ESS 424 Exam 2 | Questions with Verified Answers

ESS 424 Exam 2 | Questions with Verified Answers What is the definition of a watershed? land draining streams and rainfall into a common outlet How do the runoff rate per unit area and runoff volume per unit area generally change as the watershed area increases? Runoff rate per unit increase Runoff volume per unit area decreases What are some cases in which the watershed area may be very hard to define? - Area with unusual geology or where groundwater may be contributing to base flow. Know the component of a storm hydrograph inducing the axes. Indicate 3 things (pieces of information) about the storm hydrograph we are generally most interested in. - Axes = Intensity/Time - Generally interested in Effective Rainfall, Lag Time, and Peak Discharge. Why does runoff not begin immediately with the beginning of rainfall? Initial abstraction Define time of concentration duration for water to flow from the most hydraulically remote point to the outlet In most relationships to estimate time of concentration, the travel time for overland flow and for channel flow are treated separately. Why? Overland flow is not concentrated and will be significantly slower than concentrated flow in a channel. What factors are important in the relationship we saw to estimate overland flow time? Type of land, soil, and vegetation. What factors are important in the relationship we saw to estimate channel flow time? Length & Steepness Explain why a design storm duration equal to the time of concentration gives the maximum design runoff rate? Entire drainage area is contributing to the outlet simultaneously, resulting in peak flow and the most intense rainfall scenario. Know the factors which control runoff. Local infiltration controls, Topography, Watershed orientation and shape How does the watershed size affect how the watershed responds to storms? Larger watersheds will have less concentration than smaller watersheds. Describe why a storm that is first light then heavy produces more runoff than the opposite pattern Because soils will usually only infiltrate the first 1" of rainfall. It absorbs the light rainfall and then will cause runoff in the heavy portion of the storm Explain why a 50-year rainfall event may not produce a 50-year runoff event. Precipitation and runoff are not controlled by all of the same factors. What are the two common uses of runoff estimation? 1) Design: We want to know how big to make something. 2) Control: We have a system in place but need to limit how fast water comes out. A "design storm" is a hypothetical rainfall depth on which we will base our runoff estimation. What two factors are used to determine the design storm depth? Return Period and Duration On what basis is the design storm return period chosen? Depends on cost of failure The method we discussed for estimating runoff volume is called what? "Curve Number" Method The runoff volume estimation method we discussed uses what 2 parameters as inputs? ___P, CN,_____ and area of land. Let's say that you have 4 rainfall maps to work with: A) 2-yr 1-hr, B) 2-yr 24-hr, C) 50-yr 1 hr, D) 50-yr 24-hr. Which would you most likely pull information from if, for your small watershed, you were designing: a) a culvert under a small country road - 2-yr 1 hr b) a culvert under a major highway - 50-yr 1 hr c) a livestock pond - 2-yr 24 hr d) a reservoir above a subdivision - 50-yr 24 hr The initial abstraction in most cases is Ia = 0.2 * S. What does the S in this relationship represent (not just the name, but what it means)? S- all things that control infiltration or how the whole system responds to rainfall The initial abstraction in most cases is Ia = 0.2 * S. What does the 0.2 in this relationship represent (not just the name, but what it means)? 0.2 is the "initial abstraction" or what fraction of storage must be filled up before runoff begins For our method of estimating runoff volume, as the precipitation depth increases from 2" to 6" for CN = 80, does the percentage of precipitation leaving as runoff increase or decrease, and is that change relatively minor or substantial? It increases and it is very substantial. CN = 80 @ 2in = 28% runoff CN=80 @ 6in = 63% runoff If our watershed has several different land uses or soils (different CN values), what is the best way of handling that? Either calculating the runoffs separately and adding them up together, or calculating a area weighted CN value One significant problem with the method we used for estimating runoff volume is that it doesn't take into account what parameter? The intensity of the rainfall for a storm What are the reasons that the method that we used for estimating runoff volume is NOT appropriate for estimating small-storm infiltration? Because the data used to develop the SCS curve number method was wholly obtained from large storms The method we discussed for estimating peak runoff rate is called what? Using the Ration Method which is q=CiA How will we decide on a rainfall duration and return period to use in peak runoff rate estimation? Peak runoff rate only considers choke points. Use duration equal to ToC What 3 parameters serve as inputs to our peak runoff rate estimation method? C= area weighted from land use/conditions i= max rainfall intensity in inches/hour A = Area in Acres What is the primary addition of the TR55 Method to what we have seen in the other methods, and what impact does that have on estimated runoff rate? - It includes a pond/swamp adjustment factor - An increase in this factor will cause a decrease in peak runoff rate What is the "frequency method" of runoff estimation based on? Watersheds & historical data What is meant by "effective rainfall"? rainfall that contributes to runoff or infiltration, excluding losses like evaporation or interception What two things must be done by a stormwater conveyance? Handle the peak flow without failure, not have any erosion If I have steady-state flow, the friction force acting upslope is equal to what other force? The force of gravity down the channel Write out the Manning's Equation, and define the terms. V = (1.49 / n ) R2 / 3 S1 / 2 V - velocity in fps N - mannings roughness S - channel grade R - hydraulic radius Describe (in words) what the "hydraulic radius" really means. cross sectional area divided by wetted perimeter Designing a channel requires solving Manning's Equation and what other equation simultaneously? The continuity equation The pipe flow equation presented in class says that the flow rate depends on the loss coefficient and what two other inputs? Bend loss coefficient & pipe length coefficient How do we figure out the flow rate that our channel must handle? Usually determined with either the rational method or the TR55 method Use of Manning's equation generally assumes that the flow depth is much greater than what other measurement? Roughness Describe a case in which Manning's n can change rather suddenly. For example upright grass would result in a high n value but if the grass gets laid over the value goes down very quickly How do you change your channel design if the velocity is higher than the permissible velocity for our soil type and cover? Make the channel wider What are the two reasons that we don't just make every channel very broad and shallow? Cost is a factor and also flow that goes too slow introduces new problems What is the biggest problem at a pipe or channel inlet? Conveyance - water move in the path of least resistance, pipe/channel is most resistant by design