8.2 ESTIMATING DETENTION VOLUMES

Transcription

1 8.. Plnning versus Design number of detention bsin plnning methods hve been proposed in the professionl literture. These provide estimtes of the reuired volume of detention storge. The outlet structure is sized independently of the detention volume determintion. These methods will be clssed s plnning methods, lthough they re occsionlly used for design. They re referred to s plnning methods becuse the riser chrcteristics nd volume re determined independently of ech other. Design techniues differ in two wys from the plnning methods. First, the plnning methods only reuire pek dischrge estimtes, s opposed to reuiring entire flood hydrogrphs. Thus routing hydrogrphs through the detention bsin is not necessry when using these plnning methods. Second, since routing is not reuired, stge-storge-dischrge reltionship is not reuired; insted, "stndrd" storge-dischrge reltionship is inherent in the plnning methods. design method uses flood hydrogrphs, routing, nd site-specific stge-storgedischrge reltionship. For this reson, design method will be more ccurte thn the plnning method. However, the plnning methods re much esier to pply. Hence the terms plnning nd design re used to distinguish between pproches to SWM problem solving tht reflect differences in expected ccurcy, s well s the cost nd effort involved. The problem of plnning the detention fcility is seprted into two prts, estimting the volume of storge nd sizing the chrcteristics of the outlet fcility. 8. ESTIMTING DETENTION VOLUMES number of methods hve been proposed nd re being used for estimting detention volumes. ecognizing tht these methods often yield widely different estimtes, brief comprison of some of the more widely used methods is in order. reltionship between the rtio of the storge volume to the runoff volume nd the rtio of the "pre-development" nd "post-development" pek dischrges is the bsis for mny of these methods. For SWM policies tht reuire the pek dischrge out of the SWM bsin to be no greter thn the predevelopment pek dischrge, the before-to-fter rtio is often referred to s the rtio of the outflow to inflow since the pek of the outflow from the detention bsin euls the predevelopment pek dischrge nd the inflow to the detention bsin euls the post-development pek dischrge. 8.. The Loss-of-Nturl-Storge Method The loss-of-nturl-storge method for estimting detention volumes is bsed on the ide tht the volume of mnmde storge (Q s ) euls the volume of lost nturl storge: Q s = Q - Q b (8.) in which Q nd Q b re the depths (mm or in) of runoff for the post-development nd predevelopment wtershed conditions. It is importnt to note tht the vrible Q is often referred to s volume even though it hs the dimension of depth. While it is ctully depth, when it is referred to s volume, the ssumption is mde tht it is n euivlent depth spred uniformly over the entire wtershed. The volume of storge, V s, in m (ft ), is computed by multiplying Q s by the dringe re in hectres (cres): VS = α Q S (8.) 8-4

2 where α is conversion constnt eul to 0 in SI nd,60 in CU units. The runoff depths Q nd Q b of Eution 8. cn be computed using ny one of number of methods. For the SCS method, the SCS runoff eution (Eution 5.9) cn be used with the post-development nd pre-development curve numbers (CNs). If the rtionl method is used to estimte pek dischrges, runoff depths Q cn be estimted using the pek dischrge p in m /s (ft /s), the time of concentrtion t c in minutes, nd the dringe re in h (c): p t c Q = α (8.) where α is conversion constnt eul to 6 in SI nd /60.5 in CU units. Eution 8. cn be solved for both the pre- nd post-development conditions using the pproprite vlues of p nd t c. Then the vlues re entered into Eution 8. to compute the depth of storge, which is then used to compute the volume of storge with Eution 8.. Exmple 8... h (5.7 c) wtershed is being developed. Existing conditions hve rtionl coefficient C of 0. nd time of concentrtion of 8 minutes. In the developed condition, the coefficient C will be 0.45, nd the time of concentrtion will be minutes. Using the locl IDF curve, the rinfll intensities for the existing nd developed conditions re 79 mm/h (. in/h) nd 0 mm/h (4.0 in/h), respectively. The pek dischrges for the existing nd developed conditions re: pb p = C α = C α b i i b ( 0. )( 79 )(. ) ( 0. )(. )( 5.7 ) = = 0.0 m s = =.5 ft s 60 ( 0.45 )( 0 )(. ) ( 0.45 )( 4.0 )( 5.7 ) = = 0.9 m s = = 0. ft / s 60 Thus, the depths of runoff re computed with Eution 8.: pb t c Q b = α 0.0 ( 8 ).5 ( 8 ) = 6 = 4.7 mm = = 0.8 in p t c 0.9 ( ) Q = α = 6 = 8. mm. 0.( ) = = 0. in

3 The depth of storge (Eution 8.) nd volume of storge (Eution 8.) re: Q s = Q - Q b = =.6 mm = = 0.5 in V s = α()(q s ) =0(.)(6) = 8 m =60(5.7)(0.5) = 00 ft 8.. The tionl Formul Hydrogrph Method Given the populrity of the rtionl method, number of detention volume estimtion methods hve been developed using the rtionl method. These methods typiclly ssume tringulrshped hydrogrph with time bse eul to t c. One method uses the difference between the post-development nd pre-development pek dischrges nd the post-development time of concentrtion t c : V s = 60 ( - ) p pb t c (8.4) where, V s = storge volume, m (ft ) t c = post-development time of concentrtion, min p nd pb = post- nd pre-development pek dischrges, m /s (ft /s). The reltionship between these prmeters is shown in Figure 8.. Both p nd pb re computed with the rtionl formul of Eution 5.. V s p pb t c t cb t c t cb t Figure 8.. Volume of storge (V s ) determintion for the rtionl formul hydrogrph method 8-6

4 Exmple 8.. Becuse of development within 6-h (4.8-c) wtershed, detention bsin is plnned upstrem of n existing rodwy to prevent ponding t the culvert. Pre- nd postdevelopment pek dischrges of 0.4 nd 0.8 m /s ( nd 9 ft /s) were computed with the rtionl method. The post-development time of concentrtion is minutes. Thus, the reuired volume of storge is: V s = 60( )() = 8 m = 60(9 - )() =,00 ft t n verge depth of.4 m (4.6 ft), the pond will hve n verge re of 7 m (,890 ft ). 8.. The SCS T-55 Method Chpter 6 of SCS Technicl elese 55, or T-55 (SCS, 986), provides method for uickly nlyzing effects of storge reservoir on pek dischrges. It is bsed on verge storge nd routing effects for mny structures tht were evluted using the T-0 method (SCS, 984). The rtio of the depth of storge to the depth of runoff (Q s /Q ) is given s function of the rtio of the pek rte of outflow to the pek rte of inflow ( ). The reltionship between Q s /Q nd is: Q s s = = C Q o (8.5) in which C O, C l, C, nd C re coefficients (see Tble 8.) tht re function of the SCS rinfll distribution. The volume of storge (m or ft ) is computed by: V s = α s Q (8.6) where, α = conversion constnt eul to 0 in SI nd,60 in CU units Q = post-development depth of runoff, mm (in) = dringe re, h (c). Tble 8.. Coefficients for the SCS Detention Volume Method infll Distribution C o C C C I or I II or III Exmple 8.. Development within 7.-h (8-c) wtershed is plnned ner locl rodwy. plnning estimte of the storge reuired to detin runoff from 00-mm (.9-in) storm is needed. The curve number for existing conditions is 70, nd development within the wtershed will increse the CN to 80. The pre- nd post-development times of concentrtion re 0.55 hour nd 0.7 hour, respectively. 8-7

5 The pre- nd post-development runoff depths re obtined from the SCS runoff eution with vlues of mm (. in) nd 5 mm (.0 in), respectively. The I /P rtios re 0. nd 0.. From Eution 5., the unit pek dischrges re 0.94 m /s/km /mm nd 0.8 m /s/km /mm (449 ft /s/mi /in nd 55 ft /s/mi /in), ssuming type II rinfll distribution. Thus, the predevelopment nd post-development pek dischrges re: pb = 0.94 (0.07 km ) ( mm) = m /s = 449 (8/640 mi ) (. in) = 6.4 ft /s p = 0.8 (0.07 km ) (5 mm) = m /s = 55 (8/640 mi ) (.0 in) =. ft /s These vlues yield dischrge rtio of 0.57, which is used s input to Eution 8.5 to obtin the volume rtio s : s = (0.57 ) +.64(0.57 ) (0.57 = 0.7 Thus, the volume of storge is computed using Eution 8.6: SI Unit CU Unit V s =α s Q = 0(0.7)(5)(7.) = 000 m = 60 (0.7)(.0)(8) = 5,00 ft ) 8..4 ctul Inflow/Estimted elese The ctul inflow/estimted relese method reuires n inflow hydrogrph nd trget mximum relese vlue. n estimted relese rte from the storge fcility is drwn on grph of the inflow hydrogrph s shown in Figure 8.. The relese rte is usully ssumed to be Q Dischrge Q b Estimted relese Time Figure 8.. Storge volume estimte using ctul inflow/estimted relese 8-8