Hydraulic Modeling of Stream Enhancement Methods

Transcription

1 Hydraulic Modeling of Stream Enhancement Methods Matthew J. Curry John J. Levitsky Abstract Development within watersheds increases the amounts of runoff causing stream erosion and degradation of stream habitat. The key to mitigation of these unnatural processes is understanding the existing watershed attributes to define stream morphology characteristics and how stream degradation countermeasures can improve the quality of streams. Specifically, this involves improved hydraulic analyses of stream enhancement methods such as the Rosgen Method. This paper will discuss hydraulic analyses of such enhancement methods using the U.S. Army Corps of Engineer s HEC-RAS computer model (Reference 1). HEC-RAS can be used to model simple and complex stream enhancement methods to reflect potential changes in stream morphology. This paper will discuss a stream enhancement project for the Lackawanna River in an urbanized portion of the watershed. Introduction The Lackawanna River is considered a top trout fishery in the State with a naturally reproducing brown trout population. A section of the river [Figure 1.] was selected for enhancement by the Pennsylvania Fish and Boat Commission and the U.S. Fish and Wildlife Service (USFWS) due to a lack of general trout habitat. The stream section contains little habitat diversity due to its shallow water depths, flat slope, steep channel banks, and little floodplain [Figure 2.]. These characteristics of the river s morphology can be attributed to its highly urbanized watershed. The design concept focused on creating feeding lanes for trout, spawning locations, and varying water depths to develop habitat for various size classes of brown trout using stream enhancement measures. This paper discusses the use of the U.S. Army Corps of Engineer s HEC-RAS hydraulic computer program to model stream enhancement measures compliant with the USFWS s Habitat Suitability Index for Brown Trout. Existing Morphology of the Lackawanna River The subject reach of the Lackawanna River closely represents that of a man-made, channelized stream with its gentle slope, steep channel banks, wide channel with a lack of floodplain, and relatively low normal water depths. Using Rosgen s Method (Reference 2) of stream classification, this stream represents an F type stream [Figure 3.] 1

2 Figure 1. Lackawanna River Figure 2. Lackawanna River Stream Banks 2

3 Figure 3. Rosgen s Stream Classification (Rosgen, 1996) Specifically, the Lackawanna River within the subject reach can be classified as a F3 type stream. This type of stream is defined as cobble-dominated with lesser accumulations of gravel and sand, and entrenched and deeply incised with a gentle slope. Accordingly, this type of stream can be incised in alluvial valleys resulting in the abandonment of its former floodplain. Design Applications Stream enhancement methods that were evaluated include a rock W weir and cross vane designs [Figure 4.] in accordance with Rosgen s Method. These designs involve the installations of rock W weir and cross vane construction that would create sections of deeper slow waters with faster riffles interspersed in-between. Changes in water velocity and depths encourage higher oxygen levels in the stream and greater habitat diversity throughout the seasonal uses of the stream. Installation of rock weirs in the current will vary the water depths, creating locations for varying sizes of trout. Installation of weirs and cross vanes should be carefully designed to avoid impacting the migration of fish species and recreational canoe or kayak traffic. 3

4 W Weir Plan View Cross Vane Plan View Figure 4. Schematics of Rock W Weir and Cross Vane (Rosgen, 1996) Table 1. shows expected proposed mitigation measures, or treatments for trout habitat enhancement. Accordingly, a combination of the cross vane (boulder vane) and W weir (boulder vortex) designs were selected as a proposed mitigation measures for the subject reach of the Lackawanna River due to their habitat enhancement capabilities. Table 1. Trout Habitat Treatments (Reference 3) EXPECTED BENEFITS TROUT HABITAT TREATMENTS Reduce Bank Increase Trout RIVER HABITAT Erosion Habitat Log spurs Pool, Run, Riffle Root wads Pool & Run Stumps Riffle & Run Horizontal logs Riffle Random boulders Pool, Run, Riffle Boulder clusters Run & Riffle *Boulder vanes Run & Riffle Boulder J-hook structures Upper end of pool *Boulder Vortex structures ( W weir) Run & Riffle 4

5 Hydraulic Modeling of Stream Enhancement Measures The HEC-RAS hydraulic computer model was used to model the both the rock cross vane and W weir stream enhancement methods. Cross sections were interpolated using HEC-RAS s cross section interpolation module between surveyed cross sections [Figure 5.]. Two sets of four interpolated cross sections were set at varying distances ranging from 1.5 meters (5 feet) to 3.0 meters (10 feet) between each other at each site. These cross sections were then modified to reflect the obstructions of the cross vanes and rock weirs to show the locations of high points and areas of channel excavation [Figure 6.]. This method was repeated at other sets of cross sections at specified locations along the river. Figure 5. Existing Cross Section for the Lackawanna River Figure 6. Cross Section Modified with Mitigation Measure 5

6 The heights of the top of the cross vanes and rock weirs were set with respect to the depth of bank-full conditions. For the Lackawanna River, bank-full flow was evaluated as approximately a 1.37-year frequency storm. A separate HEC-RAS run was made to determine the elevations associated with this storm event at the proposed mitigation sites. The height of the proposed cross vanes and rock weirs outside of the normal high water mark were set at the bank-full water-surface elevation. Additionally, elevations of the high points on the proposed cross vanes and rock weirs within the stream channel were set at 1/2 of the bank-full water-surface elevations. This design was implemented to allow for maximum obstruction to flow for causing backwater. Thus, deeper pools and riffles are established for habitat enhancement [Figure 5.]. Figure 5. Cross Vein and W Weir Design 6

7 References 1. U.S. Army Corps of Engineers, HEC-RAS Hydraulic Reference Manual and Applications Guide (version 3.0), January 2001, Hydrologic Engineering Center, Davis, CA. 2. Rosgen, D.L. Applied River Morphology, Wildland Hydrology, Pagosa Springs, CO, Stream Habitat Investigation, State of Colorado, Division of Wildlife. 7