Lake Shawnee Spillway and Outlet Channel Repairs Preserve Historic Site
For those in
and around Topeka, KS, Lake Shawnee is a wonderful place to enjoy time outdoors.
This recreational lake has been a vital part of the community for nearly 70
years, allowing people to fish, swim, ski, and sail. The area surrounding the
lake includes walking trails, campsites, botanical gardens, shelter houses,
baseball/softball diamonds, and a golf course. The area hosts popular annual
events each year, such as the Great Topeka Duck Race, the Topeka Tinman
Triathlon, the Spirit of Kansas Fourth of July Celebrations, the Winter
Wonderland Celebration of Lights, and more. The 410-acre Lake Shawnee was built
as a Works Progress Administration (WPA) project, started in 1935 and completed
in 1939. More than 5,000 fishermen celebrated opening day.
Along the north
end of the lake lies 2,200 feet of earthen dam. Three drop structures control
discharge before it flows under an arched culvert bridge. The first structure is
a 200-foot-long control weir with a 2-foot drop. The next structure is a
200-foot curved weir with a 6-foot drop. The main spillway is a 120-foot curved
weir with a 20-foot drop into a stilling basin. The outlet channel was lined
with hand-placed stone that still exists along most of the banks.
Only one major
repair project has been completed since the lake opened. Years of channel
erosion had exposed the main spillway footing, and in the late 1970s an apron
and retaining wall were added 7 feet below the main spillway stilling basin.
In 2004, a dam
inspection report indicated that a majority of the protective armoring of the
outlet channel and side slopes near the main spillway had washed away. This
produced scour around the weir pylons and adjoining wingwalls to the point that
the wingwall footings were exposed. Fissures in the underlying bedrock were
seeping, and bank stability was in question. Major renovations became necessary
after a significant storm event in October 2005 caused more channel erosion to
occur, exposing the downstream bridge footing.
|The goal was to design a wall that minimized excavation, adequately sized to prevent overturning and sliding in wet conditions, optimizing construction time and having a stacked-stone appearance.
Engineering Consultants P.A. (PEC) was entrusted with providing civil
engineering design services for this project. Along with the Federal Emergency
Management Agency (FEMA), the state of Kansas and Shawnee County provided the
necessary funds for the renovation work. The most notable change was the
construction of observation areas on the downstream side of the main spillway.
During large rainfall events, the resulting waterfall over the spillway
attracted many spectators. Steep banks and slippery conditions created a safety
risk for the County Parks and Recreation Department. PEC engineers worked with
the county to provide a safe viewing area and to mitigate erosion at the
spillway. We knew the observations areas would be a gathering point and wanted
an aesthetically pleasing look to fit the site.
The banks are a
combination of sandstone and shale, and PEC engineers did not want a contractor
to cut into the bank more than necessary, especially near the existing exposed
wingwall and pylon footings. Site conditions limited the size of equipment
available to do the work—large equipment could not be used at the top of the
bank because of safety concerns, and smaller equipment didn’t have the reach
capabilities. This limitation forced the contractor, EmCon Construction, to work
from within the channel.
Our goals were
to design a wall that minimized excavation, one that was adequately sized to
prevent overturning and sliding in wet conditions, that optimized construction
time, and that had a stacked stone appearance. We decided that a hybrid
wall—combining the aesthetics of a segmental wall with the strength of
reinforced concrete—would best accomplish these goals.
blocks were specified for this project because the limestone-texture block
allowed us to design the wall with a castle-type look that we couldn’t achieve
with smaller segmental wall systems. Drilled piers were used to contend with
sliding forces. After footings were constructed, the large block units were
stacked as a 12-foot-tall gravity wall that became the front formwork for a
poured-in-place “concrete backfill” wall. This could not have been achieved
using smaller units. A rebar cage was constructed behind the Redi-Rock wall and
tied into the Redi-Rock blocks’ lifting eyes.
forms were erected to form the back face. Concrete was poured in two lifts, and,
when combined with the Redi-Rock units, created a solid mass for the bottom
portion of the wall to counteract overturning. The void behind the wall was
backfilled with aggregate and a drainage system to collect water migrating
through the rock strata.
portion of the wall was designed as a typical geogrid-reinforced retaining wall
using the mass below as a footing. This reduced the length and amount of geogrid
that would have been required for a full-height modular wall. After the wall was
capped with a concrete slab, Redi-Rock’s freestanding wall units and column
units were used to anchor a metal railing. Provisions were put in place to add
light fixtures to the tops of columns in the future. The completed wall
resembles stacked limestone, which blends well with the surrounding environment.
The resulting twin observation decks allow visitors an even closer view of the
spillway than before construction, and we achieved the effect we wanted.
March 2007, the wall was tested when a storm event approaching the 100-year
level came through the area. The renovations prevented what could have been a
catastrophic failure. Instead, the storm provided a spectacular show. In the
spring of 2008, the Parks and Recreation Department added a sidewalk connecting
the observation area to the walking trail and added another Redi-Rock wall just
off the east observation area to provide a picnic area. Thanks to these
renovations, residents will be able to enjoy the natural beauty of Lake Shawnee
for generations to come.
Author's Bio: S. Earl Tast, P.E., was the lead designer for Professional Engineering Consultants P.A. of Topeka, KS, on the Lake Shawnee dam