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Real-Time Stormwater Management Using Depth, Duration, Frequency Thresholds

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Real-Time Stormwater Management Using Depth, Duration, Frequency Thresholds WEFTEC®.06 REAL-TIME STORMWATER MANAGEMENT USING DEPTH, DURATION, FREQUENCY THRESHOLDS Susan Janek, P.E., George Oswald, P.E., Baxter Vieux, P.E., Ph.D. City of Austin Watershed Protection and Development Review Department 505 Barton Springs Road, Suite 1200, Austin, Texas 78704 City of Austin Watershed Protection and Development Review Department Vieux and Associates, Inc. ABSTRACT The City of Austin (City) operates a Flood Early Warning System (FEWS) to reduce public exposure to flash flood hazards. The system is operated and maintained by the Watershed Protection and Development Review Department, Watershed Engineering Division. Advanced information and warning of urban storm water flooding is achieved by the FEWS. A critical component of the system is rainfall detection and interpretation. Radar rainfall detection and forecasting also utilizes the existing ALERT (Automated Local Evaluation in Real Time) rain gauge network. An innovative web-based hydrologic information system built upon radar hydrology and implemented in 2004 has proved to be useful during storms producing heavy precipitation and flooding. This presentation will describe the development and use of Depth, Duration, Frequency (DDF) values in near real-time. KEYWORDS Flood warning, DDF, real-time, internet, hydrologic information system, radar hydrology BACKGROUND Central Texas is often called "Flash Flood Alley" because of its frequent, intense storms. While large events seem to happen every decade, lesser events also cause public safety concerns. Coordination between the Watershed Protection and Development Review Department (WPRDR) and Office of Emergency Management (OEM), both City of Austin agencies, results in hydrologists and emergency managers working together in the Emergency Operations Center (EOC) to warn of and respond to flooding along creeks and at major intersections in the City. Because of the challenge with identification and timely response during high-risk flood events, an integrated warning system is needed so that appropriate actions and deployment of resources can be effectively managed. Operation of the Flood Early Warning System (FEWS) helps reduce public exposure to flash flood hazards. The system is operated and maintained by the WPDRD, Watershed Engineering Division. The FEWS is used to identify the development of flood hazard conditions that warrant public safety response for roadway closings and building evacuations. The FEWS consists of a variety of field sensors which transmit stream water level and rainfall data to a central data management computer system for decision-making. Copyright © 2006 Water Environment Foundation. All Rights Reserved 3757 WEFTEC®.06 The FEWS has been in operation since 1985 and is considered an ALERT system. Portions of the FEWS have been expanded and upgraded with growth in the service area and advancing technology. Currently, the field sensor network consists of 86 rain gauges, 41 stream depth gauges, and ten automated low-water crossing barricades. Information from the sensors is transmitted by a low frequency FM radio to a repeater station. From the radio repeater station, the information is then retransmitted on a different frequency to the base station – the Emergency Operations Center, where it is managed by the Hydrolynx Novastar software system. The FEWS provides rainfall and stream stage information for 26 major watersheds and it extends over 1400 square miles. The gauge components themselves are provided by both Hydrolynx Systems and High Sierra Electronics. Data received from the gauges is pushed to a File Transfer Protocol Site (.FTP) that is maintained by Hydrolynx Novastar. This information is monitored and used by the National Weather Service and the City of Austin’s publicly available internet site. The information is also sent to the City’s rainfall processing contractor Vieux and Associates, Inc. (VAI). Radar-based rainfall data services are provided via customized Internet access operated as a web service by VAI. RADAR RAINFALL PROCESSING Radar hydrology is the use of weather radar measurements for hydrologic applications. It deals with the estimation of current spatial variability and rainfall intensity and accumulation over a selected time interval for deriving estimates of runoff and stream flow. The FEWS includes the application of near real-time, bias-adjusted NEXRAD based rainfall estimates at square kilometer horizontal resolution to augment ground-based data for flood prediction. Initial deployment of a flood hazard prediction system supports real-time hydrologic modeling driven by precipitation values derived from radar and the heterogeneous rain gauge network observations at 15-minute increments. The objective of the system is to quickly identify specific basins that are at high risk of flooding during a storm event. Multiple end-users need access to such information simultaneously and potentially at multiple locations. The information needs to be easy to interpret and appropriate actions need to be predefined and displayed unambiguously. The methodology used to achieve the objective for identification of flooding in specific basins is described below. METHODOLOGY Prior studies performed by the U.S. Geological Survey (USGS) for the Austin area led to identification of the rainfall depth, duration, and frequency (DDF) information. Over a period of time with numerous hydrologic and hydraulic models developed by and for the City of Austin plus the calibration of those models to real storm events has produced identification of the specific DDF information per each watershed that causes localized flooding in relatively small basins within the City and in areas that extend beyond the corporate limits. This experience along with existing DDF data was incorporated in a website that displays real-time rainfall information. The rainfall information is processed by combining National Weather Service (NWS) radar data with City rain gauge data. The processed rainfall is then displayed using a web site that was created by Vieux and Associates. Processed rainfall maps are generated every 15 minutes and displayed via automatic updates by the website. The rainfall is compiled as gridded maps with Copyright © 2006 Water Environment Foundation. All Rights Reserved 3758 WEFTEC®.06 1x1 km resolution of processed rainfall information displayed over the City and Travis County. In addition, gridded rainfall information may be displayed with pan and zoom capabilities to display rainfall information over a high-resolution street network, or aggregated within watershed basin boundaries. The City developed the flood threat thresholds and action plans, critical basin response information, and requirements for information to be displayed by the website. With 15-minute updates, each basin is displayed with colors that are coded to show flood threat thresholds based on accumulated rainfall within specific time periods. The response time for a watershed can be characterized by the time of concentration, Tc. The time response depends on terrain slope, length, and channel characteristics. Table 1 presents selected watershed characteristics used to derive Tc. Smaller basins react more rapidly than large basins, though with smaller discharge. Recommended actions are obtained from the website by dragging the cursor over any basin. When the basin is highlighted, it displays the basin DDF thresholds and the action plan for the current DDF status of the basin. Watershed basin colors advance from green to purple based on the status of the DDF as shown in Table 2. The ready display of color-coded information makes it easy provide specific responses during critical storm conditions, and saves valuable time for the Watershed Engineering Division employees to assess problems and recommend actions during a flooding event. In addition to the quantitative precipitation estimate, the Vieux and Associates website also implements a program that will forecast precipitation one hour in advance using radar that extends beyond the corporate limits of the City. Knowing whether more rainfall is expected to approach the City is useful for anticipating the appropriate response actions. The predictive rainfall is accomplished by advancing the radar imagery for the next hour while accounting for intensification and decay of precipitation. Table 1-Data for selected basins used for Tc development Stream- Plane U/s D/s Elev Avg. Tc INT(Tc) INT Critical L (mi) Length Elev (ft) Slope (hour) (Tc+0.5) Time (mi) (ft) (ft/mi) (hour) 51.433 25.693 1398.8 428.42 37.77 5.83 6 6 6 4.356 2.783 1010.6 842.17 60.53 1.34 1 2 1 3.984 3.009 847.32 483.19 121.02 0.91 1 1 1 17.59 11.339 1140.9 590.82 48.51 3.01 3 4 3 3.182 2.41 644.48 429.12 89.36 0.94 1 1 1 2.416 1.907 699.8 524.71 91.8 0.81 1 1 1 7.881 4.709 638.65 410.1 48.54 2.01 2 3 2 23.54 17.394 1107.3 652.37 26.15 4.74 5 5 6 A 10-year event or larger on a watershed scale will in most cases cause significant flooding so those events have been assigned a magenta color as seen in Table 2. Copyright © 2006 Water Environment Foundation. All Rights Reserved 3759 WEFTEC®.06 Table 2 - DDF rainfall amounts for a specific basin. The color shown is also displayed on the website for quick visual reference. Recurrence Interval 15 30 (YR) min min 1 hr 2 hr 3 hr 6 hr 8 hr 12 hr 1 day <2 2 0.98 1.32 1.72 2.16 2.32 2.67 2.87 3.06 3.44 2.5 1.05 1.42 1.86 2.35 2.53 2.91 3.12 3.33 3.84 3.33 1.14 1.54 2.04 2.58 2.79 3.19 3.42 3.64 4.33 5 1.26 1.71 2.28 2.89 3.13 3.56 3.82 4.07 4.99 10 1.47 1.98 2.68 3.42 3.71 4.21 4.51 4.81 6.10 25 1.76 2.36 3.28 4.20 4.55 5.14 5.52 5.90 7.64 50 2.01 2.68 3.79 4.88 5.28 5.94 6.40 6.86 8.87 100 2.29 3.04 4.37 5.66 6.11 6.85 7.41 7.96 10.20 250 2.73 3.57 5.26 6.86 7.38 8.24 8.96 9.67 12.00 500 3.11 4.02 6.06 7.94 8.51 9.47 10.34 11.20 13.50 Table 3 -Threat and category and corresponding action plan NO FLOODING PREDICTED.
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