HomeMy WebLinkAboutSDP201000054 Review Comments Critical Slope Waiver 2010-08-04*-&A
County of Albemarle
Department of Community Development
Memorandum
To: Elizabeth Marotta, Current Development planning and zoning review
From: Phil Custer, Current Development engineering review
Date: 4 August 2010
Subject: Ourada Building Site (SDP- 2010 - 00054) critical slope waiver request
The critical slope waiver request has been reviewed. The engineering analysis of the request follows:
Description of critical slope area and proposed disturbance:
The proposed plan disturbs critical slopes to construct a 1,023sf single family home, driveway, and septic
field on the northern end of TMP 88 -14A. The average slope of the area disturbed is around 28 %.
Smaller areas of steeper critical slope disturbance (50% grade) will be required off of the existing private
road when the sideslope is graded to construct the driveway in to the hillside.
Areas
Acres
Total site
2.0 acres
Critical slopes
1.18
59% of site
Critical slopes disturbed
0.15
12.7% of critical slopes
The numbers in the above table are an approximation provided by Albemarle County Engineering based
on all of the applicant's exhibits. It should be noted that there are discrepancies between the graphics of
Exhibit 6 and Exhibits 4 and 7. Exhibit 7 shows the septic drainfield in the northeast corner of the site
while Exhibit 6 shows the drainfield below the house, centered over the 740 contour. Exhibit 6 shows the
house centered on the 750 contour, but on Exhibit 4 the house is shown centered on the 744 contour. One
singular document with the proposed location of the driveway, septic field, and building with limits of
clearing/ grading would be helpful in having more confidence in the total disturbance being proposed.
Exemptions to critical slopes waivers for driveways, roads and utilities without reasonable
alternative locations:
If the flatter area in the southern portion of the site was determined not to be a satisfactory building site, the
driveway (accessway) could be considered exempt per 4.2.6.c.
Compliance with Zoning Ordinance 18 -4.2:
"movement of soil and rock"
The USDA soil classification for all areas of the parcel in question is Chester very stony loam, 15 to 25
percent slopes (15D). Because of the steepness of the grade, the USDA soil survey notes that the
hazard for erosion of this soil as severe.
In order for any building permit to be approved in the rural area, the property owner is required to sign
an Agreement in Lieu of Plan as provided in the Virginia ESC Law. This agreement commits the
applicant to following all state ESC standards during construction. With agreements however, ESC
bonds are not held by the county and site inspection is more infrequent due to the wide dispersion of
construction sites.
The applicant has included in the waiver request package a letter from a professional engineer, Mr.
Albemarle County Community Development
Engineering Review comments
Page 2 of 3
Michael Craun, which claims that construction in the flatter (15% grade), southeast corner of the site
would cause more erosion because the earth disturbance would occur in an area where water from the
uphill lands concentrate. While this is a noteworthy concern, I cannot confirm the truth of the
statement because there is no accepted method for estimating the soil loss of either situation. While it
is a concern to have an abundant amount of clean water flowing through a disturbed site, most of
erosion and sediment suspension occurs from "raindrop" erosion. Considering this, the comparison
between the soil loss of the two slopes (15% and 28 %) becomes more critical. Unfortunately, there are
insufficient resources to conclusively make any sort of quantitative evaluation. However, one
document I did come across on a USDA website evaluated the soil loss of existing established slopes
and provided the following table:
Uble 3. AvmvV annul soil la® tm t&o and normdU ed to 25% dope
1
$l
83
Ainiml
Ann" Sail Loss
Plot
2
10
Runoff
510
(NMT ft"
LQumbn
(dog=)
(91a)
OUN
Olka)
1025%)
1.124
4
20
Suide gilt
553
103.26
32
8,6
13.1
203
5"
Q336
18
14,7
25,2
20.0
219r3
1171
11
28.7
543
17.8
42.71
2.476
1.217
13
17,7
A=4 $4
15.7
36.25
1
5
83
0.1
15.32
0.222
2
10
17.5
510
4421
Ok4l
3
15
26.9
523
7737
1.124
4
20
36.4
553
103.26
1.497
5
25
46A
55.2
139.87
2A77
G
28
532
553
140.70
2.039
12
17.4
313
Tianslrui Silt
30.37
1.217
15
5.4
9.4
93
8.01
0.316
16
5.1
8.9
16.8
72#
03M
17
4.7
83
13.4
13.51
0,533
7
14,2
753
103
24.45
0.964
8
14.1
252
14_D
25.29
Q.996
9
13.9
20
125
23.95
M944
12
17.4
313
12.3
30.37
1.217
13
17,7
31.8
15.7
36.25
1,429
14
V3.5
31.4
9.9
30,23
1.193
18
2.3.7
43.9
16.1
65.19
2.570
(B.Y. Liu, M. A. Nearing, L.M. Risse "Slope Gradient Effects on Soil Loss for Steep Slopes ")
The experiment which collected the data above is not in the exact context of the evaluation currently at
hand (the experiment did not evaluate soil loss of disturbed slopes), but it does provide some
quantifiable distinction between slopes of varying grades. From this table alone, we can estimate that
28% slope erodes 3x more than a 15% slope (1.124/0.336).
For the majority of home construction projects, building contractors will wrap the perimeter of the
Albemarle County Community Development
Engineering Review comments
Page 3 of 3
construction site with silt fence as the sole sediment control measure. I anticipate that the Erosion and
Sediment Control plan for the house as currently proposed by the applicant would use this same
philosophy because the existing grades prohibit any other measure. If the home were to be constructed
in the southeast corner, using just silt fence would not be satisfactory because the large drainage area
would cause the silt fence to fail. The contractor would have to construct a channel diversion north of
the house to keep the clean water of the uphill watershed away from the building foundation and
disturbed soil. This practice is not difficult and would eliminate any increased erosion concern caused
by offsite water if the channel is lined with riprap for instant stabilization. The construction of both
house sites would likely rely on silt fence as the sole means to protect the construction site but if the
house were to be constructed in the southeast corner of the parcel a diversion channel would be
necessary to bypass the water around the construction site.
"excessive stormwater runoff'
As with nearly all single family home construction, stormwater runoff from the site will be increased,
though the addition will likely be negligible. The applicant has indicated that they are thinking of
including a cistern on the property which will help bring the property closer to the pre - development
hydrology.
"siltation"
Please refer to "movement of soil and rock" section.
"loss of aesthetic resource"
Engineering review provides no comment on the aesthetic resources on site.
"septic effluent"
County engineering review will defer to the Virginia Health Department with regards to the
construction of septic systems on critical slopes. After reading several emails from VDOH officials,
it's evident that there is no objection to the placement of the septic system on critical slopes. In fact,
VDOH is prohibiting the construction of a septic system in the only area of the site where critical
slopes are not present.
Engineering review cannot conclusively agree with the applicant that all five "public health, safety, and
welfare factors" are better served with the approval of the waiver. I suspect that the construction of the
building on the 28% slope as proposed will result in more erosion than if the building was constructed in
the southeast corner with a properly sized and lined diversion channel around the house. Admittedly, there
is not a lot of evidence available to support either position. (For septic effluent, I will defer to VDOH who
has weighed in already.)
However, I think it is important to remember that total soil loss from a site is influenced not only by
existing and proposed grades but also by total disturbed area and time of construction. Considering these
factors, the current proposal is likely well below the county average and, therefore, will lose less soil than
many other properties that do not require waivers. This should be kept in perspective when the agent and
Planning Commission are evaluating whether this waiver should or should not be granted.
file: El csw PBC Ouranda.doc