PRELIMINARY ESTIMATES

1.

Select a centerline of fill

2.

Estimate a permanent pool level

3. Using Table No. 1-1 or 1-2, estimate required stage plus freeboardand pipe diameter.

4. Measure gully depth. gully top and bottom width. total fill heightand top length of the fill. These measurements can be obtained witha hand level for depth and by pacing for length.

5. Using the simplified cross-section, compute earth fill yardage usingyardage tables in appendix or on page 450, Ready Reference Handbook.

6.

Use Table 2 on pages 5 and 6 to obtain estimated materials costs(Should be re-issued as materials costs change)

7. Structure cost equals the sum of the following:

a.b.c.d.e.

Earth fill yardage from Step 5 times the cost per yard.Cost of principal spillway pipe from Step 6.Estimated cost of clearing and grubbing.Estimated cost of embankment drainage where needed.Other extra costs such as livestock watering systems) etc

Estimated materials costs in Table 2 are based on the following assumptions:

1.

Fill-slope ratios are measured from constructed fill elevation.

2.

Settlement = 5%,

3. Controlled head is measured from crest of inlet to outlet of pipe.

4. Pipe with factory caulked seams and asphalt coated connecting bandsused throughout.

-1 -

5.

24" long watertight bands with 4 rods per band used onpipes 18" diameter and larger. 24" long bands with angleiron and bolts used on pipes from 8" through 15" diameter.

6. Riser height is equal to the controlled head or 5D (D ~ pipediameter). whichever is less.

7. Top widths, anti-seep collars, and other features conformto Iowa Standards and Specifications for ConservationPractices for Pond (378).

8. Pipe prices used are current. check if price sheet isfor present year.

9. Two-piece anti-seep collars are used on all sizes.

10.

Anti-seep collars are based upon a dimension to extend24" beyond the outside surface of the pipe.

11. 0 -72 ft. pipe -2 anti-seep collars -3 bands72 -100 ft. pipe -3 anti-seep collars -5 bands100 -128 ft. pipe -4 anti-seep collars -7 bands128 ft. and up pipe -5 anti-seep collars -9 bands

-2 -

Drainage i Design -Pipe' Min. * St;ge+ F;eeboardf'MA;. iVArea Storm Size Stage Mi~~-O- .~~~~~~~a~~'~'

30-80 10" Min.(UsuallyWorks)

Q5 4.01.5 3.5

80-120 1511 Min.(UsuallyWorks)

QI0 2.3 4.3 4.5

120-250 15" Min.QI0 2.3 4.3 4.5

24'1 6.03.6 5.6

30.1 6.5 7.04.5

7.0250+ 18" Min. 2.7 4.7Q25

24" 7.03.6 5.6

30" 7.04.5 6.5

36" 5.4 7.4 7.5

* Stage required for full pipe flow using CMP hood inlet.

** Use for average conditions.

Table No. 1-1

ESTIMATING PIPE SIZE. STAGE AND FREEBOARD -DETENTION TYPE STRUCTURES

:,,~

3 -

MinimumStage +

Freeboard*(ft)

PipeSize(in)

Area Handled at Minimum Stage(Acres)Head

(it),OutflowI

(cfs)I

RCN = 70 RCN = 75mRCN = 80

Tr10152025

11121414

4.254.254.254.25

5677

15"

10152025

16182122

4.74.74.74.7

899

11

6788

5566

18"

10152025

23272932

5.25.25.25.2

12161922

10111314

QI0 789

10

21"

Q~o10152025

32384144

5.65.65.65.6

22283134

10131415

14182022

24" QI0

10152025

61728292

3.63.63.63.6

55708080

34435262

24303642

30"

- r--10152025

95112124130

3.93.93.93.9

808093

100

65808080

44556470

--36"

10152025

4.24.24.24.2

137161172185

108138152168

Q25 808998

110

77807680

~42"

~25

~-8093

110120

-10152025

185218238252

4.54.54.54.5

168213247270

110143163177

48"

QZ5.* Based upon Hood Inlet 15" through 24", Freeboard = 2.0 ft.

Drop Inlet 30" through 48", Freeboard = 2.0 ft.Read is difference in elevation between water surface in reservoir and tailwater.(Min. tailwater = Outlet elevation plus 0.6D)

ESTIMATED PIPE SIZE -FULL FLOW STRUCTURES.Table 1-2

-4

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SURVEY-.!~~

The following check list may be used to determine if all surveyinformation needed for structure design has been obtained.

INFORMATION DETENTION FULL FLOW

1. xxProfile showing average bank and bottomelevations, culverts, overfalls, bridges andother prominent features. Continue down-stream from structure site to stable grade.Continue upstream to the top elevation ofthe proposed dam.

2. Cross section on centerline of fill and anyother cross sections necessary for design.

x x

x3. Contour map of dam site. x

x4. Contour map of temporary storage area.

-7 -

StEPS IN DESIGN OF A ~TRUCTUR~

~ DETENTION FULL FLOW1.

x xObtain photos and all availablecropping information, terraces, soils,etc. Stereo coverage is desirable.

2. Compute run-off curve number. x x

3.

Compute sediment losses. x4. Compute and draw curves for available

storage, both sediment and temporary.x

s. Make hydraulic design. x x6. Draft plan. x x7. Compllte quantities. x x8. Make cost estimate. x x

9.

x xHave plan checked and approved byEngineer if it is above your approvalclassification.

10.

x xSummarize needed information forapplications to Iowa NaturalResources Council for storage andconstruction permits, if applicable.

Note:

For farm ponds with drainage areas less than 30 acres,Step 3 may be omitted and the procedures shown in Table10 may be substituted for Steps 4 and 5.

-8 -

COMPUTATION OF RUNOFF CURVE NUMBER

Outline the drainage area on a soil survey sheet of a published soils report,a reproduction of the soil field sheet, or on the soil map in the conservationplan. Care should be taken to include all of the drainage area.

2.

Delineate the hydrologic soil group areas. See Chapter 2, Engineering FieldManual, for the hydrologic classification of soils. Runoff curve numbers fora given land use in a watershed are weighted forlthe percentage of soils ineach hydrologic class. The Area Engineer may prepare a list of soils withcorresponding hydrologic classification to supplement this handbook, ifdesired.

3. Separate and determine the areas of different land use, conservation practiceand condition or rotation within each hydrologic soil group. Use the totalacreage in each category for determining the runoff curve number.

Multiply individual areas times the respective curve numbers from Figure 1(Form IA-ENG-9), add the products, and divide this total by the total areain the watershed to obtain a weighted runoff curve number. Adjust theweighted curve number downward a maximum of one number or upward a maximumof four numbers to obtain a runoff curve number for design. For example -Weighted Curve Number = 70.9, use 70; Weighted Curve Number = 71.2, use 75.

4.

Rev. 11/81-9-

SEDIMENT COMPUTATIONS

1.

The rotation and land use information used for determination of theRunoff Curve Number will also be used in determining sediment yield.

2. Using the Universal Soil Loss Equation procedure in Section III-A ofthe Field Office Technical Guide, compute the total tons of soil lostper year from the cropland portion of the drainage area.

3. Record acres of other classes of land use such as pasture, woods, lotsroads, etc. The total tons of soil lost from these areas can becomputed using the following minimums as guides:

Pasture, permanent meadow and woods From Section III-A of theField Office Technical Guide -Usually 1 T/Ac/Yr

Buildings and Lots 4 T/Ac/Yr

Roads 5 T/Ac/Yr

4. Use Form Iowa-WS-5 to record soil loss information. The soil lost fromeach segment of cropland. woods. pasture. etc. can be computed andtotaled on this form giving the gross sheet erosion from the drainagearea.

5. Estimate the percent of the gross sheet erosion from the drainage areathat will actually reach the site using Figure 2.

Gross Sheet Erosion Delivered to SiteDrainage AreaAcres Remainder

of StateDeep Loess

Area

70%50%40%

90%70%50%

0 -4040 -100Over 100

Figure 2

-11

When active uncontrolled gullies are present, the gross sheet erosiondelivered to the site should be increased by the following amounts:

35%70%

100%

Moderately active gullyActive gullyExtremely active gully

6. With the total tons of soil lost per year from Form Iowa-WS-5 (Step 4above), and the delivery rate (Step 5 above), enter ~igure 3 andread left to acre feet of sediment storage required below crestelevation and right from the same point to acre feet of sedimentstorage required above crest elevation.

7. The estimate of sediment storage requirements is probably the mostimportant decision made in the design of a detention dam. Make itthoughtfully.

.~:~'\..

12 -