Bentle Branch Storm Water and Stream Bank Stability Study ABSTRACT

Transcription

1 Bentle Branch Storm Water and Stream Bank Stability Study Walter Skipwith, PE 1, Jeffrey Alvarez, EIT 2 and Dr. Peter Allen, PhD 3 ABSTRACT In the summer of 2004, flooding caused 2 deaths and an estimated $20,000,000 in damages throughout southern Dallas County and surrounding areas as reported by the National Weather Service. Property owners along Bentle Branch in Cedar Hill and Duncanville became concerned about flooding and stream bank erosion that occurred as a result of the July 2004 storm. As a result, both the cities of Duncanville and Cedar Hill commissioned a storm water management/stream bank stability study of the watershed. In the study, new hydrology and hydraulic models were developed and floodplain delineations were updated. Several road and driveway crossings were found to be inundated by the 10- and 100-year flood events. Existing storm water detention facilities are located at multiple sites throughout the upper basin. These facilities were evaluated as to their effectiveness in reducing peak flood events. As a result of the study, a storm water master plan has been developed for the basin. A Regional Storm Water Detention Basin is a key component of the plan. In addition, stream stability problem areas were identified and solutions proposed in both cities. 1 Vice President, Halff Associates, Inc., 8616 Northwest Plaza Drive, Dallas, Texas 75225, Phone (214) , FAX (214) , ; wskipwith@halff.com 2 Engineer, Halff Associates, Inc., 8616 Northwest Plaza Drive, Dallas, Texas 75225, Phone (214) , FAX (214) , ; jalvarez@halff.com 3 Professor of Geology, Baylor University, Department of Geology, 500 Speight, Waco, TX , Phone (254) , Fax (254) ; Peter_Allen@baylor.edu 1

2 INTRODUCTION Bentle Branch is one of three watersheds in the City of Cedar Hill that drain to Tenmile Creek. Portions of Stewart Branch, Bee Branch, and Bentle Branch drainage basins are located within the Cedar Hill city limits and are part of the 101 square mile Tenmile Creek watershed. Bentle Branch drains approximately 3.65 square miles of the Tenmile Creek watershed from its headwaters near Beltline Road to its confluence with Tenmile Creek. This stormwater management study covers Tributaries 2, 3, and an Unnamed Tributary of Bentle Branch in addition to Bentle Branch from its headwaters to just upstream of Joe Wilson Road. Bentle Branch is an attractive stream with a limestone channel overlain by mostly alluvial material covered with thick vegetation and trees. The Bentle Branch riparian corridor in Cedar Hill is primarily composed of American Elm, Black Walnut, Bois d Arc, Cedar Elm, Eastern Red Cedar, Green Ash, Hackberry, and Shumard Oak underlain by the Austin Chalk outcrop and the Frio silty clay soils. Erosion is occurring due to the natural channel evolutionary process on Bentle Branch, but this process has been accelerated due to increasing urbanization in the watershed. Much of this development has occurred within the last ten years, but large undeveloped tracts still remain. Currently, plans exist to convert most of the remaining vacant and agricultural acreage to residential, commercial, and industrial development. One such plan 2

3 is the Uptown Village development currently under construction, which will affect both Bentle Branch and Tributary 2 between Pleasant Run Road and FM HYDROLOGY AND HYDRAULICS For this study, the floodplain is defined by the 100-year flood based on ultimate (fully developed) land use. New hydrology and hydraulic models were developed based upon ultimate conditions land use determined from zoning information provided by the City of Cedar Hill. Field surveys were conducted to obtain cross sections along the channel and at all stream crossings. Water surface elevations were computed and the floodplain delineated based on 100-year flood discharges. Figure 1 shows floodplain delineations within the study area. Ten private road crossings are inundated by the 10- and 100-year floods and are considered dangerous for the 100-year flood. However, only 5 of the crossings are considered dangerous for the smaller, 10-year flood. This flood has a much greater chance (10% vs. 1%) of occurring in any given year compared to the 100-year flood and thus is an increased risk to pedestrians and motorists. Figure 1: Example Floodplain Delineation STORM WATER DETENTION AT EXISTING SITES Site detention facilities are located throughout the basin in the study area. These facilities were evaluated as to their effectiveness in reducing peak flood events. The analysis 3

4 shows that most of the existing site detention facilities along Tributary 2 do reduce discharges just downstream of their outfall location, but the effects are greatly reduced downstream at Pleasant Run Road and the confluence with Bentle Branch. Commercial Area Stormwater Detention Basin in Cedar Hill STORMWATER MANAGEMENT ALTERNATIVES Stormwater management alternatives were considered to address the erosion and flooding concerns. Results of the hydrology and hydraulic analyses showed that there are no homes inundated by the 100-year flood in the Bentle Branch watershed in Cedar Hill. Channel alterations for flood reduction were considered, but eliminated due to their probable significant environmental impact. Stormwater detention was also investigated as a stormwater management alternative. Several of the existing basins were evaluated to determine if reconfiguration would enhance their performance in terms of flood reduction especially along Tributary 2. This analysis shows that enhancements to the Plaza at Cedar Hill Mall pond could reduce 100-year discharges by up to 6% (fully developed landuse conditions) downstream at Pleasant Run Road by further restricting the outfall and raising the elevation of the exterior walls of the pond. Another stormwater detention alternative was evaluated on Tributary 2. The detention facility is created by restricting the culvert openings underneath Pleasant Run Road. Analysis shows that for (fully developed landuse conditions) as much as a 12% reduction in peak 100-year discharges could be achieved downstream of Pleasant Run Road. Some facilities, such as the site detention ponds on the Wal-Mart property along Bentle Branch and the two ponds located on the Target Shopping Center property along Tributary 2 could be altered to provide water quality enhancement, in addition to flood reduction benefits. These detention facility enhancements analyzed here do reduce discharges somewhat 4

5 downstream, but are not recommended as part of the Stormwater Management Plan because the reductions and therefore the benefits are very limited. REGIONAL STORMWATER DETENTION Regional stormwater detention was also evaluated from a flood reduction perspective. A location for a regional detention facility was identified that benefits both Cedar Hill and Duncanville. This facility is proposed just upstream of FM 1382 on Bentle Branch and would reduce flood discharges and base flood elevations on Bentle Branch. The regional stormwater detention facility concept was evaluated as to its effect on reducing peak flood discharges. The facility would provide peak 100-year flood discharge reductions ranging from 4 and 43 % through Cedar Hill and up to 4% reductions through Duncanville based on a comparison with current landuse conditions. Figure 2: Proposed Regional Stormwater Detention Facility 5

6 STREAM BANK STABILITY Halff Associates, in conjunction with Dr. Peter Allen, performed geomorphology and hydraulic analyses to evaluate and prioritize stream stability problems along Bentle Branch. The analyses included field assessment and stream hydraulic computations. Stability assessments were performed on Bentle Branch main stem and Tributary 2 only. Both Tributary 3 and Unnamed Tributary did not exhibit significant erosion and properties along the banks did not appear to be threatened at this time. Specifically, drainage/erosion complaints at sixteen locations along Bentle Branch and Tributary 2 were recorded by the City of Cedar Hill after the July 2004 flood. The field assessment entailed a walk-through of the entire reach of stream bed, including preparing detailed field notes, taking photographs, and identifying certain definitive characteristics of channel erosion. The geomorphology review also included a study of the channel evolution and potential degradation with time. A detailed Geomorphology Report was prepared by Dr. Peter Allen for use in the stream bank stability evaluation. Figure 3 illustrates Bentle Branch s different stages of channel evolution. Bentle Branch is experiencing an accelerated evolutionary process due to increasing urbanization within the watershed. The Channel Stability Assessment part of this study categorizes erosion sites based on location within the 30-year potential erosion zone. The encroachment zone is calculated based on standard setbacks used for channels in Dallas County. From the rock line, a 3:1 setback is extended away from the stream. The amount of encroachment within this setback is recorded as (-) if the structure falls within the setback zone and (+) if the structure falls outside the setback zone. Another simple weighting technique is used to establish future priorities along the channel. Based on the erosion category mapped in the field, the lateral erosion rates are assumed. Using a design lifetime of 30 years, the encroachment is adjusted for erosion of the alluvial material in the channel during this time. Figure 4 shows how the limit of this zone was determined. 6

7 Hydraulic analyses were performed to obtain parameters indicative of erosion. The HEC-RAS hydraulic model of Bentle Branch and Tributaries developed by Halff Associates (2005) for this study was executed for the 1-, 10-, and 100-year flood events to represent small, bank-full (1-year), and larger, rarer (10- and 100-year) floods. Table 1 tabulates discharge, velocity, energy grade, and shear stress for the bankfull (approximate 1-year) flood event in the areas identified in the field with moderate to severe erosion. The results do show erosive velocities in these areas and maximum shear stresses in excess of 3 pounds per square foot. Table 1 Bentle Branch and Tributary 2 Slope Stability Parameters (1-yr Event) Q Total Energy Grade Velocity Shear Stress Location (cfs) (ft/ft) (ft/s) (lb/sq ft) Bentle Branch MS (Erosion Sites) Tributary Note: shear stress = ãrs f Figure 3: Bentle Branch Channel Evolution Model 7

8 Figure 4: Encroachment Zone Determination Erosion sites identified from field observations were ranked based on severity of erosion, likelihood of slope failure and potential for damages to public and private improvements. Severity of erosion was identified in the field as slight, moderate, and severe erosion. The geomorphology report includes a ranking of erosion sites based on the erosion potential and a field score. The field score is how far the structure lay within the encroachment zone. Furthermore the structures were broken down into three categories: houses, garage, sheds and fences. Halff Associates utilized these rankings to establish a prioritization of erosion sites. Priority was given to the structures based upon the channel s severity of erosion, slope failure likelihood, field score, and engineering judgment as to the structure s potential for failure. Repeated priority numbers indicate that severity of the erosion threat was indistinguishable between structures. Figure 5 shows an example of the field identified erosion sites and encroachment zones color coded by severity of erosion. 8

9 Figure 5: Field Identified Erosion Sites 9

10 RECOMMENDED STORMWATER MANAGEMENT PLAN Storm Water Management and Stream Stabilization Plans that address increased flood discharges due to urbanization and erosion concerns along Bentle Branch in Duncanville and Cedar Hill have been developed. In Cedar Hill, the Stormwater Management Plan and stream stability alternatives combine regional detention with stream bank stabilization. A regional stormwater detention facility is recommended in the upper basin of Bentle Branch in Cedar Hill. The basin as proposed significantly reduces flooding along Bentle Branch in Cedar Hill, stabilizes the flood risk in Duncanville even with future urbanization and reduces stream erosion in downstream reaches. The plan also proposes that the stream banks in areas where private property is susceptible to erosion be stabilized with appropriate measures that preserve Bentle Branch in a natural condition. Three erosion site locations were identified as needing stream bank stabilization. Existing channel erosion has placed several structures adjacent to the stream banks in danger at these sites. The three erosion sites are located on Bentle Branch and Tributary 2 along Cedar Hill Road, Nottingham Road, Joe Wilson Road, and Balfour Drive. Recommended Stream Bank stabilization alternatives are to construct retaining wall type structures using gabions, vegetated geogrid cells, vegetated geoweb cells, or stone faced retaining walls to prevent further erosion from occurring. The estimated cost of the Stormwater Management Plan and stream stability alternatives are shown on Table 2. Table 2 Cedar Hill Stormwater Management Recommended Plan and Stream Bank Stabilization Alternative Costs Alternative Cost Regional Stormwater Detention Facility $500,000 Site 1 (Bentle Branch Along Nottingham Dr. and Cedar Hill Rd.) Gabion Option $450,000 Site 1 (Bentle Branch Along Nottingham Dr. and Cedar Hill Rd.) - Geogrid/SGRW * Option $230,000 Site 1 (Bentle Branch Along Nottingham Dr. and Cedar Hill Rd.) Geoweb Option $340,000 Site 2 (Tributary 2 Along Balfour Drive) Gabion Option $430,000 Site 2 (Tributary 2 Along Balfour Drive) - Geogrid/SGRW * Option $290,000 Site 2 (Tributary 2 Along Balfour Drive) Geoweb Option $360,000 Site 3 ( Bentle Branch Along Cedar Hill Rd. and Joe Wilson Rd.) Gabion Option $300,000 Site 3 ( Bentle Branch Along Cedar Hill Rd. and Joe Wilson Rd.) Geogrid Option $100,000 Site 3 ( Bentle Branch Along Cedar Hill Rd. and Joe Wilson Rd.) Geoweb Option $240,000 *SGRW = Stone Gravity Retaining Wall Note: Recommended alternative shown in bold. In Duncanville, the Recommended Management Plan combines property buyout, regional stormwater detention, and stream bank stabilization. The Duncanville plan depends on the regional stormwater detention basin proposed at the upstream end of Bentle Branch in Cedar Hill to provide up to a 7% reduction in 100-year flood discharges through Duncanville. The plan includes stream bank stabilization in areas where private property improvements are threatened by erosion. The proposed stabilization measures should keep Bentle Branch in a 10

11 more natural condition. The purchase of residential properties at 303 and 307 W. Cedar Creek Dr. is also recommended to reduce the flood risk. However, the properties should be acquired by voluntary purchase, allowing residents to remain until they are ready to sell their property. Plan costs are summarized in Table 3. Table 3 Duncanville Recommended Stormwater Management and Streambank Stabilization Plan Costs Recommended Plan Component Cost Purchase 303 and 307 W. Cedar Creek Dr. $265,000 Streambank Stabilization Recommendations Site 1 - Geoweb Wall $380,000 Site 2 - Geoweb Wall $390,000 Site 3 - Geoweb Wall $350,000 Total (w/o Regional Stormwater Detention Facility cost) $1,385,000 Other recommendations include updating the City s flood damage prevention ordinance to reflect higher regulatory standards providing additional protection to lives and properties similar to other cities in the area. The ordinance should institute the no rise criteria where floodplain development must not cause any rise in the 100-year flood water surface elevation as a result of the floodplain encroachment. Valley storage requirements should also be included requiring that development within the floodplain preserve valley storage. Guidelines such as these are useful in reducing the impact of urbanization on flooding. Both Cities should also consider adopting parts of iswm in their Stormwater Management program. It may be helpful to form a Technical Review Committee to consider iswm components and local criteria to be included. This is the approach currently being pursued by the city of Fort Worth. It may also be helpful to jointly pursue aspects of iswm with neighboring cities such as Desoto, especially in the area of soliciting input from the development community. To be most effective, the adoption of iswm should be accompanied by the development of stormwater master plans on a watershed basis in conjunction with adjoining communities if possible. These studies also identified several private bridge crossings inundated by the 10- and 100-year storms that are categorized as exhibiting a high risk of danger to vehicles and pedestrians. Because these structures are privately owned, this study does not recommend that the City replace these structures. However, the City should mandate certain flood hazard guidelines be adhered to if replaced in the future by the property owner. Replacement structures should be designed so as to not cause increased flooding on neighboring property Existing structures as well as replacement structures should be made safer by installing guardrails and flood gages with easy to read flood depth markings. Also replacement 11

12 structures should be designed to some level of flood protection, such as the 10-year flood event. CONCLUSIONS A comprehensive Storm Water Management and Stream Stability Study was conducted for the Bentle Branch Watershed in the Cities of Duncanville and Cedar Hill. As a result, several important points and accomplishments were made: Existing Site Stormwater Detention Basins were found to be of limited effectiveness in reducing downstream flood discharges in this watershed. Basin-wide Stormwater Management Master Plans were developed that received citizen and council support as far as flood reduction is concerned. The principal component of the Stormwater Management Master Plan, a Regional Stormwater Detention Basin, was proven effective at achieving both flood damage reduction and stream stability goals. Specific Stream Bank Stability problems were identified and appropriate corrective measures, including soil biotechnical methods, were recommended. 12

13 BIBLIOGRAPHY Annandale, G.W., (1995) Erodibility. Journal of Hydraulic Research. Volume 33, No. 4, Allen, P.M., Arnold, J.G., Skipwith, W. (2002) Erodibility of Urban Bedrock and Alluvial Channels, North, Texas. Journal American Water Resources Association, Vol. 38, No. 5. pp Allen, P.M. (1975) Urban Geology of an Interstate Growth Corridor from Hillsboro to Dallas County, Texas. Baylor Geological Studies Bulletin No. 28., 36p. Coffee, D.R., Hill, R.H., and Ressel, D.D.(1980) Soil Survey of Dallas County. USDA, SCS., US Gov. Printing Office,153p. Dallas Geological Society (1965) Geology of Dallas County, DGS. Dallas, Tx., 211pp. Federal Emergency Management Agency, Flood Insurance Study, Dallas County, Texas and Incorporated Areas, Revised August 4, 2004, 8 Volumes. Fischenich, C. (2001) Stability Thresholds for Stream Restoration Materials. USAE Research and Development Center, ERDC TN-EMRRP-SR-29. Halff, Albert H., Associates, Inc. Hydrology and Hydraulics of Floodplain Studies. Prepared for the City of Dallas. Dallas, Texas: Albert H. Halff Associates, Inc., January, Halff, Albert H., Associates, Inc. A Flood Control Feasibility Study of Tenmile Creek and Tributaries. Prepared for the City of Duncanville. Duncaville, Texas: Albert H. Halff Associates, Inc., January, Shakoor, A., and Rodgers, J.P. (1992) Predicting the rate of Shale Undercutting Along Highway Cuts. Association of Engineering Geologists, Vol. 29, No. 1. United States Geological Survey, Atlas of Depth-Duration Frequency of Precipitation Annual Maxima for Texas, Scientific Investigations Report Prepared for Texas Department of Transportation. 13