1.#QNAN error C++

I am new to programming and trying to write a new program. While checking through my program it is returning the error code 1.#QNAN. I have tried isolating the variable and researching for answers but cannot find any solutions.

My code:

 // This is a program to decide what culvert should be properly used for a specific installation
// using given measurements and data
//
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>

using namespace std;
// initializing functions 
double slope_function();
double cbasin();
// initializing classes: Subdivisions specs

//intitializing global variables
double edge_road =0;
double up_stream  =0;
double down_stream =0;
double tbm =0.0;

//double culv_length =0;
double slope = 0.0 ;
char street_name[1001];
int min_culv = 15;
double up_strm_culv =0;
double dwn_strm_culv =0;


int main (int nNumberofArgs, char* pszArgs[])
{
    cout<< "This program will allow the surveyor to double check their calculations\n";
    cout << "in deciding what size, type, and requirements are allowed for the\n";
    cout << "installation of culverts in Terrebonne Parish.\n\n";



// begin input
   cout << "what is the name of the street\nwhere the culverts will be installed: ";
   cin.getline (street_name,1000);
   cout << endl;
   cout << "What is the Benchmark: ";
   cin >> tbm;
   cout << endl;
   cout << "What is the elevation of the edge of the road: ";
   cin >> edge_road;
   cout << endl;
   cout << "What is the up-stream culvert size: ";
   cin >> up_strm_culv;
   cout << endl;
   cout << "What is the culverts up-stream inverted elevation: ";
   cin >> up_stream;
   cout << endl;
   cout << "What is the down-stream culvert size: ";
   cin >> dwn_strm_culv;
   cout << endl;
   cout << "What is the culverts down-stream inverted elevation: ";
   cin >