Drainage Design Manual, Hydrology - Flood Control District of ...

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Drainage Design Manual for Maricopa County Hydrology: Application 9.5.3 Application of Muskingum Routing 1. The Muskingum Routing method can be used where flood peak attenuation is expected. The best application of this method is for larger rivers with relatively flat slopes. 2. The parameters, K and X, are best determined by the analysis of stream gage data, if available. Where such data are available, K and X can be determined by analytic methods as presented in many hydrology textbooks, or the HEC-1 parameter optimization option can be used. Other regional flood studies (by the U.S. Army Corps of Engineers and others) may contain the results of such analyses for larger rivers in the County. 3. The following parameter estimation procedures apply primarily to natural stream channels which convey a significant amount of flow in the overbank areas during design-frequency events: a. NSTPS: The choice of a number of subreaches for a particular stream reach can be checked for computational stability using the following equation from the HEC-1 Manual: 1 -------------------- 21 ( – X) ≤ K × 60 1 ---------------------------------------- NSTPS × NMIN ≤ 2 ----------- ( X) where: (9.2) K = the travel time through the entire reach, in hours, X = Muskingum ‘X’, NMIN = the computational time step, (in hours) and NSTPS = the integer number of subreaches. b. K: K is the travel time of the floodwave peak through the entire reach. Calculation using Manning’s equation is usually an appropriate method for estimating the floodwave velocity, V m , with the following provisions: i. Use an average channel area and wetted perimeter for the reach, assuming bankfull conditions. ii. Choose an ‘n’ value representative of the main channel only. Do not include the overbank roughness in a weighted average. iii. Calculate an average flow velocity for the reach ( V ). 9-64 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Application iv. Use the following ratios (Cudworth, 1989) to estimate V m , the velocity of the floodwave: Channel Geometry V m V Wide rectangular 1.67 Wide parabolic 1.44 Triangular 1.33 The value of K is then estimated by dividing the reach length by V m . c. X: For wide, shallow channels with low to moderate slopes and significant overbank flow during the design flood being modeled, choose X = 0.15 to 0.25. For steep to very steep, narrow, deep channels with little overbank flow, choose X = 0.25 to 0.40. 9.5.4 Application of Muskingum-Cunge Routing 1. For constructed channels and some natural channels, this routing option can be used by providing all input on the RD record only. This requires selection of a predetermined channel shape (see the HEC-1 User’s Manual). Complex channel geometry and/or variable channel roughness (channel and overbank) can be modeled with the additional use of RC, RX and RY records. An eight-point cross section is input on the RX and RY records to describe the representative channel geometry. 2. Execution of the HEC-1 program may terminate with a math error message if the inflow to the routing reach is zero (no runoff generated from the upstream watershed). This may occur in situations that have either very low rainfall depth (intensities) or exceptionally high rainfall losses, or zero diversion (most often). 9.6 INDIRECT METHODS 9.6.1 Procedures The following instructions should be followed for verifying peak discharges that are derived by analytic methods (Rational Method or rainfall-runoff modeling). A. Verification with Unit Peak Discharge Curves: 1. For a given watershed of drainage area (A), in square miles, divide the 100-year primary peak discharge estimate by A. August 15, 2013 9-65
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