Page 97 - Bernardon Response to RFP-Chestnut Run Campus Master Plan

LARGE-SCALE LIFE SCIENCES CAMPUS SWM, WILMINGTON, DELAWARE

Before: Overflow discharge caused a total
loss of rock armor at the emergency

After: Successful gabion basket structure design and repair. spillway.
CHESTNUT RUN STORMWATER REPAIRS
DU PONT CHESTNUT RUN PLAZA

Chestnut Run Plaza is located in northern New Castle County, fronting on Centre Road from LOCATION
Lancaster Pike to Faulkland Road. A large stormwater pond and an emergency spillway are WILMINGTON, NEW
located in the southeasterly side of the site, near the Chestnut Run creek. Landmark was CASTLE COUNTY, DE
hired by the owner to address issues with a failing outfall structure from the stormwater
pond that had been previously repaired in place (by others). Additionally, overflow YEAR COMPLETED
discharge from the stormwater pond to an emergency spillway system had caused major 2016 Outfall Repair
erosion that required innovative design and repair. 2017 Erosion Repair
SIZE
Outfall Repair N/A
The principal outfall from an in-line stormwater facility was repaired in-place with lining
technology. Subsequently, the outfall began leaking in the vicinity of a former corroded ACRES
joint within the original outfall cribbing. It was estimated that the leak was under 14 feet of 218.7 total site
head pressure and subsidence from soil loss (piping) had been recently observed. Parcel A 218.7 AC
Landmark reviewed the alternatives and recommended a pressure-injected expanding foam Parcel B 4.0 AC
solution to the problem. Landmark, in conjunction with the contractor, developed a
protocol for injecting the expanding foam to arrest the leak and not require the pond to be OWNER
removed from service and did not require extensive excavation or exhumation of the E.I. DU PONT DE
existing infrastructure. Accordingly, the leak was arrested and pond performance and NEMOURS & COMPANY
functionality was, once again, in conformance with local and state regulations.
Outfall Erosion COST
During a heavy rainfall event, the principal stormwater pond for the facility experienced an UNDISCLOSED
overflow discharge to its emergency spillway system. Accordingly, due to the timing of the
overflow event, very little tailwater was encountered in the overflow structure tie-in. The
resulting discharge and combined elevation change created velocities that resulted in a
total loss of the rock armor at the inlet which was composed of R-4 rip-rap or equivalent.
Based on an assessment of the hydraulics made by Landmark staff during a forensic
evaluation after the event, it was recommended that Gabions and Reno Mattresses would
provide the necessary stability to withstand a similar event, if it were to occur again. Design
Plans were developed and the repair was constructed, with successful results.