concept design san antonio river improvements project

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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 126 SAN ANTONIO RIVER DESIGN GUIDELINES FLUVIAL GEOMORPHOLOGY DESIGN CRITERIA Prepared by Biohabitats, Inc. January 2001 The two primary functions of a stream or river are to convey the water and sediment supplied to it by its drainage basin. Because water is usually the dominant factor of the two and because water discharge is easier and more accurate to measure than sediment discharge, most of our knowledge of fluvial systems deals with water discharge. (A good way to point-out this fact is that whenever we talk about “discharge” of a stream it is always implied that it is water discharge.) Many investigators have shown that most natural channels are sized to carry a discharge associated with frequent, relatively small storm events rather than large, infrequent storm flows. These frequent, channel-forming discharges are often referred to as the bankfull discharge because the water surface elevation at this discharge is at the top of bank. The importance of the bankfull discharge was expressed by Dunne and Leopold (1978): “The bankfull stage corresponds to the discharge at which channel maintenance is the most effective, that is, the discharge at which moving sediment, forming or removing bars, forming or changing bends and meanders, and generally doing work that results in the average morphologic characteristics of channels.” On average, bankfull discharge has a recurrence interval of 1.0 - 1.5 years on a flood frequency analysis. Because the bankfull discharge is associated with channel formation/ channel maintenance, many morphological parameters and channel stability indices are expressed as a function of bankfull width. Therefore, the first step in testing the applicability of creating a stable, natural channel in the San Antonio River Corps of Engineer’s flood control channel, using fluvial geomorphological principles, was to determine the bankfull discharge. On a natural channel, one of the first means of determining bankfull discharge would be to measure the bankfull cross section and slope in the field and compute the bankfull discharge utilizing Manning’s Equation. Because the San Antonio River had been modified for flood control, this process was not possible. Without an existing bankfull channel to indicate bankfull stage, the next best approximation would be to use the 1.5 year discharge. The 1.5 year discharges were obtained utilizing the Army Corps of Engineers, preliminary hydrology model for the San Antonio River Basin, dated May 29, 1997. Downstream of the confluence with San Pedro Creek, the 1.5 year peak discharge was computed at 12,700 cfs. Before this bankfull discharge can be utilized to compute stable geometry and pattern, the stream type must first be determined. From historic photographs, field investigation of the natural channel upstream and downstream of the flood control channel, and existing channel slope measurements the San Antonio River should be restored to a “C” type stream according to the Rosgen Stream Classification System. Type “C” streams have low slopes (.001 - .02), (The flood control channel downstream of the confluence with San Pedro Creek has a slope of .002.) high sinuosity (>1.2), moderate to high width/depth ratio (>12) and are only slightly entrenched (entrenchment ratio >2.2). The high entrenchment ratio (floodprone width/bankfull width) is probably the most important parameter to achieve for a “C” channel and the least probable for the San Antonio River flood control channel. This is because a “C” stream requires a broad floodplain while a flood control channel eliminates the need for a floodplain. To create a “C” stream with the minimum requirements (which would then call into question the stability of the channel) below the confluence with San Pedro Creek, would require a bankfull width of 150 feet and a floodprone width of 340 feet. This is wider than the existing flood control channel in this area and would only convey the 25-year storm. Therefore the channel would have to be even wider to convey the 100-year discharge. Also, restoring natural meanders in the flood control channel would be difficult. Not only would the meanders require more space than the straight channel, they would also require a larger channel. This is because meanders reduce flow velocities. The lower velocities mean more area is needed to convey the same discharge. Due to the extensive amount of land needed outside the existing right-of-way and the large quantity of excavation, it was apparent that fully restoring the San Antonio River flood control channel to a stable, natural river was not feasible. The question then became, “what can be done to partially restore the flood control channel to enhance stability, habitat, and aesthetics?” The immediate response to this question is to add vegetation to the system. Native vegetation that is adapted to growing along river banks provides erosion protection through root system which binds and reinforces the soil and through leaf and stem system which reduces velocities and provides a physical barrier to water erosion. Vegetation enhances habitat by shading the water, providing aquatic and riparian refugia, and supplying food sources to both aquatic and riparian wildlife. The appearance of the river will change dramatically simply by the addition of woody vegetation on the banks. Streams and rivers naturally maintain a vegetated riparian zone that people associate with streams. Just as the vegetation along the River Walk is essential to its popularity, so too will a vegetated riparian zone within the flood control channel immediately transform it into a river. The problem with adding woody vegetation to a flood control channel is that the vegetation increases roughness, which increases storm flow water surface elevations. In order to maintain existing water surface elevations and add vegetation the cross sectional area of the channel would have to be increased. It was decided that any modifications to the flood control channel should be designed utilizing two different scenarios. The first – keep all modifications within the existing right-of-way. The second – modifications can be outside of the existing right-of-way only where feasible and reasonable. Because the right-of-way is very close to the top of bank of the flood control channel, only minimal changes to the channel could be made and therefore only minimal amounts of vegetation could be used. Vegetation would have to be limited to native grasses and shrubs.
Trees would increase the roughness beyond what could be compensated for in the cross section modifications. Cross section modifications would be limited to creating floodprone terraces (which could double as equipment access) and meandering the base flow pilot channel within the flood control channel. The outside of the meanders would be stabilized with 5 – 6’ tall boulder revetments which would also allow the meanders to be graded into the channel banks rather than just to the toe of slope. A property ownership search indicated that a lot of land along the San Antonio River was publicly owned. This meant that it should be feasible to grade outside of the existing right-of-way in many locations along the river. Although it appeared that there was ample opportunity to widen the existing channel, a natural channel based on bankfull discharge was still not feasible. But could some intermediate size channel be constructed in the widened flood control channel. Due to the large storm flows experienced by the San Antonio River, especially downstream of the confluence with San Pedro Creek, it was decided that any channel smaller than a bankfull channel would not be stable. Therefore it would be best, in terms of stability and maintenance, to design only for the base flow pilot channel, but to design a plan form for the base flow channel that would mimic the natural bankfull channel. Also, the flood control channel should be widened as much as reasonable to allow for the establishment of a more natural vegetated riparian buffer along the base flow channel. The numerous, tight meanders of the old San Antonio River would not be stable under the current, highly urbanized storm flows (as indicated by the meander cutoffs taking place in the channel downstream of the flood control channel). Without a good reference reach in the area it was decided that the geometry would be based on empirical formulas generated from various studies of streams and rivers around the country. Specifically, the meander radius should be 2.3 times the bankfull width and the meander wavelength should be 11 times the bankfull width. With a computed minimum bankfull width of 150 feet, the range for meander radii was computed as 400 to 700 feet and the range for meander wavelength was 1500 to 2000 feet. While none of the streams and rivers in the area were deemed suitable for reference reaches (Salado Creek, Cibilo Creek, Guadalupe River, San Antonio River), they did provide insight into the type of features which should be designed into the improved flood control channel. Many of the outside meanders were stabilized by bedrock outcrops, therefore it would be appropriate to stabilize the base flow channel meanders with boulder revetments. The pools were very long, often running through two or three meander wavelengths. The riffles were very short, often created by bedrock outcrops. The base flow channel should have very long pools with boulder/concrete drop structures to lose grade and provide bed control. These long pools can be widened in places to provide the “large water” effect and the drop structures can be designed as canoe chutes. The inside meanders had no or only small point bars and were well vegetated. The inside meanders appeared to be an integral part of the floodprone area rather than a depositional feature. This should work well with the enhanced flood control channel because while the base flow channel will meander the storm flows will still have a fairly straight flow path through the existing flood control channel alignment. This means the inside meanders should be well vegetated (even if it is only native grasses to keep roughness low) to provide a stable floodprone area. The banks and floodplains of these streams were also well vegetated. The banks usually contained species such as willows, sycamores, and cedars. The width/depth ratio will be the same as the existing flood control channel, except in a few locations where the channel will be widened. In those areas the width/ depth ratio will be larger. A cross section between Mission Road and S. Roosevelt Street was used to calculate width/depth ratio. Using a bankfull discharge of 12,700cfs the width/depth ratio is about 25 and the entrenchment ratio is about 1.3. No sinuosity range was used to design the baseflow channel; however, the sinuosity range works out to be 1.2 – 1.4. Bed material of the existing channel was not measured. This would be difficult with all the stabilization material lining the pilot channel. The majority of the material observed in the channel was sand sized, although some gravel and cobble exists downstream of the project area. With a width/depth ratio of 25, an entrenchment ratio of 1.3, a slope of 0.2%, and a sand substrate the channel would classify as an F. This makes sense because the flood control channel is over-widened and entrenched by design to carry the 100-year discharge. The base flow meander wavelength was computed using a national average of 11 times bankfull width. The range of meander wavelengths for the baseflow channel is 1000 – 1500 feet. Channel slope vs. valley slope will be the same as the existing flood control channel. Because the flood control channel is fairly straight the channel slope must be very close to the valley slope. The channel slope used for computations (which was measured from the HEC- RAS geometry below the confluence with San Pedro Creek) is 0.2%. No belt width was used for the baseflow channel except to fit it into the space available. In most areas the belt width is about 300 feet but it is as large as 500 feet in a few areas. The radius of curvature used to design the baseflow channel ranges from 400 feet to 700 feet. This range was obtained from using a national average of 2.3 times the bankfull width. My recommendation for where to monitor bedload would be as many locations as possible; however, a compromise to this would be to monitor at Alamo Street, Lone Star Boulevard, E. Theo Boulevard, S. Roosevelt Street, SE Military Drive, and Ashley Road. Existing grades and alignments at bridges will be maintained to keep flow through these structures as efficient as possible. The velocities and shear stresses to design bank armoring should be decided by SARA. If the armoring must withstand 100-year velocities and shear stresses then those are the shear stresses that should be used by the design team. If SARA decides that the 25-year velocities and shear stresses are all they want to design to then those are the shear stresses that should be used by the design team. Whichever year is designed to, those velocities and shear stresses can be computed using the HEC-RAS model, designing to the 100-year forces. SAN ANTONIO RIVER DESIGN GUIDELINES 127
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SAN ANTONIO RIVER IMPROVEMENTS PROJ
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EXECUTIVE SUMMARY INTRODUCTION AND
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PROJECT BACKGROUND Within downtown
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INTRODUCTION AND PURPOSE 8 SAN ANTO
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INTRODUCTION AND PURPOSE PROJECT AR
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PROJECT OVERVIEW 12 SAN ANTONIO RIV
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PROJECT OVERVIEW EXISTING CONDITION
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PROJECT OVERVIEW ECONOMIC DEVELOPME
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PROJECT OVERVIEW 18 SAN ANTONIO RIV
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PROJECT OVERVIEW INVENTORY PROCESS
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HISTORICAL MISSION (SOUTHERN) REACH
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MUSEUM (NORTHERN) “URBAN” REACH
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