We must understand city watersheds to manage them

Management of storm drainage in the city of Half Moon Bay has recently become of concern because of the risk from climate change bringing us “cyclone bombs” and “atmospheric rivers of rain.” We wanted to provide an overview of issues specific to the Miramar Watershed and to encourage others to provide their experiences with storm drainage. 

Half Moon Bay began as a rural agriculture area, primarily used by Mission San Francisco de Asis. After World War II numerous subdivisions were developed, including Miramar, leading to the city's incorporation in 1959. Streams in Half Moon Bay include Naples Creek, Frenchmans Creek and Pilarcitos Creek. The watershed areas (with percent area) from north to south include Roosevelt (8 percent), Pullman (6 percent), Frenchmans (2 percent), Pilarcitos (23 percent), Kehoe (11 percent), Beachwood (2 percent), and Kelly/Metzgar (10 percent). The term “watershed” refers to a land area that channels rainfall to creeks and ditches and eventually to outflow points such as the ocean.

Combining the Roosevelt and Pullman watersheds as the Miramar Watershed (14 percent of the drainage area of Half Moon Bay) is logical since both drain through the Miramar Division. The Miramar Watershed supports family dwellings of Miramar, which are on a flood plain, and Rocket Farms, which is one of the largest employers in the city. This combined watershed is bounded by Miramar Street to the north, the Corral de Tierra mountain ridge to the east, and Rocket Farms to the south. Naples Creek begins high in the Corral de Tierra mountain ridge and passes through Rocket Farms, where the water combines with runoff from the impermeable surfaces of farm property to a series of culverts, sumps and reservoirs on the east side of Highway 1.  

Flow is directed west through two pipes under Highway 1, one going to Roosevelt Ditch and the other to Pullman Ditch and eventually into the Pacific Ocean. The flow volume distribution is determined by the gates on the culverts on farm property. 

The Pullman ditch is man-made and 1,000 feet long running from the east side of Highway 1 on a straight line to Roosevelt Beach. The city of Half Moon Bay allowed building on the lots that included the Pullman ditch by undersizing the building space on the lots so the ditch actually passes through five private property owners who own 500 feet, 100 feet per lot. At Alameda Avenue comes 40 feet of city property, a site of the most serious flooding. This ditch has almost tripled in depth and width in the last three years. The city is expecting roughly a dozen private citizens to assume the dangerous consequences, though the city owns the site of the historical worst flooding at the south end of Alameda Avenue. 

One solution is relatively simple and could start immediately. The city should work with the citizens living along the ditches to prevent erosion and property damage. The other solution is described in the Los Angeles Times:  The flood prevention approach previously applied all over the state of dumping storm runover into the ocean is not appropriate in times of drought. The aim should now be to store the water on the land. 

There are things we can do as private citizens to participate in programs to help citizens handle excessive rain water as well as the drought. Barrels and larger water storage tanks would always be collecting rainwater for use during drought months, but during the rainy months they could be slowly emptied before and after big rains. Governmental actions could include supporting rain barrel, cisterns and water tank projects on existing properties and mandating them on new construction similar to the mandates on solar panels. 

Susan Suer, Danielle De Young, who is a hydraulic engineer, and Vic Froelicher are all Half Moon Bay residents and live in the Miramar Watershed.