Ashville Park Stormwater Management System Scenarios with Village C

Transcription

1 Memorandum o: From: Greg Johnson P.E. Mike Morgan P.E. Date: February Subject: Ashville Park Stormwater Management System Scenarios with Village Introduction his memo presents analysis performed for the Ashville Park Subdivision stormwater management system supplementing the evaluations documented in Draft Ashville Park Stormwater Management System Flood Mitigation Plan (referred to as the previous study in this report). he technical methodology for this analysis is the same as was used in the previous study. For the analysis presented in this memo DM Smith evaluated five scenarios described below that include future Village in combination with various stormwater management system improvements. Each scenario was analyzed using the 10-year and 100-year design storms as documented in the previous study. Attachment 1 provides schematics for each scenario. he existing condition model described in the previous study was used as a starting model for the analyses documented in this memo. Scenario 1 his scenario builds on the existing condition model as described in the previous study. It includes a fully developed Ashville Park Village (as presented in the developer s proposed lot configuration) and primary stormwater system elements. Additionally this scenario includes the following modifications from the developer s conceptual plans (see schematic in Attachment 1): hannel connecting BMP 10 and 15 that is approximately 900 linear feet (lf) with a 4-footwide bottom at elevation 1 foot orth American Vertical Datum of 1988 (AVD 88). he side slopes range from: H:1V from approximately 8 feet AVD 88 to 4 feet AVD 88; 4H:1V from feet AVD 88 to 1 foot AVD 88 and 6H:1V safety shelf from feet AVD 88 to 4 feet AVD 88. his channel geometry is subsequently be referred in this memo as the developer proposed channel. Memorandum_Additional Svcs_Feb201 JG.docx

2 Ashville Park Stormwater Management System Analysis Support February Page 2 BMP 10A located south of Ashville Park Boulevard and directly connected to BMP 10 (modeled as two separated storage units: BMP 10A-1 and BMP 10-A-2 which are connected with a 1050-lf channel proposed by the developer). BMP 16 proposed in future Village. Pipes shown in the developer s conceptual plans for Village which connect BMP 16 to BMP 6 (see previous study build out conditions model for additional details). BMP 1 proposed in future Village E (though a built out Village E is not modeled). 400 linear feet of 60-inch diameter pipe and 55 lf of channel connecting BMP 1 to BMP 10A-2. For this analysis the model hydrology and subcatchment delineation for proposed Village is considered the same as the build out model presented in the previous study. For all the other locations existing condition subcatchments are used to represent the model areas with a few edits to account for the portions inside the developer-proposed BMP ponds and ditch berms. Subcatchment delineation has been revised in the vicinity of BMPs 10A and 1 with a small berm around each so that precipitation received by each pond runs off directly to the pond (storage) node. Since the areas surrounding these BMPs are not anticipated to be regraded runoff will behave as it does in the existing condition. he hydrologic parameters (impervious area width and slope) are set based on engineering judgment and developer conceptual plans. he normal water surface sets the impervious area percentage within the BMP subbasins but with the steep bank slope and low infiltration the model will not be highly sensitive to impervious percentage area in these new subbasins. he hydraulic elements of Scenario 1 match the hydraulic elements in the build out condition model described in the previous study. he difference between Scenario 1 and the build out model is that built out Villages D and E are not included in the development only their hydraulic elements. Scenario 2 his scenario includes all the elements in Scenario 1 in addition to expansion of BMP 6 and construction of BMP 2A. he expansion of BMP 6 adds approximately acres of storage at a normal pool elevation of 2 feet AVD 88. he BMP is expected to be dredged at the northern edge expanding west toward ilshire Village. BMP 2A is located west of anier Village east of Princess Anne oad and south of Ashville Park Boulevard. his BMP is estimated to provide 2 acres of additional storage at a normal water surface elevation of 2 feet AVD 88. In this scenario the BMP is modeled as offline storage to the ditch extending between the Princess Anne oad culvert and the ditch draining to existing BMP 2. Flow Memorandum_Additional Svcs_Feb201 JG.docx

3 Ashville Park Stormwater Management System Analysis Support February Page is diverted to the BMP using a weir structure and a short ditch that outfalls to the BMP. Since the ditch is at a higher elevation than the normal water surface elevation in BMP 2 the normal water surface elevation in BMP 2A would be regulated by water table elevations. In the previous study BMP 2A was connected to the anier Village primary stormwater management system and therefore regulated as the rest of the system. It is beyond the scope of this analysis to determine the operating normal water surface of this BMP since a groundwater model would likely be necessary. For this scenario an initial elevation of.1 ft AVD 88 was used to match existing nearby lake levels. Scenario his scenario includes all elements of Scenario 1 as well as a 1-cfs pump station and gated control structure between BMP 14 and Flanagan s ane. his flood control pump station was set to operate throughout the design storm simulation and may be used to draw down the upstream ponds from the normal water surface elevation of 2 feet AVD 88 to a level of 1 foot AVD 88 prior to the storm. his draw down is not simulated in the model; however model nodes upstream of the pump have initial depths set at 1 foot AVD 88. he gated control structure is used to maintain water levels in the subdivision under normal conditions. For the design storm simulation the structure may only operate when the stage in BMP 14 is greater than the stage in the Flanagan s ane ditch at which point the pump station flows increase. Scenario 4 his scenario includes all the elements in Scenario 2 with the addition of the 1 cfs pump station and gated control structure as described in Scenario. he initial depth in BMP 2A is set to an elevation of feet AVD 88 since the pump station drawdown cannot directly affect this BMP under this configuration. It is unknown how the groundwater will respond to 24 to 48 hours of drawdown in the nearby lakes. Scenario 5 his scenario includes all the elements in Scenario 4 with the removal of BMP 1 and the associated hydraulic elements connecting it to BMP 10A (as described in Scenario 1). he subcatchment delineation has been revised to direct the runoff that flowed to BMP 1 in the previous four scenarios to the existing node ASH_88 (similar to existing condition models described in the previous study). Analysis esults An overview of the results for each scenario are provided in the sections below. he tables included in Attachment 2 provide detailed model results. able 1-1 provides critical elevation peak stage delta from the critical elevation and duration for existing conditions and the five scenarios evaluated. he five locations included in the table provide representative results for flood durations expected in the ilshire and anier Villages. ables 2-2 and -1 in Attachment 2 provide a comprehensive listing of peak stages and the difference between existing conditions and each scenario throughout the hydraulic model. Memorandum_Additional Svcs_Feb201 JG.docx

4 Ashville Park Stormwater Management System Analysis Support February Page 4 Scenarios 1 and 2 For Scenario 1 there are very slight increases in peak stage in some nodes over the existing condition. he increases are minimal and generally within the range of 0.01 to 0.02 ft. Flood durations above road flowline during the 10-year storm are slightly lower for Scenario 1 as compared to the existing condition but flood durations above road crown are similar to the existing condition for the 100-year storm. Analysis of Scenario 2 shows a reduction in peak stage increases in many nodes as compared to Scenario 1; however there remain a few spots at 0.01 ft increase. For the 10-year storm the flood durations above the road flowlines continue to shorten in Scenario 2 but for the 100-year storm flood durations increase for at least one of the locations. his could be attributed to the bottleneck being downstream of the additional storage included in this scenario. he 100-year storm takes over 24 hours to drain. As storage is increased between the ilshire and anier Villages and the outfall there is more volume to drain out of the subdivision outfall before the neighborhoods can eventually drain. Scenarios and 4 Both these scenarios significantly reduce peak flood stages in ilshire and anier Villages. However both scenarios also increase stages as compared to the existing condition offsite in ditches along Flanagan s ane and Sandbridge oad. Generally these increased stages are maintained within the ditch banks; however the increase does encroach onto Sandbridge oad at its intersection with ew Bridge oad. he durations of flooding above road flowline for the 10- year storm and the road crown for the 100-year storm are significantly reduced in this alternative. he 10-year flood durations have been reduced from half a day to 1 day in the existing condition to less than an hour at some intersections for both the scenarios. For the 100-year storm the flood durations are reduced from over 6 hours to under 20 hours. Scenario 5 his scenario provides results similar to Scenario 4 with the exception of longer flood durations above road crown for the 100-year storm. he results suggest BMP 1 is not critical to the Scenario 4 result. he removal of BMP 1 provides a result between the expected result from Scenario and the additional storage from BMP 2A and expansion of BMP 6 in Scenario 4. cc:.j. Bodnar ity of Virginia Beach Shanda Davenport ity of Virginia Beach Memorandum_Additional Svcs_Feb201 JG.docx

5 AAHME 1

6 FU Ü Y OS AY egend BE AU X E BU E Y Planned Storage AY Planned Ditch Parcel MI K H AM PIO Flood ocation (Attachment 2: able 1-1 &1-2) Proposed Improvements I EI " 8 2 ID GE D Pump Station ontrol Structure Stormwater Pipe Dredge Existing Ditch Ditch BMP Addition or Expansion Village 5 15 D PI ES D PEPPEI D E SA QUIY AY MOEIO D PESO D " 14 AS HV I EP A K B VD 10A 11 EAMA Existing eir D GADO OOP AG A S SD FA ainier Village 1 6 HAIG BIGHOOD 5 O U 4 2 AY MA 1 16 EMEIA D ASH VI E PA K B SADB ID GE D AMAIO UB AO ilshire Village 4 KEO KI D BY HE K KI AY Existing Stormwater Pipe PS O AES SEAB OAD ri d g Ash vi lle B US Fish and ildlife Property e r eek 1 D Elbow Farms Property E 1 in = 00 ft Feet B DG I E D Ashville Park Scenario 1 ity of Virginia Beach February 201

7 Ü egend BE AU X E BU BMP 6 Expansion: approx. footprint at normal pool elv. E Existing Stormwater Pipe Y Planned Storage AY Planned Ditch Parcel MI K H AM PIO Flood ocation (Attachment 2: able 1-1 &1-2) Proposed Improvements I PS EI " 8 2 ID GE D ontrol Structure Stormwater Pipe Dredge Existing Ditch Ditch BMP Addition or Expansion 5 PESO D PI ES D PEPPEI D E SA QUIY AY AY MA SADB ID 10 1 " 14 AS HV I EP A K B VD 10A 11 EAMA Existing eir 12 9 AG A S 15 D 18 ' 'Φ D GADO OOP ainier Village 1 6 SD 5 FA MOEIO D " ASH VI E PA K B HAIG BIGHOOD 16 EMEIA D O U 4 2 GE D AMAIO 1 Gated ontrol Structure Village ilshire Village 4 KEO KI D UB AO BY HE K KI Pump Station SEAB OAD D ri d g Ash vi lle B US Fish and ildlife Property e r eek 1 BMP 2A: approx. footprint at normal pool elv. Elbow Farms Property E 1 in = 00 ft Feet B DG I E D Ashville Park Scenario 2 ity of Virginia Beach February 201

8 FU E X Ü BE A BU E U Y AY egend Existing Stormwater Pipe Y Planned Storage AY Planned Ditch Parcel M IK Flood ocation (Attachment 2: able 1-1 &1-2) Proposed Improvements PS EI D 16 6 B VD 1 cfs Pump Station 15 D PESO D 9 QUIY AY ainier Village PEPPEI D AS HV I EP A K B VD 10A PS " Gated ontrol Structure EAMA 1 Ash vi lle B 1 AYMA e r eek 5 ri d g 5 AG A S K B FA SD ASHV IE PA GADO OOP Ditch GE D EMEIA D HAIG MOEIO D O U KE OK I K 4 2 AMAIO UB AO BEEIA D 1 Dredge Existing Ditch Village 4 ilshire Village SADB ID D BY HE I SHI ADEA I Stormwater Pipe E A MA AMAI O ontrol Structure BMP Addition or Expansion I EP A K GE D AS HV ID 8 US Fish and ildlife Property PIES S AE D KI I O 2 " Pump Station Elbow Farms Property E 1 in = 00 ft Feet B DG I E D Ashville Park Scenario ity of Virginia Beach February 201

9 Ü egend BE AU X E BU E Planned Storage AY MI K H AM PIO PS " 8 Pump Station ontrol Structure Stormwater Pipe Dredge Existing Ditch Ditch BMP Addition or Expansion SADB ID 6 15 D PESO D PI ES D 1 PEPPEI D E SA QUIY AY ainier Village AY MA AG A S cfs Pump Station 10 1 PS " 14 AS HV I EP A K B VD 10A EAMA 5 5 SD GADO OOP MOEIO D " FA D ASH VI E PA K B HAIG BIGHOOD 16 EMEIA D O U 4 2 GE D AMAIO 1 Gated ontrol Structure Village ilshire Village 4 KEO KI D UB AO BY HE K GE D Flood ocation (Attachment 2: able 1-1 &1-2) Proposed Improvements I 2 ID Planned Ditch Parcel BMP 6 Expansion: approx. footprint at normal pool elv. KI Y EI Existing Stormwater Pipe 11 Gated ontrol Structure D lle B BMP 2A: approx. footprint at normal pool elv. Ash vi SEAB OAD ri d g US Fish and ildlife Property e r eek 1 Elbow Farms Property E 1 in = 00 ft Feet B DG I E D Ashville Park Scenario 4 ity of Virginia Beach February 201

10 Ü egend BE AU X E BU E Planned Storage AY MI K H AM PIO PS " 8 Stormwater Pipe Dredge Existing Ditch Ditch BMP Addition or Expansion SADB ID PI ES D 1 PEPPEI D E SA QUIY AY ainier Village AY MA 10 1 PS " 14 AS HV I EP A K B VD 11 10A Gated ontrol Structure EAMA US Fish and ildlife Property Ash vi lle B BMP 2A: approx. footprint at normal pool elv. e r eek 15 D AG A S GADO OOP 1 cfs Pump Station SD 5 5 FA MOEIO D " PESO D ASH VI E PA K B HAIG D 16 EMEIA D O U 4 2 GE D AMAIO Village BIGHOOD Gated ontrol Structure K 4 ri d g ilshire Village 1 D ontrol Structure KEO KI D UB AO BY HE SEAB OAD Pump Station GE D Flood ocation (Attachment 2: able 1-1 &1-2) Proposed Improvements I 2 ID Planned Ditch Parcel BMP 6 Expansion: approx. footprint at normal pool elv. KI Y EI Existing Stormwater Pipe Elbow Farms Property E 1 in = 00 ft Feet B DG I E D Ashville Park Scenario 5 ity of Virginia Beach February 201

11 AAHME 2

12 able 1-1 Selected Flood s for the 100-year design storm ritical Elevations Exsiting (ft AVD) ocation Model ode rown rown (100-yr) (ft AVD) (1) Quincy ay & Pepperlin Dr * (2) Kittridge Dr oop () hanning n & Grandon oop A * * * (4) ilshire Dr & Blythe Dr * * * (5) BMP BMP_5 5.5** * * * * he stage has not receeded below the critical elevation by the end of the simulation ** owest Indicator Elevation Upstream BMP (ft AVD) Scenario 1 rown (ft AVD) Scenario 2 rown (ft AVD) Scenario rown (ft AVD) Scenario 4 rown (ft AVD) Scenario 5 rown able 1-2 Selected Flood s for the 10-year design storm ocation Model ode Flow ine (10- yr) (1) Quincy ay & Pepperlin Dr (2) Kittridge Dr oop () hanning n & Grandon oop A (4) ilshire Dr & Blythe Dr (5) BMP BMP_5 5.0** ** owest Indicator Elevation Upstream BMP ritical Elevations (ft AVD) (ft AVD) Existing Flow ine (ft AVD) Scenario 1 Flow ine (ft AVD) Scenario 2 Scenario Scenario 4 Flow ine (ft AVD) Flow ine (ft AVD) Flow ine (ft AVD) Scenario 5 Flow ine

13 able 2-1 Summary of Scenario Simulation esults ocation Model ode ritical Elevations (ft AVD) (ft AVD) Flowline Scenario 1 (ft AVD) rown (ft AVD) Flowline Scenario 2 Scenario Scenario 4 Scenario 5 (ft AVD) rown Flowline (10-yr) rown (100-yr) Quincy ay & Pepperlin Dr Kittridge Dr oop hanning n & Grandon oop A ilshire Dr & Blythe Dr ubao Dr & Aldea ir amarillo ane orthwest oop erramar n east of Pepperlin Blythe Dr & Aldea ir ubao Dr west of BMP est Quincy y & Grandon oop 0910-A East Quincy y & Grandon oop erramar n & Pepperlin Dr Grandon oop & Pepperlin Dr 0910-A18B Bencia Dr east of Keokirk n est hanning n B Montecito Dr 0910-A14B est Grandon oop d A Brighwood Dr & Ashville Park Bvld 0910-A ubao Dr north of Kittridge Dr Brighwood Dr & Grandon oop 0910-A4A Ashville Park Blvd & ubao Dr 0910-A Ashville Park Blvd south of BMP amarillo n east of Keokirk n amarillo ane ortheast oop (ft AVD) Flowline (ft AVD) rown (ft AVD) Flowline (ft AVD) rown (ft AVD) Flowline (ft AVD) rown

14 able -1 Model esults Existing onditions Model ode (ft AVD) (ft AVD) Scenario 1 onditions (ft AVD) (ft AVD) Scenario 2 onditions (ft AVD) (ft AVD) Scenario onditions Scenario 4 onditions Scenario 5 onditions A ASH_ A OF B J J J J os S9_A_18_S J J D (ft AVD) (ft AVD) (ft AVD) (ft AVD) (ft AVD)

15 able -1 Model esults Existing onditions Model ode (ft AVD) (ft AVD) Scenario 1 onditions (ft AVD) (ft AVD) Scenario 2 onditions (ft AVD) Scenario 4 onditions (ft AVD) Scenario onditions (ft AVD) (ft AVD) (ft AVD) (ft AVD) Scenario 5 onditions D F01A A A A A A A A A A A A A ASH_ A ASH_ D (ft AVD)

16 able -1 Model esults Existing onditions Model ode (ft AVD) (ft AVD) Scenario 1 onditions (ft AVD) (ft AVD) Scenario 2 onditions (ft AVD) Scenario 4 onditions (ft AVD) Scenario onditions (ft AVD) (ft AVD) (ft AVD) (ft AVD) Scenario 5 onditions A A A A11A (ft AVD)

17 able -1 Model esults Existing onditions Model ode (ft AVD) (ft AVD) Scenario 1 onditions (ft AVD) (ft AVD) Scenario 2 onditions (ft AVD) Scenario 4 onditions (ft AVD) Scenario onditions (ft AVD) (ft AVD) (ft AVD) (ft AVD) Scenario 5 onditions I_ A E E A18B A A A10A A1A A A A A A08A A0D (ft AVD)

18 able -1 Model esults Existing onditions Model ode (ft AVD) (ft AVD) Scenario 1 onditions (ft AVD) (ft AVD) Scenario 2 onditions (ft AVD) Scenario 4 onditions (ft AVD) Scenario onditions (ft AVD) (ft AVD) (ft AVD) (ft AVD) Scenario 5 onditions E A14B A15B A1B A06A A A12B A A05A A A A4A E A0A E A A A E E A B02A B0A BMP2_ BMP2_ BMP2_ E A B A BMP1_ BMP1_ (ft AVD)

19 able -1 Model esults Existing onditions Model ode (ft AVD) (ft AVD) Scenario 1 onditions (ft AVD) (ft AVD) Scenario 2 onditions (ft AVD) Scenario 4 onditions (ft AVD) Scenario onditions (ft AVD) (ft AVD) (ft AVD) (ft AVD) Scenario 5 onditions BMP1_ BMP1_ E E E E BMP BMP BMP D0A E E BMP4_ BMP4_ BMP4_ D02B BMP_ BMP_ BMP_ BMP_ BMP_ (ft AVD)