capacity of and attenuation behind burke bridge: · pdf file · 2006-04-13the...

Adobe Creek- Burke Rd. Bridge Capacity and Attenuation March 3, 2006 Prepared by E. Zedler, L. Xu, K. Oven – CPSD of 13

Adobe Creek Burke Road Bridge Capacity and Upstream Attenuation

Methodology and Results March 2006

Introduction and Background In early 2003, the residents in the vicinity of Adobe Creek Reach 5 (between West Edith Avenue and Foothill Expressway) opposed a District plan to improve flow conveyance capacity in Reach 5. The local municipal governments agreed with the residents and did not grant the District the rights of way necessary to construct the proposed project. After further discussions with representatives of the local community, in late 2003 the District proposed using a collaborative approach to work more closely with the community in formulating a channel improvement plan that would be acceptable to all parties. Local creekside residents, other interested members of the community, the local municipalities, state and federal resource agencies, and the Santa Clara Valley Water District formed a Collaborative to work together on developing and identifying a preferred alternative for improving the creek channel in Reach 5. The Adobe Creek Upper Reach 5 Restoration Project was initiated in April 2004. The objectives of this project were to develop plans for improving the Adobe Creek channel between West Edith Avenue and Foothill Expressway, and to re-evaluate the impact of replacing the Burke Road Bridge crossing further upstream. A 1999 Adobe Creek Watershed Planning Study outlined Adobe Creek improvements between El Camino Real and Rhus Ridge, and had included a proposal to replace bridge crossings at Burke Road (Reach 6) and O’Keefe Road (Reach 7) to address debris buildup on the upstream side of both bridges and to improve flow conveyance. However, community members were divided in their support for replacement of the Burke Road Bridge in Reach 6 and requested that the Adobe Upper Reach 5 project re-evaluate the impact of replacing this bridge crossing. A number of residents in the vicinity of Adobe Creek believe that the Burke Road Bridge acts as a dam, attenuating upstream flows, creating an in-stream reservoir, and thus protecting downstream residents from flood flows. This belief has been reinforced by long-term residents’ collective memories. According to those who have lived next to the creek, over the past 30 to 50 years Adobe Creek flows have never overtopped the Burke Road Bridge.


Residents have also expressed the belief that replacing the Burke Road Bridge with a larger capacity culvert will reduce the bridge’s ability to attenuate upstream flows and will allow more water to reach the downstream channel, thus inducing flooding in the downstream reach. This document summarizes the results of District analyses to determine the conveyance capacity and upstream flow attenuation at the Burke Road Bridge crossing of Adobe Creek. The results presented herein were computed using the unsteady HEC-RAS hydraulic model. The modeling was performed using existing channel cross-section data extending approximately 1,040 feet upstream and 1,450 feet downstream of Burke Road Bridge. The modeling was applied for existing bridge conditions as well as for the 1999 proposed bridge culvert enlargement. This document first presents the geometric dimensions of the existing bridge and the proposed replacement bridge. The flow characteristics through the bridge culvert and the flow volume at which the bridge will be overtopped are then presented. Finally, the results of attenuation analysis upstream of the bridge are provided. Methodology The modeling analyses that were performed included the following elements

• 2005 survey data of the Adobe Creek channel and surrounding floodplain • Flow hydrographs for Adobe Creek generated for various storm events using District’s

hydrology procedures • Completed design plans for the 1999 proposed Burke Road Bridge replacement • The unsteady HEC-RAS hydraulic model to simulate flow characteristics in Adobe Creek

Burke Road Bridge: Description of Existing and Proposed Design Parameters The existing and proposed design parameters for the culvert under Burke Road Bridge are listed in Table 1. The proposed bridge culvert width and height increases significantly from that of the existing bridge. The elevation of the soffit (the roof of the culvert) also increases with the 1999 proposed bridge design. As part of the culvert replacement, the creek would be widened at the approach and exit to the culvert. The extent of this channel widening would be approximately 25 feet upstream and downstream of the bridge.


Table 1. Burke Road Bridge Characteristics

Culvert Height (ft)

Culvert

Width (ft)

Soffit Elevation

(ft, NAVD88)

Low Point on Burke Road

(ft, NAVD88) Existing Dimensions 6.9 15.0 189.9 192.7

1999 Proposed Bridge Replacement

8.6

24.6

191.9

192.7

Burke Road Bridge Flow Characteristics Stream flows are conveyed through a bridge culvert under open air or “open channel” conditions until the water surface at the upstream end of the culvert reaches the culvert roof, or soffit. If upstream flows continue to increase, water flowing through the culvert will continue to rise until there is no air pocket between the water surface and the roof of the culvert. With the loss of the air pocket, flow through the culvert becomes pressurized flow. Under pressure flow, water may build up upstream of the bridge and the subsequent “head” that is created by this buildup forces more water through the culvert. In Table 1, under existing bridge conditions, the soffit elevation is 189.9 ft NAVD. The low point of the bridge road crossing is at elevation192.7 ft NAVD. If flows in Adobe Creek are high enough to fill the culvert and cause pressure flow, water could build up on the upstream side of the bridge for up to 3 feet (192.7-189.9), which would greatly increase the volume of flow pushed or pressured through the culvert. The ultimate bridge flow capacity, is the volume of flow that the pressure head can force through the culvert before the bridge is overtopped. The Burke Road Bridge flow characteristics are presented in Table 2, and were obtained by using the unsteady HEC-RAS model for a number of flow hydrograph scenarios. A range of flows is presented for the various characteristics due to the variations in flow hydrographs for different rainfall-runoff events. Table 2 results show that, under existing conditions, a flow in Adobe Creek of approximately 620± cfs results in a water surface that reaches the upstream bridge soffit. As the water builds up behind the bridge, flow through the culvert increases to 800cfs, 900 cfs, 1000 cfs, and so forth. If flows in Adobe Creek increase beyond 1100 cfs, the water buildup upstream of the bridge reaches its limiting ability to force flows through the culvert. The HEC-RAS modeling indicates that the Burke Road Bridge overtops when Adobe Creek flows reach 1155± cfs.


Table 2. Burke Road Bridge Flow Characteristics

Flow at which water surface reaches the upstream soffit

(cfs)

Flow at which bridge is overtopped

(cfs)

Existing Bridge 605 - 625 1150 - 1160

1999 Proposed Bridge 1630-1645 1755 - 1760 With the larger culvert as proposed in the 1999 Adobe Creek Planning Study, the modeling results indicate flows in Adobe Creek are conveyed through the enlarged culvert in “open channel” conditions until a flow of about 1640± cfs. If pressure flow occurs with the new culvert in place, the soffit is only 0.8 feet below the low point of Burke Road (see Table 1). Thus, water cannot build up too much behind the bridge before overtopping at the low point of Burke Road occurs. As shown in Table 2, flow overtops the low point of Burke Road when a flow of 1755± cfs is observed in Adobe Creek. The bridge flow characteristics presented in Table 2 differ from the values presented in the 1999 Adobe Creek Watershed Planning Study. In that report, a value of 920 cfs was determined as the flow at which the water surface made contact with the soffit on the upstream end of Burke Road Bridge. The 1999 value was obtained with a HEC-2 model, which treats culverts (bottom, sides, and top) with a single Manning’s n value. Because the sides and top of the culvert are concrete, a value of n = 0.015 was chosen for the HEC-2 model. However, the bottom is natural channel, and has a higher n value of about n = 0.03. The HEC-RAS model used in this current analysis allows for Manning’s n values to be different for the bottom and remaining sides and top. Manning’s n values for this modeling analysis were set to n = 0.03 on the bottom and n = 0.015 on the sides and top, which yielded a lower flow (605-625 cfs) at which the water surface reaches the upstream soffit. Flow Attenuation Upstream of Burke Road Bridge The unsteady HEC-RAS model was used to simulate, over a 30-hour period, the flow characteristics in the vicinity of the Burke Road Bridge as a result of flow hydrographs generated for various rainfall-runoff events. The flow events that were generated resulted in flow hydrographs with peak flow rates of 800 cfs, 1,155 cfs and 2,400 cfs.


1. Flow Event with a Peak Flow of 800 cfs Figures 1 and 2 provide the results of the HEC-RAS model simulation at Burke Road Bridge for a flow hydrograph with a peak flow of approximately 800 cfs, which is the estimated peak flow of the February 1998 storm event. The modeling results are shown for both the existing bridge condition (Figure 1) and the 1999 proposed bridge condition (Figure 2). In Figure 1, the dark blue line traces the flow in Adobe Creek upstream of the Burke Road Bridge. At the beginning of the simulation, the creek flow is approximately 40 cfs, which increases as the rainfall increases over time. Flow volumes rise rapidly between hours 15 and 20 of the simulation. For this flow event, which peaks at 800 cfs, all creek flows are conveyed through the existing bridge culvert. Some “attenuation” of flow upstream of the bridge can be seen close to the time of peak flow by tracing the dashed light-blue line (representing creek flow downstream of the bridge). The dashed light-blue line is slightly lower than the dark blue line at the time that the peak flow occurs, indicating that the presence of the bridge structure holds back a small portion of flow. In Figure 2, which reflects 1999 proposed bridge conditions, all flow for an 800 cfs peak event pass through the larger culvert. Table 2 indicated that the new bridge culvert could pass approximately 1,640 cfs before water levels reached the bridge soffit.


Figure 1. Flow time series for the Existing Burke Road Bridge, 800 cfs peak flow event.

Figure 2. Flow time series for the 1999 Proposed Burke Road Bridge, 800 cfs peak flow event.


2. Flow Event with a Peak Flow of 1155 cfs Figures 3 and 4 provide the results of the HEC-RAS model simulation for a flow hydrograph with a peak flow of approximately 1155 cfs, which represents the largest simulated flow event in which flows do not overtop the existing Burke Road As seen in Figure 3, under existing bridge conditions, there is a minor reduction in the peak flow downstream of the bridge, which is due to storage, or attenuation behind the bridge. As seen in Figure 4, the 1999 proposed bridge condition passes all the flow through the enlarged culvert (open channel capacity of approximately 1,640 cfs). There is no discernible attenuation of flows by the bridge, and flows do not overtop the road.

AdobPrep



e Creek- Burke Rd. Bridge Capacity and Attenuation March 3, 2006 ared by E. Zedler, L. Xu, K. Oven – CPSD of 13


3. Flow Event with a Peak Flow of 2400 cfs Figures 5 and 6 provide the results of the HEC-RAS model simulation for a flow hydrograph with a peak flow of approximately 2,400 cfs. As seen in Figure 5, under existing conditions, the existing bridge culvert can only pass flows until hour 18. For the next 12 hours of this flow event, the flow will overtop Burke Road Bridge (red dashed line), and approximately half of the peak flow of 2,400 cfs will flow over the road as the remainder flows through the bridge culvert. In Figure 6, the 1999 proposed bridge will convey flows through the culvert until approximately hour 22. Flows overtopping the road will reach a peak of approximately 500 cfs. The results of these HEC-RAS simulations indicate that a minor amount of flow attenuation is expected to occur upstream of Burke Road Bridge when water elevations begin to build up on the upstream side of the bridge.


Flow Impacts Downstream of Burke Road Bridge The HEC-RAS simulation results also indicate that flow downstream of the Burke Road Bridge will not increase significantly if the 1999 proposed bridge is constructed. Figure 7 provides an overlay comparison of the flow downstream of the bridge for the existing bridge and the 1999 proposed bridge. Figures 8 and 9 provide similar comparisons for the peak flow events of 1155 cfs and 2,400 cfs, respectively.

Figure 7. Comparison of flow time series downstream of Burke Road Bridge for existing and

proposed conditions for the 800 cfs peak flow event

Figure 8. Comparison of flow time series downstream of Burke Road Bridge for existing and proposed conditions for the 1155 cfs peak flow event.

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Figure. 9. Comparison of flow time series downstream of Burke Road Bridge for existing and proposed conditions for the 1155 cfs peak flow event
dobe Creek- Burke Rd. Bridge Capacity and Attenuation March 3, 2006 repared by E. Zedler, L. Xu, K. Oven – CPSD of 13


Modeling Conclusions The findings to date of unsteady HEC-RAS simulation of bridge flow characteristics for the Burke Road Bridge on Adobe Creek are:

• The existing bridge culvert conveys Adobe Creek flows under “open channel” conditions up to approximately 620 cfs.

• For the existing bridge, flows greater than 1155 cfs overtop the bridge.

• The 1999 proposed bridge design allows open-channel flow conditions up to

approximately 1640 cfs.

• The 1999 proposed bridge design conveys pressure flows of up to 1755 cfs before the bridge overtops.

• Flow characteristics for the existing bridge under various flow hydrograph scenarios

indicate minimal attenuation, or upstream storage, of flows in Adobe Creek (Figures 1 through 6).

• The overlay of flow hydrographs comparing flow volumes downstream of the bridge

(Figures 7 through 9) indicate insignificant differences in flow volume between existing and proposed bridge configurations.

capacity of and attenuation behind burke bridge: · pdf file · 2006-04-13the...

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