University of California, Merced
IN-15: Stormwater Modeling

All of UC Merced’s development goes through hydrology modeling which includes assessing Low Impact Development or LID practices to manage stormwater by minimizing impervious cover and by using natural or man-made systems to filter and recharge stormwater into the ground. Roads, parking lots, and other types of impervious cover are the most significant contributors to stormwater runoff and we include LID practices such as gravel parking lots in our most recent parking lots developed to capture and filter stormwater. Bio swells mentioned in the original submittal are additional green infrastructures that we developed into the campus throughout.

The goal of our LID assessment through hydrology modeling is to reduce runoff and to mimic a site’s predevelopment hydrology by minimizing disturbed areas and impervious cover and then infiltrating, filtering, storing, evaporating, and detaining stormwater runoff close to its source. This is why we have 2 retention ponds on campus that collect, filter and store 100% of stormwater runoff on campus. These ponds then recharge our local sub-basin. Other LID practices we look at during our modeling assessment include measures such as preserving undeveloped open space, rain gardens, green roofs, porous pavement, and biofiltration. The campus holdings actually include 7,492 acreas which we are only building on 1026 acres. The remaining 6,446 preserved acres are in a conservation easement which will protect the property’s conservation values in perpetuity.

Our storm drainage management system is designed to capture and treat 100% of our campus storm water runoff and specifically Total Suspended Solids (TSS) and Total Phosphorus (TP) from which these two areas comprise of four components: bio-swales, catch basins, hydraulic separators, and detention ponds in a treatment train. During a storm event, storm water runoff from the campus impervious areas (buildings, sidewalks, parking lots and other hardscapes) is routed to bio-swale area then to a storm drainage system. As runoff flows through the vegetation of the bio-swale, it reduces the velocity of the storm water runoff, allowing some percolation and filtration to take place. After the storm water runoff travels through bio-swale areas it enters a typical storm drainage system, consisting of inlets, catch basins, area drains and piping for conveyance. The system uses inlets as part of the treatment train to trap the largest particles that might enter the downstream system. So Typically, sediments that do not pass through a No. 4 sieve as well as large floatables, are trapped at the inlet. Storm water is conveyed downstream through a system of piping that terminated at the Continuous Deflective Sepertaion (CDS) unit. The CDS unit is sized to treat the first flush of storm run-off from the site and bypass storm run-off flow rates that are higher. The CDS unit routes storm run-off through a separation chamber with a cylindrical screen. A natural vortex is formed and the high velocities push out the suspended solids in the chamber. The sediment then settles into a sump while floatable particles remain in the separation chamber. The unit will capture 95% of solid particles down to those passing a No. 10 Seive and 100% of floatable materials that flow through the unit.