Reservoirs

When thinking about where rain goes in a watershed, it helps to think of a watershed as a series of reservoirs. In my last blog post, I mentioned this concept--anywhere water fills up and spills can be considered a reservoir. A bucket in your backyard, a rain barrel, and a swale would be obvious ones. Less obvious would be the soil profile itself, and the surfaces of leaves on plants and trees--it takes a certain amount of rain to get these surfaces wet before they start dripping--over an entire watershed, that adds up to a lot of water. Especially a watershed with giant redwoods.



Many of these reservoirs are leaky--not the bucket or rain barrel, but the soil and leaves can be. So it isn't always a threshold that is reached (the lip of the bucket) before water spills out--it can be a gradual increase (think of a bucket made out of sponges). In the case of the soil profile, as the soil pore space fills up, the water moving through the soil is under greater pressure, and the water leaking out of the soil downhill in springs or seeps or directly into creeks gradually increases.

Urbanization and impervious paved surfaces generally eliminate reservoirs and speed up the flow of water from raindrop impact to when it enters the nearest creek. They key for reducing peak flows and recharging groundwater and allowing water to seep out into the creeks later, after the peak flood, is to maximize the "reservoirs" in the watershed. Eliminating impervious surfaces creates a reservoir in the soil. Creating bioswales allows water to collect in depressions and soak in, instead of running right off into the nearest storm drain or creek.

Hydraulic connectivity is when a continuous path of saturated surface is present. Once hydraulic connectivity occurs, any additional rainfall will be conveyed rapidly to the nearest stream. When the reservoirs are full, hydraulic connectivity is present, and each raindrop will travel rapidly to the creeks.

The largest floods will always occur when the largest rainstorms have filled all the reservoirs, and there is no more capacity for the watershed to store rainfall. You can't stop these floods, although reducing the extent of impervious surfaces in the watershed may help slow the runoff down slightly. The best mitigation for these floods is to remove infrastructure from floodplains and areas susceptible to erosion. Get out of the way. It is too expensive to design a system of reservoirs that will capture the largest floods. Fortunately they are rare enough that the destruction, FEMA payouts, and rebuilding may not happen very often (in most areas). But the medium-sized floods can be mitigated with swales and removal of impervious surfaces.

In my watershed, I'm trying to hone in on the threshold rain events (duration and intensity) that fill the reservoirs and get everything hydraulically connected, resulting in major floods. In my last blog post, I was identifying 1/3 of an inch per hour following a day with at least an inch of rain as a threshold above which major floods can occur relatively quickly--as long as the duration is long enough and the antecedent conditions are wet enough.

The January 3rd, 2017 storm supports that hypothesis. With little rain the day before, rainfall rates between noon and 10 pm were generally under 1/4 inch per hour in San Geronimo and on Mt. Barnabe (even less in Woodacre), and San Geronimo Creek rose from 20 cfs to almost 400 cfs during those 10 hours (200 cfs to a little over 300 cfs in 2 hours). Compare that to my last blog post, which cited a flood when the creek rose from 200 cfs to 2000 cfs in 2 hours, thanks to over an inch of rain the day before and rain rates closer to 1" per hour at the peak. The January 3rd storm had neither the antecedent conditions (at least an inch of rain the day before) nor the intensity (over 1/3 inch per hour) required to reach the bankfull flood of approximately 1,500 cfs (with a 2-year recurrence interval).

My next hypothesis: the windier the storm, the lower the rain rate needed to generate a rapid rise in the creek. I suspect this may be the case since the biggest wind gusts shake the trees and drive the rain off the leaves more quickly than if there had been no wind. If the volume of leaf "reservoir" storage in the watershed is significant, then this should be the case. If the leaf "reservoir" effect is not significant, then the effects of wind should not be noticeable.