O-2020-0271 - 10/8/2020ORDINANCE NO. 0=2020=0271
AN ORDINANCE AMENDING THE "CITY OF ROUND ROCK DESIGN
AND CONSTRUCTION STANDARDS -DRAINAGE CRITERIA
MANUAL" ADOPTED BY ORDINANCE NO. =04=12=16=13AI BY
AMENDING SECTION 2 "DETERMINATION OF STORM RUNOFF"s
AND PROVIDING FOR A SAVINGS CLAUSE AND REPEALING
CONFLICTING ORDINANCES AND RESOLUTIONS.
BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY OF ROUND ROCK,
TEXAS:
That the "City of Round Rock Design and Construction Standards - Drainage
Criteria Manual," Section 2 entitled "Determination of Storm Runoff," is hereby amended
and same is attached hereto as Exhibit "A,° and made a part hereof as if fully set out in
this ordinance.
A. All ordinances, parts of ordinances, or resolutions in conflict herewith are
expressly repealed.
B. The invalidity of any section or provision of this ordinance shall not
invalidate other sections or provisions thereof.
C. The City Council hereby finds and declares that written notice of the date,
hour, place and subject of the meeting at which this Ordinance was adopted was posted
and that such meeting was open to the public as required by law at all times during
which this Ordinance and the subject matter hereof were discussed, considered and
formally acted upon, all as required by the Open Meetings Act, Chapter 551, Texas
Government Code, as amended.
0112..20202; 00454877
Alternative 1.
By motion duly made, seconded and passed with an affirmative vote of all the
Council members present, the requirement for reading this ordinance on two separate
days was dispensed with.
READ, PASSED, and ADOPTED on first reading this day of
, 2020.
Alternative 2.
READ and APPROVED on first reading this the 24t day of
, 2020.
READ, APPROVED and ADOPTED on second reading this the day of
AF 4 = /�
ATTEST:
Is
SARA L. WHITE, City Clerk
4 �0/
CRAIG VORGAWAMayor
City of Round R ck, Texas
2
EXHIBIT
„A»
SECTION 2 - DETERMINATION OF STORM RUNOFF
Table of Contents
SECTION 2 — DETERMINATION OF STORM RUNOFF Table- of —Contents,, 0 0 2- 1
2,91.0 GENERAL...................................................................................................................2-2
2,02.0 EFFECTS OF URBANIZATION...............................................................................2-2
2.2.1 Design Assumption � or Storm ow Analysis..................................................2-2
2.3.0 METHOD OF ANALYSIS.........................................................................................2-3
2.490 RATIONAL METHOD......... *0000000000000000 00000000000*000 00090000000000000 0000000**066009 00000000000900000 000,092-4.....................................2-4
2.4.1 Runoff Coefficient(C)......................................................................................2-5
2.4.2 Time of Concentration....................................................................................2-�8
2,04.3 Rainfall Intensity...........................................................................................2---�-A 11
%ft
2.4.4 Drainage Area (A)........................................................................................2--�? 11
r "W �s A .,.1% 1 ..I A
------ 4
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r
ATION SERVICE METHODS
2.5*0 "fK: 7. A.. I .. -NA.'1`UR.A1..
R.ES"0'LJ'R.CESR P RMA %
2.5.1 ^ � .: �: °A -1.14r�,�.,u. � :a 1: X 1k.un �f,,-1 ---
..,.Y, Rainfall Distribution;�....................................2-?�}}15
2.592 N*atutval Conservation Service Runoff Curve Numbers
2.5.3 Time of 2(}
I
--------- 2-1-
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00454484"X
� Drainage Criteria Manual �
SECTIO-N-----2 -DETERMINATION OF STORM RUNOFF
SECTION 2 - DETERMINATION OF STORM RUNOFF
2.1.0 GENERAL
If continuous records of the amounts of runoff from urban areas were as readily available
as records of precipitation, they would provide the best source of data on which to base
the design of storm drainage and flood protection systems. Unfortunately, such records are
available in very few areas in sufficient quantity to permit an accurate prediction of the
stormwater runoff. The accepted practice, therefore, is to relate runoff to rainfall, thereby
providing a means for predicting the amount of runoff to be expected from urban
watersheds at given recurrence intervals.
Numerous methods of rainfall runoff computations are available on which the design of
storm drainage systems may be based. The method chosen is dependent upon the
Engineer's technical familiarity and the size of the area to be analyzed. For the
method chosen the Engineer will be responsible for making reasonable assumptions as to
boomm
the development characteristics of the study area.
2.2.0 EFFECTS OF URBANIZATION
It has long been recognized that urban development has a pronounced effect on the rate
of runoff from a given rainfall, event. The hydraulic efficiency of a drainage area is generally
++e increased as a byproduct of urbanization which in effect reduces the storage
capacity of a watershed. This reduction of a watershed's storage capacity is a direct result
of the elimination of oerv'l'ous surfaces, small ponds, and holding areas. This comes
about by the grading and paving of building sites, streets, drives, parking lots, and
sidewalks and by construction of buildings and other facilities characteristic of urban
development. The result of the improved hydraulic efficiency is illustrated graphically in
Figure 2-1 in Appendix B of this Manual, which is a plot of the runoff rate versus time for
the same storm with two different stages of watershed development.
2.2.1 Design Assumptions Fceor Stormflow Analysis
A. When analyzing an area for channel design purposes, urbanization of the full
watershed without detention ponds shall be assumed (except as noted in paragraph
E. below). Zoning maps, future land use maps, and master plans should be used as
aids in establishing the anticipated surface character of the ultimate development.
The selection of design runoff coefficients and/or percent impervious cover factors
are explained in the following discussions of runoff calculation.
B. An exception to paragraph A. above may be granted if the channel is immediately
downstream of a regional detention pond and written approval is obtained from the
rs+,� E49,�o�,nQr Director of theUt'liites and Environmental Services Department
------------
(hereinafter, the "ICES Director'.
2
Co In designing a storm sewer system W1h'tn a residential subdivision, full development
of adjoining and interior tracts without detention must be assumed.
D. In designing a storm sewer system within a commercial or multifamily subdivision,
}ear stormflows, can, at the Engineer's discretion, reflect the flow reduction
anticipated by future detention ponds. This applies exclusively to the flows generated
by those properties contained within the subdivision. Provisions for conveyance of
theMN! % annual chance ,100-year, undeta'ined flows within the right-of-way or drainage
easements still apply (See Section 1.2.2C....'.).
WO&MMV
E. In the event the Engineer desires to incorporate the flow reduction benefits of existing
upstream detention ponds, the following field investigations and hydrologic analysis
will be required: (Please note that under no circumstances will the previously
approved construction plans of the upstream ponds suffice as an adequate analysis.
While the responsibility of the individual slate or subdivision plans rests with the
Engineer of record, any subsequent engineering analysis must assure that all the
incorporated ponds work collectively.)
1. A field survey of the existing physical characteristics of both the outlet structure
and ponding volume. Any departure from the original Engineer's design must be
accounted for. If a dual use for the detention pond exists, (e.g., storage of
equipment) then this too should be accounted for.
2.A comprehensive hydrologic analysis which simulates the attenuation of the
contributing area ponds. This should not be limited to a linear additive analysis but
rather a network of hydrographs,which considers incremental timing of discharge
and potential coincidence of outlet peaks.
2.3.0 METHOD OF ANAL
YSIS
3
method of analysis must remain consistent when drainage areas are combined and the
method which applies to the largest combined drainage area should be used. T.�"'o 0-1 acil� ------- F—T
SLAMIVILM-A:iiii:' :tee: Y
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....... .................... ......t........... A ............ .............
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2w4nO RATIONAL METHOD
The Rational Method is based on the direct relationship between rainfall and runoff, and
is expressed by the following equation:
Qp= C@A
Where:
(Eq. 2-1)
QP is defined as the peak runoff in cubic feet per second. Actually, QP is in units -of
inches per hour per acre. Since this rate of in/hr/ac differs from cubic feet per
second by less than one (1) percent (1 ign/hr/ac = 18008 cfs), the more common
units of cfs are used.
C is the coefficient of runoff representing the ratio of peak runoff rate "Qp" to average
rainfall intensity rate "i" for a specified area "A".
I is the average intensity of rainfall in inches per hour for a period of time equal -to
the time of concentration (tc) for the drainage area to the point under
consideration.
A is the area in acres contributing runoff to the point of design.
4
The following basic assumptions are associated with the Rational Method:
A. The storm duration is equal to the time of concentration.
B. The computed peak rate of runoff to at the design point is a function of the
average rainfall rate during the time of concentration at that point.
C. The return period or frequency of the computed peak flow is the same as that
for the design storm.
D. The necessary basin characteristics can be identified and the runoff coefficient
does not vary during a storm.
E. Rainfall intensity is constant during the storm duration and spatially uniform for
the area under analysis.
2.4.1 Runoff Coefficient (C)
The proportion of the total rainfall that will reach the drainage system depends on the
imperviousness of the surface and the slope and ponding characteristics of the area.
Impervious surfaces, such as asphalt pavements and roofs of buildings, will be subject to
approximately one hundred (100) percent runoff (regardless of the slope). On -site
inspections and aerial photographs may prove valuable in estimating the nature of the
surfaces within the drainage area.
The runoff coeffcient "C" in the Rational Formula is also dependent on the character of the
soil. The type and condition of the soil determines its ability to absorb precipitation. The
rate at which a soil absorbs precipitation generally decreases as the rainfall continues for
an extended period of time. The soil infiltration rate is influenced by the presence of soil
moisture (antecedent precipitation), the rainfall intensity, the proximity of the ground water
table, the degree of soil compaction, the porosity of the subsoil, and ground slopes.
It should be noted that the runoff coefficient"C" is the variable of the Rational Method which
9
is least susceptible to precise determination. A reasonable coefficient must be chosen to
represent the integrated effects of infiltration, detention storage, evaporation, retention,
flow routing and interception, all of which affect the time distribution and peak rate of runoff.
Table 2--2 1 presents recommended ranges for "C" values based on specific land use types.
106060SW
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TABLE2m 1 {Cinue )
R-A-TIO-NAL THUD RUN COEFFICIENTS FOR COMPO-S-ITE
Runoff Coefficient (C)
Return Period
Character
Of Surface
2
5
1 fl
54
1005
Years
Years
Years
Years
Years
Years
Years
,Pasture/Range
Flat, 0.wo2%
0,225
0,628
Ow3O
0,534
0,537
Os4l
0.653
2-7°l0
Avera eOWN
0433
0036
---------
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.37
0640
2---
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0653
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0,022
00.25
00,28
Or-31
0,635
0-n39
Ox48
Av-----
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04,40
04-43
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---
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0.35
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uX45
-------------
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Ox58
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--
1. Compasite "C" value for developed conditions (1CDEv) = IC7 + (1-1)C2 where:
400mI
Impervious cover rcenta e�decimal value
cJ%
*&Mae*= � _ "C" value tar impervious cover
Cz = "C" value for pervious caner
nnn
2. Maximum allowable im ervousicover values m------------ ------------- — -------------------------------a e iimrt--------------- ----ed b Ind use t
-------------------------------- --------------- ------------------------------------------------------------------------------------------------------ —applicable
e� refer to
Ci--------------
ty of Round Rock Zo �nq an for Development Ordinances
No-tes
-----------
Grass cover less than 50 percent of the area.
- - ------------
�* Grass cover on 50 to 75 percent of the area.
Grass cover qre4nter than 75 percent of the area.
S-o-urce% 1--. ROSSMiller R.L. "The Rational FormulaRevisited."
2.4.2 Time of Concentration
TY MwwOW" Tt (r
Sheet Tt Lshallow concentrate�j + Tt ichannei
I (Eq. 2
tom"--n- %%%%%% 0,5('SIO.'41
Tt(st,eei?= 0.4 nL)0'8/f(P2,)
where,
(Eq.2-2-3)
&%AF~
0 6
tsW*VWWWVWWW~ --m-m-ft-ft-W TIVUTNIA ---- NO.V'f 4 Sheet Flow travel time in minutes
L = Length of the reach in ft.
2-�2)
n ManningIs n (see Table
P2 2x= ear jamhour ra'I'nfall hes "n *nci i RAIn as in 2.4.3)
s =Slope of the ground in ft/ft
%of 54,jUnpaved Tt hallow concentratedl L/(60)( 16 .1345 sOIa(Eq 2-�4)
AMMAR AMR- - -- - AN ")fMA)AARAAAA
Paved Tt(shallawconcentrated) = L.J )0.51 (Eq.
(O��C}.H�S�-5)
------------------ ---------------------------------------------------- -------------
where,
--- --- TI-M.An 1-flY -1-11-1- """Aworf ---- Shallow GOCICentrated Flow travel time
tG Ttishallow concentrated)-------- ---
in minutes
L = Length of the reach in ft.
6 4
- I"11014 -- ---
s IVT-F r%s�---j-,"-a,ur
Slope of the Shallow Concentrated Flow in ft/ft
C. Channel or Storm Sewer Flow. The velocity in an open channel or a storm
drain not flowing full can be determined by using Manning's Equation. Channel
t -1
velocities can also be determined by using backwater profiles. span
channel flow, average flow velocityis usual!y determained b%, aassuming abankmfull
------------
condition.Noce that the channel flow component cif the time cif concentration may
need to be divided into multiple segments in order to re►�resent significantin
channei characterolst'I'ese The details of usaing ManningIs equation and selecting
--- ---------- - ----------------- Manning's "n" values for channels can be obtained from Section 6 of this Manual.
IM
0 Ic
If"
For Tufl storm AV dradrain flow unde riemcisure conditions (^�rt��°° r� 6V6ydraull'c
g.rade Ii%ne is hi Nqher than the lowest crown of a storm drain) the following equation
ago
should be applied:
V=Q/A
where:
004%W~
V = Average velocity, ft/s
Q = Design discharge, cfs
A = Crossmsectional area, ft2
9
(Eq. A 2.A
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1,000�*W)
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2.4.3 Rainfall Intensity (Ni)
Rainfalll intensity (i) is average rainfall rate in inches per .... n-n hour and is selected
on the basis of design rainfall duration and design frequency of occurrence. The design
is
duration equal to the time of concentration for the drainage area under consioderatioon,
The design frequency of occurrenceis a statistical variable which is established by design
standards or chosen by the Engineer as a design parameter,
is
The selection of the frequency cri eria necessary before applying any hydrologic method.
improvements
Storm drainage ion Round Rock must be desiagned to sintercept and carry the
runoff m a 41710 AnnualC
hance {finrenty-five (25) year} frequency storm, with an auxiliaryor overflow system capable of carryingcarrying a lv/o AnnualChance lone hundred (100) year},
frequency storm.
A .-A
M
V, AVIWA -�:VAL
-it
AT
Mo T. "..M. -ow-via TV= ra-M VN:
L'��AIAL M XW2WW,-,%W K am
VIFWTT� rNW 1--n.f a. -du. -Irr mc,-% r^ 'n, Q -
M;F TV 4 -3nie��ey4
am
L --- O-A --- ----- h--
------ Ar.-C.M. MazalmshMAD -us MOMMARX-lai'm -- ------
%A I MAI-I ------ UWAI ------ %V1 %IJO kVV-%AI §%,y fW%AI
uen fr selected,and desurat
shall be based on the design e CY' dion
determi'ned, b the En iAneer,, u lect to the a roval ofthy: IJES Director,and shall be
-AMARI - ---- determined fram the City of Round Rock Kainfall Application Instructions ARM n)fear
droiagicdesigns and analyses,issued and amendedbytie Eb Uirector,
0'
13
........ .......
... ... ......
MML
' 2.4.4 K...,.........''`
a a The size of .-.....-i.-..n,.........t`....n..for .
."''...Rational
vn...-/
|
. Method, -h..i.....^.......m�i"-...i"v,....n^. mPsuPPIa..,..o...
----- - ---------- W-0 --------------
by fleld surveys where topographic data has changed or where the contour interval is toon'_.v..,.,,
great ...{`...,JMARMARA topog,rap ---- h_Y ----�.,n`fl°..'..^�,.":..:e.-,....`�mii.. bil-I - based
on existi 0 ng [A L-I
Could.......bealtereduY,,,,,2roOsqu%jStreet ',/.v. lot `.,.------------
..
AMR RAMPAR A
structure configuration and orientation, and many other features that-!ar-n --- a r. na result
from,.the urban"izati*on process.
Example 2ml
An urbanized watershed is shown 41"n the followiang figure, Three types of flow condit"ions
exist between the most distant Polent in the watershed and the outlet. The calculation of
and ..
time of concentration `. time . each reach 'is as follows:
1
5
16*%L I 'AN& MAE& 0 nAft 2h&,& AL" AM& I
RudAR h D e-ma&ft Sffim& c ri t 51 Le n' h DFU na28e value/
a am A APMMM I calla MACRAMAN A
90ton
S& is AM.
0 W ���� ����Area � *�~�� � Acreaae
A to Sheet t'low 1.58 50 uwm'*" Am 1 003
Dow
lawn) (3 acres dense/Bermuda grasses
1( ----------- -- :� i s
B to C
Shallow
29.0
840
DAa=2
Paved
concentrated (20 acresl
flow (autter) I I
to D Channel Flow 10,5 --i-A.200 D A am 3 OwOl 5
fSto - rm d-ral'-n (30 acres.),
.,e
VW.IM
with i"nlets-% Dia 04~
. |3'eet)
For i.'.-,.-�`n..'... time..can .....'........
2-1
Equatisons 2 4hp% and 58
..... MA -----
%%%*M b Ub Ux
n () I - W - 8 f De, %0. 5 \00, 44
W.542(�Jvllxj x %bQF aE.2
O-C:
0 Aa ' OW 1 8,8282/(H2) min.,
#%%%%,W 0 -0 0.5
Tt(B-C) %beam 840/(60)(2Os32o2)(O-,2)
AMON
XWOM 4.87 mi*n.
The .".' in ..''-...n............/'....[',..Manniong's Equation,.n...-.
assumption of full pipe flow as follows:
0.67 O—GL
0, 1
If 77- Ur 71 E97
VC-DG4WID = (1.49/n) R f --- 1--- A--- -7----sO..
16
n - --- -- 7-(1.49/0.015) (3/4)°�7(0.015)°=�
10.04 ft/s
xxxxxxo
IN le
The channel flow travel time is calculated by dividingthe I nthe velacityand dividingby
gth b
60 to convert to minutes:
--- ---------- - ------------------ -------------
Tc(C-D) 1200/(0
1x.04 L6(0--------------------- -- . ) - -----
xxxw 1.99 min.
--- - ------ - AMAMI
The total time of concentration is calculated by adciin-Cl --- all of the calculated sheet flaw,shallow
........ -------- ----- ---------------- -------------- concentrated flow, and channel flow components:
A
XONO
Tc WMIM Tijsheetj + Tt hallow concentrated -I +---Tqchannel)
ROWN ROW .Is MMM
Waft
000M 0 � +Tits—c�
Tt(ATt�c-o�
-LU + RON
%Oaam v 4 1
**am 1862821 P2%)-fl'5 + .87 + .099
---------------- -- - ------ -- I MW
- - - - ------------------------------------------------- MARMAR a "&,~
intsi
.5686M+,.
tothe nearest wholeTime of concentrat'ion 'in decimal minutes mav be used but roundi'nq
number of minutes (greater than ar equal to 5) is generallyacce table,. 12
For this exam IcDrainage Area D-1 trvrseb reach Aw*Bj is a rssed lawn area in fair
condition Drains Area DA-2 traversedreachB~C is cmercral e
com osed
impervious+inw 9of 76% (ConcreLv Haved) area and 24% ervious rassed(Loiod condition ver e
... MOCCOM
slope) areaand Drainffle Area DAx&3 raversed bv reach C*wD} isan i'ndustrl"al develo ment
lip
composedof6% 1 ervious concrm+e paved area and32% ervious rasse Lqood
condi*t*iol IIaverac�e slope area,, q 1 1 Ommmmi ---%I - -- - - - -
eG
The co pasite runoff c fficienis for Drainage Areas DA- and DA-3 are calculated as
-----------------------------------
follows:
CDA-2 (Ow76)(0,597) + (1,.aO,57 it 6
tQ-4
V%*W& (0276 597 .924,) 461
OW~
VAMM a.a4r6
Use &"%& U0 C
.00
iF
CDA-3 %Paw*,% 0 .6 04
6 8,)f Q, 9 7 8
-- -- -- - ---------------------- A
AM" 0 0% ^ %
Oa668 iz;7 + z i(
'( O'S 0 W. 0,
--------------- --------- )(0 It "W"L
Abom 0,9 8 0 6 8
....................
Use 0,.81
The runoff coeffici*ents (C) for the three (3) areas are gioven as follows for the,.1 %----- ITY-1 O.C1 main poor. .11 !at 0
Ast W . -1-9 W] §W%.,* & I Owi-
Annual Chance 00 ear� U-11-3-4 event, - - T_ f All$
--- ------------ Y .1 ---------------- ktG)4 Is I b
DRAINA Ve
G E Rea%., 0 C in_ Area
DANC CAR
AREA _L_e_n,qth f S acres)
-----------------------
(Reach) I
-1.5
DAmml 50 180282LLP21 C, . | .
� 0441 3
840 4a,87 0*85 20
DA.m3 1200 1 OX0 1 $C99 0*81 30
m m m a a m A -------- --- m A m 9 m m I -------
� .c-_. IIIIII
WEIG-HTED a"r f"N aaa&&&f Am
TOTAL i u][AL
AVERAGE53
-1 8,282/(P2)0_'5+7 0.80
Re
88_��_ IFT ----
fp
_A_Q
rn) ---- -- -A-
.840
A"JIM
1196
9;.85
-20
4,
0 ------ -_
--- ---
-1 n
poll
The intensity (*I) of the 1 % Annual Chance (100 yearl sW. ra'Infall event 'is obta'ined from the
WOMAN
A Is
lei% e% --fpk% Aengnt-a-In V- e%
I n
RAIn as discussum Ien bection Z.4 f� F= IrVI-IF PA-M-SW�d-17V --- 1�� --- 1�7f-AT-4-1-i�U----
! N F -- ------------- - ---'IF j
Therunoff ,.°..(C)..n.-(0.41...+...``-.'...'.
30)/53= 0.80 Thus the peak flow Qpp C x I x A 0.80 X 0 �'in/hr) X 53 acre Cfs
18
Mi
M.:I- :Ali. 7i1*• iM► :M► silk
::--- -- -
TV
i
.
:�►: ':,r►:� :�,:' ':w�:' :��►'� .:mac: ,�+�►:. .:+�+� :ijrz
:pit:
P.�►.
. �:..
f: :l:�: :.+.: !: !: .: 'mac:
'ter••' ���:' . .:��.. .:�:. .�:.. .•:�:'.•�:.. .:�:`•�::. ..�::. .,+I.. ... ..,
:ilk• �i1�►: :i : Aki. :i :ilk. �Ik►. :i:
AL
ill►. aM• il►. �MiOilly :;M►. :W : =+�: '�R� �: :�: '�: ��: :�: :�: .�A4 : :fit. :.: �►:
Xv.Alt:• i1�1►. :; + ;�w. alli:;►. :+ilt• * allk: aNl►: :iil
��� :� ��' :�: �: :+A4:. :�R:�• :#A .
TV -
INK:
Q I rl n ^r e--l- ftn�CU a C
ii��
V-1-7ki-I ---- Y- v Z7 9 p
19
0
CN
�Z
z;u-
------
'5LT,
UY
UL --- V
UY
L_G ---- V,
-Gu ----- P"
UZ14P
!jU
5u 6--u- v LL ------ V
coo L V v
Tu MAP UO ---- Z;
uu--* u ZVP -V
A V__Iu ----- V
J7
UY ---- L, D_G---
G= AL= _X AVW.
------ v
C,
Aft
VVI
%R %AA %domm %0 %A
Av --- %.
---- 14-#%AwU
%4 %o %AA %#an *0 tq
Immmmm-mmr-M - V.
p%7* 1 ow" I I %F '111"150
101,1112 11^21 a
]r%Tf 4_7 M W & q6V
v
A 0- Mhalk
ILA fUTI
OC
x ---- v
MT%J
J9,91 AM MNM Aft P AMkX
T
QS
Lu ..... v
_t7
zz
........... ......... ...... ..... ... ............... .... ...... ... ...
.......... ... ... ...........
—jw.. NI,
AL
................. .......
W1 A
IMA
W.
*A:
'lilt.
jr 51W,
-*..W W X
V. X; 7 "
M-M W k., M
^*-, M 1, T- �:J""N.W. W, 5il�*-=. 7..=:, W.M.
W W4 WMW
WIL
.Shoke.
--- trnL-1-
---- - --- --- - t It - if I ---- 4-N"A- -- I ----- I-- &-&*HILO - A --- e-1 C. A
lltoa-��f " d-e4A WeAvrk4 Z�f IFbl7AJ4-JIU7X uw§ UlW%juea
.LI.L a 4tb V IT 91 1
lo
ij
AL
FAMWO.- W..
IM-2 OW-M
W
------ I :WWWWW
.............. ............... .......... ........ ...... .................... .. . . ...............
LIP
P--
m I 111111: r F. *,-,-.
IM
W. r�8w
TA: - . - U::
:
..........
ONE
1 .0-4-1-96
x
Alw:
IW
1, M ^ - elf
' xi 4" ut - ik
a
.1all. P-M
?dOW6t
UAW*, ";w A-01*00M AMR
A --- rm-&
R ai
T'w I -a is N%pv
t
KRA
--
t �t
Q4*
R 11 &aqk 1
XMAS ?A
lu A
144)-
111 ---- - --- IAA
xw ---- ------ IVIRW517
.7
f
1 w% ILJ
AA
(3)
40-
CA
171
x
R
I ---- A
Q Q
it
7
6
b
1 4-,e
44
Y.4
IRA
8
4 _7A
O
%
O
u O
AVAL
O--
A
4
WP O
---- ----
r)
42-9
4 WO
6
b
A ------ 7---
Y
s- _z .... n.
Si
-1 - ------- q_
4
Y
---- q-7- --------- &_
44 W_
-- - ---
I -If --- I
x
---- - -- - R
_q
e
a
x
Ill --- I
x
I ____ A
x
x
---- 4 ----- i n__
--- -- -- qf-1
O
O
------ r--,L
x
x
-1-4- ----- A
a
x
--- ----
---
__
x
--- ---- 7- A ----A-
---
--- ---- 0-11
--- Z;�Vov'
--- 0
0
0
% 9-
1 a ra,
x
777
x
.n if
- ---------
-
K
x
x
�iM►. :.iiit �iAi. illyWI: ai�1i: ,iAt►.
.. : i
,rprl
24
NEH=4: "Hydrology," Section 4, National Engineerinq Handbook
TR_'3(1• !'�rsmne I#or C3rra rnm fr.r Drevizoat-%'f rsW 1wmL e2 1 a %A l.'�°iexwr 4.larr3rrmlrervae
TR-55: Urban Hvdroloqv for Small Watersheds
TP=1 49:A Method for Estimating Volume and Rate of Runoff in Small Watersheds
The FIFIH dwwmroloqi'c Engineerin CPROMenter - H,ydrolo is elfiMAIMn stemAAAAAHEC-HMS comafar
2 rograms include the ability tj'*� apply the NRCS method and may be downloaded from the
US AnV0Cr S of E qineers websteiat h--!3c-Ip://www,,hec.usace..army,.Mll/. M am
RAPI
2.5.1 Is D'Istr'lbuti'o
AQRlicati4on. Instructions RAIn forW ------ d--- rolo is desl"gnsand ana,1MYSIZ�S'MOW-as issued and
All
amended by.the UES Director.
2n5n2 'Natural Resources Conservation Service Runoff Curve Numbers
4
The CSC has developed an index, the runoff curve number
(-C-N--), to represent the combined hydrologic effect of soil type, land use, agricultural land
treatment class, hydrologic condition, and antecedent soil moisture. These watershed
factors have the most significant impact in estimating the volume of runoff, and can be
assessed from soil surveys, site investigations and land use maps.
AM
V 0.!^4a3H �iOV
or
SL
"0-4-19)j
-
26
N)
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(Vn
8
A -SS,
an
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(t
8
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a.
6
as
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a.
ON
VIVO-]
x
x
am I
3c
c D%.-C
k%4
r 0
x%,,,4
IL
ow
(P
dim&
AL
A'96
PC
4rlr42
sue
x
x
K
PH"
(Nq"&
---- qV----U-r
I.A.M. A-
if v 0 %A
VON&
--- %-utr -----
-ILIAAM --- ILM-1-1-1
a V,%e WIPFIR %A %ooww
29
of a %Do ho %W V40P
Imam&
&ft 0 06 AM, 20%4
--- �M--M-r
--- V�mm-r
- 29M n--.'am
15 -1w----
--- --n- n-
-- ------
-- ------ 97
1',')' --------- -G.
4
-ID -----
------ 7---
AQ
o- ni.
0- 10
1 ----- 14
1 4
A A q
*4
79
1 sl
rw
4-7-
4 --- wq-
4"--x --- 9--w-
.0 w
4-al ----
-------- 0 --- A
I - ------- rm%,
-- ------ q- -0---
- - ----- - Q-
----- q-
..-) ---------- A-
-- ------- 4 ---- 0--
-- ---- q
-- ---- A --- 7 ---
--- ----
Ike%
---) ---------- 1A
-1 - ----- 7
-9 ---- 7-0
---- ji�
-r) ------ At
�"-W-r --- U,
n"7
-1 ---- - A A
-W ---- %;JF
- -- ----- ------ 1-
R- - ----- *-in
---- QA
0 ---- A A
---- -----
Nvo 6 4-7
&W .6 %,X %of-
41?W4
4w a -W
Ok. &"Xr
46
4-tot
A &*NW
- --------- q-
---- ----
M ----- I -A
.5 f--w
---- ----- 7 -------
NO a.
---- ---
-- ---- 9-
-0 ------- �qa
-q - ------ RA.
A ---- RA.
-- -----
`3-4-
A Q
28
00
:!1►: :,: , *!: :!�M.'` MIS`mo
r.r' now*:
..........
..* I i RJRIA�R
:�► <!!R:• l !:. :
•T•
'!AB•
fir:
�i ..... !=
:AR• I- - : -- - - - -.:�j :?lli� .: :-:.ce. :- -
!11!! `''M! `: =! �!M ` *.• �M�!` =�M!: '' :i!M!:
;'!Uk• :: .: i :fit= :*:
AMR:
29
560.
of
ir. +oc .i. it i, i► �c w ir, w
Ab: M.
WoW7,A�=r:
4*0 aw3i
6�t1
Ic
i:RAI
MW M - 'M -1 WAIllk W1 .00'WA&WAIP. WAK W W 6k•mck 51,05056*
woli rm-om 1",
Im. OJPAWKJV, III wm&v
tr�
lid
N
a it :9 a 11 a No a UNA. A all 9 11 LA- a N a 19 2 0 1 9 11 991 )ON on% I I
IA-IFTO --- XXT
it 10 10 do(
JnL-m I'm I I a 1 640%
a Alga i UT &L.A..
%A
U-P
JIM
g��fl 'gt7 I
III, fil M
DR..*
ww r, W
L OWN -A r.
AR
�ii�i7��71
F3i�i�iil�'ll
81 v .1116
TOM
..
KW MWA'
hw; INVP wAilM hominoW
MS., "i,
AM
It. Kx
IF5
Mol".
VA
U011S."W"A"llimsm: ... v A; --., EVA
;-77� wiv-swi-A
1 W., -� � .2 t
Sol
i -4
Air
A Lw ,A
140
14-75
)AW
V::"w
P?
ism
ZYM
tEM
0-1.4-030
0.Q26
G-�457-
WWA
RAI MA W, M4�
LOEWE M-iffOHEI-I
32
0:t151
r o: I
v . ff.i.
L
0:tl6 4..
Kim
0,07-0.
Group B: Soils having a moderate infiltration rate when thoroughly wetted and
consisting ch'Iefly of moderately deep to deep, moderately well to well -drained soils with
moderately fine to moderately coarse texture.
Group C: Soils having a slow infiltration rate when thoroughly wetted and consisting
chiefly of soils with a layer that impedes downward movement of water or soil with
moderately fine to fine texture.
Group Do: (High runoff potential). Soils having a very slow infiltration rate when
thoroughly wetted and consisting chiefly of clay soils with a high swelling potential, soils
with a permanent high water table, soils with a claypan or clay layer at or near the
surface and shallow soils over nearly impervious material.
ari=...,.
------------
------ --- - --- n- ------- I- f --- I
1^0,6 1-5
1) - r)-9-- --- () ---- 1---q
8n___ - n- - ----
33
eve
--------------- ---- -------- ------
( I(ua��e�abGA ou A:AIUO seam
- --- ---- ---- - ----
t76 16 913 $00000000M ------------ --- ------
sr�o�na�CI) SeGJe papeJb Alm9N
-------------- ----------- ---
---------------
seeie ueqjn bUIC1019Aea
--------- --------------
MAW^ anfiftv
C6 1 12 9 1 !R I Z % z I ieia}snpul
56' 176 Z6 69 99 ssaursnq -_ pug lenaawuaoC)
---------------------------------
s;�ra.�srp uegrn
------
613 L9 Z9 ZL (AeMlo qbis buipnioui iI 161 ) liq?(] I
------------- 00#WMv0w.M~
------
1 �n��ui --16 69 99 9L -------- b_u_IGAeJE)
__ I
)
-- - ---- ------------
-4000000000"al em 10 ---Iq -b li ---- bu�pnlous - ) C6 Z6 612 ci3 I9000090' ",u4 P.part
I I I I sac���E�a GdI
- -------------------------------------------- - -------------- - ----------------------------------------------
100MMAOCAMSaW ------------- �rv��o;yblj bUlpnloxx Ic6 IR6 126 96 ODOMsuiejp pug sgano &,p9Aecj
speod pug sear;S
-------- - -- --- - -- ----- ----------------------
10 -1 4-b-11
----------------- 0 -
136 96 96 96 '�xie ja 'SAeM9AIJP
xFS100i
01 bct paned
- --------- ------ ------------
seam snorn.ra cu,
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-- - --------------------------- --- nt�L----------------- - ------------
------ ----- -------------
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tr'a 6L 69 6f7 ------------ -- mehaseasalMa ------------------
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------------- ----------
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epos;------------------ --------------
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------- - ------- ----- --------------- .. ........ ------
�ipuo �Ib0j0J
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isnoi/u;Dd..
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o'��a� I!OIINf---------------- Dui nsse
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-- - ---- -
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------------ X000006momm 14C
anui4uo:)) je =Z elqejL
Table 23= iue
1011C
NRCS o Curve I
NumbersCN's forUrban AreasandAqricultural Lands
A& C 1cnd*Itl'onj
{assuming1 a , . AK
:048
Cover Dascription CN for Hyr2ll cSo*11 11 rr� I
Cover lyRex a n d Average% CY rlicConditiondoo IM enviousA, qiAreal I
Farmsteads ae buildfng, lanes,
driveways and surrorlina lots
Notes
..........
impervious
rr%ent area shown was used to devel AAPG<he average ge,01Pthe com osl'te CN's.
Other assul M1 0%1 t9ions areas follows, impervious areas are directlyconnected to the
MARAROMMOR PIPIR
drainage system; imperviaus areas have a C of ninety-eight (98)and pervious areas
6 dequivalent-8 d y i i ion. CN's for other
are considere to open in goo hq c cond't"combi'nati'ons of conditi"ons maybe cornputei,4 using methods in NRCS TRew55 Urban
---------
Hydrology for Small Watersheds.
M"
CI&InAt2" 812afkL I&A&fk.L A VN s a, --re qulvalent to those Ofpasture. e CN'5 M... be COM"""'Uted for
Q R A R m A Ong ARA.,RARARNR00000000000000" 000000000M OC 00 A Dole ------------ RAX
other combi*nati'ons of open sgace cover type.
3 Poor:less than 0percent cover or l�avilYw►thno mulch.. 5graz....,.,Arwl
percent
FFair: 50 to 75 grawnd cover and not heav►ly
Good:nneater than 75ercentground cover and lighily...or onl occasionalrazed.
4 Poor,: less than 50 rc around cover.
'0 YOW
Fair: oganv to 7b percent ground cover..
Good: greater than 75 percent ground cover.,
5computedCN's shown were for areas wl"th 50 A-eQkercent woods and 50 -1%^1,.Ient rass�O LA-- --------------- ------------- M0.0
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9, 6 Poor,:Forest littersmall trees &brd b h
ush aredestroyeeavygrazinq r rggular burning r Y. -- -A"
fair: Woods are razed but not burned, and some forest, litter covers the soil.
.0 ~0
Good.: Woods are protected from razil&&,ql & IItter and brush ade-wately cover theso^11.
q,-
------ --------------- DOOM
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7 Actual C is less than 30: use GN *66m 30 for runoff computations.
Source: NRCS TR-55: Urban Hvdrology for Small Watersheds
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