Downtown Shall Not Flood Again
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.
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.
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 Shockoe Bottom to handle the 2-yr storm event, identify system constraints and provide recommendations for improvement.