UFC 3-230-17FA Drainage in Areas Other Than Airfields (01!16!2004)

8/14/2019 UFC 3-230-17FA Drainage in Areas Other Than Airfields (01!16!2004)

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UFC 3-230-17FA16 January 2004

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UNIFIED FACILITIES CRITERIA (UFC)

DRAINAGE IN AREAS OTHER THAN AIRFIELDS

Any copyrighted material included in this UFC is identified at its point of use.Use of the copyrighted material apart from this UFC must have the permission of thecopyright holder.

U.S. ARMY CORPS OF ENGINEERS (Preparing Activity)NAVAL FACILITIES ENGINEERING COMMAND

AIR FORCE CIVIL ENGINEER SUPPORT AGENCY

Record of Changes (changes are indicated by \1\ ... /1/)

Change No. Date Location

This UFC supersedes TM 5-820-4, dated 14 October 1983. The format of this UFC does notconform to UFC 1-300-01; however, the format will be adjusted to conform at the next revision.The body of this UFC is the previous TM 5-820-4, dated 14 October 1983.


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UFC 3-230-17FA16 January 2004

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FOREWORD\1\The Unified Facilities Criteria (UFC) system is prescribed by MIL-STD 3007 and providesplanning, design, construction, sustainment, restoration, and modernization criteria, and appliesto the Military Departments, the Defense Agencies, and the DoD Field Activities in accordancewith USD(AT&L) Memorandum dated 29 May 2002. UFC will be used for all DoD projects and

work for other customers where appropriate. All construction outside of the United States isalso governed by Status of forces Agreements (SOFA), Host Nation Funded ConstructionAgreements (HNFA), and in some instances, Bilateral Infrastructure Agreements (BIA.)Therefore, the acquisition team must ensure compliance with the more stringent of the UFC, theSOFA, the HNFA, and the BIA, as applicable.

UFC are living documents and will be periodically reviewed, updated, and made available tousers as part of the Services responsibility for providing technical criteria for militaryconstruction. Headquarters, U.S. Army Corps of Engineers (HQUSACE), Naval FacilitiesEngineering Command (NAVFAC), and Air Force Civil Engineer Support Agency (AFCESA) areresponsible for administration of the UFC system. Defense agencies should contact thepreparing service for document interpretation and improvements. Technical content of UFC isthe responsibility of the cognizant DoD working group. Recommended changes with supportingrationale should be sent to the respective service proponent office by the following electronicform: Criteria Change Request (CCR). The form is also accessible from the Internet sites listedbelow.

UFC are effective upon issuance and are distributed only in electronic media from the followingsource:

Whole Building Design Guide web site http://dod.wbdg.org/.

Hard copies of UFC printed from electronic media should be checked against the currentelectronic version prior to use to ensure that they are current.

AUTHORIZED BY:

______________________________________DONALD L. BASHAM, P.E.Chief, Engineering and ConstructionU.S. Army Corps of Engineers

______________________________________DR. JAMES W WRIGHT, P.E.Chief EngineerNaval Facilities Engineering Command

______________________________________KATHLEEN I. FERGUSON, P.E.The Deputy Civil Engineer

DCS/Installations & LogisticsDepartment of the Air Force

______________________________________Dr. GET W. MOY, P.E.Director, Installations Requirements and

ManagementOffice of the Deputy Under Secretary of Defense

(Installations and Environment)
http://www.wbdg.org/pdfs/ufc_implementation.pdfhttps://65.204.17.188/projnet/cms/version2/index.cfm?WORKFLOW=CMS_CCRQAdd&Action=IDFORM&SecureTry=1http://dod.wbdg.org/http://dod.wbdg.org/https://65.204.17.188/projnet/cms/version2/index.cfm?WORKFLOW=CMS_CCRQAdd&Action=IDFORM&SecureTry=1http://www.wbdg.org/pdfs/ufc_implementation.pdf


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REPRODUCTION AUTHORIZATION/ RES TRICTIONS

This man ual h as been prepar ed by or for t he Governm ent a nd is publicproperty and not subject to copyright.

Reprints or republications of this manual should include a credit substan-tially as follows: Joint Departments of the Army and Air Force, USA,Techn ical Ma nu al TM 5-820-AFM 88-5, Chapt er 4, Drain age for Area sOther Than Airfields, date.


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TM 5-820-4

AFM 88-5, chap 4C-1

Change

No. 1

DEPARTMENTS OF THE ARMY,

AND THE AIR FORCE

Washington, DC 16 July 1985

DRAINAGE FOR AREAS OTHER THAN AIRFIELDS

TM 5-820-4/AFM 88-5, Chapter 4, 14 October 1983, is changed as follows:

1. Remove old pages and insert new pages as indicated below. New or changed material is indicated by avertical bar in the margin of the page.

2. File this change sheet in front of the publication for reference purposes.

By Order of the Secretaries of the Army and the Air Force

J OHN A. WICKHAM, J R.General, United States Army

Official: Chief of S taffDONALD J. DELANDRO

Brigadier General, United States Army

The Adjutant General

CHARLES A. GABRIEL

General, Un ited S tates Air Force

Official: Chief of S taff

JAMES H. DELANEYColonel, Unit ed S tates Air Force

Director of Adm inist ration

DISTRIBUTION:

To be distribut ed in a ccordan ce with DA Form 12-34B requirem ent s for TM 5-800 Series: Engineer ing

and Design for Real Property Facilities.


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Technical Manu al

No. 5-820-4

Air Force Manual

No. 88-5, Chapter

qTM 5-820-4/AFM 88-5, Chap. 4

HEADQUARTERS

DEPARTMENTS OF THE ARMY

AND THE AIR FORCE

4 Washin gton, D.C. 14 October1983

DRAINAGE FOR AREAS OTHER THAN AIRFIELDS

CHAPTER 1.

2.

3.

4.

5.

6.

APPENDIX A.B.

c.D.

E.

Figure 3-13-23-3

3-4

3-5

3-63-7

4-1

4-2

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4-4

B-1B-2B-3

B-4

INTRODUCTION Paragraph Page

*This manual supersedes TM 5-820-4/AFM 86-5, Chap 4, 14 August 1964


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TM 5-820-4/AFM 88-5, Chap. 4

LIST OF TABLES

Table Page


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TM 5820-4/AFM 885, Chap 4

CHAPTER 1

INTRODUCTION

11. Purpose and scope. The purpose of this

manual is to discuss normal requirements for de-

sign of surface and subsurface drainage systems

for military construction other than airfields and

heliports at Army, Air Force and similar instal-

lations. Sound engineering practice should be fol-

lowed when unusual or special requirements not

covered by these instructions are encountered.

12. General investigations. An on-site inves-

tigation of the system site and tributary area is

a prerequisite for study of drainage requirements.

Information regarding capacity, elevations, and

condition of existing drains will be obtained. To-

pography, size and shape of drainage area, and

extent and type of development; profiles, cross

sections, and roughness data on pertinent exist-

ing streams and watercourses; and location of pos-

sible ponding areas will be determined. Thorough

knowledge of climatic conditions and precipitation

characteristics is essential. Adequate information

regarding soil conditions, including types, perme-

ability on perviousness, vegetative cover, depth

to and movement of subsurface water, and depth

of frost will be secured. outfall and downstreamflow conditions, including high-water occurrences

and frequencies, also must be determined. Effect

of base drainage construction on local interests

facilities and local requirements that will affect

the design of the drainage works will be evalu-

ated. Where diversion of runoff is proposed, par-

t icular effort wil l be made to avoid resultant

downstream condit ions leading to unfavorable

public relat ions, costly l i t igations, or damage

claims. Any agreements needed to obtain drain-

age easement s an d/or avoid inter ference with wat er

rights will be determined at the time of design

and consummated prior to initiation of construc-

tion. Possible adverse effects on water quality due

to disposal of drainage in waterways involved in

water-supply systems will be evaluated.

13. Environmental considerations.

a. Surface drainage systems have either bene-

ficial or adverse environmental impacts affectingwater, land, ecology, and socio-economic consid-

erations. Effects on surrounding land and vege-

tation may cause changes in various conditions in

the existing environment, such as surface water

quant i ty and qual i ty , groundwater levels and

quality, drainage areas, animal and aquatic life,

and land use. Environmental attributes related

to water could include such items as erosion, flood

potential, flow variations, biochemical oxygen de-

mand, content of dissolved solids, nutrients and

coliform organisms. These are among many pos-

sible attributes to be considered in evaluating en-

vironmental impacts, both beneficial and adverse,

including effects on surface water and ground-

water.

b. Federal agencies shall initiate measures to

direct their policies, plans, and programs so as to

meet nat ional envi ronmenta l goals and s tand-

ards.

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TM 5820-4/FM 88-5, Chap 4

CHAPTER 2

HYDROLOGY

21. General. Hydrologic studies include a care-ful appr aisa l of factors a ffectin g storm ru noff toinsur e th e development of a dra inage system orcont rol work s capable of providing th e requ ireddegree of protection. The selection of design stormmagnitudes depends not only on the protectionsought but also onthe type of construction con-templated and the consequences of storms ofgreater magnitude than the design storm. Groundconditions affecting runoff must be selected to beconsistent with existing and anticipated arel de-velopment and also with the characteristics andseasonal time of occurrence of the design rainfall.For areas of up to about 1 square mile, where onlypeak discharges ar e required for design a nd ex-tensive pondig is not involved, computation ofrunoff will normally be accomplished by the s-called Rational Method. For larger areas, whensuitable unt-hydrograph data are available orwhere detailed consideration of pondng is re-quired, computation should be by uit-hydro-graph and flow-routing procedures.

22. Design storm.

a. For such developed portions of military in-stallations as administrative, industrial, andhousing areas, the design storm will normally bebased on rainfall of 10-year frequency. Potentialdamage or operational requirements may war-ran t a more severe criterion; in cert ain st orageand recreational areas a lesser criterion may beappr opriate. (With concur rence of th e using Ser v-ice, a lesser criterion may also be employed inregions where storms of an appreciable magni-tude are infrequent and either damages or oper-ational capabilities are such that large expendi-tures for drainage are not justified.)

b. The design of roadway culverts will normallybe based on 10-year rainfall. Examples of condi-tions where greater than 10-year rainfall may beused are areas of steep slope in which overflowswould cause severe erosion damage; high road fillsthat impound large quantities of water; and pri-mary diversion structures, important bridges, and

critical facilities where uninterrupted operationis imperative.

c. Protection of military installations againstfloodflows originating from areas exterior to theinstallation will normally be based on 25-year orgreater rainfall, again depending on operationalrequirements, cost-benefit considerations, andnature and consequences of flood damage result-ing from t he failure of protective work s. J ust ifi-cat ion for t he selected d esign storm will be pre-sented, and, if appropriate, comparative costs anddamages for alternative designs should be in-cluded.

d. Rainfall intensity will be determined from thebest available intensity-duration-frequency data.Basic information of this type will be taken fromsuch publications as (see app A for referenced pub-lications);

Rainfall Frequency Atlas of the United States.

Technical Paper No. 40.Generalized Estimates of Probable Maximum

Precipitation and Rainfall-Frequency Data

for Puerto Rico and Virgin Islands. Tech-nical Paper No. 42.

Rainfall-Frequency Atlas of the Hawaiian Is-

lands. Technical Paper No. 43.Probable Maximum Precipitation and Rain-

fall Frequency Data for Alaska. TechnicalPaper No. 47.

TM 5-785/AFM 88-29/NAVFAC P-89.

These publications may be supplemented as ap-propriate by more detailed publications of the En-vironmental Data and In forma tion Center a nd bystu dies of local r ain fall records. F or la rge ar easand in studies involving unit hydrography and flow-routing procedures, appropriate design stormsmust be synthesized from areal and time-distri-bution characteristics of typical regional rainfalls.

e. For some areas, it might reasonably be as-sumed that the ground would be covered with snow

when th e design r ainfall occur s. If so, snowmeltwould add to the runoff. Detailed procedures forestimating snowmelt runoff are given in TM 5-852-7/AFM 88-19, Chap 7. It should be noted, how-

ever, that the rate of snowmelt under the rangeof hydro-meteorological conditions normally en-countered in military drainage design would sel-

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TM 5820-4/AFM 88-5, Chap 4

dom exceed 0.2 inches per hour and could be sub-stantially less than that rate.

f. In selecting the design storm and making otherdesign decisions, particular attention must be givento the hazard to life and other disastrous conse-quences resulting from the failure of protectiveworks during a great flood. Potentially hazardous

situations must be brought to the attention of theusing service an d others concerned so tha t ap-propriate steps can be taken.

Table 21. Typical Values of Infiltration Rates

S oil gr ou p I nfi l t ra tion ,

D es cr ip t i on sym bol in ch es / h ou r

Sand and gravel mixture GW, GP 0.8-1 .0

SW, SP

Silty gravels and silty sands to GM, SM 0.30.6

inorganic silt, and well-devel- ML, MH

oped loam s OL

Silty clay sand to sandy clay SC, CL 0.20.3Clays, in organ ic an d or ga nic CH , OH 0.10.2

B a r e r o c k , n o t h i g h l y f r a c t u r e d - - - - - - - - 0 .0 -0 .1

U.S. Army Corps of Engineers

23. Infiltration and other losses.

a. Principal factors affecting the computation ofrunoff from rainfall for the design of militarydrainage systems comprise initial losses, infiltra-tion, transitory storage, and, in some areas, per-colation into natural streambeds. If necessary data

are available, an excellent indication of the mag-nitu des of th ese factors can be derived from t hor-

ough analysis of past storms and recorded flowsby the unit-hydrograph approach. At the onset ofa storm, some rainfall is effectively retained inwetting down vegetation and other surfaces, insatisfying soil moisture deficiencies, and in fillingsurface depressions. Retention capacities varyconsiderably according to surface, soil type, cover,

and antecedent moisture conditions. For high in-tensity design storms of the convective, thunder-storm type, a maximum initial loss of up to 1 inchmay be assu med for t he first hour of storm pr e-cipitation, but the usual values are in the rangeof 0.25 to 0.50 inches per h our . If the design r ain-

fall intensity is expected to occur during a stormof long duration, after substantial amounts of im-mediately prior rain, the retention capacity would

have been satisfied by the prior rain and no fur-ther assumption of loss should be made.

b. Infiltration rates depend on type of soils, veg-etal cover, and the use to which the areas aresubjected. Also, the rates decrease as the durationof rainfall increases. Typical values of infiltrationfor generalized soil classifications are shown inable 2-1. The soil group symbols are those givenn MIL-STD-619, Unified Soil Classification Sys-

tem for Roads, Airfields, Embankments, andFoundations. These infiltration rates are for un-compacted soils. Studies indicate that compactedsoils decrease infiltration values from 25 to 75 per-cent, the difference depending on the degree ofcompaction an d th e soil type. Vegetat ion gener -ally decreases the infiltration capacity of coarsesoils and increases that of clayey soils.

c. Peak ra tes of run off ar e redu ced by t he effectof transitory storage in watercourses and minor

ponds along the drainage route. The effects arereflected in the C factor of the Rational Formula(given below) or in th e sha pe of th e un it h ydro-graphy. Flow-routing techniques must be used topredict major storage effects caused by naturaltopography or man-made developments in the area.

d. Streambed percolation losses to direct runoffneed to be considered only for sandy, alluvial wa-tercourses, such as those found in arid and semi-ar id regions. Rat es of str eambed p ercolation com-monly range from 0.15 to 0.5 cubic feet per second

per acre of wetted area.

2-4. Runoff computations.

a. Design procedures for drainage facilities in-volve computations to convert rainfall intensitiesexpected during the design storm into runoff rates

which can be used to size the various elements ofth e storm draina ge system. There are t wo basicapproaches: first, direct estimates of the propor-tion of average rainfall intensity that will appearas the peak runoff rate; and, second, hydrographymethods that depict the time-distribution of run-off events a fter account ing for losses an d at ten -uation of the flow over the surface to the point ofdesign. The first appr oach is exemplified by theRational Method which is u sed in th e large ma-

jority of engineering offices in the United States.It can be employed successfully and consistentlyby experienced designers for drainage areas up to

1 square mile in size. Design and Construction ofSanitary and Stem Sewers, ASCE Manual No.37, and Airport Drainage, FAA AC 150/5320-5B,explain and illustrate use of the method. A mod-ified method is outlined below. The second ap-proach encompasses the analysis of unit-hydro-graph techniques to synthesize complete runoffhydrography.

b. To compute peak runoff the empirical formula

Q=C(1-F)A ca n be used; the terms are defined

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TM 5820-4/AFM 88-5, Chap 4

in appendix D. This equation is known as the mod-

ified rational method.(1) C is a coefficient expressing the percentage

to which the peak runoff is reduced by losses (otherthan infiltration) and by attenuation owing totransitory storage. Its value depends primarily onsurface slopes and irregularities of the tributaryarea, although accurate values of C cannot readilybe determined. For most developed areas, the ap-

parent values range from 0.6 to 1.0. However, val-ues as low as 0.20 for C may be assumed in areaswith low intensity design rainfall and high infil-tr ation ra tes on flat terr ain. A value of 0.6 maybe assumed for areas left ungraded where mean-dering-flow and appreciable natural-ponding ex-ists, slopes are 1 percent or less, and vegetal cover

is relatively dense. A value of 1.0 may be assumedapplicable to paved areas and to smooth areas ofsubstantial slope with virtually no potential forsurface storage and little or no vegetal cover.

(2) The design intensity is selected from theappropriate intensity-duration-frequency rela-

tionship for the critical time of concentration andfor th e design st orm frequ ency. Time of concen-tra tion is usually defined as t he time required,under design storm conditions, for runoff to travelfrom the most remote point of the drainage areato the point in question. In computing time of con-centr ation, it sh ould be kept in m ind th at, evenfor uniformly graded bare or turfed ground, over-land flow in sheet form will rarely travel morethan 300 or 400 feet before becoming channelizedand thence move relatively faster; a method whichmay be used for determining travel-time for sheet

flow is given in TM 5-820-1/AFM 88-5, Chap 1.Also, for design, the practical minimum time ofconcentr at ion for roofs or paved ar eas a nd for rel-atively small unpaved areas upstream of the up-permost inlet of a drainage system is 10 minutes;smaller values are rarely justifiable; values up to20 minutes may be used if resulting runoff ex-cesses will not cause appreciable damage. A min-imum time of 20 minutes is generally applicablefor turfed areas. Further, the configuration of themost remote portion of the drainage area may besuch that the time of concentration would belengthened markedly and thus design intensity

and peak runoff would be decreased substantially.

In such cases, the upper portion of the drainagear eas sh ould be ignored an d th e peak flow com-putation should be based only on the more effi-cient, downstream portion.

(3) For all durations, the infiltration rate isassumed to be the constant amount that is estab-lished following a rainfall of 1 hour duration. WhereF varies considerably within a given drainage area,

a weighted rate may be used; it must be remem-

bered, however, that previous portions may re-quire individual consideration, because a weightedoverall value for F is proper only if rainfall in-tensities are equal to or greater than the highestinfiltration rate within the drainage area.

In design of military construction drainage sys-tems, factors such as initial rainfall losses andchannel percolation rarely enter into runoff com-putations involving the Rational Method. Suchlosses are accounted for in the selection of the Ccoefficient.

c. Where basic hydrologic data on concurrent

rainfall and runoff are adequate to determine unithydrography for a drainage area, the uncertain-ties inherent in application of the Rational Methodcan largely be eliminated. Apparent l0S S rates de-termined from unit-hydrograph analyses of re-corded floods provide a good basis for estimatingloss ra tes for st orms of design ma gnitu de. Also,flow times a nd stora ge effects ar e account ed forin the shape of the unit-hydrograph. Where basicdat a a re inadequa te for direct det erminat ion ofunit-hydrographs, use may be made of empiricalmethods for synthesis. Use of the unit-hydro-graph method is particularly desirable where de-

signs are being developed for ponds, detention res-ervoirs, and pump stations; where peak runoff from

large tributa ry ar eas is involved in design; andwhere large-scale protective works are under con-

sideration. Here, the volume and duration of stormrunoff, as opposed to peak flow, may be the prin-cipal design criteria for determining the dimen-sions of hydraulic structures.

d. Procedures for routing storm runoff throughreservoir-type storage and through stream chan-nels can be found in publications listed in appen-dix E and in the available publications on these

subjects.

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TM 5820-4/AFM 88-5, Chap 4

CHAPTER 3

HYDRAULICS

3-1. General. Hydraulic design of the requiredelements of a system for drainage or for protectiveworks may be initiated after functional design cri-

teria and basic hydrologic data have been deter-mined. The hydraulic design continual y involvestwo prime considerations, namely, the flow quan-tities to which the system will be subjected, andthe potential and kinetic energy and the momen-tum that are present. These considerations re-quire that the hydraulic grade line and, in manycases, the energy grade line for design and per-tinent relative quantities of flow be computed, and

that conditions whereby energy is lost or dissi-pated must be carefully analyzed. The phenom-ena that occur in flow of water at, above, or below

critical depth and in change from one of these flowclasses to another must be recognized. Water ve-locities m ust be carefully comput ed n ot only inconnection with energy and momentum consid-erations, but also in order to establish the extentto which the drainage lines and water-courses may

be subjected to erosion or deposition of sediment,thus enabling determination of countermeasuresneeded. The computed velocities and possible re-sulting adjustments to the basic design layout often

affect cert ain par ts of th e h ydrology. Mann ingsequation is most commonly used to compute themean velocities of essentially horizontal flow thatoccurs in most elements of a system:

n

The terms are defined in appendix D. Values of nfor use in the formula are listed in chapters 2 and

9 of TM 5-820-3/AFM 88-5, Chapter 3.

3-2. Channels.

a. open channels on military installations rangein form from graded swales and bladed ditches tolarge channels of rectangular or trapezoidal crosssection. Swales are commonly used for surfacedraina ge of graded a reas a round buildings andwithin housing developments. They are essen-tially triangular in cross section, with some bot-tom rounding and very flat side slopes, and nor-mally no detailed computation of their flow-

carrying capacity is required. Ditches are com-monly used for collection of surface water in out-lying areas and along roadway shoulders. Largeropen channels, which may be either wholly withinthe ground or partly formed by levees, are usedprincipally for perimeter drains, for upstream flow

diversion or for those parts of the drainage systemwithin a built-up area where construction of a cov-

ered drain would be unduly costly or otherwiseimpractical. They are also used for rainfall drain-age disposal. Whether a channel will be lined ornot depen ds on erosion cha ra cter istics, possible

grades, maintenance requirements, availablespa ce, overall compa ra tive costs , and other fac-tors. The need for providing a safety fence not lessthan 4 feet high along the larger channels (es-pecially those carrying water at high velocity) willbe considered, particularly in the vicinity of hous-ing areas.

b. The discussion that follows will not attemptto cover all items in the design of an open channel;however, it will cite types of structures and designfeatures that require special consideration.

c. Apart from limitations on gradient imposed

by available space, existing utilities, and drainageconfluences is the desirability of avoiding flow ator near critical depths. At such depths, smallchanges in cross section, roughness, or sedimenttransport will cause instability, with the flow depth

varying widely above and below critical. To insurereasonable flow stability, the ratio of invert slopeto critical slope should be not less t ha n 1.29 forsupercritical flow and not greater than 0.76 forsubcritical flow. Unlined earth channel gradientsshould be chosen that will product stable subcrit-ical flow at nonerosive velocities. In regions wheremosquito-borne diseases are prevalent, special at-

tention must be given in the selection of gradientsfor open channels to minimize formation of breed-ing ar eas; pertinent informa tion on th is subjectis given in TM 5-632/AFM 9116.

d. Recommended maximum permissible veloci-ties and Froude numbers for nonerosive flow aregiven in chapter 4 of TM 5-820-3/AFM 88-5, Chap-

ter 3. Channel velocities and Froude numbers of

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TM 5820-4/AFM 885, Chap 4

h. For most open channel confluences, proper

design can be accomplished satisfactorily by com-

putations based on the principle of conservation

of momentum. If the channel flows are supercrit-

ical, excessive waves and turbulence are likely to

occur unless a close balance of forces is achieved.

In such confluences, minimum disturbances will

result if the tributary inflow is made to enter the

main channel in a direction parallel to the main

flow, and if the design depth and velocity of thetributary inflow are made equal to those in the

main channel . Fur ther , even though minimum

disturbances appear likely under such design con-

ditions, it must be remembered that natural flood-

flows are highly variable, both in magnitude and

distribution. Since this variability leads to unbal-

anced forces and accompanying turbulence, a need

may well exist for some additional wall height or

freeboard allowance at and downstream from the

confluence structure.

i. Side inflows to channels generally enter over

the tops of the walls or in covered drains throughthe walls. If the main channel is earth, erosion

protection frequently is required at (and perhaps

opposite) the point of entry. If the sides of a chan-

nel through an erosible area are made of concrete

or other dura ble materials an d inflows are brought

in over th em, care m ust be taken to insure positive

entry. There are two methods of conducting storm

water into a concrete-lined channel. Entry of large

flows over the top is provided by a spillway built

as an integral part of the side slope while smaller

flows are admitted to the channel by a conduit

through the side slope. Gating of conduit is not

required at this location because any pending isbrief and not damaging. Where covered tributary

drains enter, examination must be made to see

whether the water in the main channel, if full,

would cause damaging backflooding of the tribu-

tary area, which would be more damaging than

temporary stoppage of the tributary flow. If so,

means for precluding backflow must be employed;

this can often be accomplished by a flap gate at

the drain outfall, and if positive closure is re-

quired, a slide gate can be u sed. If flow in th e ma in

channel is supercritical, the design of side inlet

structures may require special provisions to min-

imize turbulence effects.

33. Bridges.

a. A bridge is a structure, including supports,

erected over a depression or an obstruction, such

as water, a highway, or a railway, having a track

or passageway for carrying traffic or other mov-

ng loads, and having an opening measured along

the center of the roadway of more than 20 feet

between undercopings of abutments or spring lines

of arches, or extreme ends of the openings for mul-

tiple boxes; it may include multiple pipes where

the clear distance between openings is less than

half of the smaller contiguous opening.

b. Sufficient capacity will be provided to pass

the runoff from the design storm determined in

accordance with principles given in chapter 2.Normally such capacity is provided entirely in the

waterway beneath the bridge. Sometimes this is

not practical, and it may be expedient to design

one or both approach roadways as overflow sec-

tions for excess runoff. In such an event, it must

be remembered that automobile traffic wil l be

impeded, and will be stopped altogether if the

overflow depth is much more than 6 inches. How-

ever, for the bridge proper, a waterway opening

smaller than that required for 10-year storm run-

off will be justifiable.

c. In general, the lowest point of the bridge su-

perstructure shall clear the design water surface

by not less than 2 feet for average flow and trash

conditions. This may be reduced to as little as 6

inches if the flow is quiet, with low velocity and

little or n o trash . More t ha n 2 feet will be required

if flows are rough or large-size floating trash is

anticipated.

d. The bridge waterway will normally be alined

to result in the least obstruction to streamflow,

except that for natural streams consideration will

be given to realinement of the channel to avoid

costly skews, To the maximum extent practicable,

abutment wings will be alined to improve flowconditions. If a bridge is to span an improved trap-

ezoidal channel of considerable width, the need

for overall economy may require consideration of

the relative structural and hydraulic merits of on-

bank abutments with or without piers and warped

channel walls with vertical abutments.

e. To preclude failure by underscour, abutment

and pier footings will usually be placed either to

a depth of not less than 5 feet below the antici-

pated depth of scour, or on firm rock if such is

encountered at a higher elevation. Large multi-

span structures crossing alluvial streams may re-quire extensive pile foundations. To protect the

channel against the increased velocities, turbu-

lence, and eddies expected to occur locally, re-

vetment of channel sides or bottom consisting of

concrete, grouted rock, loose riprap, or sacked con-

crete will be placed as required. Criteria for se-

lection of revetment are given in chapter 5.

f. Where flow velocities are high, bridges should

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TM 5820-4/AFM 88-5, Chap 4

be of clear span , if at all pr acticable, in order topreclude serious problems attending debris lodg-ment and to minimize chan nel const ruction a ndmaintenance costs.

g. It is important that storm runoff be controlled

over as m uch of the contr ibuting watersh ed aspra cticable. Diversion chan nels, ter ra ces, checkdams, and similar conventional soil conservingfeatures will be installed, implemented, or im-

proved to reduce velocities and prevent silting ofchannels and other downstream facilities. Whenpracticable, unprotected soil surfaces within thedrainage area will be planted with appropriateerosion-resisting plants. These parts of the drain-age area which are located on private property orotherwise under control of others will be consid-ered fully in the planning stages, and coordinatedefforts will be taken to assure soil stabilizationboth upstream and downstream from the con-struction site.

h. Engineering criteria and design principles re-lated to traffic, size, load capacity, materials, andstru ctur al requirements for highway and ra ilroadbridges are given in TM 5-820-2/AFM 88-5, Chap-ter 2, and in AASHTO Standard Specifications forHighway Bridges, design m an uals of the differentrailroad companies, and recommended practicesof AREA Manual for Railway Engineering.

3-4. Curb-and-gutter sections.

a. Precipitation which occurs upon city streetsand adjacent areas must be rapidly and econom-ically removed before it becomes a hazard to traffic.

Water falling on the pavement surface itself is re-

moved from the surface and concentrated in thegutters by the provision of an adequate crown.The surface channel formed by the curb and gut-ter must be designed to adequat ely convey therunoff from the pavement and adjacent areas toa suitable collection point. The capacity can becomput ed by using t he n omograph for flow in atriangular channel, figure 3-2. This figure can alsobe used for a ba tt ered curb face section, since th ebattering has negligible effect on the cross sec-tional area. Limited data from field tests with clearwater show that a Mannings n of 0.013 is appli-cable for pavement. The n value should be raised

when appreciable quantities of sediment are pres-ent. Figure 3-2 also applies to composite sectionscomprising two or more rates of cross slope.

b. Good roadway drainage practice requires theextensive use of curb-and-gutter sections in com-bination with spillway chutes or inlets and down-spouts for adequate control of surface runoff, par-ticularly in hilly and mountainous terrain where

it is necessary to protect roadway embankmentsagainst formation of rivulets and channels by con-centrated flows. Materials used in such construc-tion include portland-cement concrete, asphalticconcrete, stone rubble, sod checks, and prefabri-cat ed concret e or met al sections, Typical of thelat ter are the entrance tapers and embankmentprotectors made by manufacturers of corrugatedmetal products. Downspouts as small as 8 inchesin diameter may be u sed, unless a considerabletrash problem exists, in which case a large sizewill be required. When frequent mowing is re-quired, consider at ion will be given t o the u se ofburied pipe in lieu of open paved channels or ex-posed pipe. The hydrologic and hydraulic designand the provision of outfall erosion protection willbe accomplished in accordance with principlesoutlined for similar component structures dis-cussed in this manual.

c. Curbs ar e used t o deter vehicles from leavingthe pavement at hazardous points as well as tocontrol drainage. The two general classes of curbs

are known as barrier and mountable and each hasnumerous types and detail designs. Barrier curbsare relatively high and steep faced and designedto inhibit and to at least discourage vehicles fromleaving the roadway. They are considered unde-sirable on high speed arterials. Mountable curbsare designed so that vehicles can cross them withvarying degrees of ease.

d. Curbs, gutters, and storm drains will not beprovided for dr ainage ar oun d ta nk-car or t ank -tru ck u nloading areas, ta nk-truck loading stan ds,and ta nks in bulk-fuel-storage areas. Safety re-

quires that fuel spillage must not be collected instorm or sanitary sewers. Safe disposal of fuelspillage of this nature may be facilitated by pro-vision of ponded areas for drainage so that anyfuel spilled can be removed from th e water sur -face.

35. Culverts.

a. A drainage culvert is defined as any structure

under the roadway with a clear opening of twentyfeet or less measured along the center of the road-

way. Culverts ar e genera lly of circular, oval, el-liptical, ar ch, or box cross section an d m ay be ofeither single or multiple construction, the choicedepending on available headroom and economy.Culvert materials for permanent-type installa-tions include plain concrete, reinforced concrete,corrugated metal, asbestos cement, and clay. Con-

crete culverts may be either precast or cast inplace, and corru gated meta l culvert s ma y haveeither annular or helical corrugations and be con-

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TM 5-820-4/AFM 88-5, Chap 4

structed of steel or aluminum. For the metal cul-

verts, different kinds of coatings and linings are

available for improvement of durability and hydrau-

lic characteristics. The design of economical culverts

involves consider at ion of man y factors r elatin g to re-

quirements of hydrology, hydraulics, physical envi-

ronment, imposed exterior loads, construction, and

maintenance . With the design discharge and gen-

eral layout determined, the design r equires detailedconsideration of such hydraulic factors as shape and

slope of approach and exit channels, allowable head

at entrance (and pending capacity, if appreciable),

tailwater levels, hydraulic and energy grade lines,

and erosion potential. A selection from possible

alternative designs may depend on practical con-

siderations such as minimum acceptable size, avail-

able m at erials, local experience concernin g corr osion

and erosion, and construction and maintenance

aspects. If two or more alternative designs involving

competitive materials of equivalent merit appear to

be about equ al in est imat ed cost, plan s will be devel-

oped to permit cont ra ctors options or a ltern at e bids,so that the least construction cost will result.

b. In most localities, culvert pipe is available in

sizes to 36 inches diameter for plain concrete, 144

inches or larger for reinforced concrete, 120 inches

for standard and helically corrugated metal (plain,

polymer coated, bituminous coated, part paved, and

fully paved interior), 36 inches for asbestos cement

or clay, and 24 inches for corrugated polyethylene

pipe. Concrete elliptical in sizes up to 116 x 180

inches, concrete arch in sizes up to 107 x 169 inches

and reinforced concrete box sections in sizes from

3 x 2 feet to 12 x 12 feet are available. Structural

plate, corrugated metal pipe can be fabricated with

diameters from 60 to 312 inches or more. Corru-

gated metal pipe arches are generally available in

sizes to 142 by 91 inches, and corrugated, structural

plat e pipe ar ches in spa ns t o 40 feet. Reinforced con-

crete vertical oval (elliptical) pipe is available in sizes

to 87 by 136 inches, and horizontal oval (elliptical)

pipe is available in sizes to 136 by 87 inches. De-

signs for extra large sizes or for special shapes or

structural requirements may be submitted by manu-

facturers for approval and fabrication. Short culverts

un der sidewalks (not entra nces or driveways) ma y be

as small as 8 inches in diameter if placed so as to becomparatively free from accumulation of debris or

ice. Pipe diameters or pipe-arch rises should be not

less than 18 inches. A diameter or pipe-arch of not

less than 24 inches should be used in areas where

wind-blown materials such as weeds and sand may

tend to block t he wat erway. Within t he a bove ranges

of sizes, stru ctural r equirements m ay limit the maxi-

mu m size tha t can be u sed for a s pecific inst allat ion.

c. The selection of culvert materials to withstand

deterioration from corrosion or abrasion will be

based on the following considerations:

(1) Rigid culvert is preferable where indu strial

wastes, spilled petroleum products, or other sub-

stan ces ha rmful to bituminous paving and coating in

corrugated metal pipe are apt to be present. Con-

crete pipe genera lly should not be u sed wher e soil ismore acidic than pH 5.5 or where the fluid carried

has a pH less than 5.5 or h igher t han 9.0. Polyethyl-

ene pipe is unaffected by acidic or alkaline soil condi-

tions. Concrete pipe can be engineered to perform

very satisfactorily in the more severe acidic or alka-

l ine envi ronments . Type II or Type V cements

should be used where soils and/or wat er h ave a m od-

erate or high sulfate concentration, respectively;

criteria are given in Federal Specification SS-C-

1960/GEN. High-density concrete pipe is recom-

mended when the culvert will be subject to tidal

drainage and salt-water spray. Where highly cor-

rosive substances are to be carried, the resistive

qualities of vitrified clay pipe or plastic lined con-

crete pipe should be considered.

(2) Flexible culvert su ch as corr ugat ed-steel pipe

will be galvanized and generally will be bituminous

coated for perman ent installations. Bitum inous coat-

ing or polymeric coating is recommended for corru-

gated steel pipe subjected to stagnan t wat er; where

dense decaying vegetation is present to form organic

acids; where there is continuous wetness or contin-

uous flow; and in well-drained, normally dry, alkali

soils. The polymeric coated pipe is not damaged by

spilled petroleum products or industrial wastes.Asbestos-fiber treatment with bituminous coated or

a polymeric coated pipe is recommended for corru-

gated-steel pipe subjected to highly corrosive soils,

cinder fills, mine drain age, tidal draina ge, salt -wat er

spray, certain industrial wastes, and other severely

corrosive conditions; or where extra-long life is de-

sirable. Cathodic protection is rarely required for

corrugated-steel-pipe instal lat ions; in some in-

stances, its use may be justified. Corrugated-alu-

minu m-alloy pipe, fabricated in a ll of th e sha pes an d

sizes of the more familiar corrugated-steel pipe,

evidences corrosion resistance in clear granular

mat erials even when subjected to sea wat er. Corru -gated-aluminum pipe will not be installed in soils

tha t a re highly acid (pH less th an 5) or a lkaline (pH

greater than 9), or in metallic contact with other

metals or metallic deposits, or where known cor-

rosive conditions are present or where bacterial cor-

rosion is known to exist. Similarly, this type pipe will

Change 1 3-7


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TM 5-820-4/AFM 88-5, Chap 4

not be installed in material classified as OH or OLaccording to the Unified Soil Classification System aspresented in MIL-STD 619. Although bituminouscoatings can be applied to aluminum-alloy pipe, suchcoatings do not afford adequate protection (bitumi-nous adhesion is poor) under the aforementionedcorrosive conditions. Suitable protective coatings foraluminum alloy have been developed, but are noteconomically feasible for culverts or storm drains.

For flow carrying debris and abrasives at moderateto high velocity, paved-invert pipe may be appro-priate. When protection from both corrosion andabrasion is required, smooth-interior corrugated-steel pipe may be desirable, since in addition to pro-viding the desired protection, improved hydraulicefficiency of the pipe will usually allow a reduction inpipe size. When considering a coating for use, per-formance data from users in the area can be helpful.Performance history indicates various successes orfailures of coatings and their probable cause and areavailable from local highway departments.

d. The capacity of a culvert is determined by itsability to admit, convey, and discharge water underspecified conditions of potential and kinetic energyupstream and downstream. The hydraulic design ofa culvert for a specified design discharge involvesselection of a type and size, determination of theposition of hydraulic control, and hydraulic computa-tions to determine whether acceptable headwaterdepths and outfall conditions will result. In consider-ing what degree of detailed refinement is appropri-ate in selecting culvert sizes, the relative accuracy ofthe estimated design discharge should be taken intoaccount. Hydraulic computations will be carried out

by standard methods based on pressure, energy,momentum, and loss considerations. Appropriate

formulas, coefficients, and charts for culvert designare given in appendix B.

e. Rounding or beveling the entrance in any way willincrease the capacity of a culvert for every designcondition. Some degree of entrance improvement

should always be considered for incorporation indesign. A headwall will improve entrance flow overthat of a projecting culvert. They arc particularlydesirable as a cutoff to prevent saturation sloughingand/or erosion of the embankment. Provisions fordrainage should be made over the center of the head-

wall to prevent scouring along the sides of the walls.A mitered entrance conforming to the fill slope pro-

duces little if any improvement in efficiency over thatof the straight, sharp-edged, projecting inlet, andmay be structurally unsafe due to uplift forces. Bothtypes of inlets tend to inhibit the culvert from flow-ing full when the inlet is submerged. The most effi-

3-8 Cha nge 1

cient entrances incorporate such geometric featuresas elliptical arcs, circular arcs, tapers, and para-bolic drop-down curves. In general elaborate inlet

designs for culverts are justifiable only in unusualcircumstances.

f. Outlets and endwalls must be protected againstundermining, bottom scour, damaging lateral ero-sion and degradation of the downstream channel.The presence of tailwater higher than the culvert

crown will affect the culvert performance and maypossibly require protection of the adjacent embank-ment against wave or eddy scour. Endwalls (outfallheadwalls) and wingwalls should be used wherepractical, and wingwalls should flare one on eightfrom one diameter width to that required for theformation of a hydraulic jump and the establishmentof a Froude number in the exit channel that will in-sure stability. Two general types of channel instabil-ity can develop downstream of a culvert. The con-ditions are known as either gully scour or a localizederosion referred to as a scour hole. Gully scour is tobe expected when the Froude number of flow in thechannel exceeds that required for stability. Erosionof this type maybe of considerable extent dependingupon the location of the stable channel section rela-tive to that of the outlet in both the vertical anddownstream directions. A scour hole can be ex-pected downstream of an outlet even if the down-stream channel is stable. The severity of damage tobe anticipated depends upon the conditions existingor created at the outlet. See chapter 5 for additionalinformation on erosion protection.

g. In the design and construction of any drainagesystem it is necessary to consider the minimum and

maximum earth cover allowable in the undergroundconduits to be placed under both flexible and rigidpavements. Minimum-maximum cover require-ments for asbestos-cement pipe, corrugated-steelpipe, reinforced concrete culverts and storm drains,standard strength clay and non-reinforced concretepipe are given in appendix C. The cover depthsrecommended are valid for average bedding andbackfill conditions. Deviations from these conditionsmay result in significant minimum cover require-ments.

h. Infiltration of fine-grained soils into drainage

pipelines through joint openings is one of the major causes of ineffective drainage facilities. Thisis particularly a problem along pipes on relativelysteep slopes such as those encountered with bro-ken back culverts. Infiltration of backfill andsubgrade material can be controled by watertightflexible joint materials in rigid pipe and with wa-tertight coupling bands in flexible pipe. The re-


21/115

sults of laboratory research concerning soil infil-tration through pipe joints and the effectivenessof gasketing tapes for waterproofing joints andseams are available.

36. Underground hydraulic de sign.

a. The storm-drain system will have sufficientcapa city t o convey run off from t he design st orm

(usually a 10-year frequency for permanent in-stallations) within the barrel of the conduit. De-sign ru noff will be comput ed by the m eth ods in-dicated in chapter 2. Concentration times willincrease and average rainfall intensities will de-crease as the design is carried to successive down-

str eam points. In general, th e increment al con-centration times and the point-by-point totalsshould be estimated to the nearest minute. Thesetotals should be rounded to the nearest 5 minutesin selecting design intensities from the intensity-duration curve. Advantage will be taken of anypermanently available surface ponding areas, and

their effectiveness determined, in order to holddesign discharges and storm-drain sizes to a min-imum. Experience indicates that it is feasible andpractical in t he actua l design of storm dra ins toadopt minimum values of concentration times of10 minut es for pa ved areas a nd 20 m inutes forturfed areas. Minimum times of concentrationshould be selected by weighting for combined pavedand turfed areas.

b. Storm-drain systems will be so designed thatthe hydraulic gradeline for the computed designdischarge in as near optimum depth as practicableand velocities are not less than 2.5 feet per second(nominal minimum for cleansing) when the drainsare one-third or more full. To minimize the pos-sibility of clogging and to facilitat e clean ing, theminimum pipe diameter or box section height will

generally be not less than 12 inches; use of smallersize must be fully justified. Tentative size selec-tions for capacity flow may be made from the nom-

ogra phy for comput ing r equired size of circulardrains in appendix B, TM 5-820-l/AFM 88-5,Chapter 1. Problems attending high-velocity flowshould be carefully analyzed, and appropriate pro-

visions made to insure a fully functional project.

c. Site topography will dictate the location ofpossible outlets an d t he general limiting gradesfor the system. Storm drain depths will be held tothe minimum consistent with limitations imposedby cover requirements, proximity of other struc-tures, interference with other utilities, and veloc-ity requir ement s becau se deep excavation is ex-pensive. Usually in profile, proceeding downstream,the crowns of conduits whose sizes progressively

TM 5820-4/AFM 88-5, Chap 4

increase will be matched, the invert grade drop-ping across the junction structure; similarly, thecrowns of incoming laterals will be matched tothat of the main line. If the downstream conduitis smaller as on a steep slope, its invert will bemat ched to th at of the upst ream conduit . Someaddit iona l lowering of an outgoing pipe m ay berequired to compensate for pressure loss within a

junction structure.

d. Manh oles or jun ction boxes u sua lly will beprovided at points of change in conduit grade orsize, at junctions with laterals of branches andwherever entry for maintenance is required. Dis-tance between points of entry will be not moreth an appr oximat ely 300 feet for condu its with aminimum dimension smaller than 30 inches. If the

storm drain will be carrying water at a velocityof 20 feet per second or great er, with high ener gyand strong forces present, special attention mustbe given such items as alinement, junctions, an-chorage requirements, joints, and selection of ma-

terials.

37. Inlets.

a. Storm-drain inlet structures to intercept sur-face flow are of th ree gener al t ypes: drop, cur b,an d combina tion. Hydra ulically, they may fun c-tion as either weirs or orifices depending mostly

on th e inflowing wat er. The a llowable depth fordesign storm conditions and consequently the type,size and sp acing of inlets will depend on t he t o-pography of the surrounding area, its use, andconsequences of excessive depths. Drop inlets,

which are provided with a grated entrance open-ing, are in general more efficient than curb inletsand are u seful in su mps, roadway sags, swales,and gutters. Such inlets are commonly depressedbelow the adjacent grade for improved intercep-tion or increased capacity. Curb inlets along slop-ing gutters require a depression for adequate in-terception. Combination inlets may be used where

some additional capacity in a restricted space isdesired. Simple grated inlets are most susceptibleto blocking by tras h. Also, in h ousing ar eas, th euse of grated drop inlets should be kept to a rea-sonable minimum, preference being given to the

curb type of opening. Where an abnormally highcurb opening is needed, pedestrian safety may re-quire one or more protective bars across the open-

ing. Although curb openings are less susceptibleto blocking by t ra sh, t hey ar e also less efficientfor interception on hydraulically steep slopes, be-cause of the difficulty of tu rn ing th e flow int o them .Assurance of satisfactory performance by anysystem of inlets requires careful consideration of

3-9


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TM 5820-4/AFM 885, Chap 4

the several factors involved. The final selection ofinlet types will be based on overall hydraulic per-formance, safety requirements, and reasonable-ness of cost for construction and maintenance.

b. In placing inlets to give an optimum arrange-ment for flow interception, the following guidesapply:

(1) At street intersections and crosswalks, in-

lets are usually placed on the upstream side. Gut-ters to transport flow across streets or roadwayswill not be used.

(2) At intermediate points on grades, thegreatest efficiency and economy commonly resultif either grated or curb inlets are designed to in-tercept only about three-fourths of the flow.

(3) In sag vertical curves, three inlets are oftendesirable, one at the low point and one on eachside of the low point where the gutter grade isabout 0.2 foot above the low point. Such a layouteffectively reduces pond buildup and deposition ofsediment in the low area.

(4) Large quantities of surface runoff flowingtoward ma in t horoughfares normally should beintercepted before reaching them.

(5) At a bridge with curbed approaches, gutterflow should be int ercepted before it r eaches th ebridge, particularly where freezing weather oc-curs.

(6) Where a r oad pavement on a cont inu ousgrade is warped in transitions between super-elevated and normal sections, surface water shouldnormally be intercepted upstream of the pointwhere the pavement cross slope begins to change,

especially in areas where icing occurs.(7) On roads where curbs are used, runoff fromcut slopes an d from off-site a rea s sh ould, wher-ever possible, be intercepted by ditches at the topsof slopes or in swales along the shoulders and notbe allowed to flow onto the roadway. This practiceminimizes the amount of water to be interceptedby gutter inlets and helps to prevent mud anddebris from being carried onto the pavement.

c. Inlets placed in sumps have a greater poten-tial capacity than inlets on a slope because of thepossible submergence in the sump. Capacities of

grated, curb, and combination inlets in sumps willbe computed as outlined below. To allow for block-

age by trash, the size of inlet opening selected forconstruction will be increased above the computedsize by 100 percent for grated inlets and 25 to 75percent, depending on trash conditions, for curbinlets and combination inlets.

(1) Grated type (in sump).(a) For depths of water up to 0.4 foot use

the weir formula:

If one side of a rectangular grate is against a curb,this side must be omitted in computing the perim-e t e r .

(b) For depths of water above 1.4 feet use theorifice formula:

(c) For dept hs bet ween 0.4 and 1.4 feet, op-erat ion is indefinite due t o vortices an d other dis-turbances. Capacity will be somewhere betweenthose given by the preceding formulas.

(d ) Problems involving the above criteria maybe solved graphically by use of figure 3-3.

(2) Curb Type (in sump). For a curb inlet in asump, the above listed general concepts for weirand orifice flow apply, the latter being in effectfor depths greater than about 1.4h (where h is the

height of curb opening entrance). Figure 3-4 pre-sents a graphic method for estimating capacity.

(3) Combination Type (in sump). For a com-bination inlet in a sump no specific formulas aregiven. Some increase in capacity over that pro-vided singly by either a grated opening or a curbopenin g may be expected, an d th e cur b openingwill operate as a relief opening if the grate be-comes clogged by debris. In estimating the capac-ity, the inlet will be treated a s a simple grat edinlet, bu t a safety factor of 25 to 75 percent willbe applied.

(4) Slotted drain type. For a slotted drain inletin a sump, the flow will enter the slot as either all

orifice type or all weir type, depending on the depthof water at the edge of the slot. If the depth is lessthan .18 feet, the length of slot required to inter-cept total flow is equal to:

If the depth is greater than .18 feet, the length ofslot required to intercept total flow is equal to:

d = depth of flow-inchesw = width of slot---.l46 feet

d. Each of a series of inlets placed on a slope isusually, for optimum efficiency, designed to in-tercept somewhat less than the design gutter flow,

the remainder being passed to downstream inlets.The amount that must be intercepted is governed

by whatever width and depth of bypassed flow canbe tolerated from a traffic and safety viewpoint.

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TM 5-820-4 /AFM 88-5 , Chap 4

P R E P A R E D

u UREAU OF PUBLIC ROADS

Figure 3-3. Capacity of grate inlet in sump water pond on grate.

311


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HEIGHT

OF

OP

ENING

(h)

IN

FEET


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TM 5820-4/AFM 885, Chap 4

Such toleration levels will nearly always be influ-enced by costs of drainage construction. With theflat street crowns prevalent in modern construc-tion, many gutter flows are relatively wide and inbuilt-up areas some inconveniences are inevita-ble, especially in regions of high rainfall, unlessan elaborate inlet system is provided. Theachievement of a satisfactory system at reason-able cost requires careful consideration of use fac-

tors and careful design of the inlets themselves.However, it must also be remembered that a lim-itation on types and sizes for a given project isalso desirable, for standardization will lead to lowerconstruction costs. Design of grated, curb, andcombination inlets on slopes will be based on prin-ciples outlined below.

(1) Grated type (on slope). A grated inlet placedin a sloping gutter will provide optimum inter-ception of flow if th e bar s ar e placed par allel tothe direction of flow, if the openings total at least50 percent of th e width of the gra te (i.e. normalto the direction of flow), and if the unobstructed

opening is long enough (parallel to the directionof flow) th at th e wat er falling thr ough will clearthe downstream end of the opening. The minimumlength of clear opening requ ired depends on th edepth and velocity of flow in the approach gutteran d th e thickness of th e grate at th e end of th eslot. This minimum length may be estimated bythe partly empirical formula:

A rectan gular grat ed inlet in a gutter on a con-tinuous grade can be expected to intercept all thewater flowing in that part of the gutter cross sec-tion that is occupied by the grating plus an amount

that will flow in along the exposed sides. However,unless the grate is over 3 feet long or greatly de-pressed (extreme warping of the pavement is sel-dom permissible), any water flowing outside thegrate width can be considered to bypass the inlet.The quantity of flow in the prism intercepted bysuch a grate can be computed by following in-struction 3 in figure 3-2. For a long grate the in-flow along the side can be estimated by consid-ering the edge of the grate as a curb opening whose

effective length is the total grate length (ignoringcrossbars) reduced by the length of the jet directlyintercepted at the upstream end of the grate. Toattain the optimum capacity of an inlet consistingof two grates separated by a short length of pavedgutter, the grates should be so spaced that thecarryover from the upstream grate will move suf-ficiently toward the curb to be intercepted by thedownstream grate.

(2) Curb type (on slope). In general, a curbinlet pla ced on a gra de is a h ydrau lically ineffi-cient structure for flow interception. A relativelylong opening is required for complete interceptionbecause t he heads are n orma lly low and th e di-rection of oncoming flows is not favorable. Thecost of a long curb inlet must be weighed againstthat of a drop type with potentially costly grate.The capacity of a curb inlet intercepting all the

flow can be calculated by an empirical equation.The equation is a function of length of clear open-ing of the inlet, depth of depression of flow line atinlet in feet, and the depth of flow in approachgutter in feet. Depression of the inlet flow line isan essent ial par t of good design, for a cur b inletwith no depression is very inefficient. The flowintercepted may be markedly increased withoutcha nging t he opening length if th e flow line canbe depressed by one times the depth of flow in the

approach gutter. The use of long curb openingswith inter mediate supports sh ould generally beavoided because of the tendency for the supports

to accumulate trash. If supports are essential, theyshould be set back several inches from the gutterl ine.

(3) Combination type (on slope). The capacityof a combination inlet on a continuous grade isnot much greater than that of the grated portionitself, and should be computed as a separate grated

inlet except in the following situations. If the curbopening is placed upstr eam from t he grat e, thecombination inlet can be considered to operate astwo separate inlets and the capacities can be com-puted accordingly. Such an arrangement is some-times desirable, for in addition to the increased

capacity the curb opening will tend to interceptdebris an d th ereby redu ce clogging of th e grat e.If the curb opening is placed downstream fromthe grate, effective operation as two separate in-lets requires that the curb opening be sufficientlydownstream to allow flow bypassing the grate tomove into the curb opening. The minimum sepa-ration will vary with both the cross slope and thelongitudinal slope.

e. Structural aspects of inlet construction should

generally be as indicated in figures 3-5, 3-6, and3-7 which show respectively, standard circular

grate inlets, types A and B; typical rectangulargrate combination inlet, type C; and curb inlet,type D. It will be noted that the type D inlet pro-vides for extension of the opening by the additionof a collecting trough whose backwall is cantile-vered to the curb face. Availability of gratings and

standards of municipalities in a given region maylimit the choice of inlet types. Grated inlets sub-

ject to heavy wheel loads will require grates of

3-13


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TM 5820-4/AFM 88-5, Chap 4

SECTION-TYPE A INLET SECTION TYPE B INLET

Figure 3-5. Standard type "A" and type "B" inlets.

3 - 1 4


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TM 5820-4/AFM 885, Chap 4


28/115

TM 5820-4/AFM 885, Chap 4

3-16

U. S. Army Corps of Engineers

Figure 37. St and ard type D inlet.


29/115

precast steel or of built-up, welded steel. Steelgrates will be galvanized or bituminous coated.Unusual inlet conditions will require special de-s ign.

38. Vehicular safety and hydraulically effi-

cient drainage practice.

a. Some drainage structures are potentially

hazardous and, if located in the path of an errantvehicle, can substa nt ially increase t he pr obabilityof an accident. Inlets should be flush with theground, or should present no obstacle to a vehiclethat is out of control. End structures or culvertsshould be placed outside the designated recoveryarea wherever possible. If grates are necessary to

TM 5820-4/AFM 885, Chap 4

cover culvert inlets, care must be taken to designthe grate so that the inlet will not clog duringperiods of high water. Where curb inlet systemsare used, setbacks should be minimal, and gratesshould be designed for hydraulic efficiency andsafe passa ge of vehicles. Haza rdous cha nn els orenergy dissipating devices should be located out-side the designated recovery area or adequateguard-rail protection should be provided.

b. It is necessary to emphasize that libertiesshould not be taken with the hydraulic design ofdrainage structures to make them safer unless itis clear that their function and efficiency will notbe impaired by the contemplated changes. Evenminor changes at culvert inlets can seriously dis-rupt hydraulic performance.

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TM 5820-4/AFM 885, Chap 4

CHAPTER 4

HYDRAULIC STRUCTURES

41. Manholes and junction boxes. Drainage

systems require a variety of appurtenances to as-

sure proper operations. Most numerous appurte-

nances are manholes and junction boxes. Man-

holes and junction boxes are generally constructed

of any suitable materials such as brick, concrete

block, reinforced concrete, precast reinforced-con-

crete sections, or preformed corrugated metal sec-

t ions . M anho les a re l oca t ed a t i n t e r sec t ions ,

changes in alignment or grade, and at interme-

diate joints in the system up to every 500 feet.

Junction boxes for large pipes are located as nec-

essary to assure proper operation of the drainage

system. Inside dimensions of manholes will not be

less than 2.5 feet. Inside dimensions of junction

boxes will provide for not less than 3 inches of

wall on either side of the outside diameter of the

largest pipes involved. Manhole frames and cover

will be provided as required; rounded manhole and

box covers are preferred to square covers. Slab

top covers will be provided for large manholes and

junction boxes too shallow to permit corbeling of

the upper part of the structure. A typical large

box drain cover is shown in figure 3-5, TM 5-820-

3/AFM 88-5, Chapter 3. Fixed ladders will be pro-vided depending on the depth of the structures.

Access to man hole and junction boxes with out fixed

ladders will be by portable ladders. Manhole and

junction box design will insure minimum hy-

draulic losses through them. Typical manhole and

junction box construction is shown in figures 4-1

through 4-3.

42. Detention pond storage. Hydrologic stud-

ies of the drainage area will reveal if detention

ponds are required. Temporary storage or pend-

ing may be required if the outflow from a drainage

area is limited by the capacity of the drainage

system serving a given area. A full discussion of

temporary storage or ponding design will be found

in appendix B, TM 5-820-l/AFM 88-5, Chapter 1.

Pending areas should be designed to avoid crea-

tion of a facility that would be unsightly, difficult

to maintain, or a menace to health or safety.

4-3. Outlet energy dissipators.

a. Most drainage systems are designed to op-

erate under normal free outfall conditions. Tail-

water conditions are generally absent. However,

i t is possible for a discharge result ing from a

drainage system to possess kinetic energy in ex-

cess of that which normally occurs in waterways.

To reduce the kinetic energy, and thereby reduce

downstream scour, outfalls may sometimes be re-

quired to reduce streambed scour. Scour may oc-

cur in the streambed if discharge velocities exceed

the values listed in table 4-1. These values are tobe used only as guides; studies of local materials

must be made prior to a decision to install energy

dissipation devices. Protection against scour may

be provided by plain outlets, transitions and still-

ing basins . Plain out lets pr ovide no protective works

and depend on natural material to resist erosion.

Transitions provide little or no dissipation of en-

ergy themselves, but by spreading the effluent jet

to approximately the flow cross-section of the nat-

ural channel, the energy is greatly reduced prior

to releasing the effluent into the outlet channel.

Stilling basins dissipate the high kinetic energy

of flow by a hydraulic jump or other means. Rip-rap may be required at any of the three types of

outfalls.

(1) Plain type.

(a) If the discharge channel is in rock or a

material highly resistant to erosion, no special

erosion protection is required. However, since flow

from the culvert will spread with a resultant drop

Table 4-1. Maximum Permissible Mean Velocities to

Prevent Scour

Ma ximu m

Permiss ib le

Materia l Mean Velocity

Uniform graded sand

and cohesionless silts 1.5 fps

Well-graded sand 2.5 fps

Silty sand 3.0 fp s

Clay 4.0 fp s

Gravel 6.0 fps

U.S. Army Corps of Engineers

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TM 5820-4/AFM 885, Chap 4

ELEVATION

HALF PLAN - MANHOLE COVER AND FRAME

4 -2


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TM 5820-4/AFM 885, Chap 4

SECTION A-A SECTION B-B

4 -4


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TM 5820-4/AFM 885, Chap 4

in water surface and increase in velocity, this type

of outlet should be used without riprap only if the

material in the outlet channel can withstand ve-

locities about 1.5 times the velocity in the culvert.

At such an outlet, side erosion due to eddy action

or turbulence is more likely to prove troublesome

than is bottom scour.

(b) Cantilevered culvert outlets may be used

to discharge a free-falling jet onto the bed of the

outlet channel. A plunge pool will be developed,the depth and size of which will depend on the

energy of the falling jet at the tailwater and the

erodibility of the bed material.

(2) Transition type. Endwalls (outfal l head-

walls) serve the dual purpose of retaining the em-

b a n k m e n t a n d l i m i t i n g t h e o u t l e t t r a n s i t i o n

boundary. Erosion of embankment toes usually

can be traced to eddy attack at the ends of such

walls. A flared transition is very effective, if pro-

portioned so that eddies induced by the effluent

jet do not continue beyond the end of the wall or

overtop a sloped wall. As a guide, it is suggested

that the product of velocity and flare angle should

not exceed 150. That is, if effluent velocity is 5 feet

per second each wingwall may flare 30 degrees;

but if velocity is 15 feet per second, t he flar e sh ould

not exceed 10 degrees. Unless wingwalls can be

anchored on a stable foundation, a paved apron

between the wingwalls is required. Special care

must be taken in design of the structure to pre-

clude undermining. A newly excavated channel

may be expected to degrade, and proper allowance

for this action should be included in establishing

the apron elevation and depth of cutoff wall. Warped

endwalls provide excellent transitions in that theyresult in the release of flow in a trapezoidal sec-

tion, which generally approximates the cross sec-

tion of the outlet channel. If a warped transition

is placed at the end of a curved section below a

culvert, the transition is made at the end of the

curved section to minimize the possibility of ov-

ertopping due to superelevation of the water sur-

face. A paved apron is required with warped end-

walls. Riprap usually is required at the end of a

transition-type outlet.

(3) Stilling basins. A detailed discussion of

stilling basins for circular storm drain outlets can

be found in chapter 7, TM 58203.

b. Improved channels, especially the paved ones,

commonly carry water at velocities higher than

those prevailing in the n atu ral chann els into which

they discharge. Often riprap will suffice for dis-

sipation of excess energy. A cutoff wall may be

required at the end of a paved channel to preclude

undermining. In extreme cases a flared transi-

tion, stilling basin, or impact device may be re-

quired.

44. Drop structures and check dams. Drop

structures and check dams are designed to check

channel erosion by controlling the effective gra-

dient, and to provide for abrupt changes in chan-

nel gradient by means of a vert ical drop. The

structures also provide satisfactory means for dis-

charging accumulated surface runoff over fills withheights not exceeding about 5 feet and over em-

bankments higher than 5 feet provided the end

sill of the drop structure extends beyond the toe

of the embankment. The check dam is a modifi-

cation of the drop structure used for erosion con-

trol in small channels where a less elaborate

structure is permissible. Pert inent design fea-

tures are covered in chapter 5, TM 58203/AFM

885, Chapter 3.

45. Miscellaneous structures.

a. A chute is a steep open channel which pro-

vides a method of discharging accumulated sur-

face runoff over fills and embankments. A typical

design is included in chapter 6, TM 58203/AFM

885, Chapter 3.

b. When a conduit or channel passes through or

beneath a security fence and forms an opening

greater than 96 square inches in area a security

barrier must be installed. Barriers are usually of

bars, grillwork, or chain-link screens, Parallel bars

used to prevent access will be spaced not more

than 6 inches apart , and will be of sufficient

strength to preclude bending by hand after as-

sembly.(1) Where fences enclose maximum security

areas such as exclus ion and res t r ic ted areas ,

drainage channels, ditches, and equalizers will,

wherever possible, be carried under the fence in

one or more pipes having an internal diameter of

not more than 10 inches. Where the volume of flow

is such that the multipipe arrangement is not fea-

sible, the conduit or culvert will be protected by

a security grill composed of 3/4-inch-diameter rods

or 1/2-inch bars spaced not more than 6 inches on

center, set and welded in an internal frame. Where

rods or bars exceed 18 inches in length, suitable

spacer bars will be provided at not more than 18inches on center, welded at all intersections. Se-

curity grills will be located inside the protected

area. Where the grill is on the downstream end of

the culvert, the grill will be hinged to facilitate

cleaning and provided with a latch and padlock,

and a debris catcher will be installed in the up-

stream end of the conduit or culvert. Elsewhere

the grill will be permanently attached to the cul-

4-5


35/115


36/115

vert. Security regulations normally require th eguard to inspect such grills at least once everyshift. For culverts in rough terrain, steps will beprovided to the grill to facilitate inspection andcleaning.

(2) For culverts and storm drains, barriers atthe int akes would be preferable to barr iers a t theoutlets becau se of th e relat ive ease of debris r e-moval. However, barriers at the outfalls are usu-

ally essential; in t hese cases considera tion shouldbe given to placing debris interceptors at the in-lets. Bars constituting a barrier should be placedin a horizontal position, and the number of ver-tical members should be limited in order to min-imize clogging; th e tota l clear a rea should be a tleast twice the area of the conduit or larger undersevere debris conditions. For large conduits anelaborate cagelike structure may be required.Provisions to facilitate cleaning during or imme-diately after heavy runoff should be made. Figure44 shows a typical barrier for the outlet of a pipedrain. It will be noted t hat a 6-inch un derclear-

ance is provided to permit passage of normal bed-load mat erial, and tha t t he apron between t he

TM 5820-4/AFM 885, Chap 4

conduit outlet and the barrier is placed on a slopeto minimize deposition of sediment on the aprondur ing ordinar y flow. Erosion pr otection, whererequired, is placed immediately downstream fromthe barrier.

(3) If manholes must be located in the im-mediat e vicinity of a securit y fence th eir coversmust be so fast ened as to prevent un aut horizedopening.

(4) Open channels may present special prob-lems due to the relatively large size of the water-way and the possible requirements for passage oflarge floating debris. For such channels a barriershould be provided tha t can be u nfastened an dopened or lifted during periods of heavy runoff orwhen clogged. The barrier is hinged at the top and

an empty tank is welded to it at the bottom toserve as a float . Open cha nn els or swa les whichdrain relatively small areas and whose flows carryonly minor quantities of debris may be securedmerely by extending the fence down to a concretesill set into the sides and across the bottom of the

channel.

47


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TM 5-820-4/AFM 88-5, Chap 4

CHAPTER 5

EROSION CONTROL AND RIPRAP PROTECTION

5-1. General.

a. Hydraulic structures discharging into openchannels will be provided with riprap protectionto prevent erosion. Two general types of channelinstability can develop downstream from a culvert

and stormdrain outlet. The conditions are knownas either gully scour or a localized erosion referred

to as a scour hole. Distinction between the twocondit ions of scour an d pr ediction of th e type t obe anticipated for a given field situation can bemade by a comparison of the original or existingslope of the channel or drainage basin down-

stream of the outlet relative to that required forstability.

b. Gully scour is to be expected when the Froudenumber of flow in the channel exceeds that re-quired for stability. It begins at a point down-stream where the channel is stable and pro-gresses upstream. If sufficient differential inelevation exists between t he outlet a nd t he sec-tion of stable channel, the outlet structure will becompletely undermined. Erosion of this type maybe of considerable extent depending upon the lo-cation of the stable channel section relative to

tha t of th e out let in both th e vertical and down-stream directions.c. A scour h ole or localized erosion is t o be ex-

pected downstream of an outlet even if the down-stream channel is stable. The severity of damageto be anticipated depends upon the conditions ex-isting or created at the outlet. In some instances,the extent of the scour hole may be insufficientto produce either instability of the embankmentor st ructur al dam age to the outlet. However, inmany situations flow conditions produce scour ofthe extent that embankment erosion as well asstructural damage of the apron, end wall, and cul-

vert are evident.d. The results of research conducted at US Army

Engineer Waterways Experiment St ation to de-termine the extent of localized scour that may beanticipated downstream of culvert and storm-drain

outlets has also been published. Empirical equa-tions were developed for estimating the extent ofthe anticipated scour hole based on knowledge ofth e design discharge, the culvert diameter, an d

the duration and Froude number of the designflow at th e culvert outlet. These equat ions andthose for the maximum depth, width, length andvolume of scour and comparisons of predicted andobserved values are discussed in chapter 10, TM5-820-3/AFM 88-5, Chapter 3. Examples of rec-ommended application to estimate the extent ofscour in a cohesionless soil and several alternateschemes of protection required to prevent localscour downstream of a circular and rectangularoutlet are illustrated in Practical Guidance for De-

sign of Lined Channel Expansions at Culvert Out-

lets, Technical Report H-74-9.

5-2. Riprap protection,

a. Riprap protection should be provided adja-cent to all hydraulic structures placed in erosivematerials to prevent scour at the ends of the struc-tur e, The protection is r equired on t he bed andbanks for a sufficient distance to establish velocitygradients and turbulence levels at the end of theriprap approximating conditions in the naturalchannel. Riprap can also be used for lining thechannel banks to prevent lateral erosion and un-desirable meandering. Consideration should begiven to providing an expansion in either or boththe horizontal and vertical direction immediatelydownstream from hydraulic structures such as dropstructures, energy dissipators, culvert outlets orother devices in wh ich flow can expan d an d dis-sipate its excess energy in turbulence rather than

in a direct attack on the channel bottom and sides.

b. There are three ways in which riprap hasbeen kn own t o fail: movement of the individua lstones by a combination of velocity and turbu-lence; movement of the natural bed materialth rough the ripra p resulting in slumping of th e

blanket; and undercutting and raveling of the rip-rap by scour at the end of the blanket. Therefore,in design, consideration must be given to selectionof an adequat e size stone, use of an a dequatelygraded riprap or provision of a filter blanket, andproper treatment of the end of the riprap blanket.

53. Selection of stone size. There are curvesavailable for the selection of stone size required

51


38/115

TM 5-820-4/AFM 885, Chap 4

for protection as a function of the Froude number.

(See TM 5-820-3AFM 88-5, Chapter 3. Two curvesare given, one to be used for riprap subject todirect attack or adjacent to hydraulic structuressuch as side inlets, confluences, and energy dis-sipators, where tu rbulence levels are high, andthe other for riprap on the banks of a straightchannel where flows are relatively quiet and par-allel to the banks. With the depth of flow and av-

erage velocity in the channel known, the Froudenumber can be computed and a stone size deter-mined from the appropriate curve. Curves for de-termining the riprap size required to prevent scourdownstream from culvert outlets with scour holesof various depths are also available. The thicknessof the riprap blanket should be equal to the long-est dimension of the maximum size stone or 1.5times the stone diameter (50 percent size), which-ever is great er. When t he u se of very large rockis desirable but

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