ABE 527 Computer Models in Environmental and Natural Resources Engineering HEC-1 Flood hydrograph package developed by the Hydrologic Engineering Center (HEC) Release 1, U.S. Army Corps of Engineers. Model components A river basin is subdivided into an interconnected system of stream network components. Fig. 2.1-2.2 Steps in the analysis. 1. Watershed boundary delineation 2. Subbasins division based on study purpose and degree of heterogeneity. Each subbasin represents a hydrologic unit that is homogeneous. 3. Each subbasin is represented by a combination of model components a. Runoff b. River routing c. Reservoir All are available in HEC-1 d. Diversion e. Pump 4. Subbasins and their components are linked together to represent the connectivity of the river basin. Runoff component Input to this component is a precipitation hyetograph. Precipitation excess is computed based on (subtracting) infiltration and detention. Infiltration and detention are uniform over the subbasin. The rainfall excesses are routed by using the unit hydrograph or kinematic wave techniques to the outlet of the subbasin producing runoff hydrograph. Base flow is also computed and measured. River Routing Simulation of flood wave movement in a river channel. Input to this component is upstream hydrograph. This routing depends on the characteristic of the channel. Combined use of river routing and subbasin runoff components This is achieved via tree structure Reservoir component This component is used to represent the storage-outflow characteristics of a reservoir, lake, detention pond, highway, culvert, etc. It takes an inflow hydrograph and routes it. Reservoir outflow is a function of storage and not on downstream controls. Diversion components Used to represent channel diversions, stream bifurcations or any transfer of flow from one point of a river basin to another point in or out of the basin. It receives an upstream inflow and divides the flow according to a specified rating curve. Pump Characteristics and on-off elevations, number, capabilities, pumpflow can be retrieved in the same manner as directed flow. Hydrograph transformation This includes ratio of ordinates and hydrograph balance Differences between detention and retention in reservoir routing Detention: temporary storage, released while mass balance is conserved Retention: storage, may be released by main balance is not conserved Typical cross section schematic of reservoir out flows Derivation of flood flow routing Continuity Equation: S Iavg Oavg T Inflow – Outflow = change of storage rate I t-1 I t O t-1 O t St St-1 2 2 t ‘I’s (Inflow info=I) are known from design storm runoff hydrograph ordinates. Rearranging the above equation: 2St 2S O t I t-1 I t t-1 O t-1 t t t-1 values are known; t=0 values are initial values. The above equation is one equation with two unknowns, Storage (S) outflow (O) relationship is needed. How? See figure 49 below: Figure (E) is derived from figure (D). For a certain elevation, figure (C) provide total flow where figure (E) provides the corresponding volume. These generate (F). The example that follows on reservoir routing will demonstrate the use of Figure 49. Principles of flood routing Inflow = Outflow + Storage Or: idt = odt + sdt i=inflow rate for small increment of time o = outflow rate for same increment of time s = storage rate for same increment of time dt = time differential i1 i 2 O O2 t 1 S2 S1 2 2 1, 2 are subscripts denoting beginning and end of the time interval. t Problem Statement Route the inflow hydrograph below through a reservoir structure with outflow rating. Curve through weir and pipe flowing full shown in Figure 2. The stage storage configuration of the reservoir is shown in Figure 3. Example of reservoir routing Figure 1. Inflow and outflow hydrographs Figure 2. Outflow-stage relationship based on the outflow design configurations. Figure 3. Storage-stage configuration of the reservoir. Solution: O Table 1. Computing S 0 relationship. t 2 Note 3.4 not 34 in column 2, row 2 Column 1 flow from figure 2 Column 2 corresponding storage (Figure 3) for each stage height of Figure 2. S O Figure 4. O relationship; column 1 and 5 of Table 1. t 2 (PEAK OUTFLOW) Table 2. Flood routing through a reservoir computation Column 2 is using inflow hydrograph of Figure 1. Column 4 is flow from Figure 4. O2 Note that O2 2 O2 2 Weir flow formula used in the example problem 3/2 q = 0.55 CLh q = discharge in m3/s c = weir coefficient (3.0 typical) L = weir length h = depth of flow over the crest in m Pipe flow formula used in the example problem a 2 gH q 1 K e Kb K c L q = flow capacity L3/T a = cross sectional area L2 H = head causing flow (L) Ke = entrance cross coefficient Kb = bend coefficient Kc = friction loss coefficient 1. 2. 3. 4. 5. Pond Design criteria 100-year post development peak must not exceed the existing peak The emergency spillway must not be used for the 100-year event or smaller Pond is empty at the beginning of the storm Max. depth is 6 ft. for safety Outlet is 24” min diameter for easy access How to find a pond that meets the requirements: A. Compute the predevelopment design event hydrograph B. Compute post development design hydrograph C. Select a pond site and a relationship of water depth and storage D. Select outlet configuration and develop a relationship of depth and outflow for the configuration E. Select routing interval ∆t (5-6 ordinates at least including inflow peak) F. Construct storage outflow relationship by combining the relationship from steps C and D. G. Compute outflow hydrograph H. Compare max outflow rate with target rate I. Adjust pond and/or outlet structure configuration. J. Repeat the procedure steps C-J for alternative design.