Hey pal, look at the rest of the posts where there's actually math that justifies it before you post and make yourself look like an idiot.
Add to that most Riven players have CDR in their Mastery and Rune pages, plus what gets built to give Riven 40% basically every game into the late game, I know combined I start with 12.5% CDR, Others have scaling CDR that rapidly drops that cooldown to nothing.
Let's do some different math here, according to the math you neglected to look at, Riven's Shield is approximately 35 damage blocked beyond what Rumble's upgraded shield does at early levels. 117 compared to 153
Rumble has about 5 hp5 more than Riven, meaning that every 5 seconds Rumble will heal 5 more hp than Riven will. With Riven's Shield on a 10 second CD compared to a 6 for Rumble, we won't even factor in the CDR. Rumble would block 117 and then heal 10 for a combined benefit of 127. Riven still gets more out of her shield alone remember the number is 153
How's that for backup? Try looking at base stats of those shields before you post as well, since early game oppressiveness was the complaint remember? 75 on Rumble's enhanced shield, which he also gets silenced from using too much anyway, 90 all the time for Riven. Add the scalings and it shows the deviation rather quickly for that early game power
It's still not a proper comparison. Have a nice day.