Cleaning out a Trech drain

Folks,

I have one of these just on my door step about a meter width
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There are 2 bolts on each side which have it secured down. It is filling up with dust, dirt and miscellaneous debris and I was wondering how could I go about cleaning it out and what exactly are they for. I thought the bolts would be unscrewable but I tried this to no avail. I have been using a wrench and pliers ato try and pull them up and unscrew them (its hard enough getting a good grasp on the bolts) and I have had no success either. I was intending to pull the drain cover up (made of metal) and then hoover out all the dirt and other stuff. Any advice as to what I can do