Chapter 5: Open Ditches for Drainage- Design, Construction …

1 NATIONAL ENGINEERING HANDBOOK. SECTION 16. drainage OF AGRICULTURAL LAND. Chapter 5 . OPEN Ditches FOR drainage - DESIGN, Construction AND MAINTENANCE. Contents Page General 5-1. Location 5-1. Channel l o c a t i o n under nonerosive c o n d i t i o n s 5-2. Channel l o c a t i o n under e r o s i v e c o n d i t i o n s 5-2. L o c a t i o n of d i v e r s i o n d i t c h e s 5-3. Layout of d i t c h e s i n humid a r e a s 5-3. L o c a t i o n of d r a i n a g e d i t c h e s i n w e s t e r n i r r i g a t e d l a n d s 5-3. Curves i n d i t c h e s 5-3. Required C a p a c i t i e s 5-4. drainage c o e f f i c i e n t s 5-4. General 5-4. E f f e c t of o u t l e t c a p a c i t y on s e l e c t i o n of d r a i n a g e c o e f f i c i e n t s 5-5. Coefficients f o r subsurface drainage 5-5. Coefficients f o r surface drainage 5-6. Determination of c o e f f i c i e n t "C" f o r u s e i n s u r f a c e d r a i n a g e formula 5-6. Example f o r computing "C" v a l u e s 5-14.

2 Computation of d e s i g n flow 5-14. Combining flows from a r e a s on which d i f f e r e n t c o e f f i c i e n t s a r e used t o compute d e s i g n flow 5-14. drainage c o e f f i c i e n t s f o r s t e e p and o t h e r a r e a s 5-15. T o t a l storm runoff and peak flow 5-15. Volume of runoff 5-15. Peak runoff and hydrographs 5-18. Design Standards 5-18. Channel d e s i g n 5-18. Value of "n" f o r d e s i g n 5-18. Channel s e c t i o n 5-19. Depth 5-19. Bottom width 5-19. Side slopes 5-19. Ditch s t a b i l i t y 5-20. Berms and s p o i l banks 5-20. Design Procedure 5-21. General 5-21. Establishing the hydraulic gradeline 5-22. Computing d i t c h s i z e s a t j u n c t i o n s - 20-40 r u l e 5-23. Computing e q u i v a l e n t d r a i n a g e a r e a 5-24. Flow from r e s e r v o i r s i n t o d r a i n a g e systems 5-27. 1. Hydraulic design a t c u l v e r t s Hydraulic design a t bridges Computing c r o s s s e c t i o n of d i t c h Allowance f o r i n i t i a l s e d i m e n t a t i o n E s t a b l i s h i n g bottom g r a d e of d i t c h Design of l a r g e open-ditch system A u x i l i a r y S t r u c t u r e s and P r a c t i c e s J u n c t i o n s of l a t e r a l d i t c h e s O v e r f a l l p i p e s and s t r u c t u r e s H y d r a u l i c d e s i g n of " i s l a n d - t y p e c o n s t r u c t i o n ".

3 Drop s p i l l w a y s Chutes Sod c h u t e s Grade-control s t r u c t u r e s C u l v e r t s and b r i d g e s Culvert depth W a t e r g a t e s , c a t t l e g u a r d s and ramps Construction Plans General drainage p l a n maps Profiles Cross s e c t i o n s S o i l borings Ditch-design c a l c u l a t i o n s Structure details Specifications Maintenance of Open D i t c h e s R e s p o n s i b i l i t y f o r maintenance Working o u t a maintenance p l a n P a s t h i s t o r y of maintenance Economics of maintenance Methods of maintenance Using c o n s t r u c t i o n equipment f o r maintenance Mowing Pasturing Burning u n d e s i r a b l e v e g e t a t i o n Chemical c o n t r o l of v e g e t a t i o n References Figures F i g . 5-1 Key map showing d r a i n a g e c o e f f i c i e n t s f o r u s e i n drainage design F i g . 5-2 drainage r u n o f f c u r v e s F i g . 5-3 drainage r u n o f f c u r v e s F i g . 5-4 D e t e r m i n a t i o n of c o e f f i c e n t , C , i n t h e d r a i n a g e formula: Q - CM 516.

4 F i g . 5-5 drainage r u n o f f c u r v e s f o r sample d r a i n a g e d i t c h d e s i g n F i g . 5-6 Procedure f o r d e s i g n of d r a i n a g e d i t c h e s a t c u l v e r t s F i g . 5-7 Sample--Condensed p l a n p r o f i l e Page Tables T a b l e 5-1 Suggested minimum r a d i u s of c u r v a t u r e i n s t a b l e s o i l w i t h o u t bank p r o t e c t i o n 5-4. T a b l e 5-2 Value of "n" f o r d r a i n a g e d i t c h d e s i g n 5-18. Table 5-3 D i t c h s i d e s l o p e s f o r u s e w i t h v a r i o u s maintenance methods 5-19. T a b l e 5-4 Sample-- drainage d i t c h d e s i g n 5-37. NATIONAL ENGINEERING HANDBOOK. SECTION 16. drainage OF AGRICULTURAL LAND. Chapter 5. OPEN Ditches FOR drainage - DESIGN, Construction AND. MAINTENANCE. General T h i s c h a p t e r o u t l i n e s procedures f o r d e s i g n i n g , c o n s t r u c t i n g , and m a i n t a i n i n g open d i t c h e s f o r a g r i c u l t u r a l drainage .

5 It c o v e r s d i t c h e s and r e c o n s t r u c t e d channels used p r i m a r i l y a s o u t l e t s f o r d r a i n a g e systems occupying broad r i v e r bottoms, d e l t a s , c o a s t a l p l a i n s , l a k e p l a i n s and upland p r a i r i e s where t h e g e n e r a l topography i s f l a t t o m i l d l y s l o p i n g and where s u r f a c e w a t e r s a r e d i f f u s e d . Where channels extend from such a r e a s i n t o narrowing bottoms and s t e e p e r s l o p e s a d j o i n i n g o r i n t o uplands, a d d i t i o n a l guidance f o r d e s i g n and s t a b i l i t y checks a s covered i n SCS Technical Release No. 25 should be used t o a s s u r e p r o t e c t i o n a g a i n s t d e g r a d a t i o n and bank e r o s i o n . The proce- d u r e and c r i t e r i a a l s o i s a p p l i c a b l e t o t h e d e s i g n of d r a i n a g e d i t c h e s used f o r i n t e r c e p t i o n drainage . Chapter 3 , Surface drainage , d e a l s w i t h s m a l l f i e l d d i t c h e s.

6 Chapter 4 , Subsurface drainage , c o n t a i n s c r i t e r i a f o r plan- n i n g d i t c h e s f o r u s e i n s u b s u r f a c e d r a i n a g e of a g r i c u l t u r a l land. The d e s i g n of d r a i n a g e d i t c h e s must g i v e due c o n s i d e r a t i o n t o t h e equipment and methods t o be used f o r c o n s t r u c t i o n , and t o t h e needs f o r and methods t o be used i n m a i n t a i n i n g t h e d i t c h e s . The d e s i g n must be based on adequate c o n s i d e r a t i o n of t h e f o l l o w i n g i n t e r r e l a t e d f a c t o r s : 1. The d i t c h must be designed t o meet t h e p r o j e c t needs w i t h o u t a g g r a d a t i o n o r d e g r a d a t i o n of t h e channel bed o r e r o s i o n of t h e channel banks. 2. I t must be c a p a b l e of b e i n g maintained t o t h e s i z e and condi- t i o n r e q u i r e d t o c o n t i n u a l l y meet t h e p r o j , e c t needs. 3.

7 The c o s t of c o n s t r u c t i o n and m a i n t a i n i n g t h e d i t c h must be l e s s t h a n t h e b e n e f i t s which i t i s expected t o produce. 4, The c o n s t r u c t i o n , o p e r a t i o n , and maintenance of t h e d i t c h must be c a r r i e d o u t i n a manner which w i l l n o t c o n t r i b u t e s i g n i f i c a n t l y t o downstream sediment l o a d s o r o n - s i t e d e t e r i o r a t i o n i n q u a l i t y of t h e environment. Design and c o n s t r u c t i o n of d i t c h e s t o meet t h e s e requirements a r e complex jobs, P o s i t i v e c o n s i d e r a t i o n o f a l l f a c t o r s w i l l r e s u l t i n an improvement t o t h e environment and t h e a g r i c u l t u r a l economy of t h e a r e a served. Inadequate c o n s i d e r a t i o n of any of t h e f a c t o r s l i s t e d w i l l r e s u l t i n disappointment and f i n a n c i a l l o s s t o t h e owners. Location drainage d i t c h e s should be l o c a t e d t o provide t h e most e f f e c t i v e d r a i n a g e of t h e a g r i c u l t u r a l wetland.

8 Topography, e x i s t i n g d i t c h e s and d r a i n s , b r i d g e s , 5-1. farm boundaries and o t h e r physical f e a t u r e s a l l i n f l u e n c e d i t c h l o c a t i o n . Natural o u t l e t s such a s e s t u a r i e s , r i v e r s , l a k e s o r swamps, o r o l d d i t c h e s u s u a l l y f i x t h e general l o c a t i o n of an open d i t c h , but t h e alignment and e f - f i c i e n c y of t h e channel may be improved by t h e use of c u t o f f s , long tangents, and smooth curves. Open d i t c h e s should terminate i n an adequate o u t l e t . The c a p a c i t y of t h e o u t l e t must be adequate t o c a r r y t h e design discharge from t h e p r o j e c t with- o u t i t r e s u l t i n g i n s t a g e i n c r e a s e s which would cause s i g n i f i c a n t damage downstream. This may r e q u i r e extending t h e channel improvement f u r t h e r down- stream. A comparison of a l t e r n a t e l o c a t i o n s of t h e p o i n t of o u t l e t may a l s o be needed.

9 The s t a g e of a stream during t h e storm when t h e drainage system i s d i s c h a r g i n g a t t h e design r a t e determines t h e adequacy of t h e stream a s an o u t l e t . A study of t h e frequency of high water s t a g e i s needed f o r l a r g e streams, l a k e s , and t i d a l waters t o determine t h e i r adequacy as an o u t l e t and t o e s t a b l i s h t h e e l e v a t i o n of t h e design h y d r a u l i c g r a d e l i n e f o r t h e open d i t c h a t t h e o u t l e t . See Chapter 2 f o r more d e t a i l s regarding t h e r e q u i r e - ments of o u t l e t s f o r a g r i c u l t u r a l drainage systems, Channel l o c a t i o n under nonerosive c o n d i t i o n s Where t h e topography i s f l a t and s o i l s and v e l o c i t y a r e w e l l w i t h i n t h e range of c o n d i t i o n s where channel s t a b i l i t y w i l l be no problem, alignment changes can be made t o f i t t h e area.

10 Some f a c t o r s t o consider when changing a l i g n - ment of a d i t c h a r e : ( a ) S t r a i g h t d i t c h e s permit r e c t a n g u l a r f i e l d s and e f f i c i e n t farming. (b) A s h o r t e r channel w i l l have more s l o p e , g r e a t e r v e l o c i t y and l e s s c r o s s - s e c t i o n a l a r e a and w i l l be l e s s l i k e l y t o accumulate sediment than a longer channel between t h e same terminal p o i n t s . (c) Chang- i n g t h e e x i s t i n g l o c a t i o n may r e q u i r e p l a c i n g t h e d i t c h on h i g h e r land, c r o s s i n g farm boundaries, i s o l a t i n g p a r t s of f i e l d s from t h e r e s t of t h e farm, and i n s t a l l i n g new b r i d g e s and c u l v e r t s not otherwise needed. (d) The l o c a t i o n may r e s u l t i n placing the d i t c h i n more o r l e s s s t a b l e s o i l s . Channel l o c a t i o n under e r o s i v e c o n d i t i o n s Some drainage d i t c h e s may be needed where s i t e c o n d i t i o n s a r e l i k e l y t o cause s t a b i l i t y problems.