concept design san antonio river improvements project

APPENDIX
• Glossary of Fluvial Geomorphologic & Other Relevant Terms
• Economic and Market Overview
• Recommended Plant List for Restoration of Riparian Corridors
• Plant List for Use on the San Antonio River
• A Brief Study of Archaeologically Sensitive Areas
• HEC-RAS Model
• Hike and Bike Pathways
• Fluvial Geomorphology Design Criteria
• Master Cost Analysis and Gant Chart
• Naming and Dedication Rights
• Summary of Community Participation
• Brooklyn Avenue Dam
• Operations and Maintenance Requirements
118 SAN ANTONIO RIVER DESIGN GUIDELINES
planned along the river. The basic procedure was to
incorporate a 20-foot wide section cut into both bridge
abutments (sides of the bridge opening) except at South
Cross Avenue where the trail is only being planned on the
east side. Elevations of the trail through the bridges were
set between 5 and 10 feet above the river flowline elevations
depending on conditions at the respective bridges and it
was assumed that vertical walls adjacent to the trail would
be constructed to stabilize the abutments through the
selected bridge structures.
• CPS Utility
• CPS Vehicle
• MKT & Southern Pacific Railroad
• Steves Avenue
• East Mitchell Street
• East Theo Avenue
• Mission Road
• South Cross Avenue
• White Avenue
As stated above, a summary of existing versus proposed
conditions modeling output is attached.
Comparison of Existing and Proposed Conditions
A comparison of results obtained from the HEC-RAS
existing and proposed modeling is provided in the attached
modeling summary and the items described below:
1) Modeling indicates that in all locations the energy grade line
elevations for proposed conditions are below those for
existing conditions. Energy grade line elevations for
proposed conditions average about 2.2 feet below those for
existing conditions.
2) Proposed condition water surface elevations are, on
average, 1.7 feet below those for existing conditions.
3) Modeled water surface elevations for proposed conditions
are below those for existing conditions at all but 10 cross
sections out of the 124 cross section used in the modeling.
It is important to understand the reasons and/or situations
associated with these 10 instances before judging if they
represent problems or not.
First, six of the cross section locations where this occurs
have constricted cross sections in existing conditions where
flows are accelerated such that water elevations are at
undesirable conditions. Hydraulic conditions at these cross
sections are unstable with high velocities and significant
erosion potential. Additionally, the high velocities also
enable existing conditions at these locations to have a very
low water surface elevation that is impractical for
comparing to proposed conditions that has a more stable
flow profile. It is undesirable and unneeded to expect (or
require) proposed conditions to be at (or below) these
particular existing condition levels.
Second, the remaining four locations are upstream of
Ashley Road and are influenced by a somewhat undersized
bridge. At these four sections, proposed conditions call for
a significantly widened channel that introduces contraction
and expansion losses that have an affect on proposed water
surface elevations. However, proposed elevations are within
the channel for two of the four locations. It is also pointed
out again that the proposed energy grade line is below that
for existing conditions at all cross sections. In final design
modeling, it is suggested that detail evaluations be made in
the “big water” areas to better define the true limits of flow
conveyance in these areas. Expansion and contraction
losses may be too high if effective widths of the modeled
cross sections are hydraulically overstated. Overstated
widths result in excessively low velocities which, in turn,
increase expansion/contraction losses as flows are modeled
through these areas.
4) The maximum amount that the proposed energy grade
elevation and water surface elevation are below that for
existing conditions is about 6.7 feet and 7.8 feet,
respectively. This occurs just upstream of the Espada
Dam’s present location and is the result of the dam being
removed in proposed conditions. Much of this difference
that is due to removing the dam is lost as you move to the
upstream side of the S.E. Military Drive Bridge where the
energy grade and water surface elevation differences are
reduced to as low as 2.5 feet and 1.0 feet, respectively.
Conclusions
The basic conclusion that can be drawn from the HEC-RAS
modeling comparison is that proposed conditions would
provide improved 100-year flooding conditions compared to
existing flooding conditions. Further, the HEC-RAS modeling
of existing and proposed conditions reveals that it is feasible
to establish a river restoration plan by utilizing the selected
concept design that has been established for the studied
southern reach of the San Antonio River that extends from
South Alamo Street to Mission Espada. The HEC-RAS
modeling performed strongly suggests that if the concept
design guidelines and features are followed during final design,
the present level of flood protection in the study reach will be
improved or, at least, maintained.