Sediment Responses During Storm Events in Small Forested Watersheds

SEDIMENT RESPONSES DURING STORM EVENTS I N SMALL FORESTED WATERSHEDS W.A.

RIEGER and L.J. OLIVE

Department o f Geography Royal M i l i t a r y C o l l e g e ACT A u s t r a l i a

ABSTRACT Measurements o f s u s p e n d e d s e d i m e n t c o n c e n t r a t i o n and d i s c h a r g e d u r i n g storm e v e n t s a r e examined t o d e t e r m i n e t h e possible p a t t e r n s i n response of sediment t o flow i n f i v e s m a l l f o r e s t e d w a t e r s h e d s . The e x a m i n a t i o n o f s e d i m e n t r e s p o n s e i s c a r r i e d o u t i n t w o c o n t e x t s : ( a ) The r e s p o n s e o f s u s p e n d e d s e d i m e n t t o t o t a l d i s c h a r g e ( b a s e f l o w and q u i c k f l o w or s t o r m f l o w ) , o r i n t h e framework commonly u s e d f o r sediment p r e d i c t i o n modelling. ( b ) The r e s p o n s e o f s u s p e n d e d s e d i m e n t t o q u i c k f l o w , where q u i c k f l o w i s p o s t u l a t e d as a p o s s i b l e mechanism o f s e d i m e n t d e l i v e r y t o t h e channel. I n b o t h c o n t e x t s , h y s t e r e s i s d i a g r a m s are f i r s t u s e d t o d e t e r m i n e t h e b r o a d p a t t e r n s between s u s p e n d e d s e d i m e n t c o n c e n t r a t i o n and f l o w R e s u l t s i n d i c a t e t h a t seven d i f f e r e n t r e s p o n s e i n t h e time domain. Spectral analysis is then t y p e s are o p e r a t i n g i n t h e w a t e r s h e d s . used on t h e s t o r m e v e n t d a t a i n an a t t e m p t t o i s o l a t e p o s s i b l e f a c t o r s which may be c a u s i n g t h e d i f f e r e n t r e s p o n s e types. The temporal and s p a t i a l v a r i a t i o n s found t o be o p e r a t i n g i n t h e w a t e r s h e d s have i m p o r t a n t i m p l i c a t i o n s f o r b o t h t h e d e s i g n o f m o n i t o r i n g n e t w o r k s and t h e a s s o c i a t e d water s a m p l i n g t e c h n i q u e s ; and f o r t h e commonly u s e d l i n e a r p r e d i c t i v e methods of e s t i m a t i n g sediment l o a d s .

1.

INTRODUCTION

Suspended s e d i m e n t c o n c e n t r a t i o n s (mg 1-1) i n s t r e a m c h a n n e l s h a v e been used by r e s e a r c h e r s a s m e a s u r e s o f r a t e s o f e r o s i o n and s o i l During s t o r m e v e n t s i n a b a s i n , complex l o s s from d r a i n a g e b a s i n s . e r o s i o n a l p r o c e s s e s o c c u r over p a r t s o f t h e s l o p e s of t h e b a s i n and t h e r e s u l t a n t o f t h e s e processes i s s e d i m e n t d e l i v e r y t o t h e c h a n n e l ( W a l l i n g , 1 9 8 3 ) . By m o n i t o r i n g b o t h d i s c h a r g e (m3sec-l) and suspended s e d i m e n t c o n c e n t r a t i o n s a t a p o i n t i n t h e c h a n n e l , a measure o f s e d i m e n t d e l i v e r y c a n be o b t a i n e d f o r t h e c o r r e s p o n d i n g w a t e r s h e d area.

491 While such d a t a can be used f o r bulk e s t i m a t e s of e r o s i o n , t h e y can a l s o be used f o r t h e determination of t h e behaviour of suspended sediment c o n c e n t r a t i o n s , o r sediment responses, d u r i n g storm events. Once t h e responses a r e known and f u l l y understood, they can form t h e b a s i s f o r water q u a l i t y r e s e a r c h on suspended sediment. Networks can be designed s o a l l important a s p e c t s of sediment response a r e monitored, o r p o s s i b l e p r e d i c t i o n models developed based on t h e known response p a t t e r n s . I n t h e p a s t , however, t h e bulk of r e s e a r c h on sediment response has been hampered by t h e r e l a t i v e l y s i m p l i s t i c models of suspended sediment behaviour. The models a r e based on a simple l i n e a r r e l a t i o n s h i p between d i s c h a r g e and suspended sediment and w e r e o r i g i n a l l y developed f o r t h e p r e d i c t i o n of suspended sediment i n t h e form of a r a t i n g curve. The curve t a k e s t h e form:

c = a g b ,

(1)

where C i s suspended sediment c o n c e n t r a t i o n , Q i s discharge, a and b a r e c o n s t a n t s f o r a p a r t i c u l a r watershed. The curves a r e estimated from a sample of f i e l d d a t a c o n s i s t i n g of a wide range of discharges and t h e corresponding c o n c e n t r a t i o n s , u s i n g l e a s t squares r e g r e s s i o n on t h e l o g a r i t h m i c a l l y transformed d a t a . Though r a t i n g curves are convenient t o use, t h e i r simple l i n e a r framework g i v e s l i t t l e i n d i c a t i o n of t h e dynamic behaviour of t h e r e l a t i o n s h i p between c o n c e n t r a t i o n and discharge. Slope e r o s i o n , and thus sediment d e l i v e r y , i s a storm event based phenomenon with important temporal v a r i a t i o n s occuring throughout t h e storm. To use a simple l i n e a r model t o d e s c r i b e t h e behaviour of sediment d e l i v e r y ignores t h e s e q u e n t i a l n a t u r e of t h e v a r i a b l e s C and Q, and t h e f i x e d c o e f f i c i e n t s of t h e r a t i n g curve do not allow f o r p o s s i b l e v a r i a t i o n s i n t h e response of suspended sediment c o n c e n t r a t i o n a t d i f f e r e n t s c a l e s o r l e v e l s of d i s c h a r g e .

I n t h e following d i s c u s s i o n , t h e s e two important a s p e c t s of suspended sediment response a r e examined f o r d a t a obtained d u r i n g storm events. The s e q u e n t i a l n a t u r e of sediment response t o both t o t a l d i s c h a r g e and t o quickflow (stormflow) i s considered i n terms of h y s t e r e s i s diagrams which g i v e an i n d i c a t i o n of t h e behaviour of t h e v a r i a b l e s i n t h e time domain. P o s s i b l e s c a l e v a r i a t i o n s between suspended sediment and d i s c h a r g e are then considered by a t r a n s f e r t o t h e frequency domain, o r v i a s p e c t r a l a n a l y s i s . Conclusions a r e then drawn concerning t h e i m p l i c a t i o n s of varying response p a t t e r n s t o water q u a l i t y monitoring.

492 2.

STUDY AREA AND DATA

Data f o r t h e a n a l y s i s a r e from f i v e small f o r e s t e d watersheds i n south e a s t e r n New South Wales, with a l l f i v e streams flowing i n t o t h e Wallagaraugh River. The watersheds a r e a d j a c e n t t o one another and vary i n s i z e from 76ha t o 225ha. Within each watershed, a 140° V-notch weir has been i n s t a l l e d , s t a g e was measured w i t h a Rimco Sumner Mark I1 f l o a t r e c o r d e r , and water samples were taken with a Gamet automatic water sampler. The water samplers take p o i n t samples and a r e f l o a t switch o p e r a t e d . Concentration of suspended sediment f o r each water sample was determined using a membrane f i l t r a t i o n technique. The p r e s e n t a n a l y s i s i s based on t h e p e r i o d J u l y 1977 t o June 1979. During t h i s p e r i o d , 20 storm e v e n t s , with r a i n f a l l s varying from 12 t o 339mm were sampled i n t h e f i v e watersheds. Due t o equipment f a i l u r e and i n some i n s t a n c e s , l i t t l e o r no sediment response i n t h e watersheds during storm e v e n t s , a t o t a l of 39 i n d i v i d u a l storm hydrographs have been analysed. Both t h e d i s c h a r g e s e r i e s and t h e suspended sediment c o n c e n t r a t i o n s e r i e s f o r t h e s e 39 storm events were i n t e r p o l a t e d t o one hour time i n t e r v a l s before t h e a n a l y s i s was carried out. 3.

TIME DOMAIN ANALYSIS

3.1 Suspended Sediment Response t o Discharge To o b t a i n some idea of t h e broad behaviour between suspended sediment c o n c e n t r a t i o n and t o t a l d i s c h a r g e (baseflow and q u i c k f l o w ) , These p l o t s a r e simply a s c a t t e r h y s t e r e s i s p l o t s w e r e used. diagram f o r t h e two v a r i a b l e s , with t h e s z q u e n t i a l a s p e c t of t h e d a t a denoted by j o i n i n g a d j a c e n t p o i n t s i n t h e t i m e series with a straight line. Before t h e p l o t s were c o n s t r u c t e d , a t h r e e p o i n t moving average f i l t e r was a p p l i e d t o t h e two s e r i e s , t h u s removing high frequency components s o only broad p a t t e r n s of sediment response were i n d i c a t e d with t h e h y s t e r e s i s diagrams. The h y s t e r e s i s p l o t s f o r t h e d a t a from t h e 39 storm e v e n t s i n d i c a t e d t h a t seven d i f f e r e n t suspended sediment response types were o p e r a t i n g i n t h e watersheds ( F i g u r e 1 ) . A f u l l d e s c r i p t i o n of each of t h e s e responses i s given by Olive and Rieger ( 1 9 8 5 ) , and a b r i e f summary of t h e i r c h a r a c t e r i s t i c s i s a s follows: ( a ) S i n g l e rise storm events with sediment l e a d , o r a simple clockwise loop, occurred i n f o u r of t h e watersheds and made up 23% of the storm events analysed ( b ) S i n g l e rise storm events with sediment l a g , o r a c o u n t e r clockwise loop, occurred i n t h r e e watersheds and made up 8% of t o t a l events

493 SINGLE RISE

DD (a) Sediment lead

(b) Sediment lag

9

(c) Sediment-discharge correlation

MULTIPLE RISE

(d) Sediment lead

(e) Sediment lag

(f) Sediment lead-lag

(g) No recognisable pattern

DISCHARGE (cumecs)

F i g u r e 1:

)7

Sediment response t y p e s f o r storm e v e n t s .

494 ( c ) S i n g l e r i s e with t h e sediment and d i s c h a r g e peaks i n phase occurred i n two watershed s and made up 5% of t h e t o t a l e v e n t s ( d ) Multiple r i s e with sediment l e a d and sediment d e p l e t i o n occurred i n two watershed s and made up 10% of t o t a l e v e n t s ( e ) Multiple r i s e with sediment l a g and sediment d e p l e t i o n occurred i n two watershed s and made up 5% of t o t a l e v e n t s ( f ) Multiple rise with sediment l e a d and l a g occurred i n t h r e e watershed s and made up 8% of t o t a l e v e n t s ( g ) Responses i n which t h e r e was no i d e n t i f i a b l e p a t t e r n occurred i n f o u r watershed s and made up 41% of t h e t o t a l e v e n t s . These r e s u l t s a r e made more complicat ed when i n d i v i d u a l watershed s and storm e v e n t s a r e taken i n t o c o n s i d e r a t i o n . Over t h e two year study p e r i o d , p a r t i c u l a r streams demonstra ted up t o f i v e d i f f e r e n t response types during storm e v e n t s and no stream showed t h e same There were a l s o major d i f f e r e n c e s i n response type f o r a l l storms. s f o r p a r t i c u l a r storm watershed e v i f e h t among type response The dominance of response types with no i d e n t i f i a b l e events. p a t t e r n ( 4 1 % of t h e t o t a l storms a n a l y s e d ) p o i n t s f u r t h e r t o t h e complexit y of t h e behaviour of suspended sediment c o n c e n t r a t i o n s . 3 . 2 Suspended Sediment Response t o Quickflow

The examinati on of t h e response of suspended sediment t o quickflow , o r storm flow, i s i n t h e realm of p r o c e s s s t u d i e s i n t h a t t h e complexit y of t h e sediment d e l i v e r y problem i s reduced t o a form where quickflow i s p o s t u l a t e d a s a p o s s i b l e d e l i v e r y mechanism (Walling and Webb, 1982). Since t h e source of sediment i s w i t h i n a watershed and t h e sediment i s t r a n s p o r t e d t o t h e channel by s u r f a c e runoff and i n t e r f l o w , quickflow has appeal a s a p o s s i b l e d e l i v e r y mechanism. Baseflow s e p a r a t i o n was c a r r i e d out on t h e d i s c h a r g e series f o r storm e v e n t s using a r e c u r s i v e d i g i t a l f i l t e r proposed by Lyne and Hollich ( 1 9 7 9 ) . The f i l t e r i n g process t a k e s t h e form: Qq(t)

=

a

*

Qs(t-1)

+

(l+a)/2

*

[Q(t)-Q(t-l)],

(2)

where Q q ( t ) i s t h e quickflow component Q ( t ) is t o t a l streamflow a is the f i l t e r i n g coefficient. The values used f o r t h e c o e f f i c i e n t , a , were i n t h e range 0.7 t o 0.9 and phase c h a r a c t e r i s t i c s of t h e s e r i e s were p r e s e r v e d with a two pass forward and backward a p p l i c a t i o n of t h e f i l t e r . H y s t e r e s i s p l o t s were generated f o r suspended sediment c o n c e n t r a t i o n The and quickflow i n t h e same f a s h i o n a s o u t l i n e d i n S e c t i o n 3.1. follows: s a e r a s t n e v e storm 39 e h t r results fo

( a ) Those responses which showed i d e n t i f i a b l e p a t t e r n s , o r types ( a ) through ( f ) i n Section 3.1, showed s i m i l a r behaviour i n t h e sediment responses t o quickflow ( b ) Approximately h a l f of t h e storm events which showed no i d e n t i f i a b l e p a t t e r n i n t h e sediment - d i s c h a r g e p l o t s had i d e n t i f i a b l e p a t t e r n s i n sediment response t o quickflow. Thus i n using quickflow a s a p o s s i b l e sediment d e l i v e r y mechanism, sediment responses a r e almost a s complex a s were t h e responses t o t o t a l discharge. I n d i v i d u a l watersheds d i s p l a y e d d i f f e r i n g responses throughout t h e study p e r i o d and t h e r e was v a r i a t i o n i n response among t h e f i v e watersheds t o p a r t i c u l a r storm e v e n t s . The major d i f f e r e n c e with response t o quickflow was a r e d u c t i o n of t h e responses showing no i d e n t i f i a b l e p a t t e r n t o 2 1 % of t h e t o t a l storms studied. 4.

FREQUENCY DOMAIN ANALYSIS

The o b j e c t of t r a n s f e r r i n g t o the frequency domain v i a s p e c t r a l a n a l y s i s , i s t o i s o l a t e t h e important frequency components, o r s c a l e s , which might be p a r t of the p r o c e s s o p e r a t i n g between discharge and suspended sediment c o n c e n t r a t i o n . Frequency domain a n a l y s i s a l s o has t h e p o s s i b l e b e n e f i t of reducing t h e complexity of sediment response a s i n d i c a t e d by t h e time domain a n a l y s i s i n t h e above d i s c u s s i o n . The maximum Entropy Method ( M E M ) was used f o r t h e c a l c u l a t i o n of s p e c t r a l e s t i m a t e s . MEM was p r e f e r r e d t o t h e more t r a d i t i o n a l methods of s p e c t r a l e s t i m a t i o n ( J e n k i n s and Watts, 1968) because it can be used f o r s h o r t time s e r i e s (Ulrych and Bishop, 1975), which i s t h e case f o r storm event d a t a . Most of t h e 39 e v e n t s , used h e r e , have fewer than 1 0 0 o b s e r v a t i o n s i n t h e d i s c h a r g e and suspended sediment c o n c e n t r a t i o n series. A s t h e frequency domain a n a l y s i s i s t o be used f o r t h e study of t h e process o p e r a t i n g between d i s c h a r g e and c o n c e n t r a t i o n , t h e d a t a f o r each of t h e s e v a r i a b l e s w e r e combined t o g i v e a new v a r i a b l e which was r e p r e s e n t a t i v e of t h a t p r o c e s s . This new v a r i a b l e was generated by c a l c u l a t i n g t h e s l o p e between c o n c e n t r a t i o n and d i s c h a r g e f o r adjacent p o i n t s i n t i m e , g i v i n g a new series which measures t h e changing s e q u e n t i a l r e l a t i o n s h i p between t h e two v a r i a b l e s . I n e f f e c t , t h e new s e r i e s r e p r e s e n t s t h e s l o p e angle of a d j a c e n t p o i n t s along t h e h y s t e r e s i s p l o t f o r a storm event.

The g e n e r a l i s e d r e s u l t s of t h e s p e c t r a l a n a l y s i s of t h e 39 storm events a r e shown i n Figure 2 which can be summarised a s follows: ( a ) A l l storm e v e n t s a r e dominated by a low frequency component which i s l i k e l y a m a n i f e s t a t i o n of t h e broad loop i n t h e hysteresis plots.

-

.........

0

.1

identifiable sediment responses unidentifiable sediment responses

.2 FREQUENCY

Figure 2:

.3 (CYCLES HR-’)

Generalised spectra f o r storm event data.

.4

.5

497 ( b ) A l l storm events showed a high frequency component i n t h e range 0.37 t o 0.50 cycles h r - l . This frequency l i k e l y corresponds t o t h e f l u c t u a t i o n s about t h e broad h y s t e r e s i s loop, and was removed by t h e moving average f i l t e r a p p l i e d t o t h e d a t a when t h e h y s t e r e s i s p l o t s were generated i n Section 3 . ( c ) Those storm e v e n t s which showed no i d e n t i f i a b l e p a t t e r n i n t h e time domain were d i f f e r e n t i a t e d from t h e e v e n t s with recognisable p a t t e r n s by a mid-erequency component i n t h e range 0.20-0.33 cycles hr-l. 5.

CONCLUSIONS

The examination of t h e behaviour of suspend sediment c o n c e n t r a t i o n s i n stream channels d u r i n g storm e v e n t s has i n d i c a t e d a complex set of responses which have some important i m p l i c a t i o n s f o r water q u a l i t y monitoring and p r e d i c t i o n models. I n t h e c a s e of monitoring, both t h e temporal and t h e s p a t i a l v a r i a t i o n i n sediment Since response have t o be considered i n any sampling network. sediment response v a r i e s between watersheds f o r i n d i v i d u a l storms, sampling a p a r t i c u l a r watershed and e x t r a p o l a t i n g t h e r e s u l t s t o a d j a c e n t watersheds may prove t o be i n v a l i d . The a c t u a l sampling r a t e a t p o i n t s w i t h i n t h e network a l s o needs c a r e f u l c o n s i d e r a t i o n . For example, r a t e - o f - r i s e water samplers assume t h a t sediment response t o d i s c h a r g e is a l i n e a r f u n c t i o n , and would not c o r r e c t l y sample f o r responses with sediment l e a d s or l a g s . I n t h e case of p r e d i c t i o n models f o r suspended sediment, it i s obvious t h a t t h e commonly used r a t i n g curve does not manifest t h e t r u e behaviour between c o n c e n t r a t i o n and discharge. The l i n e a r response assumed by t h e r a t i n g methodology occurred i n only 5% of t h e storm e v e n t s s t u d i e d . However, t h e s p e c t r a of t h e storm event s e r i e s showed some s i m u l a r i t y i n dominant frequency components f o r t h e storm e v e n t s which had such v a r i e d responses i n t h e time domain. A l l events contained a high and a low frequency component, and events which showed no i d e n t i f i a b l e p a t t e r n i n t h e t i m e domain contained an a d d i t i o n a l middle frequency component.

ACKNOWLEDGEMENTS

The a u t h o r s would l i k e t o thank t h e A u s t r a l i a n Research Grants Scheme and t h e F o r e s t r y Commission of N e w South Wales f o r t h e i r assistance.

498

REFERENCES Jenkins, G.M. and Watts, D.G., 1968: Spectral Analysis and its Applications, Prentice-Hall, Englewood Cliffs, N.J. Lyne, V.D. and Hollick, M., 1979: Stochastic time-varying rainfall-runoff modelling, Hydrology and Water Resources Symposium, 89-92, Institution of Engineers, Australia. Olive, L.J. and Rieger, W.A., 1985: Variation in suspended sediment concentration during storms in five small catchments in south east New South Wales. Australian Geographical Studies, 23, 38-51. Ulrych, T.J. and Bishop, T.N., 1975: Maximum entropy spectral analysis and autoregressive decomposition, Reviews of Geophysics and Space Physics, 13, 1 83-20 0.

Walling, D.E. and Webb, B.W., 1982: Sediment availability and the prediction of storm period sediment yields, IAHS Publication No.137, 327-340. Walling, D.E. , 1983: The sediment delivery problem, Journal of Hydrology, 69, 209-237.