# Lag time and of concentration relationship questions

### VICAIRE - Module 1B - Chapter 4

rect runoff, determined on the basis of the relationship between the time lag and the time of the time of concentration on very small watersheds in which surface flow dominated: . problems, but has been used frequently for overland flow in. Describes several methods and equations useful for time of concentration computations. One estimate of lag time as provided by NRCS is that: tl = *tc; SHEET FLOW: It is Above equation is applicable for sheet flow lengths of upto ft. . Sustainability; Water Resources; Statistics; Questions/Discussion · Contact. this lesson, a relationship between the direct runoff hydrograph of a catchment observed at a Continuing in this fashion, it can be concluded that after a lapse of time '4Δt', concentration, which is the time that is taken by the water from the furthest point of the .. the widest possible range of problems. This includes large .

## An Exercise In the Use of the HEC1 Hydrologic Simulation Model

Below is an example of the spread sheet calculations for time of concentration: Curve Number The curve number is the parameter used by the model to estimate the potential maximum retention of rainfall. In other words, with the Curve Number the model determines the amount of excess rainfall that will result in direct runoff. The Curve Number depends on soil type, land use and antecedent moisture conditions. The land use is urban with a mix of industrial and dense residential development.

A value in the range of is a reasonable estimate for the Curve Number. The length value to use in this equation was difficult to determine. The furthest point from the outlet for this basin is approximately 19, feet. This value of length would not be representative of the length intended for this equation since, judging from the topographic maps and seeing the basin, most of the flow through Northwest Park has traveled a considerable distance in channels.

Therefore I consider a length of approximately feet to be more representative of the basin. Design Storm Design storms are precipitation patterns used for hydrologic modeling.

Design storms are based on historical precipitation data for the area of interest. The design storm used for the input for the model is the alternating block hyetograph.

The incremental depth is calculated using discrete time intervals. A design storm of any frequency or duration can be calculated and converted to an alternating block hyetograph. See the sample spreadsheet below.

The hyetograph can be shown graphically as follows: The illustration below is a typical HEC1 input file used in my study. The following parameters were explored: The tables were inserted into Excel where they were rearranged into a useful form for analysis. Design Storms and HEC1 Keeping the Curve Number and the time of concentration constant, 5 different design storms were run in the model.

### Time lags in watershed-scale nutrient transport: an exploration of dominant controls - IOPscience

The following graph illustrates the different resulting outflows for each design storm. The year storm had the highest peak and volume the area under the curve and the 5 year storm had the lowest.

They each had the same duration time. The following hydrographs resulted. The higher curve numbers result in a larger amount of runoff and therefore a higher peak flow and flow volume. The curve number of 93 would be representative of the Shoal Creek watershed because of it being an urban watershed with substantial development. The curve number of 77 would be representative of a undeveloped watershed with good soil infiltration and slower overland flow velocity.

Note how the peak of CN77 is much lower and the curve is flattened out. A smaller lag time smaller time of concentration results in a higher peak flow over a smaller time interval. Discharge is normally measured at gauging stations that are situated at different points along the river.

The discharge of a river changes over time depending on a few factors. The most influential factor is the weather. A storm hydrograph is a specific type of hydrograph that, surprise, surprise, shows precipitation and discharge during and after a storm. The main difference between a normal hydrograph and a storm hydrograph is that a storm hydrograph is over a much shorter period of time. Below is a storm hydrograph for the fictional River Shui: First is the lag time.

The lag time is the time difference between the peak precipitation and and the peak discharge. Conversely, a short lag time indicates that the precipitation is entering the river fairly quickly. The rising limb is the steep part of the discharge line that has a positive gradient, indicating that the discharge is increasing. The falling limb is the opposite showing that the discharge is falling.

One factor is the shape of the drainage basin.

• Time lags in watershed-scale nutrient transport: an exploration of dominant controls
• Lag Time versus Time of Concentration
• Discharge & Hydrographs

Drainage basins come in a wide assortment of shapes. Roughly Circular shapes are common as are more elongated and narrow shapes. This is because all points in the drainage basin are again, roughly equidistant from the river so all the precipitation reaches the river at the same time.

The size of the drainage basin obviously has an impact on the hydrograph.