Understanding and Estimating Travel Times of Overland Flows on Planes
Type of Degreedissertation
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Estimating travel time of overland flow is one of the important studies in hydrology. A quasi-two-dimensional overland flow model (OFM) with the options of dynamic, diffusion, and kinematic wave approximation integrated with particle tracking model and rainfall loss models for both impervious and pervious surfaces (planes) was developed to study flow travel times. There are many formulas for estimating time of concentration (Tc) of overland flows, but these formulas predict large unrealistic Tc as the topographic slope (So) approaches zero. Based on numerical modeling and a review of relevant literature, a lower bound for slope (Slb) of 0.1% was identified as a threshold below which traditional Tc estimation formulas become unreliable. The rainfall-runoff data collected in a relatively low slope field were used for model validation. The validated model was used to generate 750 Tc data using diverse combinations of the four physically based input variables: length (L), slope, roughness coefficient (n) of the surfaces, and effective rainfall intensity (i). The dataset of 750 Tc for a range of slopes was used to develop Tc regression formulas for standard slopes (So ≥ 0.1%) and low slopes (So < 0.1%). The OFM was also used for numerical study of travel times of overland flow using particle tracking model. Although Tc is commonly defined as the time for the runoff to travel to the outlet from the most remote part of the catchment, most researchers have used an indirect method such as hydrograph analysis to estimate Tc. Travel times for 85%, 95% and 100% of particles arrival at the outlet of impervious surfaces (i.e., Tt85, Tt95, and Tt100) were calculated from 530 model runs directly tracking both slow and fast moving particles on the flow plane. Regression equations of Tt85, Tt95, and Tt100 were developed using the four physically based input variables (L, So, n, and i). Stormwater with velocity equal or close to equilibrium velocity (Veq) can cause more soil erosion on pervious surfaces and transport significant amounts of dissolved and particulate materials on impervious surfaces to downstream receiving waters. The diffusion wave approximation of OFM was used to simulate Veq of impervious overland flows using diverse combinations of the four physically based input variables. A dataset of 530 Veq estimates was developed and the relation between Veq and input variables was developed.