UCSF Mount Sutro Management EIR
4.11 Wind
This section describes the existing site conditions, methodology and results used to quantify the current (“baseline”) wind environment, and CEQA regulatory setting for the 61 acre Mount Sutro
Open Space Reserve (“Reserve”). This section also evaluates wind-related impacts associated with the proposed management activities across the Reserve. It includes an analysis of impacts associated with windthrow in the Reserve, and wind impacts to surrounding neighborhoods.
Comments related to wind received during the Initial Study/EIR scoping process included concerns about the following:

Reducing the effectiveness of the windbreak provided by the dense forest, thereby increasing wind speeds within the Reserve and adjacent neighborhoods

Effects on tree health as result of exposure to winds

A potential increase in the spread of wildfire by reducing the windbreak formed by the dense forest and by altering wind patterns (this topic is discussed in Section 4.7 Hazards
– Fire Hazards)
Climate is the multi-year long-term average of daily weather cycles that occur over a large geographical area. Regional climate is the average of long term weather cycles over a smaller and more specific area. Often times, these smaller areas are defined by air basins, in which air currents may normally flow freely until confronted by elevated terrain or air mass differences from other basins. Regional climate can be affected by large scale changes in the atmosphere, local changes in vegetation cover or short-term weather changes.
The climate of the San Francisco Bay region, along with much of coastal California, is controlled by a semi-permanent high-pressure system that is centered over the northeastern Pacific Ocean.
Beginning in the autumn and continuing through the winter, the high-pressure system weakens and moves south, allowing storm systems originating from the Gulf of Alaska and the Pacific
Ocean into the area. Temperature, winds, and rainfall are more variable during these months.
Occasionally the broad scale circulation pattern permits a series of storm centers to move into
California from the southwest. This type of storm pattern is responsible for occasional heavy rains that may cause serious flooding (WRCC 2012a). Average high and low temperatures and precipitation data are presented in Table 4.11-1 for the San Francisco Mission Dolores weather station. The Mission Dolores station is the nearest station to the project site that currently measures temperature and precipitation, and is readily available by the Western Regional
Climate Center (WRCC).
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UCSF Mount Sutro Management EIR
4.11 Wind
Winds in San Francisco are generally from the west, off the Pacific Ocean. The average wind speed is greatest in the spring and summer months, and lower in the fall and winter, except during storm events. Daily variation in wind speed demonstrates the strongest wind speeds in the late afternoon and lightest winds in the morning. Windthrow (the uprooting and overthrowing of trees by wind) is often more important in mid and upper slopes than in lower slopes (Navratil 1995), and wind blowing perpendicular to edges are most damaging (Ruel 1989).
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Average
Max.
Temperature
(F)
Average
Min.
Temperature
(F)
Average
Total
Precipitation
(in.)
56.5 59.8 61.5 62.8 63.9 66.1 65.8 66.7 69.8 69.1 63.7 57.3
45.7 47.9 48.8 49.7 51.1 52.9 53.7 54.5 55.5 54.4
4.35 3.81
Period of Record: 1914-2012
Source: WRCC, 2012b.
2.9
51 46.8
1.42 0.57 0.16 0.02 0.05 0.22 1.05 2.57 4.04
63.6
51
21.15
Table 4.11-2 presents wind data from the San Francisco airport. The prevailing wind direction at the San Francisco Airport is westerly. The consistent westerly winds are caused by the combination of the high pressure offshore and a thermal low pressure resulting from higher temperatures inland. On average, the wind speed is around 4.7 meters per second (m/s) or 10.5 miles per hour (mph). Wind gusts are strongest in the winter months from November to March; the highest recorded at the airport was 33 m/s (73.8 mph) in December 1995.
Strong winter storms bring powerful wind events to the San Francisco area, where wind gusts can reach 45 m/s (100 mph) along the northern coast of California; wind gusts at these speeds have been known to cause the windthrow of trees (McBride and Leffingwell, 2006). Sometimes these wind events are combined with heavy precipitation, which can result in falling tree branches and windthrown, uprooted trees.
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4.11 Wind
(a) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC YEAR
WIND
Mean Speed
(m/s)
Mean Speed
(mph)
Prevailing
Direction
68 3.2
68 7.2
3.8
8.5
4.7 5.5 6.0
10.5 12.3 13.4
6.3 6.1 5.7
14.1 13.6 12.7
5.0 4.2
11.2 9.4
3.4
7.6
3.2
7.2
4.7
10.5 through 1964
WN
W
WN
W
WN
W
WN
W
W W NW NW NW
WN
W
WN
W
WN
W
WNW
Peak Gust
Direction
Speed (m/s)
Speed (mph)
12 S S S NW NW
12 27.3 30.8 28.6 24.1 25.9
12 61.1 68.9 64.0 53.9 57.9
W W W
22.8 23.2 20.1
51.0 51.9 45.0
NW S S SW SW
19.7 25.9 26.8 33.1 33.1
44.1 57.9 60.0 74.0 74.0
-Date 1995 1986 1995 1984 1984 1984 1987 1992 1994 1989 1984 1995 Dec-95
(a) - Length of Record in Years, although individual months may be missing.
0.* or * - The value is between 0.0 and 0.05.
Normals - Based on the 1961 - 1990 record period.
Extremes - Dates are the most recent occurrence.
Wind Dir.- Numerals show tens of degrees clockwise from true north.
Resultant Directions are given to whole degrees.
DATA FROM WRCC: http://www.wrcc.dri.edu/cgibin/clilcd.pl?ca2324
Wind data is also readily available from National Ocean and Atmospheric Administration
(NOAA) for buoys near the San Francisco coast line (NOAA 2012). A buoy is located approximately 0.75 miles east of the Golden Gate Bridge in the San Francisco Bay. The buoy is near the southern shore, just north of the Gulf of the Farallons National Marine Sanctuary (37.807
N, 122.465 W).
NOAA buoy data has been quality assured, and has measurement for wind direction, average wind speed, and wind gusts collected approximately every 6 minutes, or 10 observations per hour. Data from 2007, 2008, 2009, and 2011 were collected from the National Buoy Data Center.
Buoy data from 2010 was not used for this analysis, as the year had insufficient data capture. A windrose for the data is shown in Figure 4.11-1 . The average wind speed is approximately 3.3 m/s (7.4 mph) at the buoy, with winds mainly from the west and southwest.
The slight differences in wind speed and direction between the NOAA data and the SFO data may be attributed to a number of different site conditions including topography, obstructions to wind fetches, site characteristics and instrument heights. The NOAA data is from a buoy right on top of the water in the bay, while SFO’s data is from a 10 meter tower located in the middle of an airstrip.
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4.11 Wind
Figure 4.11-1
San Francisco NOAA Buoy Wind Rose
2007-2009,2011
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4.11 Wind
Gusty wind events exceeding 22.4 m/s (50 mph) from the west and north occur throughout the year at the buoy station. These gusts are usually short, instantaneous wind speed bursts, but may occur several times in one day. Table 4.11-3 shows the number of days in a year where winds equaled or exceeded 22.4 m/s (50 mph) during 2007-2009 and 2011.
2007 2008 2009 2011
50 1 20 48
Source: National Buoy Data Center (NOAA, 2012)
One potential result of high winds is windthrow, defined as the uprooting and overthrowing of trees by wind. Wind disturbances, from small-scale to stand-replacing, can exert strong impacts on forest structure in forested ecosystems (White and Pickett 1985, Foster et al 1998; Kramer et al
2001). Many factors affect windthrow. Research has shown that both biotic (of or related to living things) and abiotic (physical rather than biological) factors determine how windstorms impact forested ecosystems (Everham and Brokaw 1996); researchers have long debated the relative influence of these. Abiotic factors affecting the amount of wind damage include wind speed, topography and soils. Examples of biotic factors that influence wind events in forests include tree composition, interactions with other mortality factors like forest pathogens and pests, and tree density. These factors interact in complex patterns and their relative importance can change with each new wind event.
Windthrow events typically occur during storm or high wind events. While high and low wind speeds are not officially defined, for the purposes of this report, low wind events are defined as average wind speeds of less than 8 mph, where wind can be felt on the face, but is not sufficient to lift material. At low-moderate wind speeds (8-13 mph) wind will disturb hair, can cause clothing to flap, or can extend a light flag mounted on a pole. Moderately high winds (13-26 mph) can raise loose paper and dry soil from the ground and can be felt on the body. High wind events
(greater than approximately 26 mph) are defined here as those that impede walking and lift items larger than loose paper from the ground (based on definitions from Arenas 1981).
High wind velocities are common along the north coast of California; high velocity wind gusts sometimes reach 45 m/s (100 mph) in the San Francisco area, and are highest in the winter months of November to March (see Section 4.11.2.1, Regional Climate, San Francisco Airport). For instance, a severe windstorm occurred in San Francisco on December 13, 1995, with winds of 33 m/s (93 mph) recorded on the Golden Gate Bridge. This storm resulted in the windthrow of an
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UCSF Mount Sutro Management EIR
4.11 Wind estimated 6,000 trees in the Presidio causing damage to many structures, road closures, and one death (McBride and Leffingwell 2006). On the basis of field observations and a literature review,
McBride and Leffingwell evaluated the role of the three most important landscape characteristics associated with windthrow of the Presidio trees: soil type, topography, and tree species. These variables are discussed below followed by an analysis of tree density and other biotic factors as they pertain to windthrow impacts in the Reserve.
Windthrow is often associated with shallow soils that are subject to saturation; shear strength of soils decreases with increasing moisture content and is also very low in soil textures dominated by sand. A total of one soil map unit, the Candlestick-Kron-Buriburi Complex 30-75% slopes, is present within the Reserve (NRCS, 2012). This soil map unit is primarily composed of three soil series, 40% of which is the Candlestick series (or similar soils), 25% Kron series (or similar soils), and 20% Buriburi series soils (or similar soils). These soil series primarily have a loam texture
(texture varies from fine sandy loam to sandy clay loam). Both the Candlestick and the Buriburi series have moderate depth to bedrock (20-40 inches to bedrock and 30 inches to bedrock, respectively) whereas the shallow Kron series typically has a depth of 10-20 inches above bedrock. All three series are well drained. Overall, the soils are not excessively shallow nor are they excessively sandy. However, in certain areas, such as within the more shallow areas covered by Kron series soils, the soil shear strength could decrease sufficiently after a significant precipitation event. Such an event would lead to increased risk of windthrow. For more information on soils, see Chapter 4.5 Geology and Soils.
A geotechnical and geological evaluation report prepared for the site identifies site geology and analyzes associated slope stability (R&C 2011). Generally areas identified as having unstable colluvium were determined to be less stable and more susceptible to landslides compared to areas underlain with Franciscan Chert (R&C 2011). Areas with unstable geology are prone to landslide events and could be higher risk areas for windthrow.
The topographic setting of a site can also greatly influence exposure to wind. Sites located on high ridges, in topographic saddles, and at the heads of canyons are sites that experience high wind velocities (Harris 1989). Aspect is important; areas more prone to windthrow would be expected to be slopes directly facing prevailing winds (Harris 1989). Windthrow is often more important in mid and upper slopes than in lower slopes (Navratil 1995), and wind blowing perpendicular to edges are most damaging (Ruel 1989). Topography varies throughout the
Reserve and may play an important role in wind speeds within the Reserve.
Tree density is one of the biotic factors that impacts windthrow risk. Windthrow risk is exacerbated by the opening of forests stands to accommodate roads and provide building sites during urbanization (McBride and Leffingwell 2006). Though gap sizes left by thinning will be on average smaller than those left by building sites or road building activities, thinning of trees
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UCSF Mount Sutro Management EIR
4.11 Wind can be expected to have a similar impact in allowing increased wind penetration into the Reserve.
Trees once protected by neighboring trees would now be exposed to greater wind speeds.
One study in the Pacific Northwest found that, in a comparison of thinned versus non-thinned plots, the overall level of wind damage across all thinned plots after two growing seasons was not statistically greater than in unthinned control plots. However, where gaps (e.g. thinning areas) were located in topographically vulnerable positions, greater wind damage did occur
(Roberts et al 2007). This suggests that tree thinning will not have a consistent impact on windthrow risk, but will depend on topographical vulnerability.
In addition to this, there is a timing factor. While thinning can initially increase the risk of wind damage to trees, trees can become adapted to their new environment. Trees eventually adjust the allocation of stem and crown growth, resulting in a more stable configuration (Wonn and O’Hara
2001). Thinning promotes diameter growth more than height growth, so thinned stands develop lower height to diameter ratios over time. In this way, the risk of wind damage can be greatly reduced following thinning, and studies have shown that reducing stand density can lead to a greater long-term wind-firmness (Cremer et al 1982, Wonn and O’Hara 2001).
Another biotic factor predicting risk of windthrow is species composition. Tree species vary in their susceptibility to windthrow due to factors like shear strength of wood, root structure, and canopy structure (McBride and Leffingwell 2006). McBride and Leffingwell (2006) found that, based on these factors, the relative risk of windthrow for the dominant species in the Presidio was Monterey cypress (Cupressus macrocarpa), Monterey pine (Pinus radiata), then, most robust, blue-gum eucalyptus (Eucalyptus globulus). The Reserve is dominated (82%) by blue-gum eucalyptus, with small amounts of other tree species including Monterey pine and Monterey cypress, as well as blackwood acacia (Acacia melanoxylon), and coast redwood (Sequoia
sempervirens) (Hortscience 1999, UCSF 2010). Though blue-gum eucalyptus fares well compared to other species found in the Reserve, their shallow root systems are of concern. Trees with shallower root systems are more susceptible than trees with deeper root systems (Burns and
Honkala 1990). Indeed, downed trees have already been seen throughout the Reserve.
HortScience, in their 1999 report, noted that tree failures, particularly windthrow of entire trees, were common occurrences. The proposed management actions may lead to a conversion in species composition (e.g. through planting) that adds species that are more robust (e.g. coast live oak (Quercus agrifolia), bay laurel (Umbellularia californica) or less robust (e.g. Monterey cypress,
Monterey pine) to windthrow. Any changes in species composition could impact the Reserve’s resiliency to windthrow.
Finally, removal of dead, dying, unhealthy, and hazardous trees (see Chapter 3, Project
Description) would be expected to reduce windthrow risk in the Reserve in at least two ways.
First, dead and diseased trees are structurally compromised and would be most likely to fail during high wind events, often falling onto other trees (even healthy trees) and increasing their risk of failure. This increases the size of gaps left by windthrow, which increases the risk of future greater wind damage (Roberts et al 2007). Second, as stated above, the remaining healthy trees are predicted to grow larger through release from competition, and this increase in growth would
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UCSF Mount Sutro Management EIR
4.11 Wind occur through diameter growth more than height growth. Remaining healthy trees would be more robust to windthrow.
Trees may serve as windbreaks that can reduce wind speeds in adjacent areas. A windbreak is defined as is a tall, dense, continuous wall of vegetation. The height determines how far wind protection extends, and the density determines the degree of protection. The general rule is that a windbreak will reduce wind to a distance 10 times its height and reduce wind speed 70 to 80 percent immediately inside the barrier (Kansas State University 2004; Wisconsin Department of
Natural Resources 2003). The degree of protection from a windbreak gradually decreases with distance (Kansas State University 2004). Four basic factors are considered when designing a windbreak: its orientation in relation to the prevailing winds, its width, plant and density arrangement, and the species of plants selected (Ohio State University 2012).
Windbreak density affects the pattern of air movement around the windbreak. Wind velocity is reduced as the density of the windbreak is increased, and, as a result, the area protected tends to be increased. Density depends upon the type of trees and shrubs and the number of rows planted. Density within a windbreak can be increased by planting multiple rows of evergreen trees. Finally, wind eddies (or, small-scale air turbulence) will form around the edges of a windbreak. Therefore, windbreaks should extend at least 100 feet beyond the area to be protected. Any gaps will funnel the wind, eliminating much of the windbreak’s effectiveness
(Kansas State University 2004).
A standard farmstead windbreak has at least three rows: the outside or windward row; one or more interior rows; and the inside or leeward row. Four to six rows provide greater protection, but even one or two rows are beneficial. The standard Wisconsin Department of Natural
Resources windbreak packet contains 200 spruce and 100 white or red pine- enough stock to plant a 3-row windbreak that is 800 feet long (Wisconsin Department of Natural Resources 2003).
Three hundred (300) feet is the minimum windbreak width recommended (Hess and Bay 1994).
Mount Sutro is roughly 400 to 900 feet above sea level with exposed high terrain above the city level, featuring distinct topographic features and forested areas. It can be expected that higher wind gusts occur throughout the year at exposed locations on Mount Sutro compared with winds measured at the buoy station or San Francisco Airport. In addition, the highly variable topography and vegetation cover within the Reserve affects both the wind speed and direction variability. Densely forested areas of Mount Sutro would be expected to experience reduced winds compared to sparser areas, all other variables held equal. Topographically sheltered locations would also be expected to experience reduced winds.
Daily (diurnal) wind patterns occur around elevated terrain like the higher areas of the Reserve.
In the morning the sun warms the air at the land surface and causes it to expand and rise
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UCSF Mount Sutro Management EIR
4.11 Wind vertically. The atmosphere near the elevated terrain is slightly less dense because it is higher in elevation. As this air is slightly less dense and frequently drier than the air at sea level it heats more quickly, buoyantly rising. The atmosphere constantly tries to balance between areas of low pressure and areas of high pressure, and the winds flow from higher pressure areas to lower pressure areas. In the daytime this causes wind flow from sea level toward the mountains, or upslope winds. In the evening the wind pattern is reversed. The air in elevated terrain cools more quickly than air near sea level, and this cooler air will start flowing downhill. This pattern is called downslope winds.
Onshore and offshore wind patterns also impact the wind patterns at the Reserve. These are created by temperature differences at the land surface, similar to the upslope and downslope wind pattern mechanisms. In the morning the land heats more quickly than the water causing the air over the land to heat, expand and rise. This creates an area of lower pressure over land and causes air to move from the water to the land at the surface; this phenomenon is called an onshore wind or sea breeze. In the evening, the land cools faster than the water causing the air over the land to cool, get denser and flow toward the relatively lower pressure over water, thus creating an offshore wind or land breeze. The sea breeze-land breeze flow pattern is often the primary wind pattern mechanism in coastal California; it may be interrupted only by high and low pressure systems, frontal passages, and storms.
In the winter, when the Pacific high pressure system weakens and moves towards the south, low pressure systems frequently pass through the project area. The counterclockwise wind flow around low pressure systems creates a sometimes easterly flow during the winter months.
To investigate the wind environment in the Reserve, Vantage Pro2 Weather Stations
(manufactured by Davis Instruments) were installed in the Reserve. The stations were established in two paired plots for a total of four wind stations. The goal in placement of the paired plots was to locate one station in an area of sparse tree cover and the other nearby (within 150 feet) in an area of dense tree cover, controlling for the other major variables that impact wind speed and direction (e.g. topo-position, aspect, elevation) wherever possible. Further, the sparsely-forested plots were chosen to match the 30 foot average spacing between trees proposed in UCSF’s vegetation management plan (UCSF 2010).
Figure 4.11-2 identifies the location of the wind stations in the Reserve. Wind stations 1 and 2
(referred to as dataset 1) were located just north of Medical Center Way and west of the Surge
Building close to the bottom of Mount Sutro. Wind station 1 was sited in an area of sparse (S) forest cover approximately 100 feet due west of wind station 2 which was located in an area of dense (D) forest cover. The two stations were located at equal elevation (610 feet), aspect (N), and topoposition.
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UCSF Mount Sutro Management EIR
4.11 Wind
Source: URS
Page 4.11-10
Figure 4.11-2
Wind Station Locations

UCSF Mount Sutro Management EIR
4.11 Wind
Wind stations 3 and 4 or dataset 2, were located near the Rotary Garden close to the top of the
Mount Sutro Open Space Reserve. Wind station 3 was located in an area of dense (D) forest and wind station 4 was located approximately 50 feet east in a sparse (S) forest. The two stations were at an approximately equal elevation (855 feet) and aspect (NNW)); station 4 was in a slightly different topo-position at the top of the hill where a clearing (the Rotary Native Plant Garden) begins, whereas Station 3 was located just below the crest of the hill. Station 4 was abutted by bushes of an average height of 8 feet on its west and north sides, and by a meadow to its east and south.
All four wind stations were installed for a total of 39 days between May 31 and July 9, 2012.
However, (station 1 malfunctioned for the first 8 days of collection and was replaced). For all stations, measurements were taken every 10 minutes for 6 observations per hour per machine.
Wind direction and wind speed were measured by an anemometer located at approximately 6.5 feet above the ground. Wind speed data was collected in miles per hour (mph); because the windrose software (WRPLOT View- Lake Environmental Software) converts to meters per second (m/s), all wind speeds in this section have also been written in m/s. One m/s is equal to
2.237 mph. Wind direction data was taken in a sector format (e.g. N, NNE, NE) for every 22.5 degrees or 16 separate sectors. The data was converted to degrees for the windroses software and petals were created for the 16 different sectors. Table 4.11-4 presents a data summary for all four wind stations described above. Windroses and wind speed and direction frequency tables for each station can be found in Appendix J , Wind Data.
Number of
Observations
Data
Capture
Average
Wind
Direction
Average Wind
Speed, m/s
(mph)
Percent
Calm
Max Gust
Speed, m/s
(mph)
Station 1 (Sparse)
Station 2 (Dense)
4618
5654
82%
100%
WNW
NNW
2.3 (5.21)
0.01* (0.02)
23%
97%
16.1 (36.0)
4.0* (8.9)
Station 4 (Sparse) 5654 100% S 0.14 (0.31) 85% 5.8 (13.0)
Station 3 (Dense) 5632 100% SE 1.87 (4.18) 27% 10.3 (23.0)
*The same numbers were also computed for the common 31-day measurement period shared between station 1 and station
2.
Results indicate that wind data from the different stations on Mount Sutro were highly variable, though higher wind speed gusts consistently occurred in afternoon to late evening hours, and, in general, calmer winds occurred during night hours when the atmosphere was less turbulent.
Variable wind direction measurements seemed to depend on obstructions found near each particular station, such as vegetation or topographic features.
Station 2 (located in a very densely forested area) measured winds that were calm (low wind speeds) approximately 99% of the time during the 39-day measurement period. During the small number of times when non-zero wind speeds were measured at station 2, wind speeds were very light (0.4-0.9 m/s, 1-2 mph) and limited to the NNW direction, presumably due to the obstructing forest surrounding the station. The average wind speed at station 2 was only 0.01 m/s (0.02 mph)
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UCSF Mount Sutro Management EIR
4.11 Wind over the whole data period due to the large number of calms, or zeros, in the wind speed data.
The maximum wind gust reading at station 2 was 4 m/s (9 mph). Just to the west of station 2, station 1 (located in sparse forest) experienced an average wind speed of 2.3 m/s (5.2 mph), with much higher gusts than measured at station 2. Here the maximum gust speed was 16.1 m/s (36 mph). According to these averages, wind station 2 experienced 23 times lighter average wind speeds, and 4 times lighter wind gusts, than the neighboring station 1. Wind directions for station
1 were mainly from the western direction.
Table 4.11-5 , Station Winds Analysis by Time Correlation, presents data from each station by controlling the date and time variable to compare wind speed and wind direction measurements occurring at the same time at each station. Four local timestamps were chosen: morning (7 AM), afternoon (1 PM), evening (7 PM), and night (1 AM); while four random dates were chosen for this correlation study: June 10, 17, 25; and July 5.
For dataset 1, Table 4.11-5 confirms that station 2 experienced calm winds during all times of the day with occasional short-term light gusts from the north or northwest during the afternoon or evening hours. Station 1 measured more fluctuating wind directions in the morning and night hours, while during the afternoon and evening, wind direction was consistently from the WNW.
Wind gusts at station 1 for these times were on the order of 2.2-13 m/s (5-29 mph), mostly from the WNW. Station 1 also measured average wind speeds of 0.9-6.7 m/s (2-15 mph) higher than station 2, while wind gusts at station 1 were 2.2-11.2 m/s (5-25 mph) higher than measured at station 2.
For dataset 2, stations 3 and 4 had average wind directions from the southern sectors. As shown in Table 4.11-5 , station 3 experienced higher wind speeds than station 4, with an average wind speed of 1.87 m/s (4.2 mph) compared to station 4’s average wind speed of 0.14 m/s (0.3 mph).
The maximum wind gust at station 3 (10.3 m/s, 23 mph) was nearly double that of station 4 (5.8 m/s, 13 mph). The timestamp data reveals that station 4, located in sparse forest, measured average winds 0-4.5 m/s (0-10 mph) higher than station 3. Winds for station 3 were from the southeast during all times of day, with the exception of during night hours, when wind direction was more variable. Wind directions at station 4 were consistently from the south for most times of day, while similarly to Station 3, wind directions during night hours also varied. These findings were counter to expectation as station 3 was located under denser forest cover than station 4. However, as described above, station 4 was located near high bushes and at a different topoposition than station 3; both factors most likely obstructed wind flow measurements compared to the conditions experienced at station 3.
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UCSF Mount Sutro Management EIR
4.11 Wind
Date
6/10/2012 7:00 AM
6/17/2012 7:00 AM
6/25/2012 7:00 AM
7/5/2012 7:00 AM
6/10/2012 1:00 PM
6/17/2012 1:00 PM
6/25/2012 1:00 PM
7/5/2012 1:00 PM
6/10/2012 7:00 PM
6/17/2012 7:00 PM
6/25/2012 7:00 PM
7/5/2012 7:00 PM
6/10/2012 1:00 AM
6/17/2012 1:00 AM
6/25/2012 1:00 AM
7/5/2012 1:00 AM
Time Time of Day
Morning
Afternoon
Evening
Night
4.0
5.8
1.3
6.7
2.7
4.9
3.1
3.6
Dataset 1 Dataset 2
Station 1/Sparse Forest Station 2/Dense Forest Station 4/Sparse Forest Station 3/Dense Forest
Wind
Spd
(m/s)
Wind
Dir
High
Spd
(m/s)
Wind
Spd
(m/s)
Wind
Dir
High
Spd
(m/s)
Wind
Spd
(m/s)
Wind
Dir
High
Spd
(m/s)
Wind
Spd
(m/s)
Wind
Dir
High
Spd
(m/s)
0.9
2.2
0.9
2.7
2.7
5.4
SE
NW
SW
WNW
WNW
WNW
2.2
3.6
2.7
6.7
6.7
11.2
0
0
0
0
0
0
--
--
--
--
0
0
0
0
-- 0
NNW 2.7
0
0
0
0
0
0
NNW 0.9
NW
---
---
0.9
0
0
NNW 0.9
WNW 0.4
0
0
0
0
0.9
1.8
SSE
SE
---
ESE
SSE
SE
1.3
1.3
0
1.3
2.7
3.6
WNW 5.8
WNW 10.3
WNW 6.7
W
ENE
W
8.0
WNW 9.8
WNW 13.0
4.0
11.2
0
0
0
0
0
0
0
0
NNW 0.9
--
--
--
--
--
0
0
0
N 1.8
NNW 2.2
0
0
0
0
0
0
0
0.4
0.4
1.8
---
NW
NW
W
---
0
0.9
0.4
0.4
0
WNW 2.2
N
S
1.8
3.6
0.9
3.6
1.8
1.3
3.6
4.9
2.2
2.7
SE
SE
SE
SE
SE
SE
SE
NW
4.9
6.7
4.0
4.5
2.7
5.4
4.0
3.6
2.2
2.7
WNW
WNW
4.9
8.5
0
0
--
NNW
0
0.4
0
0.4
---
S
0
2.7
3.1
0
SE
N
4.0
1.8
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4.11 Wind
It is important to note that the 39-day and 31-day measurement period at these stations occurred during summer months when average wind speeds tend to be higher, but during which no major wind events occurred. Wind events during strong storm systems or frontal passages, or during strong Diablo wind events, would likely give conditions that may lead to windthrow on Mount
Sutro. Both wind direction and wind speed would differ during these events as compared to this dataset.
The Mount Sutro Open Space Reserve weather station data suggests that, all else held constant, tree density can play an important role in determining wind speed and direction. Dataset 1 demonstrated a 4-fold difference in highest gust wind speed in a sparse forest compared to a neighboring dense forest. The data referenced above suggested that topography and local obstructions can be overriding factors to tree density in controlling wind measurements.
There are no regulatory controls over vegetation management activities that pertain to wind conditions. CEQA does not identify any specific criterion for the evaluation of wind effects of a project. The City of San Francisco has established standards and criteria for the evaluation of wind impacts in downtown and the Van Ness Avenue Corridor; however, these standards are not considered relevant as they were developed for new buildings rather than for vegetation management projects.
An extensive literature review of wind risks (e.g., windthrow) and factors contributing to windrelated risks was performed and summarized in Section 4.11.2.2 Windthrow. The review identified biotic and abiotic factors that determine how windstorms impact forested ecosystems and result in windthrow. In addition, wind impacts on built environments and the efficacy of trees as windbreaks was studied.
The City of San Francisco has established criteria for the evaluation of wind impacts. To provide a comfortable wind environment for people in San Francisco, the City has established specific pedestrian-comfort, sitting-area-comfort, and wind-hazard criteria to be used in evaluating development proposed for certain areas of the City and near downtown (C-3 districts) under
Section 148 of the Planning Code. However, as these criteria were developed for new buildings rather than for vegetation management projects, and are not applied to districts within the vicinity of the project, they are not applicable to establish criteria for the evaluation of wind impacts for the proposed project.
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4.11 Wind
CEQA does not identify any specific criterion for the evaluation of wind effects of a project.
However, based on the extensive literature review of impacts related to wind, for the purposes of this analysis a project would have a significant wind effect if it would:

Have a substantial adverse short-term windthrow-related effect on the safety of people or structures;

Have a substantial adverse long-term windthrow-related effect on the safety of people or structures; or

Have a substantial adverse effect on wind environments in local neighborhoods.
Impact WIND-1: The proposed project could have a substantial adverse short-term windthrow-related effect on the safety of people or structures. (Potentially Significant; Less than Significant with Mitigation)
Tree removal activities could potentially alter the risk of windthrow in the Reserve and the potential for uprooted trees to affect the safety of people or structures. As detailed in Section
4.11.2.2 Windthrow, factors contributing to windthrow include wind speeds, topography, soils, forest tree composition, tree health, and tree density. Windthrow could affect the safety of people or damage structures within or adjacent to the Reserve from falling overthrown trees. Windthrow events in the Reserve could most likely occur during storm or high wind events. Tree removal activities may affect short-term windthrow risks by allowing for increased wind penetration, and by reducing the quantity of diseased or dead trees in the Reserve.
Thinning of trees would allow increased wind penetration into the Reserve, which could expose trees once protected by neighboring trees to greater wind speeds and an increased risk of windthrow. The potential for tree thinning activities to result in adverse windthrow risks would be greater in areas with other abiotic or biotic windthrow risk factors (e.g.,. steep topography).
As part of the project, UCSF would determine specifically which trees to remove with the assistance of a professional urban forester or arborist, and would make those determinations by considering the size and health of the trees, location relative to other trees and vegetation, effect on windthrow, and aesthetics, among other considerations. Removal of hazardous or diseased trees that are most likely to fail in high wind events would reduce short-term windthrow risks.
The potential adverse short-term impacts of thinning and the subsequent consequences of greater exposure of Reserve trees to higher wind speeds may be greater than the short-term benefits of removing hazardous trees in some areas of the Reserve, and could result in a net increase in windthrow risk. Therefore, this impact would be potentially significant. Implementation of
Mitigation Measure WIND-1 would reduce this potential impact to a less-than-significant level.
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Mitigation Measure WIND-1: After thinning, the project area would be regularly monitored by an urban forester or arborist to access tree health and condition. Trees prone to windthrow, e.g. dead or diseased trees or those occurring on steep slopes with limited soil for rooting, and considered a hazard to people or structures would be removed. The implementation of this measure would reduce the short-term impact of windthrow to a less than significant level.
Significance after Mitigation: Less than Significant
Impact WIND-2: The proposed project could have a substantial adverse long-term windthrowrelated effect on the safety of people or structures. (Less than Significant)
Similar to Impact WIND-1, tree removal activities may affect long-term windthrow risks by allowing for increased wind penetration, by reducing the quantity of diseased or dead trees in the
Reserve, and by altering the Reserve’s tree composition. Thinning of trees can be expected to allow increased wind penetration into the Reserve so that trees once protected by neighboring trees would be exposed to greater wind speeds and an increased risk of windthrow. However, as described in Section 4.11.2.2 Windthrow, while thinning can initially increase the risk of wind damage to trees, trees can become adapted to their new environment and become more resilient to windthrow. Removal of hazardous or diseased trees that are most likely to fail in high wind events would reduce long-term windthrow risks by allowing the remaining healthy trees to grow larger through release from competition, and this increase in growth would occur through diameter growth more than height growth. The remaining healthy trees would be more robust to windthrow. The proposed management actions may lead to a conversion in species composition
(e.g. through planting) that adds species that are more robust (e.g. coast live oak, bay laurel) or less robust (e.g. Monterey cypress, Monterey pine) to windthrow. Due to the presumed long-term resiliency to windthrow, this impact is considered less than significant.
Mitigation: None required.
Impact WIND-3: The project could have a substantial adverse effect on wind environments in local neighborhoods. (Less than Significant)
The Reserve serves as a windbreak to many of the neighborhoods surrounding it, and the removal of trees under the Proposed Management Activities has the potential to increase wind speeds in the neighborhoods. Wind speed effects would depend on a number of factors that include wind gust direction, wind velocity, the configuration of the Reserve and local neighborhoods, and the local topography. Wind gusts perpendicular to the thinned forest may cause houses on the leeward side of a portion of thinned forest to experience increased wind velocity. Wind gusts that are parallel to the forest would not result in a noticeable difference in wind velocity. Higher wind velocities would be expected to result in greater impacts on the local neighborhoods.
The regional climate data indicate that the strongest wind gusts observed at the San Francisco
Airport occur during fall and winter (October- March) and their direction is from the south as shown in Table 4.11-2 . If such events also derived from the south in the Mount Sutro area,
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4.11 Wind neighborhoods to the leeward side of the Reserve would be Woodland Avenue, Edgewood
Avenue, Koret Way, UCSF Parnassus campus, and Willard Street. During spring and summer months (April-September), the peak gusts at the San Francisco Airport are consistently lower and derive from the west or northwest. During these events, neighborhoods to the leeward side of the Reserve would be Woodhaven Court, Christopher Drive, Clarendon Avenue, and Forest
Knolls Drive (to the south), and again, Belmont Avenue, Edgewood Avenue, Woodland Avenue
(to the east). Although these are the peak gust directional patterns observed at the San Francisco
Airport, strong wind gusts could emanate from any direction in any season. If large storm events derive from the north, the same neighborhoods south of the Reserve would to the leeward side of the Reserve and from the west. The Stanyan Street and Belgrave Avenue neighborhoods would be to the leeward side of the Reserve. Some portions of Christopher Drive have only a small width of trees to serve as a windbreak if the wind direction is from the west. These areas may be more susceptible than areas with a thicker windbreak.
Wind stations discussed in Section 4.11.2.3 did not take measurements in neighborhoods nor did they measure during high wind events. Such measurements are impractical and unnecessary for this analysis, as regional wind speed data discussed in Section 4.11.2.1 Regional Climate -
Existing Conditions, combined with the data obtained in the Reserve, are sufficient to inform this analysis. Wind gusts could come from several directions depending on the relative location of storm events to Mount Sutro. The Mount Sutro weather data reveals that topographical differences between the various monitoring stations as well as local obstructions played a strong role and may have overridden any signal produced by differences in tree density.
The proposed project is expected to have a limited impact on the wind environments in select neighborhoods. Based on the dominant wind gust directions from the south (winter) and west/northwest (summer), windbreak effects from trees in the Reserve are likely most significant for the neighborhoods located to the east and north of the Reserve. The literature search suggests that the size of the windbreak provided by the Reserve is large enough (substantially greater than the minimum recommended 300 feet) that the proposed changes in stand density would have limited to no impact on the wind environment in adjacent neighborhoods. Further, trees would not be removed uniformly from all portions of the Reserve. For instance, areas that are too steep, or are inaccessible would not be thinned (UCSF 2010). The following discussion identifies the anticipated effect of the proposed project on each of the neighborhoods surrounding the Reserve.
Thinning in the reserve could potentially modify the wind environment for the UCSF Medical
Center, located to the north of the Reserve, during southerly gusts; however trees directly adjacent to and south of the medical center will likely not be thinned due to steep slopes. As a result, the expanse of the Reserve located south of the Medical Center is expected to continue to serve as a windbreak despite some thinning. The wind environment within Edgemont Avenue,
Belmont Avenue, and Woodland Avenue (to the northeast of the Reserve) is also not expected to change as a result of the proposed project. In this area, protection from the west is already limited by an existing paved parking lot and a narrow wooded portion of the reserve. Protection from the south would be unchanged due to the preservation of the Interior Greenbelt open space. The wind environment around Belgrave Avenue and 17 th Street (to the east) is also not expected to
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4.11 Wind change as a result of the proposed project due to preservation of the Interior Greenbelt open space which provides a buffer between the neighborhood and the Reserve. Similarly, the wind environment within the communities to the southeast is not expected to change because the communities are buffered from the Reserve by the Interior Greenbelt open space, the Aldea San
Miguel Housing development, and the Summit Reservoir. These three features provide protection from northwesterly winds for the communities located southeast of the Reserve.
Crestmont Drive and Christopher Drive (located immediately south of the Reserve) could be affected by strong north/northeast winds; however, these winds are less common than those originating from other directions. The Reserve extends more than 1200 feet north of these roads, and therefore would still provide protection to these communities even if it was thinned. The wind environment around Crestmont Drive to the west, particularly near the bend just north of
Oakhurst Lane, is the area most likely to be affected by proposed project. During east winds this neighborhood will be less protected after completion of the proposed project; however these winds are uncommon and would likely only be associated with storm events. In addition, even when thinned, the Reserve is wider than 300 feet at this location and would be expected to function as a modified windbreak for Crestmont Drive upon completion of the proposed project..
Further, the climate data suggests that the neighborhoods most impacted by southerly storms are leeward of sufficient swaths of forest that they would experience little to no change in wind penetration. The same appears to be true for storms that derive from the north or west. This impact would be less than significant.
Based upon the discussion above, the effect of the proposed project on the wind environment in the neighborhoods adjacent to the Reserve would not be significant.
Mitigation: None required
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Foster, D.R. 1988. Disturbance history, community organization, and vegetation dynamics of the old-growth Pisgah forest, southwestern New Hampshire. USA. J. Ecol. 76 (1), 105-134.
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UCSF. 2010. Notice of Preparation/ Initial Study for EIR. December 10, 2010. Accessible at: http://campusplanning.ucsf.edu/pdf/Initial_Study_UCSF_Mt_Sutro_Mgmt.pdf
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