Basic Hydrology: Rainfall-runoff based methods – III

Subdivision The term subdivision occurs in two contexts Watershed subdivision into connected sub-basins Stream channel subdivision into subsections with different properties (Roughness, shape, etc.) This lesson refers to the first meaning: Dividing a watershed into sub-basins

Subdivision Watersheds are divided into sub-basins when Ground conditions vary substantially within the watershed (e.g. part urban, part undeveloped) Total area is so large that variations in precipitation depth within the watershed must be modeled Other situations where treating the watershed as a lumped unit is unsuitable.

Subdivision Watersheds are divided into sub-basins when Ground conditions vary substantially within the watershed (e.g. part urban, part undeveloped) Total area is so large that variations in precipitation depth within the watershed must be modeled Other situations where treating the watershed as a lumped unit is unsuitable.

Subdivision Each sub-basin has an outlet which will generate its own hydrograph. These hydrographs are “routed” to represent the effect of storage and travel time to the basin outlet.

Subdivision The computation of flow or conveyance for each water-surface application requires a hydraulic radius, and is intended as an average depth of a conveyance. If there is significant irregularity in the depth across the section, the hydraulic radius may not accurately represent the flow conditions. Divide the cross section into sufficient subsections so that realistic hydraulic radii are derived.

Routing-Hydrologic and Hydraulic Routing simulates movement of a discharge signal (flood wave) through stream reaches. Accounts for storage within the reach and flow resistance. Allows modeling of a basin comprised of interconnected sub-basins Hydrologic routing – uses continuity equation Hydraulic routing – uses continuity and momentum (St. Venant Equations)

Routing-channel and reservoir Reservoir routing Account for storage in a reservoir Unique storage-discharge relationship Channel routing Account for storage in channel as well as travel time Storage-discharge relation in channel is non-unique (looped; HDM pp4-113) Can treat channel as a series of reservoirs to mitigate looped effect.

Flow routed through a storage element (reservoir, pond, or stream reach)

Basic reservoir routing equations From continuity equation, basic relationship is: where DS = the change is storage during some time period Dt; Qin = the average inflow during the period; Qout = the inflow during the period For application, rewrite as: It-1, It = inflow at times t-1 and t, respectively; Ot-1, Ot = outflow at t-1 and t; St-1, St = storage at t-1, t

Storage indication curve Rather than trial and error with rating, convenient to create function relating O to [2S /t + O ]. Get from manipulating rating. Then, knowing [2S /t + O ], find O. Proceed.

Rating curves to storage indication curve Compute reservoir elevation-outflow curve – orifice and/or weir: Outflow = CA(2gh)0.5 + CLH1.5 Compute elevation-storage curve based on shape of reservoir (e.g. surface area x depth) Combine elevation-outflow and elevation storage to get storage-outflow At each storage-outflow level, compute 2s/t+O using the same change in time as the inflow hydrograph ordinates. Make sure time converted to seconds!

Storage routing calculation table Step It It+It-1 2St/Dt - Ot 2St/Dt + Ot Ot 2St/Dt St -- 1 62 55-7=48 7 62-7=55 2 130 3 394 4 388 Look up in storage indication function Compute if needed. Compute with computed outflow and storage for this period