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Little Mill Creek Flood Risk Management

Background

Little Mill Creek, which is located in the northern portion of New Castle County, Delaware, has seen recurrent flooding events over the past 100 years.  These flooding events, specifically September 1971, January 1978, and July 1989, resulted in several Federal and Private watershed studies of the areas surrounding Little Mill Creek. 

The most recent Federal study, a Section 205 Flood Control Feasibility Study, was completed by the United States Army Corps of Engineers (USACE) in 1995.  This study proposed significant channel improvements for two separate portions of Little Mill Creek.  The channel improvements for the upper portion, which extended from the Kirkwood Highway Bridge to the CSX Wilsmere Railroad Yard Bridge, were completed in 2007. The lower portion investigated within the 1995 USACE Feasibility Study extends from approximately the Maryland Avenue (Route 4) Bridge to the Amtrak Railroad Bridge.  

The proposed project for the lower Little Mill Creek is a flood risk management project utilizing channel modification to reduce flood stages in Little Mill Creek.

The majority of the flood damage potential can be attributed to heavy development on the flood plain. New commercial development in the lower portion has also encroached on the flood plain. In addition, numerous bridges and culverts obstruct flood flows. These structures are prone to clogging with debris, and often cause backwater due to their inability to pass the flood flows. As a result, flood waters pond upstream of these structures and overflow the streambanks. Flood damages in the lower portion occur extensively to the many commercial establishments and few residences that line the creek in these reaches.

Project Description

The plan for the lower portion involves the modification of the Little Mill Creek channel located near Elsmere, New Castle County, Delaware.

The channel modification plan for the lower portion was modified considerably from the plan as presented in the 1995 feasibility study with a reduction in affected length from a total length of 6,225 feet to a total of 2,100 feet. This reduction is mainly due to the elimination of the hydraulic dredging portion that was part of the originally proposed plan that extended from the Amtrak Railroad bridge downstream to the Penn Central Conrail Railroad bridge (below 1-95 crossing).

The plan involves widening of only the right bank (facing downstream) from approximately 381 feet downstream of the Maryland Avenue Bridge (DE State Route 4) downstream for a length of approximately 500 feet. This segment does not include any deepening of the channel or widening along the left bank. Riprap will be placed along the right bank toe for toe protection and it will extend up the right bank slope to approximately 6 feet above the channel bottom. Channel modification will resume further downstream where the channel will be widened to 30 feet and deepened by 3 feet, which includes clearing and widening of both channel banks for a length of approximately 1,670 feet (approximately 100 ft. upstream of the Amtrak railroad bridge). Channel widening is variable with a maximum widening of the channel by about 17 feet with an average channel bottom width of39 feet (existing average width is 36 ft.). At the upstream end of this segment, riprap will be placed from the toes of both banks to 6-feet above the channel bottom for a distance of approximately 100 feet.

Approximately 30,000 cubic yards of channel bottom and bank materials would be excavated from the project area, transported, and disposed of at the U.S. Army Corps of Engineers Wilmington Harbor South Disposal Areas. This area is currently used for the disposal of dredged material from the existing Christina River and Delaware River navigation channels.