HomeMy WebLinkAboutWPO201800043 Correspondence 2018-06-01 •
MEMORANDUM - Liberty Hall Hydrologic Analysis
1739 Allied St.,Suite A
Charlottesville,VA 22903
a j t d y 540.239.1428
I� l - ;f.; www.ecosystemservices.us
info@ecosystemservices.us
k
June 1, 2018
FROM: Ecosystem Services, LLC
SUBJECT: Liberty Hall Hydrologic Analysis Overview—Objectives, Procedure, and Outputs
Objectives:
• Create a calibrated existing conditions hydrologic model
• Iterate a proposed conditions hydrologic model to optimize for hydraulic, geotechnical, and
aesthetic considerations
Procedure:
• Installed stream gage and rain gage
• Parameterized existing conditions model using VGIN landuse data, WSS soils data, total station
survey data, and watershed assessment data collected on the pond upstream of the project
• Calibrated the existing conditions model to the storm event on 10/29/2017 during which 1.62
inches of rain fell over 24 hours
• Parameterized the proposed conditions model based on the initial proposed grading and
iterated the dam height and outlet channel dimensions and slope to optimize for design
objectives including: maintaining 0.5' of freeboard in the 100 year storm, maintaining an all-cut
spillway channel, avoiding excessive earthwork, and avoiding the need for a riser structure.
• Derived dry year dataset from Richmond hourly rain data that spanned from 1948 to 2013 and
ran it through the proposed conditions model to ensure sufficient pond depth in proposed
• conditions
Outputs:
• Base Flow for Monitoring Period = 1.39 cfs
• Soils and landuse maps (see attachments)
• Peak pond inflow rate comparison between proposed and existing conditions:
Recurrence Interval
Model Output Description 1YR 2YR 5YR 10YR 25YR 50YR 100YR
Existing Peak Outflow(cfs) 135.7 207.6 334.2 450.5 611.6 767.8 952.5
Proposed Peak Outflow (cfs) 102.4 161 286.1 406.1 554.9 696.2 840.4
Attachments:
1. Watershed soils map
2. Watershed landuse map
•
''::‘1:'''''11:\s'-'\''‘'''''''' ''''4'4 ;1\
. \\ Legend
Proposed Pond Boundary
Landuse
,; r Description
� � / Cropland4 Pasture
-Alli‘ Vi '' _ Forest
iNNW ram a \
eliScrub Shrub
o a �\
Tree7,,,, ,.. .. ,
44', ' Turf Grass
\\ f ` ,-'%, Open Water
� `°`` .• \ Impervious
J
„y Landuse Description Area (ac) Percent
4 4- i
4 Tree 42.77 15.4°�
a Turf Grass
v,y, g 29.10 10.5°�
\ ,
Impervious 6.48 2.3°�
Pasture 48.45 17.5°k
Forest 144.31 52.1
3 Open Water 5.97 2.2%
•
� AV'
� SCALE: 1 INCH=500 FEET
V�\. `� �� I,. PROJECT NO: 17-0035
\\ , a
�� , � �, ' I PM:WKM
��. \ti` \ i WATERSHED LANDUSE
DRAWN BY: DCL
"` ,- 41 LIBERTY HALL
\ ,,„,,, l „.,,t' SOURCE:VGIN
• �e DATE:6 /B2018
,. 4 yn 1739 ALLIEDSTSUITE A,CHARLOTTESVILLE,VIRGINIA MAP EXHIBIT
)540)2391428 ECOSYSTEMSERVICES.US
•
Soil ID Soil Description Area(cc) Legend
14B Chester loam,2 to 7 percent slopes 1.65
a' 14C Chester loam,7 to 15 percent slopes 3.38 �__ i Proposed Pond Boundary
a 14D Chester loam, 15 to 25 percent slopes 29.90 Soils
� � 19B Cullen loam,2 to 7 percent slopes 56.85 Soil ID
\b\ �14B
Z ., 19C Cullen loam,7 to 15 percent slopes 1.47
36B HayesWlle loam,2 to 7 percent slopes 36.68
§ 36C HayesWlle loam,7 to 15 percent slopes 54.85 III 14C
� 36D HayesWlle loam,15 to 25 percent slopes 4.77 14D
�, �� 37B3 HayesWlle clay loam,2 to 7 percent slopes,severely eroded 4.20 19B
37C3 HayesWlle clay loam,7 to 15 percent slopes,severely eroded 15.68 tsc
37D3 HayesWlle clay loam, 15 to 25 percent slopes,severely eroded 33.261111-3s6
°' � ems, 4E Ashe loam,25 to 45 percent slopes kk
13.97
56B Meadowville loam,2 to 7 percent slopes 14.21 3sC
77 Ri�serview-Chewacla complex 2.11 36D
�� 83 Toccoa fine sandy loam 0.03 37B3
_-
W Water 4.07
, -37C3
\ e • -37D3
\a. Ell
4E
-56B
I
Vie. i ....
. i."' �� SCALE: 1 INCH=500 FEET
� 14y,_ ,� PROJECT NO: 17-0035
�', � � . n ). " PM:WKM
- �; � �� ,i WATERSHED SOILS DRAWN BY: DCL
. Y' hi i,,,E(415 LIBERTY HALL
&4 0 SOURCE:VGIN,WSS
o - a ► DATE:6/1/2018
1739 ALLIED ST SUITE A,CHARLOTTESVILLE,VIRGINIA
154012391428 ECOSVSTIIMSERVICES.US MAP EXHIBIT