DOWNTOWN SHALL NOT FLOOD AGAIN

Transcription

1 DOWNTOWN SHALL NOT FLOOD AGAIN Robert C. Steidel**, Robert Stone**, Lin Liang*, Edward J. Cronin*, Federico E. Maisch* *Greeley and Hansen LLC 2116 W Laburnum Avenue, Suite 100 Richmond, Virginia ABSTRACT ** Department of Public Utilities, City of Richmond, Virginia The City of Richmond, Virginia conducted a study to evaluate local drainage system in its downtown business district to remedy conditions that cause flooding more than once in a twoyear period. The main goals of this study are to evaluate the capacity of the storm water collection system in the downtown area to handle the 2-year storm event, identify system constraints and provide recommendations for improvement. The study area, Shockoe Bottom watershed, is a 65-acre sub-basin located within the City s largest combined sewer watershed, Shockoe Creek watershed, which has an existing land area of approximately 8,000 acres. Extensive hydrologic and hydraulic modeling and analyses on the existing Shockoe Bottom drainage system, including the Dock Street Pumping Station, the Northeast Interceptor, and the stormwater inlets in the Shockoe Bottom watershed and in adjacent watersheds, were conducted. Field investigations included an inventory of the number and size of the existing stormwater inlets, and connectivity of roof rain leaders. The study showed that the storm water ponding in the downtown low-lying areas may be caused by: (1) Flows discharging into the Shockoe Box Sewer exceed the capacity of Dock Street Pumping Station. This is caused by: a) Excess flows transferred into the Shockoe Box Sewer from the Northeast Interceptor watershed and, b) Flow transferred from the Shockoe Arch Sewer into the Shockoe Box Sewer through existing gates between the Northeast Interceptor and the Shockoe Arch Sewer. (2) Excess overland runoff flow, which exceeds the capture capacity of the existing storm water catchments. This excess overland flow is from the downtown watershed itself and from the adjacent watershed to the east. Based on these findings, a series of capital improvement projects were identified and evaluated to minimize the nuisance flooding in the downtown area. Benefit-to-cost analyses showed that the Northeast Interceptor improvements, installation of more stormwater inlets near the low points susceptible to flooding, as well as modification of Dock Street pumping station operation are the most cost effective measures. KEYWORDS Flood, CSO, drainage 3769

2 INTRODUCTION The City of Richmond is located at the falls of the James River. The older portion of the City is served by a combined sewer system (CSS) that comprises about 12,000 acres or 30% of the City s total area. There are currently 29 CSO outfalls identified in City s VPDES permit, most of which are located along the James River and its tributary, Gillies Creek. The largest basin in the system is the Shockoe Creek combined sewer area, which is approximately 8,000 acres or about 2/3 of the CSO system. The downtown Shockoe Bottom area is a 65-acre watershed tributary to the Shockoe Box sewer, as shown on Figure 1. Figure 1 - CSO Areas at Richmond, Virginia Shockoe Bottom Shockoe CSO Area Other CSO Area James River Being the lowest point on the north side of James River, the Shockoe Bottom area is subject to flood from both the James River and interior rainfall event. In 1994, the City completed the flood wall project which protects Shockoe Bottom from the river flooding. Since then, business and development have quickly thrived in the bottom area. However, it has been observed that the low-lying areas in Shockoe Bottom, such as Pine Alley, Walnut Alley and the Farmers Market, have experienced local flooding more than once in a two-year period (see Figure 2). The purpose of this study was to evaluate the capacity of the storm water collection system in the 3770

3 Shockoe Bottom to handle the 2-yr storm event, identify system constraints and provide recommendations for improvement. Figure 2 Local Flooding in Shockoe Bottom Area Walnut Alley Pine Alley Farmer's Market 17th St I-95 (Above) Cary St 18th St James River City Dock Flood Wall Main St Grace St 19th St 20th St Franklin St Broad St Marshall St 21th St DRAINAGE SYSTEM EVALUATION Shockoe Watershed History The original combined sewer system in the Shockoe watershed, initially built in late 1800 s, carried the sanitary sewage and runoff from storms to the James River through three natural creeks Bacon s Quarter Branch, Cannon s Branch and Shockoe Creek. During mid-1920 s through early 1930 s, the City completed a series of flood control projects for the Shockoe watershed. The major components of the project, as shown on Figure 3, include: (1) the Shockoe Arch pressure conduit that conveys runoff from the upper portions of the Shockoe watershed to the river; (2) a 17- by 12-foot concrete box (Shockoe Box) sewer with a pumping station (Byrd Street Pumping Station) and upper and lower gate houses, which removed storm water runoff and sanitary wastewater from the low-lying areas (Shockoe Bottom) not drained by the pressure conduit; (3) a 63-inch sewer along 19th Street and a flow regulator at 19th Street and Main Street diverting the majority of the wet weather flow in CSO 034 and CSO 035 to the City Dock, which reduced the drainage area of Shockoe Bottom; (4) Cannon Branch and 3771

4 Bacon s Quarter Branch combined sewers, which collected the combined sewage from upper portion of Shockoe Creek Watershed areas to Shockoe Arch Sewer. Figure 3 - Shockoe Watershed Improvements in Mid-1920 s and Early 1930 s Shockoe Watershed (8,000 Acres, including Area Tributary to Box) Cannon s Branch Bacon s Quarter Branch Shockoe Creek Shockoe Bottom Tributary to Box Sewer (65 Acres) Shockoe Arch James River CSO 034 & 035: WWF redirect to City Dock. DWF conveyed to Box Sewer 63 Concrete Pipe Shockoe Box In mid-1950s, the City began the construction of the Wastewater Treatment Plant (WWTP) to provide primary treatment and to treat sludge collected in the primary facilities. Beginning the late 1950s through the early 1960s, the City constructed the interceptor system to convey the sanitary sewage and portion of the combined sewage to the WWTP. The 96-inch Shockoe Creek Interceptor and the twin 66-inch City Dock River Crossing were constructed during this period, as shown on Figure 4. In late-1970 to early-1980 s the City s Shockoe Retention Facilities were installed to capture the first flush from the Shockoe Creek combined sewer system. These facilities included the diversion structures, the retention basin, and flap gate connections to the Shockoe Box installed on storm drains located in Pine Alley, Creek Alley and Walnut Alley. (See Figure 4). Flooding from the James River continued to be a threat for the City until the U.S. Army Corps of Engineers (Corps) built the floodwall, which was designed in the 1980 s and completed in the fall of The floodwall project relocated storm sewers from the Shockoe Arch sewer to the Shockoe Box sewer to limit the discharge and prevent over pressurizing the Arch sewer during river floods. 3772

5 East gravity outlet (EGO) was constructed just east of the East Diversion Structure to convey all storm water runoff which is not carried by the other conduits through the floodwall. The EGO was designed to reduce ponding up to the 100-year storm. West Interceptor was constructed to collect storm water runoff from the existing railroad yard north of Marshall Street, from a large inlet in the Main Street Station parking lot, and from new and existing separated storm drains west of Arch Sewer and convey it through the floodwall to the James River. Northeast Interceptor was constructed to collect existing combined sewers which are east of the Arch Sewer and which flow to the Arch Sewer and convey them to the existing Box Sewer at just downstream of the Upper Gate House. This was designed to isolate them from the existing Arch Sewer during high river stages, to prevent flooding within the pressurization areas, and to prevent damage to the existing combined sewers from the internal pressure. The large combined sewers retain a gated connection to the Arch Sewer, which allows most flows to continue to flow to the Arch Sewer and allows the design of Northeast Interceptor to be based on a design storm during high river stages, yielding a much lower design flow and hence smaller pipe size. Other improvements associated with the floodwall project included, upgrading the Upper and Lower Gate Houses, the demolition of the Byrd Street Pumping Station, and construction of the new Dock Street Pumping Station. Figure 4 shows the existing Shockoe Watershed flood control and interior drainage system. Figure 4 - Shockoe Watershed Flood Control and Interior Drainage System (Present) 3773

6 Shockoe Watershed (8,000 Acres, including Area Tributary to Box) Cannon s Branch Shockoe Creek James River Bacon s Quarter Branch Shockoe Arch West Interceptor Diversion Structures Shockoe Retention Basin WWTP Area Tributary To NE Interceptor (Main Portion Flows To Arch Sewer) NE Interceptor Shockoe Bottom Tributary to Box Sewer (65 Acres) Shockoe Box Two 66 River Crossing (Siphon) Operation of Shockoe Retention Facilities Shockoe Retention Facilities is the most critical component in the Shockoe Watershed drainage system. It can be operated in two modes: pollution prevention mode which is the normal operational mode, or flood protection mode which is exercised during less frequent interior rain event (25-year or higher). In this paper, we only address the normal operation mode. Figure 5 shows the normal operation during dry and wet weather conditions. Figure 5 Normal Operation of Shockoe Retention Facilities 3774

7 SHOCKOE ARCH SEWER WEST DOWNTOWN INTERCEPTOR CANAL SHOCKOE DIVERSION STRUCTURES SLUICE GATES OPEN BASCULE GATE P NORTHEAST INTECEPTOR SHOCKOE BOX SEWER DOCK ST PS DOCK STREET DROP INLET CITY DOCK FLOODWALL GATES OPEN DRY WEATHER DIVERSION CONDUITS SHOCKOE RETENTION BASIN GATES 50 MG (Including System Storage) CLOSED CROSSOVER CHAMBER SHOCKOE INTERCEPTOR JAMES RIVER SLUICE GATE ROLLER GATE FLAP GATE DRY WEATHER FLOW GILLIES CREEK INTERCEPTOR TWIN 66 RIVER CROSSING SHOCKOE COMBINED SEWER OUTFALL STRUCTURE EAST GRAVITY OUTLET TO WWTP SHOCKOE ARCH SEWER WEST DOWNTOWN INTERCEPTOR CANAL SHOCKOE DIVERSION STRUCTURES SLUICE GATES OPEN CLOSED BASCULE GATE P NORTHEAST INTECEPTOR SHOCKOE BOX SEWER DOCK ST PS DOCK STREET DROP INLET CITY DOCK FLOODWALL GATES CLOSED CROSSOVER CHAMBER GATES OPEN DIVERSION CONDUITS WET WEATHER SHOCKOE RETENTION BASIN 50 MG (Including System Storage) SLUICE GATE ROLLER GATE FLAP GATE WET DRY WEATHER FLOW GILLIES CREEK INTERCEPTOR SHOCKOE INTERCEPTOR TWIN 66 JAMES RIVER RIVER CROSSING SHOCKOE COMBINED SEWER OUTFALL STRUCTURE EAST GRAVITY OUTLET TO WWTP Normal dry weather flow from Shockoe Creek CSO area will flow through the Shockoe Arch Sewer and the Shockoe Diversion Structures (SDS) to the 96 Shockoe Creek Interceptor, and ultimately to the WWTP for treatment. During wet weather events, the combined sewer flows coming down through the Shockoe Arch and Box sewers will build up in the SDS. At the same time, the WWTP will pump the water out of the system up to its wet weather treatment capacity (75 MGD). When the water level in the SDS approaches the crest elevation of the Bascule Gates (El +16.0), the roller gates will open and allow the flow into the SRB through three Diversion Conduits. Once the basin is filled, the roller gates close and excessive flow will spill over the Bascule Gates, then over the Overflow Weir and discharge into the James River. After the storm event, the WWTP will keep pumping at 75 MGD until the SDS and SRB are emptied. 3775

8 In the normal operation mode, both the upper and lower gates are closed to isolate Shockoe Box from the pressure conduit Shockoe Arch, and the combined wastewater in the Shockoe Box sewer is pumped around the lower gate house to the Diversion Structure by Dock Street Pumping Station, which has a firm capacity of 134 cubic feet per second (cfs). Operation of Shockoe Box Sewer Figure 6 shows the drainage system in the 65-acre Shockoe Bottom area. The Shockoe Box receives flow from combined trunk sewers as well as street curb inlets connected directly to it. The low-lying spots such as Pine Alley and Walnut Alley have flap gate connections to the Shockoe Box to prevent the combined sewage in the Box sewer from backing up to these areas. Figure 6 Shockoe Bottom Drainage System Diversion Structures Lower Gate House James.River Floodwall Cary St Main St Dock St PS Pine Box Sewer Alley Arch Sewer Flap Gated Connections Upper Gate House 17 th St 18 th St I-95 Northeast Interceptor I-95 City Dock Area Tributary To Box Sewer Figure 7 shows the hydraulic profile of the Box Sewer from the East Diversion Structure s crest gate through the drop inlet in Pine Alley. The drop inlet has a flap gate to prevent the water from exiting the Box Sewer and discharging into the alley. The top of the grating at Pine Alley drop inlet is about elevation 14.0 ft, which is below the normal crest gate elevation of 16.0 ft. To the north of the drop inlet, there is an underground parking garage. The low point of the ramp to this parking garage is about elevation 14.3 ft. If the water elevation in the Box Sewer is greater than the water elevation in Pine Alley, the flap gate will close. Thus the only way to remove the storm water in Pine Alley is to operate the Dock Street Pumping Station. As shown in Figure 7, during low intensity rainfall event, the Dock Street Pumping Station is able to control the water level in Box Sewer at set point of 12.5 ft, so the storm water runoff in Pine Alley will drain into the Box Sewer. However, during higher intensity rainfall (greater than 3 inch per hour for 10 to 15 minutes), the Dock Street Pumping Station has not been able to control the level in the Box Sewer, and the excess flow may be relieved through the six flap gates located in the Combination Flood Control Roller Gate at the Lower Gate House (Figure 8). Once the water elevation in the 3776

9 Box Sewer exceeds about 14.0 ft, the flap gate will close, and the storm water runoff from the parking lot west of Pine Alley plus the roof drains will discharge to the alley and eventually flow into the underground parking garage. Figure 7 Operation of Shockoe Box Sewer During Low Intensity Rainfall 60 Dock St Pumping Station Controls WSEL at 12.5 feet Elevation in Feet Bascule Gate East Diversion Structure Lower Gate House Bar Rack Flap Gates Flood Closed Roller Gate Underground Parking Garage Entrance 18 Flap Gate Shockoe Box Sewer Pine Alley Drop Inlet -20 Dock St. Pumping Station -30 Figure 8 Operation of Shockoe Box Sewer During Higher Intensity Rainfall 3777

10 Elevation in Feet Bascule Gate East Diversion Structure Bar Rack Flap Gates Open Lower Gate House Flood Roller Gate Runoff Exceeds Capacity of Dock St Pumping Station Underground Parking Garage Fills with Rain Water 18 Flap Gate Closed Shockoe Box Sewer Pine Alley Drop Inlet Dock St. Pumping Station SHOCKOE BOTTOM WATERSHED HYDRAULICS Based on the understanding of the existing Shockoe system, the storm water ponding in the lowlying areas in Shockoe Bottom may be caused by the following: Excess overland runoff flow, which exceeds the capture capacity of the existing storm water catchments. This excess overland flow is from the Shockoe Bottom watershed itself and from the adjacent watershed to the east. Flows discharging into the Box Sewer exceed the capacity of Dock Street Pumping Station. This is caused by: o Excess flows transferred into the Box Sewer from the Northeast Interceptor watershed, and, o Flow transferred from the Arch Sewer through existing gates between the Northeast Interceptor and the Arch Sewer. Hydraulic Analyses of Existing Storm Water Inlets A field investigation was conducted on the stormwater inlets in both Shockoe Bottom drainage area and adjacent areas that may contribute overland flow to the bottom, as shown on Figure 9. There are a total of 84 stormwater inlets identified within the 65-acre Shockoe Box Sewer watershed, of which 58 are within the watershed tributary to Farmer s Market and Walnut Alley. Thirty (30) curb inlets were identified in the 32-acre adjacent watershed east of Shockoe Bottom. There are about 74 curb inlets identified outside of the Shockoe Bottom and the 32-acre adjacent watersheds. Over 95% of these inlets are curb-opening inlets, and the rest are grate inlets and grate-curb opening combination inlets. It was found that more than 50% of the curb inlets in the bottom and adjacent area are smaller than 4-ft. Given the steep topography and high imperviousness, the number and size of existing curb inlets in the Shockoe Bottom and adjacent watersheds may not be adequate to capture a 2-year stormwater runoff. 3778

11 Figure 9 Existing Storm Water Inlets in Shockoe Bottom and Adjacent Watersheds Diversion Structures Lower Gate House I-95 (Above) Dock St Pump Station Cary St Main St Upper Gate House Franklin St 17th St 18th St 19th St Box Sewer Grace St Broad St Marshall St Arch Sewer Northeast Interceptor Box Box Sewer Sewer Watershed City Dock 20th St 21st St 22nd St 23rd St Adjacent Watershed Figure 10 and Figure 11 show simplified hydrographs for Shockoe Bottom watershed and adjacent watershed for various storm events. 3779

12 Figure 10 Hydrograph for Existing Shockoe Bottom Watershed Runoff (cfs) Time (min) 1-yr Event 2-yr Event 5-yr Event 10-yr Event 25-yr Event\ Figure 11 Hydrograph for Existing Watershed Adjacent to Shockoe Bottom Runoff (cfs) Excess Runoff Transferred to Shockoe Bottom Estimated Capacity (1) of Existing Catchments In Adjacent Watershed East of Shockoe Bottom Time (min) 1-yr Event 2-yr Event 5-yr Event 10-yr Event 25-yr Event Note: (1) Assumes all catchments are clean 3780

13 Hydraulic Analyses of Northeast Interceptor Figure 12 shows the delineation of Northeast Interceptor watersheds. There are seven sub-basins tributary to the Northeast Interceptor. Three combined trunk sewers (Marshall Street, Washington Street, and Williams Street) are also connected to the Arch Sewer through gated structures (Gate 11, Gate 14 and Gate 15, respectively). As described above, the NE Interceptor and gated structures were designed to isolate the combined trunk sewers from the Arch Sewer during high river stages (when the bascule gates at Shockoe Diversion Structures are lowered, and the Upper and Lower Gates on the Box Sewer are open). Under normal conditions, these gates are open to allow most of the flow from these sub-basins to drain into the Arch Sewer. Figure 12 Northeast Interceptor Watersheds Martin Luther King Bridge G15 G14 Williams St Sewer Washington St Sewer Arch Sewer Northeast Interceptor Upper Gate House I-95 G11 Marshall St Broad St Venable St Box Sewer Shockoe Bottom Watershed CS Pipes Connected to Arch & NE Interceptor CS Pipes Connected to NE Interceptor SD Pipes Connected to NE Interceptor Extensive hydraulic analyses were conducted for the Northeast Interceptor and its sub-basins. Figure 13 shows the 2-yr storm hydraulic profile of Northeast Interceptor at low water level in Arch Sewer (shown as dashed lines). The majority of the runoffs from Williams Street, 3781

14 Washington Street and Marshall Street will flow into the Arch with free discharge. When the Arch level is low, the flow into the Box Sewer through the Northeast Interceptor is approximately cfs. Figure 13 Hydraulic Profile of Northeast Interceptor at Low Water Level in Arch Sewer 60 cfs to 70 cfs 60 cfs to 70 cfs 66 cfs 17 cfs 17 cfs When the Arch Sewer is surcharged and the gates are open, flow may be transferred from the Arch Sewer to the Northeast Interceptor, and discharged into the Box Sewer. Figure 14 shows the hydraulic profile in the Arch Sewer at various flow rates, and Figure 15 shows the hydraulic profiles of Northeast Interceptor given an Arch flow of 7000 cfs under low water level (blue line) and surcharged water level (green line) in the Box Sewer. 3782

15 Figure 14 Hydraulic Profile of Arch Sewer at Various Flow Rates 7,800 cfs 7,000 cfs 6,000 cfs 5,000 cfs Figure 15 Hydraulic Profile of Northeast Interceptor at Arch Flow of 7,000 cfs Figure16 shows the flow transferred from Arch to Box through Northeast Interceptor as a function of Arch flow rate over the bascule gate (at elevation 16.0). The invert of Northeast Interceptor at the Marshall Street gate is only 17.72, which allows flow to transfer even during low intensity rainfall event. 3783

16 Figure 16 Flow Transfer from Arch Sewer to Box Sewer Arch Flow over Bascule Gate (cfs) 9,000 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0 Box WSEL at 19.5 feet Free Discharge Into Box Sewer NE Interceptor Invert at Flow Transfer from Arch to Box (cfs) Figure 17 shows the hydrograph of flow transfer from Arch to Northeast Interceptor derived from once per summer hydrograph for Arch Sewer. Figure 17 Hydrograph of Flow Transferred from Arch Sewer to Box Sewer Shockoe Arch Runoff (cfs) 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0 Arch Sewer Once per Summer Hydrograph NE Interceptor Hydrograph Time (min) Arch Flow Transferred to NE Interceptor A composite hydrograph was developed for Shockoe Bottom using the individual hydrographs for Shockoe Bottom watershed itself, flow from the CSO 034 dry weather regulator at 19 th and 3784

17 Main Streets, runoff transferred from adjacent watershed, and transfer flow from Arch to Box via Northeast Interceptor, as illustrated on Figure 18 for 1-year storm event. Figure 18 Composite Shockoe Bottom Hydrograph for 1-Year Storm Event Composite Hydrograph 250 Runoff (cfs) Arch Transfer Flow to Box Via NE Interceptor Shockoe Bottom Transferred from CSO 034 DWF Regulator Time (min) Runoff Transferred from Adjacent Watershed Figure 19 shows how the existing system would perform under a 1-year storm event with hydrograph shown on Figure 18. This example assumes that the rainfall starts in the Shockoe Bottom at about the same time as the rainfall starts in the entire Shockoe watershed. Flow from the Northeast Interceptor and CSO 034 may surcharge the Box Sewer, discharge through the combination roller/flap gates in Lower Gate House, and into Farmers Market. 3785

18 Figure 19 Performance of Existing System for 1-Year Storm Event Box Volume of 3.2 MG at Elevation 16.0 Runoff (cfs) Box Volume of 2.5 MG at Elevation Volume Captured by Dock Street Pumping Station Excess Flow Not All Volume Pumped May Surcharge Box Sewer & Discharge through Combination Roller/Flap Gate in Lower Gate House May Surcharge into Farmers Market Dock St PS at 134 cfs Time (min) FLOOD CONTROL ALTERNATIVES EVALUATIONS Potential improvement projects for flood controls in the Shockoe Bottom area were evaluated for three design conditions: Normal condition interior rainfall only River flooding River flooding with interior rainfall In this paper, only the alternatives under normal operational conditions will be discussed. Adjacent Watershed Improvements The adjacent watershed improvements would minimize excess overland flow from this area to the Shockoe Bottom low-lying areas such as Farmers Market and Walnut Alley. The improvement projects may be implemented in 2 phases. Phase 1 project will add new large cub inlets near 19 th Street at the boundary of adjacent watershed and Shockoe Bottom watershed, along 15 th Street, and on Main Street between 14 th Street and 15 Street. These new curb inlets would intercept as much bypass flow from upstream as possible, but may still pass some excess flow to the bottom area under the 2-year storm event. Phase 2 would address the entire adjacent watershed and install new curb inlets throughout the watershed to capture the 2-year runoff. The existing curb inlets will be rehabilitated during the implementation of the improvement projects. Figure 20 shows the project area, and summarizes the design features, preliminary cost, advantages and disadvantages of this alternative. Figure 21 shows the design criteria for the curb inlets in the adjacent watershed. 3786

19 Figure 20 Adjacent Watershed Improvements 19th St 20th St Features: Rehab existing inlets Part 1: New curb inlets in adjacent watershed along 19 th Street, 15 th Street, and Main Street Between 14 th and 15 th St. Part 2: Remainder of New curb inlets in adjacent watershed to capture 2-yr storm Cary St Main St 21st St 22nd St 23rd St Grace St Franklin St Box Box Sewer Sewer Watershed Broad St Marshall St Preliminary Estimated Costs: Capital Cost Part 1 = $ 550,000 Part 2 = $1,180,000 O&M Cost Part 1 = $5,000/yr Part 2 = $9,000/yr Annual Cost Part 1 = $ 53,000/yr Part 2 = $112,000/yr Advantage: Potential to Implement in Phases Reduce excess overland flow from adjacent watershed to Shockoe Bottom Disadvantages: Increased number of catchments to maintain Existing Curb Inlets Proposed New Curb Inlets - Part 1 Proposed New Curb Inlets Part 2 Figure 21 Curb Inlets Design in the Adjacent Watershed 200 Runoff (cfs) Excess Runoff Transferred to Shockoe Bottom Design Catchments in Adjacent Watershed to Capture 2-yr Event Time (min) 1-yr Event 2-yr Event 5-yr Event 10-yr Event 25-yr Event 3787

20 Shockoe Bottom Watershed Drainage Improvements The goal of these improvements is to keep the water off the street and minimize excess runoff to Farmers Market and Pine Alley. The potential projects may be implemented in 3 phases. Phase 1 focuses on drainage improvements in Farmers Market and Pine Alley. New large curb inlets may be installed near and in Farmers Market and Walnut Alley to capture flows greater than 2-year event. In Pine Alley and Walnut Alley, new drop inlets with flap gate connection to the Box may be installed or the existing drop inlets may be enlarged. The rain leaders from the buildings in Pine Alley will be hard piped to the Box Sewer to reduce surface runoff. Phase 2 will install new large curb inlets in Shockoe Bottom along elevation Phase 3 would address the remainder area of Shockoe Bottom with new curb inlets installed to capture 100% of 2-year runoff. Figure 22 and Figure 23 summarize the improvement projects and storm water inlet design criteria. Figure 22 Shockoe Bottom Watershed Improvements Cary St Main St Existing Curb Inlets Proposed New Curb Inlets Part 1 Proposed New Curb Inlets Part 2 Proposed New Curb Inlets Part 3 Franklin St Grace St Arch Sewer Box Sewer Broad St Northeast Interceptor 17th St 18th St 19th St Box Box Sewer Sewer Watershed Marshall St Features: Rehab existing inlets Part 1: Drainage improvements in Farmers Market & Pine Alley Part 2: New curb inlets in Shockoe Bottom watershed above elevation 22.0 Part 3: Remainder of New curb inlets in Shockoe Bottom watershed to capture 2-yr storm Preliminary Estimated Costs: Total Capital Cost Part 1 = $344,000 Part 2 = $220,000 Part 3 = $680,000 O&M Cost Part 1 = $14,000/yr Part 2 = $ 2,000/yr Part 3 = $ 4,000/yr Annual Cost Part 1 = $43,000/yr Part 2 = $22,000/yr Part 3 = $64,000/yr Advantage: Potential to Implement in Phases Minimizes Excess Runoff to Farmers Market Disadvantages: Increased number of catchments to maintain 3788

21 Figure 23 Curb Inlets Design in the Shockoe Bottom Watershed Design Catchments in Farmers Market to capture flows greater than 2-yr Event Runoff (cfs) Design Catchments above Elevation 22.0 In Shockoe Bottom to Capture 2-yr Event Dock St PS Firm Capacity At 134 cfs Time (min) 1-yr Event 2-yr Event 5-yr Event 10-yr Event 25-yr Event Northeast Interceptor Improvements As discussed above, the majority of excess flow into the Box Sewer under normal condition (interior rainfall only) appears to be from the Northeast Interceptor watershed and the flow transferred from the Arch Sewer. It is proposed that gate structures be installed on the Northeast Interceptor to control the flow transfer between the Arch Sewer and the Northeast Interceptor, as shown on Figure 24. The improvements include three new gate structures: 1) on the Washington Street combined sewer near the existing Gate G14 two new sluice gates (G14b and G14c) may be installed and are normally closed so that the flow from Washington Street watershed is directed to the Arch Sewer; 2) on the Marshall Street combined sewer near the existing gate G11; and 3) just upstream of the new gate structure on Marshall Street to control the flow from Clay and Venable Street watershed. The existing manhole covers on the new gate structure influent sewers would be bolted down. Gate G15 would need to be raised to handle surcharged Arch Sewer hydraulic grade line. During normal conditions, all gates on the Northeast Interceptor would be closed and all gates connecting to the Arch Sewer would be open, which allows the Dock Street Pumping Station to pump only the runoff tributary to the Shockoe Bottom watershed plus small amount of runoff from the Broad Street watershed and CSO 034. During high river stage, all gates on Northeast Interceptor would be open, and the gates connecting to Arch Sewer would be closed. Figure 24 also shows the flood trigger elevation for the operation of the gates. 3789

22 Figure 24 Northeast Interceptor Improvements Broad St G11b Marshall St G11a Clay St G11d Venable St G11c Northeast Interceptor 17th St 18th St Martin Luther King Bridge Washington Street Sewer Arch Sewer G14a G14b Williams Street Sewer I-95 G14c G15 Features: Three New Gate Structures Five New Sluice Gates One Connection to Arch Sewer One Manhole Bolt Down MH Covers Raise Gate 15 Structure Preliminary Estimated Costs: Capital Cost: $902,000 O&M Cost: $18,000/yr Annual Cost: $97,000/yr Advantage: Potential to Implement in Phases Prevents transfer of water from Arch Sewer to Box Sewer Allows Dock St PS to pump runoff tributary only to Shockoe Bottom Disadvantages: More sluice gates to maintain & operate during flood events Description G11a G11b G11c G11d G14a G14b G14c G15 Legend Old Gate No. G11 New New New G14 New New G15 Existing Gate Structure Gate Size 5x5 5x5 5x5 5x5 2x3 4x4 4x4 7.5x7.125 New Gate Structure Normal Position Flood Position O C C O O C C O O C C O C O O C Gate Normally Open Gate Normally Closed Flood Trigger Elev Modification of Operation of Dock Street Pumping Station Figure 25 shows the composite Shockoe Bottom watershed hydrograph after the improvements to the Northeast Interceptor and adjacent watershed. Figure 26 shows that approximately 1.8 MG of storage would be needed to capture 10-yr to 25-yr event peak flow after the improvements to the Northeast Interceptor and adjacent watershed. Currently the set point of the Dock Street Pumping Station is at elevation 12.5, which would allow Pine Alley to drain into the Box Sewer. Figure 27 shows that by dropping the pumping set point to elevation 5.3, there will be additional 1.8 MG storage available in the Box Sewer, which can be used to capture the 10-yr to 25-yr storm event peak without causing flooding in Pine Alley. 3790

23 Figure 25 Shockoe Bottom Watershed Hydrograph after Improvements to Northeast Interceptor and Adjacent Watershed 450 Runoff (cfs) Hydrographs after improvements to NE Interceptor & adjacent watershed Dock St PS Firm Capacity At 134 cfs Time (min) 1-yr Event 2-yr Event 5-yr Event 10-yr Event 25-yr Event Figure 26 Storage Needed in Box Sewer to Capture 10-yr to 25-yr Event 450 Runoff (cfs) Approximately 1.8 MG of Storage Needed to Capture 10-yr to 25-yr Event Peak Volume Captured by Dock Street Pumping Station Hydrograph after improvements to NE Interceptor & adjacent watershed Time (min) Dock St PS Firm Capacity At 134 cfs 3791

24 Figure 27 Storage Needed in Box Sewer to Capture 10-yr to 25-yr Event Shockoe Box Storage in MG Potential Operating Range: Approximately 1.8 MG of Storage Needed to Capture 10-yr to 25-yr Storm Event Peak 0.5 Wet Well Set Point at Elevation in feet Box Level of 12.5 Pine Alley Grating at Elevation 14.0 New Pumping Station Near Lower Gate House New pumping station with firm capacity of 120 cfs can be constructed around the Lower Gate House to compensate the inadequate capacity at Dock Street Pumping Station, as shown on Figure 28. This pumping station can provide additional CSO storage volume in the Box Sewer, and the diesel-powered pumps can serve as back-up for Dock Street Pumping Station. 3792

25 Figure 28 Potential New Pumping Station Near Lower Gate House Site Plan Diversion Structures Floodwall CSXT Railroad New Pumping Station NS Railroad RMA (Above) Dock St Lower Gate House I-95 (Above) 15 th St Arch Sewer CSXT Railroad New Gate Connections Dock St Cary St Flap Gate Creek Alley Dock St Pump Station Box Sewer 17 th St Main St Flap Gate Pine Alley Features: Six Stand-by Pumps Wet Well Suction & Discharge Piping with Valves Sluice Gated Connections to Box Sewer Operated Primarily During River Floods Greater than 25- year event Preliminary Estimated Cost: Capital Cost = $ 5.0 M O&M Cost = $151,000/yr Annual Cost: $582,000/yr Advantage: Provides additional CSO Storage Volume Provides backup for Dock Street Pumping Station Disadvantages: More complex operation & maintenance than other alternatives Stand-by Alley Pumping Station A stand-by pumping station may also be installed to collect and pump the ponding water in Pine, Creek and Walnut Alleys to the Arch Sewer, as shown on Figure 29. This alternative does not solve the main watershed issues and may not be needed if other improvements are implemented. In addition, it will not prevent flooding in Farmers Market if the Box Sewer is surcharged (water can not enter the Box Sewer and will flow overland to the low-lying Farmers Market area). 3793

26 Figure 29 Stand-by Alley Pumping Station I-95 (Above) Arch Sewer Dock St Wet Well 24 SD from Creek Alley Inv Elev -6.0 Box Sewer Cary St Connect to Arch Sewer Two Pumps in Enclosure 17 th St Grade Elev Existing Drop Inlet Box Sewer 18 Pump Discharge 2-48 SD (See Section at Box) 36 Storm Drain Main St Section at Box Sewer Main St Station Drop Inlet Curb Inlet 36 SD from Farmers Market 24 SD from Pine Alley Two 48 Pipes Franklin St Features: Hard Pipe Rain Leaders to Box Curb & Drop Inlets 36 & 48 Storm Drains 10 Dia Wet Well Two Stand-by Pumps (Each: feet) Pump Enclosure Suction & Discharge Valves Connection to Arch Sewer Preliminary Estimated Costs: Capital Cost: $ 4.2 M O&M Cost: $ 84,000/yr Annual Cost: $ 451,000/yr Advantage: Minimizes Flooding in Shockoe Bottom by pumping storm runoff to Arch Sewer Disadvantages: Will not prevent flooding in Farmer s Market if Box Surcharges. May not be needed if other improvements implemented Operating remote stand-by pumping station has lower longterm reliability compared to other alternatives FINDINGS AND RECOMMENDATIONS The potential improvement components evaluated above are grouped into 7 projects and planned to be implemented in the next 5 years. The project descriptions, the results of the benefit-to-cost analysis, as well as corresponding project rankings are summarized in Table 1. The benefits of the projects are estimated based on real estate value, business revenue and city revenue, and damage prevented in the Shockoe Bottom area. Projects SBD-1 through SBD-6 address the sources of flooding in the Shockoe Bottom and thus are recommended for phased implementation in the Fiscal Years shown in Table 1. Projects SBD-4 and SBD-5 (stormwater inlet improvements) contribute to a higher level of flood protection and could be further divided, into smaller projects, to match the funding availability, as needed. Project SBD-7 was considered and exhibits the lowest cost-to-benefit ratio. This project may be implemented as a partial solution for Pine Alley and Walnut Alley. However, it may not be needed after projects SBD-1 through SBD-6 are implemented. Thus Project SBD-7 is not recommended as an integral element of the overall solution for the Shockoe Bottom at this time. The most cost effective projects are the Northeast Interceptor improvements, modification of Dock Street Pumping Station operation, and storm water catchment improvements near the low-lying areas (Pine Alley, Walnut Alley and Farmers Market). Other recommendations include updating the flood wall O& M manual to change the operation of the Shockoe Box and the gates on the Northeast Interceptor, developing educational 3794

27 information to help the public better understand the system, and developing emergency plans for interior rainfall greater than 50-yr event, floodwall overtopping, and combination of James River flood with interior rainfall, such as early warning system, public notification and evacuation plan, and operational protocols of the emergency management, police, fire, public works and public utilities departments. Table 1 Project Description, Benefit-to-Cost Ratio and Project Ranking Normal: Preliminary Estimated Total Total Flood Estimate Cummulative Estimated Benefit Fiscal Project Protection of Capital Annualized Annual to Cumul Project Year ID Project Description Range Cost Cost Benefit (1) Cost Ratio Ranking SBD-1 NE Interceptor Improvements: - Washington St, Marshall St & Venable St Gate Structures Shockoe Bottom Watershed Part 1: - Connect Pine Alley Rain Leaders to Box Sewer - Add Stormwater Inlets in Farmer's Market & Walnut Alley - Modify Operation of Dock Street Pumping Station 1-yr to 2-yr $1,246,000 $140,000 $733, SBD SBD SBD-4 Adjacent Watershed Improvements Part 1: - Add Stormwater Inlets Along 19th Street, 15th Street, and Main Street Between 14th Street and 15th Street Shockoe Bottom Watershed Improvements Part 2: Add Stormwater Inlets Along El 22 Contour Adjacent Watershed Improvements Part 2: Add Remainder of Stormwater Inlets to Capture 2-yr Event 2-yr to 5-yr 2-yr to 5-yr 5-yr to 10-yr $550,000 $193,000 $733, $220,000 $215,000 $733, $1,180,000 $327,000 $733, SBD-5 Shockoe Bottom Watershed Improvements Part 3: Add Remainder of Stormwater Inlets to Capture 2-yr Event 10-yr to 25-yr $680,000 $391,000 $733, SBD-6 Lower Gate House Pumping Station 10-yr to 25-yr $4,939,000 $973,000 $733, SBD-7 Alley Pumping Station 10-yr to 25-yr $4,198,000 $1,424,000 $733, Note: (1) Total Annual Benefit includes estimated annual damages plus 50% of the City's annual revenue. Assumes that 50% of Buisnesses would leave Shockoe Bottom if flooding was not controlled to at least two year event. 3795