small stream water supply capacity

8/6/2019 Small Stream Water Supply Capacity

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Examples: Water Supply Capacity of a SmallStream, North Fork Yachats River

Location: The Yachats River is located in the Mid Coast of Oregon. The river flowssouthwest.

Description of Project: Assume that three water rights permit applications are

under review for a combined 7 cfs to be issued to (1) the city of Yachats and (2)two private landowners. Assume that the city applicant is to receive 3 cfs whilethe two landowner applicants are to receive 2 cfs each. Further, assume thatthere is a minimum 2 cfs instream flow requirement for fish habitat and passage.

The diversion for all three applicants is to be placed on the North Fork of theYachats River two kilometers north of the North Fork/Mainstem confluence. TheYachats is an ungaged river.

Objectives: To determine during what portion of the year the diversion of 7 cfscan be achieved while maintaining an instream flow of at least 2 cfs.

Plan of Action: Compare nearby gaged systems to choose a data set to performthe hydrologic analyses for this project. Determine how much flow is passed perseason using a monthly analysis. Develop a flow duration curve based on meandaily discharge data to know what percent of time flow will be at or below 2 cfs(no water available for diversion). Ascertain the number of months that havedays with flow at or below 2 cfs using mean monthly discharge averaged over the

entire period of record. Construct flow duration curves on a daily basis for criticalmonths to determine the percent of time flow can and cannot be diverted duringeach critical period.

As you read through this example, you may wish to follow along with the analysissteps in an MS Excel file. You candownload the data file by clicking here.

Step 1: Delineate the watershed and determine the drainage

area of the North Fork Yachats River using topographic maps.

Topographic maps used in this example are published by the U.S. Geologic Survey(USGS) and were obtained for use here from the online map source,

topozone.com. The watershed was delineated using identifiable ridges wherepossible. For the areas of the watershed where the ridgetops are not clearlyidentified, headwaters of streams in adjacent watersheds were used to determinethe correct location of the drainage divide. For this example, drainage area wascalculated using an overlay grid of boxes with known area. The boxes werecounted and the number of boxes within the watershed multiplied the area of anindividual box. The watershed drainage area was found to be 10 mi2.

Step 2: Review the preliminary estimations page to determine
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a rough estimate of streamflow and precipitation values in thisregion.

The preliminary estimations for the North Coast will appear as follows:

MID COAST BASIN

Range for Annual Precipitation

70 160 in

1778 4064 mm

Annual discharge per unit area

5.17 ft3/mi2

0.057 m3/km2

Monthly flow as a percent of Annual Flow (%)

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEPT

623

37

38

35

2617

9 5 2 1 2

Note that the values for monthly flow as a percentage of annual flow do not addup to 100%. This is due to overlapping of drainage areas (i.e., nesting) of someor all gages used to calculate the percentages.

The preliminary estimations for this basin show that you can expect the annualrainfall to be approximately 70 inches. The lower value of the range was chosenbecause the study site is at an elevation that would not be influenced byorographic lift experienced higher in the mountain range. Had the study site beenlocated at or near the headwaters of the stream, the upper value would have beenchosen. With a drainage area of 10 mi2, the annual discharge is expected to beapproximately 52 cfs. The flow regime for a typical water year is anticipated tofollow the general trend for west of the Cascades, namely, low flow during thesummer months, with peak flows occurring during the winter months.Furthermore, the most days of inundation are expected to occur during themonths of December, January, and February.

Step 3: Identify and list the characteristics of all nearby gages.

A review of the Table of USGS gages for the mid coast of Oregon shows three

gages that may be appropriate choices for data in which to perform the hydrologicanalysis of North Fork Yachats River: North Fork Alsea, Five Rivers, and BigCreek.

Statio

n

Numb

er

Station Name Fro

m

To N

u

m

be

r

Dr

ain

ag

e

Ar

Al

tit

ud

e

(ft

Me

an

An

nua

l

Discha

rge/Un

it Area

(cfs/sq
http://streamflow.engr.oregonstate.edu/links/gages_mainx.htmhttp://streamflow.engr.oregonstate.edu/links/gages_mainx.htm


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of

Wat

er

Y

ears

ea

(sq

mi)

ab

ov

ese

a

le

vel)

Dis

cha

rge

(cfs) mi)

14306100

N FK ALSEAR

AT ALSEA,

OREG.

1957-

10-

01

1989-

09-

30 31 63

27

2

27

6 4.38

143

06400

FIVE RIVERS

NR FISHER,OREG.

196

0-

10-01

199

0-

09-30 30

114

130

539 4.73

143069

00

BIG CREEK

NR

ROOSEVELTBEACH,

OREG.

197

2-10-

01

199

1-09-

30 19

11.

9

14

1

90.

5 7.60

It is advantageous to select a gaged river that has a long period of record. Thetable shows that the North Fork Alsea, Five Rivers, and Big Creek have 31, 30,

and 19 years in the period of record, respectively. Thus far, North Fork Alsea andFive Rivers are potential candidates for sources of data. However, drainage areais another vital factor to consider. If the drainage area of the gaged stream issimilar to that of the study stream, then the uncertainty involved with scaling upor down can be avoided. Observing the table, North Fork Alsea, Five Rivers, and

Big Creek have 63, 114, and 12 mi2 of drainage area, respectively. If all threerivers exhibit the same flow regime, then the Big Creek gage is the one to choosesince it has such a comparable drainage area to that for the North Fork of theYachats (10 mi2). The problem with the shorter period of record can be solvedthrough a validation procedure using normalized data from the long period ofrecord of North Fork Alsea. Nevertheless, it is wise to perform further analysescomparing the data sets for more than one gage before making a final decision.

Step 4: Perform simple statistics on data to choose the most

appropriate gage.

The general pattern of streamflow for each river can be viewed by plotting mean

monthly discharge versus time for the period of record. To compare the patternsof more than one system, normalization of the data is done by dividing each valueby the drainage area of the respective system. As can be seen in the figure, thepatterns for all three systems are similar. The drastic differences seen between

Big Creek and the other two systems could be explained by basin geology(bedrock types), infiltration rates, or differences in annual precipitation due totopographical influence. The figure also emphasizes the substantial difference inthe size of the drainage area of Big Creek versus the two rivers; nonetheless, theimportant item to note is the similar pattern.
http://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306100&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306100&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306400&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306400&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306400&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306900&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306900&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306900&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306100&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306100&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306400&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306400&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306400&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306900&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306900&agency_cd=USGShttp://waterdata.usgs.gov/or/nwis/nwisman/?site_no=14306900&agency_cd=USGS


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Plotting mean monthly discharge normalized by mean annual discharge for atypical water year, shows that the pattern still holds, but in addition to thisvalidation, it also suggests slight dissimilarities between the three sets of data. Itappears that Big Creek and Five Rivers peak early while only Big Creek maintainsa higher base flow during the dry months. The discrepancy could also havedeveloped due to gaps (missing years) in the period of record for one or allgages. However, there are great ramifications for the gage selection processbecause it shows that the two large rivers, even after scaling, cannot accuratelyrepresent the behavior of a small stream. For these reasons the Big Creek gagewill be used for the remainder of the analyses.


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longer period of record.

North Fork Alsea with 31 years in the period of record was chosen for theanalysis. By normalizing the mean annual discharge values for Big Creek andNorth Fork Alsea and plotting discharge/drainage area versus water year, the usercan once again see pattern similarities. Although, the most useful information thisgraph displays is that Big Creek data does not occur in a particularly dry or wet

cycle. The line for the mean annual discharge is straddled by the data for eachsystem. This gives confidence that even with the shorter period of record; thelikelihood of overestimation or underestimation is small.

Step 6: Develop a visual representation of the dischargeassociated with a typical year

Using the mean monthly discharge/mean annual discharge for the period of recordplotted vs. month for a typical water year was compiled. From the figure, whichfollows, it is concluded that if diversion will be allowed, it will most likely be

allowed during the period of December through March. April through October allpass 8 % or less of the mean annual discharge, therefore it is this period whendiversion could become interrupted by small flows and the need to maintain the 2cfs instream flow.


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Step 7: Construct a flow duration curve to determine thepercent of time water is available/unavailable for diversion.

The 2 cfs minimum instream water rights and 9 cfs adjusted combined water right(should applications be granted) were labeled on the graph of the flow durationcurve. It is noted that approximately 12 % of the time the flow in the river wouldbe too low to provide full supply to the diversion. According to the figure tofollow, minimum instream flow of 2 cfs is likely to be available 100 % of the timefor an average year. However, the demand of 9 cfs will only be available 88 % ofthe time. This implies that full supply will be available during a portion of thewater year while a reduced supply will be available during other times of the

year. To decide the specific dates when full and partial supplies are likely to beavailable, a monthly analysis and daily analysis must be conducted.


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Step 8: Perform monthly analysis to search for critical monthswhere flow is insufficient for full capacity.

Monthly averages for the period of record were used to search for critical months.From the following figures, it is noted that the period of July through October canbe considered critical months. Caution must be taken when allowing for diversionduring this period. Therefore, further analysis on a daily time step must beconducted to determine how often if ever full diversion can take place.


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Step 9: Construct flow duration curves based on mean dailyvalues for each of the critical months to determine whatpercent of time during each month flow cannot be fullydiverted.

The results of the flow duration curves for each of the critical months weretabulated. The amount of time flow is insufficient is equal to 100 minus theexceedence frequency of 9 cfs. It must be determined if diversion will bepermitted at all during months such as September when flow is insufficient 45% ofthe month.


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Critical month Amount of time flow is insufficient (%)

July 6

August 36

September 45

October 22

Step 10: Perform flood peak analysis to design protection of

intake

For an intake operating on such a small stream, the diverters may choose not todesign protection against the 100-year design flood usually used in such projects.Instead, assume that a flood with a return period of 20 years will be used as apreliminary estimate for the design flow. Later, during more detailed design, acost/risk analysis can be made for a range of design flows and return periods. Fornow, a Log Pearson type III flood frequency analysis regionalized for the midcoast of Oregon was conducted using the methods outlined in the AnalysisTechniques section of this web site was completed. According to the floodfrequency curve, the 20-year flood for North Fork Yachats is 2,000 cfs.

Remember, this value was calculated using instantaneous peak values, making it
http://streamflow.engr.oregonstate.edu/analysis/floodfreq/index.htmhttp://streamflow.engr.oregonstate.edu/analysis/floodfreq/index.htmhttp://streamflow.engr.oregonstate.edu/analysis/floodfreq/index.htmhttp://streamflow.engr.oregonstate.edu/analysis/floodfreq/index.htm


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a brief event and perhaps a conservative estimate.

Step 11: Summary of analysis and conclusions

The analysis can be summarized as follows:

North Fork Yachats watershed is 10 mi2.

Big Creek is the gaged system that is used for the analyses.

12% of the time flow in the river is expected to be too low to providefull supply to the city and landowner.

Base flow is approximately 5 cfs, 3 cfs above the 2 cfs minimuminstream requirement.

Full supply cannot be met at all times, however, reduced supply ispossible during critical months.

The intake must have protection constructed for a design flood of2,000 cfs.


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