South Christiana Sewer Basin Study & Plan
Location: New Castle County, Delaware
Client: New Castle County
JMT provided a hydraulic model for a 45 square-mile sewershed to
help minimize sewer capacity issues.
Expected impacts from the operations growth at the Aberdeen
Proving Ground in nearby Aberdeen, Maryland, led New Castle County
to accelerate the infrastructure expansion planning for this 1.75
million foot sewer basin on the County's western border. As
part of the study phase of the project, JMT was requested to
perform two evaluations in concert to address impacts from the Base
Realignment and Closure (BRAC) of Fort Monmouth, New Jersey with
relocations to Aberdeen and a full evaluation of the South
Christiana Basin. The South Christiana Sewer Basin
encompassed more than 45 square miles and included more than 40
pump stations and 8,000 manholes and is home to precious
environmental resources, including the sensitive Churchman's
Marsh.
The County asked JMT to assess the risk of SSOs due to capacity
overloading from system age, infiltration, and future growth.
The challenge of creating a physical elevation model of the sewer
network required a combined effort of field surveys, GIS analysis,
and record drawing reviews. Sewer inverts were established on
a single datum and captured in a GIS layer for importing to the
selected hydraulic modeling platform.
JMT used a sophisticated GIS Build-Out analysis as well as
results from a permanent flow monitoring network to construct a
hydraulic model to assess the expected Level of Service from the
system in a series of hypothetical development futures and storm
intensities. The Level of Service analysis enabled the County
to make important judgments about capital resource allocation,
sewer capacity management, and moratorium release for
development.
The study concluded that system capacity limitations were caused
by RDII (rain-derived inflow and infiltration) entering the sewers
and not future development. JMT developed a sanitary sewer
evaluation survey (SSES) workplan to identify the sources of inflow
and infiltration in the lower level of service sub-basins
exhibiting high RDII. The SSES program will be used by the
County to identify system defects and sources of I/I through smoke
testing, pipeline CCTV inspections, manhole inspections, and night
flow isolation weiring.
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