Scalable Website Backend

We have an app that collects data from a users body. Some data has 10 readings per second while other have only one reading every 10 or 30 seconds. There are approximately 15 values being measured during a test. Each test can last from 10 seconds to hours of data being collected. There is also data about each user such as their weight, height, body type, medical conditions etc.. Additionally each session will have information about their state such as activity level, stress level, energy level. The information will be sent from our app to our server. You need to also create an api or have one created to receive information from the phone and send to the phone app if necessary as well as for their login and password. We are expecting a lot of users so we want to start with an architecture designed for high volume. Additionally we want to analyze all of the data in many ways in individual accounts as well as across all accounts to find useful patterns. The main part of this project is to: 1) Create the user interface 2) Allow people to see a list of each of their sessions 3) Click within each session to explore the data. There will be about 6 graphs and other data related to their session. I'm thinking of something like Hadoop or Cassandra will be the best but I'm open to suggestions. Something that will scale with us as our userbase and data increases.