Z20-09 - Preliminary Stormwater Report - Cape Fear Engineering
Cape Fear Engineering (CFE) was requested to analyze independently the conceptual stormwater
management plan for the proposed Demarest Pointe neighborhood prepared by Middlesound LLC, Scott
D Stewart RLA ASLA and was requested to further provide CFE’s professional opinion regarding the 5-13-
20 CSD Engineering conceptual stormwater evaluation (attached as Exhibit A).
CFE further prepared preliminary hydrologic analysis (Water Quantity), evaluated potential flooding
impacts (Flood Zone Information), Watershed Impacts (Watershed Characteristics) and water quality
impacts (Water Quality) on neighboring and downstream properties.
WATER QUANTITY
The Demarest Pointe project would be required to comply with New Hanover County stormwater
ordinance which requires attenuation post-development runoff rates from the 2-year (4.5”), 10-year
(7.0”), and 25-year (8.05”) 24-hour storm events to pre-development values.
CFE performed a preliminary routing analysis of the concept stormwater plan (attached as Exhibit B)
utilizing Hydraflow Hydrographs modeling software. Model results show that the Demarest Pointe
neighborhood is expected to reduce runoff for not only the NHC required design storms but also the 500-
year (18.10”) and 1,000-year (21.1”) storms.
A summary of results from CFE’s routing analysis is shown in the table below.
Design
Storm
Pre-Dev.
Runoff (CFS)
Post-Dev.
Discharge (CFS)
Difference (CFS)
2-year 0.15 0.10 -0.05 (33% reduction)
10-year 2.40 1.46 -0.94 (39% reduction)
25-year 4.12 2.35 -1.77 (43% reduction)
50-year 5.92 2.92 -3.0 (51% reduction)
100-year 8.02 4.07 -3.95 (49% reduction)
500-year 29.00 22.69 -6.31 (22% reduction)
1000-year 37.92 35.26 -2.66 (7% reduction)
A similar analysis was performed by CSD Engineering (attached as Exhibit A) which also showed
attenuation of post-development runoff rates for up to the 1,000-year storm. The analysis performed by
CSD Engineering was performed using the same methodology and modeling software as CFE; however,
there were very minor differences in model inputs for drainage area due to the conceptual nature of the
study, SCS curve numbers, times of concentrations, pond contour surface areas, and pond outlet
configuration/elevations.
In conclusion, CFE agrees with the overall findings of the 5-13-20 CSD Engineering report. CFE recognizes
the Demarest Pointe neighborhood stormwater program will confidently manage a minimum of a 500-
year storm event and through the required permitting process, it is feasible to approach or meet
attenuation of the 1,000 year storm event.
FLOOD ZONE INFORMATION
According to FEMA flood maps, (attached as Exhibit C) the Demarest Pointe neighborhood is located
outside of the 100-year flood zone. Elevations onsite range from approximately +27’ to +29’. The nearest
100-year flood zone is located approximately 2,400’ to the southeast with a reported food elevation of
+13’.
We understand that there have been concerns raised about the Demarest Pointe neighborhood proposal
because there has been a history of downstream flooding. Review of FEMA maps and available elevation
information shows that the downstream properties are at significantly lower elevations than the proposed
Demarest Pointe project site whereas many downstream properties have been built within the 100 year
flood zone and experience frequent flooding due to the existing elevations of the homes that were built
within the 100 year flood zone of Pages Creek.
One specific location of historical flooding is at the intersection of Parliament Drive and E. Bedford Drive.
According to survey information provided by Atlantic Coast Surveying (attached as Exhibit D) the
elevation of this intersection is approximately 6.0’ msl as compared to flood zone elevation of 13’ msl.
WATER QUALITY
Because the proposed Demarest Pointe neighborhood will disturb more than one acre and construct more
than 10,000 sf it will require a State Stormwater permit from NCDEQ. The required water quality design
storm standards are based on the location of the Demarest Pointe neighborhood, classification of
receiving waterbody, and proximity of the site to the receiving water body.
Typically, projects are required to store and treat runoff volume from the 1.5” design storm; however,
Projects draining to and withing ½ mile of SA waters are required to store and treat runoff volume equal
to the difference in post-development and pre-development runoff from the 1-year 24-hour storm.
Pages Creek is classified as SA waters but since the Demarest Pointe neighborhood is greater than ½ mile
from Pages Creek, approximately 0.65 miles, (attached as Exhibit E) the Demarest Pointe neighborhood
is only required to be designed for the 1.5” water quality design storm.
Preliminary calculations for the project indicate a water quality design volume of approximately 8,300 CF
for the 1.5” event compared to 16,200 CF for the 1-year 24-hour storm event.
Due to low infiltration rates and relatively shallow seasonal highwater table (SHWT), (attached as Exhibit
F), a wet detention basis is expected to be the primary water quality treatment BMP. Based on the
contributing drainage area, proposed impervious coverage, and anticipated pond depth it is estimated
that a pond surface area of 4,800 sf will be required. The conceptual stormwater plan includes a pond
with estimated surface area of 12,500 sf (approximately 260% of requirement).
Additionally, the conceptual stormwater plan incorporates additional storage and treatment in four rain
gardens and one water quality swale (attached as Exhibit G).
It is the opinion of CFE that the proposed Demarest Pointe neighborhood will provide significantly more
stormwater treatment than required by NCDEQ. It is anticipated that the Demarest Pointe neighborhood
will provide total treatment volume equal to the more stringent SA waters requirements, though not
required. Demarest Pointe will have no adverse or negative impacts on downstream water quality.
WATERSHED CHRACTERISTICS
Utilizing USGS Quad maps and NCDEQ Online Stream Mapping application it was determined that the
Demarest Pointe neighborhood is part of the Pages Creek watershed. According to the USGS StreamStats
online application the contributing drainage area at the upper end of Pages creek is 2.21 square miles
(~1400 acres) with an estimated impervious coverage of 14.4% (~202 acres) (attached as Exhibit H).
The entire Demarest Pointe site area (including drainage easement) is approximately 4.7 acres, which
makes up less than 0.035% of the contributing watershed at the point of analysis on Pages Creek.
Additionally, the Demarest Pointe neighborhood is expected to have impervious coverage of 65,000 sf.
Construction of this impervious will result in a new watershed impervious coverage of 14.54% (~0.14%%
increase).
EXHIBIT A
CSD Engineering Conceptual Stormwater Evaluation
EXHIBIT B
Cape Fear Engineering Preliminary Routing Analysis
49 713
Demarest Pointe Preliminary 2-year
f 2
h
PRE-DEVELOPMENT
1 - 2
2
Post-Development
No. 2 - 2 Year
2
POST DEV.
POND#1
N2
402 894
Demarest Pointe Preliminary 10-year
4.124 20,445
Demarest Pointe Preliminary 25-year
916 7,103
Demarest Pointe Preliminary 50-year
8.022 4,756
Demarest Pointe Preliminary 100-year
9.00 12,699
Demarest Pointe Preliminary 500-year
500
h
R
500
500
POST DEVELOPMENT
No. 2 - 500 year
500
POST DEV.POND#1
No.500
7.92 46,063
Demarest Pointe Preliminary 1000 year
f 1000
h
R
1000 Y
1000
POST-DEVELOPMENT
N1000
1000
POST-DEV= POND#1
N1000
EXHIBIT C
FEMA Floodmaps
DEVELOPER:Middlesound LLC6933 Running Brook TerraceWilmington, North Carolina 28411
LAND PLANNING LANDSCAPE ARCHITECTURE:
BUILDER
SURVEYOR:Atlantic Coast Survey1200 N 23rd Street, Suite 107
CDS Engineering3805 Cherry AvenueWilmington, North Carolina 28403
314 Walnut Street Wilmington, North Carolina 28401
Wilmington, North Carolina 28405
CIVIL ENGINEERING:
PROJECTTITLE:
DEMAREST POINTE
MASTERDEVELOPMENTPLAN
New HanoverCounty,NorthCarolina
ThisisaConceptualrenderingpreparedbasedonpreliminaryinformation.Allimagesandusesareexhibitedsolelyfor the
purposeof representingscale,texture,spatialrelationships,potentiallandusesandthephysicalrelationshipsintended
betweenexistingnaturalknownlandfeaturesandproposedfutureimprovementswhicharesubjecttoadditions,deletions,
totalmodificationsand/ormaynotbeimprovedordevelopedatall.Allimprovementswillbeinaccordancewithdetailed
designplansbyrequiredprofessionalsandsubjecttopermitsandapprovalsbyapplicablegovernmentagenciesbefore
considerationtobedeveloped.Allplanssubjecttochange.
DKC|NWD|etaD|noitpircseD|veR
PROJECT NO.
GRAPHIC SCALE: APPROXIMATE
DWG TITLE
:
REF. NO.
DATE: 3/28/20
DRAWN
DRAWING NO.
: SDS
CHECKED: SDS
DRAWING SCALE: NTS
CHECKED:
DEVRESERSTHGIRLLA0202THGIRYPOC
DEMARESTCOMPANYLANDSCAPEARCHITECTS
MIDDLE SOUND COMMUNITY
TOWNHOMES FLOOD MAP
6933 Running Brook Terrace Wilmington, NC 28411 910•231•2428
Landscape ArchitectsLand Planners
ASLA
DEMAREST COMPANY
15
LEGEND:
TOWNHOMES_(12Acres)____________________________________
101RESIDENCES(12ACRES@8/ACRE)
101–2STORY(20’X45’)RESIDENCES73TWOBEDROOM@2.5PARKINGSPACES/RESIDENCEOR183SPACES
28THREEBEDROOM@3.5PARKINGSPACES/RESIDENCEOR98SPACESTOTAL:101RESIDENCESWITH281PARKINGSPACES(290PROVIDED)
7000SQFTCLUBHOUSEW/POSTOFFICE
2,500SQ.FT.1LANELAPPOOL,450SQ.FT.OUTDOORKITCHEN1FIREPIT,2GAZEBO’S,2,500SQ.FT.PLAYGROUND;4,000SQ.FTDOGPARK
1MILEOFSIDEWALKS;10BYCYLCERACKS:65,000SQFTWATERFEATURE2-12’X24’PARKGAZEBO;1-12’X30’LANDSCAPEPARKARBORPONDOVERLOOOK
1,500LFPERIMETERMULTIUSEPATHSTO1.5MILEDEMARESTPARKPATH
MULTIFAMILY_(22Acres)_____________________________________
336RESIDENCES(22ACRES@15/ACRE)14-3STORY(70’X170’)RESIDENCES,24RESIDENCES/BUILDING
168ONEBEDROOM@1.5PARKINGSPACES/RESIDENCEOR252SPACES168TWOBEDROOM@2.5PARKINGSPACES/RESIDENCEOR420SPACES
TOTAL:336RESIDENCESWITH672PARKINGSPACES(690PROVIDED)
8,000SQ.FT.CLUBHOUSE;2,000SQ.FT.POSTOFFICE2,500SQ.FT.2LANELAPPOOL,600SQ.FT.OUTDOORKITCHEN
2FIREPITS,8GAZEBO’S,3,600SQ.FT.PLAYGROUND;6,000SQ.FTDOGPARK200LFCLUBARBOR;75LFPOOLARBOR;
2.75MILESOFSIDEWALKS; 20BICYCLERACKS: 1ACREWATERFEATURE500LF 6’WIDETIMBERPEDESTRIANBRIDGES;4-18’X24’PARKGAZEBO’S
26-12’X26’GARAGEPARKING SPACES;25’X40’MAINTENANCEBUILDING20’X30’CARWASH;25’X50’COMPACTOR;18’X24’RECYCLECOMPACTOR
1,100LFPERIMETERMULTIUSEPATHTO1.5MILEDEMARESTPARKPATH
COMMERCIAL/RETAIL/OFFICE(36Acres)_________________________
271,000SQ.FT(36ACRES)
3 X 41,700 SQ.FT = 125,000 SQ.; 2 X 37,150 SQ.FT. = 74,300 SQ.FT.1 @ 21,450 SQ.FT; 1 @ 19,350 SQ.FT.; 1 @ 16,850 SQ.FT.1 @ 8,000 SQ.FT.; 1 @ 6,000 SQ.FT.1,300 PARKING SPACES (4.8 SPACES/1000 SQ. FT.)
HOTEL(2Acres)_______________________________________________
150ROOMS;160PARKINGSPACES
ATHLETICCLUB_(18.95Acres)___________________________________
154,000SQ.FT.ATHLETICCLUBW/8LANE50METERINDOORPOOL450LFLAZYRIVERW/8,000SQ.FT.ZEROENTRYBEACH&THELAZYISLAND
4,000SQ.FT.SLASHPARK,SLIDETOWERW/3FLUMES,7-20’DIAWATERUMBRELLAS3,800SQ.FT.2LANELAPPOOLW/SPA,60LFSWINGARBOROVERLOOKING1.5ACRELAKE
320LFSHADEDARBORW/20’40’COVEREDOVERLOOKPAVILION5INDOORTENNISCOURTS,5INDOORBEACHVOLLEYBALLCOURTS
1.5MILEMULTIUSETRAILPARK,325PRIMARYPARKINGSPACES&250OFFSITEPARKING
DEMAREST POINTE
A Classic American Neighborhood
Sustainability-Diversity-Ecology-Connectivity
™
31
THAI’S TRAIL TOWNHOMES 2003
DEMAREST POINTE TOWNHOMES 2020
DEMAREST VILLAGE TOWNHOMES 2000
ANCHORS BEND TOWNHOMES 2020
EXHIBIT D
Atlantic Coast Survey – Downstream Survey Information
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EXHIBIT E
Receiving Watershed Information
EXHIBIT F
ECS Soils Report
June 3, 2020 Mr. Scott D. Stewart Middle Sound, LLC 6933 Running Brook Terrace Wilmington, North Carolina 28411 Reference: Report of Seasonal High Water Table Estimation and Infiltration Testing Demarest Pointe Wilmington, New Hanover County, North Carolina ECS Project No. 49.11777 Dear Mr. Stewart: ECS Southeast, LLP (ECS) recently conducted a seasonal high water table (SHWT) estimation and infiltration testing within the stormwater control measure (SCM) area(s) off of Middle Sound Loop Road in Wilmington, New Hanover County, North Carolina. This letter, with attachments, is the report of our testing. Field Testing On June 1, 2020, ECS conducted an exploration of the subsurface soil and groundwater conditions, in accordance with the NCDEQ Stormwater Design Manual section A-2, at two requested locations shown on the attached Boring Location Plan (Figure 1). ECS used GPS equipment in order to determine the boring locations. The purpose of this exploration was to obtain subsurface information of the in situ soils for the SCM area(s). ECS explored the subsurface soil and groundwater conditions by advancing one hand auger boring into the existing ground surface at each of the requested boring locations. ECS visually classified the subsurface soils and obtained representative samples of each soil type encountered. ECS also recorded the SHWT and groundwater elevation observed at the time of the hand auger borings. The attached Infiltration Testing Form provides a summary of the subsurface conditions encountered at the hand auger boring locations. The SHWT and groundwater elevation was estimated at the boring locations below the existing grade elevation. A summary of the findings are as follows:
Location SHWT Groundwater
I-1 24 inches 38 inches
I-2 26 inches 55 inches
ECS has conducted two infiltration tests utilizing a compact constant head permeameter near the hand auger borings in order to estimate the infiltration rate for the subsurface soils. Infiltration tests are typically conducted at two feet above the SHWT or in the most restrictive soil horizon. Tests in clayey conditions are conducted for durations of up to 30 minutes. If a more precise hydraulic conductivity value is desired for these locations, then ECS recommends collecting samples and performing laboratory permeability testing.
Report of SHWT Estimation and Infiltration Testing Demarest Pointe
Wilmington, New Hanover County, North Carolina ECS Project No. 49.11777
June 3, 2020
2
Field Test Results Below is a summary of the infiltration test results: Location Description Depth Inches/ hour
I-1 Gray clayey SAND 26 inches 0.008
I-2 Tan/orange/gray CLAY 24 inches <0.001
Infiltration rates and SHWT may vary within the proposed site due to changes in elevation, soil classification and subsurface conditions. ECS recommends that a licensed surveyor provide the elevations of the boring locations. Closure ECS’s analysis of the site has been based on our understanding of the site, the project
information provided to us, and the data obtained during our exploration. If the project information provided to us is changed, please contact us so that our recommendations can be reviewed and appropriate revisions provided, if necessary. The discovery of any site or subsurface conditions during construction which deviate from the data outlined in this exploration should be reported to us for our review, analysis and revision of our recommendations, if necessary. The assessment of site environmental conditions for the presence of pollutants in the soil and groundwater of the site is beyond the scope of this geotechnical exploration.
ECS appreciates the opportunity to provide our services to you on this project. If you have any questions concerning this report or this project, please contact us. Respectfully, ECS SOUTHEAST, LLP K. Brooks Wall W. Brandon Fulton, PSC, PWS, LSS Project Manager Environmental Department Manager bwall@ecslimited.com bfulton@ecslimited.com 910-686-9114 704-525-5152
Winslow E. Goins, PE
Geotech Department Manager
wgoins@ecslimited.com
910-686-9114 Attachments: Figure 1 - Boring Location Plan Infiltration Testing Form GBA Document
APPROXIMATE BORING LOCATIONS
SCALE SHOWN ABOVE
Demarest Pointe
Wilmington, New Hanover County,
North Carolina
ECS Project # 49.11777
June 2, 2020
KBW
Figure 1–Boring Location Plan
Provided by: Google Earth
I-1
I-2
N
W
S
E
N
W
S
E
Infiltration Testing Form
Demarest Pointe Wilmington, New Hanover County, North Carolina
ECS Project No. 49.11777
June 1, 2020
Location Depth USCS Soil Description
I-1 0-24” SM Black/gray silty SAND
24”-38” SC Gray clayey SAND
38”-48” CL Gray/orange sandy CLAY Seasonal High Water Table was estimated to be at 24 inches below the existing grade elevation. Groundwater was encountered at 38 inches below the existing grade elevation.
Test was conducted at 26 inches below existing grade elevation
Infiltration Rate: 0.008 inches per hour
Location Depth USCS Soil Description
I-2 0-15” SM Black silty SAND 15”-24” SM Black silty SAND (hardpan) 24”-48” CL Tan/orange/gray sandy CLAY 48”-55” SC Gray clayey SAND Seasonal High Water Table was estimated to be at 26 inches below the
existing grade elevation.
Groundwater was encountered at 55 inches below the existing grade
elevation.
Test was conducted at 24 inches below existing grade elevation
Infiltration Rate: <0.001 inches per hour
Geotechnical-Engineering Report
Important Information about This
Subsurface problems are a principal cause of construction delays, cost overruns, claims, and disputes.
While you cannot eliminate all such risks, you can manage them. The following information is provided to help.
The Geoprofessional Business Association (GBA) has prepared this advisory to help you – assumedly a client representative – interpret and apply this geotechnical-engineering report as effectively as possible. In that way, clients can benefit from a lowered exposure to the subsurface problems that, for decades, have been a principal cause of construction delays, cost overruns, claims, and disputes. If you have questions or want more information about any of the issues discussed below, contact your GBA-member geotechnical engineer. Active involvement in the Geoprofessional Business Association exposes geotechnical engineers to a wide array of risk-confrontation techniques that can be of genuine benefit for everyone involved with a construction project.
Geotechnical-Engineering Services Are Performed for Specific Purposes, Persons, and ProjectsGeotechnical engineers structure their services to meet the specific needs of their clients. A geotechnical-engineering study conducted for a given civil engineer will not likely meet the needs of a civil-works constructor or even a different civil engineer. Because each geotechnical-engineering study is unique, each geotechnical-engineering report is unique, prepared solely for the client. Those who rely on a geotechnical-engineering report prepared for a different client can be seriously misled. No one except authorized client representatives should rely on this geotechnical-engineering report without first conferring with the geotechnical engineer who prepared it. And no one – not even you – should apply this report for any purpose or project except the one originally contemplated.
Read this Report in FullCostly problems have occurred because those relying on a geotechnical-engineering report did not read it in its entirety. Do not rely on an executive summary. Do not read selected elements only. Read this report in full.
You Need to Inform Your Geotechnical Engineer about ChangeYour geotechnical engineer considered unique, project-specific factors when designing the study behind this report and developing the confirmation-dependent recommendations the report conveys. A few typical factors include: • the client’s goals, objectives, budget, schedule, and risk-management preferences; • the general nature of the structure involved, its size, configuration, and performance criteria; • the structure’s location and orientation on the site; and • other planned or existing site improvements, such as retaining walls, access roads, parking lots, and underground utilities.
Typical changes that could erode the reliability of this report include
those that affect:
• the site’s size or shape;
• the function of the proposed structure, as when it’s
changed from a parking garage to an office building, or
from a light-industrial plant to a refrigerated warehouse;
• the elevation, configuration, location, orientation, or
weight of the proposed structure;
• the composition of the design team; or
• project ownership.
As a general rule, always inform your geotechnical engineer of project
changes – even minor ones – and request an assessment of their
impact. The geotechnical engineer who prepared this report cannot accept responsibility or liability for problems that arise because the geotechnical engineer was not informed about developments the engineer otherwise would have considered.
This Report May Not Be ReliableDo not rely on this report if your geotechnical engineer prepared it:
• for a different client;
• for a different project;
• for a different site (that may or may not include all or a
portion of the original site); or
• before important events occurred at the site or adjacent
to it; e.g., man-made events like construction or
environmental remediation, or natural events like floods,
droughts, earthquakes, or groundwater fluctuations.
Note, too, that it could be unwise to rely on a geotechnical-engineering
report whose reliability may have been affected by the passage of time,
because of factors like changed subsurface conditions; new or modified
codes, standards, or regulations; or new techniques or tools. If your geotechnical engineer has not indicated an “apply-by” date on the report, ask what it should be, and, in general, if you are the least bit uncertain
about the continued reliability of this report, contact your geotechnical
engineer before applying it. A minor amount of additional testing or
analysis – if any is required at all – could prevent major problems.
Most of the “Findings” Related in This Report Are Professional Opinions
Before construction begins, geotechnical engineers explore a site’s
subsurface through various sampling and testing procedures. Geotechnical engineers can observe actual subsurface conditions only at those specific locations where sampling and testing were performed. The
data derived from that sampling and testing were reviewed by your
geotechnical engineer, who then applied professional judgment to
form opinions about subsurface conditions throughout the site. Actual
sitewide-subsurface conditions may differ – maybe significantly – from
those indicated in this report. Confront that risk by retaining your
geotechnical engineer to serve on the design team from project start to
project finish, so the individual can provide informed guidance quickly,
whenever needed.
This Report’s Recommendations Are Confirmation-DependentThe recommendations included in this report – including any options or alternatives – are confirmation-dependent. In other words, they are not final, because the geotechnical engineer who developed them relied heavily on judgment and opinion to do so. Your geotechnical engineer can finalize the recommendations only after observing actual subsurface conditions revealed during construction. If through observation your geotechnical engineer confirms that the conditions assumed to exist actually do exist, the recommendations can be relied upon, assuming no other changes have occurred. The geotechnical engineer who prepared this report cannot assume responsibility or liability for confirmation-dependent recommendations if you fail to retain that engineer to perform construction observation.
This Report Could Be MisinterpretedOther design professionals’ misinterpretation of geotechnical-engineering reports has resulted in costly problems. Confront that risk by having your geotechnical engineer serve as a full-time member of the design team, to: • confer with other design-team members, • help develop specifications, • review pertinent elements of other design professionals’ plans and specifications, and • be on hand quickly whenever geotechnical-engineering guidance is needed. You should also confront the risk of constructors misinterpreting this report. Do so by retaining your geotechnical engineer to participate in prebid and preconstruction conferences and to perform construction observation.
Give Constructors a Complete Report and GuidanceSome owners and design professionals mistakenly believe they can shift unanticipated-subsurface-conditions liability to constructors by limiting the information they provide for bid preparation. To help prevent the costly, contentious problems this practice has caused, include the complete geotechnical-engineering report, along with any attachments or appendices, with your contract documents, but be certain to note conspicuously that you’ve included the material for informational purposes only. To avoid misunderstanding, you may also want to note that “informational purposes” means constructors have no right to rely on the interpretations, opinions, conclusions, or recommendations in the report, but they may rely on the factual data relative to the specific times, locations, and depths/elevations referenced. Be certain that constructors know they may learn about specific project requirements, including options selected from the report, only from the design drawings and specifications. Remind constructors that they may
perform their own studies if they want to, and be sure to allow enough
time to permit them to do so. Only then might you be in a position
to give constructors the information available to you, while requiring
them to at least share some of the financial responsibilities stemming
from unanticipated conditions. Conducting prebid and preconstruction
conferences can also be valuable in this respect.
Read Responsibility Provisions Closely
Some client representatives, design professionals, and constructors do
not realize that geotechnical engineering is far less exact than other
engineering disciplines. That lack of understanding has nurtured
unrealistic expectations that have resulted in disappointments, delays,
cost overruns, claims, and disputes. To confront that risk, geotechnical
engineers commonly include explanatory provisions in their reports.
Sometimes labeled “limitations,” many of these provisions indicate
where geotechnical engineers’ responsibilities begin and end, to help
others recognize their own responsibilities and risks. Read these
provisions closely. Ask questions. Your geotechnical engineer should
respond fully and frankly.
Geoenvironmental Concerns Are Not Covered
The personnel, equipment, and techniques used to perform an
environmental study – e.g., a “phase-one” or “phase-two” environmental
site assessment – differ significantly from those used to perform
a geotechnical-engineering study. For that reason, a geotechnical-
engineering report does not usually relate any environmental findings,
conclusions, or recommendations; e.g., about the likelihood of
encountering underground storage tanks or regulated contaminants.
Unanticipated subsurface environmental problems have led to project
failures. If you have not yet obtained your own environmental
information, ask your geotechnical consultant for risk-management
guidance. As a general rule, do not rely on an environmental report
prepared for a different client, site, or project, or that is more than six
months old.
Obtain Professional Assistance to Deal with Moisture Infiltration and Mold
While your geotechnical engineer may have addressed groundwater,
water infiltration, or similar issues in this report, none of the engineer’s
services were designed, conducted, or intended to prevent uncontrolled
migration of moisture – including water vapor – from the soil through
building slabs and walls and into the building interior, where it can
cause mold growth and material-performance deficiencies. Accordingly, proper implementation of the geotechnical engineer’s recommendations will not of itself be sufficient to prevent moisture infiltration. Confront
the risk of moisture infiltration by including building-envelope or mold
specialists on the design team. Geotechnical engineers are not building-envelope or mold specialists.
Copyright 2016 by Geoprofessional Business Association (GBA). Duplication, reproduction, or copying of this document, in whole or in part, by any means whatsoever, is strictly
prohibited, except with GBA’s specific written permission. Excerpting, quoting, or otherwise extracting wording from this document is permitted only with the express written permission of GBA, and only for purposes of scholarly research or book review. Only members of GBA may use this document or its wording as a complement to or as an element of a report of any kind. Any other firm, individual, or other entity that so uses this document without being a GBA member could be committing negligent
Telephone: 301/565-2733
e-mail: info@geoprofessional.org www.geoprofessional.org
EXHIBIT G
Conceptual Water Quality Plan
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EXHIBIT H
USGS StreamStats Report
Demarast Pointe - Downstream Drainage Analysis
Basin Characteristics
Parameter
CodeParameter Description ValueUnit
DRNAREAArea that drains to a point on a stream 2.21square
miles
PCTREG1Percentage of drainage area located in Region 1 0percent
PCTREG2Percentage of drainage area located in Region 2 0percent
PCTREG3Percentage of drainage area located in Region 3 0percent
PCTREG4Percentage of drainage area located in Region 4 100percent
PCTREG5Percentage of drainage area located in Region 5 0percent
Region ID:NC
Workspace ID:NC20200609181643573000
Clicked Point (Latitude, Longitude):34.27594, -77.79775
Time:2020-06-09 14:16:30 -0400
Parameter
CodeParameter Description ValueUnit
LC06IMPPercentage of impervious area determined from NLCD 2006
impervious dataset
14.41percent
I24H50YMaximum 24-hour precipitation that occurs on average once
in 50 years
10.8inches
Peak-Flow Statistics Parameters[Peak Southeast US over 1 sqmi 2009 5158]
Parameter CodeParameter Name ValueUnits Min LimitMax Limit
DRNAREADrainage Area 2.21square miles1 9000
PCTREG1Percent Area in Region 10percent0 100
PCTREG2Percent Area in Region 20percent0 100
PCTREG3Percent Area in Region 30percent0 100
PCTREG4Percent Area in Region 4100percent0 100
PCTREG5Percent Area in Region 50percent0 100
Peak-Flow Statistics Flow Report[Peak Southeast US over 1 sqmi 2009 5158]
PIl: Prediction Interval-Lower, PIu: Prediction Interval-Upper, SEp: Standard Error of Prediction, SE:
Standard Error (other -- see report)
Statistic ValueUnitPIlPIuSEp
2 Year Peak Flood 101ft^3/s57.917634.5
5 Year Peak Flood 202ft^3/s11735034
10 Year Peak Flood 283ft^3/s16149835.1
25 Year Peak Flood 397ft^3/s21872337.5
50 Year Peak Flood 497ft^3/s26493539.6
100 Year Peak Flood 609ft^3/s313118041.9
200 Year Peak Flood 713ft^3/s353144044.3
500 Year Peak Flood 873ft^3/s412185047.7
Peak-Flow Statistics Citations
Weaver, J.C., Feaster, T.D., and Gotvald, A.J.,2009, Magnitude and frequency of rural floods
in the Southeastern United States, through 2006—Volume 2, North Carolina: U.S. Geological
Survey Scientific Investigations Report 2009–5158, 111 p.
(http://pubs.usgs.gov/sir/2009/5158/)
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