1. Consultant Report
On the Hydrologic Impacts of San Elijo Hills
Development on Carr Property
Prepared by:
Soroosh Sorooshian, Ph.D.
33 Village Way, Irvine CA, 92603
August 5, 2009
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2. Consultant report related to the hydrologic impacts of San Elijo Hills development
on Carr property prepared by:
Soroosh Sorooshian, Ph.D., NAE
33 Village Way, Irvine CA, 92603
August 5, 2009
I have had the opportunity to review a number of available reports which have been
prepared in relation to the hydrologic assessment of Carr property provided to me by Mr.
Carr. This included the Hunsaker & Associates Rough Grading Drainage Study for San
Elijo Hills of September 9, 2005; the Chang Consultants report of June 25, 2008; and the
PIRO Engineering report of June 4, 2009. Furthermore, I paid a site visit to Carr property
(accompanied by Mr. David Carr) on July 21st 2009, and inspected the site, took some
photos and made the necessary channel cross section measurements. Some of the other
required data and estimates of necessary parameters were obtained from the existing
reports made available to me by Mr. Carr and from hydrologic handbooks.
Some detailed analysis has been conducted by Chang and PIRO Engineering to evaluate
the hydrologic impact of San Elijo Hills development on Carr property. This includes the
subareas flow analysis, using the rational formula, by Chang. It is my conclusion that
much of Chang’s analysis while accurate is not relevant to the issue which is the key
factor to this case. In my professional judgment the primary hydrologic concern to Carr
property relates to a section of the relatively steep natural stream which passes through
the upper left corner of the watershed and is marked as section X on the Google map
(Fig. 1 below).
Figure 1: Google Map of the Carr property with the red line showing the approximate boundary of the
property; the light blue showing the natural stream channels and the yellow section marked as X showing the
segment of the stream channel passing trough Carr property which will be carrying the additional flow from the
San Elijo Hills Development phases III & V storm Drain Line labeled as “N”.
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3. The segment of the natural stream marked as X in the above figure is approximately 120
feet in length with the average slope of nearly 30%, which is relatively steep. It is my
judgment that the influence of any land use change above Carr property will have its
greatest impact on segment X and the sediments transported to the pond. Therefore, the
primary questions I would pose are whether the resulting land use change due to San
Elijo Hills Development phases III & V:
1. Has increased the magnitude of the 100 year flow (Q100) within the watershed
passing through the section marked as X in Fig. 1. and, if so, then
2. Will the increase in Q100 cause an increase in the flow velocity, energy and
erosion, and hence result in either deepening or widening of the stream segment in
the Carr property marked as X?
3. Will the increase in erosion result in build up of additional sediments in the pond
on Carr property?
It is my conclusion that hydrologically speaking, the answers to all three questions above
are YES. As for the first question (increase in post-development Q100), there is no dispute
and all the studies confirm it. The calculated post-development Q100, post-development are very
close to each other (52.4 cfs in Hunsaker analysis; 55.0 cfs in Chang analysis and PIRO
value is 53 cfs). Exception is in the magnitude of pre-development Q100. The most
noticeable difference is in the estimate in Chang’s report (Q100, pre-development =11cfs), while
the PIRO report calculated a value of (Q100, pre-development = 4 cfs). PIRO estimate is based on
the pre-development contribution of approximately 2.8 acres of drainage area above the
point of entry to channel segment X (see Fig. 1) of Carr property. Based on my
evaluation, it is my judgment that PIRO Engineering analysis is a more careful study in
this regard. Therefore, I have selected the estimates of (Q100, pre-development = 4 cfs and Q100,
post-development =52.4 cfs) in my flow analysis for segment X on Carr property. The purpose
of my analysis is to provide answer to questions 2 and 3 listed above.
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4. Flow analysis for channel segment X in Figure 1
The purpose of the flow analysis is to see by how much the flow velocity and depth of
water increases in channel segment X, due to the increase in Q100 from pre- to post-
development condition. Figure 2 shows two photos related to channel segment marked as
X in Figure 1.
Figure 2A Figure 2B
Figure 2 represents photos of the stream channel marked X in Figure 1, in two locations. Fig. 2A shows the
stream at the fence and entry point to Carr’s property and Fig. 2B shows the channel looking downstream and
towards the point of confluence. The red lines represent visualizations of the approximate channel cross sections
and the blue arrow shows the direction of flow.
Figure 2A shows the point of flow entry to Carr property and 2B is looking down slope
towards the point of exit from Carr property and towards the pond. The channel cross
sectional profiles (in red) and direction of flow (in blue) are shown. This is a relatively
steep (approximately 30% slope) and very rugged channel with some natural vegetation
primarily on its banks.
In the current state of engineering practice, the Manning equation is the primary model
used for open channel analysis which was also used in this study. The details of the
calculations are provided in Appendix 1.
The relevant parameters related to this case are provided in the following table.
Q100 Flow velocity Water Depth Width of Water Surface
(V ft/sec) (Y ft) (T ft)
Pre-Development 4.0 8.0 0.45 2.23
Post-Development 52.4 15.0 1.17 5.90
Percentage change 1200 88% 160% 165%
(% increase) %
Table 1: Calculation results for changes (increases) in flow velocity; water depth and width of water
surface from pre- to post-development
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5. Based on the results shown above, it is clear that with the almost approximately13 fold
increase in the magnitude of Q100 in segment X the flow velocity increases by 80%,
flow depth increases by nearly 160%, and the width of water surface will most likely
increase by 165%. With the increase in these parameters and the resulting increase in
flow energy, the potential for significant channel erosion, and hence loss of sediment will
increase. Given that channel segment X in Carr property is a natural steep stream, there
are two possible scenarios for erosion and sediment delivery:
1- If the bottom of the channel is of “native rock formation”, as suggested in the
Chang report, then much of the flow energy will result in eroding the soil from the
banks of the channel. This scenario assumes a uniform rock formation at the
channel bottom.
2- If the bottom of the channel is a mix of native rocks and soil, then the erosion will
take place from both the banks and the bottom of the channel.
It is very clear that the latter scenario is the dominating factor. Therefore, the increase in
Q100 will cause significant erosion from both the bottom and the banks. While a
significant amount the sediments will be carried towards the pond, some of it may also
get deposited in the stream and in front of any obstacles (e.g., rocks and debris)
temporarily.
Experience and evidence from the June 2009 flooding due to fire hydrant break incident
In June 2009, there was an incident on San Elijo Rd. and a fire hydrant near the fire
station was knocked out, causing a minor flooding event lasting nearly 20 minutes.
According to the information Mr. Carr has obtained, the flow rate for this “man-made”
flood event was approximated Q = 9.8 cfs. The water, after discharging from the culvert
above Carr property, made its way down hill towards the pond and passed through
channel segment X shown in Fig. 1.
Figure 3A (taken January 2008) Figure 3B(taken June 2009)
Figure 3 showing pictures of the same channel location in segment X. Picture 3A was taken in January 2008 and
picture 3B was taken on June 2009 and after the fire Hydrant incident. (Provided by Mr. Carr)
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6. Comparing figures 3A&B clearly shows the impact of the relatively minor flood caused
by the fire hydrant incident. As can be seen, the amount of sediment build up in front of
the small boulder in the channel segment X of Carr property is relatively significant. Note
that this erosion is due to a flow of only twice the amount of pre-development (Q100, pre-
development = 4 cfs) and smaller than Chang’s estimated (Q100, pre-development =11cfs). There
should be no doubt that in the event of any storm reaching and/or exceeding the post-
development 100-year flow of nearly 52.4 cfs (13 fold increase), it will cause significant
erosion along this channel and the transported sediment will eventfully reach and get
deposited in the pond, hence reducing the water holding capacity of the pond.
In order to avoid any doubt about the adverse impacts of increased flow rate, two more
pictures taken after the June 2009 fire hydrant incident along the stream channel in Carr
property are included in Figure 4. The circled areas show clear evidence of erosion and
soil loss.
Figure 4A Figure 4B (Provided by Mr. Carr)
Figure 4A&B show the evidence of erosion impact due to June 2009 small flood event caused by the
fire hydrant incident.
Figure 5 shows a picture of the pond soon after the fire hydrant incident in June 2009.
The muddy color of the water, in the right side of the picture, which is located in front of
Carr’s property, is further evidence of the impact of transported sediments into the pond
and answer to question number 3 posed above.
Figure 5 is a picture of the pond taken in June 2009 and soon after the fire hydrant break. The muddy color of water on the
right side of the picture is evidence of sediment plum carried by the minor flood caused by the hydrant incident. (Provided by
Mr. Carr)
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7. Summary and final Conclusions
Based on my review of the analysis conducted by Chang and PIRO consultants and my
own analysis, I make the following conclusions:
- The nearly 13 fold increase in the amount of Q100 due to San Elijo Hills
development, will significantly increase the potential for erosion in the stream
channel (marked X in figure 1) in Carr’s property. The potential for sediment load
on the pond will also increase with increased flow and erosion.
- My analysis also strongly supports the conclusions reached in the PIRO
Engineering Report dated June 4th 2009. While the parameter values may be
different (due to differences in a few of our assumptions), the overall conclusions
that damage to the Carr property along the stream channel X will occur and
additional sediments will get deposited in the pond, are in agreement.
- Chang report makes several conclusions that either ignore the main problem
and/or are not relevant to the key issues related to the Carr property. Quoting
form Chang report; “A flow increase occurs within the hillside stream in the
northwesterly portion of the Carr property. However, this increase can be
conveyed in the stream…”. There is no dispute that the increase in flow will be
conveyed by segment X. In fact one can argue that due to the nature of the
topography of the banks of stream channel, much larger flows (perhaps as high as,
or higher than Q1000 ) can be conveyed. However, the issue is not the conveyance
of flow, but the damage it will do to Carr property. The Chang statement that the
additional flow “…will not appreciably impact erosion or sediment delivery due
to the underlying native rock formation…..”, is not correct.
Again, quoting from Chang report, “ In fact, the San Elijo Hills development
has created an overall reduction in sediment delivery to the Carr property and
pond because the development has replaced many natural surfaces that could
generate sediment with impervious surfaces an landscaping that resist
erosion.”, this conclusion again misses the key issue - namely the impact of
increased Q100 on the stream channel marked as X in figure 1. As shown above,
contrary to Chang’s suggestion, both erosion and sediment transport to the pond
will occur and increase as result of 13 fold increase in Q100.
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8. Appendix 1: Flow analysis for stream channel section X (Figure 1) of Carr property
We use the most commonly used equation for analyzing open channels - the Manning equation.
Manning equ. :
T
Q = A / n * R 2 / 3 * S 1/ 2
y
Given:
-Triangular cross-section
-Slope (S): approx. %29
-Manning Coef (n): 0.035 (for Stony, Cobbles)
S
Case 1: Pre-development 1
Q 100 =4 cfs=0.1133 cms
Assumption:
For triangular cross-section:
A = z * y2
z* y
R=
2* 1+ z2
T = 2* z * y
Therefore:
0.1133 = 2.5 * y 2 / 0.035 * (0.464 * y ) 2 / 3 * 0.291 / 2
y = 0.136m = 0.448 ft
V = Q / A = 0.1133 /( 2.5 * 0.136 2 ) = 2.45m / s = 8.04 ft / s
T = 2 * z * y = 2 * 2.5 * 0.136 = 0.68m = 2.23 ft
Case 2: Post-development
Q 100 =52.4 cfs=1.484 cms
From Manning equ. :
1.484 = 2.5 * y 2 / 0.035 * (0.464 * y ) 2 / 3 * 0.291 / 2
y = 0.36m = 1.17 ft
V = Q / A = 1.484 /(2.5 * 0.36 2 ) = 4.58m / s = 15.02 ft / s
T = 2 * z * y = 2 * 2.5 * 0.36 = 1.8m = 5.9 ft
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