Following an intense
rainfall event on June 21, 1996, the City of Midland experienced
widespread flooding from both the storm system (overland)
and the sanitary system (basements). Approximately
1,500 residents responded to a questionnaire sent out by
the City claiming to have incurred flooding due to the large
3.96” in 3 hour event. This intensity has been associated
with a 200-year flooding event.
HRC was hired to
conduct a 3-phase study to do the following:
- Determine the cause
of both the storm and sanitary system flooding
- Determine the existing
capacities of both systems
- Identify alternatives
for improvement to minimize the risk of flooding if such
an event were to occur again.
HRC developed and utilized
an XPSWMM model to evaluate the three largest stormwater
basins (George Street Basin, Snake Creek Basin and Sturgeon
Creek Basin) where instances of flooding occurred.
XPSWMM was also used to develop a sanitary model of the
two largest trunkline sanitary sewers.
In
the later phases of the study, due to extremely high costs
of a record nearly 4” rainfall event, efforts were re-directed
towards addressing 25-year and 50-year events. The
design and construction of sanitary system improvements
and a 43.5 Million Gallon storage basin at the WWTP resulted
from this portion of the study. The basin allowed
the City to comply with their consent order issued by the
MDEQ, accompanied with a 24 month flow metering project.
The objective of the 2-year flow metering project
was to identify areas with excessive infiltration/inflow
and to determine if Midland's sanitary system could convey
the peak flow generated from the 50-year, 1-hour event.
Data from 11 flow monitoring devices and six (6)
rain gauges was utilized.
A Phase 4 Study was later added to address
the storm water system for a 50-year event and other specific
issues such as culvert capacities and localized instances
of flooding. As a result of this study, the City
and HRC are designing an estimated $3 Million Dollar storm
water system improvement project in the George Street Basin
to nearly double the existing trunkline sewer capacity.
