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Coyote Valley Not Only Stores Water It Absorbs Water: Natural Infrastructure and Flood Protection

Updated: Oct 20, 2018

When the average person thinks of climate change the first thing that comes to mind is“hot.” That is true, but according to NASA, climate change will not only bring extreme weather resulting in increased fire, drought, and hurricanes, but we will also see a change in precipitation patterns. This means inland areas will flood more often, not only due to heavy rainfall, but because humans have altered natural landscapes, foothills and valleys designed for infiltration, percolation and absorption.

NASA Global Climate Change Vital Signs of the Planet

Taken as a whole, the range of published evidence indicates that the net damage costs of climate change are likely to be significant and to increase over time.
- Intergovernmental Panel on Climate Change

After five of the driest years on record, in 2017 we had the wettest year on record. The amount of groundwater recharge is really about the runoff produced in the upper watershed, not just the rain that falls on Coyote Valley directly. In 2017, after the Coyote Creek President’s Day Flood on Feb. 20, the Water District released water from Anderson Reservoir for about two months at 800 ac-ft/day. That water refilled not only the Coyote Valley Sub-basin but flowed down gradient to refill the main Santa Clara Sub-basin and reduced our drought response.

(note: 1-acre foot (ac-ft) of water covers a football field 1 foot deep, 325,851 gallons of water)

City of San José Coyote Valley Water Supply Assessment

The Coyote Valley Sub-basin is roughly 7 miles long and 2 miles wide, with a corresponding surface area of about 15 square miles, and contributes groundwater through the Coyote Narrows into the Santa Clara Sub-basin, which covers a surface area of 225 square miles (City of San Jose).

This is such an important topic that we decided to interview San Jose State University Environmental Studies Professor Pat Ferraro who is an expert in water resources, a distinguished engineer, and prior Water District board member.

Pat, why is Coyote Valley so important? "Having the Coyote Valley Sub -basin as permanent open space will allow rainfall to percolate into the basin, rather than become rapid runoff that causes floods downstream in downtown San Jose."

Awesome Pat, but you didn't think I would let you off that easy, did you? You're the Water Wizard and the people need to know. How much water are we talking about? Let's just say that Coyote Valley was impervious at the time of the flood on Coyote Creek. Let's do the math, square miles, amount of rain, millions of gallons, some engineering calculations over my head. How much water would have hit downtown San Jose if Coyote Valley was developed? One times, two times, ten times?

Coyote Valley from Coyote Valley Open Space Preserve

“Let’s start with the higher runoff factor that happens when open space is urbanized. Because of the Sig Sanchez 8-lane speedway (101), Monterey Rd and the feeder streets. The runoff factor is probably now about 10%. Fully developed, the factor could rise to 80%. So for any amount of rainfall, runoff would increase 8 times: Runoff today, as Open Space with the 3-inch rainfall in Feb 2017: 3/12 ft x 7000 acres x 0.10 = 135 ac-ft or roughly 45 million gallons. If that runoff enters Coyote Creek at an even pace, the streamflow would increase about 65-70 cubic feet per second. A fully developed Coyote Valley would increase all these numbers by 8 times.

(note: 1 cubic foot per second (cfs) equates to 7.5 gallons per second)

The timing of this flow would precede the flows generated in the upper watershed above both the dams (Anderson and Coyote). If the dams have flood storage available, some or all that upstream runoff could be captured behind the dams. But, when both the reservoirs are full and spilling, only a small amount of the peak discharge is stored for a short term. This is called surcharge storage. The flow backs up and raises the reservoir level above the spillway elevation. So, a couple of feet of surcharge storage on Anderson Reservoir is about 2000 ac-ft. This will reduce the discharge from the spillway by about 1,000 cfs over 24 hours. So, now look at the potential for Coyote Valley to buffer the flood flow as the upper peak moves through about 12-15 hours after the storm. If the water table is below grade (at the surface) some small amount of water will percolate into the basin and be removed from the flood peak.

Anderson Dam spillway 02/20/17

Coyote Valley becomes most valuable for flood protection when the groundwater basin is completely full. Last year, it got to within one foot of the surface AFTER the reservoirs were drained down to their mandated storage limits. Besides the uncontrolled flood flows after the storm, about 35,000 ac-ft was released slowly at 400-450 cfs (800-900 ac-ft/day). Once Coyote Valley is full, it then officially becomes a vernal wetland and the entire 7,000 acres acts like the upstream reservoirs and begins its surcharge storage. Each foot of storage stores 7,000 ac-ft of the flood flow, possibly taking 3,500 cfs off the peak flood flow.

Coyote Valley acts as a flood plain when Coyote Creek overflows 02/20/17

If this had happened in 2017, the flow through San Jose could have been reduced to about 4-5,000 cfs and no homes would have flooded. Also, no flooding would have occurred in downtown San Jose if the mandate storage levels set by the Board and the State Division of Dam Safety had not been exceeded by about 35,000 ac-ft.

One inch of rain in the 200 square miles above the dams produces about 10,000 ac-ft of water when fully saturated, as it was on Feb 20, 2017. The three-inch storm that occurred the next day produced about 30,000 ac-ft of runoff, all of which flowed uncontrolled over the spillways and into Coyote Creek, through Coyote Valley and into downtown San Jose, causing 14,000 homes to flood.”

San Jose flood victims

When we asked Pat if there is more flood protection potential within Coyote Valley he said, "Flooding can also be mitigated by restoring Laguna Seca, south of the Metcalf Energy Plant, to capture the flow of Fisher Creek running on the west side of Coyote Valley. Right now, it flows into Coyote Creek and out to the Bay and may cause flooding along the way."

Laguna Seca seasonal lake in Coyote Valley, Santa Clara County

Coyote Valley provides free environmental services for the residents of Santa Clara County, green infrastructure, that not only provides drinking water, but crucial flood protection in a time of extreme, unforeseen weather patterns.

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