31 Mar 95
complete levee design, with a "minimum facility" being the first
step. Because of several borrow areas and some natural storage
Case Study for Interior Flood Damage
located inside the levee alignment, it was believed that a
Reduction Measures, Valley Park,
minimum facility would mainly consist of gravity outlets and
existing ponding. The duration of flooding for the Meramec
River is short, with 4 to 6 days duration above flood stage for
duration of blocked drainage, it was believed that interior
facilities beyond the minimum would not be needed.
Valley Park is an incorporated community of about 4,300,
The approximate quantities of material to be removed
situated in southwestern St. Louis County, Missouri. A portion
from the potential borrow sites, as well as the amount of
of the city lies in the Meramec River floodplain, and is subject
undeveloped areas usable for ponding, were known early in the
to flooding from events rarer than about a 10-percent annual
interior analysis. The volume of the 1-percent chance flood,
chance of flooding. Valley Park is located about 22 miles
4-day-duration storm was estimated, with the resulting runoff
upstream from the mouth of the Meramec River, which empties
volume (about 200 acre-ft) filling the ponding storage.
into the Mississippi River just downstream from St. Louis. The
Consequently, it was decided to initially size the interior system
drainage area of the river at Valley Park is about 3,800 sq miles.
for this storage, using a 1-percent chance event as the design
Periodic flooding has been a problem, with significant flooding
standard. No economic incremental analysis was judged
occurring in 1915, 1945, and 1957, and lesser amounts in other
necessary for the interior analysis, because the borrow pit
years. In December 1982, the flood of record occurred.
storage would be available for any design flood and changes in
Estimated as a 1-percent chance flood at that time, it flooded
gravity outlet size(s) would be expected to show little reduction
many low-lying areas of the community with 8 to 10 ft of water.
in peak ponding stages.
In May 1983, another significant event (about a 4-percent
chance flood) resulted in widespread flooding. In the
To fully test the design and the new program, both the
mid-1980's, the St. Louis District investigated various flood
HEA and CSA methods were used. A series of 4-day-duration
mitigation projects for Valley Park and other communities along
the lower 50 miles of the Meramec. Only a levee for Valley
frequency analysis for both open and closed gravity outlet
conditions. The continuous period-of-record method (CSA) was
willing to cost-share the project. The Design Memorandum for
then applied to establish the minimum facility and to compare
the levee and accompanying interior flood control project was
against the stage-frequency relationship developed through the
completed in February 1993. Construction began in the autumn
Basic Data Requirements
Interior flood hydrology analyses are very data intensive,
The proposed levee project will protect about 461 acres of the
especially when both HEA and CSA techniques will be used.
city of Valley Park. It will protect against the 1-percent chance
The following paragraphs identify the major data needs:
event from the Meramec River, and from coincident flooding
from two tributaries: Fishpot Creek and Grand Glaize Creek.
Subareas. Five interior drainage basins were
Almost no hillside area is included within the levee alignment.
identified, based on urban storm drainage systems and
The protected interior area will be drained by six gravity outlets,
topographic contour mapping. These areas are identified as: the
with five ponding areas providing storage during blocked
Fishpot, Highway 141, Glass Plant, Simpson Lake, and Grand
gravity outlet conditions. Open channels and drainage
Glaize interior areas. The Highway 141 subarea consisted of
structures were also sized to convey the storm waters to the
two subbasins, with a diversion to the Fishpot subarea during
ponding areas. Although the interior analysis was fairly routine,
blocked outlet conditions. The other four subareas each
it was the first application of the HEC-IFH computer program
consisted of a single subbasin. Separate HEC-IFH analysis
to analyze and design an interior system. The original beta test
would be performed for each of the five subareas, with each
version of HEC-IFH was first used, with the updated versions
including gravity outlet and ponding storage. Table E-1 gives
incorporated as they became available.
pertinent data for the interior areas and Figure E-1 shows a
schematic diagram. Two-foot contour interval topographic
mapping was available for the lower 50 miles of the Meramec
from the earlier analysis.
Interior flood control analysis is an essential part of the