The Merriam Mountains project is a proposed 2,300-acre residential development by NNP-Stonegate Merriam in northern San Diego County, Calif., approximately 40 miles north of downtown San Diego. The project is near the end of a multi-year environmental clearance process during which the county of San Diego implemented several changes to its stormwater regulations. As a result, the focus of the project’s stormwater management widened from attenuation of peak runoff from the 100-year storm event to include the impacts of runoff from lower-magnitude events.
The goals of managing this broad range of events are protection of downstream water quality and avoidance of hydromodification of receiving waters caused by development. This change in focus approximately halfway through the clearance process resulted in the addition of several detention basins designed to treat the water quality design storm. And as the project neared final clearance, modifications to the design of detention basin outlet structures were required to consider the range of flows significant to the geomorphic sensitivities of receiving channels, streams, and creeks.
Since the project had already progressed through much of the environmental process, a key challenge was ensuring that detention basin design modifications did not result in a change in habitat disturbance, usable area for the project, detention basin footprints, number of dwelling units, or change the view simulations already produced for the project.
Fuscoe Engineering recently completed a hydromodification analysis for the project — the largest such analysis completed to date in San Diego County. The analysis was accomplished using the Hydrologic Simulation Program – FORTRAN (HSPF) in conjunction with the U.S. Environmental Protection Agency (EPA) Basins 4.0 software. A total of 18 detention basins were analyzed per the requirements set forth in the county of San Diego Standard Urban Storm Water Mitigation Plan and in accordance with the county Interim Hydromodification Criteria (IHC). The performance standard from the IHC requires the following:
- For flow rates from 20 percent of the pre-project five-year runoff event (0.2Q5) to the pre-project 10-year runoff event (Q10), the post-project discharge rates and durations shall not deviate above the pre-project rates and durations by more than 10 percent over more than 10 percent of the length of the flow duration curve.
- For flow rates from 0.2Q5 to Q5, the post-project peak flows shall not exceed pre-project peak flows. For flow rates from Q5 to Q10, post-project peak flows may exceed pre-project peak flows by up to 10 percent for a one-year frequency interval. For example, post-project flows could exceed pre-project flows by as much as 10 percent for the interval from Q9 to Q10 or from Q5.5 to Q6.5, but not from Q8 to Q10.
Because of the size of the Merriam Mountains development, it was necessary to draw upon historical data from three county rain gauge stations. Rainfall data was provided in one-hour intervals from May 1, 1979 to Dec. 31, 2005 (therefore data for complete hydrologic years beginning Oct. 1 and ending Sept. 30 was available from 1980 to 2004). GIS data CA042863 for the same time period and station was used to determine input for the parameters of evapotranspiration, wind, and temperature.
Model calibration was necessary to perform the analysis. Calibration parameters are still being developed for areas within San Diego County, so the model was calibrated using the Contra Costa County guidelines. After calibration of the model parameters, 26 years of data was modeled for the pre-project, post-project, and post-project with detention provided. Per a threshold developed by a county of San Diego technical advisory committee during the processing of the study, the flow data was separated into discrete storm events based on a threshold of 0.002 cubic feet per second per acre (cfs/acre). Once the flow rate dropped below 0.002 cfs/acre for a period of 24 hours or longer, it was deemed a separate storm event. The peak of each event was then obtained to develop the peak flow frequency curves for each detention basin. Flow duration curves were also developed for each of the detention basins by utilizing the entire 26-year data set.
To develop the peak flow frequency and flow duration curves for the post-project scenario with detention considered, stage-storage-discharge data was developed for the detention basins. Through an iterative approach, the outlet pipe from the detention basins was designed to consider the interim hydromodification criteria, the water quality control volume, and the 100-year flow event, as necessary. Discharge through the culvert was determined using standard orifice calculation.
The proposed detention basins were designed to provide hydromodification impact management, extended detention of the water quality control volume (WQCV), and/or attenuation of the 100-year storm. Not all basins are required to provide detention for all three concerns. Table 1 provides a matrix illustrating the functions and purposes of each detention basin proposed with the project.
In general, outlet structures consisted of large-diameter riser pipes that were designed in a manner that reserved the lower portion of the detention basin for hydromodification management and extended detention of the WQCV. Above the upper limit of the WQCV, an orifice was designed to convey 10-year storm events, and the 100-year storm event was designed to discharge through either the top of the riser pipe, or in some cases through a spillway channel designed in the top of the detention basin.
WQCV, storage depth, and drawdown time were determined for each detention basin as the starting point for outlet structure design. The outlet structures were initially designed with a target drawdown time of 48 hours for the WQCV. To achieve the target drawdown time, small orifice holes were designed at varying diameters and depths. This initial design was then used as a first iteration for hydromodification flow management.
Quite often, the most difficult part of the modeling effort was to control the flows associated with design storms ranging between the five-year and 10-year events. Iteration was required, and diameters of orifices at the bottom of the riser structure were typically increased, and the elevation for the 10-year orifice control typically had to be raised at least 1 to 2 feet above the WQCV depth. In some cases, the drawdown time for the WQCV was decreased by approximately half a day, but in all cases a drawdown time greater than 24 hours was preserved.
Prior to changes in county stormwater requirements, design for attenuation of peak runoff from 100-year events was completed and approved at the staff level. Provision for hydromodification management and water quality detention required a complete remodeling of the 100-year event.
The HSPF model showed that with detention considered and outlet structures provided, post-project flow rates would be decreased to below pre-project levels. Thus, it was determined that the detention basins are adequate to attenuate the runoff from smaller, more frequent and geomorphologically significant flow rates. Attenuated 100-year flow rates were substantially similar to previously approved discharges, and in all cases, the attenuated flows were less than or equal to existing conditions.
Most importantly, the footprints of the detention basins were not revised in a manner that impacted the environmental clearance for the project and did not lead to additional disturbance to natural areas. View simulations for the project, number of lots, amount of usable area, and total number of dwelling units also remained unchanged. In late October 2009, the county of San Diego Planning Commission accepted the county engineering staff proposal and recommended the project for approval by the county Board of Supervisors.
Paul Haaland, P.E., CPSWQ, is a project manager with Fuscoe Engineering, Inc. He can be contacted at firstname.lastname@example.org.