An evaluation of small scale dispersed technologies for managing urban stormwater quality

AN EVALUATION OF SMALL SCALE DISPERSED TECHNOLOGIES FOR MANAGING

URBAN

STOKMWATER QUALITY

David H. Moreau, P r o f e s s o r and

Thomas F. L a P o i n t e , Research A s s o c i a t e Department of C i t y and Regional P l a n n i n g

U n i v e r s i t y of N o r t h C a r o l i n a a t Chapel H i l l , N , C , 27514

The work upon which t h i s p u b l i c a t i o n i s based was s u p p o r t e d i n p a r t by funds p r o v i d e d by t h e O f f i c e of Water Research and Technology,

U.S. Department of t h e I n t e r i o r , Washington, D . C . , t h r o u g h t h e Water Resources Research I n s t i t u t e of The U n i v e r s i t y of N o r t h C a r o l i n a a s a u t h o r i z e d by t h e Water R e s e a r c h and Development Act of 1978, and by t h e D i v i s i o n of Environmental Management, North C a r o l i n a Department o f N a t u r a l Resources and Community Development, However, c o n t e n t s o f t h i s p u b l i c a t i o n do n o t n e c e s s a r i l y r e f l e c t t h e views and p o l i c i e s o f any of t h e s e a g e n c i e s , n o r does mention of t r a d e names o r commercial p r o d u c t s c o n s t i t u t e t h e i r endorsement sr recommendation f o r u s e by t h e U.S. o r North C a r o l i n a Governments.

P r o j e c t No. 50019

ACKNOWLEDGMENTS

The a u t h o r s would l i k e t o e x p r e s s t h e i r a p p r e c i a t i o n t o a number of p e r s o n s who have c o n t r i b u t e d t o t h i s s t u d y a s i t was i n i t i a t e d , as i t was conducted, and a s t h e f i n a l r e p o r t was p r e p a r e d , David H. Howells,

P r o f e s s o r E m e r i t u s and former d i r e c t o r of t h e Water Resources Research I n s t i t u t e (WRRI) of t h e U n i v e r s i t y o f North C a r o l i n a , was most h e l p f u l i n i n i t i a t i n g t h e p r o j e c t and a r r a n g i n g t h e f i n a n c i a l s u p p o r t . D r . N e i l Grigg, A s s i s t a n t S e c r e t a r y of t h e N o r t h C a r o l i n a Department of N a t u r a l Resources and Community Development (on l e a v e from h i s p o s t a s D i r e c t o r of WRRI), h a s c o n t i n u e d t o l e n d s t r o n g s u p p o r t f o r t h e p r o j e c t .

P r o f e s s o r s Maynard M. Hufschmidt and D a n i e l A. Okun of UNC a t Chapel H i l l made s i g n i f i c a n t c o n t r i b u t i o n s by s e r v i n g a s members of D r . L a P o i n t e ' s d i s s e r t a t i o n committee.

S e v e r a l members of t h e s t a f f o f t h e N o r t h C a r o l i n a D i v i s i o n o f

Environmental Management gave v a l u a b l e a s s i s t a n c e i n s u g g e s t i n g d i r e c t i o n s t h a t would a s s i s t t h e D i v i s i o n i n f o r m u l a t i n g s t r a t e g i e s f o r improving t h e q u a l i t y o f urban s t o r m w a t e r r u n o f f i n t h e s t a t e . S p e c i a l t h a n k s a r e g i v e n t o M r . Mike McGee and M r . Dean Cunningham.

ABSTRACT

T h i s s t u d y i s a n e x p l o r a t i o n of s e l e c t e d s t r a t e g i e s f o r improving t h e q u a l i t y o f stormwater r u n o f f from urban a r e a s . S t r a t e g i e s of

p a r t i c u l a r i n t e r e s t a r e t h o s e which a r e based o n s m a l l s c a l e t e c h n o l o g i e s , w i d e l y deployed o v e r u r b a n s p a c e , and a d a p t e d t o v a r i a t i o n s i n t h e l o a d i n g

i n t e n s i t i e s and p h y s i c a l c h a r a c t e r i s t i c s of p o l l u t a n t s o n v a r i o u s t y p e s of u r b a n l a n d u s e s . The e x p e r i m e n t a l approach i s t h a t o f d i g i t a l computer s i m u l a t i o n on w a t e r s h e d s and d r a i n a g e s y s t e m s h a v i n g h y d r o l o g i c , h y d r a u l i c , and p o l l u t a n t l o a d i n g c h a r a c t e r i s t i c s of w a t e r s h e d s i n t h e S o u t h e a s t e r n U n i t e d S t a t e s . The mix of l a n d u s e t y p e s i s r e p r e s e n t a t i v e of t h o s e found

i n u r b a n a r e a s t h r o u g h o u t t h e U n i t e d S t a t e s .

TABLE OF CONTENTS

Page

. . .

ACKNOWLEDGMENTS iii

ABSTRACT

. . .

v

. . .

LIST OF FIGURES i x

. . .

LIST OF TABLES

x i

. . .

SUMMARY AND CONCLUSIONS x i i i

. . .

INTRODUCTION 1

. . .

P u r p o s e 1

. . .

Background 1

. . .

Scope 2

. . .

Approach 4

. . .

The Report 5

. . .

THE MODEL AND EXPERIMENTS

7

. . .

The Model 7 Watersheds

. . .

7

T y p e s a n d A m o u n t s

. . .

7

. . .

S i z e s and S p a t i a l P a t t e r n s 8

. . .

Runoff Mechanisms 11

. . .

Hydrographs 11

. . .

P o l l u t a n t T r a n s p o r t 1 3

. . .

E x p e r i m e n t a l C o n d i t i o n s 13

. . .

Storm E v e n t s 1 4

. . .

P o l l u t a n t Loads 1 5

. . .

P a r a m e t e r s and Drainage System Geometry 16

. . .

The T r e a t m e n t s 19

. . .

C o n v e n t i o n a l Drainage 20 Reduced Runoff D r a i n a g e

. . .

20

G r a s s S w a l e s

. . .

21

S t r e e t Sweeping

. . .

21

S t o r a g e and S e t t l i n g B a s i n s

. . .

22

The Experiments

. . .

25

. . .

RESULTS

. . . .

Load D i s t r i b u t i o n by Land Use

. . .

Pre-S torm Loads

. . .

Loads i n Runoff

. . . .

E f f e c t s o f S t r e e t Sweeping

Hydrographs and T r a n s p o r t P r o p e r t i e s

. . .

G r a s s Swales

. . .

S t o r a g e B a s i n s

. . .

S e t t l i n g B a s i n s

Page

.

IV

A

COST-EFFECTIVE COMPARISON OF ALTERNATIVES

. . .

49

. . .

Cost and Cost-Effectiveness of Individual Technologies 50

Erosion Control at Construction Sites

. . .

50 On-Site Retention

. . .

50 Street Sweeping

. . .

51 Settling and Storage Basins

. . .

5 1 Cost-Effectiveness of Alternative Comprehensive 53 Strategies

. . .

53

.

V SUMMARY AND CONCLUSIONS

. . .

57

REFERENCES

.

LIST OF FIGURES

Page

. . .

Figure 1,

Unit Catchments for Experimental Watersheds

9

Figure 2.

Larger Watersheds as Aggregates of Unit

. . .

Catchment Areas

12

Figure

3.

Dimensions of Street Gutters and Grassed Swales

. . .

19

Figure

4.

Removal Efficiencies as a Function of Sweeping

. . .

I n t e n s i t y . . .

24

Figure 5.

Removal Efficiencies for Given Particle Sizes

. . .

SUMMARY AND CONCLUSIONS

I n t h i s r e p o r t t h e e f f e c t i v e n e s s of s m a l l s c a l e d i s p e r s e d

t e c h n o l o g i e s f o r improving t h e q u a l i t y of u r b a n s t o r m w a t e r r u n o f f have been examined. The a n a l y s i s i s based on computer s i m u l a t i o n e x p e r i m e n t s on h y p o t h e t i c a l u r b a n w a t e r s h e d s u s i n g h y d r o l o g i c c h a r a c t e r i s t i c s ,

h y d r a u l i c p a r a m e t e r s , s t o r m e v e n t s , and p o l l u t a n t l o a d i n g r a t e s r e p o r t e d i n t h e l i t e r a t u r e f o r a r e a s i n t h e S o u t h e a s t e r n United S t a t e s . Cost e s t i m a t e s a v a i l a b l e from d i v e r s e s o u r c e s have a l s o been compiled i n a n e f f o r t t o make r e l a t i v e comparisons of c o s t - e f f e c t i v e n e s s ,

Although f i n d i n g s from t h i s s t u d y s h o u l d n o t b e r e g a r d e d a s b e i n g d e f i n i t i v e f o r s i t e - s p e c i f i c a p p l i c a t i o n s , t h e r e s u l t s do p r o v i d e s i g n i - f i c a n t i n s i g h t s i n t o b a s i c p r i n c i p l e s f o r d e s i g n i n g management s y s t e m s f o r c o n t r o l l i n g t h e q u a l i t y o f u r b a n stormwater r u n o f f , Major f i n d i n g s a r e d i s c u s s e d i n t h e f o l l o w i n g p a r a g r a p h s .

1. P o l l u t a n t l o a d i n g i n t e n s i t i e s and p a r t i c l e s i z e d i s t r i b u t i o n a r e q u i t e v a r i a b l e a c r o s s t y p e s of u r b a n l a n d u s e ; a p p r o x i m a t e l y 40 p e r c e n t of pre-storm l o a d s d e p o s i t e d on urban s u r f a c e s w e r e found i n commercial a r e a s ; t h o s e a r e a s a c c o u n t f o r l e s s t h a n 1 0 p e r c e n t of u r b a n l a n d . Another 20 p e r c e n t of t h e t o t a l pre-storm l o a d was found i n h i g h d e n s i t y r e s i d e n t i a l a r e a s ; t h o s e a r e a s a l s o account f o r l e s s t h a n 1 0 p e r c e n t of u r b a n l a n d .

2, For s t o r m s w i t h a f r e q u e n c y a s h i g h a s once e v e r y two y e a r s , p o l l u t a n t l o a d s t r a n s p o r t e d i n t h e r u n o f f a r e a p p r o x i m a t e l y t h e same as pre-storm l o a d s ; f o r h i g h e r f r e q u e n c y s t o r m s (more s p e c i f i c a l l y , two-month s t o r m s ) t h e r e l a t i v e importance of commercial a r e a s i s g r e a t l y m a g n i f i e d , and, t o a l e s s e r e x t e n t , h i g h d e n s i t y r e s i d e n t i a l a r e a s a r e of g r e a t e r r e l a t i v e

importance. T h i s e f f e c t can b e e x p l a i n e d by v a r i a t i o n s i n p a r t i c l e s i z e d i s t r i b u t i o n , I n commercial and h i g h d e n s i t y r e s i d e n t i a l a r e a s l a r g e p o r t i o n s of t h e t o t a l l o a d of s o l i d s a r e s m a l l s i z e p a r t i c l e s which a r e r e a d i l y suspended and t r a n s - p o r t e d by s m a l l s t o r m s .

O n - s i t e d r a i n a g e c o n t r o l s can s i g n i f i c a n t l y r e d u c e b o t h t h e t o t a l volume and peak r a t e s of r u n o f f f o r high-frequency (two- y e a r and two-month) s t o r m s . Those r e d u c t i o n s a r e of l i t t l e b e n e f i t i n c o n t r o l l i n g f l o o d s a s s o c i a t e d w i t h l a r g e low-

f r e q u e n c y s t o r m s , b u t t h e y can b e s i g n i f i c a n t i n r e d u c i n g d e s i g n volumes f o r s e t t l i n g b a s i n s d e s i g n e d t o t r e a t high-frequency s t o r m s .

Where s t r e e t sweeping i s used f o r r e d u c i n g pre-storm l o a d s , vacuum sweepers a p p e a r t o b e p r e f e r a b l e t o m e c h a n i c a l b r u s h sweepers, and t h e y a p p e a r t o b e most e f f i c i e n t when deployed w i t h e q u a l e f f o r t o v e r c o m e r c i a l and h i g h - d e n s i t y l a n d u s e s .

Grass s w a l e s a s a n a l t e r n a t i v e t o c u r b and g u t t e r i n low-

d e n s i t y r e s i d e n t i a l a r e a s show c o n s i d e r a b l e promise i n r e d u c i n g t h e l o a d of s o l i d s t r a n s p o r t e d i n t h e r u n o f f .

S e t t l i n g b a s i n s d e s i g n e d f o r o v e r f l o w r a t e s a s s o c i a t e d w i t h t h e peak r u n o f f of two-month s t o r m s can b e e f f e c t i v e i n improving t h e q u a l i t y of r u n o f f ; t h e r e a r e d e c r e a s i n g r e t u r n s t o s c a l e a s d e s i g n f l o w s a r e i n c r e a s e d t o two-year s t o r m s , a r e s u l t which can b e a t t r i b u t e d t o t h e s o - c a l l e d " f i r s t f l u s h " e f f e c t .

S t o r a g e b a s i n s d e s i g n e d f o r f l o o d c o n t r o l puznoses c a n b e e f f e c t i v e i n r e d u c i n g p o l l u t a n t l o a d s i f t h e b a s i n s a r e w e l l - p o s i t i o n e d i n r e l a t i o n s h i p t o h i g h i n t e n s i t y l o a d s i n

commercial and r e s i d e n t i a l a r e a s .

A c o s t - e f f e c t i v e r a n k i n g of a l t e r n a t i v e s would b e s u b j e c t t o c o n s i d e r a b l e u n c e r t a i n t y u s i n g t h e a v a i l a b l e l i t e r a t u r e on c o s t s . Comparisons made i n t h i s s t u d y i n d i c a t e t h a t :

( a ) a c o n t r o l s t r a t e g y i n which t h e primary t e c h n i q u e i s s t r e e t sweeping i s l i k e l y t o b e i n f e r i o r t o o t h e r a l t e r n a t i v e s ;

(b) a s t r a t e g y which s e e k s a h i g h d e g r e e of l o a d r e d u c t i o n ( g r e a t e r t h a n 60 t o 65 p e r c e n t ) u s i n g a v a i l a b l e t e c h n o l o g i e s is n o t l i k e l y t o b e c o s t - e f f e c t i v e ; and ( c ) t h e most c o s t - e f f e c t i v e a p p r o a c h e s a p p e a r t o b e t h o s e which u s e s e t t l i n g b a s i n s o r vacuum sweeping i n commercial and h i g h d e n s i t y r e s i d e n t i a l a r e a s , g r a s s s w a l e s i n low d e n s i t y r e s i d e n t i a l a r e a s , and e r o s i o n c o n t r o l s a t c o n s t r u c t i o n s i t e s .

The b a s i c p r i n c i p l e which emerges from t h e s e f i n d i n g s i s t h a t

s t r a t e g i e s f o r c o n t r o l l i n g t h e q u a l i t y of urban stormwater r u n o f f s h o u l d e x p l o i t v a r i a t i o n s i n l o a d i n g i n t e n s i t i e s and p a r t i c l e s i z e d i s t r i b u t i o n s , and t h e s t r a t e g y s h o u l d a p p l y a r a n g e of s m a l l s c a l e t e c h n o l o g i e s t o d i f f e r e n t l a n d u s e t y p e s . Uniform r e q u i r e m e n t s a p p l i e d o v e r a l l

of l a n d u s e s a r e i n f e r i o r a p p r o a c h e s , p a r t i c u l a r l y when a p p l i e d o n l y t o new, low-density r e s i d e n t i a l developments.

U n c e r t a i n t i e s a s s o c i a t e d w i t h r e s u l t s of t h e s e experiments s h o u l d b e reduced by e x p e r i m e n t s conducted on r e a l w a t e r s h e d s and by improved c o s t e s t i m a t e s . F i n d i n g s from t h i s s t u d y s u g g e s t t h a t t h e most f r u i t f u l avenues t o p u r s u e i n f i e l d e x p e r i m e n t s would be: ( a ) c o m p a r a t i v e s t u d i e s on commercial a r e a s u s i n g no sweeping, mechanical sweepers, and vacuum sweepers; (b) c o m p a r a t i v e s t u d i e s of g r a s s s w a l e s and c u r b and g u t t e r i n low d e n s i t y r e s i d e n t i a l a r e a s ; and ( c ) t h e e f f e c t i v e n e s s of s e t t l i n g b a s i n s d e s i g n e d f o r h i g h frequency s t o r m s on c o m e r c i a l and h i g h d e n s i t y

CHAPTER I INTRODUCTION

Purpose

The p u r p o s e of t h i s s t u d y i s t o e x p l o r e a s t r a t e g y f o r c o n t r o l l i n g t h e f l o w o f m a t e r i a l s d e p o s i t e d on u r b a n s u r f a c e s t h a t a r e t r a n s p o r t e d by storm- w a t e r r u n o f f and r e s u l t i n t h e d e g r a d a t i o n of t h e q u a l i t y of r e c e i v i n g s t r e a m s , The s t r a t e g y of i n t e r e s t h e r e i s one t h a t i s based on small s c a l e t e c h n o l o g i e s , w i d e l y deployed throughout u r b a n s p a c e , and a d a p t e d t o t h e s p a t i a l d i s t r i b u t i o n and p h y s i c a l p r o p e r t i e s of t h e p o l l u t i o n l o a d a s t h e y a r e determined by t h e v a r i e t y of l a n d u s e s w i t h i n t h e u r b a n a r e a . A s s u c h t h i s s t r a t e g y i s a n a l t e r n a t i v e t o l a r g e - s c a l e c o l l e c t i o n and c e n t r a l i z e d s t o r a g e - t r e a t m e n t schemes u n i v e r s a l l y employed f o r t h e management o f

domestic sewage and s u g g e s t e d by many a s t h e p r o p e r a p p r o a c h f o r c o n t r o l l i n g p o l l u t i o n from stormwater r u n o f f .

Background

Only w i t h i n t h e p a s t f i f t e e n y e a r s have p o l l u t i o n l o a d s c a r r i e d by t h e r u n o f f from s t o r m s o v e r u r b a n a r e a s been f u l l y r e c o g n i z e d a s major c o n t r i - b u t o r s t o t h e d e g r a d a t i o n of w a t e r q u a l i t y . Among t h e e a r l y q u a n t i t a t i v e a s s e s s m e n t s of t h e problem was work by Weibel e t a l . (1964). A l a t e r s t u d y by Whipple (1970) i n d i c a t e d t h a t l e s s t h a n o n e - t h i r d of t h e b i o c h e m i c a l oxygen demand i n s e l e c t e d s t r e a m s i n New J e r s e y c o u l d b e a c c o u n t e d f o r by known d i s c h a r g e s from m u n i c i p a l and i n d u s t r i a l sewage t r e a t m e n t p l a n t s . R e s u l t s of a m o n i t o r i n g s t u d y of u r b a n r u n o f f i n Durham, North C a r o l i n a

(Bryan, 1970) i n d i c a t e d t h a t indeed r u n o f f c o n t r i b u t i o n s d u r i n g s t o r m e v e n t s c o u l d e q u a l o r exceed t h o s e of d o m e s t i c sewage.

Concern f o r t h i s s o u r c e of p o l l u t i o n and t h e need t o i n c o r p o r a t e i t

(b) ( 2 ) (A) "Any p l a n . , . s h a l l i n c l u d e , . . t h e n e c e s s a r y , . . u r b a n s t o r m w a t e r r u n o f f systems."

A d d i t i o n a l l y , t h a t s e c t i o n c a l l e d f o r t h e i n c l u s i o n of measures t o i d e n t i f y and c o n t r o l n o n p o i n t s o u r c e s o f p o l l u t i o n from a g r i c u l t u r a l and f o r e s t a c t i v i t i e s , c o n s t r u c t i o n a c t i v i t i e s , and t h e d i s p o s a l of w a s t e m a t e r i a l s on l a n d s u r f a c e s . The p r i n c i p a l t r a n s p o r t p r o c e s s f o r most p o l l u t a n t s from t h e s e s o u r c e s i s , of c o u r s e , stormwater r u n o f f ,

With implementation of S e c t i o n 208 i n a l l s t a t e s and n e a r l y 200 m e t r o p o l i t a n a r e a s , i n t e r e s t i n t h e g e n e r a l problem of n o n p o i n t p o l l u t i o n c o n t r o l h a s grown. T e c h n o l o g i c a l s o l u t i o n s have been f o c u s e d on two approaches. F i r s t , some have pursued t h e u s e of l a r g e - s c a l e c o l l e c t i o n and c e n t r a l i z e d t r e a t m e n t works w i t h complex p h y s i c a l , c h e m i c a l , and bio- l o g i c a l p r o c e s s e s t h a t have been developed f o r t h e management of d o m e s t i c sewage and i n d u s t r i a l w a s t e w a t e r s , Whipple (1977) r e v i e w s s e v e r a l

s t u d i e s where l a r g e - s c a l e t e c h n o l o g i c a l a l t e r n a t i v e s w e r e d e s i g n e d f o r San F r a n c i s c o , Chicago, and Boston, The second approach h a s been t o a d a p t s o i l e r o s i o n c o n t r o l p r a c t i c e s t h a t have been developed i n a g r i - c u l t u r a l and highway c o n s t r u c t i o n a p p l i c a t i o n s , based on t h e f a c t t h a t much of t h e n o n p o i n t p o l l u t i o n i s e i t h e r s o i l p a r t i c l e s o r s u b s t a n c e s

t h a t a d h e r e t o t h e s u r f a c e s of s o i l p a r t i c l e s . how eve^, l i t t l e r e s e a r c h h a s been devoted t o a c o m p a r a t i v e a n a l y s i s of a l t e r n a t i v e c o n t r o l

s t r a t e g i e s , i n c l u d i n g e s t i m a t e s of t h e i r e f f e c t i v e n e s s and t h e i r r e l a t i v e c o s t s .

Scope

I n t h i s s t u d y a t t e n t i o n i s f o c u s e d on t h e f o r m u l a t i o n and e v a l u a t i o n of a s t r a t e g y t h a t i s p r e d i c a t e d on two b a s i c h y p o t h e s e s :

(1) t h e r e a r e v a r i a t i o n s i n t h e i n t e n s i t y and p h y s i c a l p r o p e r t i e s of s u r f a c e l o a d s among u r b a n l a n d u s e s t h a t c a n b e e x p l o i t e d t o improve t h e c o s t - e f f e c t i v e n e s s of management s t r a t e g i e s ; and

S p e c i f i c i n v e s t i g a t i o n s i n c l u d e d :

(1) e s t i m a t i o n of t h e d i s t r i b u t i o n of t o t a l u r b a n l o a d s by l a n d u s e c a t e g o r i e s ;

(2) e s t i m a t i o n of t h e e f f e c t i v e n e s s of s e v e r a l s m a l l - s c a l e tech- n o l o g i e s on e a c h of s e v e r a l w a t e r s h e d s of d i f f e r e n t s i z e s and l a n d u s e ;

(3) s u r v e y s of a v a i l a b l e c o s t i n f o r m a t i o n f o r t h e a l t e r n a t i v e s ; and

(4)

e s t i m a t i o n of t h e e f f e c t i v e n e s s of s e v e r a l a l t e r n a t i v e composite

s t r a t e g i e s a p p l i e d t o a l l w a t e r s h e d s i n a c i t y o f 100,000 p e r s o n s ,

S m a l l - s c a l e t e c h n o l o g i e s i n c l u d e d i n t h e a n a l y s i s a r e :

(1) l a n d r e d u c t i o n through mechanical broom and vacuum s t r e e t sweeping;

(2) o n - s i t e r e t e n t i o n of stormwater u s i n g r o o f t o p s t o r a g e , porous pavements, and r o o f t o p d i s c h a r g e t o g r a s s e d a r e a s ;

(3) s t o r a g e and s e t t l i n g b a s i n s ; and

The c u r r e n t s t u d y does n o t f u l l y a d d r e s s t h e b r o a d e r t a s k of i n t e g r a t i n g u r b a n stormwater management s t r a t e g i e s t h a t would s i m u l t a - n e o u s l y r e d u c e l o s s e s from f l o o d i n g w i t h i n t h e u r b a n a r e a and t h e i m p a c t s of stormwater r u n o f f on r e c e i v i n g s t r e a m s , I t d o e s , however, i n c l u d e a n e v a l u a t i o n o f t h e w a t e r q u a l i t y management s t r a t e g y w i t h and w i t h o u t t h e u s e of o n - s i t e f l o w r e d u c t i o n t e c h n i q u e s a s d i s c u s s e d above,

Approach

The e x p e r i m e n t a l method used i n t h i s s t u d y was d i g i t a l computer s i m u l a t i o n on s y n t h e t i c w a t e r s h e d s t o which homogenous l a n d u s e s w e r e a s s i g n e d , Four k i n d s of l a n d u s e s were a s s i g n e d t o t h e w a t e r s h e d s ; r e s u l t s were a g g r e g a t e d t o r e p r e s e n t t h e a v e r a g e mix of t h o s e l a n d u s e s i n a U.S. c i t y of 100,000 p e r s o n s . R a i n f a l l , h y d r o l o g i c p r o p e r t i e s o f w a t e r s h e d s , and l o a d i n g r a t e s f o r p o l l u t a n t s were based on c o n d i t i o n s r e p o r t e d i n t h e l i t e r a t u r e f o r c i t i e s i n t h e S o u t h e a s t , p a r t i c u l a r l y r e - s u l t s of f i e l d s t u d i e s c a r r i e d o u t i n t h e 208 s t u d y a r e a i n t h e R e s e a r c h T r i a n g l e r e g i o n o f North C a r o l i n a ,

These c h o i c e s of m e t h o d o l o g i c a l a p p r o a c h e s r e p r e s e n t compromises between c o s t and t i m e r e q u i r e d t o r u n f i e l d e x p e r i m e n t s and t h e l i m i t a - t i o n s of s i m u l a t i o n . For e x p e r i m e n t s on l a r g e - s c a l e nvstems where t h e c o s t s of c o n s t r u c t i n g a l t e r n a t i v e management t e c h n i q u e s a r e l a r g e , d i g i t a l computer s i m u l a t i o n , l i k e l a b o r a t o r y s i m u l a t i o n s u s i n g p h y s i c a l s c a l e models, o f f e r s a much l e s s c o s t l y means of a n a l y s i s , It may p r o v i d e pre-

d i c t i o n s of t h e consequences of a l t e r n a t i v e s t h a t a r e s u f f i c i e n t l y

a c c u r a t e t o p e r m i t v a l i d d i s c r i m i n a t i o n between good a l t e r n a t i v e s and bad o n e s , e s p e c i a l l y when t h e o b j e c t i v e i s t o d e t e r m i n e o n l y c o a r s e d i f f e r - e n c e s among a l t e r n a t i v e s . O f c o u r s e , t h e s c i e n t i f i c v a l i d i t y of p r e - d i c t i o n s based on s i m u l a t i o n a r e l i m i t e d by t h e f i d e l i t y of models t o a c t u a l f i e l d c o n d i t i o n s and p r o c e s s e s . I m p o r t a n t a d d i t i o n a l a d v a n t a g e s of t h e u s e of s i m u l a t i o n i n s y n t h e t i c a l l y c o n s t r u c t e d w a t e r s h e d s a r e :

(1) t h e c a p a c i t y t o c o n t r o l h y d r o l o g i c i n p u t s ;

( 2 ) t h e a b i l i t y t o r e a d i l y a d j u s t h y d r a u l i c p r o p e r t i e s of t h e w a t e r s h e d , p a r titularly ground s l o p e , i m p e r v i o u s n e s s o f

( 3 ) t h e c a p a c i t y t o s e l e c t w a t e r s h e d s w i t h homogenous l a n d u s e s , t h e r e b y a v o i d i n g t h e o b s e r v a t i o n of mixed e f f e c t s t h a t i n v a r i a b l y p l a g u e f i e l d experiments on l a r g e w a t e r s h e d s ,

The Report

Subsequent p o r t i o n s of t h i s r e p o r t i n c l u d e f u r t h e r d e t a i l s of t h e methodology, a summary of t h e p r i n c i p a l f i n d i n g s , and c o n c l u s i o n s and

recommendations. Chapter

I1

c o n t a i n s a d i s c u s s i o n of ( a ) e x p e r i m e n t a l w a t e r s h e d s and l a n d u s e d i s t r i b u t i o n s w i t h u r b a n a r e a s ; (b) t h e hydro-

l o g i c - h y d r a u l i c s i m u l a t i o n model and p o l l u t a n t t r a n s p o r t ; (c) p a r a m e t e r s and e x p e r i m e n t a l c o n d i t i o n s i n c l u d i n g c h a r a c t e r i s t i c s of t h e w a t e r s h e d s , p o l l u t a n t l o a d s , and s t o r m e v e n t s ; (d) d e s c r i p t i o n s of how e f f e c t s of t h e c o n t r o l s a r e r e p r e s e n t e d i n t h e model; and ( e ) a d e s c r i p t i o n of t h e

experiments. Chapter I11 r e p o r t s t h e r e s u l t s of e x p e r i m e n t s . Chapter I V c o n t a i n s s u r v e y s of c o s t i n f o r m a t i o n and comparisons of a l t e r n a t i v e

CHAPTER I1

THE MOCEL AND EXPERIMENTS

A f u l l d e s c r i p t i o n of methods used t o i n v e s t i g a t e t h e e f f e c t i v e n e s s o f management s t r a t e g i e s under examination i n t h i s s t u d y can b e broken i n t o t h r e e c a t e g o r i e s :

( a ) a d e s c r i p t i o n of t h e s i m u l a t i o n model;

(b) a d e s c r i p t i o n of e x p e r i m e n t a l c o n d i t i o n s , p a r t i c u l a r l y t h e p r o p e r t i e s and l o a d i n g r a t e s of p o l l u t a n t s , s t o r m e v e n t s , and h y d r a u l i c p a r a m e t e r s assumed f o r t h e w a t e r s h e d s ; and

(c) a d e s c r i p t i o n of t h e s e r i e s of e x p e r i m e n t s t h a t were conducted. These t o p i c s a r e d i s c u s s e d i n s u b s e q u e n t p o r t i o n s of t h i s c h a p t e r i n t h e o r d e r t h a t t h e y a r e l i s t e d above,

The Model

The e x p e r i m e n t a l model c o n s i s t s of t h r e e b a s i c components: (1) t h e w a t e r s h e d s ;

(2) t h e p r o c e s s e s by which stormwater i s r o u t e d o v e r t h e w a t e r s h e d s ; and

(3) t h e b e h a v i o r of stormwater and t h e p o l l u t a n t l o a d i t b e a r s a s t h e y p a s s t h r o u g h c o n t r o l d e v i c e s ,

Watersheds

Types and Amounts

d e n s i t y r e s i d e n t i a l ; (3) h i g h d e n s i t y r e s i d e n t i a l ; (4) commercial, i n c l u d i n g c e n t r a l b u s i n e s s d i s t r i c t s (CBDs), and i n d u s t r i a l . T a b l e 1, based on m e t r o p o l i t a n l a n d u s e s u r v e y s r e p o r t e d by Bartholomew (1955, p. 1 2 1 ) , g i v e s t h e d i s t r i b u t i o n of u r b a n l a n d a r e a and r e s i d e n t popula- t i o n by c a t e g o r y f o r a c i t y o f a p p r o x i m a t e l y 100,000 p e r s o n s . T h a t d i s t r i b u t i o n i s used f o r t h e purpose of e s t i m a t i n g a g g r e g a t e p o l l u t a n t l o a d s and t h e i r d i s t r i b u t i o n by l a n d u s e t y p e w i t h i n a n u r b a n a r e a . The s o u r c e o f t h a t d i s t r i b u t i o n i s d a t e d , b u t i t c o n t i n u e s t o b e widely r e f e r e n c e d i n t h e l i t e r a t u r e .

TABLE

1.

D i s t r i b u t i o n of Urban Land A r e a s f o r a C i t y of 100,000 P e r s o n s

Area P o p u l a t i o n

Land Use Acres % of T o t a l

Undeveloped 9 , 6 0 0 50.0 0

Low-density 6,620 34.5

r e s i d e n t i a l High-density

r e s i d e n t i a l

Commercial 690 3.6 18,80u

I n d u s t r i a l 1 , 5 0 0

-

7 . 8 0

T o t a l s 1 9 , 2 0 0 100.0 100,000

S i z e s and S p a t i a l P a t t e r n s

( a ) Low D e n s i t y ~ i s i d e n t i a l

( b ) H i g h D e n s i t y R e s i d e n t i a l

( c ) C o m m e r c i a l

( d ) U n d e v e l o p e d

Legend

- - -

S t r e e t boundaries

0-

-

I n l e t s and s t o r m s e w e r s

V 7 7 Houses .and b u i l d i n g s P a r k i n g a r e a s

l a r g e r w a t e r s h e d s , and hydrographs from l a r g e r w a t e r s h e d s can b e o b t a i n e d by s u p e r p o s i t i o n of hydrographs from u n i t catchments.

S p a t i a l p a t t e r n s of l a n d u s e s on t h e u n i t catchments a r e a l s o shown i n F i g u r e 1. Undeveloped a r e a s a r e s i m p l y g r a s s e d a r e a s w i t h impervious a r e a s assumed t o b e 2 p e r c e n t of t h e u n i t catchment, A d r a i n a g e c h a n n e l i s assumed t o b e l o c a t e d a l o n g t h e c e n t e r l i n e a s shown. The hydrograph f o r t h e catchment i s c a l c u l a t e d a t t h e downstream end where t h e c a t c h - ment d i s c h a r g e s t o e i t h e r a n a t u r a l o r a r t i f i c i a l c h a n n e l .

U n i t catchments f o r low d e n s i t y r e s i d e n t i a l w a t e r s h e d s c o n t a i n e i g h t 2

r e s i d e n c e s e a c h w i t h 900 f t of roof s u r f a c e and connected t o a driveway 2

w i t h 200 f t

.

A 20-foot s t r e e t p a s s e s a l o n g t h e c e n t e r l i n e and t h e r e a r e 20-foot h a l f - s t r e e t s on e a c h end, Stormwater i n l e t s a r e l o c a t e d a t 260-ft i n t e r v a l s a l o n g t h e c e n t e r l i n e s t r e e t . O t h e r p o r t i o n s o f t h e catchment a r e g r a s s e d , Two r o o f t o p d r a i n a g e a l t e r n a t i v e s were p e r m i t t e d : ( a ) r o o f t o p t o driveway t o s t r e e t ; and (b) r o o f t o p t o f r o n t lawn t o s t r e e t . L i k e w i s e two s t r e e t d r a i n a g e a l t e r n a t i v e s were p e r m i t t e d : ( a ) s t a n d a r d c u r b and g u t t e r ; and (b) g r a s s e d s w a l e s .

F o r h i g h d e n s i t y r e s i d e n t i a l w a t e r s h e d s , u n i t catchments, s t r e e t s and a s s o c i a t e d d r a i n a g e a r e t h e same a s above. I n d i v i d u a l r e s i d e n c e s a r e r e p l a c e d w i t h two m u l t i - f a m i l y d w e l l i n g u n i t s on e a c h s i d e of t h e

L

s t r e e t , each w i t h 32,000 f t of roof s u r f a c e and e a c h f r o ~ i t e d by a park-

2

i n g a r e a of 4,000 f t

,

Other a r e a s a r e g r a s s e d . Two b u i l d i n g d r a i n a g e schemes a r e p e r m i t t e d : ( a ) roof and p a r k i n g d r a i n a g e d i r e c t l y t o t h e s t r e e t ; and (b) r o o f t o p s t o r a g e w i t h r e l e a s e t o a porous p a r k i n g a r e a w i t h d r a i n s t o t h e s t r e e t . S t r e e t d r a i n a g e a l t e r n a t i v e s a r e t h e same a s above.

Commercial catchments a s shown i n F i g u r e 1 c o n t a i n f o u r complexes each c o n t a i n i n g a b u i l d i n g ( 2 0 0 f t x 5 0 f t ) , a p a r k i n g l o t ( 1 0 0 f t x 5 0 f t ) and a g r a s s e d a r e a ( 1 0 0 f t x 5 0 f t ) surrounded by a 1 0 - f t s i d e w a l k ,

L a r g e r w a t e r s h e d s of 25 a c r e s , 200 a c r e s , and 800 a c r e s used f o r some of t h e e x p e r i m e n t s were c o n s t r u c t e d by a r r a y i n g t h e u n i t catchments a s shown i n F i g u r e 2.

Runoff Mechanisms Hydrographs

S i m u l a t i o n of t h e e f f e c t s of c o n t r o l d e v i c e s on stormwater r u n o f f q u a l i t y r e q u i r e s t h e s i m u l a t i o n of w a t e r f l o w s o v e r u r b a n s u r f a c e s , i n d r a i n a g e c h a n n e l s of v a r i o u s t y p e s , and t h r o u g h t h e c o n t r o l d e v i c e s themselves. Because o f t h e l o n g - s t a n d i n g i n t e r e s t i n u r b a n d r a i n a g e , t h e r e a r e many models a v a i l a b l e f o r t h i s k i n d of s i m u l a t i o n , e a c h of which h a s s p e c i a l c h a r a c t e r i s t i c s , and t h e y e x i s t a t v a r i o u s l e v e l s of development and a v a i l a b i l i t y . The s o - c a l l e d Stormwater Management Model

(SWMM), developed by a t r i a d of r e s e a r c h e r s w i t h s u p p o r t from t h e U.S. Environmental P r o t e c t i o n Agency (EPA, 1 9 7 1 ) , i s a n a p p r o p r i a t e model f o r t h e purpose of t h i s s t u d y . It was s e l e c t e d b e c a u s e of i t s c a p a c i t y t o s i m u l a t e w a t e r q u a l i t y , i t s r e s p o n s i v e n e s s t o t h e c o n t r o l d e v i c e s under i n v e s t i g a t i o n h e r e , and b e c a u s e of p r i o r e x p e r i e n c e and a v a i l a b i l i t y i n North C a r o l i n a (TJCOG, 1 9 7 6 ) , Some minor a d a p t a t i o n s t o t h e model were n e c e s s a r y t o make i t more r e s p o n s i v e t o t h e s e t of c o n t r o l d e v i c e s b e i n g examined (LaPointe, 1979).

Unsteady r u n o f f from r o o f t o p s , p a r k i n g l o t s , g r a s s e d a r e a s , and o t h e r s u r f a c e s i s approximated by t h e s o l u t i o n of a sequence of s t e a d y - s t a t e mass b a l a n c e e q u a t i o n s which a c c o u n t f o r p r e c i p i t a t i o n , d e p r e s s i o n s t o r a g e , i n f i l t r a t i o n ( f o r p e r v i o u s s u r f a c e s ) , and o u t f l o w . Outflow r a t e s and i n f i l t r a t i o n a r e e s t i m a t e d u s i n g t h e w e l l known Manning and Horton e q u a t i o n s , r e s p e c t i v e l y . S p a t i a l v a r i a t i o n s w i t h i n r o o f t o p , p a r k i n g , and o t h e r s u r f a c e s upon which r a i n f a l l s a r e i g n o r e d .

P o l l u t a n t T r a n s p o r t

The t r a n s p o r t of p o l l u t a n t s i n stormwater r u n o f f i s p r o b a b l y t h e l e a s t w e l l u n d e r s t o o d phenomenon s i m u l a t e d by t h e model. A s i m p l e e x p r e s s i o n i s used t o d e s c r i b e t h e r a t e of "pick-up" of p o l l u t a n t s , a l l of which a r e assumed t o b e a t t a c h e d t o s o l i d p a r t i c l e s d e p o s i t e d on urban s u r f a c e s , P a r t i c l e s i z e s a r e assumed t o v a r y ( t h e p a r t i c l e s i z e d i s t r i b u t i o n and o t h e r p r o p e r t i e s of t h e l o a d a r e d i s c u s s e d l a t e r ) , b u t i n t h e a b s e n c e of more d e t a i l e d i n f o r m a t i o n , a l l p a r t i c l e s a r e assumed t o b e p i c k e d up a t t h e same r a t e , That r a t e i s assumed t o b e d i r e c t l y p r o p o r t i o n a l t o r a i n f a l l i n t e n s i t y , i, and t h e amount of l o a d remaining on t h e s u r f a c e , P ( S a r t o r and Boyd, 1972, and Metcalf and Eddy, I n c . , 1971, pp, 75-76). Expressed m a t h e m a t i c a l l y t h e r a t e of "pick-up" i s

where k i s a r a t e c o n s t a n t , I t f o l l o w s t h a t t h e l o a d r e m a i n i n g a t any t i m e t from t h e s t a r t o f a s t o r m i s P ( t ) = P exp ( - k i t ) where Po i s t h e

0

i n i t i a l l o a d a t t h e s t a r t of t h e storm. Thus, t h e c u m u l a t i v e amount p i c k e d up by t h e r u n o f f a t t i m e t i s P

-P

( t ) = Po [1-exp ( - k i t ) ]

.

0

A l l m a t e r i a l suspended i n stormwater r u n o f f i s assumed t o b e t r a n s - p o r t e d t o t h e d r a i n a g e network; i t s b e h a v i o r w i t h i n t h e d r a i n a g e s y s t e m depends upon p r o p e r t i e s of t h e v a r i o u s e l e m e n t s i n t h a t system. For s t r e e t g u t t e r s and d r a i n a g e c h a n n e l s from r e s i d e n t i a l and commercial a r e a s , i t i s assumed t h a t t h e S h i e l d s e q u a t i o n h o l d s (EPA, 1971,

pp, 346-352). The b e h a v i o r of suspended m a t t e r i n o t h e r e l e m e n t s s u c h a s g r a s s s w a l e s , s t o r a g e b a s i n s , and s e t t l i n g b a s i n s i s d i s c u s s e d l a t e r when t h o s e c o n t r o l methods o r w a t e r s h e d " t r e a t m e n t s " a r e c o n s i d e r e d ,

E x p e r i m e n t a l C o n d i t i o n s

The e f f e c t s of c o n t r o l d e v i c e s on t h e q u a n t i t y and q u a l i t y o f storm- w a t e r r u n o f f c a n b e e s t i m a t e d by d i f f e r e n c i n g t h e outcomes of e x p e r i m e n t s w i t h and w i t h o u t t h e v a r i o u s t r e a t m e n t s o r c o n t r o l d e v i c e s . However,

t h o s e outcomes a r e i n f l u e n c e d by some v a r i a b l e s which a r e e i t h e r uncon- t r o l l e d i n r e a l w a t e r s h e d s o r which a r e beyond t h e s c o p e of t h i s

(1) t h e c h a r a c t e r i s t i c s of s t o r m e v e n t s t o w h i c h , t h e w a t e r s h e d s a r e s u b j e c t ; (2) t h e r a t e s a t which p o l l u t a n t l o a d s a r e d e p o s i t e d on u r b a n s u r f a c e s and t h e p r o p e r t i e s of t h o s e l o a d s ; and (3) l a n d s l o p e , ground c o v e r , t h e geometry of d r a i n a g e c h a n n e l s , and o t h e r f a c t o r s a f f e c t i n g t h e h y d r a u l i c s of r u n o f f , To r e d u c e v a r i a b i l i t y i n e x p e r i m e n t a l o u t - comes which c o u l d mask t h e e f f e c t s of primary c o n c e r n h e r e , many of t h e s e v a r i a b l e s have been s e t a p r e d e t e r m i n e d v a l u e s , I n d o i n g s o , t h e r a n g e of c o n d i t i o n s under which t h e outcomes a r e v a l i d i s o b v i o u s l y l h i t e d .

Starm E v e n t s

Two s t o r m s were used t o e s t i m a t e t h e e f f e c t s of c o n t r o l d e v i c e s . T o t a l r a i n f a l l f o r t h e two s t o r m s , e a c h w i t h a one-hour d u r a t i o n and c o n s t a n t i n t e n s i t y , were 0.6 and 1 . 6 i n c h e s . These s t o r m s w e r e s e l e c t e d w i t h r e c u r r e n c e i n t e r v a l s o f a p p r o x i m a t e l y two months and two years., r e s p e c t i v e l y , f o r a r e a s i n t h e Piedmont p l a t e a u of r i v e r b a s i n s i n t h e South A t l a n t i c s l o p e of t h e U.S.

Choices of t h e s e s t o r m s were somewhat a r b i t r a r y , b u t t h e y do r e f l e c t t h e c o n s i d e r a t i o n s d i s c u s s e d below. F i r s t , w h i l e i t i s n o t p o s s i b l e t o experiment under a l a r g e number of s t o r m s , no s i n g l e s t o r m c o u l d ade- q u a t e l y r e f l e c t t h e e f f e c t s of c o n t r o l s . D i f f e r e n t sto:ms would o b v i o u s l y produce d i f f e r e n t hydrographs and d i f f e r e n t r a t e s a t which p o l l u t a n t s a r e washed from u r b a n s u r f a c e s , A lower bound was e s t a b l i s h e d on t h e b a s i s of a n a n a l y s i s of stormwater d a t a c o l l e c t e d by C o l s t o n i n Durham, N . C .

P o l l u t a n t Loads

P o l l u t a n t s covered by t h i s a n a l y s i s a r e t h o s e a t t a c h e d t o s o l i d p a r t i c l e s d e p o s i t e d on r o o f t o p s , p a r k i n g l o t s , and s t r e e t s u r f a c e s . S e v e r a l a t t r i b u t e s of t h e s e p o l l u t a n t s a r e r e l e v a n t t o f o r m u l a t i o n of a l t e r n a t i v e c o n t r o l s t r a t e g i e s , and t h e s e a t t r i b u t e s must b e f u l l y s p e c i f i e d p r i o r t o e v a l u a t i o n of t h o s e s t r a t e g i e s , Among t h o s e a t t r i b u t e s a r e :

r a t e s of a c c u m u l a t i o n of s o l i d s on e a c h of t h e t h r e e t y p e s of s u r f a c e s , i n c l u d i n g any v a r i a t i o n i n t h e s e r a t e s a c r o s s l a n d u s e s ;

p a r t i c l e s i z e d i s t r i b u t i o n s of accumulated s o l i d s ; and

t h e c o m p o s i t i o n of accumulated s o l i d s a s t h e y a r e c l a s s i f i e d and measured by s t a n d a r d i n d i c a t o r s o f w a t e r p o l l u t i o n . Of t h e t h r e e t y p e s of s u r f a c e s i n c l u d e d i n t h i s a n a l y s i s , s t r e e t s have r e c e i v e d g r e a t e r a t t e n t i o n i n t h e w a t e r q u a l i t y l i t e r a t u r e . S e v e r a l s t u d i e s (American P u b l i c Works A s s o c i a t i o n , 1969; S a r t o r and Boyd, 1972; and U.S. Army Corps of E n g i n e e r s , 1975) have been u n d e r t a k e n t o e s t i m a t e t h e r a t e s of a c c u m u l a t i o n on s t r e e t s . Mid-range v a l u e s t a k e n from t h e APWA s t u d y i n Chicago were s p e c i f i e d f o r t h i s a n a l y s i s . Those r a t e s a r e :

(1) 81.4 k g l c u r b m i l e l d a y f o r commercial a r e a s ; ( 2 ) 56.7 f o r h i g h - d e n s i t y r e s i d e n t i a l a r e a s ; and (3) 1 7 . 3 f o r low-density r e s i d e n t i a l a r e a s .

D u s t f a l l r a t e s on r o o f t o p s were s e t a t 0.645 k g / a c r e / d a y , a v a l u e r e p o r t e d by EPA (1971, Vol. I , p , 1 8 6 ) . Although t h e s e r a t e s v a r y from one community t o a n o t h e r , t h e v a l u e used h e r e i s of t h e same o r d e r of magnitude found i n s e v e r a l c i t i e s (Betson, 1978). D e p o s i t i o n r a t e s on p a r k i n g l o t s were s e t a t 1 0 k g / a c r e / d a y based on measured r u n o f f l o a d s i n t h e R e s e a r c h T r i a n g l e 208 s t u d y (TJCOG, 1976, pp. I1

.K.

1-17)

.

The d i s t r i b u t i o n of t h e weight of s t r e e t s u r f a c e s o l i d s by p a r t i c l e s i z e was assumed t o b e t h e same a s t h a t found by Shaheen i n Washington,

D . C . (1975). O t h e r s t u d i e s s u c h a s t h o s e by t h e Corps of E n g i n e e r s (1975, pp. 30-43) have found d i s t r i b u t i o n s s i m i l a r t o t h e one e s t i m a t e d by Shaheen. T h a t d i s t r i b u t i o n , c o n s i s t i n g of f i v e s i z e r a n g e s , i s

shown i n T a b l e 2 ; a s i x t h c a t e g o r y f o r p a r t i c l e s l e s s t h a n one micron i n s i z e h a s been added i n t h i s a n a l y s i s , The s o l u b l e f r a c t i o n was

TABLE 2. D i s t r i b u t i o n of S t r e e t S u r f a c e S o l i d s by P a r t i c l e S i z e s (from Shaheen, 1975)

P a r t i c l e s i z e

<

1 1-75 76-250 251-420 421-850 851-3300

(microns )

P e r c e n t o f

t o t a l l o a d 7.0 9 . 5 29.7 22.0 1 7 . 8 1 4 . 0

I n t h e a b s e n c e of d a t a on p a r t i c l e s i z e d i s t r i b u t i o n s f o r r o o f t o p and p a r k i n g l o t l o a d s , t h e d i s t r i b u t i o n s shown i n T a b l e 2 were a s s i g n e d t o t h o s e l o a d s .

F i n a l l y , r e s u l t s of s e v e r a l i n v e s t i g a t i o n s i n t o t h e c o m p o s i t i o n of s t r e e t s u r f a c e s o l i d s a r e shown i n T a b l e 3. Each s t u d y r e p o r t e d i n t h a t t a b l e c o n t a i n s o n e o r more v a l u e s which d i f f e r from t h e norm f o r t h a t c o n s t i t u e n t , b u t t h e d a t a a r e s u f f i c i e n t t o e s t a b l i s h a n o r d e r of magni- t u d e f o r e a c h p o l l u t a n t c a t e g o r y . The Shaheen d a t a (1975, p. 8 4 ) were used i n t h i s a n a l y s i s . F u r t h e r m o r e , a n a n a l y s i s of C o l s t o n ' s r e s u l t s showed l i t t l e v a r i a t i o n i n composition from o n e l a n d u s e LEI a n o t h e r

(LaPointe, 1979, Appendix A), and i n t h e a b s e n c e of o t h e r d a t a , T a b l e 3 i s assumed t o a p p l y t o a l l c a t e g o r i e s of l a n d u s e .

A d i s t r i b u t i o n of p o l l u t a n t mass by p a r t i c l e s i z e was o b t a i n e d by combining T a b l e 2 and t h e a p p r o p r i a t e columns of T a b l e 3 . R e s u l t s a r e g i v e n i n T a b l e 4.

P a r a m e t e r s and D r a i n a g e System Geometry

TABLE 3. Composition of S t r e e t S u r f a c e S o l i d s Cfrom Shaheen, 1975)

Ave. of C i t i e s Omaha

Surveyed by

b Washington, New Old

P o l l u t a n t c h i c a g o a S a r t o r and Boyd D.C.C Areas Areas

Suspended 1.00 1 . 0 0 1.00 1 . 0 0 1 . 0 0

s o l i d s

BOD ,005 ,0097 ,0032 .061 .030

COD .040 .067

T o t a l ,00079

phosphorous

Lead

--

,00041 ,004 ,0005 ,00019

a -American P u b l i c Works A s s o c i a t i o n , 1969, p. 55. b - S a r t o r and Boyd, 1972, p . 3.

c -Shaheen, 1975, p. 84

d -U,S. Army Corps o f E n g i n e e r s , 1975, p. 45.

TABLE 4. D i s t r i b u t i o n of P o l l u t a n t Masses from S t r e e t S u r f a c e s by P a r t i c l e S i z e

P a r t i c l e S i z e

Cumulative P e r c e n t a g e of T o t a l Load

Suspended T o t a l

(Microns) s o i i d s BOD COD Phosphorous Lead

c la 0 2 8 1 4 1 2 4

a - S o l u b l e f r a c t i o n s were a s s i g n e d t o t h i s s i z e r a n g e .

Both t h e c r o s s - s e c t i o n a l geometry and t h e d r a i n a g e network must b e s p e c i f i e d . Except f o r s t r e e t g u t t e r s , a l l d r a i n a g e c h a n n e l s were

a s s i g n e d symmetric t r a p e z o i d a l c r o s s - s e c t i o n s , dimensions f o r which a r e shown i n T a b l e 5 . S t r e e t g u t t e r s and g r a s s s w a l e s were s i z e d a s shown i n F i g u r e 3.

TABLE 5. Assumed P a r a m e t e r s f o r Drainage Channels

Watersheds Manning Bottom S i d e

S i z e ( a c . ) Type C o e f f i c i e n t Width ( f t ) S l o p e s

3.6 Undeveloped .10 2 .O . 3

A l l c o n v e n t i o n a l .013 p r a c t i c e

G r a s s . s w a l e s .20 1 . 0 . 5

2 5 Undeveloped .10 2.0 1 . 0

Commercial O t h e r s Undeveloped Commercial

O t h e r s .035 3.0 1 . 0

Undeveloped O t h e r s

The geometry o f t h e d r a i n a g e network i s d e p i c t e d i n F i g u r e 2.

w

Curb and Gutter

FIGURE 3 . DIMENSIONS OF STREET GUTTERS AND GRASSED SWALES

Land slopes were t r e a t e d a s a v a r i a b l e i n t h e experiments. Channel s l o p e s were s e t equal t o land s l o p e s f o r u n i t catchments and f o r water- sheds up t o 200 a c r e s i n s i z e . For experiments on 800-acre watersheds, channel s l o p e s were halved a s t h e channels e x i t e d t h e 200-acre a r e a s . The Treatments

To e s t i m a t e t h e e f f e c t s of v a r i o u s elements i n t h e stormwater management s t r a t e g y , experiments were run with v a r i o u s "treatments" o r

s l o p e s f o r which e x p e r i m e n t s were conducted a r e d i s c u s s e d l a t e r i n t h i s c h a p t e r . T h i s s e c t i o n d e s c r i b e s t h e n a t u r e of t h o s e t r e a t m e n t s , p e r t i n e n t c r i t e r i a t h a t s p e c i f y t h e i r u s e i n t h i s a n a l y s i s , and t h e manner i n which t h e y were s i m u l a t e d i n t h e model.

S i x t r e a t m e n t s . w e r e i n v e s t i g a t e d . They a r e : (1) " c o n v e n t i o n a l " s y s t e m s ;

(2) "reduced r u n o f f " d r a i n a g e s y s t e m s ;

(3) s t r e e t g u t t e r s r e p l a c e d by g r a s s s w a l e s ;

(4)

s t r e e t sweeping;

(5) s t o r a g e b a s i n s ; and ( 6 ) s e t t l i n g b a s i n s . Conventional D r a i n a g e

The d e f i n i t i o n of c o n v e n t i o n a l d r a i n a g e v a r i e s s l i g h t l y from o n e l a n d u s e t o a n o t h e r , b u t i t g e n e r a l l y r e f e r s t o t h e p r a c t i c e of conveying

r u n o f f from i n d i v i d u a l p a r c e l s of l a n d a s q u i c k l y a s p o s s i b l e . For low d e n s i t y r e s i d e n t i a l (LDR) a r e a s r o o f t o p r u n o f f i s d r a i n e d t o driveways which d r a i n d i r e c t l y t o s t r e e t g u t t e r s . All of t h e s e s u r f a c e s a r e

impervious. I n h i g h d e n s i t y r e s i d e n t i a l (HDR) w a t e r s h e d s t h e p a t t e r n i s r o o f t o p t o p a r k i n g l o t t o s t r e e t g u t t e r s ( a l l i m p e r v i o u s ) . D r a i n a g e i n commercial a r e a s i s d i r e c t l y connected t o s t o r m i n l e t s .

Reduced Runoff D r a i n a g e

For reduced r u n o f f t h e d e f i n i t i o n i s a g a i n s l i g h t l y v a r i a b l e a c r o s s l a n d u s e s , b u t i t i s based on t h e c o n c e p t of r e t a i n i n g w a t e r on i n d i v i d u a l p a r c e l s a s l o n g a s p o s s i b l e w i t h o u t c r e a t i n g o n - s i t e f l o o d i n g . I n LDR a r e a s , r o o f t o p r u n o f f d i s c h a r g e s t o y a r d s i n s t e a d of driveways a s i n con- v e n t i o n a l d r a i n a g e ; driveways a r e assumed t o b e porous; and s t r e e t

G r a s s Swales

I

G r a s s s w a l e s a r e i n c o r p o r a t e d i n t h e reduced r u n o f f t r e a t m e n t b e c a u s e of t h e i r e f f e c t s on t h e h y d r a u l i c b e h a v i o r of stormwater r u n o f f , b u t t h e y a r e a l s o c o n s i d e r e d a s a s e p a r a t e t r e a t m e n t b e c a u s e o f t h e i r e f f e c t s on t h e t r a n s p o r t of s o l i d m a t t e r i n stormwater r u n o f f .

S i m u l a t i o n of p a r t i c l e t r a n s p o r t i n g r a s s s w a l e s p r e s e n t e d s p e c i a l problems. The S h i e l d s e q u a t i o n ( F a i r , Geyer, and Okun, 1966, Vol. 1, p. 14-6) i s a p p l i c a b l e t o p i p e f l o w , n o t t o n a t u r a l c h a n n e l s . A more a p p r o p r i a t e model f o r n a t u r a l c h a n n e l s i s t h e e m p i r i c a l r e l a t i o n s h i p by 110 (1975, p. 1561) where sediment c o n c e n t r a t i o n i s e s t i m a t e d t o b e pro- p o r t i o n a l t o t h e s q u a r e o f t h e v e l o c i t y , p r o p o r t i o n a l t o t h e c h a n n e l s l o p e , and i n v e r s e l y p r o p o r t i o n a l t o g e o m e t r i c mean p a r t i c l e s i z e . However, w i t h t h e p a r t i c l e s i z e d i s t r i b u t i o n f o r s t r e e t s u r f a c e s o l i d s used i n t h i s a n a l y s i s , s m a l l s i z e p a r t i c l e s s o dominate t h e d i s t r i b u t i o n t h a t t h e g e o m e t r i c mean p a r t i c l e s i z e i s o n l y s l i g h t l y g r e a t e r t h a n t h a t of t h e s m a l l e s t - s i z e d c a t e g o r y .

Even a t t h e low v e l o c i t i e s found i n g r a s s s w a l e s , t h e 110 e q u a t i o n p r e d i c t s t h a t a l l m a t e r i a l s would remain i n s u s p e n s i o n , a n u n l i k e l y con- d i t i o n f o r t h e l a r g e r p a r t i c l e s . Although t h e S t o k e s e q u a t i o n ( F a i r , Geyer, and Okun, 1966, Vol. 2, p. 25-5) i s n o t s t r i c t l y a p p l i c a b l e t o t h i s s i t u a t i o n b e c a u s e q u i e s c e n t c o n d i t i o n s do n o t p r e v a i l , t h e low v e l o c i t i e s s u g g e s t t h a t some i n s i g h t s i n t o t h e e f f e c t of g r a s s s w a l e s on sediment t r a n s p o r t can b e gained from t h e i r t r e a t m e n t a s a s e r i e s of s e t t l i n g b a s i n s . S t r e e t Sweeping

The n a t u r e and e x t e n t of s t r e e t sweeping p r a c t i c e s i n u r b a n a r e a s have been d e s c r i b e d i n s e v e r a l s u r v e y s of U.S. c i t i e s (American P u b l i c Works A s s o c i a t i o n , 1969; S a r t o r and Boyd, 1972). While t h e most common o b j e c t i v e s e r v e d by t h e s e p r a c t i c e s i s t h e r e d u c t i o n of v i s u a l b l i g h t , t h e y have a s i g n i f i c a n t impact on t h e l e v e l of p o l l u t i o n l o a d s which a r e s u s c e p t i b l e t o b e i n g t r a n s p o r t e d i n t o s t r e a m s by t h e f l u s h i n g a c t i o n o f stormwater r u n o f f .

c o n d i t i o n s , and t h e frequency of sweeping, and mechanical p r o p e r t i e s and o p e r a t i o n o f t h e sweeper i t s e l f . These e f f e c t s a r e i n c o r p o r a t e d i n t o t h e model f o r e s t i m a t i n g r e s i d u a l l o a d s a s d i s c u s s e d below.

As g i v e n by Donigian and Crawford (1976, pp. 46-47) t h e l o a d accumu- l a t e d a t t h e end o f t h e tth day a f t e r a s t o r m i s g i v e n by:

where L i s t h e d a i l y l o a d i n g r a t e and r i s t h e f r a c t i o n of t h e accumulated r a t e which i s removed e a c h day, That model assumes e i t h e r t h a t t h e f r a c - t i o n removed i s c o n s t a n t from day t o day o r t h a t t h e l o a d c a n b e

a d e q u a t e l y e s t i m a t e d by r e p l a c i n g r by t h e a v e r a g e r a t e of removal, I n t h e p r e s e n t s t u d y t h e model i s a p p l i e d t o e a c h of t h e p a r t i c l e s i z e c a t e g o r i e s , and t h e f r a c t i o n removed i s made v a r i a b l e a c r o s s t h o s e c a t e g o r i e s .

The f r a c t i o n removed i s e s t i m a t e d

as

f o l l o w s : r = e ( p / f )

where e is t h e e f f i c i e n c y of a p a r t i c u l a r t y p e of sweeper f o r c a p t u r i n g a p a r t i c l e of a g i v e n s i z e , p i s t h e f r a c t i o n o f s t r e e t s u r f a c e covered by a sweeper i n o n e day, and f i s t h e sweeping i n t e r v a l i n d a y s ,

E f f i c i e n c i e s f o r b o t h b r u s h and vacuum sweepers as r e p o r t e d by S a r t o r and Boyd (1972, p. 112, 123) a r e g i v e n i n T a b l e 6. When chose s i z e - s p e c i f i c e f f i c i e n c i e s a r e combined w i t h t h e d i s t r i b u t i o n of p o l l u t a n t s by p a r t i c l e s i z e , o v e r a l l removal e f f i c i e n c i e s may b e c a l c u l a t e d and d i s p l a y e d a s i n F i g u r e 4.

S t o r a g e and S e t t l i n g B a s i n s

Two of t h e most f r e q u e n t l y mentioned t e c h n i q u e s f o r c o n t r o l l i n g t h e q u a n t i t y and q u a l i t y of stormwater r u n o f f a r e s t o r a g e b a s i n s and

s e t t l i n g b a s i n s ( P o e r t n e r , 1974, pp. 113-123, and Lager and Smith, 1974, pp. 215-219). While t h e y have been c i t e d a s t e c h n i q u e s a p p l i c a b l e t o l a r g e w a t e r s h e d s , t h i s s t u d y e x p l o r e s t h e p o s s i b i l i t y o f a d a p t i n g them t o s m a l l e r w a t e r s h e d s where t h e y may b e s t r a t e g i c a l l y l o c a t e d t o s e r v e h i g h a c t i v i t y u r b a n a r e a s on which p o l l u t i o n l o a d s a r e c o n c e n t r a t e d .

TABLE 6. C o l l e c t i o n E f f i c i e n c i e s f o r Brush and Vacuum S t r e e t Sweepers ( f r o m S a r t o r and Boyd, 1972)

P a r t i c l e

S i z e Brush Vacuum

(Microns) Sweepers Sweepers

A s t o r a g e b a s i n i s d e s i g n e d t o s t o r e a s p e c i f i e d volume o f s t o r m w a t e r r u n o f f , and i t s e f f e c t i v e n e s s a s a w a t e r q u a l i t y c o n t r o l d e v i c e i s gen- e r a l l y measured by t h e f r a c t i o n of t h e t o t a l mass o f p o l l u t a n t s i n t h e r u n o f f t h a t i s d i v e r t e d t o t h e s t o r a g e b a s i n . On t h e o t h e r hand,

a s e t t l i n g b a s i n i s u s u a l l y s i z e d t o remove a g i v e n p e r c e n t a g e of s o l i d s from t h e r u n o f f t h a t i s r o u t e d t h r o u g h t h e b a s i n by s e l e c t i n g a n appro- p r i a t e s u r f a c e o v e r f l o w r a t e . I n t h i s c a s e t h e c h o i c e i s based upon S t o k e s ' law f o r d i s c r e t e p a r t i c l e s e t t l i n g ( F a i r , Geyer, and Okun, 1966, p. 587).

For e a c h of t h e two t y p e s of b a s i n s , two v a r i a t i o n s i n f l o w r o u t i n g a r e c o n s i d e r e d . S t o r a g e b a s i n s a r e assumed t o b e empty a t t h e s t a r t of a s t o r m . I n t h e f i r s t c a s e i n f l o w s a r e by-passed a f t e r t h e b a s i n i s f i l l e d . I n t h e second c a s e , when t h e b a s i n i s l e s s t h a n f u l l , o u t f l o w s a r e c o n s t r a i n e d t o b e e q u a l t o o r l e s s t h a n t h e peak r u n o f f from a n undeveloped w a t e r s h e d w i t h c h a r a c t e r i s t i c s i d e n t i c a l t o t h o s e used i n computing t h e i n f l o w hydrograph; when t h e b a s i n i s f u l l , e x c e s s i n f l o w s a r e by-passed. These b a s i n s a r e r e f e r r e d t o a s Types W and VC,

0-

0-

-0--0- vacuum sweepers

-a-0-

mechanical broom

I I

s

I I

FIGURE 4

.

. 4 .6 . 8

I

..o

L e v e l o f S w e e p i n g E f f o r t

s w e e p e r s

b e f u l l a t a l l t i m e s and t h e e n t i r e i n p u t hydrograph i s r o u t e d through t h e b a s i n . The second c a s e assumes t h e b a s i n i s empty i n i t i a l l y , and, when t h e b a s i n i s l e s s t h a n f u l l , t h e o u t f l o w i s c o n s t r a i n e d a s i n t h e Type VC s t o r a g e b a s i n ; when t h e b a s i n i s f u l l , t h e i n p u t hydrograph i s passed through t h e b a s i n w i t h o u t f l o w s e q u a l t o i n f l o w s . These two c a s e s of s e t t l i n g b a s i n s a r e r e f e r r e d t o a s Types SU and SC, r e s p e c t i v e l y .

S t o r a g e l e v e l s , by-pass r a t e s , and o u t f l o w r a t e s f o r e a c h t y p e of b a s i n a r e e s t i m a t e d by n u m e r i c a l s o l u t i o n s t o t h e a p p r o p r i a t e u n s t e a d y mass b a l a n c e e q u a t i o n s . For t h e s t o r a g e b a s i n s , any w a t e r e n t e r i n g t h e b a s i n i s assumed t o remain i n s t o r a g e f o r a s u f f i c i e n t l e n g t h o f t i m e t o e f f e c t i v e l y c a p t u r e a l l o f i t s p o l l u t i o n l o a d . Thus, t h e i r e f f e c t i v e - n e s s i n r e d u c i n g p o l l u t i o n l o a d s c a n b e r e a d i l y e s t i m a t e d by computing t h e r a t i o of t h e mass of p o l l u t a n t s e n t e r i n g t h e b a s i n t o t h e mass c o n t a i n e d i n t h e t o t a l r u n o f f . S o l i d s removed i n t h e s e t t l i n g b a s i n s a r e c a l c u l a t e d by e s t i m a t i n g t h e f r a c t i o n of s o l i d s w i t h i n t h e b a s i n removed d u r i n g any t i m e i n t e r v a l f o r t h e s u r f a c e o v e r f l o w r a t e t h a t i s computed f o r t h a t i n t e r v a l u s i n g t h e S t o k e s r e l a t i o n s h i p .

I n a l l c a s e s t h e b a s i n s were assumed t o h a v e a d e p t h of t e n f e e t w i t h v e r t i c a l s i d e w a l l s . F i g u r e 5 shows t h e r e l a t i o n s h i p between s u r f a c e o v e r f l o w r a t e s and t h e e f f i c i e n c y of s e t t l i n g b a s i n s f o r t h e p a r t i c l e s i z e d i s t r i b u t i o n s shown i n T a b l e 2. Those c u r v e s were d e r i v e d u s i n g S t o k e s ' law w i t h t h e t e m p e r a t u r e s e t a t 2 0 ' ~ and a s p e c i f i c g r a v i t y f o r p a r t i c l e s a t 2.65. They r e v e a l d i m i n i s h i n g economies a s o v e r f l o w r a t e s d e c r e a s e below a p p r o x i m a t e l y 0.3 f t / s e c . T h a t o v e r f l o w r a t e c o r r e s p o n d s t o t h e removal o f a l l p a r t i c l e s g r e a t e r t h a n a p p r o x i m a t e l y 100 microns i n s i z e ; f o r t h e d i s t r i b u t i o n s shown i n T a b l e 2 , t h a t would amount t o t h e removal o f 87 p e r c e n t o f t h e suspended s o l i d s . These c r i t e r i a were a d o p t e d f o r s i z i n g t h e s e t t l i n g b a s i n s .

S i z i n g c r i t e r i a f o r s t o r a g e b a s i n s a r e d i s c u s s e d i n t h e f o l l o w i n g s e c t i o n .

The Experiments

O v e r f l o w R a t e , f t . / s e c .

I I I 1 I I

1000 500 2 5 0 100 50 10

s u s p e n d e d s o l i d s

----

lead

---

_COD

C r i t i c a l P a r t i c l e D i a m e t e r f o r G i v e n O v e r f l o w , m i c r o n s

FIGURE 5,

REMOVAL EFFICIENCIES FOR GIVEN PARTICLE SIZES AS A FUNCTION

t h e v a r i o u s t y p e s of w a t e r s h e d s f o r s e v e r a l l e v e l s of s e l e c t e d t r e a t m e n t s . Some of t h o s e e x p e r i m e n t s r e q u i r e d t h e u s e of t h e s i m u l a t i o n model des- c r i b e d e a r l i e r t o r o u t e s e l e c t e d s t o r m s and p o l l u t a n t l o a d s o v e r t h e w a t e r s h e d s and t h r o u g h t h e d r a i n a g e network. E s t i m a t i o n of t h e e f f e c t s o f s t r e e t sweeping r e q u i r e d o n l y a s i m p l e m a n i p u l a t i o n of t h e l o a d s ; a n a l y s i s of s t o r a g e and s e t t l i n g b a s i n s i n v o l v e d t h e r o u t i n g of hydro- g r a p h s t h a t were g e n e r a t e d by t h e s i m u l a t i o n model t h r o u g h t h e b a s i n s a c c o r d i n g t o a p p r o p r i a t e mass b a l a n c e models.

The b a s i c s e t o f e x p e r i m e n t s i s i d e n t i f i e d by t h e c o m b i n a t i o n s o f w a t e r s h e d t y p e s , t r e a t m e n t s , r a i n f a l l i n t e n s i t i e s , s u r f a c e s l o p e s , and w a t e r s h e d s i z e s l i s t e d i n T a b l e 7 . For e a c h w a t e r s h e d t y p e - t r e a t m e n t combination t h e r e w e r e 1 8 e x p e r i m e n t s , one of e a c h of two r a i n f a l l i n t e n - s i t i e s f o r e a c h of t h r e e s l o p e s f o r e a c h of t h r e e s i z e s of t h e l a r g e r w a t e r s h e d s . S i n c e T a b l e 7 i n c l u d e s e i g h t t y p e - t r e a t m e n t c o m b i n a t i o n s ,

a n a l y s i s covered 144 i n d i v i d u a l s i m u l a t i o n e x p e r i m e n t s .

I n a second set of e x p e r i m e n t s t h e performance of s t o r a g e b a s i n s and s e t t l i n g b a s i n s w e r e examined under l o a d i n g c o n d i t i o n s which were g e n e r a t e d a s o u t p u t s of t h e b a s i c e x p e r i m e n t s d e s c r i b e d above. Both Type W and VC were s i z e d t o c a p t u r e 30, 60, and 1 0 0 p e r c e n t of t h e r u n o f f f o r e a c h o f t h e two-month and two-year s t o r m s f o r e a c h o f t h e w a t e r s h e d t y p e s , s i z e s , and t r e a t m e n t s shown i n T a b l e

7.

Type SU s e t t l i n g b a s i n s (where o u t f l o w r a t e s a r e e q u a l t o i n f l o w r a t e s ) w e r e s i z e d t o accomodate t h e peak r u n o f f r a t e s f o r e a c h of t h e combinations shown i n T a b l e 7 . Type SC s e t t l i n g b a s i n s (where o u t f l o w r a t e s a r e r e - s t r i c t e d t o b e e q u a l t o o r l e s s t h a n peak f l o w s from similar undeveloped w a t e r s h e d s ) w e r e a l s o s i z e d f o r t h o s e t y p e - t r e a t m e n t - s i z e c o m b i n a t i o n s g i v e n i n t h e t a b l e .

F i n a l l y , t h e e f f e c t s of s t r e e t sweeping were e x p l o r e d by c a l c u l a t - i n g removal e f f i c i e n c i e s and r e l a t e d l o a d r e d u c t i o n s f o r t h e set of e x p e r i m e n t a l c o n d i t i o n s shown i n T a b l e 8. Those c o n d i t i o n s were

s e l e c t e d t o r e v e a l t h e d i f f e r e n c e s i n e f f i c i e n c i e s (1) between b r u s h and vacuum sweepers o