Downtown Flood Mitigation Project

The City of Alton’s Downtown Flood Mitigation Project

The increasing recurrence and duration of high-water events (floods) is adversely impacting the City of Alton, specifically the downtown central business district and industrial corridor by limiting pedestrian/vehicular access, creating costly business disruptions and draining local resources to address flood mitigation and recovery efforts. 2019 was the 2nd highest flood on record after 1993. Local flood mitigation and recovery costs exceeded $700,000. Alton incurred over $4,000,000 in property damages. Construction costs for temporary structures of approx. $400,000. Reflective of the recurrent nature of the problem, 4 of Alton’s Top 10 flood events on record have occurred since 2013.

The project includes flood control measures (flood wall and sewer system improvements) to protect the downtown central business district. Although described separately, the improvements are interdependent. The project scope also includes infrastructure protective measures to Chessen Lane. The proposed improvements are described in greater detail below:

Permanent Flood Wall

Currently, the City of Alton Publick Works Department monitors the Corps of Engineers predictions for Mississippi River stages and upon forecast of flooding, the City will construct a temporary flood wall to protect the downtown area. This temporary wall is approx. 1,250 feet in length and averages 5 feet in height. This temporary wall construction requires weeks of manpower to build and thousands of tons of barrier blocks and gravel to resist the forces of flood water. The project includes the construction of a new permanent reinforced concrete flood wall along Sugar Alley from William Street to Piasa Street (US 67). This wall will be approximately 1050′ long, 5′ tall, and include a grout curtain in the soil beneath the wall to limit underseepage.

Subsurface Infrastructure Improvements

As part of the flooding problems the downtown Alton area experiences, the existing concrete and clay pipe storm sewer system allows river water to back up into the pipes and free flow out of the various inlets, manholes, and catch basins located at the intersections of State/Broadway, West Third/Piasa, and west Third/Belle St. All of these intersections are behind the temporary flood wall during high water events. These structures must be sand bagged, extended in height to flood stage with concrete risers, or the sewers themselves must be blocked off with inflatable rubber plugs. Because the sewers in the area are combined with both sanitary and storm sewer flows, any plugging of the flow results in raw sewage backing into basements of the affected area. Construct valves in the combined sewer area along West 3rd Street and seal old, non-functioning connections to the Piasa Sewer. Isolate storm sewer inlets on the streets by installing shut-off valves that can be closed during high river stages. In tandem with the valve system, the project includes the construction of a permanent pump station near the 3rd Street fountain. This work will effectively isolate the gravity sewers from backup associated with rising flood waters and will allow the Downtown sanitary facilities to function during flooding events. The pump station will be activated at the same time as the combined sewer valves are closed to provide uninterrupted sewer service to the 25 businesses and 23 residential units in the project area. The permanent flood wall, sewer valve system, and pump station improvements are interdependent.

Chessen Lane

The project improvements include rebuilding Chessen Lane from the railroad tracks at the bottom of the hill south of Broadway to a point 600 feet south of the Alton Steel crossroad. The work will include re-establishing ditches and replacing culverts on both sides of the road. The pavement surface will be raised, and reconstruction will include new aggregate base course and bituminous binder and surface courses. The total length of the reconstruction is 1,500 lineal feet and the proposed width of pavement is 24 feet. Ditching cross section will typically be a 2′ wide bottom, 2′ deep with 2:1 side slope. The subject roadway is a major link to the following businesses: Alton Steel, Azcon Corporation, Eagle Tubular Products, and National Vinegar Co. In addition, it is the only vehicular access to Illinois American Water Company’s Wastewater Treatment Facility.

The proposed project seeks to protect the downtown central business district and portions of the industrial corridor. Downtown Alton has a 20-minute trade area of approx. 100,000 people. Key metrics of the project area include: 20-Min. Trade Area of approx. 100,000 people. 45 Buildings (90% occupancy) includes 23 residential units. Approx. 25 businesses and 2 public institutions in 3 block downtown central business district. 4 businesses with total employment over 330 within industrial corridor project area. Avg. Daily Traffic counts > 19K vehicles. Trade area purchasing power of $790M. Equalized Assessed Value of $2.47M ($248K property tax revenue). Est. annual payroll taxes in excess of $1.2M. Chessen Lane provides the only direct access to the wastewater treatment plant. Annual Gross Tax Revenue (sales tax, gaming, etc.) of $500K.