Page 99 - Bernardon Response to RFP-Chestnut Run Campus Master Plan

AVENUE NORTH CHANNEL RESTORATION
Channel Erosion Caused by Degraded Stormwater Outfall Pipe

As part of a Land Development Proposal, the property Owner and Developer were required LOCATION
by New Castle County to assess and correct severe ongoing erosion within an existing FAIRFAX, NEW CASTLE
drainage channel as part of the 80-acre Avenue North mixed-use redevelopment of the COUNTY, DE
former AstraZeneca campus. Landmark was charged with developing a solution. YEAR COMPLETED
This erosion was a result of long-term uncontrolled discharges during storm events from an 2019 Recordation
existing 48-inch concrete pipe outlet. The existing underground system piped stormwater 2020 Parking Garage
runoff from a ±60-acre drainage area consisting mainly of rooftops, lawns and paved 2020 Channel
surfaces. These pipes converged into the 48-inch concrete pipe that discharged up to 120 Restoration
cubic feet per second of stormwater directly into the existing earthen channel which
scoured the banks and undercut large trees over time. This 210-foot drainage channel SIZE
flowed southward through a forested area and exited the site through four 12” pipes 1,860,000 SF
randomly embedded in a concrete structure beneath a steel security fence. The concrete 960,000 SF office*
structure was severely undercut and eroded from historical stormwater surges and the (*100,000 SF new)
small pipes easily became clogged with debris. Beyond the concrete structure, the 200,000 retail
stormwater entered the existing drainage system associated with Powder Mill Road. 175,000 hotel
335 apartment units
The project involved hydraulic grade line and stormwater volume analyses, site-specific 7-story parking garage
design of concrete culvert to control site outflow, a two-tiered gabion basket weir/scour-
hole structure to control in-flow, and two J-hook rock vanes in the channel bed to direct ACRES
water flow. Our bank restoration design reduced the steep slopes of the existing channel 80
and protected them with rip-rap, geotextile fabric, live stakes, native plant landscape design COST
and seeding for long-term stabilization.
$400,000,000+ Total
The Delaware Department of Natural Resources & Environmental Control Wetlands Section Project Construction
issued a Subaqueous Lands Permit and the U.S. Army Corps of Engineers granted a $250,000+ Restoration
Nationwide Permit 13 for this project in February 2020. Compensatory mitigation was
waived by both agencies as the benefits of the project (stemming erosion and improving
downstream water quality) outweighed the minor impacts to jurisdictional waters.
Construction of the new outfall structures and restoration of the channel was completed in
August 2020.

Landmark performed all planning, topographic surveying, wetland and forest stand
delineations, environmental permitting, civil engineering, landscape design, construction
stakeout and as-built surveys required for the successful and timely completion of project.