC Line

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 IMA Allocation (mgd)Average Flow (mgd) Connection Point Meter NameAvg.Peak Scenario 1 – No Diversion to Noman Cole Scenario 2 –3 mgd Diversion from Difficult Run to Noman Cole Horsepen A1Sully Road # Horsepen A2Sully Road # Horsepen A3Rock Hill Road Sugerland Run Difficult RunGreat Falls Scotts Run Pimmit Total31N/A Amount above IMA Allocation Amount above IMA Allocation after 1mgd flow swap with LCSA under negotiation8.55.5

12 Summary of Model Runs Dry Weather FlowsWet Weather Peaks PI Capacity Assessment RunFairfaxAll OthersFairfaxAll Others System Changes5-yr10-yr15-yr 1COG 6.3 Up to IMA limit NoneOK 2COG 6.3 Per regression NoneOKNo mgdCOG 6.3Per regression NoneNo mgdCOG 6.3Per regression NoneOKNo mgdCOG 6.3Ann. Avg x 2.3Up to IMA limitNoneOK No mgdCOG 6.3Ann. Avg x 2.3Up to IMA limitNoneOK No mgdCOG 6.3Per regression Relieve PIOK Impact of peaks > IMA Add Fairfax flow but limit peaks to IMA Addit. CSO impacts Add Fairfax flow 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 +6.5 mgd, <5 yr +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