How to set value greater than zero for water level in simulink model so that i could go through with linmode?

Hi, i have a model of system with two tanks and i made a simulink model to get flow rate, volume and water height in tanks over the time. That part is ok.

Now i need to linearise that model, but when i use linemod i have a problem whit my simulink model in which i have values less that zero in my sqrt block, therefore i get my A matrix of ss with NaN's. I included my m file with simulink models.

My question is, how could i set height of water to not be less that zero and avoid this problem, or is there some other way to fix this.

Greatly appreciate any help. Thanks in advance. Milan