DRAFT Memorandum. Background

Transcription

1 Barr Engineering Company 4700 West 77th Street Minneapolis, MN Phone: Fax: An EEO Employer Minneapolis, MN Hibbing, MN Duluth, MN Ann Arbor, MI Jefferson City, MO Bismarck, ND DRAFT Memorandum To: John Sachi, City of, and Scott Thureen, City of From: Janna Kieffer, Barr Engineering Co. Subject: Allowable Flow Cost Apportionment for Dawn Way Storm Sewer Improvement Project Date: October 22, 2008 Project: 23/ JMK2 001 c: Ted Witkowski, WSB & Associates, Inc. The cities of (SSP) and (IGH) have requested that Barr Engineering Company assist them in determining the recommended cost allocation for the Dawn Way Storm Sewer Improvement Project using the Allowable Flow methodology. Allowable Flow is the cost apportionment method defined in the Lower Mississippi River Watershed Management Organization (LMRWMO) Joint Powers Agreement as a means of determining cost sharing among municipalities when stormwater management projects include drainage from more than one municipality. Background The Allowable Flow method is defined in the current LMRWMO Joint Powers Agreement (JPA), and has been further clarified in several technical memos throughout the past two decades. The Allowable Flow method is based on the basic principles that 1) upstream communities have the right to discharge some flow ( allowable flow ) downstream without financial obligation, and 2) upstream communities should share in downstream costs of handling their excess flows. The proposed Dawn Way Storm Sewer Improvement Project involves adding capacity to the Dawn Way storm sewer system via addition of a parallel system that begins approximately 250 feet north of the intersection of Dawn Way and 61 st Street East and continues in a northeasterly direction toward CSAH 56. The proposed storm sewer system was modeled in XP-SWMM by WSB & Associates for the 10-year and 100-year frequency events. Results from WSB s hydrologic and hydraulic model were used to complete the Allowable Flow analysis. 1

2 To: John Sachi, City of, and Scott Thureen, City of From: Janna Kieffer, Barr Engineering Co. Subject: Allowable Flow Cost Apportionment for Dawn Way Storm Sewer Improvement Project Date: October 22, 2008 Project: 23/ JMK2 001 c: Ted Witkowski, WSB & Associates Allowable Flow Methodology The allowable flow cost allocation for the storm sewer improvement project was based on flows from the 10-year frequency event, as the storm sewer design is based on providing a 10-year level of service (i.e., the system is designed with sufficient capacity to handle the runoff from a 10-year critical storm event). The allowable flows ( A ) from the municipality upstream of each segment were calculated using the following equation: A = C*I*A with C = Runoff Factor, 0.15, A = Drainage Area (ac), and I= Rainfall Intensity (in/hr) based on the time of concentration. The time of concentration for each case was determined based on results from the XP- SWMM model and the corresponding rainfall intensity was determined using precipitation frequency curves that were generated based on the U.S. Weather Bureau s Technical Paper 40 and NOAA Technical Memorandum NWS Hydro-35. The excess flows from the upstream municipalities were calculated using the following equation, with the exception of flows from the drainage area: Excess Flow ( E U/S City ) = Total Flow ( T U/S City ) Allowable Flow ( A U/S City ) The cost share was then determined using the following equation: Cost Share U/S City = E U/SCity T where T is the total design flow for each storm sewer segment. The total design flow ( T ) to each storm sewer segment was determined by combining the 10-year peak flow from the upstream pipe segment(s) and the 10-year peak flow from the direct drainage area. Where the tributary drainage areas included portions of each municipality, the total design flow was proportioned based on the percentage of runoff generated from each municipality. A portion of the drainage area upstream of the proposed Dawn Way storm sewer improvements is tributary to the, which is located in and receives runoff from portions of 2

3 To: John Sachi, City of, and Scott Thureen, City of From: Janna Kieffer, Barr Engineering Co. Subject: Allowable Flow Cost Apportionment for Dawn Way Storm Sewer Improvement Project Date: October 22, 2008 Project: 23/ JMK2 001 c: Ted Witkowski, WSB & Associates and. For cost apportionment downstream of ponds, excess flow from the pond is calculated using the following formula from Exhibit A, page 9 of 9 of the JPA: E( outlet ) = E ( inlet ) * T ( outlet ) T ( inlet ) with the proportioning T(outlet) / T(inlet) being computed on the same return frequency event used for pond design. Based on the assumption that the was designed for a 100-year frequency event, the following equation was used to calculate excess flow from the : = E( outlet)10y E( inlet )10Y * T ( outlet )100Y T ( inlet )100Y Results As mentioned above, the excess flows from the were calculated separately from the excess flows for the areas draining directly to the Dawn Way storm sewer system. Table 1 summarizes the allowable and excess flows from the area tributary to the. The flows calculated for the City of apply where the storm sewer construction is in. The flows calculated for the City of apply when construction is located in. Table 1. Allowable and Excess Flows from City of South St. Paul City of Inver Grove Heights Drainage Area tributary to 4.9 acres 11.3 acres Allowable Flow ( A ) from area tributary to 3.1 cfs 6.4 cfs E inlet ) 10Y ( 7.8 cfs 15.9 cfs T outlet ) 100Y ( 7.8 cfs 7.8 cfs T inlet ) 100Y ( 45.1 cfs 45.1 cfs Excess Flow from ( E outlet) 10Y ( ) 1.3 cfs 2.7 cfs Total 10-year Design Flow ( T ) from 3.3 cfs 3.3 cfs 3

4 To: John Sachi, City of, and Scott Thureen, City of From: Janna Kieffer, Barr Engineering Co. Subject: Allowable Flow Cost Apportionment for Dawn Way Storm Sewer Improvement Project Date: October 22, 2008 Project: 23/ JMK2 001 c: Ted Witkowski, WSB & Associates For purposes of the Allowable Flow analysis, the proposed Dawn Way storm sewer system has been split into Segments A through F (see Figure 1) based on the addition of flow to the system by lateral storm sewers or other inlets. A cost allocation was calculated for each segment. The allowable and excess flows for the areas draining directly to the Dawn Way storm sewer system are summarized by segment below. Segment A Segment A has a tributary drainage area of 20.4 acres, excluding the watershed (see Figure 2). Since Segment A of the proposed system is being constructed entirely within the City of South St. Paul, the City of was considered the upstream municipality for Segment A and was assigned an allowable flow. The portion of the drainage area tributary to Segment A, excluding the watershed, is approximately 15.8 acres. The allowable flow ( A ) and excess flow ( E ) from the area tributary to Segment A (excluding the watershed) are summarized in Table 2. Segment B Segment B has a tributary drainage area of 44.6 acres, excluding the watershed (see Figure 3). Since Segment B of the proposed system is predominantly located within the City of Inver Grove Heights, the City of was considered the upstream municipality for Segment B and was assigned an allowable flow. The portion of the drainage area tributary to Segment B, excluding the watershed, is approximately 24.3 acres. The allowable flow ( A ) and excess flow ( E ) from the area tributary to Segment B (excluding the watershed) are summarized in Table 2. Segments C, D, E, and F Segments C, D, E and F of the proposed system are located entirely within the City of Inver Grove Heights. As such, the City of was considered the upstream municipality for these Segments and was assigned an allowable flow, accordingly. The drainage areas to Segments C, D, E, and F are summarized in Table 2 and depicted in Figures 4-7. The area tributary to these segments is approximately 25.1 acres. The allowable flow ( A ) and excess flow ( E ) from the areas tributary to Segments C, D, E, and F (excluding the watershed) are also summarized in Table 2. 4

5 To: John Sachi, City of, and Scott Thureen, City of From: Janna Kieffer, Barr Engineering Co. Subject: Allowable Flow Cost Apportionment for Dawn Way Storm Sewer Improvement Project Date: October 22, 2008 Project: 23/ JMK2 001 c: Ted Witkowski, WSB & Associates Table 2. Summary of Drainage Areas, Allowable and Excess Flows for Proposed Dawn Way Storm Sewer Improvements (Excluding Watershed) Parameter Segment A Segment B Segment C Segment D Segment E Segment F IGH Tributary Drainage Area ac 20.3 ac 23.3 ac 24.3 ac 25.4 ac 27.6 ac SSP Tributary Drainage Area ac 24.3 ac 25.1 ac 25.1 ac 25.1 ac 25.1 ac Total Tributary Drainage Area ac 44.6 ac 48.4 ac 49.4 ac 50.5 ac 52.7 ac IGH Allowable Flow ( A ) cfs na na na na na IGH Excess Flow ( E ) cfs na na na na na SSP Allowable Flow ( A ) 2 na 14.2 cfs 14.7 cfs 14.7 cfs 14.7 cfs 14.7 cfs SSP Excess Flow ( E ) 2 na 15.5 cfs 15.5 cfs 15.5 cfs 15.5 cfs 15.5 cfs Total Design Flow ( T ) cfs 55.7 cfs 61.5 cfs 62.4 cfs 63.6 cfs 67.4 cfs 1 Excludes area tributary to upstream 2 Excludes flows from upstream Cost Allocation To determine the recommended cost allocations, the excess flow from the upstream municipality for each segment were combined with the excess flow from the. Cost share was then determined based on the proportion of excess flow ( E ) to total design flow ( T ), including the 10-year frequency design flow from the. The resultant cost allocations, based on application of the Allowable Flow methodology described above, are summarized in Table 3. 5

6 To: John Sachi, City of, and Scott Thureen, City of From: Janna Kieffer, Barr Engineering Co. Subject: Allowable Flow Cost Apportionment for Dawn Way Storm Sewer Improvement Project Date: October 22, 2008 Project: 23/ JMK2 001 c: Ted Witkowski, WSB & Associates Table 3. Summary of Recommended Cost Allocations Segment of Proposed System City of Cost Share City of Cost Share A 55.4 % 44.6% B 28.5% 71.5% C 25.9% 74.1% D 25.6% 74.4% E 25.1% 74.9% F 23.8% 76.2% 6

7 Segment F Segment E Segment D Segment C Segment B Segment A Barr Footer: Date: 10/21/ :24:25 PM File: I:\Projects\23\19\1003\Maps\Figure1_Segments.mxd User: jmk2 Storm Sewer Segments Drainage Area to Dawn Way System Figure 1 Subwatersheds Existing Storm Sewer Proposed Storm Sewer Segments of Proposed Storm Sewer and

8 Segment A Barr Footer: Date: 6/3/2004 4:10:56 PM File: C:\Temp\Test.mxd User: bal Segment A of Proposed System Areas Tributary to Segment A Areas Tributary to Figure 2 Segment A Tributary Areas and

9 Segment B Barr Footer: Date: 11/5/2008 9:28:41 AM File: I:\Projects\23\19\1003\Maps\Figure3_SegmentB.mxd User: jmk2 Segment B of Proposed System Areas Tributary to Segment B Areas Tributary to Figure 3 Segment B Tributary Areas and

10 Segment C Barr Footer: Date: 11/5/2008 9:29:46 AM File: I:\Projects\23\19\1003\Maps\Figure4_SegmentC.mxd User: jmk2 Segment C of Proposed System Areas Tributary to Segment C Areas Tributary to Figure 4 Segment C Tributary Areas and

11 Segment D Barr Footer: Date: 6/3/2004 4:10:56 PM File: C:\Temp\Test.mxd User: bal Segment D of Proposed System Areas Tributary to Segment D Areas Tributary to Figure 5 Segment D Tributary Areas and

12 Segment E Barr Footer: Date: 6/3/2004 4:10:56 PM File: C:\Temp\Test.mxd User: bal Segment E of Proposed System Areas Tributary to Segment E Areas Tributary to Figure 6 Segment E Tributary Areas and

13 Segment F Barr Footer: Date: 6/3/2004 4:10:56 PM File: C:\Temp\Test.mxd User: bal Segment F of Proposed System Areas Tributary to Segment F Areas Tributary to Figure 7 Segment F Tributary Areas and