Surface Water Hydrology

Transcription

1 Suface Wate Hydology Intoduction Wate is a key component in the life cycles of all oganisms because of its ability to dissolve many substances (univesal solvent) and as a cooling agent. It is also an integal pat of photosynthesis fo plants and espiation fo animals and micooganisms. The stong self-attaction of wate molecules leads to capillay suction that help plants such as tees to povide life sustaining wate to thei highest banches, as well as helping hold wate in soil fo plant oots to access it. These ae just a few of the many easons why wate is of geat inteest to humans. Hydologic Cycle The study of the distibution and movement of wate on eath is known as hydology. Humans have leaned much about the availability and movement of fesh wate (elatively fee of dissolved impuities), because being depived of it fo only a numbe of days can theaten human life. The neve ending ecycling of wate above and beneath the eath s suface, tansfoming fom liquid to gaseous foms and back, is known as the hydologic cycle. Wate tapped as ice o snow is emoved fom the cyclic movement fom anywhee days to thousands of yeas of yeas until melting fees it again fo movement within the cycle. Figue 1 is a depiction of this cycle. The diving foce Figue 1. Hydologic cycle components. fo all wate movement on eath is the sun. It causes edistibution of wate vapo in the ai by winds and weathe systems. The enegy of sunlight is also esponsible fo a constant supply of wate vapo to the atmosphee though evapoation fom collected wate on the eath s suface such as lakes o the oceans. It is indiectly esponsible fo plant

2 elease of wate vapo due to photosynthetic metabolism in what is known as tanspiation. Due to the difficulty in sepaating these two vapo foming mechanisms, hydologists combine the two and efe to the combination of the two as evapotanspiation. Wate vapo condenses back to liquid in the atmosphee in the fom of small doplets in collections that make up clouds. When clouds collect sufficient moistue that they no longe can stably suspend them, they ae eleased as pecipitation (ain o snow), and move back fom the atmosphee back to the eaths suface. ecipitation collects on the land sufaces in small puddles (depession stoage), seeps into the soil and feeds deepe levels though infiltation, o flows towad lowe elevations as unoff. Runoff collects into ditches o small steams which eventually ae collected into lage steams o ives. Feshwate lakes ae fed by these steams and ives, and emain as feshwate esevois so long as they have an outflow to flush out dissolved solids fed though the lake. The ultimate destination fo steam and ive flow is an ocean body, whee the dissolved solids have no outlet and contibute to seawate salinity. Wate vapo evapoating fom seawate is a significant wate souce feeding back into the cycle. The Rational Equation Civil and envionmental enginees and scientists ae inteested in pedicting the ate of unoff flow fom pecipitation fo seveal easons. The most obvious is to povide sufficient flow capacity in conveyance pipes and channels to avoid flooding. Anothe impotant eason is the need to pedict flows though ives and lakes to descibe the movement of tanspoted contaminants. In many cases elated to conveyance, meely knowing a peak expected flow is sufficient infomation fo decision making. The simplest appoach is to assume a simple linea elation to ainfall intensity (in/h o cm/h). The Rational Equation is a atio elationship descibing the popotionality between the peak collected unoff Q p (ft 3 /s o m 3 /s) fom an aea A, due to a ainfall distibuted unifomly ove the aea at a steady intensity i. It is stated as Qp Ci A whee C is a non-dimensional unoff coefficient ( = 1 fo impevious). It is best applied when the collecting aea is less than 20 aces in size. Application of the equation is most easily undestood by consideing unoff fom an asphalt paking lot whose suface aea is easonable appoximated as a ectangle (see Figue 2). If ainfall begins to fall steadily at a unifom intensity ove the aea, almost all of the wate will un off, with only a small amount seeping into the

3 asphalt o though cacks in it (C = 0.9). Figue 3 is a plot of the collected outflow Q as a function of time assuming ainfall continues at intensity i fo a long time. Once even the most distant point (flow-wise) is contibuting to the outflow Q, it will level off at the peak outflow Q p. The flow path fo this case is shown in Figue 2 by the aow showing the flow path Figue 2. aking lot unoff schematic. fom the most elevated paking spot to a collecting channel in the middle of the lot, followed by the path aow down the middle of the lot. The time the wate takes to flow oveland along this path to the outflow point is called the time of concentation. It should be a popety only of the collecting suface, and not the ainfall intensity. Thee ae a numbe of empiical elations that have been developed to pedict t c, but the most diectly applicable to paved aeas is the FAA equation based on the total maximum tavel length L (ft) and the mean slope S (ft/ft) along that path. Figue 3. Runoff flow appoach to maximum.

4 t c 1/3 1/ C L S whee C is the Rational Eq. constant and t c is in minutes. Example 1. The south Engineeing Building paking lot at SIUC can be appoximated as a ectangle sketched in Figue 4. Assuming C = 0.9, L = 540 ft, w = 150 ft, and the slope is along the length fom elevation 439ft above sea level to 436 ft. Find the time of concentation fo the lot. Solution: S t 439 ft 436ft 540ft (elatively flat) 1/2 1/3 S c 1/3 Figue 4. aking Lot C L min Geneal pactice fo using the ational equation on small aeas such as this is to limit the time of concentation to minutes with an abitay 15 minutes used without efeence to topogaphy. The pactical implication in this instance is that only one inlet would be enough fo this lot, but typically seveal ae used. Intensity-Duation-Fequency Cuves The time of concentation can be used in application of the Rational Eq. to detemine a design stom intensity by use of Intensity-Duation-Fequency (I-D-F) cuves. Detailed examination of pecipitation ecods all the constuction of these cuves. Fo a measued ainfall event of duation D and total collected pecipitation measued as, the aveage stom intensity is

5 i avg D The pobability of occuence of a given intensity event in any given yea is estimated by anking the intensities ove as many yeas of eliable ecods as available, and then elating to a etun peiod of T (ys) as T 1 y An I-D-F constucted in such a fashion fo the egion of ou paking lot example is shown in Figue 5 taken fom an asphalt manufactues manual. The fequency of the I-D-F cuve is actually 1/T, but the etun peiod is commonly Figue 5. I-D-F fo SIUC lot. efeed to as the fequency. Designes of paking lot dainage systems use a etun peiod of 5 o 10 yeas. By choosing one of these cuves and using the time of concentation as the duation, a design stom intensity can be detemined. Example 2. The taking the time of concentation detemined fo the south Engineeing Building paking in Ex. 1, estimate the design peak flow. Remembe that t c = 10.1 min, C = 0.9, L = 540 ft, and w = 150 ft. Solution: Fom Fig.5, with D 47 min & T 10 y i 6.2in / h 2 2 A 540ft 150ft 81,000ft 1.86ac 1ac 43,500ft 3 ac in ft Qp in / h1.86ac h s 3 ac in ft note : cfs h s

6 Hydogaphs Thee ae times when plannes need moe infomation about unoff than just the peak flow. Fo lage collecting suface aeas on the ode of squae miles (watesheds), a time seies of flow values a collection point of the wateshed descibes what is known as a hydogaph. A pedictive tool that can poduce an accuate hydogaphic esponse to exteme ainfall events can help planning to mitigate flooding effects. It is also valuable to know flow vaiations in collecting steams, so that the effects of contaminant loads in unoff wate o dischaged diectly to the steams can be assessed. Figue 6 shows a hydogaph depicting the unoff poduced fom a ainfall event fo the wateshed aea to the left. The outline of the wateshed descibes a divide, which is a cest on the land suface about which pecipitation landing inside the wateshed bounday poduces unoff flowing towad the collection point (noted by the aow), and pecipitation outside the bounday poduces unoff fo some othe wateshed. Wate is collected inside the wateshed into the steams indicated as the intenal solid lines. Divides indicating aeas contibuting to the thee steam banches ae noted as dashed lines inside the wateshed. Figue 6. Hydogaph (ight) fo wateshed (left). One of the most common methods of pedicting unoff hydogaphs is to assume that a chaacteistic unit hydogaph (UH) can be constucted, and hydogaphic esponse to any magnitude ainfall event can be descibed in tems of multiples of this base esponse. The key aspect of the unit hydogaph is that it poduces one unit of unoff (one in o one cm). Thee ae two UH foms descibed by NRCS. Figue 7 is a gaphical desciption of the time vaiation pescibed fo its UHs. The shape moe like that of Fig. 6 is called the natual UH, and the tiangula UH is a piecewise linea appoximation to the natual shape. Both foms will poduce 1 in of unoff if one uses the following equations to calculate Q p and t p.

7 464A 2 Q cfs & t t h p p c tp 3 fo a stom of duation D Using the above equations, the base time length of the tiangula UH must be 2.67 times t p to poduce 1 in of unoff. Befoe a UH can be applied, an estimate of the faction of the ainfall poducing unoff must be made. A simplified NRCS appoach is pesented hee as an attempt to simplify the discussion, but the eade is Q/Qp tc & A in mi NRCS Dimensionless UHs t/tp 2 Natual Tiangula Figue 7. NRCS natual and tiangula UHs. diected to the moe complete discussions available in a vaiety of hydology textbooks. A constant simila to the ational C is the NRCS cuve numbe (CN). It is a function both of the soil popeties and how the land suface is being used. Fo example a gassed aea on swelling clay soils would have a CN = 80, wheeas fo sandy silt type soils that dain moe easily, CN = 40. The cuve numbe can be thought of as a pecentage ating fo unoff fom pecipitation. If the aveage intensity i avg of a stom of duation D is iavg D CN then Qavg iavg A 100 Whee is the total (ain gauge collected) pecipitation. This desciption can also be thought of as unoff being caused by excess pecipitation, whee it is calculated as excess = (CN/100).

8 Example 3. Teating swelling clay soil undelying the paking lot of Ex. 2, estimate the unoff flow hydogaph pio to the gassed aea being paved ove, fo a stom of intensity 3 in/h ove 40 minutes. Assume t c inceases to 60 min. Solution: Take CN 80 D i c excess avg t 60 min 8min in 2 h 100 h 3 1.6in 1.6in 40 min 0.04in / min ove D( 8min) 0.04 in excess (8 min) min 0.32 in So the 40 minute stom is 5 applications of 8 min UHs Q (cfs) cfs cfs cfs time (min) Figue 8. Outflow hydogaph. Fo each UH Q p 1.86 ac ac / mi 2 / 3h cfs Each 8 min peak 0.32(0.675cfs) 40 min Q 0.675cfs cfs complete time histoy by speadsheet, plotted above.