San Luis Rey River watershed to be pilot project for flood forecasts

The San Luis Rey River watershed will be the pilot project for a flood forecast model to be used by the San Diego County Flood Control District.

The San Diego County Board of Supervisors, who act as the board for the San Diego County Flood Control District, voted March 26 to enter into negotiations with DHI, Inc., and upon completion of negotiations to award a contract to DHI for the development of a real-time flood forecast model for the San Luis Rey River watershed. The director of the county’s Department of Public Works, which includes flood control, has the authority to amend the contracts to reflect changes to services and funding as needed.

“If this proves to be successful we’ll expand it to other regions in the county, other waterways in the county,” said Department of Public Works program manager Cid Tesoro.

The county’s flood forecasting program currently relies on data provided by the Automated Local Emergency in Real Time (ALERT) flood warning system consisting of more than 100 real-time weather stations which measure rainfall and can identify flooding or rainfall events. Although information provided by the ALERT system assists in response to existing flood events, the stations only monitor existing conditions and cannot provide more than a few minutes of advance warning if a flood were to occur.

Since the ALERT system cannot provide the Flood Control District with the magnitude of the flooding or predict when the event will occur, county staff must calculate estimates which would direct emergency response resources.

While flooding often is prevalent in periods of heavy rainfall, the areas burned by the October 2007 fires were subject to debris flows and flooding as the result of lighter storms. The San Diego County Flood Control District worked with the county’s Office of Emergency Services and the county’s Geographical Information System staff to issue advance warnings to residents in impacted areas. The system worked best when the lead time was maximized, and the county believes that the new program will provide several hours of advance warning to residents who reside in floodplains or are affected by low water crossings.

DHI’s flood forecasting will use real-time telemetry, weather radar, and forecast data to create a forecast shell. The modeling tools incorporated include runoff, routing, and error prediction. “You can use it to predict the extent of the flooding,” Tesoro said. “If you know that information, you can tell the public.”

The system allows not only for advance notification of residents but also for positioning emergency resources and manpower to be nearby when the flooding occurs. The modeling will include the assessment of tributaries as well as the river itself.

Flood Control District staff conducted a search among prominent engineering companies involved in flood forecasting and model development and determined that only DHI, Inc., had the qualifications and software systems required for the necessary level of modeling. DHI’s modeling experience includes major rivers in Great Britain and continental Europe as well as in the United States, and in addition to pinpointing locations DHI’s Web-based flood forecasting capabilities can provide two-dimensional modeling which can identify specific streets and parcels subject to flooding. The county’s competitive procurement policy was waived due to the unique ability of DHI to provide the specifications.

The DHI hydrologic model will access real-time data from the ALERT system, although that implementation will require computer modeling the county’s system currently does not have. The contract will call for DHI to provide a model of the San Luis Rey River which will enable the Flood Control District to manage its own forecasts, automatically integrate the system with data from the county’s weather stations and existing warning system, automatically retrieve rainfall forecast data from the National Weather Service or from a private weather forecast company, automatically adjust forecast flow based on observed flow at select stream gages, allow easy adjustments to the model parameters, provide two-dimensional real-time flow forecast graphics, and be Internet-based so that the system can be operated from outside a specific office.

“Our goal is to try to have it before the start of the rainy season,” Tesoro said. “Might be a little difficult to do that, but we’ll do our best.”

Although it is not part of the initial contract, DHI also has additional modules which can later be integrated to produce runoff forecasts in urban storm drain systems and would be able to forecast water quality changes.