1、WaterWastewaterInfrastructureOffices Throughout New England | www.wright- 99 Main StreetTopsham, ME 04086 USAPhone 207.725.8721 | Fax 207.729.8414January 23, 2007W-P Project No. MTM07Mr. Patrick CloutierDirector, Water Resource Protection25 Cottage Road, P.O. Box 9422South Portland, Maine 04116-9422
2、Subject: Proposal to Conduct Additional Services related to the Combined Sewer Overflow (CSO) Facilities Plan UpdateDear Pat:As you are aware, a number of unfortunate issues have been encountered during the work on the CSO Facilities Plan update project which has precipitated the need for funding of
3、 additional data collection, study and physical equipment replacement needs. While it was envisioned that we could generate an updated Facilities Plan that would serve as a solid “blueprint“ for eliminating or significantly reducing CSOs under our original budget, it will not be possible without add
4、itional efforts. We, like you, truly believe that sound science with proven results must guide our decision-making efforts so that the publics money is spent wisely.Please find below a discussion of the relevant project background including the issues encountered during execution of the project; the
5、 proposed additional scope of services to generate the data that we need to conduct an effective alternatives analysis; and the associated compensation for those services. PROJECT BACKGROUNDWright-Pierce was retained by the City on May 22, 2007 to update the Citys 1994 CSO Facilities Plan (Plan). Th
6、is update to the Plan is a requirement of the Citys current Maine Pollution Discharge Elimination System (MEPDES) permit. A completion date of December 31, 2007 has been stipulated by the Maine Department of Environmental Protection (DEP).In general, Wright-Pierces scope of services included the fol
7、lowing major items:1. Conduct continuous flow monitoring at key locations with the Citys collection system using 9 flow meters over an 8-week period (conducted May 30, 2007 to July 25, 2007);2. Conduct short-term flow monitoring at multiple locations during 6 rainfall events;3. Develop portions of a
8、 collection system model and utilize the monitoring data collected to calibrate the model. The model is useful in predicting how the collection system will respond to various storm events;4. Utilize the model to predict changes in CSO overflow volumes with various upgrades to the collection system,
9、pump stations and WWTF; andMr. Patrick CloutierJanuary 23, 2008Page 25. Evaluate alternatives for technical and economic feasibility and select a recommended CSO abatement plan for the City.During the course of Wright-Pierces work on Scope Items 1 and 3 above, it was determined that it would be more
10、 beneficial to expand these scope items rather than conduct Scope Item 2. This translated to 4 additional weeks of monitoring with 3 flow meters (September 26, 2007 to October 23, 2007) as well as the development of a complete Stormwater Management Model (SWMM).Prior to the commencement of the flow
11、monitoring program, both the City and Wright-Pierce were hopeful, but speculative, that a significant rain event of sufficient duration would occur in the months of June and July. Considering the end of year deadline for submission of the Plan update, it was determined that regardless of our concern
12、s with capturing a storm of significant magnitude, flow monitoring would be conducted. Unfortunately, only 6.6 inches of total precipitation fell during the 8-week monitoring period with only three events resulting in greater than 1-inch of rainfall. Only one event, approximately 2 inches of precipi
13、tation over a 48 hour period starting on June 4, resulted in a CSO event at two of five overflow locations within the City. As we have discussed with you, the SWMM model works best with intense rainfalls of appreciable magnitude to allow calibration that in turn allows the model to become a reliable
14、 predictor of future events. However, considering the end of the year deadline, Wright-Pierce embarked on the calibration and verification of the SWMM in August-September 2007 with the flow monitoring and CSO data available. During our efforts to verify that the calibrated model would predict overfl
15、ow volumes comparable to actual overflow volumes recorded by the City in the CSO structures, two issues were discovered:Item 1: With the data collected, we attempted to calibrate the model and compare the results against past reported CSO quantities. Unfortunately, it was not possible to correlate t
16、he calibrated model to previously reported events. Thus, it was determined that it would not be possible to utilize the model to evaluate the impacts of larger rain events on overflow volumes.Item 2: Based on the above discoveries, we began to review the Citys existing data collection methods. A num
17、ber of issues were discovered with these methods and the equipment being used by the City to collect the data necessary to calculated overflow volumes at the five CSO structures. Following the discovery of these issues, we met with the City staff on several occasions to discuss possible resolutions.
18、 Recently, we engaged John True, State CSO Program Coordinator for the Maine DEP at an onsite meeting with the City. We reviewed our concerns as well as the scope of service needed to collect additional data; improve existing data collection methods; and ultimately produce a Facilities Plan that is
19、founded in good science with recommendations that will result in the elimination or significant reduction in CSO activity in the City. The following Scope of Services presents a proposed resolution to these issues. Mr. Patrick CloutierJanuary 23, 2008Page 3SCOPE OF SERVICESIn order to address the tw
20、o issues discussed above, we would propose the following additional scope of services to our existing Contract:Item 1 (Scope Items 1 and 2 on the attached table)The flow monitoring efforts to date were valuable in determining relative I/I contributions from the various drainage areas and in setting
21、up and initially calibrating the Stormwater Management Model (SWMM). However, additional data is needed to refine SWMM and to allow for greater accuracy in predicting the overflow activity for storms of greater intensity (1-, 2-, 5- or 10-year, 24 hour storm event). Initial runs of the model using t
22、he minor storm for calibration did not produce overflow results that matched with larger events and CSO quantities reported in the past. As such, the current model calibration will not be an accurate predictor of future events and thus not a useful tool in CSO mitigation that will produce both relia
23、ble results and ones that are cost-effective. SWMM needs to be recalibrated and verified for larger storm events if it is to be trusted in predicting overflows for large design storms. Additionally, this recalibration and verification of SWMM will allow it to more accurately predict the impact that
24、various capital projects may have on the volume of overflows. As such, a well-calibrated and verified SWMM is a very valuable cost-saving tool for the City. SWMM will also survive this facilities plan update and can be used in the future to predict the impacts that growth may have on the collection
25、system, pump stations and treatment facility.In order to address the issues described above, we recommend that the City initiate an intensive flow monitoring effort in the spring of 2008 to capture a storm of sufficient intensity to refine SWMM. Initially, it was proposed that 22 flow meters be temp
26、orarily installed to “blanket“ the collection system to collect data simultaneously in many drainage areas during a significant storm event. However, after meeting with Dave Thomes on December 4, 2007 to discuss the proposed metering scope of work, it was determined that the number of temporary flow
27、 meters would be reduced to 13 with the understanding that calibrated flow meter data would be provided by the City from the Long Creek Pump Station, Pearl Street Pump Station, Front Street Pump Station and West High Street Pump Station in electronic format for incorporation into the SWMM. This also
28、 assumes that the City will purchase and install permanent flow meters at each of the CSOs prior to the installation of the temporary flow meters (see Item 2 discussion below).In order to calibrate the model for larger storm events, a storm event that has approximately 3 to 4 inches of rain within a
29、 24 hour period will be targeted. This range of rainfall would be similar to a 2- to 5-year, 24-hour design storm. The 13 temporary meters may be in for a short duration and could capture a significant storm quickly after installation or it may take several months of data collection before a signifi
30、cant storm occurs. We are hopeful that a two month monitoring period in March and April 2008 will result in a sufficient rainfall event. Once the flow monitoring data has been obtained, the data will be analyzed and then utilized to recalibrate and verify SWMM so that it can be used as an accurate p
31、redictor of storms of greater intensity. Once that task has been completed, the model will be a valuable tool in determining and refining the alternatives to abating or eliminating the remaining CSOs in South Portland.Mr. Patrick CloutierJanuary 23, 2008Page 4Item 2 (Scope Item 3 on the attached tab
32、le)During the development of SWMM, there were significant differences observed between the CSO volumes calculated by the model (as initially calibrated) and the CSO volumes historically reported by the City. This led W-P to closely examine the SWMM assumptions, the historic data, the method of measu
33、rement and the layout of each CSO structure as it relates to weir equations and downstream hydraulic conditions. A number of issues were found with the methods used to determine the volume of combined stormwater and wastewater overflowed. The data gathered to date is both useful and relative in that
34、 it shows the great strides that the City has taken to reduce the volume overflowed. However, an improvement in the accuracy of the flow monitoring at the overflow locations is recommended to maximize the value of the modeling effort. A well calibrated and verified model will allow Wright-Pierce and
35、 the City to select mitigation projects that have the greatest potential to abate the Citys CSOs. Thus it will be necessary to reinvest in new flow metering equipment to monitor CSO activity and record volumes.Recommendations for each CSO location are discussed below.CSO #005 (Cash Corner)It is reco
36、mmended that the City modify the method by which the CSO flow is measured at CSO #005 as soon as there is consensus on the proper means and technology. There are two methods under review to measure flows more accurately at this location:1. Install a depth (pressure)/velocity meter in the 33“ piping
37、entering the stormwater outfall chamber adjacent to the CSO #005 diversion structure. This option will need to be reviewed in additional detail due to concerns with turbulence created by the 48-inch stormwater piping also entering this structure.2. Install an overflow weir in the stormwater outfall
38、chamber adjacent to the CSO #005 diversion structure. This option would entail the installation of two pressure transducers, one on each side of the weir, to determine whether free-fall conditions exist on the downstream side of the new weir. The City and W-P will review these two options and determ
39、ine the optimal location to install a permanent meter and any modifications to the structures that are required.CSO #006 (Evans and Broadway)The City will modify the method by which the CSO flow is measured at CSO #006 by installing a new pressure/velocity sensor in the 30-inch piping downstream of
40、the current weir location. CSO #018 (Front Street)Of all of the CSOs currently monitored by the City of South Portland, CSO #018 is the most challenging and may or may not require modifications to the diversion structure. The upstream side of the diversion structure has multiple, large sanitary sewe
41、r pipes entering. The weir is located on the sanitary side of the diversion structure. Flow discharging over the weir flows through a flap gate into the stormwater side of the same diversion structure. The stormwater side Mr. Patrick CloutierJanuary 23, 2008Page 5of the diversion structure has multi
42、ple stormwater pipes entering before flow leaves through a 36-inch pipe discharging to the Fore River.There are two methods under review to measure flows more accurately at this location:1. Monitor the 36-inch stormwater pipe leaving the diversion structure and the 24-inch stormwater pipe entering t
43、he structure (while deeming two additional smaller stormwater pipes as negligible). By subtraction, the wastewater discharging over the weir could be determined. The difficulty in this approach is that there are two, additional stormwater pipes that convey catch basin flow into the structure. Based
44、on how many catch basins are captured by these two stormwater pipes, this flow could either be deemed negligible or the pipes could be diverted to a location downstream of the diversion structure.2. Reroute the two additional stormwater pipes downstream of the diversion structure and monitor the 36-
45、inch stormwater pipe leaving the diversion structure and the 24-inch stormwater pipe entering the structure. Obviously, diverting the stormwater pipes to a downstream location would increase the cost of implementing this alternative.The City and W-P will review these two options and determine the op
46、timal location to install permanent meters and any modifications to the structures that are required.CSO #019 (West High Street)In lieu of the existing pressure sensor, the City will install of a new pressure/velocity sensor on the downstream side of the flap gate at the West High Street overflow st
47、ructure in the 21-inch overflow pipe discharging to the Fore River. If the tide did rise to the point where it entered this structure (and the flap gate failed, allowing ocean water to enter the pump station), a negative velocity would be measured, which is indicative of water flowing in the opposit
48、e direction as the orientation of the sensor. The current pressure transducers cannot discern the direction of fluid flow.CSO #024 (Elm Street)The City will install a new pressure/velocity sensor in the 36-inch overflow pipe located downstream of the existing weir.It is also recommended that the Cit
49、y request a modification to its existing MEPDES permit to reopen the former CSO #004 and install a new pressure/velocity sensor in the overflow pipe. Since this CSO location was closed, flows have exceeded the capacity of the Long Creek pump station on a few occasions, allowed combined stormwater and wastewater to exit the facility through access doors. In addition to Wright-Pierces $3,198 effort to assist the City in this endeavor moving forward, it is estimated that the City will need to spend approximately $50
