Run-off and Flood Prediction for the Muddy River

Midterm Project Report

By

Erik Robison

 

 

Introduction

 

The Muddy River is located in Southern Nevada, and releases any flow into Lake Mead at the top of the Overton Arm.  This river has a small base flow that varies throughout the year.  Approximately once every ten years the river swells and floods the lower portions of Overton Nevada as shown in Figure 1. These floods result in property damage to homes located in this region.

 

DCP_0373

Figure 1: The Muddy River flooding during the year 2005

 

Objectives

The purpose of this study was to develop an understanding of the runoff, and extents of flooding that would result from a 100 year storm. To accomplish this task it was proposed to utilize ArcGIS to create a map of the drainage basin that would include the information necessary to calculate the runoff using the SCS method, and then route the flow through Moapa Valley using HEC-RAS.  To accomplish this, a drainage basin would need to be delineated, and soil types, land cover, and precipitation events determined. The resulting runoff could then be calculated, and routed through Moapa Valley.

 

ArcGIS

Using the Arc Map 10 software that was available, a map of the area was created. The first item added was a base map of the area from Bing maps, using a hybrid map of the area which included satellite imagery with roads transposed on for reference.   A DEM, Flow Accumulation, and Flow direction, for the Area was downloaded from NHDPlus (Colorado Region 15b), and added to the map.  Three HUC_8 water sheds constitute the relevant area, because Moapa Valley sits at the base of the most downstream watershed as shown in Figure 2.  After the relevant watershed basin was established, the rest of the irrelevant area was then trimmed to eliminate unnecessary data, and help the software run a little faster.

Figure 2: Relevant Watershed Basin

 

After the watershed was delineated and flow lines added (Figure 3), Land Use (National Land Cover Data Set of 2001) data was downloaded from USGS. This data was used to determine the land cover percentages by the statistics in the attribute table for the land use layer (Figure 4).  The Land Use layer first had to be trimmed to the watershed boundary, and then the count was recorded for each type of land use.  The count is the number of cells in the grid corresponding to a specific land use type.  This information was input to a spreadsheet, and percentages were developed based on the land use type count per total count (Table 1). 

Figure 3: Image of the Flow line through Moapa Valley

 

Figure 4: Image of the Land Use

 

Table 1: Percentage of each type of land, with the classification group each was placed in to correlate a curve number

 

 

 

Hydrology

Some land cover percentage values were negligible, and categorized in a group that isn’t characteristic of that cover type, such as water.  This land use data was put into 4 categories to establish an approximate Compound Curve Number (CN) based on the SCS method established in 1986 (Table 2).  The soil group was determined as group C and the corresponding CN values were used.  This information was used to develop the Compound curve number shown in Table 3.

Table 2: Table of Runoff Curve Numbers from TR-55 (http://www.lmnoeng.com)

Table 3: Resultant Compound Curve Number

 

Using the statistics of the delineated watershed, the total area of the watershed was found to be 876.4 square miles.  Precipitation for the area was taken from the NOAA website, for a 100yr 24hr storm at various locations in the watershed. These amounts were averaged to obtain the value of 2.62 in. This Information was used to determine the flow accumulation in the area.

Equations

 

 = 1.96 inches

= 1.19 inches2

Runoff = Q*Area = 55,600 acre ft

S = the potential maximum soil moisture after runoff begins in inches

P = Precipitation in inches

Q = Runoff in inches

 

Additional Research Necessary

 

The Routing portion of this research was not conducted due to the lack of sufficient cross-section data for an input to HEC-RAS.  It was originally planned to import the GIS file into HECGeo-MS and HECGeo-RAS for the analysis, but it was discovered that the software doesn’t currently support version 10 of ArcGIS.  This portion of the research could be conducted when this is made possible.  It would also be able to be conducted with an earlier version of ArcGIS. 

 

Further research could be conducted by evaluating runoff for previous precipitation events, and back calculation curve numbers for the area.  This information could then be compared to the curve number established using soil and land cover data.  A coefficient could also be back calculated with the same methodology for the area.  It would be suggested to evaluate as many precipitation events as possible to eliminate erroneous data, and determine correlation between precipitation amounts, and runoff events.

 

References

LMNO Engineering. Ken Edwards ,Ph.D., P.E. November 22, 2010. LMNO Engineering. (Dec, 1 2010) http://www.lmnoeng.com.

 

National Hydrography Dataset plus (2010) “Great Basin Region 15 b” Horizon systems Corp. < http://www.horizon-systems.com/nhdplus/>

 

USGS Land Cover Institute (LCI) (2010) “National Land Cover Dataset of 2001” < http://landcover.usgs.gov/usgslandcover.php>

 

NOAA’s National Weather Service (2010). “Hydrometerological Design Studies Center.” Precipitation Frequency Data Server, <http://hdsc.nws.noaa.gov/hdsc/pfds/sa/nv_pfds.html> (Dec, 1 2010).