concept design san antonio river improvements project

HISTORICAL MISSION (SOUTHERN) REACH
existing bends and lay back the banks and recontour
them where adequate land area is available. The intended
result is a more natural, meandering river corridor that
will also continue to provide effective flood control.
The cross sectional area of the flood channel shall
never be reduced but will, in fact, be enlarged in
several locations. The meandering pilot channel and
widening of the channel cross section will allow for
the construction of point bars and terraces within the
channel. However, the toe of the channel bank shall
never move further into the channel but may be
moved back as possible and as is necessary.
The river edges must be protected in locations where
storm flows have the potential to erode the banks. The
outside of the meanders will experience the highest
energy during storm flows and therefore are the most
likely locations for this erosion to occur. Erosion
control can be done with vegetation, soil bioengineering
methods that include the use of natural materials to
create an erosion control mat and natural rock
formations. The natural rock would be placed at the toe
of slopes where the highest shear stresses occur.
A few locations will require retaining walls using natural
materials where the river’s edge is tucked up against the
channel bank. These locations have been minimized and
only occur where adequate right-of-way does not exist
and the channel side slopes would exceed a 3:1 slope.
In order to plant additional trees within the flood
control channel, additional floodwater capacity must
be added. This has been proposed in areas where
adjacent land is owned by public entities. The Master
Plan indicates in several areas where the channel banks
34 SAN ANTONIO RIVER DESIGN GUIDELINES
can be carved back into the adjacent property and
flattened out to approximately a 1:5 or 1:6 slope.
Each final design team must investigate ways to spoil
the earth cut on adjacent properties, acquire adjacent
property as a spoil site, and/or identify agencies/
groups that need soil and will remove it. The goal is
to reduce the amount of soil that is hauled off to a
distant spoil site and therefore greatly reduce the
impact to the construction budget.
Profile Geometry
The San Antonio River has a very flat longitudinal
slope of about 0.2% in general but a greater slope
does exist in the northern extent of the Historical
Mission (Southern) Reach from Lone Star Boulevard
to the San Pedro Creek confluence. In this area, grade
control structures are needed to inhibit scouring of
the river bottom. The existing corrugated metal dams
will be replaced with stone grade control structures to
accomplish the same goal.
RIVER INFRASTRUCTURE
South Reach Pilot Channel
The Historical Mission (Southern) Reach has, or will
need, several systems of infrastructure to serve its
many purposes. A description of these systems
follows.
Grade control structure
Pilot Channel
The pilot channel is the relatively narrow depression
that carries the river base flow water. Currently, it is
configured in as straight of an alignment as possible in
order to move storm water quickly. However, this
configuration is not a natural one and the river, during
flood conditions and trying to reclaim a more natural
alignment, will erode its banks. The city has placed
broken concrete rubble rip-rap along the channel
edges south of the San Pedro Creek confluence in an
effort to reduce the erosion. When the channel is
realigned in accordance with the Master Plan, the riprap
can be removed and will be used as sub-structure
for channel bend armoring.
Base Flow Channel Grade Control Structures
A series of grade control structures exist in the
Historical Mission (Southern) Reach that consists of
corrugated sheet metal dams. They are in the
relatively steep sloped areas of the river to prevent
scour and incision of the channel bed. These existing
structures should be removed and relocated as shown
on the Master Plan, and be reconstructed with
indigenous materials. The new grade control
structures should be taller (2.5 to 3 feet tall) and
therefore fewer structures will be required.
Tributary outfall structure
S. E. Military Drive Bridge
S. E. Military Drive Bridge supports create an obstruction