Reforestation of the Mission Reach of the San Antonio River
Abstract:The Mission Reach Ecosystem Restoration and Recreation (MR-ERR), part of the overall 13-mile San Antonio River Improvements Project, is addressed as an in-progress case study of an urban riparian re-forestation project. The project is a 160 million-dollar Federal, County, and City investment within urban San Antonio, Texas. This project is one of the largest civil works project within the United States Army Corps of Engineers (USACE) Fort Worth District. The task is to restore the 8-mile MR-ERR segment to natural upland and aquatic ecosystems by creating wildlife habitat in the form of vegetation, rock riffles, embayments and restored river remnants. Ironically, the San Antonio River was originally channelized by the USACE in the 1950's as a flood damage reduction project, and the ecosystem restoration project is taking place under the original congressional project authorization.
The site's pre-project condition is best described as an earthen trapezoidal ditch with an incised channel in the bottom. The channel is actively eroding, and evidence of head cutting is evident. Due to maintenance practices aimed at ensuring the channel effectively functions as a floodway, no woody vegetation is permitted within the pre-project site. The project increases the available right-of-way for the channel, enabling the overbank slopes to be reduced, the sinuosity of the river to be increased, and the re-forestation of the floodway. This must be done while maintaining the flood damage reduction benefits of the channel. The re-forested communities consist of native species combinations based upon observations of nearby communities.
A riffle-run-pool stream model is utilized in the restoration project to evaluate and balance sediment transport concerns, evaluate sinuosity and determine proposed grade line for the re-aligned river. HEC-RAS and GEO-RAS hydraulic models are utilized to evaluate flow regimes and determine acceptable plan-form geometries and proposed vegetation densities.
To effectively mathematically model the approximately 20,000 new trees proposed to be planted in the re-aligned channel, the proposed vegetation is divided up into 4 discreet densities with corresponding roughness coefficients. Generally speaking, the higher the vegetation density, the higher the habitat “value,” therefore all vegetation in the channel that is not of the highest density is a hydraulic concession.
Upon completion, the project will contain over 140 hectares of riparian habitat and over 2.7 million cubic meters of earth will be removed from the channel to offset the proposed vegetation. No negative impacts to the flood damage reduction capabilities of the river will occur.
Document Type: Research Article
Publication date: January 1, 2010
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