HomeMy WebLinkAboutSDP201000054 Review Comments Critical Slope Waiver 2010-08-20*-&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: 20 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, carport, driveway, and
septic field on the northern end of TMP 88 -14A. The average slope of the area disturbed is around 28 %.
A smaller area of steeper critical slope disturbance ( -50% grade) will be required off of the existing private
road when the driveway is constructed into the hillside.
Areas
Acres
Total site
2.0 acres
Critical slopes
1.18
59% of site
Critical slopes disturbed
0.21
17.8% of critical slopes
The numbers in the above table are a compilation of data provided by the applicant's surveyor and
architect. These numbers do not include any disturbance necessary to establish a well and running the
waterline to the house, but I believe this disturbance to be negligible.
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 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 Virginia ESC Law. This agreement commits the
applicant to follow 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
rural area building permits.
The applicant has included in the waiver request package a letter from a professional engineer, Mr.
Michael Craun, which asserts 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 believe this disadvantage can be
mitigated by constructing a channel on the uphill side of the house to bypass this flow. A channel or
Albemarle County Community Development
Engineering Review comments
Page 2 of 3
low point on the uphill side of the house will be needed in both building locations, but the channel
would need to be larger if the lower corner of the property was the area of house construction.
In my opinion, comparing the potential soil loss of the two slopes (15% and 28 %) is more critical than
the construction of the channel around the house when evaluating potential soil loss. The Revised
Universal Soil Loss Equation (RUSLE) is a formula endorsed by the USDA that is primarily used to
estimate soil loss on tilled farmland. Though, it can be applied to construction sites as well. The
equation is represented below:
A= R *K *L *S *C *P
where, A = Net Detachment (i.e. Soil Loss)
R = Erosivity Factor
K = Soil Erodibility Factor
L = Slope Length (ft)
S = Slope Steepness (ft/ft)
C = Cover - Management Factor
P = Supporting Practices
When applying this equation to the two potential building sites being analyzed in this application, the
values R, K, C, and P will essentially be the same (assuming an upstream channel was installed properly).
So, the equation reduces to:
Soil Loss = X * Slope * Length
where, X = a constant (RKCP)
The average grade at the current proposed location of the building ( -28 %) is almost twice as steep as the
alternative at the lower end of the site (15 %). Consequently, the length of slope disturbed at the current
proposed location will be slightly longer. So, according to the RUSLE, the soil loss is estimated to be
twice as much at the current proposed building site than if the area of the property with flatter slopes were
to be the site of the house.
I acknowledge that some additional soil loss would result from the construction of the bypass channel. But
the amount is impossible to quantify or estimate. I suspect the loss of soil that would result from the ditch
construction and stabilization would be much less than the difference that can be attributed to the steepness
of the two slopes. The applicant believes there is a greater chance for soil loss with the cleanwater channel
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 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.
Albemarle County Community Development
Engineering Review comments
Page 3 of 3
"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 soil loss 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 below the county averages and, therefore, may lose less soil than
many other properties that do not require waivers for critical slope disturbance. This may be helpful to
keep in mind when the agent and Planning Commission are evaluating whether this waiver should or
should not be granted.
file: E2 csw PBC Ouranda.doc