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PRIES1; CRAVEN & ASSOCIATES, INC.

April 26, 1995

LAND USE CONSULTANTS

Mr. Terry Warren
Stormwater Engineer
Town of Cary
P.O. Box 1147
Cary, N.C. 27512

Re: Watershed Protection Plan
Coventry Woods/Coventry Glen

Dear Terry:

We respectfully request to be allowed to amend the previously approved Stormwater
management plan for the above referenced project Specifically, we request to be
allowed to use four smaller, periodically flooded, detention basins located nearer to
the discharge points rather than the single large wet detention pond previously
approved. This amendment will apply only to that portion of the subdivision identified
in the Stormwater Management Plan as " Area Three ".

The amendment requested is within the Town's jurisdiction because the percentage of
impervious cover is less than 24 %.

Our request is based on the following reasons:

1 - The pro'po~ed devices will provide a dep-ee of treatment equal to or hieher than
the currently approved wet detention basin.

We are enclosing a report from Soil and Environmental Consultants in support of this
reason. Although this report was based on the techniques of infiltration and overland
flow, the general principal principles outlined are similar. It is important to note
in this regard that we are not requesting any variance to or relaxation of the
objectives of the ordinance. We are proposing to use an alternative method of
compliance.

2 ;. Buildine the approved wet detention basin conflicts with the basic objectives for
desianation Qf the Speieht's Branch Corridor as a Public Greenway.

The Speight's Branch Corridor is a fairly wide area of floodplain and wetlands which,
despite relatively intense adjacent residential development, has retained it's
natural character. This state of preservation in such close proximity to residential
neighborhoods was the primary basis for designating this corridor asa Greenway. The
importance of the corridor and the Town's commitment to it is demonstrated by the
expenditure of a substantial sum of public funds for the construction of a Greenway
tunnel under Cary Parkway.

Construction of the currently approved wet detention basin will destroy a significant
portion of the natural setting and the visual character of the Greenway. In addition,
construction of the detention basin will result in a signifigant threat to natural
habitat, not only due to the disturbance itself, but because it forms a barrier to
free movement of species which inhabit the corridor.

PLANNERS I LANDSCAPE ARCHITECTS I ENGINEERS I SURVEYORS

3803~B Computer Drive, Suite 104, Raleigh NC 27609. Phone 919/781~0300. Fax [phone]~1288

Mr. Terry Warren
March 22,1995
Page Two

3 - ~:i~~~:~~:~~nee~:::~~aa::~o;::h:~~t:::r::i::~:ri~~lr~~r::~~t:~ will

lower the value of adjacent properties.

Although the Town requires that wet detention basins be fenced, experience shows that
standing bodies of water such as these often become unsightly, attractive nuisances,
particularly to small children. In addition, small standing ponds such as these are
inherently subject to eutrophication and accompanying odors. These safety and
aesthetic factors combine to cause the adjacent properties to be less valuable than
they would be otherwise.

4 - Approval of the requested amendment will accomplish all of the PUJ,:poses and
objectives of the Stormwater Ordinance with none of the objectionable effects.

Installation of the smaller can be accomplished with only minimal disturbance to the
existing area thereby preserving the natural character of the corridor. This
amendment will allow the Speight's Branch Greenway to function as it was intended and
will not pose any safety hazard nor will it harm or devalue any of the surrounding
properties.

It is our understanding that the Town has approved similar requests for the use of
alternative means of compliance with the Stormwater Management Ordinance.

We are attaching construction plans and other documentation for your approval If you
have any questions please let us know.

Thank you for all of your past considerations in this regard.

, 7J::~ 6'

Dennis C. Priest, P.E.

covaltsw.ltr

S&
EC

Soil & Environmental Consultants, Inc.
244 West Millbrook Road. Raleigh, North Carolina 27609. [phone]. Fax [phone]

Alternative Stormwater Treabnent System Design Parameters -
Coventry Subdivision, Cary, NC

Introduction

S&EC was retained to provide environmental consulting services for this
project Initially our concerns were focused on wetland related issues. The
project engineer (Priest Craven and Associates, Dennis Priest) informed us that
according to Town of Cary regulations stormwater treabnent facilities were
required for this site. Such facilities would not be required under state
regulations but Ca.ry's more stringent rules mandate treabnent

S&EC began looking into obtaining wetland permits for a conventional
stormwater "pond". It quickly became apparent after office and on-site
meetings with DEM staff, that it would be unlikely that we could obtain such
permits because the impacts related to pond construction would be significant
and are likely avoidable. Another issue that forced S&EC to pursue alternatives
to storm water treabnent by ponds was that the DEM Water Quality Planning
staff informed us that" stormwater ponds" were treabnent facilities which could
not be located on streams. Construction of a pond on this site would require that
such a pond or ponds be located on a small stream and in wetlands. From a
practical standpoint I do not see how DEM can maintain this position and
require construction of ponds for all projects that require them under current
regulations in the Piedmont of NC, Unfortunately there is no choice but to abide
by these rules. Therefore, S&EC, Priest Craven and the developer had no choice
but to pursue" alternative" methods for stormwater treabnent as described in
the Town of Cary's stormwater regulations.

Research of Available Technol~gy

&'

One of the reasons that has been stated for the protection of our Nations
wetlands is that they are the "Kidneys" of the earth which filter contaminants
from stormwater runoff. Therefore it seems logical that preserving such areas
would allow them to perform this function. In the last 10 years a significant
effort has gone into researching the ability of man made and natural wetlands in
treating stormwater and wastewater. Unfortunately most of the data is
incomplete, confusing and/ or contradictory. There is no hard and fast method

Soil/Site Evaluation . Mapping and Physical Analysis . Wetlands Mapping and Mitigation . Environmental Audits
On-Site Waste Treatment Systems, Evaluation and Design

for designing stormwater ponds or wetland systems that all experts agree on
and it is not clear how effective either method of stormwater treabnent is.

The town of Cary requires that TSS (Total Suspended Solids) removal in
basins be 85% for the first 1" of an average piedmont storm. The method
utilized by ponds for accomplishing this is detaining the runoff for 2 to 5 days so
that such suspended solids may settle out While much of the published data
support a 75% to 90% TSS reduction in ponds, specific studies often report
removal efficiencies less than 50% (see Table 1). It has been estimated that 90%
of most pollutants in stormwater are in the first I" flush and that most pollutants
are attenuated on suspended solids,

Basically only 3 alternatives to ponds were found in our literature search:
(1) Freshwater Marshes (2) Overland Flow and (3) Some type of filtering
mechanism. Our site does not contain a freshwater marsh and constructing one
would damage existing wetlands as much as a pond so overland flow thru the
existing wetland and filtering were evaluated further.

Examination of design criteria for overland flow systems are presented in
Table 2 and Table 4. This criteria is based on research for treating stormwater.
Design criteria for rock filters (see Table 3) was only available for wastewater
systems which typically contain a higher TSS than stormwater.

Based on a review of performance data for overland flow (see Table 1)
and rock filters see Table 5), it seemed that it could be possible to meet the 85%
TSS removal required by the Town utilizing such systems. However, it
appeared that overland flow alone would not be adequate. Based on S&EC's
knowledge in the hydraulic performance of soils and knowing that only the first
I" of stormwater was to be retained with the remainder passed, it seemed that a
combination of the two systems would be the only logical answer.

DesigI!

We first looked at the design criteria for treating 100%. of the first I" using
each system independently. The area required for overland flow using criteria
from Table 4 is presented for each discharge point in Femwood in.Table 7. At
least 1.2 to 6 times more than the minimum area is being provided for each
discharge point (see Table 8). The surface area available was determined by the
distance to the stream from the discharge point and the length of the rockfilter
trench (discussed later). The most difficult aspect of utilizing wetlands overland
flow for stormwater treabnent is uniform distribution. Point discharges, even
with flared end sections and rip rap energy dissipaters often result in erosive
forces which scour channels to the stream. When this happens overland flow
does not occur, channel flow does. Little treabnent results during channel flow.

Even though treatment is theoretically handled by the overland flow
design, the rock filter concept is being incorporated for the following reasons:

(1) Some method is needed to speed the stormwater flow out evenly so that
overland flow occurs.
(2) The TSS requirements would not be met by overland flow alone if one
believes data available in the literature.
(3) To avoid scouring the rockfilter incorporates some storage.

A debris basin is incorporated into the design to minimize trench
clogging. Table 6 indicates the length of 2' x Z trench required to hold 100% of
the first one inch of stormwater. Since our goal is to have overland flow
treatment 1/4 to 1/2 of this amount is recommended (see Table 8). Otherwise,
theoretically the trench would not overflow during the first 1". The combined
efficiencies of the rock filter and overland flow should provide equivalent
treatment as data reported in the literature for ponds.

Another reason for recommending the combined system approach is that
rockfilters efficiency reduces over time while overland flow increases as long as
% vegetated surface is maintained. Since these areas are in stream buffers it
would seem reasonable to assume they will remain vegetated. Because of the
reduced efficiency of rock filters/ infiltration devices over time they alone do not
seem to be adequate for long term performance.

Please note that even though we realize that some infiltration will occur in
these trenches similar to that in septic tank drainfields, no "credit" was given for
this phenomena since no data could be found to correlate to stormwater and
because continuation of long term infiltration at high rates is questionable. Data
presented is based on a "flow through" rock filter with a discharge point

Please note that the purpose of these design recommendations is for
stormwater treatment not flood control. It is our understanding that the
regulations which we are addressing deal with water quality not water quantity.
The overland flow system design depends on the movement of water over the
surface and just below ground, therefore areas between the trenches and the
stream will be wetter than they are now and "seepy" especially after storms.
Such a phenomena is contrary to the old way of straight piping stormwater
directly to streams however it is necessary if any treatment is to occur prior to
reaching the stream. This phenomena will also enhance the wetland in all
likelihood. If you have questions or require further information regarding these

recommendations please call.
Sincerely,

~t.<-I1Miiu @tfft1)

Kevin C Martin,
President

KCM/jmh

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PROPOSED AMENDMENT
STORMWA';I.'ER MANAGEMENT PLAN
COvtmTRY WOODS I COVENTRY GLEN
GARY, N.C.

1 - Infiltration Volumes

Average Lot Area ( A ) 1/4 Ac.
Runnof Curve No. ( Cn ) 75
Hydrologic Soil Group B
Design Rainfall ( P ) l,t
Soil storage Coeff. ( S ) S=(1000/Cn)-10
S=(1000/75)-10
8=3.333

From Sub. Plan
From S.C.S.
From S.C.S.
From Ordinance

From S.C.S.

Runoff Depth

( Q )

Q=(P-0.2S)t2 /(P+0.8S) From S.C.S. Eq.
Q=(1-(0.2)(3.333)YZ/(1+(0.8)(3.333))
Q=(0.1116)/(3.666)
Q=0.0304

Drainage Areas

( DA )

DA,=197,762 SF
DAtz= 48,352 SF
DA'!l=653,836 SF
DA+=432,551 SF

From Const. Plan
( 4.54 Ac )
( 1.11 Ac )
( 15.01 Ac )
( 9.93 Ac )

2 - Discha~ge ~asin Minimum Requirements
Minimum Detention Vol. ( V )

From Eq.

V = (Q/12 )( DA)
V.=(.0025)(197,762)=
V~=(.0025)( 48,352)=
V~=(.0025)(653,836)=
V+=(.0025)(432,551)=

495 CF
121 CF
1635 CF
1082 CF

Mimimum Surface Area

( SA )

From Eq.

SA =(1.10/100)(DA)
SAI=(0.0110)(197,762)= 2,175 SF
S~=(0.0110)( 48,352)= 532 SF
SA~=(0.0110)(653,836)= 7,192 SF
SA+=(0.0110)(432,551)= 4,758 SF

Weir Length (L) and Weir Depth (H) From Eq. Q = CLH4r~
Q = Q10
C = Weir Coeff.
L = Weir Length
H = Weir Depth

3 - Discharge Basin Design

Bulkhead
Disch Length

Basin Dim.
Area Av. Depth Volume

Weir_
Q10 Length(L) Depth(H)

-~---------~----~-------~---------------~-------------------------~------

1 138 LF 2,450 SF 0.65 FT 1,592 CF 19.13 CFS 12 FT 0.62 FT
2 18 LF 3,000 SF 1.25 FT 3,750 CF 4.37 CFS 6 FT 0.37 FT
3 126 LF 7,500 SF 0.75 FT 5,625 CF 57.04 CFS 18 FT 0.94 FT
4 122 LF 5,700 SF 0.50 FT 2,850 CF 34.95 eFS 12 FT 0.94 FT
Note: Average depth is based on 1/2 of total depth

Table 1

Overland Flow Performance (1) where area is (a) dry between storms
(b) 20% of ground vegetated
(c) bare ground 55%
(d) pockets of standing water 25%
or less

Typical Retention/Detention Pond vs. Overland Flow vs. Pond Performance
General Site Specific Site Specific
Parameter Pond(2) Overland Flow (1) Pond (1)
Sediment (TSS) 75-90% 24 45
Total Phosphorus 55-65 % -44 50
Orthophosphate 19 28
Total Nitrogen ....40% -32 -17
BODs ....40% -22 -8
Metals 0-80% -30 to +27 -86 to +83
TDS -20 -50

"'Note a negative number indicates an increase in the parameter. Data for ponds
were for sites with littoral shells, ponds without such vegetated zones do not
perform as well,

Table 2 - Design Parameters for Specific Pond vs. Specific Overland Flow (1)
Data Presented in Table 1.

Parameter Pond Overland Flow
Drainage Area 1200 ha 600 ha
Debris Basin .000125 % of drainage area .000125% of drainage area
Detention Time 1-12 days 5 to 48 hrs.
Surface Area .0071 % of drainage area .0027% of drainage area

Table 3 - Design of Subsurface Rock Filters (3)

~
Domestic Wastewater

Loading: Rate
4.7 cm/d to 18.7 cm/d*

*For polishing treatment after basic or secondary treabnent

Table 4 - Other Recommended Design Criteria For Stormwater Treatment
Systems (3)

(1) H extended detention is not possible surface area should be a minimum of
3 % of contributing drainage area.

(2) Length to width ratio of 2:1 is preferable

Table 5 - Performance of Subsurface Gravel Filters on Wastewater* (3)

Parameter % Removal
BOD 52 to 96
Suspended Solids 67 to 94
Nitrogen (Total) 12 to 38
Phosphorus -12 to 70

*Note there is only minimal data on the effectiveness of such systems on
stormwater. However, most contaminant levels (e,g. BOD, N, P, etc. will be
lower in stormwater than primarily treated wastewater).

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Recommended Minimum Design Criteria for Rock Filter vs. Overland Flow
Stormwater Treabnent System - Coventry Subdivision *

Discharge
Point
Area 1
Area 2
Area 3
Area 4

Volume of
Water To be
Treated per
Storm Event
499 .
122
1650
1092

Drainajte Area
2.2ac.
0.54 ac.
7.29 ac.
4.84 ac.

Table 6.
Length of *** '2:
x 2' Rock Filter
Needed
406'
99'
1341'
888'

Table 7
Minimum
Surface Area
of Overland
Flow Needed
2875 ft2
706 ft 2
9527 ft2
6325 ft2

* Assumes no more than one storm event per day and that no treabnent is
required beyond first one inch.

** Based on 3 % of drainage area.

*** Based on 18.7 cm/d loading rate for wastewater treabnent (This data is for
general information only.)

Table 8 - Length of 2' x 2' rock Trench Provided and Overland Flow Surface
Area Provided for Treatment of Stormwater prior to Reaching Stream

Lot
Area 1
Area 2
Area 3
Area 4

Trench *
120'
90'
200'
200'

Overland Flow **
18,000 ft2
9900 ft 2
27,000 ft2
28,000 ft2

* Trench recommended is .15 to ,5 the criteria to treat wastewater loaded at 18.7
cm/d 7 days per week. The main purpose of the trench is for distribution and
energy dissipation.

*** Overland flow area provided is 2.8 to 14 times greater than that needed to
treat stormwater volume.

Bibliography

Source:

Constructed Wetlands for Wastewater Treatment Hammer,
Donald .1989 Lewis Publisher Inc.

(1) Urban Runoff in a Fresh/Brackish Water Marsh in Fremont, CA. Emy
Chan Meiorin, Pg. 677-685

(2) Creation of Wetlands for the Improvements of Water Quality, Lewis C.
Linker. Pg. 695-701

(3) Performance Expectations and Loading Rates for Constructed Wetlands
Watson, Reed, Kadlec, Knight and Whitehouse. Pg. 317-351