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Orange County Flood Control Channel Hydraulic Physical Model - Findings

OC Watershed

In late 2010 and early 2011, PACE and OC Public Works performed several tests of a key section of the East Garden Grove Wintersburg Channel utilizing a 15:1 scale physical model. The following text describes the purpose, development and findings of the model study.

View a video on the model development and operation here.

East Garden Grove-Wintersburg Channel Physical Hydraulic Study
A sophisticated 130-foot long physical hydraulic flume model of the East Garden Grove – Wintersburg Channel (EGGWC) was developed to resolve significant engineering analysis challenges and identify channel improvement approaches that have the potential to save the County of Orange millions of dollars. Orange County Public Works (OCPW) is responsible for over $300 million in future improvements to the EGGWC and its tributary channels and infrastructure, a critical flood control network serving a 28.1-square mile watershed of Orange County. One particular area of the channel at the 405-freeway crossing was particularly challenging to analyze for hydraulic capacity with computer models due to the unique geometric configuration of the channel at that area, multiple confluences of channels and pipe networks and an adjacent retarding basin. With millions of dollars of proposed improvements at stake, the determination of the true hydraulic capacity and performance of the channel was critical and has a significant influence on design approaches to future channel system improvements. This was the first physical model study to be conducted by the County of Orange in several years.

Study Area Background
The East Garden Grove-Wintersburg (EGGW) Channel (Facility No. C05) at the I-405 Freeway is a complicated flood control system composed of an array of culverts and tributary channels. Located in the cities of Westminster and Huntington Beach, the channel system's unique geometric configuration, multiple confluences and storage basin demonstrates that this particular reach of channel is difficult to accurately analyze by conventional mathematical and computer modeling with confidence in the results. It is estimated that over $30 million in future flood control improvements are dependent upon the accuracy of the hydraulic analyses at the I-405 crossing. It was determined that a hydraulic physical model would provide the most accurate understanding of the existing and proposed channel improvements for this reach of the channel system because of the unknown nature of the hydraulic processes which are occurring.

Due to proposed channel improvements at two confluences upstream and downstream of the I-405 freeway, there was uncertainty regarding the impact these new designs would have on the undercrossing and upstream water surface. Proposed improvements to Newland Storm Channel will discharge flow both upstream and downstream of the freeway crossing through a split flow or bifurcation structure. The acceptable volume of water discharged upstream of the freeway had to be determined by the physical model. Likewise, the proposed improvements to Edinger Storm Channel located downstream of the freeway undercrossing calls for two 66-inch RCPs in addition to the existing RCP. In order to efficiently maximize the performance of both proposed channels, the I-405 culverts and tributaries needed to be scrutinized by a hydraulic physical model to fully understand the channel hydraulics.

Description of the Physical Model
The physical model was designed to represent a 1,000' reach of the EGGWC centered on the I-405 Freeway crossing from centerline Station 250+00 to 260+00. The intent of the physical model was to provide a close approximation of the channel and I-405 Freeway crossing to the extent that it could be reasonably replicated. The model was a tool used to better understand the hydraulic performance of the existing facility as opposed to an exact replication of the existing facility. The model also provided insight into the hydraulic impacts resulting from proposed improvements to Newland and Edinger storm channel confluences. The physical model scale was carefully chosen to balance the size of the model based on the construction costs and the ability to accurately measure the desired phenomenon. If the model size was greatly reduced from the prototype, there would be potential for scale effects of forces other than gravity. A 1:15 scale model was selected for several reasons including (1) it ensured the anticipated amount of water surface change could be accurately determined based on the limitations of the instrumentation and acceptable error, (2) the model approximately meets the Froude scaling for friction through use of plywood coated with urethane paint, and (3) it was the largest scale model that would satisfy the project budget requirements.

The overall model length was approximately 130 feet. The model also included a separate flume (2.5 feet wide model scale) to model the proposed future channel improvements of the Newland Channel from the confluence with the EGGWC. Additional components of the model included (1) head tanks for the EGGWC and Newland Channel flumes, (2) a water storage reservoir, (3) tailbox with adjustable weirs at the downstream end of the flume, (4) pumping and piping system for water delivery, (5) flow measurement meters, (6) in-channel piezometers, and (7) freeway culverts.

Model Findings
The model study allowed OCPW to identify improvements to culvert hydraulic performance and a superior configuration to the Newland Channel confluence with the EGGWC. In general, the physical modeling demonstrated that the existing channel system has capacity for the design discharge, but it also provided a detailed understanding of surface wave disturbance and flow distribution. However, the model testing determined certain areas of the channel have limited freeboard and culvert entrances produced surging and splashing at the headwalls. Increases to the wall height in certain locations were recommended to improve the level of protection provided by the facility.

A number of modified configurations of the culvert entrances were evaluated in order to better distribute the flows as well as for their ability to reduce surging and splashing at the headwalls which included long pier extensions, short pier extensions, splitter walls, and improvements to the culvert inlets. Most of the configurations tested created increased water surface elevations elsewhere in the channel. Ultimately, a configuration that included short pier extensions with vertical ribs added to the triple box culvert and a splitter wall between the headwalls of the elliptical pipes and triple box culvert produced the best results.

Planned improvements to the Newland Channel which intersects with the EGGWC were also evaluated. These improvements included a 25 foot rectangular concrete channel upstream of the split flow bifurcation. The analysis determined that the most critical area of the channel was the elliptical pipe headwall of the downstream bifurcation structure, which experienced splashing and surging over the top of the channel wall during the peak to peak flow condition. Following testing of multiple improvements, it was found that a curved alignment to the west well of the upstream bifurcation channel improved the flow patterns at the confluence.

The findings of the testing demonstrated that design modifications recommended could potentially save significant cost to the County compared to the originally planned improvements. With so many influences to the channel system, the physical model was able to account for far more variables than conventional computer modeling, providing greater confidence to OCPW to move forward with critical system improvements.


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