GEL 446: Applied Environmental Geology

Transcription

1 GE 446: ppled Envronmental Geology Watershed Delneaton and Geomorphology Watershed Geomorphology Watersheds are fundamental geospatal unts that provde a physal and oneptual framewor wdely used by sentsts, natural resoure managers and poly maers. The geomorphologal attrbutes of watersheds often provde valuable nsght nto ther hydrologal behavor and an be used to develop desgn hydrolog models. The purpose of ths exerse s to demonstrate how some of the most mportant and most often used geomorphologal varables are measured and used by hydrogeologsts. The upper ttle Chazy Rver watershed n the northwestern Champlan Valley wll be used as an example.. ength Measurements Watershed ength ( w ): Watershed length s measured along the ourse of the prnpal stream from the basn outlet to the dvde (Fgure ). Gray (96) (Table ) demonstrated that for small basns watershed length orrelates wth basn area..4 for w n lometers and and n m w Fgure. Watershed map showng ommon watershed length measurements. p.

2 Several other authors have developed smlar relatonshps (Table ). Table. Empral relatonshps between basn area (m ) and hannel length (m) (from Sngh, 989). w w w Ha (957): Vrgna and Maryland Brown (973): ustrala ee, et al. (97): Indana Empral relatonshps of ths type should be used autously. Gray s (96) relatonshp an be rewrtten as; w Note that as nreases the rato w dereases whh ndates that basns tend to beome more elongate as ther sze nreases (Sngh, 989). Channel ength ( ): Channel length s measured along the manstream from the basn outlet to ts soure (Fgure ). Channel length to entrod of basn ( a ): The hannel length to the entrod of the basn s used n several models for predtng pea flows. The dstane a s the dstane measured along the manstream to a pont opposte the entrod (or enter of area). The basn entrod may be found by uttng a sale mage of the watershed on paper by drawng vertal lnes from or 3 ponts loated along the watershed dvde. The nterseton of these ponts should dentfy the entrod. For best results the ponts should be spaed 90 o apart (f ponts are used) or 60 o apart (f 3 ponts are used). The dstane to the entrod of the basn may also be estmated from a graph of umulatve area versus dstane. Gray (96, 96) found that a s approxmately half the watershed length; a w Channel length between 0% and 85% ponts ( 0-85 ). Watershed Relef Watershed (or basn) relef (H w ): Watershed Relef refers to the dfferene n elevaton between the basn outlet and the hghest pont on the dranage dvde. Relef, however, an be defned between any two ponts n the watershed so areful defnton of the relef parameter beng used s mportant. Two ommon relef parameters are the relef rato and relatve relef. The relef rato s defned as; H R h b p.

3 where H s the basn relef and b s the basn length. Relatve relef s gven by; R p H p where p s the watershed permeter. 3. Slope Parameters Watershed Slope: S ΔE where Δ E s the dfferene n elevaton measured along the watershed length dvded by the watershed length. Note that ths elevaton dfferene s usually not equal to the maxmum relef. Channel Slope: S ΔE where E s the dfferene n elevaton measured along the man hannel between the outlet and the upper end of the man hannel dvded by the hannel length. ( ) Channel Slope Index (weghted average hannel slope): S n where n s the number of stream segments nto whh the stream s dvded and K s gven by; n Δ e l where Δ and l are the elevaton dfferene and length of eah segment. e p. 3

4 4. Hypsometr Curve The hypsometr urve s a measure of the relatonshp between elevaton and area wthn a basn (Fgure ). Sngle-value ndes of the hypsometr urve are; Hypsometr Integral: the area under the hypsometr urve. It s a measure of the basn s stage n the eroson yle. Profle Fator: the rato of the maxmum devaton of the hypsometr urve from a straght lne between the ponts (0,) and (,0) to the length of that lne. 5. Watershed Shape Shape Fator: ( ) 0. 3 l a Fgure. Method of hypsograph analyss (after Strahler, 964 and Sngh, 989). where and a (m) are the watershed length and length to entrod, respetvely. Crularty Rato: F P ( 4π) 0. 5 where P and are the basn permeter (ft) and area (ft ), respetvely. nother measure of rularty s gven by; R o where 0 s the area of a rle wth the same permeter as the basn. NOTE: R F p. 4

5 Elongaton Rato: The rato of the dameter of a rle (D) wth the same area as the watershed to the watershed length. The rato equals.0 for a rular watershed. R e m π 0.5 f Gray s (96) relatonshp between watershed length and area,.4 (Gray, 96), s substtuted nto the above equaton, then Re s nversely related to basn area; R e Form Fator: The form fator s defned as the rato of watershed area to the square of the basn length: w R f b Dranage Networs Dranage Densty D where s the total length of stream hannels n the basn. Constant of Channel Mantenane (CCM) CCM D p. 5

6 7. Stream Orders and Stream Magntude Stream order s a stream networ lassfaton based upon the relatve sze and poston of stream hannel segments wthn the dranage networ. The method urrently used was orgnally devsed by Horton (945) and later modfed by Strahler (957). In ths sheme, frst-order streams are defned as headwater (soure) streams or streams wthout trbutares. Seond-order streams are formed by the junton of two frst-order streams, thrd-order streams by the junton of two seond- order stream, and so on. The junton of a stream of lower order does not affet the order of the larger-order stream. The Horton- Strahler method of stream orderng s depted n Fgure 3 for a 4th order basn. Fgure 3. Comparson of Horton-Strahler stream order and Shreve stream magntude. p. 6

7 8. Horton s aws of Dranage Composton aw of Stream Numbers The aw of Stream Numbers relates the number of streams of any order () to the bfuraton raton (R b ) and the prnpal order () n the basn. N R b where s the hghest order stream, s the th order stream and R b s the bfuraton rato. The bfuraton rato s the average rato of the number of stream streams of a gven order to the number of streams of the next hghest order The rato an be determned by rearrangng the equaton above and solvng for R b ; ln R b [( ln N )( ) ] ( ) or by averagng the stream number ratos for eah set of stream orders; R b N N + N N 3 N +... N ( ) aw of Stream engths The aw of Stream engths relates the average length of stream of a gven order to the Stream ength Rato and the average length of the frst order streams. r where s the average length of the th order stream, s the average length of the frst order streams and r s the length rato. e the bfuraton rato, the ength Rato s average length of streams of any order to the average length of the next lower order. The ength Rato an be determned by; [(ln ln )( )] ln r [( ) ] or by averagng the average stream length ratos for eah set of stream orders; p. 7

8 ( ) r. aw of Stream reas: The aw of Stream verages relates the mean trbutary area of streams of order to the average area of frst order basns and the stream area rato: r where s the average area of the th order basn, s the average area of the frst-order basns and r a s the area rato (average area of basns of any order to the average area of the next lower order). ] ) [( )] )( ln [(ln ln r a or by averagng the average stream area ratos for eah set of stream orders; ( ) r a p. 8

9 EXERCISE. Delneate the watershed for one of the followng upper ttle Chazy Rver watersheds; Cold Broo at Chasm ae Robnson Broo at Chasm ae Farrell Broo at the Cty of Plattsburgh Flume Tray Broo at Slosson Road. Fgure 4 llustrates the relatonshp between topograph ontours and the loaton of the surfae-water dranage dvde. Determne the basn and subbasn areas (n m ) usng a polar planmeter. Instrutons for the proper use of the planmeter wll be provded n lass. Fgure 4. Relatonshp between topograph ontours and dranage dvde p. 9

10 . Usng rvew software and the ttle Chazy Rver GIS database provded to you determne and tabulate the followng geomorphologal parameters for the basn; Watershed ength Watershed rea Watershed Permeter Shape Fator Channel ength to Basn Centrod Crularty Rato (F and R ) Relef Rato Elongaton Rato Relatve Relef Form Fator Watershed Slope Dranage Densty Channel Slope Bfuraton Rato 3. Determne the proporton of bedro, land over, and surfal deposts usng rvew tools demonstrated n lass and the watershed shape fles provded. 4. (Optonal) Construt a hypsometr profle for the basn and determne the hypsometr ntegral. Dsusson Questons. Brefly desrbe the omposton and morphology of your watershed wth respet to the ro and sedment types pand morphologal varables that were alulated.. Comment upon the advantages and dsadvantages of usng morphologal varables as ndes of the hydrolog harater of the basns. What are possble soures of error for these analyses? p. 0