ESTIMATING COSTS OF OFF-STREAM IRRIGATION STORAGE RESERVOIRS

1

ESTIMATING COSTS
OF

OFF-STREAM IRRIGATION STORAGE RESERVOIRS
Bulletin 647 September 2001 Alabama Agricultural Experiment Station Luther Waters, Jr., Director Auburn University Auburn Alabama 36849
Printed in cooperation with the Alabama Cooperative Extension System (Alabama A&M University and Auburn University)

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CONTENTS
Page Introduction ........................................................................................................... 3 Methods ................................................................................................................. 3 Results & Discussion ............................................................................................ 4 Conclusions ........................................................................................................... 6

The authors wish to express appreciation to the following for their financial and technical support for this research project: COTTON INCORPORATED ALABAMA COTTON COMMISSION TENNESSEE VALLEY AUTHORITY

Information contained herein is available to all persons regardless of race, color, sex, or national origin. Issued in furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, and other related acts, in cooperation with the U.S. Department of Agriculture. The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) offers educational programs, materials, and equal opportunity employment to all people without regard to race, color, national origin, religion, sex, age, veteran status, or disability.

ESTIMATING COSTS OF OFF-STREAM IRRIGATION STORAGE RESERVOIRS

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Estimating Costs of Off-Stream Irrigation Storage Reservoirs
Larry M. Curtis, Marshall M. Nelson, Perry L. Oakes1

INTRODUCTION
Lack of adequate water sources has limited the ability of many Alabama farmers to adopt irrigation. In many parts of the state, ground water sources are either inadequate or impractical to develop for irrigation. Surface water sources such as streams often do not have sufficient flow during the growing season in Alabama to provide enough water for irrigation. Water harvesting, the collection and storage of surface water during the off-season, when rainfall and stream flows are high, can make irrigation possible in areas where direct pumping from streams, lakes or wells is not feasible. The most common type of water harvesting is construction of dams and reservoirs directly on streams. However, on-stream impoundments are often impractical, either because streams are not located on drainage basins suitable for reservoir construction, or because stream flows are so high in fall, winter or spring that dams of the size and complexity that would be needed are not economically feasible. Another water harvesting alternative is to build offstream storage reservoirs, pumping water from the nearby stream to fill the reservoir during the high stream flow period. Water is then pumped from this storage reservoir during the growing season to irrigate crops. Off-stream irrigation storage has potential to greatly expand agricultural irrigation capacity in Alabama. This practice has proven feasible under the right conditions, and has been put to use already at some sites in Alabama. The feasibility of off-stream water storage for irrigation depends on many factors, including seasonal stream
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flow rates, availability of suitable acreage for a reservoir, distance to crops to be irrigated, and the cost versus benefits of this type of irrigation for the crops to be grown. Reservoir construction is a key cost factor to be considered. This investigation was undertaken to examine the possibility of estimating reservoir construction costs in Alabama under various conditions likely to be encountered. Reservoir construction costs will vary, depending on terrain characteristics, geology of the site, local labor and equipment costs, and the storage capacity needed. For a given reservoir capacity, the surface slope and conformance of the terrain determine the amount of excavation and/or earth fill required, which usually is the largest cost factor where on-site soil has adequate natural sealing capability. The geology of the site determines whether some type of liner (other than compaction of on-site soil) will be needed for adequate water retention in the reservoir. Liners, of either the soil amendment type, such as Bentonite or soda ash, or of plastic or rubber, can increase costs significantly.

METHODS
Since earth moving is a primary cost factor for any constructed reservoir, the investigators first developed a spreadsheet analysis (using Microsoft Excel) to determine the amounts of excavation, borrow and fill required for any given storage volume. This model includes variables for relevant factors in producing the required excavation, borrow and fill volume, such as land slope, depth vs surface area, levee width and side slope, freeboard, and shrinkage

Curtis is Professor and Extension Engineer, Biosystems Engineering Department, Auburn University; Nelson is State Design Engineer, NRCS; and Oakes is State Conservation Engineer, NRCS.

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of borrow excavation. RESULTS AND DISCUSSION Figure 1 is a spreadsheet printout which displays the variables, with example calculations for a 300-acre-foot hill- Table 1 shows examples of the primary result of the invesside reservoir and a 100-acre-foot levee reservoir. Figure 2 tigation in terms of derived data, estimated costs for irrigais a definition sketch from the program showing the vari- tion storage reservoirs of various sizes, with and without ous dimensions and slopes used in the calculations. The liners. For 100-acre-foot capacity, these estimates range from spreadsheet program is designed for computing excavation about $35,000 for a reservoir needing no liner to about and fill requirements for reservoirs that are square in plan $282,000 for a reservoir with an EPDM rubber liner. Table and located on either flat or sloping land surfaces, where 2 shows representative costs determined for the various types of liners that might be used. Computations of estimates in levees will be required on either three or four sides. The second step in the estimating process was deter- Table 1 also include cost of establishing vegetation to stamining costs for excavation and grading equipment and la- bilize levees, determined to be about $350 per acre, and the bor, establishing vegetation to stabilize FIGURE 1. EXAMPLE CALCULATIONS SHOWING SPREADSHEET levees, and installing various liner maPROGRAM VARIABLES terials if needed. Liner materials conReservoir type Hillside Levee sidered as relevant for Alabama condiReqd. Storage (ac-ft) 300 100 tions include traditional soil amendLand slope (LS, %) 1.0 ment materials, soda ash and Bentonite, Depth (d1, ft) 9.5 and several types of plastic or rubber Depth (d2, ft)1 4.0 6 Top width (TW, ft) 12 12 liners. Side slope (Z) 2.5 2.5 The specific liner applications Profile factor (PF)2 2 1 studied were soda ash at 0.15 pounds per Freeboard (FB, ft) 2 2 square foot, 200-mesh Bentonite at 3 Length for levee reservoir (L1, ft) 735 pounds per square foot in two layers, or Fill shrinkage factor (%) 20 20 1.5 pounds per square foot in one layer, L1 974 735 30-mil PVC plastic, 40-mil HDPE plasL2 994 765 tic, and 45-mil EPDM rubber. A surL3 926 735 vey of representative suppliers was conL4 1004 775 ducted to determine prevailing costs for TW2 42 these items, including installation. A1 624.625 The different types of liner would A2 162 256 Volume of fill (VF1, cu yd) 57775 be appropriate for different sites in AlaVolume of fill (Vf2, cu yd) 24378 29848 bama, depending on the particular geTotal fill (VT, cu yd) 82153 29848 ology of the given site. It should be Volume excavated (Vexc, cu yd) 77402 35817 noted that other suitable liner materiVolume excavated (Vexc, ac-ft) 48 22 als are available. Those listed are some Reservoir surface area (ac) 22.67 13.43 of the more commonly used and the Storage (S1, ac-ft) 197 costs of other liner types should be Storage (S2, ac-ft) 89 77 within the range of those presented in Total storage (ST, ac-ft)3 286 100 this report. Depth of below ground excav. for levee reservoir (d, ft)4 1.8 The cost of earth moving was Borrow excav. reqd. (cu yd)5 21182 35817 combined with liner costs to derive an Borrow excav. reqd. (ac-ft) 13 22 estimated total construction cost for any Depth of excav. below L3 (ft) 0.7 desired water storage volume. Costs of 1 other site work such as clearing, and of When a hillside reservoir has a levee on only three sides, input d2 = 0 and adjust d1 until the desired storage is obtained. surveying and professional engineering 2 Profile factor (PF) for a levee reservoir is always 1. consultation, were not included. Also not 3 Adjust inputs until required storage is obtained. included were estimates of costs for as4 Depth required for total borrow excavation. 5 sociated equipment such as pumps and Borrow excavation for the hillside reservoir is that in excess of the excavation within the reservoir (Vexc). If land slope is very small, decrease d1 and increase d2 to keep water lines. Costs for these items are surface area reasonable. The borrow excavation required has been increased by the fill highly variable and may or may not be shrinkage factor. required on any specific site.
OUTPUTS INPUTS

ESTIMATING COSTS OF OFF-STREAM IRRIGATION STORAGE RESERVOIRS FIGURE 2. DEFINITION SKETCH FOR RESERVOIR DIMENSION VARIABLES
L4

5

Where the geology of the site indicates that more expensive liners would be called for, the cost differentials are much greater. For exL2 ample, Table 1 shows a 100-acrefoot reservoir with 7.7 acres surface L1 area would cost about $179,000 if lined with EPDM rubber, while a TW same-size reservoir with 10.7 acres FB surface area would cost about $282,000, well over $100,000 more. Z d2 The table shows estimated 1 S2 TW 2 costs for example reservoirs with all S1 liner types investigated except the d1 Existing ground profile 30-mil PVC plastic. This type liner L3 was omitted because when costs of d3 an additional soil cover needed to prevent degradation of the plastic by Average ground slope LS sunlight are included, total costs apS1 = storage between lines L1 and L3 FB = freeboard depth proach or exceed those for heavierS2 = storage between lines L1 and L2 Ls = average ground slope of the site in percent gauge and more durable HDPE plasNotes: When the existing ground profile is not a straight line, the Profile Factor indicates the tic. ratio of excavation below the existing ground line compared to the volume between line L3 and Generally, liners are considthe average ground slope. For example, a PF of 3 would indicate the actual excavation below the existing ground profile is 1/3 of that below the average ground slope. ered only when natural sealing For a levee reservoir, assume a flat ground surface. LS is then zero and d1 is zero also. Lines methods do not appear to offer adL1 and L3 become the same. Input an assumed value for L1 and d2 until the desired storage equate water retention. Table 1 is obtained. shows that while earth-moving is The Fill Shrinkage factor is a number indicating the percent increase in borrow required to produce the required fill. For example, a factor of 20% would mean 120 cu yds of borrow would generally the largest cost factor for be required to produce 100 cu yds of compacted fill. any reservoir needing no sealing beyond compaction of on-site soil, prevailing cost of earth-moving work in Alabama reservoir even the least expensive liner materials are likely to exceed construction, determined to be about $1.00 per cubic yard. earth-moving costs. Note that increasing (or decreasing) fuel prices will have a In addition to cost of liner materials and installation, direct impact on earth moving costs. other factors such as water quality and availability, ease of In general, Table 1 shows that reservoirs needing no installation, and durability of materials may be relevant in liner usually can be constructed at least cost by minimizing choosing a liner. For example, HDPE plastic requires availthe amount of fill needed. Usually, this means holding res- ability of expert installers, while EPDM rubber does not. ervoir depth to a minimum. For example, the table shows Plastic or rubber liners (types not requiring soil cover) may that a 100-acre-foot reservoir with a 10-foot depth will have provide better water quality. Water quality parameters monia surface area of 10.7 acres and require 33,000 cubic yards tored over a four-year period have shown consistently high of fill, while a same-volume reservoir with 15-foot depth water quality at a 140 acre-foot research and demonstrawill have a surface area of 7.7 acres and require 41,000 tion reservoir constructed at the Tennessee Valley Research cubic yards of fill. and Experiment Station at Belle Mina, Alabama, and lined As can be seen in Table 1, the program shows that with 40 mil HDPE. where liners will be required, minimizing the surface area The spreadsheet analysis developed in this investigaof the reservoir generally results in least cost. For the cheap- tion also produces useful curves, examples of which are est type of liner, soda ash, the cost difference is rather small shown in Figures 3-8. These figures show, for various storbut still significant. For example, Table 1 shows that a 100- age capacities of hillside reservoirs, the relationships beacre-foot reservoir with a surface area of 10.7 acres would tween depth of water and fill, and depth of water and surcost about $97,000, while a same-capacity reservoir with a face area. surface area of 7.7 acres would cost about $88,000.

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TABLE 1. ESTIMATED COSTS FOR IRRIGATION STORAGE RESERVOIRS
Storage Depth (ac-ft) (ft) 100 100 200 200 300 300
1

Area Fill (ac) (cu yd) 10.7 7.7 21 15 30 21.5 33,000 41,000 50,000 66,000 66,000 88,000

w/o liner $35,000 $350 $43,000 $430 $52,000 $260 $68,000 $340 $69,000 230 $91,000 $303

Estimated Costs (per reservoir/per ac-ft) w/bentonite1 w/bentonite2 w/HDPE w/EPDM $239,000 $2,390 $189,000 $1,890 $403,000 $2,015 $354,000 $1,770 $574,000 $1,913 $412,000 $1,373 $137,000 $1,370 $116,000 $1,160 $203,000 $1,015 $211,000 $1,055 $354,000 $1,180 $208,000 $693 $221,000 $2,210 $177,000 $1,770 $418,000 $2,090 $330,000 $1,650 $592,000 $1,973 $466,000 $1,553 $282,000 $2,820 $179,000 $1,790 $538,000 $2,690 $414,000 $2,070 $762,000 $2,540 $587,000 $1,957

w/soda ash3 $97,000 $970 $88,000 $880 $175,000 $875 $158,000 $790 $246,000 $820 $216,000 $720

10 15 10 15 10 15

Treatment rate 3 lbs/sq ft placed in two layers; 2 Treatment rate 1.5 lbs/sq ft placed in one layer; 3 Treatment rate 0.15 lbs/sq ft in one layer. NOTE: Costs in this table include costs for earth work, liner and installation, and vegetation. Clearing, embankment foundation treatment, irrigation piping and pumps, engineering services or other costs are not included. Other miscellaneous costs associated with construction of the reservoir may range from 5 to 10 percent or more depending on the complexity of the site. Unit costs used in the computations are as follows: Earth fill $1.00/cuyd Vegetation $350 /ac Bentonite $145 /tn Soda Ash $0.16 /lb HDPE $0.40 / sq ft (Installed cost) EPDM $0.53 / sq ft (Installed cost)

CONCLUSIONS

TABLE 2. TYPICAL RESERVOIR LINER COSTS

This report presents a procedure useful for Product Material Installation Total cost estimating off-stream water storage reservoir HDPE, 40 mil $0.25/sq ft $0.15/sq ft $0.40/sq ft construction costs in Alabama under variPVC, 30 mil $0.25/sq ft $0.10/sq ft $0.35/sq ft $0.38/sq ft $0.15/sq ft $0.53/sq ft ous conditions likely to be encountered. EPDM, 45 mil $0.085/lb (50lb bag) – – Exact costs of particular installations will Bentonite, 200 mesh $0.08/lb (100lb bag) – – of course vary to some extent from costs $0.0725/lb (per ton) – – predicted by any generalized estimating proSoda Ash $0.16/lb – – cedure. However, the estimates produced by Notes: Costs listed are as estimated in December 2000. PVC liners require a soil cover. the procedure outlined in this report should This cost is not included here. PVC and EPDM may be installed with local crews under prove useful to anyone considering such an the guidance of an experienced supervisor. Costs for installation supervision average undertaking and wanting to determine the $0.05/sq ft. Per ton cost for Bentonite assumes very large quantities. least cost approach suitable to their site, conditions and needs. This estimation procedure should also fied NRCS and Extension personnel assisting farmers and be useful to funding agencies and private or governmental others interested in the possibilities of off-stream irrigation agencies interested in irrigation as a planning tool for agri- storage reservoirs. The program should be used only by incultural development in Alabama. Further, the procedures dividuals familiar with the engineering principles involved presented here should be applicable to other states or re- in reservoir construction. gions with appropriate adjustments to suit conditions prePersonnel using the program should be aware of the vailing in other regions. need for a geological study of any site considered for an The spreadsheet program developed in this investiga- off-stream storage reservoir, in order to determine whether tion allows a competent user to quickly explore various sce- a liner might be needed. Qualified NRCS personnel or pronarios in reservoir construction and compare construction fessional engineering firms can provide the best possible cost estimates by changing various dimension and land con- evaluation of a site’s water-holding capacity, and a recomtour inputs. The program will be available for use by quali- mendation as to the type of liner needed, if any.

ESTIMATING COSTS OF OFF-STREAM IRRIGATION STORAGE RESERVOIRS FIGURE 3. DEPTH OF WATER VS FILL FOR 100 AC-FT HILLSIDE RESERVOIR
60 50 Fill (cu yd X 1000) 40 30 20 10 0

7

0

5

10 15 20 Depth of water (ft)

25

30

FOR

FIGURE 4. DEPTH VS SURFACE AREA 100 AC-FT HILLSIDE RESERVOIR

18 16 14

Surface area (ac)

12 10 8 6 4 2 0 0 5 10 15 20 Depth of water (ft) 25 30

FIGURE 5. DEPTH OF WATER VS FILL FOR 200 AC-FT HILLSIDE RESERVOIR
Fill (cu yd X 1000)

120 100 80 60 40 20 0 0 5 10 15 20 25 Depth of water (ft) 30 35 40

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FIGURE 6. DEPTH VS SURFACE AREA FOR 200 AC-FT HILLSIDE RESERVOIR
Surface area (ac)

30 25 20 15 10 5 0 0

ALABAMA AGRICULTURAL EXPERIMENT STATION

5

10

15 20 25 Depth of water (ft)

30

35

40

FIGURE 7. DEPTH OF WATER VS FILL FOR 300 AC-FT HILLSIDE RESERVOIR
Fill (cu yd X 1000)

200 180 160 140 120 100 80 60 40 20 0 0 10 20 30 40 Depth of water (ft) 50 60

FIGURE 8. DEPTH VS SURFACE AREA FOR 300 AC-FT HILLSIDE RESERVOIR

40 35 Surface area (ac) 30 25 20 15 10 5 0 0 10 20 30 40 Depth of water (ft) 50 60

Estimating Costs of Off-Stream Irrigation Storage Reservoirs
Bulletin 647, September 2001, Alabama Agricultural Experiment Station, Auburn University